R22 Corner
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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
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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.
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.
Last edited by Gaseous; 5th Jan 2005 at 01:33.
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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.
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.
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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.
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.
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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
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
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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
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
Last edited by Gaseous; 7th Jan 2005 at 16:23.
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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
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
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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.
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.
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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...................
Hovering autorotation? 1,2,3 chop, PULL!!! (only you do it faster than you can read it!) We had it tough in my day...................
Senis Semper Fidelis
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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
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
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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
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
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zxcvbvn
I think that this wouldn't be a problem in flight (assuming that you put the same weight on both blades
) 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.
I think that this wouldn't be a problem in flight (assuming that you put the same weight on both blades
) 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.
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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
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
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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
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
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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.
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.
Last edited by Gaseous; 8th Jan 2005 at 01:24.
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!
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!
Last edited by the coyote; 8th Jan 2005 at 07:10.
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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.
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.
Last edited by Gaseous; 8th Jan 2005 at 08:08.
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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.
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.
Last edited by 13snoopy; 8th Jan 2005 at 08:22.
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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.
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.
Last edited by Gaseous; 8th Jan 2005 at 12:28.