High Density Altitude Autorotations

[[email protected]]

I fly a R22 at high DA (5000-8500ft, depending on the season) and I am looking for any advice from those who have high altitude experience (in any type) on autos at high DA. I have experimented a little, but the most part I practice the autos by the book, 65kts in the glide and RPM in the green arc. I know I don't have a whole lot to work with up here so any help would be appreciated.

I would also be interested in a explanation of the aerodynamic / energy management issues associed with autos where the air is thin.

Thanks,

Steven

[SHortshaft]

I only did it once, in a Lama, at about 8000 DA.

Never want to do it again. It was kind of exciting, perhaps too exciting.

My lasting impression was how quick the rotor slowed down when you pulled in the lever.

[NickLappos]

The change in capability in auto follows the power-on flight effects. The power needed to hover is higher because of density effects on the rotor, so the "power" needed to stop the auto is higher, thus the rpm droop with collective pull is faster, and the collective pull is less effective.

The steady state rate of descent is the same (a trick of aerodynamics) but the collective pitch setting to get that ROD, and set the rpm properly, is much higher.

Altitude autos at gross weight are some of the most difficult things test pilots do.

[Robbo Jock]

SPW, you could probably take that as a hint: Don't Experiment.

[[email protected]]

From Nick:
"the "power" needed to stop the auto is higher, thus the rpm droop with collective pull is faster, and the collective pull is less effective. "

I have noticed this, and I have a few questions stemming from this.

Would this suggest that it would be preferable to maintain a higher airspeed in the glide (or at least towards the bottom) and accept the higher ROD in order to have a little more energy for the flare?

Any advantage to maintaing RPM in the glide near the upper limit?(I know that this will increase the ROD) Or would it be prefereable to increase it into this range in the flare? I usualy do the later and try and keep the RPM at the top of the allowable range during the flare.

Since it does take more power to stop the machine how would that affect what altitude you should flare at (higher or lower)?

Again from Nick:
"the collective pitch setting to get that ROD, and set the rpm properly, is much higher."

Agreed, the autorotative RPM of the helicopter is higher and it does take more collective to maintain the desired RPM. The little bugger wants to overspeed with all due haste upon entry with two people onboard.

Anything other advice that would make job a little easier (its never easy) if the engine should ever quite would be appreciated.

To RobboJock:
Agreed, you won't find me playing test pilot I am not trying to push the limitations, just learn a little more about the autorotavive performance of the machine at altitude. My experimenting thus far has been very conservative.

Thanks, Steven

[BlenderPilot]

Just recently an operator had a Bell instructor come to his base (MMTO at 8445 FTAMSL) to give them training in Bell 206 and 407, I just spoke to a couple of the pilots who got to do touchdowns at 8445 FT and they tell me the only noticable difference was that only more collective was required for every phase of the auto, but mostly at the end, but even then they never ran out of collective, just used more of it.

Density Altitude is usually above 10K there.

[NickLappos]

spw,

The best way to see if more speed will help is to see the effect on ROD. If the ROD does not go up measurably, than the speed will not be a bad idea, as long as the landing is to a hard surface. Extra touch down speed to a sod surface is a real problem.

If the ROD goes up a bunch, more than 10% than the landing task is getting much more difficult, and the extra speed is probably hurting you.

Remember that for the same IAS on descent, your ground speed is already increasing with altitude (about 2% per thousand feet) so that your landing task at constant speed is more difficult at altitude.

Same argument goes for rpm, exactly. If the rpm increase costs little ROD, then go for it.

Most likely, you will experiment in descent and discover that the flight manual procedures work pretty well as is.

Remember, the gross weight for the helo changes with altitude, follow the charts in the flight manual. The auto should be performed at no more than the weight that you can hover at, IGE, and preferable at a much lower weight for routine training. Those who advocate doing all routine training at the worst case conditions should also try practice bleeding, just in case they have a car accident. Same logic (THAT should keep this thread going!!)

Blender, I think the Jet ranger is the best autorotative helo on the planet, so don't use it as the acid test for altitude effects on helos! Those with low inertia and high ROD will get esty on you fast at altitude. I know of no helo that is the same at altitude as they are at sea level, ALL are worse.

[delta3]

I thought that at high altitude an R44 may have problems maintaining main rotor RPM especially if it is lightly loaded. A spiral descent would be necessary to load extra G's in order to maintain rpm

Delta3

[NickLappos]

delta3,
Backwards. At high altitude, the rpm builds more easily.

The worst place for rpm is very low altitude, some helos have very low rpm at very cold sea level conditions. That condition usually sets the low collective stops.

[emergov]

I have always been struck by the high groundspeed at touchdown at high DA, due to the TAS / IAS relationship at altitude. There is always merit in having some fwd speed to increase the amount of rotor thrust available to cushion, regardless of DA, and as previously posted, there is less pitch angle available for the cushion because you have to contain your RRPM in the flare.

I think zero speed autos at high DA are high risk, and best reserved for when there is no alternative due to obstacles - ie a real power loss. If you keep some fwd IAS, your ground speed is a lot higher than you would expect, but you will have more margin to cushion.

[BlenderPilot]

Señor Lappos,

Just today I got to do some autos to a touchdown in a 407 at 9400 FT HD, having also been at Bell just last week thus having a fresh comparasion I discovered that the main and most noticable difference was at the very end of the auto where collective wasn't as effective, other than that I couldn't feel much difference, just sharing my experience. And you are absolutely right the, 206 is very forgiving in autorotation.


Emergov,
When doing autorotations in helicopters that I am familiar with I almost never look at the IAS indicator, just look outside and you'll see how fast you're going with even more accuracy.

[delta3]

Thanks a lot Nick,

I did not yet simulate this one (nor real nor mathwise, which i will do immediately).

I experienced one day (officially a PPL may not train/check this under Belgian law, need an instructor an board) real trouble to maintain even 90% in exactly the conditions you described, so I really did not even dare think about trying this at altitude giving my misinformation (by a US based CPL btw, namely Florida, so he must have identified/confused cold and mountain....).

Added

Confirmed by simulator (or stated reversely simulator seams to be correct given your statement

Delta3

[[email protected]]

Thanks to Nick and everyone for the information. It is good to finally to get some good advice on the subject. I have have gotten quite a few pieces of bad information, like delta3, that never synched up with what I have expeienced up here.

Just curious BlenderPilot, where are you doing autos at 9400ft?

Cheers,
Steven

[BlenderPilot]

[email protected],

We have two hangars, one in MMMX (Mexico City), and one in MMTO (Toluca), 30 miles apart.

Mexico City is at 7,340 FT AMSL and Toluca is at 8445 FT AMSL, you cannot go between the two without going up to 10,500 FT AMSL,

When I said I was doing autos at 9400 FT HD I added the "HD" to indicate Density Altitude, altitude above sea level was somewhere around 7,800 FT AMSL, but HD was 9400 FT

Yesterday's HD at MMTO was almost 12,000 FT.

Check out some of my pictures by pressing the www button above.