All he's saying is that by the time you react, get the lever down and the aircraft settles into its fully developed auto with a stable rate of descent, it takes about 1000 ft.

Leading up to that, there will be various variables that take a while to settle down, as he's already described above.

That's not the same as saying you can't fly a proper auto from pretty much anywhere up the transition slope, or anywhere really - you just have to judge how much flare and how much lever lowering you can do.

Obviously from a decent height you can get the collective down, flare to achieve your desired speed, stabilise at a constant rate of descent and have a cup of tea on the way down, but from lower down you won't necessarily be able to do all those things. You may have speed that you can trade for extra potential, maybe not.

As you'll note, E86 has mentioned things like the delay involved in your brain processing what's happened, during which the Nr is decaying like a bastard. Hughes500, you obviously got the lever down quick and did what you had to do, but that's not to say everyone would. The other thing to keep in mind is that he's only talking about being established in full blown stable autorotation, not just in a position where you can pull off a landing, if you get my drift.

Arm

As an experiment today flew with a customer who had not flown for 14 months. Flew at 1000 ft agl over airfield ,70 kts in a 300C. I chopped the throttle with no prior warning. Customer ( tt 70 hours, not trained by me)dumped lever as ac yawed about 40 degrees to the left. RRPM down below red line about 380 rpm. He flared back with the cylic bringing rrpm back to about 410 rpm by 800 ft and 50 kts. By 600 ft the ac was rrpm of 480 rate of descent 1600 ft per minute, fully stabilised autorotation.
So I am afraid I do not understand where 1000 ft comes from, unless an R22 is the subject of 1000 ft . If that is the case I will eat humble pie and offer a sincere apology

Fair enough mate, I haven't done any autos recently, although I've done heaps to touchdown in the past from many different setups.

When doing maintenance test flying in Hueys we would get set up 1000 ft above the test altitude to enter auto to ensure everything was stable to note the Nr when passing through that altitude. It was part of the test flight schedule so I guess there's something in it taking perhaps that much height to get totally stable to the nth degree.

Having said that, starting from say 1000 AGL on a straight-in auto, you're certainly comfortably established in autorotation and waiting for flare height to come up after two or three hundred feet or so, so yeah, who knows.

don't know where i got this 300' of airspace thing from been carrying it around without testing the idea for some time.
just now (one up R22 almost full fuel) experimented starting from Straight and level at about 150 feet using fence posts as measure markers. from cruise at 21" to established in auto descending flight it was 250 metres.

eagle86
You will not have any risk of losing the rotor in a Bell with an aft cyclic movement, unless you know something the rest of us don't know.
Abrupt forward movement is a problem, but aft???
Please explain.
In a Bell 206 it takes less than 300' for the rotor RPM to get back into the green from a hovering engine failure (at a high enough altitude, obviously). Done it many times when teaching the H-V curve.

1000ft is total rubish.

You can start an auto at 100kts/50ft and be fully stabilised in time to make a landing, equally from 1000ft using minimum height loss technique you can be stabilised after about 50ft of descent if you are smooth.

Autorotation can be established almost instantaneously by pulling back the cyclic. This can be shown by the need to contain Nr.

If Nr rises by itself, then you are in auto.
Rate of descent is totally unimportant, only AoA relative to the disc matters.

There is absolutely no need for any descent to occur to be in auto.

Correspondents appear to believe that I am saying you can't pull of a successful landing following an engine failure from any where - go back to my original post - I have carried out many thousands of engine off landings from any of those scenarios successfully. However, as an example, I can assure you that a loss of power from an IGE hover or an OGE hover almost anywhere inside the HV curve the rotor system will NOT be in fully developed autorotation before you need to pull pitch to cushion the landing. Let's not get too pedantic the height loss maybe say 800 feet for fully developed aerodynamically stable autorotation but stable aerodynamic autorotation is not instantaneous. As AOW stated when carrying out an auto rev check test flight the auto was commenced at least 1000 feet above test height to ensure fully developed stable autorotation was achieved by that test height.
A word on underslung teetering heads - years ago the service I was attached to suffered two fatal accidents, four pilots lost, as a result of main rotor separation - one flight was simulated IF which was to include UA recovery. The other was a two ship low level tactical formation. Somehow these pilots pulled the rotor heads off - there was no sign of a pre-existing failure. A senior Bell test pilot was invited to discuss the aerodynamics of this system. He emphasised that extreme care should be taken with cyclic movement with anything less than optimum RRPM particularly at less than 1G - exactly where you will be should you drop the lever after an unalerted power loss.
When constructing the HV curve manufacturers take into account many factors including pilot reaction time - low inertia/high inertia rotor system to name a couple. The result is the avoid curve - in other words avoid flying in this zone as we don't guarantee a successful landing following a sudden power loss.
GAGS
E86

From what I read here,You all have a Big gap in what H/V tests are and how to use the controls and height /air speed to carry out a secessful power failure (autorotation), In Not just one but any of the different types of helicopters supplied to a pilot. Sad so sad.

eagle86

What is this "aerodynamic autorotation" you are talking about?

Autorotation is an aerodynamic effect. We call it "autorotation"
There is no other form of autorotation.

I have flown Seaking/Wessex/Gazelle/Squirrel/UH1/AB212/S70B/Mi17 and none of them require anything like the height you talk about even from OGE hover.

Yes from a hover you will lose significant height if you try to regain speed, but that is nothing to do with autorotation, that is just trading height for speed. Autorotation is very quick to take effect. Try an over aggressive fast-stop and watch how quick the Nr gets excited.

Could you explain what you just wrote a bit more clearly, hillberg? I don't get what you're on about.

Are you trying to say that no-one who's posted here knows how to do a successful auto? That's what it sounds like, and if so, you're way off the mark.

Tourist,
Aerodynamically stable autorotation.
GAGS
E86

Tourist again,
conservation of angular momentum ie the ice skater effect.
GAGS
E86

Hit the books, Every helicopter handles a little differently than another, From Low Gross weight/High gross, Air speed and all the other stuff that you take in account for a safe flight. From the little R-22, and the Hughes 269 to the S-58. If you are flying with such a gap in knowlage required that you are asking a Forum a question "Why aft cyclic in the start of an autorotation" Get your money back from that "Instructor". What you don't know will kill you. Learn & live.

I'm not asking that question, and before shooting from the hip you would be well advised to read the original post at the start of this thread.

There's a lot of knowledge and experience here, and I think we're all well aware of when to use cyclic in an auto.

Do you always need aft cyclic entering auto? Of course not, it depends on the situation, so go back and read what everyone's said before pontificating.

Ever flown a fixed float-equipped Bell 205 or 206? You need a bunch of forward cyclic entering auto to stop them tipping over backwards, which is what would happen if you applied aft cyclic because of all the extra surface area forward of the C of G that wants to pitch you up like crazy as the rate of descent builds up.

eagle.

I have no idea what you are refering to.


Lower the lever until the aerodynamic forces have an accelerating effect on the NR.

You are now in auto.

End of.

You can now turn off the engines and pull wingovers and bunts and spot turns or accelerate to Vmax to your hearts content as long as you raise or lower the lever to maintain Nr in the correct bracket.


You are just plain wrong about needing many hundreds of feet to establish in a stable datum type auto.

Hi Hillberg,

Perhaps you could help explain procedures for a couple of scenarios from your books?

1. Peter pilot is loitering around at 180 to 250 feet agl at around 10 to 25 knots IAS, moderate timber being 35 to 45 feet. Winds light and variable.

Captain Peter is scanning, scanning for the elusive bovine or etc. Silly sod is concentrating on the bovines, the job, the local bosses daughter, the next pi**up or something - and not thinking about the inevitable - if the noise stops. It does, what does he do with the cyclic and collective?

2. Captain Peter is a silly sod, with the same wind, the very next day he is tooling along at about four feet AGL over an open clay pan at high cruise speed, despite being told many times as a newbie last year not to, and after a great yard up or a great night with the bosses daughter or a splendid night on the turps, inflated with his own ego, and all of the above, at about 1200 hours experience (the most dangerous for mustering pilots) and - not thinking about the inevitable - and dammit the noise stops. What should he do with the collective and cyclic?

This will be any light helicopter 12E, '47, 269 or the fabled 22. He might even be in a 44 or a 206.

Regards.
TET

#1 Dump the collective and keep what RPM you have left , pull collective just above the tops of the trees and take the ride down through the branches. Keep the descent vertical as the nose of any helicopter will offer no protection,Let the structure do its job,Been there done this.(no trees)
#2 aft cyclic a little and let air speed become a little altitude and (bump) and then lower the collective a little to keep the tail rotor from hitting the ground (ARMY NOE TRICK) & keeping what RPM that will be needed for the tuochdown. Been there twice.
At cruise speed the 269 auto target is at your toes,The 206 is just above the lower windshild frame,The R-22 at 70 mph you can "ballon" and do a second auto to a new spot. The Brantley can finnish an auto with out adding collective at the bottom. and In a good stiff wind the Bell47 will float in the bottom for 300+ feet @.13" above the ground.
As for floats the tunaboat job as fun. As for the R-44 beter have the Bladders installed. Fire sucks.
A poor Peter pilot should allways fly with a way out,A keep his mind on flying. I like it use "What If?" & RPM is life.

Tourist,
I guess your public profile says a lot about you. Mate, I can't help you. Let me know where and when you intend to conduct your next autorotational display - I'd like to come and watch - from a safe distance!
GAGS
E86

Interesting hillberg, those books; I think they miss a bit of detail.

#1 Yep RPM is life no question.
Another one is knowing your winds even when you are sound asleep in bed. Getting down quickly and arresting the ROD into wind is far easier than doing it downwind. Even 2 to 3 knots is significant. No such thing as Light and Variable when your world is around the tree tops, I am sorry, but that was a planted clue.

Another clue was 'moderate' timber, reads “enough room to get the hull down straight and let the blades chew their way down”. Trunks up to five inches are OK even in a R22.

Conduct the auto to the ground, the collective control will be useless after the first decent strike but the blades will be absorbing energy like you wouldn’t believe and slowing everything down. The cross tubes may bend but you should be able to undo and step out, been there seen that.


I do not believe that to pull pitch and run out of RPM at the 25’ tree tops is a good idea, we have seen a couple of deaths from overpitching here lately, from about that height. You need that energy of RPM to use against the timber to slow down ROD.

Firstly as you say get that lever down pronto, at the same time point the cyclic directly at the spot you have already subconsciously been aware of. That sub conscious awareness as much into wind as possible, must be demonstrated prior to a low level and mustering sign off.

Get the cyclic back just before you go through the canopy. If it is into wind its forward airspeed will stop on a dime.

Forget the garbage about the low G danger to the head; the aircraft is only descending at the rate of one G. It is not in a low G push over manoeuvre, don’t forget that.

#2 There was a couple of clues here too. Notice how I changed the order of controls in the question and also a 1200 hour ego machine will not stop a machine descending at least one foot or more if it is a sudden and complete power loss like a free wheel slip.


So from anything under four foot skid height there will be no, y'll hear, NO REAR CYCLIC until the aircraft is cleared away from the ground to a safe T/R clearance height.
It is simple, a quick jerk on the collective, then start with your cyclic as you put the lever back down and then control RPM with the lever as you would in a quick stop.

Zoom it a bit if you like, maybe jump over an obstacle or some such, whatever you fancy, even a 180 can be accomplished with most types from twenty feet or so and a high cruise speed, to get back into wind. A G5 will do it from 65-70 knots in the width of an airstrip.

The thing is, in both instances the people that are usually going to be good at mustering are often a bit flamboyant, sometimes will take risks and can be counted on to forget, sometimes defy, instructions regardless in their early years. So, one solution to reduce risk is to teach the survival technique basics at an age when they usually remember the basics. Just like a young pup.

cheers tet

1. An unpowered rotor will NOT Chew its way down, The ROD will be as low as you can get it before entry in the tree tops. If you have time for in the wind I'm all for it , But adding forward cyclic will droop rpm- with a minimum of time you need to arrest the fall before the rotors are lost,After the blades are gone all control is lost and you become the battering ram at the front if any forward speed is left.

2. aft cyclic and bumping collective will rotate the aircraft attitude and raise the tail rotor clear. No aft cyclic? Some helicopters tuck on a power failure, It all depends on the way some helicopters handle.

3. Ty a right 180 auto then a left 180 auto-Tell the results.
The #1 rule is get home alive.
You have some good answers.

Some good thoughts here. Of course, the right answer is to not put yourself into a bad situation, if you can avoid it.

In general, I think of the autorotation in three phases: entry, sustained, landing. Each has its own peculiarities that we could all talk about for days. When you get into any low level situation, then you may have to omit the sustained portion.

For example, zero speed 500' AGL, lower collective and dive for airspeed, you should be able to attain your normal sustained parameters, so all is simple.

The entry and the landing both have airspeed changes and height changes. I think of the landing phase as a graph of speed and height, start high and fast, end at 0/0. If the entry speed and height approaches the landing speed/height line you just switch from entry phase to landing phase, allowing time and energy for the transition. Sounds complicated, but its really not.

Change the example to 200' and zero speed, you lower collective and dive. At 100' you have ~40knots (made up numbers), you haven't attained your sustained parameters, but your speed/height combination looks just like the middle of your normal landing phase, so you raise the nose to your decelerating attitude, allow the rpm to build a bit and continue with the landing you've trained.

topendtorque,

#1 I agree you don't want to lose all rpm or collective at the tree tops, but you also don't want to hit the trees at max speed. Seen the effects of that. I'd rather take my chances on a 35' foot fall, with a small cushion available than a 2000fpm descent into lumber. My plan if I don't think foliage will cushion me is to zero speed and minimize descent just above the tree tops, then dump collective to try and recover rpm on the way down, cushion at the bottom. Of course, that would be pointless if I thought the trees were going to destroy the blades so this is all situationally dependant.

This gets a bit more difficult on the West Coast of Canada. 200' trees that are greater than 5' diameter at the base, and with foliage not always dense enough to stop or slow your descent. Reliable engines and/or prayer is the best alternative we could come up with.

#2 Trade airspeed for altitude. Same as above, you start in the entry phase, but in this case you approach the landing phase from below. When you flare on entry, you made need to raise collective to control rpm, but that is type and situation dependant. When your altitude & height intercept the landing phase, it gets easy again.

Interesting opinion but not one I share. I agree with Tourist: you don't need anything like that to establish a steady state auto in the types I've flown.

Eagle 86

"Tourist,
I guess your public profile says a lot about you"


Nice.

Always good to play the man not the argument.:=

(anyway, what's wrong with being a wafu?!)

Come to Predannack any day of the week and you will see a variety of autos.

My personal favourite was always the
3000' 90kts into wind Seaking comedy auto invented by a certain very grumpy pilot who now flys for Loganair.
On top the target (15kts of wind was nice), enter auto and rapidly ramp it up to hit zero speed without height loss.
As soon as zero airspeed, level the cab and spot turn in auto right.
After one complete turn, bury the nose to get 70kts and continue the turn into a 360 auto right.
As you approach the into wind heading dump the nose further to get into a range auto up near Vne.
As the wind has drifted you backwards, the two stage flare should bring you nicely back to a spot landing on the H.:)

I still remember my large ginger instructor on 810sqn demonstrating that for a real double engine failure, you can do whatever necessary to get you back to the spot. His party trick was to fly over directly the spot into wind 90kts 2000' and get the student to say when he thought it was no longer possible to get back to the spot in auto. When you eventually said "now" some 20 seconds past the spot he would wait another 10 seconds before entering auto then straight into a wingover in auto, dive back towards the spot before another wingover to bring him back into wind for the flare/check/level. As he said. "if you control the Nr, then the rest is just normal flying"

R22 autorotation
 

Hi guys
One thing i am having an hard time to understand and visualize is the relationship between cyclic movements and rrpms? Why rrpms change as the helicopter pitch is changed during the autorotation?

Many thanks

Baobab72

Think of it as a windmill. If you blow towards the windmill perpendicular to the windmill rotor plane (directly into the windmill rotor), you get a lot of rotation in the system. That same windmill, if you blow from the side, you won't get anything out of it.

Cyclic alters your attitude towards the airflow or if you will alters the relative direction of the airflow in relation to the rotor disc. Flare - increase of area-airflow, cyclic forward, decrease in area through which the airflow is going (just like blowing the windmill from the side.

Baobab

When you bring the cyclic back, you are raising the nose and changing the angle the air meets the blades - this will give more lift (and drag) but the extra lift will make the blades try to fly up (cone up). As the blades cone up, their centres of gravity will move towards the rotorhub (only a small amount) enough to increase the RRPM because of the conservation of angular momentum (think of a spinning ice skater who brings her arms in to spin faster and out to spin slower).

The opposite is true when you move the cyclic forward (this can be very dangerous in a teetering head helicopter if you do it quickly as it can lead to mast bumping).

In summary - bringing the cyclic back increases RRPM in auto (and reduces speed) and moving it forward decreases RRPM (and increases speed). It is the rate of change of the airflow that determines how quickly the RRPM rises or falls.

coning 'cause' doubtful - taught but doubtful
 

Ice skaters can't keep moving their arms in forever - the coning explanation at best is only a (transient) minor contributory effect - likely to be the dominant effect is the Total Reaction becoming 'more forward' (causing more 'autorotative force') corresponding to the increased angle of attack derived from increased negative induced flow achieved by the flare.

You can see this by holding an increased loading through bank in auto where the transient coning effect does not manifest itself above the effect of the ongoing continuous bank.

or even more simply
 

like Phoinix says but more simply:

the thing that makes the blades go around in auto rotation is the air passing up through the disc.
when you pull the nose up more air goes through the disc faster, so it'll get more of what makes the blades go around - that will make the blades go faster.

It's a different sort of autorotation, eagle. You are right, it takes a few turns to get steady state, to where you can let go of the controls and the aircraft spins in a predictable fashion, and stay that way until impact if no spin recovery inputs are made. In that case, the entire aircraft is rotating about the vertical axis ... you sort of create a rotating wing out of the wings and fuselage ... is that the point you are making?

I've taught spins in FW, and autorotations in helicopters, and I'd never take the position that they are the same sort of maneuver. Not sure what you are after there.

A helicopter autorotation is a different critter from a spin. The rotating wings are not stalled during the evolution, and the rest of the aircraft, once the torque is reduced/removed by reducing pitch, doesn't flop about. You remain in controlled flight.

About autorotations to check control rigging:

I agree that it takes some altitude to establish a steady state auto, steady state ROD, and your test airspeed right on the money to take the precise data point needed to check control rigging. I think it's a bit much to use such a requirement to assert that you aren't in a stable auto in less than a thousand feet. Depending on rotor inertia, you can get stable enough in light helicopters to make a complete auto in less altitude. In the big and heavy ones, maybe not ... haven't done on in a while, so I'll not go any further on that score.

eagle, I very much liked the list of cases you provided for reference, since engine out and auto fit into a lot of different conditions, and one needs to be prepared for any of them.

Great thread, by the way. :ok: Thanks to all who have contributed.

no, they reach a steady state - just like when you stop moving the cyclic and the Nr returns to a steady state.

still struggling with English

and P of F;)