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.