Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: earl1937 on December 28, 2015, 05:16:54 AM
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:airplane: There has been a "lively" discussion about stalls with the two posts regarding the 262!
Here is another point to consider in stalls: There is only a certain of amount of "travel" in the C.G. of any aircraft! When is the aircraft the most difficult to recover from a "stalled" condition, When the C.G. is at its most forward limit or when it is at its most rearward limit?
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Keep in mind that I know little to nothing about the mechanics of aerodynamics so this is pretty much a shot in the dark. In my mind it makes the most sense for the answer to be when the C.G. is at its most rearward limit. To me this would hurt forward momentum and more likely produce a flat spin or something similar.
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The center of lift moves forward on the wing as AOA increases so a rear CG would get worse.
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I know nothing... but I do know that RC flyers live in fear of a rearward CG way, way more than a forward one. Forward just makes the plane sluggish. Rearward makes it uncontrollable.
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If your cg is to far aft you won't be able to get your nose down to recover from a stall.
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An aircraft will become less recoverable as the CG moves aft. Realize there are a lot of other factors as well, but the deal is that a more forward CG makes the plane more recoverable, but the tradeoff is that you have to have the tail surfaces create more balancing force down to keep the plane in level flight - which creates more drag. So I could have 2 identical aircraft, and a CG placement just a few inches forward on one will make the AoA on the wing and the trim settings totally different to make it cruise level. Moving the CG a few inches aft makes the same aircraft cruise a bit faster, but the tradeoff there is that if the CG gets too far back then the plane tends towards getting into a flat spin (unrecoverable).
This is why you see aircraft burning off fuel tanks in a particular order - the CG changes as the fuel is burned off and the plane will handle differently. In the P-51 it was necessary to burn off the tank behind the pilot first to keep directional stability by moving the CG forward. You could fly it straight and level with the tank full, but if you pulled any wild maneuvers with that weight back there you were taking a risk. There are a couple of stories out there about pilots who burned the tanks off in the wrong order and managed to get home to tell the tale (maybe it was Yeager?).
Anyway - realize the whole thing is a balancing act between forces (thrust, lift, drag, control surfaces), requirements (payload, range), and performance. You want your fighter aircraft to be on the ragged edge of stability / instability because you win if your ride does something that the other guys ride can't do. I give a big <S> to the guys that flew those planes, but I also have the utmost respect for the guys that were designing them on paper and doing the math with a slide rule.
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An aircraft will become less recoverable as the CG moves aft. Realize there are a lot of other factors as well, but the deal is that a more forward CG makes the plane more recoverable, but the tradeoff is that you have to have the tail surfaces create more balancing force down to keep the plane in level flight - which creates more drag. So I could have 2 identical aircraft, and a CG placement just a few inches forward on one will make the AoA on the wing and the trim settings totally different to make it cruise level. Moving the CG a few inches aft makes the same aircraft cruise a bit faster, but the tradeoff there is that if the CG gets too far back then the plane tends towards getting into a flat spin (unrecoverable).
This is why you see aircraft burning off fuel tanks in a particular order - the CG changes as the fuel is burned off and the plane will handle differently. In the P-51 it was necessary to burn off the tank behind the pilot first to keep directional stability by moving the CG forward. You could fly it straight and level with the tank full, but if you pulled any wild maneuvers with that weight back there you were taking a risk. There are a couple of stories out there about pilots who burned the tanks off in the wrong order and managed to get home to tell the tale (maybe it was Yeager?).
Anyway - realize the whole thing is a balancing act between forces (thrust, lift, drag, control surfaces), requirements (payload, range), and performance. You want your fighter aircraft to be on the ragged edge of stability / instability because you win if your ride does something that the other guys ride can't do. I give a big <S> to the guys that flew those planes, but I also have the utmost respect for the guys that were designing them on paper and doing the math with a slide rule.
:airplane: Excellent Essay on why you want the C.G. slightly forward of center of C.G. on the "moment" arm!