Autorotation

[MrIce]

I've got some questions about autorotations. Is it true a helicopter can keep his RRPM in the green with 0 speed? It sounds logical to me, but I can't find any reference on it. Secondly I saw a movie of a (low speed) no flare autorotation in a Jet Ranger (http://www.youtube.com/watch?v=CJGvm1IOgwk&mode=related&search=). Is this a know manoeuvre? What are your thoughts on it?

Grtz

[rudestuff]

Yes. You can autorotate at any airspeed. It looks to me like there is plenty of speed there - it just looks slow because of the steep descent angle. A zero airspeed auto to the ground would be over a lot quicker, and you'd need perfect timing to not get hurt.

[denlopviper]

not a helo pilot but plan to be one and own a bell-222 once earn enough with airlines :}

as i understand it, to keep the rotors turning, you need forward velocity....you can keep the rpm in the green, but then you`ll be falling like a rock :}

[aspinwing]

That is one we can add to Nick's list of urban legends.:hmm:

[Nigel Osborn]

Zero air speed or zero ground speed autos are no problem if you practise, just like any other tricky manoevre. Basically if you start at a steady 40 kts into a 20 kt wind & run on at 20 kts, you would be surprised how easy it is to gradually reduce your airspeed to achieve zero ground speed. You also should start with a light machine & slowly build up weight. Of all the dozens of pilots I've trained to do this, not one has had a problem, in fact they had more problems judging when to flare & level off.

[[email protected]]

For a Constant Attitude EOL - ie there is no flare - we used to teach that you wait until the green turns into grass (between 20 and 30' agl) and make a short, sharp check up with the lever to begin to reduce the RoD, then cushion the touchdown using the rest of the Nr. It is technique that works well in a reasonably high inertia rotor system but not something I would like to try in a robbie.

I did fly with a colleague (RN) at Shawbury who delighted in checking your bottle by leaving all the lever movement until about 6 inches from the ground and then pulling to 16 degrees of pitch in about half a second. The aircraft just shuddered a bit and plopped onto the ground quite gently but it wasn't a technique you could afford to get wrong:) JFR where are you?

Denlopviper - make sure you pay attention in groundschool - it is the rate or air movement through the rotors that produces the autorotative force to keep the blades turning - you can autorotate and any speed from zero to Vmax and the only thing that will change is the RoD. Most helicopters have an optimum auto speed between 50 and 70 knots where RoD will be least - above and below that speed the RoD will be higher.

[Tailboom]

We teach 40knots autos inthe R44 which are an absolute delight !!! if you get it spot on you can touch down almost without feeling it, with a little bit of flare at about 20ish feet to reduce spead you can almost bring it to a hover

I've also used a 40 knot auto in a 206 on low skids worked a treat :)

[Peter-RB]

Whenever I went up in R22, R44 or 206 I nearly always during the flight did a practice Auto and power recovery, I only actually went onto the ground whilst in my training time or if I had a much more experienced second pilot with me( dont ask me why) but I found that wind strength/speed was the governing input to ground speed , forward speed meant very little once I had picked the spot for my final theoretical act, A pilot called Geoff Day showed me well how to make S turns whilst under auto's situation and this helped a lot in being able to pick out and stay with a good landing spot without allowing the wind to compromise the final act of landing without power

Peter R-B
Vfrpilotpb

[Shawn Coyle]

The zero airspeed autorotation is, as mentioned earlier, quite possible, and a very useful tool for getting rid of unwanted height if you're trying to get to a specific spot.

One of the myths about this is that it generates a very high rate of descent - I've seen mention of over 4,000fpm - this would be enough to make most people not want to get close to zero airspeed.

My experience in the Bell 206 series is that if a 60 KIAS auto produces 1500 - 1600 fpm, the zero airspeed auto will produce in the vicinity of 1800-2000fpm rate of descent.

And using Microsoft Flight Sim for 'training' will give completely the wrong impression - when you go to zero airspeed, the rotor stops - not the same as real life at all!

[come in copper 1]

It was Geoff Day that taught me the constant attitude auto in the R44, at 40 knots. The first time when Geoff was doing the demo I suffered a little bit from ground rush but once used to it, it was a doddle.
Every morning Geoff would arrive at Sandtoft with a constant attitude auto to the helo parking area.

[heliski22]

Would that be Geoff Day of Yorkshire Helicopters that was? Did my PPL(H) Skills Test back in 1990.

[Graviman]

My experience in the Bell 206 series is that if a 60 KIAS auto produces 1500 - 1600 fpm, the zero airspeed auto will produce in the vicinity of 1800-2000fpm rate of descent.


Is there enough inertia in the rotor system to raise collective for landing at zero airspeed? Is there sufficient benefit in this manouvre for small helicopters to consider a compact highspeed flywheel in the rotor drivetrain?

[lelebebbel]

no flare - why would you flare at zero airspeed. you just raise at the right moment like in a hover auto.
It works in a R44*, so I would believe it could work in a 206, too.


*i've had this demonstrated to me at the RHC factory. R44 from a 1000agl hover to 0 with no forward airspeed. according to the instructor, you only get away with it if the ship is light. Don't try this at home..

[212man]

No, Crab is correct. You may be referring to transient effects during cyclic input, perhaps?

[Arm out the window]

The words 'Factors A, B & C in autorotation' popped unbidden into my head on reading the last few posts - at least a few things must have stuck over the years.
Haven't thought to look back over that stuff for a good while, but looking back at the aerody books shows a few factors in play when increasing forward speed from zero in autorotation and comparing rates of descent.
As my aerody manual would have you believe, factors A and B relate to disc tilt and act to increase rate of descent as speed is increased, whereas factor C is about the disc flying into undisturbed air and acts to decrease rate of descent.
As published in flight manuals, and as we know from experience, there are best (lowest ROD) speeds for autorotation, so what Crab says sounds right to me.

[[email protected]]

I almost mentioned A,B and C in my first post but decided not to complicate the explanation for denlopviper.

A.agincourt - I am afraid it is you who are not quite right old boy - Factor A is the disc tilt which does reduce the inflow angle - so does factor B which is the increase in the horizontal airflow BUT!!!! Factor C saves the day as the air is effectively moving faster up into the disc compared to the air that was being slowed by the disc. Factor C is essentially an increase in RoD flow which increases the inflow angle again. Therefore no loss of autorotative force and no reduction in Nr.

Graviman - flare effect is gained by changing the angle of the induced flow into the disc, reducing its magnitude and thereby generating an increase in AoA and rotor thrust. At zero speed in auto, all the RoD flow is coming straight up at the rotor and any cyclic change will reduce the RoD flow and reduce rotor thrust, thereby increasing the RoD itself - not what you want to do for an EOL. Read fewer textbooks and go flying more:)

[ThomasTheTankEngine]

Hi Mr Agincourt

In part your right;

1) As you push the cyclic forward, you reduce the angle at which the air flows into the rotor disc, this will reduce RRPM.

2) However as forward airspeed increase's the air will flow into the rotor disc quicker, this will increase RRPM.

A cyclic flare will produce the opposite effects.

[Peter-RB]

Heliski,


Could have been the same Geoff Day, I meet him about 1998ish at EGNH and he did a few of my yearly checks, plus showed me quite a few very safe but easy moves for getting down quick QUICK.

Regards

Peter R-B
Vfrpilotpb:D

[[email protected]]

A.agincourt - the transient effect of moving the cyclic which can temporarily reduce Nr is because you are unloading the disc causing a reduction in 'G'.

Application of positive G increases Nr, application of negative (eg less than 1)G reduces Nr but only temporarily.

Yes it is easily demonstrated but it only lasts while you have the reduction in G. This has absolutely nothing to do with the migration of the driving section of the blade which happens when you raise or lower the lever.

According to your theory, if you start at 60 kts in auto and then increase to range speed (say 90 kts) in auto you will see a permanent decrease in NR which just ain't the case.

[[email protected]]

A.agincourt - I presume that last page of bo**ocks is an attempt to mask your misunderstanding of this question. You can try and wind me up as much as you like but you may confuse guys who actually want to understand P of F.

[bugdevheli]

To put things in perspective would somebody indicate what is possible in say an R22 in terms of a vertical auto to ground with no airspeed. Bug

[somepitch]

eight inches? :}

never flown one, but the thought of a vertical auto in an R22 with those skinny little blades from anywhere but a low hover makes me cringe...

[Shawn Coyle]

The rotor RPM in a steady autorotation won't change much from zero to Vne (autorotation), after all the transient effects die down, and the collective is fully down (or in the same position).
At least that's my experience from doing lots of autos in the Bell 206 series.

Above Vne (autorotation), the rotor RPM can decay pretty quickly, which is why it's the Vne(autorotation).

The rate of descent increases any time you go faster or slower than the Velocity for Minimum Rate of Descent (VsubY) - that should be obvious when you think what a graph of Rate of Descent vs. Airspeed is all about.
But the rotor RPM doesn't change.
The reason the Rate of Descent increases away from VsubY is that going has more fuselage drag (pretty obvious) When you go more slowly, the reasons become too complicated for me to remember or care about, so I give up and just show that the Rate of Descent increases....

[540DEGREE TorqueTurn]

i am confused , just for the record

in a r22 and most other small helis any zero airspeed and vertical decent with no engine to the ground above say 6 ft you are going to bend something

if your talking about going vertical / backwards / s-turns / 360 /180 or just plain straight in in the auto , remember you must have the indicated airspeed to be able to flare , and reduce the rate of vertical decent ,at the bottom , this is done at approximately anywhere from 100ft - 200ft . depends on where you want to end up
there is no vertical straight down with no indicated airspeed and safetly pull of a touch down auto , from height .

If someone wants to argue this please bring your video camera

[parabellum]

Had the dubious privilege of being the first Bristow pilot from the Duri/Kotapinang operation in Sumatra to experience an engine failure during flight in a B206, (PK-HBC), in December 1968.
Certainly found plenty of energy in the blades, had got very lucky as I had an open space to go for rather than the usual 150'-200' trees and needed to pull a bit to clear the last line of trees but RPM returned almost immediately when I dumped the lever.
Seem to remember the IAS was about 50kts in descent, flared off at the bottom, big tug on the collective when the grass became obviously grass and cushion on - only problem was that the grass was 5' elephant grass and there was three foot of water! A bit of damage but it flew again a month later when the spares arrived.
Certainly would never have considered a vertical descent in the hope of recovering it all at the bottom!:=:ooh:

[540DEGREE TorqueTurn]

something like this A.Agincourt,


http://www.youtube.com/watch?v=SKC8aAN1UoM

or this

http://www.youtube.com/watch?v=AQwJUz7LVBk

[540DEGREE TorqueTurn]

or this , an oldie but i still laugh

http://uk.youtube.com/watch?v=JRZllC-qRHo

[eagle 86]

Pedantic - but engine failure in the hover (or hover taxi) is not autorotation.
GAGS
E86

[[email protected]]

A.agincourt - the use of b**llocks might have been a bit harsh but your posts are somewhat random in their content and you are the one who took us in this direction in the first place by criticising me (when I was just trying to help someone else), not the other way round.

540 and the original post on this thread were talking about very low speed or even vertical autorotations to an EOL yet you have started on about zero speed touchdowns which is not the same thing at all. You have your ideas about P of F (your hint at Boscombe implies a TP course) which are at odds with the mainstream view - if you really are so knowledgeable then please pass on that wisdom so we might all learn, but please do include some references.

Your theory (I call it that because you were the one arguing the case) is that cyclic pitch change causes a permanent decay in Nr - Shawn (who is a TP) says not.

I know that the P of F we teach is not the whole truth, it is a convenient explanation for what we know happens in the real case. However, it serves its purpose well since the alternative is many pages of greek flute music and other hard sums that make your head hurt.

As for the rather childish reference to other threads then I suppose you must be a SAR expert as well:)

540 - you are correct, if you try a vertical autorotation all the way to the ground and therefore have only the energy left in the head to save you, you will bend the aircraft rather badly.

[eagle 86]

Agree Crab - having gone to the same "school" I know I was on a QHI course and not trying to graduate as a Bach. Air Eng.
When an auto rev check is conducted as part of a MTF a KIAS to be flown is specified (as well as noting AUW/PA/OAT) - this is because different KIAS produce different auto RPM with flat pitch.
GAGS
E86

[[email protected]]

Eagle - that is true but it is because differing speeds give differing RoD and therefore affect Nr (so in that respect AA is right but increasing speed will give higher RoD and Nr) but that would also be the case if the speed was reduced below normal auto speed, the RoD would increase and so would the Nr at flat pitch.

[g0lfer]

While flying an R44 with my instructor he had me do an auto in a strong headwind which resulted in us actually flying backwards over the ground. It is the airspeed which is important not the ground speed so in effect you can still come down vertically but with 40kt airspeed and hence still flare.

[Torquetalk]

In addition to the limited amount of energy available to arrest the ROD at the bottom if a no IAS auto is taken to the ground, is there not also the risk of vortex ring state? i.e As pitch is applied at the bottom an induced flow will oppose the inflow from below; No airspeed; high ROD; pitch applied: sounds like a recipe for VRS, or at least "falling through" at the bottom.

G0lfer: not sure whether a 40kt flare is going to do much for you; not enough energy to reduce the ROD or rate of closure. That is more a constant attitude speed.

TT

[Pandalet]

Assuming you're still in autorotation at the bottom, surely no vortex ring? I seem to remember one of the possible (not necessarily advisable) escapes from vortex ring is to go straight into autorotation, as rotors being driven is one of the required factors for VRS?

[Torquetalk]

Rotors being driven is not a requirement for VRS; pitch applied is. The rotors don't care whether the drag force is overcome by the energy from the engine (as in powered flight) or rate of descent flow (as in autorotation).

In VRS an induced flow from applied pitch is opposed by an upflow of air from a high rate of descent: the lack of airspeed localising this confused airflow to the disk.

With pitch down, you won't have VRS: but if you apply pitch with a high rate of descent and no airpseed, what then?

TT

[bugdevheli]

OK Lets run through this situation again. Machine R22. In hover at ten feet. No wind in any direction. Sudden engine failure. Will it get down intact. At four feet its a bit scary. Anybody done ten feet? Bug.

[Torquetalk]

There are some high time FEs who do high hover engine failures; inc R22. Know of one who has demonstrated 12ft. But I think they lower the lever initially to preserve the RRPM; raising it again nearer the ground. That takes a lot of experience and presence of mind...

The aircraft is not in autorotation in a hover engine faillure practice, merely accelerating towards the ground. It takes 2-300 feet for the aircraft to fully establish in autorotation (watch the VSI accelerate then stabilise at a given ROD in an auto done from say 1500ft)

Doubt very much if there would be enough energy in the disk to cope with an engine failure at 12ft if the lever were left in the same position. And the odds of Joe pilot lowering the lever faced with a sudden acceleration to the ground seem slim.

TT

[bugdevheli]

Was in a 22 a few weeks ago where my friend was practising engine chops at four feet, pulling collective at the last minute. Quite exciting, but lets you know how quickly the rotor rpm decays. Definate case here for some special high enertia system for light helicopters:):):)

[[email protected]]

I don't think VRS is an issue at the end of a vertical auto to EOL - as you pull pitch you are not inducing a flow down through the rotor, you are simply slowing the rotor down by increasing AoA, producing a burst of rotor thrust and lots of rotor drag - the Relative air flow is still form underneath the disc until you hit the ground. If you had power applied to the rotor then it would be a different matter.

In a steady state vertical auto, rotor thrust equal to aircraft weight is produced (otherwise you would keep accelerating towards the ground) as well as sufficient torque to drive the rotor. At the bottom, that surplus of energy driving the rotors is used instead to augment the rotor thrust (but it doesn't last for long) and reduce RoD (hopefully) for touchdown.

As for hover EOLs I have had the lever raised on me from a 5' hover in a Gazelle as I retarded the throttle and suddenly we were at 8-10 feet with the Nr decaying - I don't think I have ever said/shouted "I have control" so forcefully:)

[rotorfossil]

In R22's, for a hover EOL demo even from 8 ft, I lower the lever first before hoisting it. However, the chances of someone actually doing this in a real situation are pretty remote. I'm also very sceptical about successful EOL's from a true vertical auto in nil wind, having fallen straight through in spite of full collective when a student instructor got a bit slow on a constant attitude EOL. I suspect that the people who say it can be done may not be doing it on flat calm days and in helicopters that the ASI under reads in low speed autos. Just a thought!

[helonorth]

This thread is about as interesting as watching paint dry. About as
relevant, too.

[nigelh]

Just to stir the paint ...i would like to be pedantic and say you can enter autorotation without losing ANY height , let alone 2-300 ft , you can in fact GAIN height during the process. So na..na...nana...na...ya ...boo....sucks .
end of contest , i win.

[eagle 86]

AA - you have given me a headache!!
GAGS
E86
PS. NH you might like to explain yourself a little more clearly - yes you can cyclic climb whilst lowering the lever post engine failure but a descent airflow is required upwards through the rotor system to establish the aerodynamic forces to sustain the rotor RPM.
PPS. Or am I wasting my time Crab!

[[email protected]]

AA -do the terms coherent and lucid mean anything in your world?:) if not I'll have a pint of what you've been drinking!! Keep taking the medication....:rolleyes:

Eagle - I think Nigel is talking about a zoom climb entry to auto, the sort of thing we used to do from 120 kts in a Gazelle, you could gain 2-300 feet if you were gentle. I think the initial maintaining of the Nr is due to flare effect - in fact if you lowered the lever too much you would overspeed the head - once you select a speed stable attitude the RoD starts to build and then you are fully in autorotation.

[nigelh]

Yes crab , i am , but by the time you have levelled off i would say you are in autorotation .....feels like it anyway ....and i reckon you could land at a higher point than where you lost power. ie if you were zooming down a river canyon, low level, you could lose power and still climb and land safely. ( i accept you would have to be very on the ball to do it !!) so my point is that i believe during the cyclic flare the aircraft can establish itself in auto.
Certainly crop spraying you could have a donk stop at 10ft and 50 knots and climb, flare and land safely ...but that may be a different thing .....:rolleyes:

[parabellum]

Well I only ever had one real engine failure during flight and any idea that you might 'zoom' climb after the failure whilst the speed drifted back would have been a joke. It was a catastrophic failure of the compressor and happened so quickly that the yaw, when corrected, had the airspeed back at 70kts and by the time the vital actions and a Mayday had gone out the IAS was stabilising at around 50kts.

Seen many fine demonstrations of the 'zoom' climb but in reality you may not have that opportunity.

[Shawn Coyle]

There was discussion of doing hovering engine failures in an R-22 at heights of up to 12 feet, involving lowering the collective before pulling pitch to cushion the touchdown. This was followed, quite correctly, by discussions that indicated the posters didn't think that anyone would react quickly enough to an engine failure at that height to be able to lower the lever first before raising it.

Quite correct - and that's why the low hover point of the HV curve is demonstrated by only allowing the lever to be moved up. No down movement first is permitted in determining the low hover point.

From experience in demonstrating this (albeit in the 206) many times to budding test pilots, and having had one (self-induced) real engine failure I can assure you that this height is a very realistic height. Hover above this height at your peril - in all probability you won't react quickly enough to do anything except cushion the touchdown.

[Graviman]

Shawn, at the risk of going off-topic: Along the same lines as your experiences with non fadec torque limiters, is there an arguement here for an automatic collective system? Again it is the thought of the collective obeying the pilots intentions rather than inputs (a TP will likely understand what i mean by this).

What i am thinking about is a system which will have already positioned the collective correctly for entry to auto before the pilot even recognises the engine problem. It could be based on N1/2 and Nr, and could have two or three sensors and channels to avoid a single point of failure. There is nothing to stop this system also altering a raised collective input for overtorque to keep the Nr at most efficient %.

The system could be the usual SCAS arrangement of parallel and series actuators, to control shape pilots input. Most of the time it would not alter the input, but when the situation demanded it it would provide just the right adjustment...