Autos: To drop, or not to drop
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Autos: To drop, or not to drop
Whilst reading some of Ray Prouty's articles I downloaded off the internet I came across this passage regarding the entry into autorotation:
I have often heard people advocate a slight cyclic flare at the same time of lowering the collective, but never *instead of* (well, at least initially instead of)
Does anyone actually adopt this theory in practice, or is it generally considered too risky for the benefit it brings?
NB the preceding paragraph had discussed how controlling the rotor speed by collective resulted in a substantial loss of altitude as potential energy is sacrificed to put kinetic energy back into the rotor. This was the alternative
There is an alternate procedure, however, that can work if the power failure has occurred at a moderately high forward speed. In this situation, the pilot can take advantage of the kinetic energy associated with forward speed by doing a mild cyclic flare before lowering the collective pitch. This puts the rotor into a nose-up attitude that reduces the decelerating torque and maintains thrust and altitude until the forward speed is decreased to the best autorotational speed. At that point the collective pitch is reduced for entry into autorotation.
I know of one test pilot who developed this technique on a UH-1 to the point where he could delay dropping the collective for nine seconds after the power chop. (I also know of one test pilot who takes exception to this alternative procedure. Bob Ferry, who worked for Hughes, says, " lower the collective pitch the very first thing! I know of several dead pilots who didn'" Bob also makes the point that messing up the autorotative landing flare is usually not fatal, but messing up the autorotative entry almost always is.
I know of one test pilot who developed this technique on a UH-1 to the point where he could delay dropping the collective for nine seconds after the power chop. (I also know of one test pilot who takes exception to this alternative procedure. Bob Ferry, who worked for Hughes, says, " lower the collective pitch the very first thing! I know of several dead pilots who didn'" Bob also makes the point that messing up the autorotative landing flare is usually not fatal, but messing up the autorotative entry almost always is.
Does anyone actually adopt this theory in practice, or is it generally considered too risky for the benefit it brings?
NB the preceding paragraph had discussed how controlling the rotor speed by collective resulted in a substantial loss of altitude as potential energy is sacrificed to put kinetic energy back into the rotor. This was the alternative
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A friend of mine flies for one of the county police departments; often at high speed and low altitude. Every year, he's goes to the Bell school to practice something similar. In his case, the point is to use the forward speed to gain enough altitude with a short cyclic flare in order to setup for a proper auto (slightly different, but same principle).
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If memory serves, the low level engine failure drill in the gazelle was similar - i seem to remember attending to the collective well after pulling the cyclic back, but perhaps that was just me!
phil
phil
Should you suffer a power failure, maintaining a safe RRPM is paramount, not altitude.
To advocate a technique that favours maintaining or gaining altitude at the likely expense of RRPM in auto is madness if you ask me.
We all know there is many ways to skin a cat. But there is a big difference between an experienced test pilot in UH-1 with high intertia who is ready for the failure, compared to an inexperienced pilot who has packed his daks and then reacted to a sudden failure in an R22 with no inertia.
Anyone who has done a zillion autos knows you can play with all sorts of stuff, but don't forget that they carry a wealth of experience and intuition with respect to the performance and RRPM behaviour of the aircraft in auto.
Not all pilots carry that experience or intuition, or have done a zillion autos.
To lead a pilot in a single to believe it is OK to do anything other than lower the collective initially when the engine fails and the adrenaline is pumping is in my opinion a very slippery slope indeed.
To advocate a technique that favours maintaining or gaining altitude at the likely expense of RRPM in auto is madness if you ask me.
We all know there is many ways to skin a cat. But there is a big difference between an experienced test pilot in UH-1 with high intertia who is ready for the failure, compared to an inexperienced pilot who has packed his daks and then reacted to a sudden failure in an R22 with no inertia.
Anyone who has done a zillion autos knows you can play with all sorts of stuff, but don't forget that they carry a wealth of experience and intuition with respect to the performance and RRPM behaviour of the aircraft in auto.
Not all pilots carry that experience or intuition, or have done a zillion autos.
To lead a pilot in a single to believe it is OK to do anything other than lower the collective initially when the engine fails and the adrenaline is pumping is in my opinion a very slippery slope indeed.
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It probably wasn't clear from my initial post because I only pasted a small passage of the article. Reading it in light of the whole article, I don't think Mr Prouty was suggesting you let RRPM decay at the entry. His suggestion is to use the kinetic energy from forward speed to obtain the same result as had you used potential energy to keep the blades spinning. So theoretically this should still keep RRPM up after engine failure without actually lowering the collective. Still, sounds a little 
to my extensive inexperience, even if I can see the theory side of it 
Off subject, whats the general consensus on constant attitude autos? Show boating, or a genuine (better?) way to do them?
to my extensive inexperience, even if I can see the theory side of it Off subject, whats the general consensus on constant attitude autos? Show boating, or a genuine (better?) way to do them?
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I was always taught to flare AT THE SAME TIME as lowering the collective. In the R22, this should give you another second or so before the RRPM decays too catastrophically...probably essential in the R22!! 
Any Robbo pilots out there do anything different?
Any Robbo pilots out there do anything different?
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On the R22, I've had 8 different instructors. Some of them strongly advocate the simulataneous aft cyclic, whilst others consider it wholly unnecessary (presumably because you know what is coming during a practice auto).
Personally, I prefer to get into the habit of slight aft cyclic. If the donk does stop one day, chances are you won't be quite so quick to lower the collective as you were when the instructor was sat beside you counting you in In which case, the RRPM probably already has fallen more than you would like and the slight flare will be instinctive
However, when you transition to other machines with higher inertia rotor systems, you will get people looking at you going what the f...?!
Personally, I prefer to get into the habit of slight aft cyclic. If the donk does stop one day, chances are you won't be quite so quick to lower the collective as you were when the instructor was sat beside you counting you in
In which case, the RRPM probably already has fallen more than you would like and the slight flare will be instinctiveHowever, when you transition to other machines with higher inertia rotor systems, you will get people looking at you going what the f...?!
Last edited by charlie s charlie; 22nd May 2004 at 10:57.
In the Huey and the Jet Banger, we practised 90 kt / 100 feet auto entries. The first action is flare to get an upward vector and keep revs with the flare effect, then the lever went smoothly down. Approaching 60 kts, adjust the attitude and allow descent at normal auto speed. (If you don't, the flare continues at 120 feet or so, you run out of flare effect and fall to the ground with 20 kts and decaying revs - could be messy. Anybody who did this was power terminated.) The flare height was lower because you didn't have that 1800'/min rate of descent, and it worked beautifully.
If you lower the lever FIRST, you are already descending at normal flare height. This results in an early flare to stop ROD, and it could not be fully developed due to ground proximity, so a high run-on speed resulted. Again, could be messy.
Constant attitude requires a certain amount of judgment, but if there is only one spot available and anywhere else is certain death, then put that spot in your chin bubble and come down at 30 kt. Go through the normal flare height.
When you think "Holy 5H1T!" and your clacker takes a bite of the seat cushion, take a bubby flare and a sharp pull on the collective to about 1/3 travel. The ROD stops, and the machine starts to move forward. Cushion on and dump the lever to stop before you hit the trees / fall off the cliff / zip into the bushfire. It might nose over, but you are on the correct side of the danger zone.
If you lower the lever FIRST, you are already descending at normal flare height. This results in an early flare to stop ROD, and it could not be fully developed due to ground proximity, so a high run-on speed resulted. Again, could be messy.
Constant attitude requires a certain amount of judgment, but if there is only one spot available and anywhere else is certain death, then put that spot in your chin bubble and come down at 30 kt. Go through the normal flare height.
When you think "Holy 5H1T!" and your clacker takes a bite of the seat cushion, take a bubby flare and a sharp pull on the collective to about 1/3 travel. The ROD stops, and the machine starts to move forward. Cushion on and dump the lever to stop before you hit the trees / fall off the cliff / zip into the bushfire. It might nose over, but you are on the correct side of the danger zone.
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In the R22, which is the only helicopter I really know about, there are two reasons for the simultaneous aft cyclic. One is to stop the nose dropping when you lower the lever...can't offhand remember the exact reason why the nose tends to drop, but it does. This just requires slight aft cyclic pressure, to hold it up rather than actually flaring. Slightly more than that to flare will raise the RRPM, and I would say it's absolutely necessary. The reason? On my instructor's course I was given a throttle chop; I was told it would happen, but not exactly when, and told to just lower the lever, not flare. The reason was to show how low the RRPM will drop in the R22 before you can get that lever down. Well, I would have sworn I dumped the lever instantly, but the RRPM was still down to around 90%. Knowing that...I teach lower the lever and flare simultanteously, and that's what I do. Hopefully it'll be instinctive, because in the R22 if the donk stops, it would have to be instinctive to do it fast enough.
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I shall probably be shot down in flames by those that know a lot more about aerodynamics than me, but I'm pretty certain the nose down is simply "flap forward" ie reduced pitch from lowering the collective - - precessed through 90 degrees - - nose down
You mention throttle chops which is another area of debate. Personally, I think they were a good thing. Where I did my primary training they were routine right from when you first started doing autos (ie pre PPL). I also know there have been accidents / fatalities caused by them. As always, a double edged sword.
Is the added realism worth the risk? To quote from that first passage again "Bob also makes the point that messing up the autorotative landing flare is usually not fatal, but messing up the autorotative entry almost always is"
Surely throttle chops have more value than full-down autos? Admitedly, they require a lot of locking of limbs on the behalf of the instructor to make sure it doesn't all go pearshaped.
You mention throttle chops which is another area of debate. Personally, I think they were a good thing. Where I did my primary training they were routine right from when you first started doing autos (ie pre PPL). I also know there have been accidents / fatalities caused by them. As always, a double edged sword.
Is the added realism worth the risk? To quote from that first passage again "Bob also makes the point that messing up the autorotative landing flare is usually not fatal, but messing up the autorotative entry almost always is"
Surely throttle chops have more value than full-down autos? Admitedly, they require a lot of locking of limbs on the behalf of the instructor to make sure it doesn't all go pearshaped.
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I thought the nose drop on entry to autorotation was because there is no rotor thrust acting on the horizontal tail surface(s) - certainly noticeable in the H300 (he says authoritatively, having made the grand total of 2 entries). Robert Mason in Chickenhawk describes how the first model would 'tuck' if the stick was moved forward on entry, requiring 1500' to recover.
As you all know, Bringing the cyclic back (Flaring) INCREASES Rotor RPM, thus you control the RRPM with the collective while bleeding of the airspeed/climbing (or whatever). In our smaller helicopters, the RRPM rises fairly rapidly (low inertia) while in other machines, it does not increase as rapidly, but then again it does not decrease as rapidly either.
Dont know if anybody has any more input?
Dont know if anybody has any more input?
All Prouty is saying here is that if you are banging along at 145kts it is not necessary to dump collective pitch for entry into the auto.
The flare to hold up the nose and maintain altitude will drive the autorotative region, reverse the flow and maintain the RRPM in the "green". When you reach the bottom of the "green" then the collective needs to be on the floor. If this coincides with airspeed at best autorotative spec, then good job.
Like Coyote said nobody is advocating decayed RRPM. That's just silly....
The other issue is a Huey has a lot of blade interia and a robbie has none. Robbies are pretty slow as well so there is not too much airspeed (kinetic energy) that can be traded for potential energy. H500 are pretty fast and can trade airspeed for RRPM.
The flare to hold up the nose and maintain altitude will drive the autorotative region, reverse the flow and maintain the RRPM in the "green". When you reach the bottom of the "green" then the collective needs to be on the floor. If this coincides with airspeed at best autorotative spec, then good job.
Like Coyote said nobody is advocating decayed RRPM. That's just silly....
The other issue is a Huey has a lot of blade interia and a robbie has none. Robbies are pretty slow as well so there is not too much airspeed (kinetic energy) that can be traded for potential energy. H500 are pretty fast and can trade airspeed for RRPM.
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What you can and can't do
What you should and shouldn't do
is all Type Specific...!!
Whatever you fly ask and learn your own helicopters capabilities!!
topilot or to..pprune
What you should and shouldn't do
is all Type Specific...!!
Whatever you fly ask and learn your own helicopters capabilities!!
topilot or to..pprune
At the end of the day, the key is RRPM awareness. If you need to flare upon entry to restore RRPM, do it. If you have plenty of height then there's no need to flare initially, the RRPM will restore from autorotation. I have seen 78% in the R22 upon entry (not that I am at all happy about or proud of it), but had the height available and it restores quickly once autorotation is established with a ROD.
Food for thought, given the original scenario: You are banging along at 50' and 75Kts in an R22. Engine stops so you flare first as per the original suggestion and then lower the collective. The aircraft will climb a bit if you do this. But what will happen to your RRPM when you apply forward cyclic to stop flaring and set the attitude for say 55Kts to complete the auto?
Firstly, forward cyclic is a very effective and rapid way to lose a bit of RRPM in auto. (I have used it to quickly correct potential RRPM overspeeds when a flare was too aggresive, and find it produces an immediate response. When you load the disc the RRPM rises, when you unload it the opposite occurs.)
Secondly, the aircraft is climbing and so has inertia taking it away from the ground. Once you set a level aircraft attitude, you will not have an autorotative force acting on the rotor system until the aircraft arcs, ie stops climbing and then begins to descend. (Admittedly a short period of time, but nonetheless a critical period of time given the proximity to the ground). With a level disc attitude you need a ROD airflow to produce the driving force on the rotor, yes?
If this all commenced at 50' then I suspect you will not have a whole lot of time or height after levelling from the flare to restore an autorotative force to drive the rotor system.
In this situation I believe what you gained on the swings you well and truly lost on the merry go round.
Food for thought, given the original scenario: You are banging along at 50' and 75Kts in an R22. Engine stops so you flare first as per the original suggestion and then lower the collective. The aircraft will climb a bit if you do this. But what will happen to your RRPM when you apply forward cyclic to stop flaring and set the attitude for say 55Kts to complete the auto?
Firstly, forward cyclic is a very effective and rapid way to lose a bit of RRPM in auto. (I have used it to quickly correct potential RRPM overspeeds when a flare was too aggresive, and find it produces an immediate response. When you load the disc the RRPM rises, when you unload it the opposite occurs.)
Secondly, the aircraft is climbing and so has inertia taking it away from the ground. Once you set a level aircraft attitude, you will not have an autorotative force acting on the rotor system until the aircraft arcs, ie stops climbing and then begins to descend. (Admittedly a short period of time, but nonetheless a critical period of time given the proximity to the ground). With a level disc attitude you need a ROD airflow to produce the driving force on the rotor, yes?
If this all commenced at 50' then I suspect you will not have a whole lot of time or height after levelling from the flare to restore an autorotative force to drive the rotor system.
In this situation I believe what you gained on the swings you well and truly lost on the merry go round.
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If you are considering this from a *smaller* helicopter point of view:
Suppose you have 25-30kts to play with above best autorotative speed, and you were thinking about trading kinetic energy rather than altitude. To have this amount of margin on a small helicopter generally is going to require a particularly high pitch angle; you won't be far off VNE. The question is, with such high pitch angles could the flare effects be sufficient to keep the RRPM in the green if you did nothing to the collective? I envisage the required flare being so aggressive that the speed will have been washed off back to best auto speed in a couple of seconds, thus meaning very little difference from lowering the collective in the first place!
Anyone (deliberately ) not lowered the collective immediately in a small helicopter like the R22 so they could trade kinetic not potential energy?
Suppose you have 25-30kts to play with above best autorotative speed, and you were thinking about trading kinetic energy rather than altitude. To have this amount of margin on a small helicopter generally is going to require a particularly high pitch angle; you won't be far off VNE. The question is, with such high pitch angles could the flare effects be sufficient to keep the RRPM in the green if you did nothing to the collective? I envisage the required flare being so aggressive that the speed will have been washed off back to best auto speed in a couple of seconds, thus meaning very little difference from lowering the collective in the first place!
Anyone (deliberately
) not lowered the collective immediately in a small helicopter like the R22 so they could trade kinetic not potential energy?
Nobody advocates "doing nothing with the collective."
Simply this:
First: sharpish cyclic input to raise the nose and get an upward vector going.
Second: smoothly lower the collective.
Third: (and i should have put this bit in before, but thought it was pretty obvious) don't let the attitude stay above that for auto speed - be it 60, 65, 75 or whatever. As the airspeed bleeds back and the trajectory arc turns back into a descent, relax the attitude to hold auto speed, and come on down.
RRPM always seemed to stay in the green, but usually by that time, the eyeballs were firmly outside on the height/landing area.
Remember why the lower shaded area of the Danger/Avoid Area is there.
Too fast, too low, too late, too bad, two bodies.
Funnily enough, an S76B doesn't have a lower area, but an A model does. Not sure why, perhaps because it has a higher permissible run-on speed and higher max speed for using brakes? Nick?
Simply this:
First: sharpish cyclic input to raise the nose and get an upward vector going.
Second: smoothly lower the collective.
Third: (and i should have put this bit in before, but thought it was pretty obvious) don't let the attitude stay above that for auto speed - be it 60, 65, 75 or whatever. As the airspeed bleeds back and the trajectory arc turns back into a descent, relax the attitude to hold auto speed, and come on down.
RRPM always seemed to stay in the green, but usually by that time, the eyeballs were firmly outside on the height/landing area.
Remember why the lower shaded area of the Danger/Avoid Area is there.
Too fast, too low, too late, too bad, two bodies.
Funnily enough, an S76B doesn't have a lower area, but an A model does. Not sure why, perhaps because it has a higher permissible run-on speed and higher max speed for using brakes? Nick?
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The question is, with such high pitch angles could the flare effects be sufficient to keep the RRPM in the green if you did nothing to the collective?
Anyway, maximum glide configuration is probably one of the more challenging maneuvers and it's something that really needs to be practiced. It actually takes LARGE AMOUNTS OF COLLECTIVE to bring the R22's RRPM down to 90% (and keep it there, that is at 75 KIAS). Hence, you get the high pitch angles mentioned above.
When we got to the bottom, he began a standard flare. As soon he pulled back on the cyclic, the RRPM shot up and we immediately began to climb (abruptly)! He corrected by dumping the collective at that time, but I was AMAZED at how different a light weight R22 will behave with only a 10 knot increase (not to mention the low RPM horn blaring in your ear for what seems to be an eternity).
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During my entire career in helicopter aviation I was always lucky to get to fly with some very experienced flight instructors. When I went for my CFI I received some special training on full touchdown autos. My flight instructor sat down with me and asked me to explain how I would do an auto and why I would do it that way. When I was finished he explained his point of view. And yes, he was right. No matter what helicopter you are flying in, aft cyclic first is absolutely essential. At this point we were not even talking about the aerodynamical reasons but here is the scenario according to Murphy's law:
When will the engine failure take place? When you are not prepared for it.
So, assuming you are out on a long x-country, you don't know how to sit any longer and your butt is aching - cause despite the fact we are flying Volkswagens for the price of a Rolls-Royce the manufacturers still haven't figured out how to produce seats that justify that kind of price - you finally come up with the idea to stretch your legs and place them on top of the pedals. Than you need to change the frequency on your radio. So you reach down with your left hand and.... the only thing which is still under control is the cyclic.
The engine quits and if you don't do anything about it it's not only the RPM which will drop but also the nose of the helicopter resulting in a further drop of RPM. If in this situation ONLY the collective was lowered the nose would drop even more resulting in a less effective increase of RPM caused by the intended lowering of the collective.
Consequently, the only way to recover from this situation is to apply aft cyclic, sufficient to keep the nose from dropping and sufficient to keep your RPM from decaying. This will give you the extra amount of time to get your hand back on the collective in order to lower it.
From the aerodynamical point of view you have to keep in mind that everytime you lower that collective the nose of the helicopter wants to tilt forward initially resulting in a decrease of RPM if no further action (e.g. aft cyclic) was taken.
When will the engine failure take place? When you are not prepared for it.
So, assuming you are out on a long x-country, you don't know how to sit any longer and your butt is aching - cause despite the fact we are flying Volkswagens for the price of a Rolls-Royce the manufacturers still haven't figured out how to produce seats that justify that kind of price - you finally come up with the idea to stretch your legs and place them on top of the pedals. Than you need to change the frequency on your radio. So you reach down with your left hand and.... the only thing which is still under control is the cyclic.
The engine quits and if you don't do anything about it it's not only the RPM which will drop but also the nose of the helicopter resulting in a further drop of RPM. If in this situation ONLY the collective was lowered the nose would drop even more resulting in a less effective increase of RPM caused by the intended lowering of the collective.
Consequently, the only way to recover from this situation is to apply aft cyclic, sufficient to keep the nose from dropping and sufficient to keep your RPM from decaying. This will give you the extra amount of time to get your hand back on the collective in order to lower it.
From the aerodynamical point of view you have to keep in mind that everytime you lower that collective the nose of the helicopter wants to tilt forward initially resulting in a decrease of RPM if no further action (e.g. aft cyclic) was taken.
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I think some people didn't really understand this guy's original post. As I read it, he wasn't wanting a discussion on the generic autorotation technique but on a very specific different method: cyclic flare to keep rotor RPM up for as long as you can - the quoted paragraphs says one pilot managed 9 secs - and then, and only then, lower collective.
The practice of aft cyclic around the same time of lowering collective is commonplace, and its merits were not the intended discussion point IMO
The practice of aft cyclic around the same time of lowering collective is commonplace, and its merits were not the intended discussion point IMO