rotor stall in dynamic turn
Originally Posted by [email protected]
BI - I assume you are taking about the inboard section where there is no actual aerofoil and not the inner part of the blade itself where, at least on the advancing side, there is both speed and an aerofoil section.
I have only ever heard reference to the root of the blade in P of F terms as only including the parts where the aerofoil section exists but then I am just an ex-mil QHI who doesn't lecture internationally........
I have only ever heard reference to the root of the blade in P of F terms as only including the parts where the aerofoil section exists but then I am just an ex-mil QHI who doesn't lecture internationally........
BTW Crab, you do yourself a dis-service - I have read many of your posts on PPRuNE and I can assure you that you do indeed lecture internationally
Lecturing in and of itself does not connote the transfer of knowledge as I have learned down through the years.
Just saying.
Just saying.
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Crab, effectively yes, you are correct. By the root I am referring to the entire section from the mast attachment point out to the point at which the blade is not stalled on the advancing side. In pure terms there will be a point at the extreme inboard section of the 'flying' blade where the blade is stalled at 89 degrees of advance, flying at 90 degrees (ie exactly perpendicular to the forward airflow), and stalled at 101 degrees of advance. The blade is incrementally flying as we move outboard from this point.
BTW Crab, you do yourself a dis-service - I have read many of your posts on PPRuNE and I can assure you that you do indeed lecture internationally
BTW Crab, you do yourself a dis-service - I have read many of your posts on PPRuNE and I can assure you that you do indeed lecture internationally
Last edited by Chock Puller; 24th Mar 2023 at 12:26. Reason: Inappropriate comment.
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thanks for all replies and sorry for not participate into discussion (a bit busy lately)
here is another example of risky maneuver (in my opinion) , as long as airflow is from underside of the rotor it's fine , but when you raise up collective so dynamically doing kind of flare , your blades will change AoA , load will increase and if your engine is not powerfull enough is there a risk of blade(rotor) stall ?
https://www.instagram.com/reel/CqRni...d=MDJmNzVkMjY=
here is another example of risky maneuver (in my opinion) , as long as airflow is from underside of the rotor it's fine , but when you raise up collective so dynamically doing kind of flare , your blades will change AoA , load will increase and if your engine is not powerfull enough is there a risk of blade(rotor) stall ?
https://www.instagram.com/reel/CqRni...d=MDJmNzVkMjY=
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thanks for all replies and sorry for not participate into discussion (a bit busy lately)
here is another example of risky maneuver (in my opinion) , as long as airflow is from underside of the rotor it's fine , but when you raise up collective so dynamically doing kind of flare , your blades will change AoA , load will increase and if your engine is not powerfull enough is there a risk of blade(rotor) stall ?
https://www.instagram.com/reel/CqRni...d=MDJmNzVkMjY=
here is another example of risky maneuver (in my opinion) , as long as airflow is from underside of the rotor it's fine , but when you raise up collective so dynamically doing kind of flare , your blades will change AoA , load will increase and if your engine is not powerfull enough is there a risk of blade(rotor) stall ?
https://www.instagram.com/reel/CqRni...d=MDJmNzVkMjY=
As far as risk goes, I doubt the wearing of a hi-viz vest trumps not wearing a seat belt and coming close to having the top of your head knocked by the blade roots.
There is nothing crazier than seeing Rainer Wilke with the Red Bull BO105 flying aerobatics. Even if he flies upside down, at no moment during his performance is the rotor stalled. Never. It may not produce lift, but there is no complete stall of the disk. And compared to the Red Bull BO105, what anybody else does in these videos is just lame. It might be not safe like that bloke in the video and the way he flies might end in tears one day, but not because the rotor stalled. It is good that you ask these questions and I invite you to ask more questions, but I think with what everybody said until now, that question about the rotor disk stall has been answered.
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got it , thanks
what paid my attention was caused by 2 situations in the past
1. coming to land at the spot , not so fast , like 60-70 kts , low like 20-30 ft above the ground with a very little tailwind - intentions to do "spectacular" 180 turn as close to landing spot as possible , almost at the spot and land there . During that 180 turn , body starts to shake , NR drop , reacted like quick drop on collective to gain NR back - all ended good but heli shows it's edge on the limit's
2.coming to land 50-60 kts , descent angle like 4-5 degree , but descent "on power" , flare phase with cyclic move to the back to slow down but without reducing collective much , rather raising it up a little to reduce vertical speed - again NR dropped to the red ( very little , easy to recover but wondering about exact rotor aerodynamics in both cases )
what paid my attention was caused by 2 situations in the past
1. coming to land at the spot , not so fast , like 60-70 kts , low like 20-30 ft above the ground with a very little tailwind - intentions to do "spectacular" 180 turn as close to landing spot as possible , almost at the spot and land there . During that 180 turn , body starts to shake , NR drop , reacted like quick drop on collective to gain NR back - all ended good but heli shows it's edge on the limit's
2.coming to land 50-60 kts , descent angle like 4-5 degree , but descent "on power" , flare phase with cyclic move to the back to slow down but without reducing collective much , rather raising it up a little to reduce vertical speed - again NR dropped to the red ( very little , easy to recover but wondering about exact rotor aerodynamics in both cases )
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Chr, what you probably saw there was overpitching, where more power was demanded than was actually available at the time.
Overpitching refers to the pitch angle of the blades, not the aircraft.
Overpitching refers to the pitch angle of the blades, not the aircraft.
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Chr, I agree with ShyTorque, you asked too much from your engine. That was not an aerodynamic problem of the rotor.
Let me paint you a picture. In the first situation you said you had very little tail wind. What, 10 kts? 15? Anyway while you initiate your turn and decelerate you fall out of translational lift (the shudder). Depending on the direction of the turn, you might use even more power because your tail rotor wants power, too. And now you are in a situation where you need more power than you have. Ergo, NR drops. The thing is, you fly by a ground track and your brain tells you that you still move fast enough for translational lift, but your rotor does not care about the ground track. The tailwind made your rotor loose translational lift earlier as you anticipated. And you are in a turn, therefore your load factor increased. You are not the first one that falls into that trap.
In the second scenario you also asked too much from the engine. Again, that has very little to do with the aerodynamics of the rotor, but with power management. If you need power in the flare to stop the sink rate, you probably did something wrong earlier. Initiated the flare too late, for example.
There is nothing gained from flying spectacular if you are not paid by Red Bull to do so. If you do that in front of passengers you are going to pick up, at least half of them will be scared. The other half think it is normal to fly like that. Nobody will be impressed. Would I stand there I would probably think that I go flying with somebody else. Or tell you to move over and let me fly. If you want to impress, try to fly with finesse. Try that for once. Pick up the ship to a two feet hover on a runway and now accelerate the ship very gently to translational lift without pulling more power and continue until you are at your cruise altitude and speed. Be careful not to touch the ground. You will be amazed, how little of height you loose, if you do that very gently. Once you find out how all that works, you can fly a whole circuit without changing the power setting. Coming down takes a bit cheating (pull carb heat for example), but it is fun. Prepares for a stuck collective. Be careful with slipping, because some helicopters with their high CG, will bite back if slipped too aggressively. Stay in the limits. If the handbook says no slipping, no slipping it is. But once you are behind the power curve, carefully reducing speed and altitude, you can come out of translational lift and to a hover just with the cyclic and pedals. Takes a while but I think it is very gratifying to fly like that. It gives you a much better feeling for the ship. You are mentally always ahead of the ship and gives you training for situations you do need finesse. If you are not experienced enough, ask a CFI to fly with you. Flying gently and with finesse is much safer. Many of us have landed in spots where we could not hover. It is not something we should do and if you crash, they will blame you for that. But coming in to a spot like that, which might be a small platform where you will not get a lot of ground effect, with your power almost maxed out, asks to be very gently. An aggressive flare and the subsequent pull on the collective will end in a more or less devastating crash.
And should you have your own helicopter, flying with finesse saves money in the long run. If you rent, finesse will impress the owner. Helicopters are not aerobatic aircrafts. They are all relatively flimsy. Any spectacular flying will cost you more beans from the jar and things will break earlier. Red Bulls maintenance requirements are eye watering for their aerobatic ships.
Let me paint you a picture. In the first situation you said you had very little tail wind. What, 10 kts? 15? Anyway while you initiate your turn and decelerate you fall out of translational lift (the shudder). Depending on the direction of the turn, you might use even more power because your tail rotor wants power, too. And now you are in a situation where you need more power than you have. Ergo, NR drops. The thing is, you fly by a ground track and your brain tells you that you still move fast enough for translational lift, but your rotor does not care about the ground track. The tailwind made your rotor loose translational lift earlier as you anticipated. And you are in a turn, therefore your load factor increased. You are not the first one that falls into that trap.
In the second scenario you also asked too much from the engine. Again, that has very little to do with the aerodynamics of the rotor, but with power management. If you need power in the flare to stop the sink rate, you probably did something wrong earlier. Initiated the flare too late, for example.
There is nothing gained from flying spectacular if you are not paid by Red Bull to do so. If you do that in front of passengers you are going to pick up, at least half of them will be scared. The other half think it is normal to fly like that. Nobody will be impressed. Would I stand there I would probably think that I go flying with somebody else. Or tell you to move over and let me fly. If you want to impress, try to fly with finesse. Try that for once. Pick up the ship to a two feet hover on a runway and now accelerate the ship very gently to translational lift without pulling more power and continue until you are at your cruise altitude and speed. Be careful not to touch the ground. You will be amazed, how little of height you loose, if you do that very gently. Once you find out how all that works, you can fly a whole circuit without changing the power setting. Coming down takes a bit cheating (pull carb heat for example), but it is fun. Prepares for a stuck collective. Be careful with slipping, because some helicopters with their high CG, will bite back if slipped too aggressively. Stay in the limits. If the handbook says no slipping, no slipping it is. But once you are behind the power curve, carefully reducing speed and altitude, you can come out of translational lift and to a hover just with the cyclic and pedals. Takes a while but I think it is very gratifying to fly like that. It gives you a much better feeling for the ship. You are mentally always ahead of the ship and gives you training for situations you do need finesse. If you are not experienced enough, ask a CFI to fly with you. Flying gently and with finesse is much safer. Many of us have landed in spots where we could not hover. It is not something we should do and if you crash, they will blame you for that. But coming in to a spot like that, which might be a small platform where you will not get a lot of ground effect, with your power almost maxed out, asks to be very gently. An aggressive flare and the subsequent pull on the collective will end in a more or less devastating crash.
And should you have your own helicopter, flying with finesse saves money in the long run. If you rent, finesse will impress the owner. Helicopters are not aerobatic aircrafts. They are all relatively flimsy. Any spectacular flying will cost you more beans from the jar and things will break earlier. Red Bulls maintenance requirements are eye watering for their aerobatic ships.
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All good stuff from Rotorbee and Shytorque there chr - running out of power doesn't mean stalling the rotor.
On that second video, you can see he nearly overcooks it on his departure - he loses translational lift as he goes through 180 degrees in the transition and is lucky not to hit the buildings.
All manoeuvring in the low speed environment requires extra power above what you need to hover - if you don't appreciate that and anticipate it, you will embarrass yourself.
If you don't have a good power margin above hover OGE then maintaining ETL is essential when manoeuvring spiritedly.
Many pilots will have learned a 'downwind' quickstop where you start downwind at speed, and then use a combination of flare and AoB to end up in the hover into wind. When taught in the military, the idea is to maintain a minimum of 30 kts until within 30 degrees of the wind - this ensures ETL until you are ready for the final decel to the hover.
On SAR we used to teach a similar idea but using yaw to start the turn from a slower speed - 30 to 40 kts - and a fair amount of nose down to maintain the speed and ETL until into wind.. This was a great technique when operating in tight valleys or where you didn't have the room to carry out a normal downwind quickstop.
On that second video, you can see he nearly overcooks it on his departure - he loses translational lift as he goes through 180 degrees in the transition and is lucky not to hit the buildings.
All manoeuvring in the low speed environment requires extra power above what you need to hover - if you don't appreciate that and anticipate it, you will embarrass yourself.
If you don't have a good power margin above hover OGE then maintaining ETL is essential when manoeuvring spiritedly.
Many pilots will have learned a 'downwind' quickstop where you start downwind at speed, and then use a combination of flare and AoB to end up in the hover into wind. When taught in the military, the idea is to maintain a minimum of 30 kts until within 30 degrees of the wind - this ensures ETL until you are ready for the final decel to the hover.
On SAR we used to teach a similar idea but using yaw to start the turn from a slower speed - 30 to 40 kts - and a fair amount of nose down to maintain the speed and ETL until into wind.. This was a great technique when operating in tight valleys or where you didn't have the room to carry out a normal downwind quickstop.
Chr, Crab is absolutely right. Always have enough power margin. Hover checks are a very good tool not to get into trouble. The power charts of every helicopter you fly are holy. Use them. On the other hand, our esteemed crustacea Crab is military trained. In the civilian world, we do not fly tactically. We do not have to rush things because nobody is shooting at us ... normally. We do not know your level of training, but let me tell you this. During all my training up to CFI, I used training material that isn't even close to the truth regarding aerodynamics of the rotor. It is good enough to make you a reasonably save pilot, but when you are interested in these things and dive deeper, you will find, that all that material were "lies to children", because the real thing is so much more complicated, that most of us would have troubles to understand them. It is so complicated that even today, helicopter manufacturers still depend on trial and error and experience what works, without knowing exactly how. Therefore we give students a overview about it and tell them, what to avoid. I am almost certain, that you never heard about dynamic stall in flight school, but to understand the ins and outs of the aerodynamics of a rotor, dynamic stall is essential. Another example would be gyroscopic precision of the rotor. You will find that in many helicopter flying handbooks (Yes FAA, I am looking at you), but that just does not apply to a helicopter rotor, or to be more precise, only in a vacuum. Otherwise every helicopter would have a 90° offset of the controls, but they don't. If you want to know more, I recommend the books from Prouty, Helicopter Aerodynamics, more Helicopter Aerodynamics and even more Helicopter Aerodynamics (Yes, these are the titles) and the books from Shawn Coyle. Yes, Shawns books are expensive, but worth every dime.
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you are mine of knowledge , thanks
looks like all becomes clear , few more analysis of different scenarios and it all should be straight and lvl in my head
Thank you
looks like all becomes clear , few more analysis of different scenarios and it all should be straight and lvl in my head
Thank you
An aircraft is blind. It is unaware if it over ocean, jungle, mountains or flat plains.
It only “Knows” the density altitude and where “g” and relative wind is.
It “knows” how much power it can produce, how much lift it can provide for both the main and tail-rotors.
Ask it to do more than it is capable of and it will disappoint you every time.
It only “Knows” the density altitude and where “g” and relative wind is.
It “knows” how much power it can produce, how much lift it can provide for both the main and tail-rotors.
Ask it to do more than it is capable of and it will disappoint you every time.
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here we go again 
another vid , any threats in manouver like this ? no overpitching risk when coming out of flare ?
https://www.instagram.com/reel/CpUWJ...d=YmMyMTA2M2Y=
another vid , any threats in manouver like this ? no overpitching risk when coming out of flare ?
https://www.instagram.com/reel/CpUWJ...d=YmMyMTA2M2Y=
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or this one , even better
whole manouver at the end of this reel
https://www.instagram.com/reel/Cobu8...MDJmNzVkMjY%3D
whole manouver at the end of this reel
https://www.instagram.com/reel/Cobu8...MDJmNzVkMjY%3D
chr - have a you got a different link to the video, I don't use instagram.
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