Helicopter Urban Myths
 

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.

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

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.

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

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.

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.

Who's they?

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". ;)

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...

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.

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!

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.

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.

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

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?

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:

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

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.

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.

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

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.

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.

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.

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

Whispering jack
 

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:

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.......

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:

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

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? ...

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:

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.

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?
:{

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.

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.

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.

Paul said:

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.

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.

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.

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!

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!

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

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.