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Autorotation ? R22 for the Experts
Here is a good question for the more experienced helicopter pilot or
mechanic. A friend of mine that flies a late model Robison R22 Beta II asked this question of a number of individuals and no one seem to have an explanation. This included people at the manufacturer. I believe I know the answer, however, I would like to put the question out and see what others think before tainting their thoughts with my ideas. So here is the question: With one person flying the R22 (170lb), simulating an autorotation by rolling off throttle, the collective goes full down and pretty-much stays there to maintain proper rotor rpm. With two people, same maneuver, some collective has to be pulled to keep the main rotor from over-speeding. The amount of collective will obviously vary dependent on a number of factors, but for this example let's say about 1 inch off the stop. One more time with the same maneuver, with two on board, except let the governor control engine speed, and not rolling off throttle. In order to maintain rotor speed the collective has to be on the stop. The question is why? Please post your thoughts. Thanks, Eric D |
#2
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In the first instance, you are not "simulating" an autorotation; with no
engine OR no engine power applied to the rotor (throttle rolled off) you are "performing" an autorotation. With the engine throttle left in a flight position, the governor is GOVERNING rotor R.P.M. due to it's linkage to the collective and the engine which is what under any circumstances what it ACTUALLY GOVERNS, and ONLY THEREFORE can govern your rotor R.P.M. SO...collective bottomed...engine "running", governor GOVERNS rotor R.P.M. NO engine running (or no throttle applied) YOU govern rotor R.P.M. The other variables you alluded to will include gross weight per fuel on board, etc. Bob |
#3
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"Bob" wrote in message ...
In the first instance, you are not "simulating" an autorotation; with no engine OR no engine power applied to the rotor (throttle rolled off) you are "performing" an autorotation. With the engine throttle left in a flight position, the governor is GOVERNING rotor R.P.M. due to it's linkage to the collective and the engine which is what under any circumstances what it ACTUALLY GOVERNS, and ONLY THEREFORE can govern your rotor R.P.M. SO...collective bottomed...engine "running", governor GOVERNS rotor R.P.M. NO engine running (or no throttle applied) YOU govern rotor R.P.M. The other variables you alluded to will include gross weight per fuel on board, etc. Bob One pob = collective fully down to maintain rotor speed. Two pob = collective not fully down to prevent overspeed. If helicopter is heavier will not the coning angle be greater and therefore the rotor speed higher because of less effective rotor diameter. Hence the need to pull some collective at heavier weight to slow blades and increase rotor disk diameter? Or maybe I'm talking b****x! It has been known!! Herbie |
#4
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Herbie...
YES. BUT...that wasn't the question. The question was given weight-throttle on vs. given weight-throttle off. Bob |
#5
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And come to think of it...actually NO. Collective down, there's no cone to
speak of or your rotor speed would be drooping. |
#6
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"Bob" wrote in message news Herbie... YES. BUT...that wasn't the question. The question was given weight-throttle on vs. given weight-throttle off. Bob I'm not sure I'm getting this either. The original poster was confused because, engine out of the equation, they had to "raise" the collective to prevent an overspeed. With the engine / governor still in the equation, they "didn't" have to raise the collective to prevent an overspeed. Now, if the collective is bottomed out and the MR blades are at an autorotative angle, wouldn't the rotor system try to outrun the engine / drive system? Or, is the engine acting as a brake in the situation, kind of like a car coasting downhill, using engine compressionas a brake? Just wondering? Fly Safe, Steve R. |
#7
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"Bob" wrote in message ... And come to think of it...actually NO. Collective down, there's no cone to speak of or your rotor speed would be drooping. No cone? I'm not sure I agree with that. Even with the collective all the way down, there's still positive lift being generated by the rotor system. Maybe the coneing angle isn't what it would be in steady state, powered flight but I'm not sure it's accurate to say that there's "no" coning with the collective full down. Fly Safe, Steve R. |
#8
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Steve...
Kindly read my post carefully. I said NO CONE TO SPEAK OF. As in...it would not to compare to the cone present when trying to yank the helicopter off the ground at Max Gross Weight. The lift generated with the collective down is what keeps your rate of decent an "autorotative" rate, rather at terminal velocity As to your other post...when the collective is down with no engine power (on a real helicopter) the pitch links are adjusted to keep the auto-R.P.M. within an acceptable RANGE. That is a standard maintenance function, but it dosn't mean the pilot can go to sleep during an auto. Collective down, if rpm is too high- it's not adjusted properly, but the pilot can jockey the collective. If its too low it's DEFINATELY not adjusted properly and rots o' ruck. In my first post in this string ALL I was trying (apparently not too clearly) to explain is that if the engine is running (at all) the governor knows it. I am a heavy helicopter maintenance type and can't even spell Robinson, BUT even the most "rudimentary" if you will, helicopters have to operate with some derivation of dynamic principles and proven hardware concept application. The bottom line is in an auto, to keep the rotor in the green right off. Too High is always better than Too Low because the pilot can fix that. Good luck, Bob |
#9
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You will have almost as much blade cone in a STEADY auto rotation as
in hover. The cone angle is the tangent of the vertical force vector verses the centrifugal l force vector generated by the rotation of the blades. The only force not supported by the blades in auto rotation is the force generated upward on the body of the helicopter by the vertical wind component. Due to this you will still have 90% or more of the cone angle when in hover. You can reduce the cone angle by speeding up the RPM of the blades since the centrifugal force generated depends on the square of the RPM. Most people keep the blade RPM at the top of the green in auto's anyway. John On Sun, 29 Feb 2004 15:14:24 GMT, "Bob" wrote: Steve... Kindly read my post carefully. I said NO CONE TO SPEAK OF. As in...it would not to compare to the cone present when trying to yank the helicopter off the ground at Max Gross Weight. The lift generated with the collective down is what keeps your rate of decent an "autorotative" rate, rather at terminal velocity As to your other post...when the collective is down with no engine power (on a real helicopter) the pitch links are adjusted to keep the auto-R.P.M. within an acceptable RANGE. That is a standard maintenance function, but it dosn't mean the pilot can go to sleep during an auto. Collective down, if rpm is too high- it's not adjusted properly, but the pilot can jockey the collective. If its too low it's DEFINATELY not adjusted properly and rots o' ruck. In my first post in this string ALL I was trying (apparently not too clearly) to explain is that if the engine is running (at all) the governor knows it. I am a heavy helicopter maintenance type and can't even spell Robinson, BUT even the most "rudimentary" if you will, helicopters have to operate with some derivation of dynamic principles and proven hardware concept application. The bottom line is in an auto, to keep the rotor in the green right off. Too High is always better than Too Low because the pilot can fix that. Good luck, Bob |
#10
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The string starts with a guy asking a question of a pilot or a mechanic. I,
being a longtime mechanic, try to answer his questionand stay ON TOPIC. The guy, and his original question, disappears! No offense to John, but we end up with a physicist. Start a new string...pullleeeeez!!!! Bob |
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