Autorotation question

[walesuk]

Quick beginner question... can someone tell me what effect strong or light wind will have on the initiation of an auto? In other words... begin the flare higher or lower with differentiation in wind strength. Thanks!

[overpitched]

C'mon you can do better than that. Why don't you ask us what effect the wind will have on a turn... eg turning downwind at 500 agl... Now thats a fun question !!!!!!

[Arm out the window]

Hopefully this one's not such a bone of contention...the reason you flare is to reduce rate of descent (wind not a factor here) and to reduce groundspeed (wind naturally comes into play for this).
Compare a windy day (say 20 kt breeze) auto to a calm day one, assuming you want to get zero groundspeed for touchdown in both. For the windy case, you only need to reduce IAS to 20 kt to get zero groundspeed as opposed to zero KIAS on the calm day, so the flare doesn't have so much work to do, so to speak.
This implies that on the windy day you could start the flare later, not flare so much, or flare slower, whichever you like.

[NickLappos]

Arm has it just right, the required airspeed reduction is much less, and also the needed collective pull is less because the rotor is more efficient at 20 knots of airspeed (at zero groundspeed) so there is much more energy and more chance to fix minor glitches in technique.
Heavy helicopters are often not able to be zeroed out at touchdown, and they rely on a 20 to 30 knot landing ina full auto. The wind therefore is a real boon to big machines when they have to make power off landings.

[teeteringhead]

Exactly so, which accounts for (certainly in the military) minimum wind limits for certain of the dodgier practice EOLs (autorotative landings). IIRC 'twas 10 kts windspeed required for hover or constant attitude EOLs. Furthermore, 10 kts "through the disc" required for all EOLs, ie you couldn't (legally) do a zero speed in less than 10 kts, a 5 kt headwind would require a 5 kt groundspeed run-on etc etc...

[TheFlyingSquirrel]

When doing type ratings or training in large twins, are autos practised to the ground or just to the recovery? Does it differ from the UK to the US?

[Head Turner]

In a real EOL and on a windy day, think of 'windshear'. You will have a benefit from the wind until near the ground when that wind could well disappear due to trees/buildings up wind of the landing site.

[212man]

TFS,
autos in large twins are conducted to a a powered recovery (in the civilian world). In fact, the previous norm was to restore the throttles to 'Flight Idle' by around 500 ft agl and use power during the landing flare but to a large extent this practice has been replaced by a simple fly away by 500 ft agl, maintaing the IAS throughout. The thought process being that the entry and Nr management in descent are the more important areas to train for, and the risk attached to the landing is out of prportion to the chances of having to carry one out for real. Double engine failures are pretty rare!

No idea what they do elsewhere.

[cl12pv2s]

In additiion to what ArmOut...and Nick have already said, I want to add another perspective around the same theme...

That is 'the amount of space you need.'

The concept is simple, but the implications, when faced with an engine failure, are big.

Simply, with no wind against you, you will require a larger 'distance over the ground' in order to slow the aircraft to a reasonable setdown speed.

This needs to be taken into account when selecting a touchdown area.

I'll try to explain what I mean...

If you think of an 'Aiming Point' as being 'the point at which you aim for when descending during the auto' (i.e. if you did no flare you'd hit this point). Then also think of a 'Termination Point', which is the point where the aircraft will stop.

In a no wind situation, the distance between the two will be significantly larger, than in a high headwind situation.

In fact, not considering the importance of this concept (and adjusting for the conditions) is the reason why I see many students who will continually overshoot their intended touchdown point. (During the auto, they tend to aim for their termination point, not the 'aiming point'.

Generally, all winds being equal, an earlier and higher and progressive flare will result in a slower ground speed. This is because of the increased time with a nose-up attitude. (Aft component of lift.) Again, during training, I see students who leave it too late, and then wonder why they always seem to run for miles on touchdown.

Well, I hope that 'ramble' sort of helps.

cl12pv2s

Disclaimer: Your instructor is the best person to advise you on technique. Don't try anything new (that you may have picked up on this list) without discussing it with him / her first!

[Arm out the window]

Good point, Lift Formula.
I also reckon it's handy to have a rough rule of thumb in aircraft lengths per 5 kt of wind, say, that will give an idea of how far back the aim point should be from the termination point, and that can be worked out over a few auto sessions in different wind conditions using runway cone markers or whatever to see what works for the particular machine you're in (will also be affected by weight and DA, but at least it's a start point).

[eagle 86]

In the real case DO NOT ever contemplate landing with any forward groundspeed - this applies to any surface - open paddock,trees, rocks, water anything!

For instance trees - imagine the top of the canopy is ground level - at flare height stand the machine on its tail - level from the flare around 10 feet above canopy and settle gracefully, vertically into the trees pulling all the available collective as the skids/wheels/lower fuselage settles into the treetops - your chance of survival from this vertical descent is much better than if your ROD is combined with only a hint of forward speed

As with fixed wing do not try to "stretch the glide" - if you screw up the auto bit recognise so early, re-establish a normal auto profile and land vertically wherever you end up.

GAGS E86

[Hiro Protagonist]

Lift Equation - I found that teaching the aiming point / termination point greatly enhanced my students ability to hit their spot, especially, if their entry point was less than ideal and they needed to maneuver to aim their glide at the aiming point.

eagle 86 - I agree with your first point about having zero (perhaps even a slight rearward) ground speed during a real (unplanned) EOL. I found this concept to be well expressed in Phil Crouchers book, "The Helicopter Pilot's Handbook". I tend to disagree however, with your contention (also, if I remember correctly, held by Mr. Croucher. You're not him are you?) that you should not attempt to extend your glide to reach a better spot to crash.

If my engine goes bang while I’m cruising along at say 2,000' AGL over the woods, and an extended glide will bring you to the soft cushy meadow, while a regular auto will bring me to a stand of timber, I know that my Nr will be at the bottom of the allowable range, and my airspeed will be close to VNEauto until I see a normal sight picture to the meadow.

Perhaps this difference of opinion has to due with the different autorotational characteristics of different helicopters. Most of my experience with modifying autos to get where I want to be is in R22s and R44s. These machines show a marked extension in glide when flown in their published max glide configurations. And the energy stored in the extra airspeed you’re carrying, can be used in the flare to extend even farther while recovering Nr as described in Shawn Coyle’s book, “Cyclic and Collective”.

Of course, part of the equation is crashworthiness. I’m much more willing to go into the woods in the Bell 214 I’m co-piloting in this summer than in an R22.

eagle 86, I'd be interested in your reasons for opposing attempts to extend your glide.

[Bravo73]

Mr. Croucher. You're not him are you?


No. paco (http://www.pprune.org/forums/member.php?s=&action=getinfo&userid=24026) is.


:E :ok:

[paco]

No that's me!

I certainly do not agree with stretching things unnecessarily - I would rather do a good auto to a bad spot than the other way round! If you overcook the stretch and can't get the revs back, you could be going for a football field and still be dead. Mind you, the comments in the HPH were based on flying over Northern Canada where there aren't so many meadows and considerably more trees!

I'm glad u like the book! :)

Phil

[eagle 86]

HP,
I'm not talking about not using your helo's performance parameters such as range configuration to make it to a better landing point - what I am saying is that when this is obviously not working (sight picture) then convert back to the "normal" auto profile so that when you arrive at flare height you have the best flare potential to reduce ROD and G/S to achieve a zero/zero touchdown.
GAGS E86

[Arm out the window]

eagle 86, totally off the track here, but what does the GAGS stand for in your posts? Obviously some computer shorthand I'm not familiar with.
The only thing that springs to mind is 'go and get stuffed' but I'm assuming that's not it!

[eagle 86]

AOTW,
Do you think you know me?
GAGS E86

[SASless]

In another life, I was told about doing good autorotations to bad places. The theory was....upon realizing your arrival was going to be to a bad place....then the choice of which bad place became of little importance. The important issue was to arrive with as much rotor rpm as possible. The arrival without bags of rotor rpm was not an option.

The suggested procedure upon making the major decision about good auto/bad place...or bad auto/good place was to be made early. The auto was to be held until just off the bad stuff....hard flare done...build all the rotor rpm possible...and when forced by gravity, inertia, and experience....pick the best looking place at hand...and carry out a vertical landing into the bad stuff.

Coming off a 10,000 foot mountain near Tok, Alaska late one evening just at dark.....the donk died with a bang leaving me in a rather awkward position. Way...way...way too high up over what appeared to be the world's larget patch of forest. Gave up trying for the one "best" spot and went for the suggested method. Arrived at treetop level...just as I began the flaring maneuver...realized the nice spot was just beyond some taller trees....and within reach with a good flare....and it worked...popped over and fell right straight down into a pussy willow thicket....with no damage to the aircraft beyond the seat cover I bit off with my teeth telling the passengers they could exit the aircraft.

It pays to listen to old guys that have been there and done that you know.

"Stretch the glide" is okay.....but don't over stretch it....helicopter pilot's live and die by rotor rpm....airspeed can be traded for rotor rpm....altitude can traded for airspeed which can be traded for rotor.......you cannot have too much rotor rpm when you are doing real engine off landings. The overspeed is secondary to the primary consideration of saving what mashes your wallet to the seat.:ok:

[TheFlyingSquirrel]

So Rambo - you're saying if you're going to land somewhere nasty, then adopt the old " the aircraft has f**** me so f*** it "notion and just go all out to make as gentle as possible zero speed landing into the trouble matter, even if it means an overspeed in the descent and or the flare?

[SASless]

If you are going to land in the trees....do you care about an overspeed? Might as well get some benefit from all that rotor system you are about to write off whacking off tree trunks. Out in my part of the world some of those trees get kind of tall....hate to think of falling down through some that are 200 feet tall. Would aim for the shorter...thicker branched style....and hope there is not a great big old stump under me.

[paco]

You need as dense a forest as possible - the biggest thing to get in to your head is that you're going to hit something.

Interestingly, this is one of the best discussions in my CRM courses

phil

[ec135driver]

Fly to survive!

Engine parameters and rotor speed limitations are there, in the main, to ensure that everything reaches TBO at the right time.

Even Frank R will tell you at the school " the engine (in an R22) is rated to 160hp - dont crash just because you didn't want to exceed the MAP limit for the day!"

Rotor overspeed, vne exceed - who cares? when it comes the time to put all your training to good effect - LIVE!

[Arm out the window]

Eagle 86, no, don't think I know you, just wondering idly about the sign off part of your posts.