Helicopter Urban Myths

[NickLappos]

Ten Urban Myths that pervade our understanding of helicopters and how they operate. Each is fundamentally incorrect, but most are generally held as gospel, because training, lore and reference documents have repeated them long enough that they are simply accepted.

1) Vortex Ring State (VRS) can happen at only 300 foot per minute descent, it does not have to be a higher descent rate

2) VRS is more likely at high altitude and high gross weight

3) Hovering with the nose off wind consumes much more power

4) Blade stall is always preceeded by vibration

5) Winds affect the power we require when we are in forward flight

6) Downwind takeoffs are absolutely forbidden

7) The Height Velocity curve is a precise guide to the engine failure danger zone

8) Engine failure is the most common accident cause, so full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters.

9) The legal definition of VFR is sufficient to assure flight control and safety using outside references

10) "They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.

11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.

12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees

13) LTE is when you run out of power pedal and can be experienced by any single rotor helicopter.

14) NVG are dangerous and should only be used by gifted military pilots.

15) You have to first learn to fly fixed wing before you take helicopter training

16) Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.

[SASless]

Number 9....Nick is dead on the money on that one....he echoes my harrangue on that topic.

A large Arab oil company's part 91 operation ignored the reality of that concept....and bought 212's without SAS (.....SASLESS....to some!) and uses them for night offshore flights....defending their decision by saying...ah...but the weather is VMC. (despite no visible horizion). DF actually thought we believed that line I think?:p

[autosync]

Good thread,

A lot of people comming out through the system don't question what they have been thought, or what they read in books, they then go on to teach what they were been thought and add in there own redundancy to be extra cautious, and the cycle continues, until before you know it, a whole generation of people think that helicopters are just slow moving airplanes.


Pedal turns, must always be done to the left (or right in European helicopters)
Is another load of B.S I have noticed from people comming out of large schools.

zxcvbn
can you explain number 1 please? are you saying it can happen at < 300ft per min descent?

May aswell open a can of worms and try and answer this.
Personally when I am demonstrating this on a flat calm day in a piston engine helicopter, it takes at least 500fpm ROD indicated, before I get the oscillations, pitching, rolling and sluggish cyclic response associated with VRS, bearing in mind that the air around the static port will be turbulent and there is also a delay on the VSI, I would assume that the ROD is nearer to 800 fpm, on a windy day it will obviously be greater, more often then not it is very very difficult to demonstrate VRS in a Robbie or huges 300 and what some people consider VRS is actually just pilot induced autorotation!

But I would never tell a PPL student that, for fear of luring them into a false sense of security, I am guilty of using the 30 30 30 system to make it easy to remember.
30% power applied
>300 fpm ROD (500fpm is greater then 300 so its not really a lie!)
<30Kts of airspeed

[Martin1234]

Nick, is there some kind of mathematical model made for some helicopter types in regards as when VRS might start to occur? I'm thinking in terms of a GPS with a nice 3D reception and an input of W/V....


...apart fram that, what other factors would such a software need to take into consideration apart from aircraft type?



"i was taught that pedal turns should be done to the left always (in the r22). what is the reasoning for teaching people this?"

Probably that if you run out of left pedel the right one will always be there for you. Interestingly, I was taught to use right pedal to consume power. I was also taught to, if possible, accept a right turn when exiting a confined area in order to use the power to get me out of there, not to feed the tail.

[fulldownauto]

That's true about making pedal turns to the right, they use less power, but require power to stop the turn, so if you make a left pedal turn, you won't find yourself in a situation where you'll be unable to stop it, the torque of the main rotor will do that for you.

Additionally, if you make the pedal turns to the left in the R22 if you happen to smack into anything, it won't be tail rotor first and you might have a chance to come away from it without losing the tailrotor.

"They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.

Who's they?

[NickLappos]

Martin1234,

I tossed some stuff together a while back at www.s-92heliport.com/vrs.htm

fulldownauto,

I tossed that one in for all those threads where "They" (and "They" know who they are!) are hiding the real effect, the real Kennedy gunman, the real cause for all those accidents, the patent for the engine that runs on water.

Actually, "They" is anyone but "Us", and I am not so sure about "You". ;)

[Ascend Charlie]

It's hard to puncture those myths when you have official stuff, such as the New Zullund CAA publishing their Good Aviation Practices (GAP) books (available as a PDF from the NZ CAA website) which says on page 15:

"on approach, VRS can be minimised by keeping the rate of descent less than 300' per minute when IAS is less than 30 kts."

Straight from the horse's arse...

[NickLappos]

We should take nothing away from the professionals who wrote all those pubs, with 99% of the info as solid and helpful (and even the VRS info is conservative, thus safe). The thing we must do is seek the "why" for things, becuase our procedures wil be tied then to understanding, not just rote memorizing.

I truth, if you have more than 300 fpm downward rate while below 30 knots, you are making a lousey approach, VRS or not! My personal gate is to be less than -300 fpm when below 150 feet and 30 knots, just for basic power conservation, since the collective pitch suck-in can be 10% above the hover power if you make a hairy flare when entering the hover.

VRS can't occur at less than 70% of the downwash velocity, which is perhaps 700FPM for many light helos. If you set -300 fpm from a hover, and let the aircraft settle without raising the collective, it will accelerate downward (that is not VRS) until it gets into VRS at maybe 700 fpm. most professional helo pilots who do photo chase and the like have made vertical descents like that without fear of VRS (and rightly so). The book is not right, but as advice for newbies, it is sound advice.

[Flingwing207]

Hi Nick,

Just to belabour the obvious (and I have looked over your website studiously), point #1 is saying that you might - and most probably will have to - be at well in excess of 300'/min descent rate to get into VRS. If so, I agree, but will continue to hammer my students to stay inside the guideline of ROD<300'/min before A/S<ETL. I'd rather be exposed to the tiny chance of engine failure than the relative certainty of VRS if they find the ROD required!

Your point #3 is interesting only in that it is commonly taught that in a CCW-rotor helo (like the Jet Ranger), a right crosswind hover will require more power than a left crosswind hover. This is usually based on the idea that the T/R is having to further accellerate the air which is already travelling right-to-left. I would think that the power demanded by the T/R is directly related to the mass of air being accellerated (not the speed), and the AOA (not blade pitch) required. So in this case, more left pedal would not necessarily mean more power required.

However, the wind itself exerts a fairly significant force on the entire empannage, so a left crosswind would offload the T/R a bit, while a right crosswind would add to the force trying to yaw the helicopter. So indeed, more power should be demanded in a right crosswind than in any other azimuth. (Of course, a right crosswind is helping counter translating tendancy... ...oh my head!)

Point number 10 is no myth - sometimes they do hide things. We all do - it's quite necessary to do so in the course of everyday life. The key is to hide the appropriate things, and there's the rub - who decides what's "appropriate"? Why me, of course!

[Shawn Coyle]

Nick:
Great summary of the myths.
Not sure I agree with no. 3, but perhaps it's just minor wording.
May I add 2
Turbine engine power available depends on density altitude?
Governors have no effect on airframe handling.

[Texdoc]

zxcvbn,

Besides
"Additionally, if you make the pedal turns to the left in the R22 if you happen to smack into anything, it won't be tail rotor first and you might have a chance to come away from it without losing the tailrotor."

It is also easier to clear your right side all the way back to avoid the previous from happening.

[Paul McKeksdown]

Hmm, a few interesting ones there!

The vortex ring state is generally written to include the 'worst state'. What this means is that rotor downwash recirculation, leading to the incipient vortex ring state, can occur below the expected value due to a variety of environmental effects. How many of us have flown a vortex ring demo perfectly? If you take this 'worst case' teaching with an aircraft on a still wind day with a perfect 0 groundspeed descent the rotor tip vortex recirculation will occur at a lower ROD due to the vortex state not being removed from above the rotor by wind/forward airspeed etc... This serves as a warning, especially in cases of photographic flights etc. where the aircraft may be doing 30/40 kts downwind with 30 kts wind and hey presto you are in still air, add in a gentle rate of descent and theres your problem.

It has been seen where rapidly rising air over, for example. a very hot concrete surface can cause a reduction in the required OBSERVED ROD.

Watch out for environmental factors that can bite.

As for the turns, the decision to turn the aircraft is based upon which pedal you use to arrest the turn as not to over torque upon stopping. (be it a US or European Helo)

Gas turbine engines work upon the principle of mass airflow and are optimised to perform at a specific mass throughput. Therefore changing the density of the incoming air will have a dramatic effect upon the engines power/performance. Why do you think we calculate density altitdue calculations for helo ops. Don't expect your Jet Ranger to leap off the pad in Dubai with the same performance as in the UK. It ain't gonna happen. OAT is a performance killer for Jet Helos!!

And Finally, coz thats all I came remember from da points, VFR places the flight visibility, conditions and maintenance therefore, terrain and aircraft collision avoidance with the aircraft captain. The weather gets too cr%^py then don't fly, Land, go home whatever. The captaincy decision is yours alone, if you go for it and screw it up then the decision was yours alone, noting to do with what the rules state. A poor decision will always come from poor information.

Pheew, sit back, don steel helmet and wait for flak!!

:p

[CRAZYBROADSWORD]

The downwind takeoff thingy. I have always done them when I had to and have always tought my students how to T/O and land downwind, yet I see lots of pilots T/O into wind and then make very early downwind turns almost as soon as they get transational lift which to me seems daft. Does anyone reckon there is a right or wrong way to get airborne downwind?

[Paul McKeksdown]

There is no problem in a heli taking off down wind, the only things you need to be aware of are a longer run to obtain translational lift and a flatter climb gradient.

Obviously when transitioning downwind your indicated airspeed is going to be lower than your ground speed which then takes its effect on circuit pattern positioning, ground rush etc...

A downwind approach, and yes they do need to be flown sometimes, must be approached with caution. The ASI will, once again, be reading low in comparison to the ground speed and the rate of descent, if flown like a normal approach, will be higher. Always have plenty of power in hand and approach the site a little slower than normal with a flatter approach path. BE WARY!!! The wind coming from behind will put you into the vortex ring area as you approach 0 kts IAS with 20Kts ground speed and you want to arrest that rate of descent :}

Taking off into wind and then turning directly downwind with little height and a low airspeed whilst feeding in a banking manoeuvre strikes me as a recipe for disaster :uhoh:

[Thomas coupling]

I would suggest, Nick that there are very few people alive today who have actually experienced actual VRS. demonstrations end at the incipient VRS and go no further.
Those who have experienced fully developed VRS are either stupid, lucky or test pilots:E

[Shawn Coyle]

Paul:
The power available from turbine engines depends on pressure altitude and (mostly) temperature, not density altitude.
5,000' Density altitude can be 9,000' and -40C or 2,000' and +40C - the power available will be wildly different.

[SASless]

TC....

Does it make you stupid if you are not a test pilot but go out and intentionally experience VRS as part of a training situation? Me thinks you put too many limitations on your catagories here.

Done at a safe altitude....is this yet another maneuver that can be demonstrated safely? I am still here after all these years....thus either I am a quirk in the laws of probability or it can be done. It does get the blood to pumping sometimes...but in my younger days.....we thrived on adrenalin.

[Paul McKeksdown]

DA=PA+/-120T

The DENSITY altitude changes dependant upon the temperature. The density of the air I.e. the mass per cubic meter as the temperature climbs. Therefore if the pressure altitude is corrected for excessive temperature the density altitude will climb and make the power from the engine less.

A gas turbine is not interested in how high it is it is interested in the masss gas flow through the combustion chamber and the cooling. If the mass per cubic meter reduces then the amount of 'burnable' air reduces, the amount of air required for cooling reduces and therefore the available power reduces and the operating temperature of the engine increases. IRRESPECTIVE of the pressure altitude.

Please note that the operating data for helicopters, especially the bigger aircraft, are calculated on density altitude.

So thanks for pointing my 'error' out and get back in the books.

Cheers:rolleyes:

Add fuel to the fire;)

Suck,squeeze,bang,blow!!!!

The amount of intake required by a gas turbine engine is approximately 10 times that required by a reciprocating engine. The air entrance is designed to conduct incoming air to the compressor with minimum energy loss resulting from drag or ram pressure loss, that is, the flow of air into the compressor should be free of turbulence to achieve maximum operating efficiency. Proper design con-tributes materially to aircraft performance by increasing the ratio of compressor discharge pressure to duct inlet pressure.

The amount of air passing through the engine depends on the--

Speed of the compressor RPM.
Forward speed of the aircraft.
*****Density of the ambient air.*****


I thank you!!!!
:E :E :E

[diethelm]

Paul:

I think that Shawn's practical point is that density altitudes derived from lower temperatures and higher pressure altitudes can produce more power than same density altitudes derived from higher temperatures and lower pressure altitudes in a typical turbine. The reason is because a typical turbine runs out of temperature before it runs out of volumetric capacity.

[FixedRotaryWing]

The power of a turbine engine does not only depend on air mass pumped through. It also depends on:

a) Temperature. If the same mass of air comes cooler to the combustion chamber, the engine deliveres more power due to better heating rate.

b) Ambient air pressure. If the exhaust gas has to fight against less ambient air pressure, the engine delivers more power.

The result is: At SAME density altitude, a turbine engine will deliver more power at high pressure altitude (that is low ambient air pressure) and therefore cool temperature.

[ShyTorque]

TC said:

I would suggest, Nick that there are very few people alive today who have actually experienced actual VRS. demonstrations end at the incipient VRS and go no further.
Those who have experienced fully developed VRS are either stupid, lucky or test pilots.
---------------------------

Or just a tired pilot over Forkhill. Felt obliged (!) to take control of a certain Puma once after the handling pilot of the night got himself in a bit of a fix and lost 3500ft in not many seconds. A long time at FL100 hovering followed by a lapse of concentration was all it needed. Slight rearward aircraft movement resulted in a slight pitch up, then suddenly increasing ROD followed by random pitching and rolling, VSI on the bottom stop against OGE hover power. Interesting for a while, including quite extreme nose down attitudes. Not sure he would have recovered it by himself. Having recognised things were going wrong and warned him twice to get airspeed, he was slow to respond and down she went. I put the cyclic to the instrument panel for him and pushed the lever down, interestingly, the nose down attitude didn't change for quite some seconds, she was doing her own thing. Once we had some airspeed, out she came. Not a lot was said on the way home, but a lot of thinking was done.

[jellycopter]

Another couple of myths:

"The R22 is a great little training helicopter."

"If you can fly an R22, you can fly anything."

J

ShyTorque,

I think quite a few Puma jockeys can recount a story very similar to that one.

I used to be the QHI out there for a while and spent a lot of time showing the new guys just how easy it was to get into VR. One particular evening, whilst demo\'ing from FL80, we lost 6500 ft, even though we knew it was coming. There was a long thread on this subject about a year ago.

It\'s quite disconcerting when the cyclic doesn\'t work quite as you\'d like it to. The few second it takes to get that indication of airspeed seems like an eternity when the cyclic is straining against the bottom of the instrument panel!:ooh:

J

[Jack Carson]

FAA Practical Test Standards for a 61.58 check for a type rated aircraft requires a demonstration of Power Settling. This maneuver has the ability to establish an aircraft in VRS. Having flown a recent Check ride in an H-64E, I was required to perform this maneuver. It goes against everything we were taught and in the case of the Crane resulted in a relatively harsh aircraft response. I questyion the FAA's requirement for this maneuver. Does anyone else share this belief?:confused:

[dammyneckhurts]

In response to the 300 fpm descent for VRS, I find that generally speaking you need a lot more then that.....however,I am a guy that has experienced I-VRS several times with the rate of descent at Zero, I know a few others that have had the same experience......The conditions condusive to this situation are rare, but some in the industry will come across it from time to time....Hovering at the top of a cliff face, with a line on, air coming up at you from below, low power, tail is twitching around, pull up just a smidge on the pole and the bottom drops out. Gets the heart pumping when your that close to the ground.......

[Paul McKeksdown]

Back to the Gas Turbine, yes I slept well thank you :E

Gas Turbines for helicopters are, generally, small fixed spool compressor turbines optimised for a specific altitude. If the turbine is operated at this design altitude it will perform at its best. Remove it from its design criteria and the ISA conditions that the egg heads tested it on and it will generally perform worse. This is due to a variety of factors:

1. If the density, i.e. the pressure and the temperature of the air, is very thick, we are looking at cold and high pressure then the turbine will still under perform. The reason being that the compressor is inefficient at compressing the thick, sticky air. The combustion chamber will love it, all that cold thick air and the cooling will be excellent however the power required to turn the compressor will rise dramatically and therefore reduce the theoretical power available to the free power turbine.

2. Going the opposite way, if the air is very thin and very hot i.e High altitude and high temperature then the compressor is happy because it doesn.t have to work so hard to compress the thin air. But, by the time the air reaches the combustion chamber it has to be split into three flows:
a. Primary, the air available for combustion
b. Secondary, the air fed into the combustion chamber to prevent the flame from contacting the sides. Flame shaping
c. Tertiary cooling air, pumped around the combustion chamber to provide cooling.

If these flows are already thin and hot then it is obvious that the overall operating temperature of the engine will increase. There is no longer, physically, as much air to burn therefore the efficiency of the combustion chamber is reduced which leaves less ooomph for the power turbines and the free power turbine. The cooling is less efficient resulting in a high operating temperature for the engine.

Have just checked the operating data manual for the S-61N, which incidentally includes Inlet Guide Vanes to assist with mass flow during startup due to the fixed compressor design and thick, slow moving air (;-)), and all of the engine graphs are entered at pressure altitude and then corrected for temperature giving you, hey presto, density altitude. So irrespective of the pressure, if it's 1045mb and 50 degrees the performance would be questionable on ECU temperature grounds.


:ok:

[Vfrpilotpb]

Hey TC,

Been there with a very experienced Cfi(Ex Mil many 0000's of hours) showed me the way things happen, and how fast they happen, and thankfully how to get out of the dreaded Vrs, then made me do it quiet a few times,

Glad I've been there! :\ (I think)

Peter R-B

[FixedRotaryWing]

Hi Paul McKeksdown,

there are many ways to combine pressure altitude with temperature. Only one of them gives you density altitude.

Have you checked your graph, whether it really results in density altitude? Is it really an engine only graph? ...

[Paul McKeksdown]

Okay, we seem to be a bit at logger heads here :rolleyes:

The problem seems to be what the definition of density altitude is and how that then aerodynamically and performance related effects the aircraft.

Density altitude is pressure altitude corrected for variations in temperature. A definition, not mine :-

"Density altitude is defined as the pressure altitude corrected for non-standard temperature variations. And while this is a correct definition, my definition is perhaps more appropriate: DENSITY ALTITUDE IS THE ALTITUDE THE AIRPLANE THINKS IT IS AT, AND PERFORMS IN ACCORDANCE WITH."

If you look at any performance graph for a commercial aircraft you will find the pressure altitude, because thats what you read off of your calibrated altimeter corrected to msl by the pressure setting. This will then be cross coupled with ISA +10, ISA +20,ISA +30 temperature lines. You enter the graph for, say, single engine performance at your pressure altitude, for example 1000'. This would then be ICAO corrected at 1.98 degrees per 1000' therefore I would EXPECT the temperature to be 13 degrees. I look at my OAT guage and see the temperature is 33 degrees and, hey presto, I have ISA+20. Then I correlate my pressure altitude line to the ISA+20 altitude line to give me ..... my density altitude, which can then be applied to the performance figures for the engines to find my value.

I have operated helos all over the world including in hot & high environments and have ALWAYS calculated my performance figures based upon density altitude because performance on a 1013 day in england at 10 degrees OAT is not the same as the performance from the same helo in Dubai on a 1013 day at 45 degrees OAT even though the barometric pressure altitude is the same!!!

Pressure altitude is used as a base line because its given by the instrument, only the uncautious would use it without taking into account the OAT.


P.s. Just checked a couple of other aircraft types, and they are all Pressure corrected for temperature, i.e. density. S61N, S76, EC135

Your turn.......:ok:

[NickLappos]

Paul McKeksdown,

The problem with your theory is that you don't see any limits. When Tzero is hotter, the T5 must go hotter for a given power, by about 3.5 degrees C per degree Tzero. That means the turbine temp limit is reached earlier, so the power output of the engine is strongly influenced by temperature. At constant pressure alt, the engine power drops off by about 1% power for each degree C Tzero rise. Big influence that temperature has. Big.

[Paul McKeksdown]

Yep, I see your point and if you look at the threads posted previously you will see that the whole debate centres upon whether pressure altitude or density altitude affects the performance.

As density altitude is DIRECTLY related to the temperature this is directly related to engine performance. As the temperature goes up, the efficiency of the turbine goes down due to lack of effective combustion chamber cooling. Therefore, as your post suggests temperature has a huge effect upon performance but the question is why? I have attempted to answer that. Hot and High is a killer for helo turbines as they are not equipped with the correct type of variable stator/rotor compressors that large bore, high bypass jets possess. Temp goes up, density alt goes up, air thins out, ecu temp goes up and performance goes ..... down!

The original 'myth' was that turbine power depended upon the density altitude, which, in my experience (3500Hrs+) is not a myth but true.

As to the limitations they are, obviously, different for each machine but the principal behind why is common to each aircraft.

Does that make sense?
:{

[Paul McKeksdown]

Found another one :E

'THE EFFECTS OF INCREASED DENSITY ALTITUDE'
On engine performance (power available to the rotor)

Turbine performance will be adversely affected by increased density altitude.

1. As DA increases, air density is less. For a constant speed compressor in a fixed spool engine the mass of air entering the combustion chamber decreases. For a given throttle opening the combustion chamber temperature increases as there is less air to cool the flame.
The throttle can be opened to give more power until the T4 (TOT) limit is reached. As DA increases this limit is reached at smaller throttle openings and the power available is less. (Note: See previous posts as to the internal workings of the turbine)

2. In a free power engine the compressor speeds up as DA increases, to maintain the same mass of air entering the combustion chamber. The throttle can be open to give more power, and the compressor speed (Ng) increases the mass of air, until a limit is set by the forces on the compressor blades. As DA increases this limit is reached at smaller throttle openings and the power available is less. Thus the power available from the gas turbine engine reduces with increasing DA (constant pressure altitude with increasing temperature for example). However at low DA when the power available is greatest there will be a limit set by the maximum transmission torques permitted'

There are your limits and the reasons why DA effects them so greatly. How it physically happens in the engine is described in a previous post but I do have a power point presentation detailing the internal workings of a gas turbine, written by me for tuition purposes, if you wish a copy Nick then PM me.

I find it interesting as I have been teaching this stuff for years now and I am surprised that there is so much opposition to the theory. Look at the common pressure versus highly differing temperature example to see how your performance will drop purely by the increase in temperature.

[NickLappos]

Paul McKeksdown,

You can't look at it like a constant throttle or cooling air issue, because that is not how the engine actually is used. The author of that piece really does not understand how an engine operates, or at least does not understand how to tell someone how it operates! To explain what is happening, you must consider the need for a fixed amount of power, and what the engine must do to the air it eats and the fuel it burns to make that power. When the pilot needs a certain amount of power, the engine bends over backwards to deliver it, varying throttle and gas generator speeds as needed to make the power. The engine is only an expensive device that heats air and then extracts the heat from it, that is all. The reason the engine runs to higher temperatuures when it eats hot air is NOT that the throttle is more or less open, it is because the air mass times the difference in temperature makes the power, so a hotter package needs higher temp to make the power, as does a lower mass need even more difference in temp to make the power. Since hotter air has less mass per unit volume the given volume must be heated up more to make the power.

If you run it at lower altitude, the air is thicker, so more mass means less temperature increase needed, and it runs cooler.

The engine is affected by density altitude, but not like a wing or rotor, because the density is only part of the engine's equation. The temperature itself is also a problem for the engine. A wing or rotor does not care about temp, it cares about density, so it behaves relatively constant with respect to constant density altitude. An engine does NOT behave constantly regarding constant density altitude, because the absolute temperature of the air package is important to the engine, regardless of its density.

[Paul McKeksdown]

Sorry but here the good will ends:*

What a complete load of b:mad: ks!

The engine operates on the principle of VELOCITY like every other damn jet engine in the aviation world. God knows where your profile comes from but if this is level of your technical knowledge then dont come knocking on my office door asking for a job!!!

The air is COMPRESSED by a COMPRESSOR (written large so you can read it) in order to provide a better package of air for the COMBUSTION CHAMBER where the COMBUSTION takes place. The resulting expansion of the gases within the COMBUSTION CHAMBER causes a high velocity airflow to inpinge a POWER TURBINE which turns the COMPRESSOR. After that any remaining energy in the gas flow would be ejected from the back of the engine as THRUST in a normal jet but in a helo it turns a FREE POWER TURBINE before exitting the EXHAUST.
So Mr Lappos engineer supreme, Heat is a necessary by product of combustion but one that destroys components therefore parts of the air are used to cool. If HEAT drove the engine we would want to run it untill melt down. Check your theory before you drivel here.
I thought that this was a professional pilots rumour network but you've burst my bubble.

At the end of the day the engine turbine temperature limitation exists at 795 degrees for the S-61 IRRESPECTIVE of the outside air temperature because thats the point at which the b:mad: y combustion chamber and the face of the power turbine would MELT. Even then on a high temp day you wouldn't be anywhere near your torque limitation.

Thanks for your prattle and search for Gas Turbine theory on the internet.

[boomerangben]

Paul said:


Have just checked the operating data manual for the S-61N, which incidentally includes Inlet Guide Vanes to assist with mass flow during startup due to the fixed compressor design and thick, slow moving air (;-)),


The IGVs on an S61N do not move until Ng exceeds 63% - well above the ground idle speed. There is an air dump valve in the compressor that is opened during the start sequence to allow the engine to start without hanging or stalling.

I am not sure what you mean by "fixed compressor design". The IGVs and the first 3 stages of guide vanes move (between 63 and 95% Ng). The CT58-140-2 Engine has a free power turbine.... Are there engines that have moveable compressor rotor blades or are there engines that have the compressor spools disconnected from their turbine spools?

As for aircraft performance graphs, I was under the impression that these had as much to do with the blade aerodynamics as engine performance. Density altitude affects the amount of lift a blade can generate and therefore the amount of drag and hence the torque required, which then determines how much power the engine needs to supply. Agreed the engine would also be affected, but I doubt it is as much as the blades are. Besides I seem to remember that the thermodynamic cycle the gas turbine engine is based on is a Temperature cycle not a volume or pressure cycle.

[Thomas coupling]

Confused or what, now:
So what is the correct answer:
turbine power is directly affected by densite alt, or pressure alt?

I thought it was always DA???



Vfr - howdy. You weren't demonstrated VRS, you were demonstrated IVRS.

Jack Carson: I agree with you it was demonstrated years ago. Sycamore pilots used to demonstrate it to ab initio RAF pilots in the 60's. I worked with one of these instructors for a while who explained that it was demonstrated only the once to each student during their course, from FL100. And very often, it took in excess of 7 thousand feet to recover. Some didnt make it. It was only when they looked into it in detail that they realised how silly it was practising it!

In fully developed VRS [which in theory most helo pilots should be aware of], you actually have NO control over the aircraft. If it decides to tip inverted, or stay fully developed all the way to the deck, then you've bought the farm! If you recover from fully developed VRS then it is not thru anything you have done..the a/c simply decides it wants to fall out of that state.

It's not to say a pilot couldn't demonstrate it...but he would be suicidal to allow it to develop fully.

juddering / wishy washy feedback from the controls / increase in ROD to excessive limits....is IVRS.
Loosing feedback from the controls completely, massive ROD's, excessive yawing and pitching...is VRS.

[Droopystop]

Paul,

And you teach this stuff?

I have done gas turbine theory twice, in two separate and unrelated educations and both tell me you are wrong.

PS If I was you I wouldn't bother asking Sikorsky for a job and I don't suppose Nick would ever have to ask you for a job.

[Paul McKeksdown]

Okay, more than happy to see your idea of how it works :}

The S61 IGV, which are fuel pressure activated, close at plus 63% ng to brinf the airflow into line with the 3 rows of movable stators which follow the row of IGV's. The primary reason being to allow the smoother transition of air into the compressor at low compressor rotation speeds. As the Ng increases the IGV's close up bringing the IGV's and Stators into line with the 7 stages after the front 3. Without this system the ECU would be damn difficult to start. Any Questions there????

To bring ECU theory to its absolute basics its the Suck, Squeeze, Bang,Blow and if you have a problem with that then, sorry, thats not my problem.

ECU's have a performance problem with density altitude full stop. They are regulated bu mass gas flow NOT by power derived from temperature. Temperature is a product of that required to produce velocity which is then useful.

If any of you doubt it have a look in the exhaust once in a while and play with the free power turbine, its what keeps you in the air.

Have fun

Oh, and just as a bit of tiger poking, if our wonderful engine runs purely on termperature differentials, can someone explain why they surge then??? Wouldn't want that power going back into the cold compressor would we!

[NickLappos]

When you get some physics lessons, let me know, Paul. A certain Mr. Carnot could tell you that your velocity is actually caused by heat, strange as it seems to your view of the world. I do have some understanding about this, having gotten 6 or 7 patents on turbine engine fuel controls. Perhaps you could look those patents up on the net while I look up the stuff you think I don't know!

[Paul McKeksdown]

I would just LOVE to know where you get the idea that HEAT is converted into power? Even the piston engine has heat as a by product, requiring bulky cooling systems where as the pressure within the cylinder is required to push the thing around.

Heat is used in this case for gaseous expansion leading to velocity. Love to see your patents:p

Please expand your wisdom to explain to all of us poor lost uneducated souls how the Lappos engine works, I'd love to see it.:E

[boomerangben]

The S61 IGV, which are fuel pressure activated, close at plus 63% ng to brinf the airflow into line with the 3 rows of movable stators which follow the row of IGV's. The primary reason being to allow the smoother transition of air into the compressor at low compressor rotation speeds. As the Ng increases the IGV's close up bringing the IGV's and Stators into line with the 7 stages after the front 3. Without this system the ECU would be damn difficult to start. Any Questions there????


Right let me get this straight, you are saying that the IGVs are closing as Ng increases. So your glorious mass flow is being obstructed by your now closed IGVs at high power?

Heat by the way is the kinetic energy of the gas particles. It is that kinetic energy that is imparted on the power turbine blades (via conservation of momentum) that generates this power. Have you never wondered why your crewmen don't see the 600 degrees C that you T5 gauges are reading? It is because the power turbine has cooled your T5 and transferred that energy to the gearbox.

[NickLappos]

Paul,
Try this, which plots the volume and pressure (mass) of air vs the temperature. It shows the area under that curve as the work the engine produces. Thus endith the first lesson:

http://www.lerc.nasa.gov/WWW/K-12/airplane/Images/carnot.gif

http://www.lerc.nasa.gov/WWW/K-12/airplane/Images/carnot.gif

[Paul McKeksdown]

Okay, if you won't go with my explaination of the theory then I have kindly added a couple of links. You'll like the first one, done for kids with lots of pretty pictures and, oddly enough, talks about velocity??? With heat only being the primer required for gaseous expansion, well theres a surprise.

http://science.howstuffworks.com/turbine.html

A little history, but then Nick with your patents you won't need that will you. You've written it.

http://www.aircav.com/histturb.html

Ironic that all of them talk about gaseous expansion and velocity. Uncanny to think that they use these engines with thrust in aircraft, whatever will they think of next. I know HEAT CONVERTORS, need one of those on a cold day.

Cheers

Oh, and thanks for the piccie but where is velocity?????


:E
p.s. From your beginners website have alook at this:-
http://www.grc.nasa.gov/WWW/K-12/airplane/bgp.html


Good website, by the way have alook at this:-
http://www.grc.nasa.gov/WWW/K-12/airplane/Animation/turbtyp/ettp.html

and notice where the highest pressure is, ie where the gas flow with the most energy, doing the most work is. Oh, its at the face of the power turbine, providing the energy to turn the compressor, whats left is THRUST my dear.

Sorry Nick, you went to the wrong page for your ideas :D

Have a look at the Brayton Cycle, thats the one that equates to Gas turbines by the way, which shows the pressure against volume. Sorry but your work done by transfer of heat energy equation that is at the end of the Carnot cycle, for reciprocation engine I believe but then I\'m not sure,

http://www.lerc.nasa.gov/WWW/K-12/airplane/brayton.html

Questions?
Sorry one last one, a quote from the NASA website:

The temperature increase depends on the type of fuel used and the fuel-air ratio. The hot exhaust is then passed through the power turbine in which work is done by the flow from station 4.

Suck, Squeeze, Bang, Blow

QPQ

http://www.lerc.nasa.gov/WWW/K-12/airplane/Images/braytonpv.gif

[Droopystop]

The Brayton Cycle. Thanks Paul for the reminder. So can you see now that a gas turbine engine depends on temperature more than anything else?

Mass flow rates are what goes into the engine and comes out the other end, but they dont explain what is going on inside the engine. Mass flow rate depends on average gas velocity. Average exhaust gas velocity depends on the temperature (Tout rises when lever is pulled). Therefore the mass flow rates depend on temperature. Increase gas temperature, increase gas velocity, hence afterburners. Afterburners only increase the amount of mass emmitted by the engine by the amount of extra fuel burnt. Insignificant. No extra air is going through the engine when the afterburners are on. Extra thrust is provided by extra heat.

[autosync]

Suck, Squeeze, Bang, Blow
At what point does the Bang occur?

[SASless]

Now I know what a dog watching television feels like!:ok:

[Paul McKeksdown]

So, does extra heat push the aircraft faster through the air???

Have a closer look at the theory, the more fuel you add the greater and more rapid the expansion of gases and therefore the greater the velocity. Temperature does indeed increase however this is not the route cause but more a problem. If temperature were not a problem then we could pump the afterburner fuel load into the combustion chamber instead and let the compressor sbenefit from the extra ooomph. Why do we burn the fuel in an after burner?? and why do we have to protect the aircraft from the heat with a convering shround like the F16 for example?

The heat is a LIMITING factor, without it we could dump all the fuel we wanted into the engine and enjoy unlimited power. Gas turbines work well in cold temperatures because the Torque limitations are reached first.

Look at start limitations eg. S61 start temp allowed 840 degrees momentairily (hate that word). Normal operating is 680, why is that? It's because during the start cycle the engine can handle a short hot blast. The overtemp checks are to look for small molten bits of the combustion chamber on the turbines and exhaust casing. Heat kills engines full stop. More fuel, more heat but more importantly a greater gaseous expansion.

p.s. on the afterburner thingy look at the back of most military aircraft and you'll see a moveable shroud. This reduces the cross section of the afterburning exhaust output to increase the available thrust. (Newtons laws) The reason being a converging duct produces an increase in velocity for a given reduction in pressure. As we are looking for pure thrust at this point a duct is introduced.

Adding to that look at the construction of the compressor and you'll find convergent ducts used to impart anergy and velocity and in the power turbine divergent ducts to extract energy. Moving air is stored energy pure and simple it doesn't matter how hot or cold it is.

Adding to the other comments, have alook again at the wording Diethelm.

In the S-61N the Free power turbine is effectively a windmill sitting in the exhaust outflow. It turns high speed shaft connected to the MRGB. There are two Power Turbines that are placed directly behind the combustion chamber and fed from the chamber through a convergent duct. These two are then connected DIRECTLY to the Gas generator or compressor if you will, how else is it going to spin. The biggest problem is that the airflow is split into tertiary which provides the combustion chamber cooling, if you reduce this you reach your temperature limitation quicker.

Sorry should have said that that comment is for a fixed lever/throttle position!!

The Bang comes when you light blue touch paper in the combustion chamber!!!!:E

Any more???:p

[NickLappos]

The story of any heat engine is the story of the flow of energy as the working fluid is heated by fuel, and then the heat is exchanged for work by some physical means. There is nothing wrong with thinking about the velocity of the gas stream as doing work, but it is not the origin of that work, only an intermediate step.

Paul, the way you are analyzing a turbine engine is as if you looked at a light bulb as the start and the electric generator as the end of a system that converted darkness into fuel.

I do suggest that you take a peek at a thermodynamics text, you are far and away over your head on this subject, although I am sure your intentions are good.

SASless, if I put up a picture of a mailman, will you bark?

[Disguise Delimit]

Pull My Keks Down has tossed a bait into the forum and hooked a couple of serious fish.(Nick et al). He has played the catch with a lot of energy and got a lot of fun out of it.

But he doesn't know when to cut the line before the fish climbs into the boat and eats him.

Is this another appearance of Flare Dammit / Pprune Fan #1??

[Paul McKeksdown]

I'm not in over my head I was just trying to answer a simple question of which limits a gas turbine. The thermal expansion dynamics of air whilst in the combustion chamber is of no great interest but I have looked into it and the dispersal of fuel particles from the injectors using laser spectro annomymetry but who cares.

The simple answer is that if the makers could build the thing to accelerate the air without wasting the stored energy of the fuel in heat we would all sing hallelugha or however its spelt. Please tell me I'm wrong.

Are you really taking this down to the transfer of stored energy within the fuel to the air mass???? How deep do we wish to go? We could start talking about the relative properties of different aviation fuel but where will that take us? We wish to extract the energy held within a fluid medium and apply that, within our turbine, to and air flow to produce thrust. The stored energy is transferred by combustion causing explosive expansion. This produces a variety of energies. 1 Velocity through expansion, Perfect. 2. Heat, waste of stored energy unfortunately necessary in energy transfer 3 sound, see point 2.


Combustion is a means to accelerate the medium to produce the motion from which the work is extracted.

Newtons law that any force has an equal and opposite force.

Heat pushes nothing it is a means to an end. It is our limitation. The more heat you produce during the combustion the more rapid the expansion and transfer of heat and the graeter the pressure and when guided the velocity. Lets pull till the thing melts shall we.

You talk about the conversion of heat to work and energy but you provide no physical evidence of how this is acheived within the engine. My theory is corroborated by just about every internet site that explains jet engines, fuel is used to accelerate the air, how it does it is, at the end of the day, irrelevant.

P.s. Light bulbs are wonderful things but I aint interested how they work just where the spares are! Ah there under the undercarriage light indicator. :ok:

[Droopystop]

Urban Myth No. 56b

You don't need to be a rocket scientist to fly a helicopter.

Urban Truth No 1

You do need to be a rocket scientist to explain how it all works.

Paul,

Look once again at the Brayton Cycle that you referenced. It is a plot of temperature against entropy. Entropy is a measure of the energy in a system that is unavailble to further processes. Gas turbine theory has NOTHING TO DO WITH MASS FLOW. You have given us all a link that unequivocally shows this.

Yes temperature is a limiting factor because of material technology - if a metal could withstand 1200 deg C, the engines would have an amazing power to weight ratio. The graph of T against S shows that the bigger the temperature change, the more work is produced, ie the more power you get. That is exactly what the Brayton cycle says. If you cannot see this then enrol on a first year engineering degree. This is first year Thermodynamics.

[Hughesy]

This is all quite interesting, but with Mr Lappos posting here there is no sign of good ole Lu. :D
Unless Paul McKeksdown is Lu. :D
No body mention the rumoured turbine robbie.
:cool: :D
Hughesy

[Lu Zuckerman]

To: Hughesy

This is all quite interesting, but with Mr Lappos posting here there is no sign of good ole Lu.

Mainly because Lu does not give a Sh!t. My knowledge of a jet engine and application of that knowledge is the function of every part of a jet engine, the interrelation of the parts within that engine and the effect of the failure of said parts on the engine and ultimately on the craft that the engine is installed in.

All of this without the understanding of mass flow, heat input, heat rejection, entropy, enthalpy, turbine airfoils and power extraction as well as many other things jet engine related.

However I do know what a convergent divergent duct is (De Laval Nozzle).

I find it interesting that each of you with similar engineering background have come up with your own interpretation of how a jet engine works and none of you agree with the other participants on this thread.

Unless Paul McKeksdown is Lu

And no, I am not Paul McKeksdown.


:E :E

[407 too]

Paul, Nick, et.al.

can a no-mind join in ??

seems to me like you are both saying the same thing from different angles

your talking different languages but tell much the same story

in my small space, heat is an agent to interact with something to get a result

bear with me as we reverse engineer this to the basics, ability to turn a power wheel, but lets use baking soda and vinegar instead of fuel and air

keg of vinegar, shovel full of baking soda, uncontained, gas goes everywhere, contain it in a baloon, baloon gets big, contain it by a steel vessell, pressure builds, open a port, gas is expelled against a wheel with slanted veins, wheel goes around, no heat, no outside air, just vinegar and soda which produces a gas.

now lets substitute vinegar with air and baking soda with jet-b (match reuired), same results as with vin and soda

the HEAT creates the expansion of air, the expansion of air drives the wheel.. so WHAT DOES THE WORK, THE HEAT ACTING ON THE AIR, OR THE AIR EXPANDED BY THE HEAT ???

depends on your slant i guess

open the other end of the vessel and continually put in vin and soda OR fuel and air and get same result as a turbine (much less power, granted, but i'm operating with a 25 watt bulb, not 500)



or should i learn how to knit ???? :O

[NickLappos]

407 too,
You do really understand it! The point is the heat from the burning fuel is the source of the energy, it is used to push against a wheel and convert the heat energy into motion which creates the velocity, then it pushes against another wheel and actually cools the airflow down (T5 is always hotter than T7 because the work has been taken out of the flow). Heat is the thing that we are tracking, it is pure energy.

[SASless]

407....very akin to our Nick thinking of Nicole Kidman.

Heat and energy!

[407 too]

but, alas, at Nick's age the result is just a lot of hot air :ok:

[407 too]

as for the Hp or Hd myth, i say - - YES

if you correct (making number up) say 5,000 ft at 10C you may get 5600 ft **AT WHAT TEMP ?? **

if you correct 4,000 ft at 20C you may get 5600 ft **AT WHAT TEMP ?? **

is it ISO or ISO corrected for std. lapse rate, either way it is an altiutde which is a temp corrected Hp !! (it can't be the starting temp or you are not corrected)

the aircraft will act as if flown at that Hp

SO, if you say Hp is the one, I think it needs to be qualified by saying it has been corrected for temp :eek:

[pa42]

. . . one wonders whether a smart dog can at least make sense of the scoreboard. At least much on telly HAS a scoreboard . . .

how 'bout if we form an umpire committee to keep score on the hot air debate and advise-at-end-of-thread? Then my occasional lapses into somnolence will interfere less with comprehension of the bottom line.

Poor Paul, to so ruthlessly take on Nick, and presumably thereafter retire forever from the forum in shame!

[R1Tamer]

"The simple answer is that if the makers could build the thing to accelerate the air without wasting the stored energy of the fuel in heat we would all sing hallelugha or however its spelt. Please tell me I'm wrong."

"Heat pushes nothing it is a means to an end. It is our limitation. The more heat you produce during the combustion the more rapid the expansion and transfer of heat and the graeter the pressure and when guided the velocity. Lets pull till the thing melts shall we."


Paul,

It seems to me in these two simple sentences you've succinctly outlined why you're failing to grasp such a simple concept. Let off some steam, have a good long look at what you've written here and you may start to see where you've become so completely unhinged!

Best of Luck

[Paul McKeksdown]

Hi all, sorry the moon came up and I had to get some sleep :}

So, after taking the advice I had a good look back through the posts and came to an interesting conclusion.

Nick, I can see what you are getting at when we look at a pinpoint section of the gas turbine combustion chamber. However, it sounds like you are an engineer who is having a problem putting across an overview. Correct me if I'm wrong, I'm sure you will.

You are stating that the thermodynamic heat and energy transfer given by the combustion of the fuel within the combustion chamber will be less due to the increased starting temperature of the combustible mass. I.e. The overall range of thermodynamic movement is restricted by the temperature limit of the engine. Start at 0 limit at 100 = 100% energy transfer, start at 10 limit at 100 = 90% energy transfer. Does that squeeze your physics into a bean can for mere mortals???
Sorry but you took a damn long time to come to a useable point ;)

If that is so I can finally see what your aiming at and I think we are looking at this from two differing perspectives. I don't know what the teaching is over in the good 'ol US of A.

My standpoint is of what we, as helo drivers, can understand within the turbine. My point to answer the question was a point of keeping the combustion chamber within its physical design limitations. If the core of the primary air is burning at 1500 degrees plus then a hike of 20 degrees in the OAT is going to make a change but will that change be significant enough to limit the engine? With the combustion chamber walls glowing at 800 degrees as the secondary flow attempts to contain the flame the hike in OAT will, probably, cause a more noticeable rise in engine operating temperature. Cool the whole thing with hotter air and you hit your engine limitation far quicker. We are talking about the MODULE here, not just the fact that your fuel burn is slightly more inefficent.

I think that both factors play a role here, the question is which is more limiting?

I have contacted an engineer in the aero engine section of Rolls Royce to ask his opinion and will post his reply here when I have it.

I must apologise to Nick for a previous, not so gentlemanly, post but it did take you a long time to get to a useable argument
:ok:

R1Tamer, the two things are mutually inclusive. If we could extract the locked in energy in the fuel without wasting it in heat transfer the world would be a better place. Take your car engine after a long drive, touch the engine and think how much petrol you have wasted getting that big lump of metal to heat up. If we could develop useful ceramic combustion chambers then okay, lets pull till we die, the problem comes in stopping the stuff melting. Unhinged? don't think so look at a video from Rolls Royce doing a PPI check on a gas turbine and watch it glow white!

[Paul McKeksdown]

From the propulsion support group:

The limitation placed upon the engine is a combination of the effects of cooling, mass airflow and fuel burn. These effects are different dependant upon whichever turbine we are talking about. A turbine requiring a large cooling factor, generally the older turbines will react badly in a hot environment due to the restrictive cooling and the restrictive throttle. A more modern engine with less cooling requirements will perform poorly with a greater emphasis placed on the restrictive throttle position caused by reduced the thermal expansion range.

SO! After a long and labourious thread it would seem that there is no winner or loser. The system reacts slightly differently dependant upon the aircraft, engine fit and age. Some aircraft, generally the older, respond poorly to cooling, hence not reaching their torque limits before the temperature. Others, with more efficient cooling systems reach the limit by running out of 'combustion space'.

It is a broad range of aircraft, situations and scenarios that we have and to flame someone for looking from a different view point as having no knowledge is a dangerous thing. There are very few 'black and white' situations in our profession.

Nick, thanks at the end I can see it from your point of view as well as my own and after flying the S-61 for too many years I have also learnt that there are other factors out there. The performance and limitations that I use have served me well in ops around the globe and I will continue to use them but I will also now include your theorys as well. It is interesting to see that a quick canvas of opinions at work resulted in 1. Blank looks, 2. Arguments for both ideas.

So..... Thanks all, I'll wait to see what comes up. :D

[212man]

SASles: Ah, but it must be a true story as it's in colour!


Paul:
"Gas turbines work well in cold temperatures because the Torque limitations are reached first"

Now you've really lost the plot.

[R1Tamer]

Paul

'Unhinged? don't think so look at a video from Rolls Royce doing a PPI check on a gas turbine and watch it glow white!'

This made me chuckle somewhat? Okay Unhinged was a bit strong...sorry. It's been a very interesting debate though.

As a final if somewhat moot point. I've no need to watch videos of Rolls Royce engines since I worked on making enough of them during my apprenticeship not to mention much larger gas turbine and nuclear reactor chambers. Try going beyond white hot - to a point where the castings become so translucent that you can literally see through them!


This is real enginering - practice meeting science!

[SASless]

212man....

Does that mean things that are Black and White are not necessarily true? We must colour our perceptions to see reality?

Oh dear....back to watching the freight trains roll by.:E

[Flash2001]

Paul said:

p.s. on the afterburner thingy look at the back of most military aircraft and you'll see a moveable shroud. This reduces the cross section of the afterburning exhaust output to increase the available thrust.

WHAT!

After an excellent landing you can use the airplane again!

[Paul McKeksdown]

R1 Tamer, thanks

:ok:

I found it fun as well!

SASless, wasn't the whole world black and white before they invented colour?

Flash, ahhh, then convergent duct to increase thrust, think I mentioned that.

Thanks all.;)

[Flash2001]

Yeah, well, the ones I remember opened when the burner was lit...

After an excellent landing you can use the airplane again (And maybe even the helicopter)!

[Aser]

Now I know what a dog watching television feels like! ROFL , good one SASless!

A dog.

[Thomas coupling]

Well:
is it DA or PA????????????????????????????????????????????:confused:

[NickLappos]

These statements are true:

Turbine maximum engine power output can be accurately told by using pressure altitude and outside air temperature.

Turbine engine maximum power varies greatly if operated at identical density altitude, but at variable outside air temperature.

[Steve76]

Well if we are going to start taking off downwind now we might as well begin learning our downwind auto's as well.
We would be able to relate to the fixed wing boys as well....

Powered pedal turning: super idea. Start doing the RH pedal turns lads and see how long you last. Like it was said, you can push in all the RH pedal you want but can you stop it? Very imperitive that you understand the concept for operations in bush and remote areas or at altitude.
Get ready to write an overtorque report.

[NickLappos]

Steve76,

You are right mostly, and sometimes not. The aircraft will not fall out of the sky if you take off downwind, and it can be perfectly safe. In fact, it can be perfectly safe to take off vertically, and tell the wind that you just don't care. Lots depend on the power margins and options available. I think our military brothers who fly attack helos or medevac can relate to those times when hovering downwind and shooting, or landing downwind and pcking up injured was the only way to do the job.

The idea behind this "urban myths" thread was to spark the value judgments that allow us to discuss these optons, and open the debate where the maneuver envelope of the helicopter is understood, and used when necessary.

[SASless]

Landing downwind is no problem....have done thousands of them...a few intentionally. The planned ones always seemed to work out much better than the impromptu version. Tail wag on the impromptu method seems to be common but missing on the planned. Torque consumption seems to increase on the downwind landing vice into to wind variants of the technique.

Torque deviations always seem to be more prevalent when doing faster...flatter approaches rather than the steeper, slower approaches.

From my experience flying offshore with pilots from both sides of the green pond....our Gomer buddies faint at steep approaches whereas our tea drinking compadres panic at Gomer style flat approaches.

Wonder why there is such variation in making a landing to an elevated deck in the same type aircraft such as the Bell 212?

Lets add one more myth....QFE altimeter settings are more accurate than QNH altimeter settings. Particularly for use around and aerodrome (think airport for you Rednecks out there).

[ShyTorque]

Jellycopter,

Pse check PMs.

[MightyGem]

Lets add one more myth....QFE altimeter settings are more accurate than QNH
Not quite sure where that myth comes from. One just gives you height above airfield instead of an arbitrary level. Yes I am happy to use both, but then again my airfield is around about the arbitrary level anyway. :)

[teeteringhead]

quote:
--------------------------------------------------------------------------------
Lets add one more myth....QFE altimeter settings are more accurate than QNH
--------------------------------------------------------------------------------
If it's an airfield QNH, then there should be no difference in accuracy, but a regional QNH is a forecast figure, so only as accurate as the Metman.:(

And of course the accuracy of the altimeter is (presumably) optimised for "normal" figures. So using QFE at Nairobi (if you could dial it up) probably wouldn't be terribly accurate. - Just guessing on the latter point though...

[SASless]

Forecast Regional QNH? Fly around using an altimeter setting based upon some boffin's best guess? Why not use an altimeter setting for an airfield nearest your location , say within 100nm's? Would that not give a more accurate setting than a mere guess?

[MightyGem]

Possibly, but try telling ATC that you want to fly on a different pressure setting to everyone else.

[ShyTorque]

SASless,

I prefer to use a local airfield QNH whenever possible as it is measured / calculated rather than estimated / forecast.

However, in UK it is allowable to fly IFR below TA on an airfield QNH ONLY up to 25 nms from the source, it's not up to personal choice.

Outside that, the regional QNH is a forecast supplied to ensure that a pilot doesn't come to grief with regard to MSA.

If flying beneath (or over) controlled airspace, then obviously one should obtain the actual QNH to make sure an altitude bust doesn't occur....

BTW, incorrect altimetry is the most common cause of a failure in the CAA IR test.

[John Eacott]

FWIW, in some parts of the globe (Australia being one) QFE is almost unheard of. All our altimetry is based on QNH, be it airfield, forecast or area.

[Steve76]

Hi Nick,

Regarding the downwind bit.

Sure, no problems landing or taking off. Like SASLESS I have done bazillions of them as it is rare to get a nice wind on the nose in day to day off field operations.

I distictly remember a 20kt downwind at night in to Simcoe cone site. What a giggle that wasn't.

But the question remains how to cope with the increased personal risk with an engine failure (here we go again! :) at say...40ft on takeoff.
In a single, where is the room for the flare to clear the tail?
In a twin, you are now behind the curve and struggling for that clearance from the rig, hospital, trees....
You are just making life very difficult for yourself.

The other issue I have noted, is that you tend to get "blown" off the bubble while doing a downwind departure. That is not a very technical term but I think you relate. This is assuming a nice level disc attitude and 6ft hover (as Shawn so excellently exposed out of the Bell 206 manual a couple of years ago.)
The aircraft settles and if over unhappy terrain that is an uncomfortable feeling requiring more power to stop the decent.

This is all assuming you have the option for an into wind departure and the downwind is a choice rather than a requirement.

Cheers for the thoughts. <):)

[SASless]

Shy...

In the dim parts of my alcohol ravaged brain....remains memories of lots of knob twirling and palavver amongst the HP and the NHP (PF or PNF, etc....) when doing a missed approach whilst IFR in the UK....throw in a handover of controls during the process and then resetting three altimeters to the correct settings in itself was a darn chore subject to some mistakes.

At least in our (and our Southern cousin's) world....we use the one QNH setting at the airport the approach was made at...it is the same on both altimeters ....no need for three here....and life is much simpler.

All obstacles are based upon MSL data and also show AGL heights....makes it all very simple even when looking at enroute terrain and obstacles....

[NickLappos]

Steve76,
Blown off the bubble maybe not, but that horrid sinking feeling certainly is real when you accelerate from the rearwind condition (lots of good hover performance because that rotor doesn't care which direction the wind is from, mostly). As you get to groundspeed that is equal to wind speed, you are now at zero airspeed, the highest power point, and you had better have enough power. This is where the takeoff really starts, but you have spent hundreds of feet just getting here! Then you accelerate in airspeed and things get better again.
Bad takeoff technique if you are marginal in power, for sure! Not impossible, but not wise unless you really have to do it.

[Droopystop]

Nick,

What exactly is ground cushion? We are taught that ground cushion is due to a slightly higher air pressure acting on the base of the fuselage than the top, but I suspect it is more complicated than that.

[Kyrilian]

Sorry to bring this up once again, but I think this myth deserves to be on the list:

"All teetering rotor blades (all rotors for that matter) respond 90degrees after their input"

I don't mean to reignite this debate here--technical discussion of it should remain on those many threads where it's been hashed to death. Just search under "phase lag".
:ugh:

[NickLappos]

Droopystop,

There really is no ground cushion, I will make that Myth #11, Thanks!!

We bashed this around quite a bit in the past. The ground effect is due to the aerodynamic effect the ground plane has on the flow under the rotor. It reduces the angle needed to achieve the lift, and it reduces the induced drag/power of the rotor.

The pressure under the helo in a hover is NOT higher than somewhere else, and the aircraft is not perched on a ball of high pressure air.

Myth #11:

The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.

[Thomas coupling]

Oh no...not again....pleeeeeeeze...I beg you not to bring this one up again, Nick.

[NickLappos]

Tom C.

I agree!! Note how I didn't start up! If someone more energetic than me wants to search the thread(s) where we beat this one to death, it would be most appreciated!

[Checkboard]

You guys may enjoy on old post of mine in TechLog:

http://www.pprune.org/forums/showthread.php?s=&threadid=10655&perpage=15

page two talks about efficiency with temp change as well.

[NickLappos]

Checkboard,

Well said. The reason why airliner jet engines are more efficient at high rpm is because they are designed that way, where the rpm and the temperature rise together. The blade area, speeds and airfoils are all designed as a delicate balance between the needs of burning little fuel in high altitude cruise and the need to crawl away from a takeoff at high power at low altitude.

The basic efficiency of an engine is determined by how hot it can get internally. The higher its heat, relative to the temp around it, the more efficient it is. That is because any heat engine must allow its heat to escape to the outside, and the greater the temperature difference between the two places (combustion chamber and outside) the more easily the heat does work as it gets out.

If you simply plot the fuel needed to perfrom one horsepower, it will drop as the combuster temperature rises. That is why engine research is devoted to new ways to cool and new materials to withstand the heat. Maximum permitted temperature is the story of engine efficiency.

The rpm of a jet engine is locked with its temperature only when you set all those blade design parameters to be that way. You can make the blades quite efficient at any rpm, it is not high rpm that makes efficiency, per se, it is the engine matching that drives temperature and rpm up at the same time, in cruise, that allows the peak cruise efficiency to be reached as peak temperature is reached. In other words, it is temperature that makes efficiency in any engine.

[[email protected]]

Steve76 - if you fly an academic downwind transition from the hover (say with 15 kts of wind) it neatly divides into 3 sections:

1. Having noted your hover power, as you move forward after you adopt an accelerative disc attitude, the power required to maintain height increases.

2. As your groundspeed reaches the same as the windspeed (ie the helo is in a zero relative wind condition) everything feels very smooth (no vibration or burble) and your power required to maintain height is at a maximum.

3. You achieve ETL with its associated burble the aircraft wants to climb. At normal climbout speed/power your angle of climb is shallower and your groundspeed is much higher than your airspeed.

I have seen many pilots overtorque or inadvertantly touchdown trying to fly a downwind departure with not enough power to do it.

[helmet fire]

Great thread. Someone postulated earlier that Flare dammit/PPrune Fan #1 may be related to Paul Mekeks, but I have never seen Flare/Poo fan come close to that sort of, perhaps, maybe, admission of Nick's point of view (though he did in the end adhere ridgidly to his original position).

I wonder if Paul might answer the point made by Sawn Coyle on the first or second page: if DA is the overiding factor, how can engine output vary so much at the same DA but at different OAT?
(9000 ft -40C, 2000ft +40C, and avoid using aircraft hover/performace charts as they include aerodynamic reasons which ARE mass flow related).

Nick, other myths if I might:
"Blade stall causes the blade to suddenly fly down and chop the tail boom off"
"Centrifugal force keeps the blades from coning"
"Phase lag is cause by gyroscopic precession"
"LTE is when you run out of power pedal"
"Full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters"
"NVG are dangerous and should only be used by gifted military pilots"
"Aeroplanes are fun to fly"
:8 :8 :8

[Steve76]

Crab;

As soon as I read Nicks reply I just realised what a brain-fart I had when I wrote that comment.....

Of course if you have 10kts up the chuff, somewhere down the departure you will end up with nil airspeed. Just didn't think about it hard enough before engaging typing devices.

Cheers boet!

[Aesir]

Hi Helmet Fire

"Centrifugal force keeps the blades from coning"

Uhh? I always thought it keept the blades from coning! What is it then that keeps the blades from coning?

This from a training handbook.

"7. BLADE CONING

It is the upward bending of the blade caused by the lift. The centrifugal force as a result of rotation keeps the blade down and stretched. At lower RPM, centrifugal force is less causing the rotor blade to bend more. This increases the stress on the blade."

I´m just curious, I´m certainly no match to the great minds that discuss various "Black magic" here.

[Rotorbee]

I do not know what Helmet Fire is refering to, but my prof always said, there is not such thing as a centrifugal force. It is an imaginary force. Using that, you can calculate the forces on anything that turns in a kind of static system. It is the centripetal force that keeps everything flying in circles. But that one is exactly opposite to the "centrifugal" - imaginary - force. There is no "force" that pulls outward on a rotorblade, but there is a force that pulls the rotorblade towards the mast and keeps it flying around. But it is an accelerated system and that is a bit more complicated to understand and calculate then the normal physics we learn in high schools. For the helicopter pilot who normaly knows his high school physics it is easier to imagine a static system where all forces are balanced. And it does not really make you a better pilot to understand that stuff.

Now I duck for cover because the last time I had to use that was a long time ago.

Have to go changing diapers. Daughter very angry.

[NickLappos]

helmet fire,

There is no such thing as CF, like there is no such thing as Santa Clause, but CF is a convienient way to describe an action, so we let it pass (I use CF all the time in discussions with everybody buy Lu!)

That being said, CF does counter coning, and flapping, and is very important in the behavior of the rotor, and the control of the craft. I think we could eliminate that myth.

The others are fun:

"Blade stall causes the blade to suddenly fly down and chop the tail boom off" Actually, stall does create large moment shifts, and the blade does fly somewhat down, usually, but not so as to cause any accidents.

"Phase lag is cause by gyroscopic precession" Nice one, it makes the list, let's add "and is always exactly 90 degrees"

"LTE is when you run out of power pedal" Yes, and lets add "and can be experienced by any single rotor helicopter."

"Full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters" I actually infer this one in the one about engine fialure is the leading cause of accidents. let e edit this in, it is a good one.

"NVG are dangerous and should only be used by gifted military pilots" Yes, it goes in!

"Aeroplanes are fun to fly" Well, like sex, flying is a Yes/No thing, the worst day of flying is better than the best day of deskwork (the worst sex experience I ever had was EXCELLENT.)

[SASless]

When Nick talks of sex.....he hums the hymn "Precious memories". Of course he has had Nicole Kidman numerous times in the shower!

[Rotorbee]

And due to the fact that Nick wants to sell his helicopter to that president he has to be politcaly correct and therefore his sexlife must be as interesting as the tv shopping channel at 2 am.

And hey, he is posting in this forum on a sunday morning.........

[NickLappos]

Norma,

The Myths list works!

In fact, many helicopters have control phase angles of less or more than 90 degrees, based on the hinge angle (delta 3) and the blade inertia vs its flap damping. I know of one helo that had a lead angle of almost 180 degrees, and the S-76 has a lead angle of 56 degrees, both not at all 90 degrees!

The way the blades flap as they whirl around, and the way the cyclic makes them flap is not gyroscopic at all, but a gyro and a rotor share the same need to make fundamental physics happy. The conservation of angular momentum is the key, so there is a bit of gyro DNA in a rotor, but not enough to make the lead angle precisely 90 degrees. In fact, it s almost never exactly 90 degrees in any helicopter. In fact, it is not even the same angle in one helicopter. let me explain:

The real phase angle of a helo can be easily found, just tilt the swash plate to the north, and watch where the rotor plane dips lowest after all settles out. The typical rotor will dip lowest somewhere around west, so we call that one a 90 degree phase angle (engineers call it gamma, the swash plate phase angle).

If you take a given helo and do that test at three different speeds, you will find three different gammas, because the airspeed has a strong effect. In other words, the "gyro precession" angle of a rotor varies with airspeed!

For a helo with delta 3, the gamma shifts for each degree of delta 3, so the Robbie, with 18 degrees of delta 3 has a gamma of 72 degrees. This has lost Lu a lot of sleep over the last few years, as he ponders the universal significance of that horrid 18 degree shortfall, all the while thousands of Robbies have flown millions of miles.

This has been beaten to death here on pprune over the last few years.

Suffice it to say, the typical rigging angle of 90 degrees for most helos is an approximation, it works, it can be different for any model helos, and it is not due to gyroscopic precession.

[Lu Zuckerman]

To: NickLappos

If it is not gyroscopic precession is it aerodynamic precession?

:E :E

[slowrotor]

Nick,
I have a mini electric RC helo. It works great for learning rotor physics because you can hold it in your hand and power it up, then tilt the rotor to feel the reaction.

I could not figure why the phase angle did not seem to be 90 degrees.
Then I noticed that it seemed to change at higher rrpm.

Your explanation confirms the real behavior I could feel and see in the model.
Thanks for that.

P.S. RC helo from Hobby Lobby called Aerohawk is about $200 complete with radio. Works well for hover. Electronic control died twice, so far they have sent parts. www.hobby-lobby.com

[NickLappos]

Lu,

If it is not an apple, is it an orange? Pear?

It is how the rotor behaves as it flaps, it is approximated in some pretty hairy equations, and it is not generally 90 degrees.

Just because our helos are rigged that way does not make their gama exactly 90 degrees, but they are close enough so that it works. I would bet that a sizable percentage of production helos have gammas that are 10 degrees different than their controls are rigged, but it is so hard to notice, nobody cares.

When we build helos, we actually look to see what it s. I personally did it on the S-76 and Comanche. It is also not a constant for an aircraft, as slowrotor has observed, it changes with rpm (because it is a factor of the blade flapping inertia, centripital field, aerodynamic damping, hinge offset and several other things that escape me (phase of the moon?) Gamma even changes short term vs long term. If the rotor is rapidly flexed with cyclic, it dips in a different place than where it ends up long term. Boelkow drivers know this, and automatically compensate.

[Lu Zuckerman]

To: NickLappos

If it is not gyroscopic precession is it aerodynamic precession?

The reason I asked is that in past postings I referenced gyroscopic precession and got jumped by guys from the UK and OZ with them telling me there is no such thing as gyroscopic precession. They insisted it was aerodynamic precession. Now settle the argument. (Which witch is which?). In reference to your posting above you reference the dynamics involved. Are these functions related to gyroscopic or aerodynamic precession or is precession even involved?


If it is not an apple, is it an orange? Pear?

So, which is it?

:E :E

[NickLappos]

Your demands fall on deaf ears. Why must it be either/or? Is there room in this world for things you have not dreamt of in your philosophy?

[Lu Zuckerman]

To: Posters from the UK and OZ

Now is the time to chime in. Nick states that there are differing philosophies which is what I have stated time and again. I stated that it depends on where you got you education.

Nick leaves the question open. Don't you want to know which witch is which?

:E :E

[Droopystop]

I am not sure what the argument is that I am about to enter, but here goes. I never liked the gyroscopic progression explaination, mainly because it is like gravitational acceleration. I don't know why or how massive bodies are attracted to one anouther, but I will accept it. I don't know why or how gyroscopic progression works, but it does and you can feel it. What I can understand is that when you change the pitch angle of a blade, you change its flight path. If you start to reduce the pitch over the tail maximise the input over the pilots right shoulder and slowly re attain "neutral" pitch over the nose, I can see the rotor disc tilts forward. Simple.

The real story though seems to be one of those engineering compromises. My betting is that there is a little bit (questionable if it is significant) of gyroscopic procession, alot of aerodynamics and a healthy dose of mechanics that go into this, hence the variety of phase angles.

Maybe I should enroll in an aeronautics engineering degree - this is fascinating!

[Kyrilian]

Lu,
"...which one it is"???

Nick is correct--it is not either/or. Gyroscopic infers only inertial effects, while aerodynamic infers only aerodynamic effects. The rotor is affected by both the dynamic (mass/inertia) and aerodynamic (external force) characteristics. A helicopter is a complex bit of machinery, not a fruit!

What Nick was getting at earlier is that even the relative importance of each (aerodynamic and 'gyroscopic'/inertial) characteristic changes as rotor speed is changed, ambient air conditions (density) change, etc, etc, etc. Those who teach that gyroscopic precession causes a 90 degree phase lag on all rotors are oversimplying, and are quite frankly, wrong. Sure, there are some rotors that are physically set up such that the blades respond 90 degrees past their input, but those just make up a sample, rather than define the general rule.

Rotor dynamics cannot be defined in simplistic ways like you want to believe, Lu. Frankly, the complexity is what drives some texts to incorrectly define the physics the way they have. And for its purposes, at least in the past when most(?) rotors did essentially behave the way you think they all behave, this explanation served its purpose. To me, the point of teaching this to pilots is for them to realize that the blades don't respond instantaneously, but instead change position some azimuthal distance past the input. Whether it's 90, 85 or 72 degrees, I don't think really matters to many. If it's rigged so that forward cyclic gets you going forward, then exactly how it got there doesn't really matter. Giving it delta-3 or larger hinge offset will shrink that phase lag, but the pilots will care more about how this effects the handling qualities than the intricacies of the head and phase lag.

However, when we have a technical (engineering) discussion about what is actually going on, you need to forget about the lessons you've learned again and again in all your technical training lessons. Just let go of your simple version of reality and open a book (I'd suggest Leishman)!!!! Asking others, many of whom have simply been taught what we were all taught at first, doesn't make what they say more correct. This is not meant as a slight to all the folks out there who were taught gyroscopic precession or the blades flying to position--I was too. It takes a good deal of engineering education to thoroughly understand this stuff, or at least a great deal of insight to understand it a little! :ok:

[NickLappos]

Thanks, Doopystop and Kyrilian, for your help in explaining it.

Lu, when you ask someone a question of one thing or the other, and the true answer is neither, do not expect them to pick up your music and dance to it. You are so far from understanding this, it is not really close.

The answer I gave you was from Hamlet, who described the ghost to Horatio, who could not understand, and so Hamlet said, "There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy."

[Lu Zuckerman]

To: Droopystop

You send your kids to school and buy them books but their understanding of the different subjects is based on the schools you send them to and the books you buy them and how well they assimilate the information presented.

If you attended engineering university 20 to 30 or more years ago you would have been taught gyroscopic precession and how it applied to differing rotational objects from a spinning rotor in an aircraft instrument, an aircraft propeller or even the rotor of a helicopter. It seems that universities are now teaching that gyroscopic precession does not apply to helicopter rotors. Instead they substitute all types of calculations and formulae to explain how the rotor system responds to input.

Even though it doesn’t apply to helicopters (any more) it still applies to gyro instruments on an aircraft, the propellers on aircraft, the wheels on a motorbike or bicycle. A propeller you ask? When an aircraft maneuvers the gyroscopic turning moment on the propeller causes the propeller to bend to the point that the blade tips depart the rotational plane. When the maneuver stops the “centrifugal” moment will restore the blade position.

I managed a training program for the US Army teaching maintenance on helicopters. We taught gyroscopic precession and the mechs. and maintenance officers would ask, “if the blades are independent from each other then how could they respond as a disc”?

We had the model shop construct a gyro that had twelve arms each weighing several ounces. These weights were attached to a disc, which in turn was driven by an electric motor. We constructed a rudimentary swashplate, which allowed perturbation of the independent weights on the disc. The disc was brought up to speed and when we input a perturbing force the independent weights would respond 90-degrees later until the rotating weights (rotor disc) were all tilted 90-degrees after the control input.

It is easier to use gyroscopic precession because it is easily understood and, it can be demonstrated. Is it right or wrong? It depends on where you went to school and of course, when.


:E :E

[NickLappos]

No, it is wrong Lu. It is wrong.

Move on, Lu, it is not the end of the world.

[Kyrilian]

Lu,
A small device like you describe is impacted mostly by its dynamic characteristics, so it really isn't acting like a larger rotor. A propeller is also more affected by inertia and less by aero forces than a rotor. So sure, a bike wheel and propeller and even helicopter rotors have some characteristics of a gyroscope--and it may be easier to teach gyroscopic precession--but that does NOT mean that gyroscopic precession is the correct description for it. Thus in answer to your question, this teaching IS WRONG, but suffices in some circles to get certain partial ideas across.

Now please let this be! Just be content that this is the way lots of people have been and continue to be taught--not just you. But don't let yourself believe that it is real.



If anyone else would like to understand this and are willing to let go of the dogma that they may have first been taught, check the other threads (search under "phase lag" for many of them or start a new thread) and continue to ask questions. I'm sure there are some of us who wouldn't mind responding.

[SASless]

Peturbing thoughts there Lu!

[SASless]

Diethelm....

Like a discussion of "limits" in math....analogy was similar...if you are allowed to step half the remaining distance to that fair lady...and then again step half the remaining distance...and again....one would never get to her...but you would get close enough....that the remaining distance would not matter.

[NickLappos]

If Lu tried to take that "rotor" he had built and fly it back then, we wouldn't have to go through this now! That was not a rotor, and the demo he gave was not illustrating rotor blade flapping.

As this thread has tried to spark, the old myths die hard, but Lu, they are dead.

It is not a "different way of saying it." It is not an "old school" vs a "new school". The experts then knew it was wrong, but it was close enough, so they let it slide, and a few generatons of plots and mechanics "knew" it was gyroscopic precession. The designers and engineers knew the real story, the old school had some very bright people in it.

[407 too]

diethelm,

your flying in a gyroscopic drink holder,

just keep the ball in the middle:ok:

[LGNYC]

Nick,

I totally agree with you about the phase angle, no gyro precession and all. Actually, I would add a reference to this discussion: p.93 (the section on "Blade flapping equation of motion") in Simon Newman's "The foundations of helicopter flight". It's all there.

HOWEVER: just try and discuss that while you take an FAA (PPL, CPL...) practical test! In the US, I dont think you would pass if you do not pronounce the phrase "gyroscopic precession". So, while it is good to understand what is really going on, it is also good not to forget the lingua franca.

LG

[Lu Zuckerman]

To: NickLappos

It is not a "different way of saying it." It is not an "old school" vs a "new school". The experts then knew it was wrong, but it was close enough, so they let it slide, and a few generatons of plots and mechanics "knew" it was gyroscopic precession. The designers and engineers knew the real story, the old school had some very bright people in it.

Nick if you really believe this then I would strongly suggest that you go to your own service school and set them straight instead of making fun of their "Blue Book" written by John Montgomery and vetted by Ralph Lightfoot.

The preface of the Blue Book says in part: “It is not intended for the engineer or aerodynamicist. Explanations given are simplified and are not in any sense offered as Sikorsky criteria, design or otherwise”.

The blue book, which you have classified as a comic book, has illustrations in it that can be found in most aerodynamic texts.

The Blue Book is intended for pilots and mechanics and suffices to educate them to a level where they understand how a helicopter works. (Or, according to you, they think they know how a helicopter works). If they and I guess myself still believe in Gyroscopic precession then I guess ignorance is bliss. I and I assume many of the posters on these forums do not understand the full engineering descriptions offered by you graduate engineers but they still fly the helicopters and if the opportunity would arise I would continue teaching gyroscopic precession. And, in the back of my mind I would know that I was lying to them.

If Lu tried to take that "rotor" he had built and fly it back then, we wouldn't have to go through this now! That was not a rotor, and the demo he gave was not illustrating rotor blade flapping.

The purpose of the device was to illustrate that a segmented gyro rotor could react in the same way a solid gyro rotor would react to a perturbing input. This was in response to the questions from the students as to how a fully articulated rotor system could act as a gyro rotor. Whether you disagree with the demonstration or not the students came away from the class with a better understanding of gyroscopic precession as applied to a helicopter rotor system.

Remember Nick this was in 1956 and everyone including the helicopter gods believed in gyroscopic precession. By the way, what were you taught in the Army flight school? And did you believe in Gyroscopic precession up until you walked through the front door of Georgia Tech?

:E :E

[ShyTorque]

Lu, I don't ever recall anyone here denying the existence of gyroscopic precession, simply that helicopters don't use gyroscopic precession, per se, as the primary control.

Similar to the model you mentioned, there is a child's "helicopter" toy that consists of a one-piece moulded plastic "rotor", with usually three or four fixed-pitch blades set at a positive pitch angle. The blades are moulded together with the hub and have a perimeter rim also moulded in, so the blades cannot move independently. Effectively, therefore, this little "helicopter" is a lightweight GYRO. The "Helicopter" is made to fly by using a launcher, a handle with a vertical spindle. The spindle can be rapidly rotated by pulling a string wound round it. These toys usually fly over the hedge into the next door neighbour's garden or onto the roof because they exhibit the qualities of a gyro and cannot be controlled....because a precessional force cannot be applied once the rotor leaves the launcher.

However, a helicopter as we know it has separate blades that can and do move independently from each other. The blades do exhibit rigidity but they ARE independent. NOT the same as that toy rotor/gyro wheel where the perimeter is joined.

Archimedes reputedly said "Give me a lever long enough and a place to stand and I will move the world". However, there has to be a place to stand to apply a force! The force has to be large enough to produce the required motion at the required rate.

IF gyroscopic precession was to be used to control the path of the rotor as the pilot wished, where would the place to stand be and how long the lever? The rigidity of the blades due to the gyroscopic properties would strongly resist him. Would the pilot be able to overcome the huge rigidity of the system? Come to think of it though, there would be no need to use cyclic pitch changes, just a push / pull rod directly connected to the underside of the blade hub for the pilot to heave on... Hate to think how long the cyclic "lever" would have to be..

Instead, the blades have a pitch change sleeve and a pitch angle control horn connected to the pilot's controls. Now, a relatively small force can be used to alter the pitch angle, via the cyclic. This causes a change in the angle of attack of the blades, which fly to a new position. The aerodynamic forces overcoming their rigidity / inertia, call it what you will. They cannot do this immediately, so the blades follow a path similar to that taken had they been gyroscopically precessed.

Kaman's servo-tab rotor control system illustrates the point quite well. Where is the gyrocopic precession there? The blades are quite clearly flown to their new positions.

So, in UK, Oz and in quite a few other places the blades are flown to the desired position, not gyroscopically precessed. That is where your opinion and that of most others here seems to differ.

[NickLappos]

And let's not have Lu's obstinacy ruin a fun thread. Any other myths to discuss out there?

[helmet fire]

D'oh!
I knew I shouldn't have mentioned gyroscopic precession. But I just couldn't help myself. Lu, the intro to the Blue Book, as you quoted, is the best explanation you will get for this, and even for centrifugal: Use an easily understood "explanation" for an observed phenomenon so that you can concentrate on imparting the important info, not the labourious detail of a means to an end. but we have explained that so many times...
oops...Sorry Nick, I'll leave it alone.

Re the Centrifugal and cone angles, you spotted the myth part - centrifugal force is not real (despite Lu and Flight Safety!) :}

Other myths, mmmm....... do we even go near
"rotor blades emit a visible light when flying through mountain/desert air at night"?

how about:

"helicopter technical texts are not limited by the understanding of their authors, rather they become factual and unquestionable explanations merely by being checked for spelling by an editor, and achieving print"

:ok:

[Dave_Jackson]

How about the myth;

Igor was so obsessed with the tail-rotor that he built a helicopter with 3 of them.


:O

[Paul McKeksdown]

:p :p :p

Fantastic, good to see that the hearty debate still rages.

Good to see :ok:

Sorry Lu, but I have to go with Nick and the others on this one.


Remeber; Debate stimulates the mind and opens the brain to new ideas.

[Aesir]

One myth which I hear often is:

You have to first learn to fly fixed wing before you take helicopter training.

Or, Helicopters can do instrument letdown vertically over a NDB and dont need to do the full approach.

Regarding all this complicated aerodynamic stuff on how the rotor behaves and works, I think my old instructor explained it best when he was trying to get us to understand some complex aerodynamics stuff. He said:

"This all is besides the point really because the only thing that makes a helicopter fly is MONEY, and lots of it. Without money no helicopter will fly!

How true : )

[SASless]

Paul....

Not in all cases does learning stem from debate.....We have Nick and Lu as prime examples of how that can be. They certainly have a spirited exchange now and then...but rarely does there seem to a veer towards the new.

[Whirlygig]

You have to first learn to fly fixed wing before you take helicopter training
Phew! Glad that's a myth 'cos I've never flown a fixed-wing thingy ;)

Cheers

Whirlygig

[NickLappos]

Whirlygig,

I agree! I had 1500 hours and a combat tour before I ever even sat in a starched wing!

[Paul McKeksdown]

:D

SASLess, or can I call you STAB out?????

Yep, totally agree, in the case of the duelling combatants, myself included, it is often difficult to see the others point of view. Especially as we get further along our individual timelines.

This thread is for the discussion of 'helicopter myths' and as such, even with a non moving debate the spectators tend to learn alot from the subject content.

It is not always having to re-invent the wheel that keeps us going forward and often a 'ahh but I saw that on PPrune' that jogs the old grey cells and pushes towards official research into the myth/phenomena/what the hell was that!!!

Cheers
:ok:

[SASless]

Yes Nick...but in those days it could have been a narrow body one.....your choices would be diminished today.

[Whirlygig]

Ouch :ouch:

Put those claws away! I am learning a lot from this thread; not least on how to improve on my bitching skills ;)

Cheers

Whirlygig

[Shawn Coyle]

Having been away for a while training, I note the clouds of dust on engine performance have cleared, thanks no doubt to the downwash created by Lu and Nick's discussions.
But seriously, the point seemed to be missed by Peter that the power available is not a function of density altitude. I hope the rest of you got the message though. It is a myth.
For those that need further education, either look at the power check charts for a turbine engine for extremes of temperature and pressure altitude that make up the same density altitude, or if you have sufficiently underpowered helicopter, look in the hover performance section and note that temperature is shown twice - with significant reductions in maximum weight to hover at high temperatures.

And I'm not sure if the list is for helicopter pilots who need to know what is true and what isn't, or for those outside the profession looking in, who have a whole other set of myths - like if the engine quits you're going to fall out of the sky like a brick.

[ShyTorque]

"like if the engine quits you're going to fall out of the sky like a brick."

Helicopters do that. They all make a sound like a Bell 47 when powered and a noise like a Stuka dive bomber when not..... :p

[Paul McKeksdown]

I seem to remember coming down quite fast in mine when the engine coughed and died.

Accompanied by the words 'ohhh f:mad: k!'

:}

[SASless]

Mine was quite the opposite....popped off the top of a 10,000 foot mountain in Alaska....got out over the flat about 9,500 feet too far below me....lowered the lever....got an almighty BANG! sound with attendent yaw swings for what seemed a few minutes.....finally decided the donk had indeed departed to where their spirits go to....and took hours to get down to the lovely lush forest below. That is entirely too high for such things to happen....would much prefer 300-500 feet.....pace of life much quicker....and not much time to think or contemplate your future. Being in a 500D....one would think they fall like a set of streamlined car keys!

[Paul McKeksdown]

Mine was in transit, 1000', birds were singing and the sun was high in the sky with white puffy clouds, the sort your kids draw.

It all went quiet, never a good sign!

Things moved fairly quickly after that and the flare recovery was executed with a little more bravado than normal but apart from that it all went swimmingly.

Seem to remember having about 2000-2500' ROD.

Thank god for farmers and mobile phones.

:uhoh:

[SASless]

Ten mile walk out to the nearest dirt road on mine...no cells...no farmers...certain amount of apprehension knowing I was at least one or two steps down the food chain in that particular neighborhood. Brought a new definition to "whistling in the dark". Cracker "Glad to be Alive" party back at camp that night! Landed without a mark on the aircraft....other than a rather soiled seat cushion!

[Paul McKeksdown]

Adrenalin is indeed brown with a rather wiffy smell!!

I noticed that the old ticker rate hit beats faster than a kids techno party as well.

Highly interested farmer 'where did you come from then?' and 'when can you move it then'

:ok:

[Homer_Jay]

SASless,

Well done. I'm always scared if it happens to me at that height I'll over think it and touch down in trees a cruel 100m short of a perfect pad.

[rotorfloat]

I'm not one to agree that there is a 'bubble' of high pressure under the rotor system in a hover, although we may all be guilty of describing it this way for simplicity. I do notice when pulling pitch during takeoff, the VSI indicates about a 150-200 fpm descent, as well as the altimeter will drop about 40 feet.

Does anyone else notice this, or can anyone explain....or am I just being silly?

[TIMTS]

The indications of a descent comes from the air being accelerated down around your static port and disturbing the signals.
I was told to use this as an indication that the static system works without leaving the pad.

Probably just stating the obvious...the more accurate description is sure to follow.

Timts

[jellycopter]

TIMTS,

If air was accelerated past a static port it would, in all probability cause the opposite indications to those described due to venturi effect.

Rotorfloat,

what you describe is common to all helicopters (well, at least all the helis I've flown anyway). What you are seeing is the build up of ground effect' ie; an increased pressure under the rotor disc.

J

[NickLappos]

jellycopter,
There is no such thing. That is why it is called a myth. Ground effect does not rely on, nor does it make a high pressure under the helicopter.

[Talk Turn]

Dammit,

I would agree with Jelly.
I thought that the induced flow is forced toward the ground and cannot escape quick enough because of the ground. Therfore a higher local static air pressure is found (maybe 2mb hence the -60' indicated in the ALT)
Because of this the induced flow is reduced increasing the Angle of Attack and so on.
:ugh:

[Paul McKeksdown]

At the risk of getting embroiled once again......

:D

Ground effect is an aerodynamic effect caused by the placement of an obstruction underneath the downward flow from the rotor.

Aerodynamically saying, the rotational airflow remains constant, the induced airflow is slightly decreased by the added restriction placed in the induced flow, thus reducing the induced flow. The relative airflow, being taken from the top of the induced airflow vector is therefore also reduced slightly in angle. As the angle of attack is taken from a constant held pitch angle the angle between the chord/pitch angle and the relative airflow also increases slightly giving greater lift.

Whether or not a small localised pressure increase exists under the helo? yes probably but it has no/or possibly very little contributory effect to 'ground effect'.

This can probably best be seen when over a rig. The small landing pad offers the same ground effect with a small plan area therefore reducing the possibility of the ground effect 'bubble'.

Sigh..... Sit back and wait for flak.

P.s. The ROD is the increased flow from the rotor over the pitot staic system resulting in a small downward deflection caused by position error. Have a look at the beanie on the S-61N to see what mr Sikorsky tried to do about it!!!!

:E

[NickLappos]

Ground effect has nothing to do wiith pressure.
If the pressuure were higher under the helicopter, the power to hover would be greater IGE, because the pressure under the rotor would mean the engine would have to work harder to push the aiir into it!

Talk Turn, try to find one reference that is from a scientific source that discusses PRESSURE as an aspect of ground effect. (NOT a popcorn training site that repeats the myth).

[VoicesFromTheCreche]

To quote directly:

Ground Effect
7. In a free air hover the airflow through the rotor disc begins at zero velocity some distance above and accelerates through the disc and into the air below. There is little resistance to the downward movement of air. If the helicopter is hovered close to the ground the downwash meets the ground, is opposed, and escapes horizontally. A divergent duct is produced causing an increase in pressure...

Not exactly from a scientific reference, but I think the Royal Air Force Manual of Flying (AP3456) has been regarded as slightly more than a popcorn training site. It would appear that regard was misplaced.

If its not an increase in pressure, how is the induced flow reduced?

And if high pressure beneath the aircraft would mean the engine had to work harder to push air into it, does that mean hovering over lower pressure would reduce power required to hover?

Our heads hurt.

[VeeAny]

Clearly there is a pressure there, however none of the texts seem to reference it as anything to do with Ground Effect.

If we work it out,

Dynamic Pressure = 1/2 rho v squared.

V goes up Pressure goes up !

V goes down Pressure goes down.

Induced flow velocity decreases (because of the presence of the ground) pressure goes down (from the OGE case).

There is a method of calculating the vi reduction in Padfields book(helicopter flight dynamics I think) , depends on Disc area, the OGE vi and disc height above the ground, reducing vi more the closer the ground is.

There is a paper on it at,
http://www.adl.gatech.edu/archives/groundeffect/literature.pdf

I haven't had time to read it all, may answer some questions tho.

V.

[NickLappos]

The ground effect is not a pressure effect, in any sense. The reduction inpower required to hover is because the blade angle is reduced, so less power is wasted on induced drag.

The idea that pressure bubbles hold the airplane up is a third-grade pop-physics approach without any technical merit. When the FAA or the RAF repeat it, it is simply the unschooled drinking their own bathwater. That is why it is on the urban myths list!

Look at a 4 engined bomber airplane moving at 150 knots, within one wingspan of the ground. It stays up by using the ground to shape the flow around its wings, allowing the wing to produce the lift at a very much lower angle of attack, at very much lower drag. Lower drag, not high pressure, folks! Yes, the bent flow has a density, pressure and velocity relationship that someone in pprune land will assume is the "pressure bubble" but they are wrong, it is the flow direction change that brings on the reduction in angle of attack of the blade, and that angle reduction eats less power - That is ground effect.

Please, all you who will now produce some training manual somewhere that says pressure bubble, don't bother.

[VeeAny]

Nick

Just in case you thought otherwise, my post about pressure calcs was to support your thoughts that it's nothing to do with it.

This is one of the best threads we've had in ages.

Here's another for the list, a wing acts like one half of a venturi !

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

Don't think anyone else has offered that one up yet.

V.

[jellycopter]

Nick L,

If there's no such thing as Ground Effect (as a function of pressure increase) why do helicopter static instruments display such characteristics? Why, when I'm sat on the ground at min collective and I increase collective pitch do I observe, what appears to the uneducated like myself, to be a pressure increase? If it's not related to the proximity of the ground, why do helicopter static instruments not behave in the same manner when in a HOGE and collective pitch is increased?

These are genuine questions Nick; please educate me........ and try and keep it simple, I didn't do higher education.

J

[Talk Turn]

I hear what you are saying Nick.

The answer would be to get something to measure static air pressure in the 2 cases or maybe that is what the altimeter does

Maybe
??

[Droopystop]

I must admit that I have never noticed a change in pressure instrument readings when lifting into the hover (maybe aircraft type, maybe because I am too busy looking out!). I suspect that these fluctuation are due to a fuselage affect ie some downwash is causing a dynamic pressure in the static system?

[Talk Turn]

Static port is in the fuselage in an R22 - transmission bay for want of a better word
??

[helmet fire]

voicesfromthecreche (and others), the veracity of the text's you quote from are limited by the understanding of the subject by the author/s. By having been published, they do not of themselves create reality, they merely reflect that author's understanding at that point in time. We can see over history how scientific papers have been proven incomplete at latter dates by more advanced understandings, and helicopter technical explainations are no different. Whilst quoting technical texts is often our only way to understand a subject, sometimes they are just plain wrong (Lu, some even refer to centrifugal force!) That's why I propose another myth :

"helicopter technical texts are not limited by the understanding of their authors, rather they become factual and unquestionable explanations merely by being checked for spelling by an editor, and achieving print"

[212man]

Does this count as a myth: the S-92 is 'all American'?

"Sikorsky is at the helm of a global partnership working together to design, build and support the S-92 to meet the needs of the worldwide helicopter industry. Sikorsky manufactures the advanced S-92 drivetrain and integrates all other parts and components at final assembly in the USA. Partners in the production of the S-92 include: Mitsubishi Heavy Industries in Japan; Aerospace Industrial Development Corporation in Taiwan; Gamesa in Spain; Jingdezhen Helicopter Group in the People’s Republic Of China and Embraer in Brazil."

[rjsquirrel]

212man,

It seems that you are mixing S-92 words about their partnership with VH-92 quotes about their partnership. Do you do that in the cockpit when you fly? With that sharp memory, you must really keep your co-pilots on their toes!

According to everything Sikorsky has put out (but obviously a bit too complex for you to grasp) the VH-92 uses Vought to do the airframe, making the VH-92 All American. Word is from the oil patch that the S-92 world-wide partners still make the airframe for the S-92, as you confused it with your snippy quote.

RJ

[212man]

RJS,
Ah, I see. So the oil workers and their pilots go second class in the slightly less safe version.

Thanks for clarifying that.

[SASless]

212man with rounded shoulders and upturned lip slinks off to the company bar to ruminate about his being several steps down from second class.....:{

[rjsquirrel]

212man,
Second class is as second class does. Give it a break, ok?

[212man]

Well, no intention of going into a downhill spiral on this one, so I guess that counts as giving it a break. However, you can expect some sniping when so much emphasis is made (check other threads plus press) on the VH-92 being safer than the US-101 by virtue of it being 'all American'. It must follow from that logic that if its sister ship is not 'all American' it must be less safe.

Personally, I would say they are more than likely equally safe and would be equally pleased to fly either.

The word 'class' was not meant to be inflammatory btw.

SASless; you should know we don't have a company bar anymore! Plus we travel Upper Class!

[SASless]

No company bar....now that is a first for a Bristow operation! There are more negative effects to being "americanized" than meets the eye.

I was trying to apply some left handed logic there....
Vxxx, 92,101,225,Tiger,155,365,Scyamore, 212 as to class.

Remind me....port side to Lagos....starboard side to London?

[NickLappos]

212man,

I lead the team and gave many of the press interviews, I know what we said, and it was nowhere near what you proport.

I do object to you stealing this thread, it is a fun one. Why don't you start your own thread?

[SASless]

212man....

Trust me young feller....on this side of the pond....we care even less about class distinctions than you might surmise. To borrow an idea from a great American Hero....Forrest Gump...."Class is as Class does!" It may be learned but it is not inherited. I do believe that is not the way it works on your side of the salt water divide.

How does that old saw go....no offense meant here Nick..."Never trust a Jewish guy with an attorney, a Frenchman with a snag tooth, a Greek wearing tennis shoes, don't do business with a Chinese, and never let an English Gentleman owe you money."

[212man]

Resisting the temptation to respond further, in defference to the original thread starter, a couple more comments that are heard from time to time (maybe not urban myths, but certainly innacuracies believed by some as true):

Regarding flight planning: "it doesn't matter that the wind is stronger than we planned for, as what we lose on the way there, we'll make up for on the way back"

Regarding max take off weight (when using a WAT graph): "we can take off a bit heavier than the graph as we have loads of runway", or "we can take off a bit heavier than the graph as there's a strong wind."

[Rotorbee]

Another thread gave me the idea. I heard that early in my training and it is still teached to new CP students.

"When flying inadvertly into IM conditions, you should immediately enter autorotation."

I had the opportunity to see in real life that this would be very, very dangerous. A few years ago I was sitting in the class room at my university and I could see an EMS helicopter flying low over the town right into a cloud. Fortunately he did not autorotate but made 180. He came out of the cloud after a few seconds very nose low. If he had autorotated he would have crashed, because the cloud touched the ground.

But here is the question, is it save to autorotate through a cloud deck? Provided you are sure that there is enough space below the clouds to continue the flight in VMC.

[JimL]

Rotorbee,

I'm sure the FAA pondered on that question before they accepted SEIMC?

[SASless]

JimL,

You place way. way, way too much faith in the FAA when it comes to things like that.

Engines do not quit on singles when you are IMC...dont'cha know...it is the single generator and other lack of redundancies that make me wonder.

It is odd how the FAA can have one view on issues if it is a three hundred seat airliner or a five seat JetRanger. Is it the sheer number of lives that matter?

Last year the EMS industry killed more people than did the airlines in this country.....and how many of them were single engined aircraft in IMC conditions?

[Rotorbee]

Ah, ups, sorry, I was talking about europe and VFR pilots. If I am not totaly wrong, the FAA training does not (yet) require a helicopter pilot to do any instrument training. The argument for entering autorotation immediately when flying inadvertly into IMC was that in an autorotation the helicopter is more stable.

Flying SE in IMC has been done for years with planks and I can see no reason why not in a helicopter. I is up to the pilot to consider all factors and a lot of SE plank drivers set their limits in a way that they don't fly IFR when they have no chance land in the event of an engine failure.
BTW in a helicopter I can not fly SEIFR commercialy and there aren't many private SEIFR helicopters around. I would probably be very difficult to find an accident report about an engine failure while flying SEIFR in a helicopter.

[GLSNightPilot]

I learned to fly instruments in a SE helicopter, and logged lots of instrument time in them. The UH1 was a decent instrument platform (and still is) and the Army flies in IMC routinely in the TH57(?), or whichever number the 206 has these days. They launch into 1/4 mile IMC all the time in training, and AFAIK have had few, if any, accidents attributable to engine failure. I had far rather fly IMC in a turbine helicopter than in a light piston SE airplane, and they fly IMC all the time. Single-engine IMC in commercial operations is another matter, though. The FAA's view is that you can kill yourself whenever you want, you can kill 9 passengers if you exercise a little care, but you can't kill 10 without jumping through more hoops - thus the 9 or few pax rules, and the 10 or more rules. Then when you get to 20 it's more stringent. This is why you'll never see 20-passenger helicopters in the GOM - 20 pax requires a paid and trained flight attendant. Whether this makes sense is moot, because the bureaucracy has to have rules, and any number they pick is arbitrary.

[JimL]

GLS,

Those numbers apply the world over.

[Talk Turn]

Surely, there is no excuse for flying in to a cloud by mistake anyway.

[SASless]

Ever been night flying away from the airport TT?

If you cannot see it....you can fly into it without knowing it ahead of time.

[Talk Turn]

Sure.

maybe if you are flying in the middle of nowhere with no visual cues you are IMC anyway and therefore need the skills and/or kit.
If you do have visual cues/ambient/celestial light etc you are night VMC and then maybe there is no excuse.

[Rotorbee]

There is always an excuse for spatial disorientation when flying during the night, even if it is VMC. During a night flight I had a false impression, that the terrain was climbing. I new it was flat, but my brain gave me another impression. There where lights everywhere but no moon. That happend about 10 miles from my home airport. I was very, very happy when I saw the airport with all those lights. That was a very disturbing experience.
Our brain produces that image of our environment and not the eye. That depends a lot from what we have learned to see or even what we want to see. It is very easy to confuse the brain.

[2beers]

Great thread, got a lot ideas confirmed. Anyway, back to VRS:

If you have a helicopter with a pilot and some fuel weighing 1000kg and go out and at altitude gets VRS at 800fpm. After that you go back and fill it up to MAUW at 2000kg and try the same thing. At what descent-rate will VRS occur?

With my understanding of VRS, I would say that it will happen at a greater descent-rate, say 1400fpm. The reason beeing that a higher weight must generate a stronger downwash, so you need to go faster downwards to catch it and enter VRS.

What actually makes me wonder is that I've read in a book (can't remember wich one) that states it's easier to encounter VRS if ligtly loaded in same machine.

So... am I right or how wrong am I? :O

Cheers!
/2beers

[NickLappos]

2beers,

You are right about the higher weight requiring more descent rate. Higher altitude also requires more descent rate, both of these are counter to the urban myth.

[Rotorbee]

And what about VRS in an autorotation or in a quick stop?
I remember a film that was shown at the Robinson Safety Course. It was during certification where they entered autorotation very low and landed very hard.
I am not quite sure if it was acctualy VRS.

Is it possible to get VRS during a flare? What conditions should one avoid to get into VRS during the flare? (If I had a choice)

[NickLappos]

Some early morning thoughts - forgive the smart a**, it is early!

The dreaded VRS cannot be experienced anywhere except when the rotor is forced to eat its own downwash, at descent speeds that are near zero forward speed and near 75% of the downwash speed (relative to the rotor axis). Nowhere else in the rotor flow regime will VRS occur.

Like Typhoid Mary, VRS has been blamed for crimes where it wasn't even in the neighborhood, but the mythology earns it the blame:

If you are in autorotation, you can't get VRS

If you do a power recovery from an auto, and you are too low, you will hit the ground. In Test Piloting circles, we have a name for that. We call it hitting the ground (HTG). HTG occurs if you encounter the ground and you have any vertical velocity.

Now, some instructor, armed with Her Majesty's Best Helicopter Aerodynamics Manual (or George Bush's Best Aerodynamic Manual) will find on page 73 that you can get VRS while asleep on Tursdays, as well as while performing a power recovery auto. No you can't. You can however get HTG, call it VRS and nobody will stop you.

The ability to establish a descent at 8 knots forward and 1200 fpm downward to get VRS is clearly outside the power recovery and quick stop regime, but that does not prevent people from declaring it to be VRS anyway.

What can happen in the recovery or quick stop is that you demmand too much thrust from the rotor, by flaring too aggressively, and you then find yourself saturating the engiine's ability to cover your tracks. This makes the rotor rpm droop, and the MP or Torque go sky high,. Shortly afterward, you get HTG.

Try this:
Do a series of quick stops, strating very gently, from the same speed, say 25 knots. At each one, note the peak MP or torque.

Use the flare angle and the rate of nose up to make the quick stop a bit more aggressive each time. Don't get silly with this, just carefully get a little more agressive with each one, until you hit your power limits (or your personal limits, which shows intelligence). Note that each ever more rapid decel requires more power. Duh. Who needs VRS when we can have a HTG the old fashioned way?

[Texdoc]

The obvious question is Nick, once you have encountered HTG what is the immediate action and recovery sequence :8

[Rotorbee]

And another myth cleared. Thank's

Could have found that out by myself - no downwasch - no VRS (and we all agree now, that this is NOT settling with power).

But the book is wrong with tuesday, it's monday, because on sunday at 3 pm you get that terrible light that pulls you down and you try to recover on monday.

I should really ask for a refund for the ground school I got. But that was during the stone age of helicopter flying.

The obvious question is Nick, once you have encountered HTG what is the immediate action and recovery sequence
In that video I mentioned it went like this. Jump out of the ship - or wreck, whichever applys - ,throw your helmet on the ground and say nasty words. I am not sure, if they had that on the test card in this sequence.

Btw: It was NOT anybody from RHC who said it was VRS, it was my instructor. He is flying planks now anyway.:E

[Rotorbee]

I just realised something very odd in Nick's list.

# 10!!!!!

Isn't Nick one of "Them"? Ha! Gotcha!

What are you trying to hide from us, Nick? Are the 15 myths acctualy true? Is the Queens rotorcraft manual right!

[NickLappos]

If I am "Them" we are all sunk, cause I am the only one of "Them" talking. Maybe I missed the memo from the Head Them.........

[ShyTorque]

If, following a f***ed up flare recovery, I encountered HTG and was able to walk or even crawl, I would make sure the tech log was on board, put my licence on what remained of the dashboard and back slowly away, striking matches and throwing them at the wreckage. :uhoh:

[helmet fire]

on the topic of "them" and what "they" tell us, how about this myth:

"Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then."

:}

[Rotorbee]

If I am "Them" we are all sunk, cause I am the only one of "Them" talking. Maybe I missed the memo from the Head Them.........
Ah, but if you are not one of "them", who is them then. Who is the head "them". But if you are "them", how will we ever find out? To be on the save side, should we - which means we are us and not them - follow SASless proposition in that other thread about ILS approaches? We are getting a witch hunt here. We could also put you on a scale and find out if you are heavier then a goose. If you are lighter, we can all pull more power and our engines will still reach TBO.

Should we give it a try?:E

[FixedRotaryWing]

@ Rotorbee:

It is a myth that they hide something... that means to me, maybe they do not hide anything.

[Rotorbee]

To FixedRotaryWing:
You are not familar with Monty Python, are you?

[NickLappos]

This is how "They" write flight manuals:
Sir Bedevere : Good. Now, why do witches burn?
Peasant 3 : ...because they're made of... wood?
Sir Bedevere : Good. So how do you tell whether she is made of wood?
Peasant 1 : Build a bridge out of her.
Sir Bedevere : But can you not also build bridges out of stone?
Peasant 1 : Oh yeah.
Sir Bedevere : Does wood sink in water?
Peasant 1 : No, no, it floats!... It floats! Throw her into the pond!
Sir Bedevere : No, no. What else floats in water?
King Arthur : A Duck.
Sir Bedevere : ...Exactly. So, logically...
Peasant 1 : If she weighed the same as a duck... she's made of wood.
Sir Bedevere : And therefore...
Peasant 2 : ...A witch!

[Rotorbee]

There are still some pilots with a classic education around.
Chapeau! You are right, it was a duck.

And now something completely different.

What about # 4? Did not know that this was a myth. Can we replace that with Shawns proposition about turbines and density altitude? That triggered one heck of a discussion.


...... Hang on? You know that by heart? How many times have you seen this movie?

[topendtorque]

hmmm, classical education?

sounds all a bit like 'them' who 'talk to goats' and try to 'walk through walls,' all this HTG stuff.

[rotorrookie]

YouTube - She's a witch! (http://www.youtube.com/watch?v=zrzMhU_4m-g)

zrzMhU_4m-g

[John R81]

Topendtorque....


Not Talk to Goats..... The Men who Stare at Goats.


The Men Who Stare at Goats (2009) - IMDb (http://www.imdb.com/title/tt1234548/)


Watch the trailer. The film is better

[paco]

But that's not a myth....

And "they" at Aiirbus didn't tell the flight crews (or anyone else as far as I can find out, because it would appear not to be in the Flight Manual) that the control response is halved when one wheel of their airliners touches the ground - that was a major factor if you watch the video of the Airbus flying sideways into Hamburg (aside form not trying to land there in the first place). I forget the link, but it's on Youtube.

Phil