Helicopter Urban Myths

[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".

[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/showthr...655&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"

[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]

Which witch is which?

To: NickLappos

If it is not gyroscopic precession is it aerodynamic precession?