Helicopter G limits/manoeuvering speed(R22) + Takeoff w/ Gs
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ifresh21,
In general, yes, the rotor would stall, and produce a load factor much less than the static system is designed for, so the maneuver would not produce damaging G, but it would produce damaging fatigue forces in the rotating system controls.
That does not mean the rotor would break or any such thing, because generally, the components from the blade to the servo are designed to take everything the blade stall forces can hand out, at least a few times. They would fatigue, and most likely, the bearings would wear out very quickly. You would find pitch link bearings, swashplate trunnions and scissors wearing very prematurely as the big loads were being routinely passed through them. I would bet helos that are maneuvered a lot see lots of bearings being replaced, as well as minor cracks on the blade root areas.
Let's answer each piece:
So if I understand correctly, you are saying that something else would break before 3Gs is achieved?
no, just that the rotor will reach a stall condition, and produce no more thrust, and so no more G. It wouldn't likely break, but it would get worn out quickly.
You also talked about rotor stall. Is it possible to stall the rotor if you remain inside the RPM limits in a helicopter like the R22? Or is it that you would run out of power(within MAP limits) to maintain RPM with before reaching a high G load?
Great question. The engine power is only a small piece of the puzzle, because big maneuvers must be fueled by decelerating and using the aircraft's kinetic energy to produce the G, temporarily, as the speed is coming way down. I have experienced 20 knots per second speed loss while in big maneuvers in helos, while the engine power was only moderate.
But if the FARs say that the helicopter has to be able to withstand a certain number of Gs, why would that not count for the entire helicopter? That means that the helicopter would not be able to handle the Gs required by FARs - right?(cause the rotor system would break or jack stall etc.)
No, because the FAR is a static system requirement, later in the FAR it even say the forces assumed to produce the G are assumed to act on the mast or rotor center, because these limits are static system, theoretical limits. Once again, the static machine cannot come apart under load factor, but that design G strength limit is no guarantee that the rotor can produce that much thrust. Most helos cannot produce 2 G's under engine power, let alone 3.5!
The R22 doesn't use hydraulics, so is there some sort of equivalent problem since jack stall is impossible?
Yes, the stick forces would rise, the cyclic would feel throbby and start to jump around in the pilot's hands. In effect, the pilot is the "jack" and he/she would find it hard to control the aircraft.
Is it that the rotor can't physically produce the load factor to bother static system because of rotor stall?
Yes, now you've got it.
Im pretty confused about this.
You should be, it is hard, nobody ever teaches or discusses it, and frankly, few folks know the rotor theory and the pilot lingo, so the two worlds don't often meet.
What about helicopter that can do a loop like the apache ah64 how many Gs would that be
A loop takes about 2.5 G when done at helo speeds, few of us who fly helo aerobatics pull much more, because we can't - the rotor will not produce that much thrust. At 80 knots, 2.5 G is a tiny maneuver radius, perhaps 300 feet, a very wonderful loop!
One macho comment, I used to demo 3+ g's in light Black Hawks using a wind-up turn, and trading airspeed for load factor at 75 degrees of bank. John Dixson taught me how, it is ok on a light machine at perhaps 13,500 lbs (and bad bad on a heavy one). Enter at 125 knots, roll out at 75 or 80, and have hit 3+G for a 2 second count. At that weight, I used to trim at 80 knots, 60 degrees of bank and 2 g's and make 720 degree slowly climbing turns while trimmed at 2 G's. A light Black Hawk is a screamer!
In general, yes, the rotor would stall, and produce a load factor much less than the static system is designed for, so the maneuver would not produce damaging G, but it would produce damaging fatigue forces in the rotating system controls.
That does not mean the rotor would break or any such thing, because generally, the components from the blade to the servo are designed to take everything the blade stall forces can hand out, at least a few times. They would fatigue, and most likely, the bearings would wear out very quickly. You would find pitch link bearings, swashplate trunnions and scissors wearing very prematurely as the big loads were being routinely passed through them. I would bet helos that are maneuvered a lot see lots of bearings being replaced, as well as minor cracks on the blade root areas.
Let's answer each piece:
So if I understand correctly, you are saying that something else would break before 3Gs is achieved?
no, just that the rotor will reach a stall condition, and produce no more thrust, and so no more G. It wouldn't likely break, but it would get worn out quickly.
You also talked about rotor stall. Is it possible to stall the rotor if you remain inside the RPM limits in a helicopter like the R22? Or is it that you would run out of power(within MAP limits) to maintain RPM with before reaching a high G load?
Great question. The engine power is only a small piece of the puzzle, because big maneuvers must be fueled by decelerating and using the aircraft's kinetic energy to produce the G, temporarily, as the speed is coming way down. I have experienced 20 knots per second speed loss while in big maneuvers in helos, while the engine power was only moderate.
But if the FARs say that the helicopter has to be able to withstand a certain number of Gs, why would that not count for the entire helicopter? That means that the helicopter would not be able to handle the Gs required by FARs - right?(cause the rotor system would break or jack stall etc.)
No, because the FAR is a static system requirement, later in the FAR it even say the forces assumed to produce the G are assumed to act on the mast or rotor center, because these limits are static system, theoretical limits. Once again, the static machine cannot come apart under load factor, but that design G strength limit is no guarantee that the rotor can produce that much thrust. Most helos cannot produce 2 G's under engine power, let alone 3.5!
The R22 doesn't use hydraulics, so is there some sort of equivalent problem since jack stall is impossible?
Yes, the stick forces would rise, the cyclic would feel throbby and start to jump around in the pilot's hands. In effect, the pilot is the "jack" and he/she would find it hard to control the aircraft.
Is it that the rotor can't physically produce the load factor to bother static system because of rotor stall?
Yes, now you've got it.
Im pretty confused about this.
You should be, it is hard, nobody ever teaches or discusses it, and frankly, few folks know the rotor theory and the pilot lingo, so the two worlds don't often meet.
What about helicopter that can do a loop like the apache ah64 how many Gs would that be
A loop takes about 2.5 G when done at helo speeds, few of us who fly helo aerobatics pull much more, because we can't - the rotor will not produce that much thrust. At 80 knots, 2.5 G is a tiny maneuver radius, perhaps 300 feet, a very wonderful loop!
One macho comment, I used to demo 3+ g's in light Black Hawks using a wind-up turn, and trading airspeed for load factor at 75 degrees of bank. John Dixson taught me how, it is ok on a light machine at perhaps 13,500 lbs (and bad bad on a heavy one). Enter at 125 knots, roll out at 75 or 80, and have hit 3+G for a 2 second count. At that weight, I used to trim at 80 knots, 60 degrees of bank and 2 g's and make 720 degree slowly climbing turns while trimmed at 2 G's. A light Black Hawk is a screamer!
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AWESOME post Nick. Thank you!
I will have to read through that a few more times
. But my preliminary question is - I don't think you answered whether or not a rotor stall is possible if RPM is maintained.
That has actually been a question since day 1 for me. I asked it to my instructor on my first intro flight. Of course, I didn't get a good answer (instructors generally keep things very simple when dealing with aerodynamics).
I just really would love to know if a stall is possible if RPM is maintained where it is supposed to be(top of green in R22/104%).
Thank you and I will probably have more to ask after a read your post again a few times
Also, you served in the military? - Thank you for your service, Sir.
I will have to read through that a few more times
. But my preliminary question is - I don't think you answered whether or not a rotor stall is possible if RPM is maintained. That has actually been a question since day 1 for me. I asked it to my instructor on my first intro flight. Of course, I didn't get a good answer (instructors generally keep things very simple when dealing with aerodynamics).
I just really would love to know if a stall is possible if RPM is maintained where it is supposed to be(top of green in R22/104%).
Thank you and I will probably have more to ask after a read your post again a few times
Also, you served in the military? - Thank you for your service, Sir.
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ifresh21,
Since each individual blade can be considered a wing, as long as it is operating at it's normal operating RPM (blade tip speed is around 400 knots), stalling the blade will be unlikely. My (fuzzy) memory on aerodynamics leads me to believe that rotor stall is only going to happen as RPM decays, I can't think of a scenario where it could happen at 100% (vortex ring state?)
Retreating blade stall, on the other hand, can occur due to excessive forward speed.
Buy and study the books, this stuff is hard! I know I need to get my head back in them!
Since each individual blade can be considered a wing, as long as it is operating at it's normal operating RPM (blade tip speed is around 400 knots), stalling the blade will be unlikely. My (fuzzy) memory on aerodynamics leads me to believe that rotor stall is only going to happen as RPM decays, I can't think of a scenario where it could happen at 100% (vortex ring state?)
Retreating blade stall, on the other hand, can occur due to excessive forward speed.
Buy and study the books, this stuff is hard! I know I need to get my head back in them!
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Any books in mind that deal with this?
I did some math 80% RPM is 366mph(318kts) - 104% is 476mph(413kts). This is tip speed in the R22.
Just a lil info since it was brought up.
I did some math 80% RPM is 366mph(318kts) - 104% is 476mph(413kts). This is tip speed in the R22.
Just a lil info since it was brought up.
Last edited by ifresh21; 12th Apr 2011 at 02:06.
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Rotorcraft Flying Handbook is a start, and free as a PDF on the FAA website.
Principles of Helicopter Flight -W.J. Wagtendonk, is a bit more advanced.
Cyclic and Collective comes highly recommended too, but I don't own that yet
Principles of Helicopter Flight -W.J. Wagtendonk, is a bit more advanced.
Cyclic and Collective comes highly recommended too, but I don't own that yet
ifresh - if you pull hard enough on a R22, you won't stall the jacks (because there aren't any) but you can experience retreating blade stall (RBS) - this can be achieved at normal RRPM but only at high speed or density altitude or in a harsh pull-up or turn (or any combination thereof).
The highest aerodynamic backloads (highest AoA) are on the retreating side of the disc so that is where the stall occurs when you load the rotor whilst manoeuvring. The symptoms are a pitch nose up and a roll to the retreating side (left in an R22) and the recovery (although it will come out itself as the speed washes off due to the pitch up) is to lower the collective and reduce the severity of the manoeuvre (unload the cyclic).
So in answer to your question - yes you can get rotor stall at normal RRPM and within engine limits but only by mishandling the aircraft. Note that unlike a FW, you have only stalled a part of your lifting area, not all of it.
Aerospatiale (now EC) always maintained that jackstall (servo transparency) was designed in to provide early warning of RBS - the symptoms and recovery are identical.
Sustained G in a helo is down to engine power - a 2G manoeuvre (60 degree level banked turn) is usually started at a higher speed (120 kts for example) and will settle at about 80kts (in Gazelle and Lynx) with almost max continuous Tq.
As Nick said, loops and the like are achieved within 2.5G and it is only on the pull up (high speed) and recovery (accelerating) that you have to be careful as the higher speed means you are much closer to RBS when you load the disc.
The highest aerodynamic backloads (highest AoA) are on the retreating side of the disc so that is where the stall occurs when you load the rotor whilst manoeuvring. The symptoms are a pitch nose up and a roll to the retreating side (left in an R22) and the recovery (although it will come out itself as the speed washes off due to the pitch up) is to lower the collective and reduce the severity of the manoeuvre (unload the cyclic).
So in answer to your question - yes you can get rotor stall at normal RRPM and within engine limits but only by mishandling the aircraft. Note that unlike a FW, you have only stalled a part of your lifting area, not all of it.
Aerospatiale (now EC) always maintained that jackstall (servo transparency) was designed in to provide early warning of RBS - the symptoms and recovery are identical.
Sustained G in a helo is down to engine power - a 2G manoeuvre (60 degree level banked turn) is usually started at a higher speed (120 kts for example) and will settle at about 80kts (in Gazelle and Lynx) with almost max continuous Tq.
As Nick said, loops and the like are achieved within 2.5G and it is only on the pull up (high speed) and recovery (accelerating) that you have to be careful as the higher speed means you are much closer to RBS when you load the disc.
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Well thats just partial rotor stall though (RBS). During almost all flight regimes, a portion of the disc is stalled. So thats not really what I meant.
I meant stall to the point where there is no recovery.
Good reply though
I meant stall to the point where there is no recovery.
Good reply though
