Autorotation Speed vs. Density Altitude

[Aucky]

I think most pilots are aware of the reduction in helicopter Vne with density altitude, primarily due to our true airspeed being higher than the indicated airspeed and therefore increasing the likelihood of retreating blade stall etc...

What is the effect of this on our autorotative speed?

In the R44 POH the suggested standard autorotative speed is 70kts. At Sea-level the Vne is 130kts (is TOM<2200lbs), and autorotative Vne of 100kts (always 30kts less than the power on Vne). At 10'000 feet the Vne could be as low as 85kts, with an autorotative Vne of 55kts.

I've always been wary of doing auto's with speed under 60kts because of the reduced flare effect, which I assume relies on dynamic pressure (IAS) as opposed to TAS? perhaps not?

So if the Autorotative Vne was 55kts, would it be wise to fly the auto at 55kts (to help with the flare, especially when the air is already thin having a detrimental affect on the flare effect) even though the blade TAS would surely be higher than in a 70kt standard auto, or would there be any negative effects of flight near power-off Vne? i.e. should the standard autorotative speed be brought back accordingly to perhaps 40kts?

I have very little experience flying at high densitly altitudes (usually T/O at 60ft AMSL in 5-15deg C) and would like to hear the opinions of those who frequently practice auto's at higher DA's :ok:

Cheers

[Jack Carson]

Typically, the airspeed for autorotation is provided in the emergency procedures section of the rotorcraft flight manual. This airspeed is presented as an IAS. It is derived from flight test. This airspeed should provide the best combination of rate of descent and decelerative flare and allow for a safe engine out landing. The testing would have been completed throughout aircrafts certified altitude range.

This should not be confused with the airspeed (IAS) provided in the maintenance check procedures section of the flight manual which provides a very repeatable procedure for checking the low collective position rigging.

The limitations section of the flight manual provides the power off Vne for the aircraft. The power off Vne may be affected by altitude.:8

[JohnDixson]

Aucky, you could call Robinson and talk to their pilots re what testing they've done with regard to autos at altitude, esp with regard to speed into the flare.

Your question is a good one, as not a lot of qualification work is directed at that specific task.

One thing that can and should be done is to keep the auto Nr at the high end of the allowable range, whatever machine you fly. Remember that as an example, the density of air at 10K is 74% of that at sea level, and aerodynamic lift is a straight line function of density, but a square function of velocity, so the idea is to offset the 26% lift loss due to density with as much increased lift due to velocity as the operating limit allows.

Thanks,
John Dixson

[Rotorgoat8]

Hard to believe no one has yet to post experienced info on this subject! Come on Man???!!!

[fadecdegraded]

NR as we all know is the primary consideration in a autorotation, depending on how the auto Rrpm has been rigged will determine what auto Rrpm you get at a specific altitude and of course the AUW of the helicopter.
ie if a machine is rigged for sea level it will be Rrpm high at altitude l ( a good thing as it can controlled by collective) but if a machine has been rigged for altitude it will under speed at sea level ( not a good thing).
In my experiance you would keep the auto speed the same as the FM states, retreating blade stall would not be problem.
If the RRpm is overspeeding no prob controll with collective.
If it is under speeding there could be a problem,personally if i had this prob i would be looking at a higher IAS in the auto to help with regaining Rrpm that would not be there at the flare and possibly some turning on the way down to keep rrpm up, have never had this happen but have had machines that have been rigged for altitude and then not hold Rrpm in the green at sea level when the throttle has been chopped.
any one else,s thoughts would be appreciated.

[Shawn Coyle]

The problem is that as you go up in altitude (DA or PA) the number of suitable places to do autorotations decreases.
Certification only requires autos to be demonstrated at 7,000' DA.
So no-one would spend a lot of time figuring this out.
There is an issue which has been little explored, which deals with blade stall in the flare/touchdown part of autorotations at high DA - several accidents that were attributed to pilot error were later determined to be stall of the main rotor, and there hasn't been much guidance published on this problem (note to self- more stuff for next edition of the book...)
Basically, you need to make sure you don't touch down with very low rotor RPM or flare too hard (when practicing, of course- if it's the real thing - who cares what happens if you can walk away?) when doing autorotations at high DA.
At high DA, your TAS is going to be higher than your IAS, so you'll have slightly more energy available in the flare.

[HeliTester]

Shawn,

Regarding the autorotative landing demonstration requirements for Transport Category Rotorcraft certification, AC 29-2C states that "the rotorcraft should be capable of conducting the all engine out landing at the takeoff and landing WAT limiting conditions up to the maximum altitude approved for takeoff and landing."

AC 27-1B states something slightly less prescriptive, but along the same lines for Normal Category Rotorcraft.

My point being that many helicopters are certified for takeoff and landing at density altitudes well above 7,000 ft, and have performed the required flight test demonstrations.

HT

[Gordy]

Correct me if I am wrong---but I believe the manufacturer is allowed to test at one altitude and then "extrapolate" the numbers to create the charts.

[HeliTester]

IGE handling qualities, height-velocity and engine operating characteristics can be extrapolated 2000 ft density altitude from the test altitude. Hover and takeoff & landing performance can be extrapolated 4000 ft.

[fijdor]

Based on my own experience, if you go back to basics you will realized that what you need in an autorotation is a rate of decent to keep the RPM up there and that is what you will need at the bottom. Speed in theory, is an optional thing because you could shoot an autorotation a zero airspeed if you wanted to. The speed you want on the way down is up to you or more like up to what you will need to make it to your intended point of landing, the usual speed we use is around 60 kts and that is the speed the book and people refer too as the speed to be at. These numbers have names like "minimum sink rate", "max glide distance " etc but whatever speed you will need is what you will use to reach the spot and keep it lower than VNE or you will start loosing RPM.

What you should worry about at altitude is RPM. As a matter of fact, it is what you should worry about in any autorotations not speed. FADECDEGRADED is exactly right about RPM at altitude and your aircraft being set up (auto RPM adjusted) to fit the DA you will be working at, read his post and what will happen if you're not set up for it, but what he forgot to mention is this if the engine quite at high altitude, and you do what you are supposed to do when it does, well I can guaranty you that within 2 or 3 seconds that collective will be right back up where it was when the engine quite just to maintain the RPM close to the green and to make sure the blades are not going to take off on you because of the overspeed or get a bigger blade tip than the blade root is (I know I tested it and also did have to deal with a real auto at a lot higher altitude than 10,000 ft) The overspeed in itself is not a major problem because you can manage that by using the collective, the problem will be at the bottom when it is time to land, remember you are already at half collective to maintain RPM and the flare at the bottom will ask for more, the problem is, there won't be enough collective left to soften the landing. That is what you need to adjust.

I don't know much about the R22 but I am assuming the auto RPM can be adjusted to the DA you will be working at, like other aircraft, at least the ones I fly can be. There is a chart in the maintenance manual to check and adjust the auto RPM according to altitude and the gross weight of the aircraft. Once you get to where you are going, I would recheck that and readjust according to your new location. Don't worry about a couple of thousand feet but if you go from sea level and base yourself at 10 thousand feet and work from there, I sure would

To answer your question now, use whatever speed you think you will need to get there, and that is as simple as that. Don't worry about speed if you go too fast the aircraft will let you know. In a real emergency, you WILL NOT HAVE TIME to calculate 26% of VNE minus 12,000 ft DA plus I didn't get much sleep last night minus how many pax I have at + 3 degrees and on and on.

Just get down there and land safely.

JD

[Aucky]

I may be being a little slow here (2 scotch's later, and limited high DA experience) but I always assumed that we would suffer lower rpm at altitude as the air is less dense and that the effect would be a reduced autorotative force... but obviously for a given IAS the TAS is higher... is that the reason for a higher rpm in auto?? or is it the reduction in drag due to less dense air perhaps? also what's the effect on ROD, is it significantly higher for a given rpm as less lift is being produced? :confused:

I remember half way through my ppl doing a little mountain flying in the Drakensbergs, SA and with my instructor - We took a 22 up to around 11,000' and auto'd the whole way down to the valley floor at around 4,000' lasting around 4min but it was too early in my experience for me to notice the intricacies. Actually the one thing I do remember in the auto was autorotating upwards, in a 2000'/min updraught but thats going off topic slightly.... :}

[[email protected]]

Aucky - to establish a steady state autorotation at high DA you need to produce sufficient rotor thrust to balance the weight of the aircraft. As you correctly state, the air is thinner so to produce that rotor thrust the RoD must be higher (ie you fall faster to push enough air through the disc).

In this condition, the inflow angles are greater, the AoA of the blades increases and the area of the blade producing autorotative force moves outboard and increases in size - therefore the Nr will increase until you raise the lever to control it.

As for speed, your TAS at 10,000 ft with 55 kts IAS will be about 75kts (using 2 kts per 1000' as a rough rule of thumb) and if that is your VNE in autorotation, you don't want to be faster than that because your Nr will decay (reduction in size of auto section caused by too much AoA and a rearward tilted lift vector).

Your flare effect will be reduced in effectiveness by the DA but it will still work although Nr will be lively as you load the disc as there is less rotor drag at high DA.

Will a high DA EOL be possible? Yes. Will it be more exciting than at SL? Yes.

Aim for a big area as low down the mountain as you can:)

[Aucky]

Crab - Thanks a lot, that has been a good help in my understanding :ok:

[that chinese fella]

All this talk about setting up Auto Nr for your area of operations DA confuses me.

All the Auto Nr charts and Maintenance Manual references dont mention anything about it.....

The charts take into account the altitude and temp at which a Auto check is taken and if you adjust the Nr to appropriately then it is my understanding that the Nr will remain within limits, no matter what altitude you find yourself. Take note of an Auto RPM chart next time you get chance.

Is the setting a high Nr a misnomer as well? High Nr = High ROD, so yes we can arrest the Nr with collective on the way down but as has been mentioned unless you lower the collective (and probably have it overspeed) before flare, level and cushion, the amount of collective remaining is reduced.

Personally I prefer Nr to be set as per the MM and auto chart. Crazy talk I know....

[fijdor]

In my earlier post I mentioned the chart in the maintenance manual and that is what you use to check and adjust auto RPM . I have a chart in front of me right now. In that chart it tells you what NR you should have at different DA and gross weight and you should be within +- 2.5% of that.

I also said you check the RPM before you go and also once you get to the altitude you will be working at, living at. you recheck and readjust accordingly by plotting the numbers on the chart.

As per my experience, it is not because the numbers you get a sea level are ok and within the chart that they will follow the chart and still be ok at a DA of 12,000 ft.

What I would suggest is simply to climb to ten, eleven thousand feet and test the system. Shoot an auto, maintain a constant speed and RPM (with collective) and I can guaranty you that you will have to lower the collective a bit at every thousand feet of altitude lost just to stay within the limits.

The infos in my posts are based on observations made working at a high DA.
In fact there are some really good infos here made by other guys/girls, Crab,Fadec etc

Enjoy the flight

JD

[Jack Carson]

The check for correct rotor speed required in the maintenance procedures is there to establish that the aircraft’s collective is rigged properly. This check is usually accomplished at some nominal weight and altitude. Most charts will indicate that DA and/or gross weight at which the rotor speed will exceed the upper limit should the collective be lowered to the full down position. As an example, the CH-53E at its maximum gross weight requires that at least 30% up collective be maintained to insure that the maximum rotor speed is not exceeded during an autorotative descent. At the other end of the scale, if the rotor speed is too high you may not be able to achieve maximum power (high collective) in level flight. The check for max power is a secondary check required after checking autorotative rotor speed. I have experienced instances in the BK-117 where high autorotative rotor speed resulted in not being able to achieve the aircraft’s true Vh. :8

[Shawn Coyle]

Helitester:
The HV curve is demo'd typically at 7,000' DA, and all the manufacturer is required to do to demo higher conditions is show that from a 'normal' airspeed in autorotation that you can arrest the rate of descent with a flare. Done it several times and it works.
The reason for this is that it's impractical to ask someone to do an HV series at altitudes where there are no paved runways (which is anywhere above 9,950' (Leadville, CO) in North America.