Heres a spin on it arrording to Frank you have 1.1s to lower collective but in fact it takes 0.6s before your brain reacts to horn sounding so with a quick bit of maths you only have 0.5s to get the lever DOWN

Heres a bit of reassurance to all Robbie drivers. I drive a dual fuel car. When it runs out of propane - no gauge- it takes a lot less than 1.1 seconds to identify the problem, locate the switch which is out of sight , and flick it over to petrol. The difference is there is no time pressure but the action is now automatic. I don't have to think about it cos I have done it hundreds of times and it is not exactly unexpected when it happens. The implication is practice is good.

I have had an engine stop in flight in an Enstrom. It didn't take very long to bottom the lever. Flying an Enstrom makes one practice for engine failures. I am confident that if I had been in a R22 the outcome would have been no different.

The autorotational characteristics of the R22 don't concern me too much. Blades falling off worries me more and Frank is fixing that one. I do wish he would put fuel injection on it as well. Ironically, it is probably the cost of certification and beaurocracy that prevent Frank from doing this. What a crazy world.

I too would not be flying without the R22 and am grateful for it.

Turn!
 

zxcvbn,
Regarding keeping NR up when low GW; consider what happens when you flare or turn. As load factor increases NR climbs. If you've done any 180 autos you'll recall that you had to pull in some collective to keep NR from exceeding limits during the turn.

Thus, if you ever find yourself in a situation where you want to bring up or maintain an NR higher than you can get with a straight ahead decent with the collective bottomed, consider s-turns or spiraling down to your target.

Careful with the turns if Nr is very low.

If my memory of PPL aerodynamics is OK, the scenario is something like this.

2 very undesirable things could happen which would make things worse.

1) Increasing the load factor is great on a flying blade, however, the stall speed of the blade increases with increased load factor as angle of attack is increased, so an overenthusiastic turn could turn a barely autorotating blade into a catastrophically stalled one. Thrust reversal due to flaring also increases angle of attack and stall speed so is risky at very low Nr.

2)The coning angle increases with load. Coning angle will already be high with low Nr. Increasing coning angle is good for Nr up to a point due to conservation of angular momentum, however, if it goes too far... things break.


Perhaps the thing to do with very low Nr is to very slowly push the stick forward to increase airspeed and decrease angle of attack, hence lowering the stall speed of the disc. This also decreases drag on the inboard, stalled area of the blade which becomes smaller. The other main problem with low Nr is high sink rates which increase angle of attack unless the disc is tilted forwards to turn sink rate to airspeed. possibly another reason to push the stick forward.

I think at very low Nr the usual rules go out of the window. This is complex.


Come on you aerodynamics people. Discuss.

Gaseous,
In regards to your last post here:
1)Pushing cyclic forward on an R22 would indeed get you more airspeed but would lower rotor rpm. A gentle aft cyclic would get the aircraft the proper attitude that is most conducive to maintaining rotor RPM. 2)And turns increase rotor rpm.
3)Also, coning angles are LESS of a factor when blades are at lower rotor rpm as far as increasing the rotor rpm is concerned.
Question: How does flaring a helicopter LOWER rotor rpm???
Think about my first point before you answer the question.
Regards,
snoopy

Yes they have, and all R22s except the very early ones do have tip weights fitted. Still a low inertia system though.

13snoopy


Question: How does flaring a helicopter LOWER rotor rpm???
Answer : If it precipitates blade stall because Nr is near stall speed.

As angle of attack increases so does aerofoil stall speed.

Aft cyclic increases the angle of attack.
Turns increase load factor which also increases angle of attack.
Increased sink rate increases angle of attack.
Decreasing Nr increases angle of attack.

If the increased angle of attack raises the stall speed to above the speed of the blade, the rotor will stall followed by a catastrophic blowback. this will result in mast bumping, blade incursion into airframe and death. (sound familiar to readers of R22 NTSB/AIBB reports??)

Attitude is irrelevant to maintaining Nr.
Angle of attack is what is important.

Take the following fixed wing analogy. If flying along at 5Kts above stall speed in a Cessna. Pull back on the stick hard and you will instantly stall. Push the stick forward to decrease angle of attack and you are flying again. No problem. Try this in an R22 with Nr at 5% above stall speed and you dont get the option of pushing the stick forward because the stall is non recoverable.

As long as the blades do not stall all the expected things happen in response to turns and aft cyclic. We all know what is supposed to happen - and it does when we practice it at 90%and above Nr.

Coning angles increase as Nr decrease or load factor increases. As coning angles increase, disc radius decreases, which tends to increase Nr. Great. The problem is there are mechanical limits on coning angle.

The point is we don't practice low RRPM recovery at 70-75% Nr in an R22. You only do it for real and if you get it wrong you die.

Where this is relevant to this thread is that if an R22 pilot has an engine failure and1.3 seconds later finds Nr at 75-80%, thinks Oh SH**T, and does a big flare as trained, it will be the last thing he does. The question is would keeping the stick still or slight forward cyclic have kept the rotor turning at such low Nr??


RRPM is life. Stall it and its all over.

Regards

Some very interesting points there Gaseous.

All I can add is that the factory doesn't teach a big flare, or even a flare at all for a low RPM recovery. The reason they say to put in aft cyclic is to hold the ship level when lowering the collective, as the nose tends to drop. They don't recommending flaring the ship at all.

It's pretty easy to visualize a blade that is close to stall, when asked to flare the ship by increasing the pitch on that blade at the 3 o'clock postion would subsequently cause it to exceed the critical angle of attack and then for the rotorsystem to blowback.

Would like to see some mathematical aerodynamic simulations of the results of cyclic inputs in near stall situations!

James

FDA

I too would like to see some maths on this but I do not know enough about it to do that. What seems obvious to me is that if everything else pushes angle of attack towards stall, the only thing the pilot can do is apply forward cyclic to counter it. If the AOA is optimal for the blade speed the rotor will autrotate wherever the cyclic is in respect to the fuselage.

I know this is somewhat controversial but what is important in low Nr autorotation is angle of attack contol.

Hands up all those who have flow an R22 without tip weights?
Hovering autorotation? 1,2,3 chop, PULL!!! (only you do it faster than you can read it!) We had it tough in my day...................;)

Good evening Chaps,

James, when I was taught to enter Autos, the instructor always indicated a gentle flair for two reasons, bring back the RRpm if you had lost any and also to keep the cabin level, Gentle flare that was, in my book it is hardly able to called a flair or Flare.


But I don't understand what you mean by increasing pitch whilst in the Auto, unless to go for distance, increasing pitch would ultimately cause you serious problems for it would errode your RRPM and at that stage RRPM is your parachute.

Peter R-B

Vfr

Auto an R22
 

Only a new PPL so go easy but..

I thought the most critical element to a succesfull autorotation entry is to establish an autorotational air flow up through the disc as quikley as possible. This can be achieved by flaring along with the lowering of collective. Airspeed is the kinetic energy instantly avaliable to maintain or build Nr unlike the airflow from ROD which can take second/s to develop. Which is why we climb at 60nts and not 53nts.

So if Nr is very low and you haven't fallen out of the sky, flaring will aid in restoring RRPM due to reduction in blade drag and an increase in rotor driving force.

But then again is it a realistic situation to be autorotating borrdering rotor stall with an established ROD and not have sufficient airflow up through the disc to build Nr. It seems unlikely to be in auto at a stable low RRPM without it decaying untill stall or RRPM building to normal auto levals. You have either reacted quikly enough or not!!!


More of a question than a statement.

Regards Eyesout

zxcvbvn

I think that this wouldn't be a problem in flight (assuming that you put the same weight on both blades:D ) but the prblem comes when the rotors stop!!

The blades would have to be much stronger to maintain structural integrity when static, this increase in blade weight would need a new head redesign, and would then require a bigger engine with more HP to recover Rrpm when pitch applied, this would then require a stronger engine frame to support the extra engine output, etc etc.

why not build a new helicopter and call it somthing like R44.



:)

Gaseous,
I am not suggesting aggressively pulling back. My advice was aimed more at what to do if you've got the collective fully down and your NR is decaying still--not dangerously low 'yet', but getting there.

However, I would still advise against pushing forward cyclic when in an auto if you're worried about low NR. Realize that the outer portion of the blades are wind milling, and providing the thrust (ie, their local total force vector has a forward (positive torque - driving NR higher) component in the disk axis). The magnitude of this force is a function of airflow through the disk, and thus reduction of flow through the disk as will occur if the disk is less perpendicular to the airflow will reduce this tip thrust and NR will decrease. Without this force the form drag on the blades will cause an NR reduction.

I understand your point about blade stall, but I don't think it should drive you to push forward for airspeed. The impact of reducing the airflow will have an immediate effect on the blade thrust and NR, but will have a slower response on airspeed. If you can get there, put it in a turn. Keep the nose down and airspeed up, but load up the disk with slight aft cyclic. You'll see a higher number on your VSI, but you'll have kinetic energy in your airspeed and in the rotor.

This is all based on my engineering background and flight experience. I don't mean to present this as fact. However, if I found myself in an auto with the NR going down through 80% I think I would still pull back gently on the cyclic. I'd like to look deeper into the numbers and perhaps model/simulate it computationally. My gut feeling is that in forward flight greater airflow up through the disk on the retreating side caused by pulling back on the stick would be better than the reverse flow already there and exacerbated by pushing forward cyclic. This is where I'm guessing stall would first show up, as opposed to the advancing side.

Don't try this at home :cool:

As far as RPM in the normal range is concerned >90, aft cyclic that raises the nose, when there's forward airspeed, will increase RPM, we all know that. Fun to play with (with due caution, of course), roll off the throttle and flare the helicopter, see how long you can maintain the RPM.

As far as increasing pitch goes, I was referring to the increase in pitch on one blade, the advancing blade, when aft cyclic is applied.

I've spent a couple hours sitting here trying to figure out a few things.

A. What is the angle of attack of the blades at full down collective, 60 knots forward speed and 80% RPM versus 104%

B. Could an aft cyclic stall the advancing blade? Lets say at full down, 60 knots, level attitude and 80% RRPM the AOA is 14 degrees. Add in two degrees to the advancing blade from the cyclic input and it would exceed a theoretical critical angle of attack of 16 degrees.

C. What factor does inflow/ROD play? You can't get 14 degrees of AOA on blades who's pitch is low without it.

D. Without a complete understanding of all the factors involved, such as what the air is actually doing over the span of a rotorblade when it stalls, it's really hard to figure this out!

My hats off to those aerodynamicists out there!

James

Krylian.

Quote: However, if I found myself in an auto with the NR going down through 80% I think I would still pull back gently on the cyclic.


If you've got the lever down and your Nr is still decaying you should come back and haunt your engineer! (or mechanic if you live in USofA). Helicopters should be trimmed to maintain autorotation with the lever down and should do to the point where the rotor stalls.

The aerodynamics of rotor stall are roughly this. There are 3 regions of the autorotating blade starting at the mast. First there is a stalled area which contributes drag to the rotor system. Moving out there is an area of autorotation where the lift vector drives the rotor. Moving out further there is an area of drag. As the angle of attack increases due to falling Nr the boundaries move outwards. At normal Nr the stalled area at the root is small and insignificant but as Nr drops this area gets bigger so that eventually drag is greater than autorotative force so the rotor stalls. In forward flight the retreating blade stalls more than the advancing blade so roll to the left is likely in a R22 as catastrophy looms.


lets assume an arbitrary rotor stall speed of 75% in stable autorotation. at this point the angle of attack is critical. Any increase will cause the rotor to stall. Pulling back on the stick makes no sense at all. You will die. At 80% there is a little latitude for aft cyclic. At 90% quite a lot and at 100% you can haul back on it with near impunity.

***The nearer Nr is to rotor stall, the less safe aft cyclic or increased load factor get. Simple as that.***

Aft cyclic causes an increase in angle of attack as the aircraft pitches aft. Check it in your PPL book. (mine is Norman Bailey's Helicopter pilots manual, P100).

I totally agree that if you don't stall the rotor all the right things will happen. Just be careful.

No one really knows what the dead pilots of all the crashed R22s did last. I suspect some of them did a big flare with low Nr because they thought it would increase Nr. Imagine their surprise when the rotor stalled.


Think about it.

FDA
You posted while I was answering Krylian.

You are asking all the right questions.

Question: What is the angle of attack of the blades at full down collective, 60 knots forward speed and 80% RPM versus 104%

Answer: A lot greater. cant give you numbers but airspeed is a lot lower. As airspeed decreases angle of attack increases.

Question: Could an aft cyclic stall the advancing blade? Lets say at full down, 60 knots, level attitude and 80% RRPM the AOA is 14 degrees. Add in two degrees to the advancing blade from the cyclic input and it would exceed a theoretical critical angle of attack of 16 degrees.

Answer Yes. And as the aircraft pitches aft add in the angle the mast tilts aft too.

Question:What factor does inflow/ROD play? You can't get 14 degrees of AOA on blades who's pitch is low without it.

Answer: Tricky one but basically increased ROD causes increased AOA. Inflow during a flare in auto comes from further below the Plane of rotation so increases AOA.

No numbers because I'm not good at that but the basic principles are good for understanding. Hope this helps.

The more I think about this the clearer it becomes. You say Robinson recomend no flare. My explanation explains why.

I would tentatively suggest, based on my crackpot theories that after an engine failure, if you find Nr very low, i.e. less than 80% you should:

1) Get the lever down. RRPM should stabilise. Do not flare. do not make any big cyclic movements.
2) Watch the tacho - it should stabilise and start to rise.
3) when it does, very carefully apply aft cyclic. watching RRPM.
If RRPM rises maintain aft cyclic and recover as normal.

Heres the really controversial bit.

If any left roll or fall in RRPM is detected apply slight forward cyclic until RRPM stabilises.

If after lowering the lever, RRPM continues to fall, the rotor has started to stall. Left roll is likely. flare will push the disc further into the stall. Judicious forward cyclic to reduce AOA is your only chance.

It is better to arrive at the the ground with 80% RRPM than 0%

Shoot me down on aerodynamic theory. Not because this is not what you were taught.

Gaseous,

Interesting point, and from a strictly aerodynamic point of view I think I agree with you.

I see some danger in what you advocate (gentle forward cyclic as the only course of action because aft cyclic may/will produce rotor stall) largely from a human factors and a 'big picture' point of view:

1 - What is important to realise is that you are talking about a specific and dire situation (Nr well below limits and close to stalling) and a last ditched attempt to survive or to not drop the bundle. A theoretically possible technique to prevent rotor stall in a situation that the pilot should never have got into in the first place.

2 - If the pilot does happen to be in this situation, their heart is going to be in their mouth and it is unlikely that any input made will be gentle, but rather an adrenalin driven excessive stab.

3 - You may be planting a seed of contradiction and confusion that could kill someone who is NOT in such a dire RRPM situation, but has suffered a power failure. You are talking about a situation that requires identification, awareness and conscious thought to override instincts. In a time critical emergency good training produces instinctive actions, without such conscious thought. Your suggestion may just open the door to enough cognitive confusion in the heat of the moment to lead to 'brain fade', similar to someone inexplicably pushing the wrong pedal upon entry into auto.

It is impossible to train for every scenrio. But I think there is a very good reason for that as well: to not jamn the noodle with too much information, so that it can produce the right results when it is critically needed.

Prevention is better than cure. This scenario to me demands prevention, simply don't let the Nr get that low when you've got a long way to fall. And if you do, its a product of pilot error and your fate is yours.

While you are trying to save a sub-standard pilots skin (in my opinion anyway) in a very rare AND preventable situation, you may well be endangering another (not sub-standard) pilot who's Nr is low but not THAT low. And for that reason your idea fails in my opinion.

And one last thing re your advice on entry to autos: "do not make any large cyclic inputs". If you do that the nose is going to drop a mile on entry, allowing the Nr to decay far more than it would if you simply kept the original attitude.

The holy grail is Nr, if a pilot can't protect it they simply shouldn't be flying. My advice: stick to your training!

Coyote.

Absolutely valid points. All of them. Vital - It does apply at very low Nr. Pilots do find themselves in this position. Some who have are dead. Who knows if they had done a differnt thing they would have survived? Knowledge of aerodynamics is power over the circumstances you find yourself in. The 1.1 second thing means Low Nr is more likely in an R22.

Substandard pilots? I think we are all that from time to time.

I really would like some input to this from people who know a lot more than me. I also suggest no one does this because of what some moron suggests on Pprune. I have no qualifications and not much experience. No one has yet shot the aerodynamics to bits. If what I suggest is right I would like people to look at the traning side of things and if a change the current thinking is appropriate. implement it that way.

As with all advice offered on PPrune - do not do this at home.

Gaseous,
I have never been trained to "flare" when the engine quits or if the low rotor RPM horn sounds. I've been taught to get the collective down pronto. I am afraid if you've been taught such you have had utterly poor instruction.
Your assertion that the helicopter's attitude has little relevance as far as rotor RPM is concerned is laughable. If you truly think this it's obvious you've never autoed in anything smaller than a Chinook. The first thing I learned was that the lightweight heli's attitude going into an auto is the most important item there is. You may want to speak w/ a CFI sometime soon to understand your errors here. And with all due respect, you are in error.
In my training the CFI would even cover the gauges with a piece of cardboard so I'd get my eyes out and ensure proper attitude during the auto.
Have you ever autoed in an R22? It doesn't sound like you have.
PS
Comparing a Cessna's actions to a Robbie's is ridiculous. And we are talking about a flare increasing rotor RPM in normal instances. If the rotor's stalled, no flare or anything else for that matter is gonna get it back. That's common sense.
And after reading all your recent posts here I am shocked at your total and very mistaken disregard for the crucial need for proper attitude when entering autororation in a light heli.
Do yourself a favor:
W/ CFI aboard, fly an R22 and enter an auto, then roll off throttle and play w/ the heli's attitude. Pushing cyclic forward w/ increase airspeed and lower RRPM. Gentle aft cyclic will immediately increase RRPm. Try it and you'll stop posting wrong info here. You will stop saying that adding aft cyclic won't help a decaying rrpm.
And I am not talking about a blade that's already stalled.
Thanks.

Snoop
Read back through the thread. The whole point of my posts is DONT FLARE!. However, there are numerous references to flare in auto. I personally wouldn't. What I suggest about ideal attitude is that it is not the same at 75% as at 100%.I agree, I phrased it badly. I have auto'd a R22 a few times but never much below 90%. The fixed wing aerofoil comparison is valid to show that large control inputs can cause stall. Rotating aerofoils are no different in that respect. CFI or not I'm not taking Nr to the point where this is relevant. You can play all you like at 90%. It proves nothing.

Edited for spelling.

Say both blades are close to stall, forward cyclic is put in, the angle of attack on the retreating blade is increased, now why would a retreating blade be less likely to stall than an advancing blade? When it does stall, the advancing blade doesn't, but the result of the aft cyclic, the disk has started to tilt aft, and the retreating blade stalls, which would have the same effect as a full rotorstall because the drag is dramatically increased. You can demonstrate rotorstall close to the ground, it's dramatic, 75% and boom, all RPM is gone. Since you're traveling forward, and the retreating blade has stalled, the rotorsystem will blowback and chop the boom.

Quote from SN-24
"When the rotor stalls, it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher airflow on the advancing blade than on the retreating blade. This causes the retreating blade to stall first, allowing it to dive as it goes aft while the advancing blade is still climbing as it goes forward. The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disk sometimes referred to as "rotor blow-back". Also, as the helicopter begins to fall, the upward flow of air under the tail surfaces tends to pitch the aircraft nose-down. These two effects, combined with aft cyclic by the pilot attempting to keep the nose from dropping, will frequently allow the rotor blades to blow back and chop off the tailboom as the stalled helicopter falls."

Tim Tucker had the RPM in the certification tests they were doing for the R22 down to 73%. I'd like to ask him, but my guess is he didn't have aft cyclic in when recovering. Too bad they didn't have mini video cams back then, would be some very interesting footage.

James

Fulldownauto,
To answer your question regarding Gaseous' assertions about retreating blade stall? Retreating blade stall has absolutely nothing to do with blade stall in an auto. Again, this Gaseous fellow is totally misinterpeting a different set of circumstances and applying them to autos. I say his poster name is correct though.

I used the term retreating blade stall. Look at the context. I clearly wasn't referring to high forward speed retreating blade stall.

What we're talking about here, and I can't figure out why this is hard to grasp, is the effect of cyclic movements when the rotorsystem is near stall. That's it! Just a moment in time. Not a sweeping rethinking of how engine failures should be handled.

In the context it was in (read the SN, they mention "This causes the retreating blade to stall first") how could you possibly think I was referring to high forward speed retreating blade stall??

We can have intelligent discussions about theoretical situations without confusing people as to what they know to do and actually thinking people would misinterpret what is said here and apply it to when they have an engine failure. We're all going to do as we're trained and have experienced.

Back to the points. Since my Socratic method hasn't worked, I'll say what I think. I think that any cyclic movement when the rotor is near stall is not good. Keep it centered, and if you've got the collective full down, it's just a matter of waiting till the aircraft descends fast enough for the RPM to start building.

Thanks,

James

Hi guys,

FWIW:

a) If I remember right from the last 2 safetycourses, the 1.1 sec figure is only true IF you want to "keep speed + altitude the same as before the power failure".
You loose all power, you immediately will start to slow down and start to sink if you do nothing. So here you already got some time coming towards you.
IF you really wanted to keep v+h you would have to get some forward cyclic and collective INCREASE at the same time, then of course everything is over in 1.1 sec,:(

b) The suggestion in the safetycourse was " to apply gentle aft pressure on the cyclic to maintain the nose more or less level. Basically don't let the helicopter accelerate."
While "gently" applying aft pressure you also would want to lower the collective:D .

c) "Slamming down the collective!" This is a no-no!!
Slamming/snapping down the collective does nothing good - it will only get you towards a low-g situation (which in an auto is not as bad as in a regular push-over, but....).
All you will do is go nearly through the roof!
Correct action is to lower the collective SMOOTH and QUICK, but please don't slam it. Give the rotor some time to change the airflow!

back to b) The faster you fly when 0-power hits, the faster Nr will decay. Worse yet the faster you fly the more inertia the helicopter will have (the whole machine, NOT the blades!), which means it will not want to really start to decend, but keep going straight (I think that Newton-guy found that out some time ago...)

So back to the safetycourse (and reality), if you are fast it will take some time to get a decent decent rate. Now as we will need something to convert to maintain Nr, the only thing we have for the moment is speed.
So if you would like to keep up Nr in the comfort range you will HAVE to do a "GENTLE" flare, produced by the "GENTLE" backpressure on the cyclic.

Those who attended the safetycourse will remember the graphs that showed the amount of energy you get from decelerating from higher speeds towards 60 kts - V2(squared) does magic,:D

However it is very important to not overdo this as you can maintain altitude perfectly well nearly all the way down to 25-30 kts by flaring hard enough. Only you have nothing left to convert if you are at some serious altitude at this point.
The idea is to just hold back enough (decelerate) to keep Nr happy until the decent-rate is good enough to start milking altitude for energy.

With enough practise you can do throttle-chops (not recommended by RHC or me) at 90 kts and never hear the low rpm-horn (98%) until the level-off at the bottom of the auto.

"With enough practise" is the key-word!!

When I do occasional pre-solo checks, all I really care for (in the auto department...) is a correct AUTOMATIC reaction from the student if something goes towards low Nr. And mostly they are surprised. After a couple of real entries they seem to asimilate the idea.

You can't expect a student to be perfect when going for a solo, especially in a R-22 and around 20 hrs total time. All I care for is that they are able to get into a stabilized auto. At the bottom they will do something and most likely they will walk away in the unlikely event.

I don't usually chop throttles, but I will start to roll off power slowly to get the low rpm horn. After a couple of trials the reaction becomes automatic. Smoothly lowering the collective and SIMULTANIOUSLY applying "GENTLE" backpressure.
At this time we are not practising perfect autos (1-2-3 lower collective and roll it off) but reaction to the unexpected.
Mostly I start to roll the throttle towards low rpm on the way back to the hangar a couple of times too, and things get more relaxed all the time. Most students even start to react before the warning horn comes on (just hearing the change in rpm) - collective starts to go down smooth and cyclic starts to apply back pressure ever so subtle.

Remember not all low Nr is because of a power failure: Some students and "done" pilots are nervious and grip the throttle too hard so the governor can't do its job - reaction should be the same.

As mentioned the R-22 is not going to pardon slow reaction time, so it is a basic requirement to get your reactions to any changes in Rotor-sound and/or Engine-sound AUTOMATIC, like putting your foot on the brake pedal if you see something unexpected in front of you on the road. You might not want to brake after all, but you want to be ready - AUTOMATICALLY.

Don't blame the machine for your lack of practise or wrongly learned maneuvers!

d) "Increasing rpm in turns" This happens mostly because of deceleration in the turn. Try it out, concentrate on keeping that 60-65 kts in the turn and your rpm hardly moves! Decent-rate will increase some, but will also come back when you straigten out. While maintaing your speed the nose will go a little low too, but rpm will hardly move.......keep it at or below 30º bank too!

You don't have to fly the R-22, constantly awaiting desaster, most likely it will never happen. Just spend some quality-time with your instructor practising autos/reactions/etc.
"You shouldn't feel right if you didn't practise at least 1 auto everytime you are out flying!:D " Autos should become fun to do, not a dreaded necessity.......
("1 auto everytime out" is mainly aimed at low time pilots, I understand that in the commercial world this will mostly not be possible. Also for low timers this should be done with your safety-course equipped CFI!!)
Just plan for some 15 min extra and do some quick-auto-warm-up with your CFI. Then kick him out, get your pax and go fly!!

Cheers,
3top,:cool:

PS: I did the "GENTLE" thing repeately and on purpose - It is very important! I don't like big stick moves!! Helicopters want as little movement on the controls as possible and if you do, you actually want to think about "applying pressure" rather than moving - whatever you do, do it smooth, no need to snap the controls anywhere ever and once you're done quit stirring the fuel...........

The static weight on the blades wouldn't be the problem.

The problem would be blade grips, spindle bearings, rotor head, mast, trans, etc.

but it ends the same, call it 44:ok:

The new R-22 blades are no longer Alu wrapped, but stainless like the R-44 blades, however to maintain the weight, it is rather thin stuff covering the honeycomb!

3top

am I right in thinking that the tip weights are roughly 6lbs? remember hearing this somewhere before... HH

Gaseous,
You say "If you've got the lever down and your Nr is still decaying you should come back and haunt your engineer! (or mechanic if you live in USofA). Helicopters should be trimmed to maintain autorotation with the lever down and should do to the point where the rotor stalls."
Agreed! ;)

Another concern beyond misrigging is the possibility of something getting stuck under the collective. Obviously this is something that we don't want to happen, but if you lose an engine and simply cannot get the collective far enough down due to a handheld radio or misrigged collective, turns were my suggestion.

Regarding the aerodynamics, I erred. You are right, the outermost region is the driven region, rather than the driving region. Shame on me. I should (do) know better. :ok:

If we agree that the retreating side is the likely area for stall then consider my remark about reverse flow. You are right in your point that the angle of attack at the blades will increase and the stall region will move outward. However, I believe the impact is relatively small and may be offset by blade angle changes caused by a push on cyclic. Airflow impinging the bottom of the disk on the retreating side in an auto is at some acute angle. In theory, if we're doing 70kts and descening at 1500 fpm (pulling number out of my butt here--actual numbers are irrelevant), the disk is seeing airflow coming at approximately 12deg angle from horizontal at the 9 o'clock position if the ship is level. If we pitch up a degree, this inflow angle becomes 13deg. Fair enough? At 80% NR the 75% rotor radius of the blade is moving in plane at roughly 423fps, or 25380fpm relative to the aircraft. Thus, at zero blade angle, where we have 12deg disk inflow, the angle of attack is 4.687deg. The blade is seeing 1500 fpm in the vertical direction and 18296 fpm in the in-plane direction (25380 - 7084). When the disk pitches back a degree, the angle of attack is 4.956deg, a change of only 0.269deg for the 1deg disk pitch up.

Also, consider that when you pull back on the stick you are increasing pitch on the advancing side (primarily--a bit over the nose as well owing to the delta-3) and decreasing it on the retreating side. Therefore pushing forward cyclic will lead you closer to stall on the retreating side. Whether pulling back enough to yield a one degree disk pitch up will give more or less than a 0.269deg reduction in local blade angle, I don't know.

Please realize the above contains many assumptions and I've been quite loose with theory (delta_blade angle is a function of delta_angle of attack for instance). I think it helps to bring out the factors affecting the angle of attack however. I'd still like to delve a bit deeper to feel better about this analysis.

"No one really knows what the dead pilots of all the crashed R22s did last. I suspect some of them did a big flare with low Nr because they thought it would increase Nr. Imagine their surprise when the rotor stalled."

Perhaps they pushed forward :hmm:

<edited to change "¡ã"s into more meaningful terms>

r22 tip weights?
 

The last set of 22 tip weight I cut out were 2 pounds per blade. held in place on the front spar by if I remember correctly, 4 quarter bolts.

I'm not trying to misrepresent anything. I'm trying to work out the complex physics that goes on at rotor stall. I can find nothing in my text books about it and I am trying to put together the bits of the puzzle.

I think we can probably all agree that the rotor stalls due to the angle of attack becoming too large.

Here are 3 extracts taken from Norman Baileys book. I hope he will not mind me using them. He knows who I am.

Cyclic input alters the plane of rotation. NOT the angle of attack. (Bailey, principles of flight, p40).

forget the red herring of cyclic affecting pitch. do not confuse pitch with angle of attack. forward cyclic will not increase angle of attack on retreating blade

When the disc is flared, a componant of the horizontal airflow will now be opposing the induced flow. The change in direction of the airflow relative to the blade will INCREASE the angle of attack and therefore an increase in total rotor thrust. (Bailey, principles of flight, P100).

It is with this in mind that I suggest that a flare will push the disc towards a stall. If this is true, the converse must be true.

...At the same time, section D at the root becomes stalled and the extra drag generated causes a reduction in the size of the autorotative area and a rpm decrease. (Bailey, p50)

There is a diagram on page 51 which shows the stalled area of the blade is larger on the retreating blade than on the advancing blade.

It is with this in mind that I make my assumption of left roll as stall approaches.

Autorotate a R22
 

The most frightening statistic that has appeared in these posts is the vast differences of opinion as to what actions to take in an emergency situation. I am informed that the most hazardous part of a test pilots job is testing the autorotational limits on any new design. It would appear that a lot of pilots test these limits for free.

Gotta question.
 

To: bugdevheli

When you "cut out" the weights was this by drawing with a specified material and were bolt locations on the weights deliniated by drawing. The reason i ask is that if the weights are not properly positioned and the static balance of the blade not verified you could run into several problems:

!) Rotor spanwise imbalance

&

2) A serious change in the pitching moment of the blades

PLEASE EXPLAIN THE TERM "CUT OUT"

If you in fact removed the 2 pound weights did you replace them with heavier weights in the same place? This too can seriously effect the pitching moment and the dynamic stability of the blades
as well as possibly effecting the spanwise balance of the blades.

:E :E

budgevheli,

This conversation is intellectual chess on what to do in a situation that in my opinion a pilot should never allow to happen.

I have seen the R22 Nr at 78% doing autos in the training environment, and it recovered promptly by waiting. No flare, turn, push or pull. I never want to see it there again though, and I think the pilot (that was me!) is to blame by letting it get that low. I learnt from that.

I wonder just how close the rotor was to stalling back then, but in my mind this conversation is talking about 75% Nr or less, ie critically low Nr.

And yes, I'm as concerned as you are that someone might take this speculation on board somehow and in confusion deviate from what they have been trained to do when the blood is pumping.

Stick to the training! Pprune is a great source of info, ideas, and increased awareness. How people choose to interpret that and what course of action they ultimately take is up to them eh?

How does the R22 MCP Limits graph work?
 

How does the R22 MCP Limits graph work?

Before taking off in the R22, one of the performance checks is to calculate limit of MCP and 5 Min Take Off power available from the graph in the aircraft.

As the OAT gets higher, and thus air less dense, more MAP is available. (And as pressure alt increases at a constant OAT less MAP is available). I have been told this is to do with the temperature of the burn of the fuel on ignition which could lead to overstressing the drive system and blades at lower temperatures.

Can someone give an informed explanation of whats happening inside the engine and why the graph reads like this please?

choppersafari

My understanding of it, stated as simply as I can put it (and correct me if I'm wrong anyone please) is that it's not that more manifold pressure is 'available' but that more manifold pressure is 'required' in order to obtain your 5 minute maximum (131hp). Cooler, more dense conditions - less MAP required. Hotter, less dense conditions - more MAP req'd.

Lets see what the more technically minded and more experienced amongst us have to say...

A good test of your instructors knowledge, ask them.

I am talking about an engine that is not boosted (turbo charged).

Call fuel/air mixture "Stuff".

MAP is the absolute pressure of Stuff measured in the intake manifold downstream of the throttle. The more the throttle restricts the airflow into the intake manifold, the more the manifold pressure will drop (as the engine 'sucks' harder) and vice versa. When the throttle is wide open, the MAP can only get as close to the ambient outside air pressure as possible. When the engine isn’t running, the MAP will be the same as the ambient air pressure.

It is common to have a ‘de-rated’ engine that is capable of producing more horsepower than the drive train can accept at lower altitudes. Horsepower is purely a function of the pressure differential inside the cylinder when it goes bang, compared to the ambient pressure outside the cylinder in the crankcase. In order to control this pressure differential, we must introduce the correct mass of Stuff into the cylinder to burn. We can only do this by varying the pressure of Stuff going into the cylinder, as we have no control over the density or the cylinder volume.

In order to limit horsepower to the maximum that the drive train can accept, the manufacturer (and the pilot) must therefore limit the intake manifold pressure to deliver a pre-determined maximum mass of Stuff to the cylinder under the given conditions.

Say full throttle will let 90% of the ambient pressure into the intake manifold (I don't know the real data). It is physically impossible to get 100%, as there is always some restriction of the air. As the pressure altitude increases, the ambient air pressure decreases and 90% of less ambient pressure is less intake manifold pressure. A lower pressure is required in the cylinder after combustion to push against a lower ambient crankcase pressure to deliver the same horsepower. This means less mass of Stuff to start with, and is why the MAP limit on the chart reduces as you climb.

As the OAT rises the density decreases, but the ambient pressure stays the same. This means that less mass of Stuff will go into the cylinder at a given manifold pressure, and less pressure differential (horsepower) will be produced after combustion. So in order to get back to the limit you must get more Stuff into the cylinder, and the only way to do that is to increase the pressure of the Stuff going in. This is why the MAP limit on the chart increases as the OAT rises, or more correctly the air density reduces.

This is also why the chart allows full throttle at altitude, because up there the engine can’t get enough Stuff to produce enough horsepower at full throttle to exceed the drive train limit. Shovel less coal, get less power.

It is all to do with finding the correct amount of Stuff to burn to create the required pressure differential between the inside and outside of the cylinder, given the ambient pressure outside the cylinder to start with.

By the way: Pilots that exceed the limit “because the engine is de-rated anyway” don’t realise the damage they may be doing by overstressing the drive train.

Hope this helps.

Great STUFF Coyote

Robinson: Tech Q's (Merged)
 

Answer for Lu Zuckerman. Cut out, as in take saw, cut off outer skin to expose internal tip weights. Remove said tip weights and weigh. Purpose. To determine where and what loads are passed through a R22 blade. Objective. To construct a blade for a homebuilt machine that outperforms r22 blade both in terms of structural integrity and inirtia value whilst decreasing the overall blade weight. Regards Bug.

This is how the big guys calculate in-flight horse power.
 

On large round engines this is how they calculate horsepower at any stage of flight to include take off.

First the engine must be equipped with a BMEP gage. BMEP is the Brake Mean Effective Pressure inside the cylinder.

To calculate horsepower the flight engineer will use a formula known as PLANK.

P= Prerssure or BMEP
L=Length of the piston stroke
A=Area of the piston
N=Number of cylinders
K=A "K" factor for that particular engine type

In a chain multipication of each of the elements the flight engineer can determine engine horsepower output.

At least that is what I remember from mechanics school back in 1949

:E :E

Lu,

'PLANK' is the old method. They now use 'GIRDER'

It was changed when they went from wooden blades to metal ones.

:E :ok:

Dave

PS. Since you have the ire and ear of Heilport, would you do me a favor? Please ask him how to search for some past technical information on a thread that is 433 posts long.

:D :D

To: Dave Jackson

If I did it might be the straw that breaks the camels butt. I think you have already asked the question.

:E :E