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#21
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You're right - the elevator produces lift (same direction as the wings) at
low speeds, not at high speeds. Got mixed up. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de om... "Bert Willing" wrote in message ... That doesn't make sense to me. At high speeds, the elevator produces lift so in case of structural failure, the bits would go upwards. -- Bert Willing ASW20 "TW" Bert, The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). |
#22
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Bert,
It has little to do with airspeed. The position of the CG will determine the force on the elevator. "Bert Willing" wrote in message ... You're right - the elevator produces lift (same direction as the wings) at low speeds, not at high speeds. Got mixed up. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de om... "Bert Willing" wrote in message ... That doesn't make sense to me. At high speeds, the elevator produces lift so in case of structural failure, the bits would go upwards. -- Bert Willing ASW20 "TW" Bert, The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). |
#23
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Not quite correct. At high angles of attack, the elevator produces lift and
at of angle of attack, it produces negative lift. The crossover (i.e. zero lift, minimum drag) is a design criterium and is usually placed at the max L/D angle of attack. But then, this will of course be influenced by a large variation of the CG. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de . com... Bert, It has little to do with airspeed. The position of the CG will determine the force on the elevator. "Bert Willing" wrote in message ... You're right - the elevator produces lift (same direction as the wings) at low speeds, not at high speeds. Got mixed up. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de om... "Bert Willing" wrote in message ... That doesn't make sense to me. At high speeds, the elevator produces lift so in case of structural failure, the bits would go upwards. -- Bert Willing ASW20 "TW" Bert, The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). |
#24
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"Bert Willing" wrote in message ...
You're right - the elevator produces lift (same direction as the wings) at low speeds, not at high speeds. Got mixed up. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de om... "Bert Willing" wrote in message ... That doesn't make sense to me. At high speeds, the elevator produces lift so in case of structural failure, the bits would go upwards. -- Bert Willing ASW20 "TW" Bert, The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). Ok, so let me see if I've got this straight now. I cruising along at 60 kts in trim and elevator close to neutral. I want to go 140kts so I push the stick forward, the elevator goes down, which pushes the tail up, which pushes the nose goes down, I go faster. And all this is because the elevator is producing more lift in the downward direction? For a fixed stab with moving elevator don't we have to consider the forces on both components separately to predict the failure mode? Andy |
#25
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Andy Durbin wrote:
"Bert Willing" wrote in message ... You're right - the elevator produces lift (same direction as the wings) at low speeds, not at high speeds. Got mixed up. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de om... "Bert Willing" wrote in message ... That doesn't make sense to me. At high speeds, the elevator produces lift so in case of structural failure, the bits would go upwards. -- Bert Willing ASW20 "TW" Bert, The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). Ok, so let me see if I've got this straight now. I cruising along at 60 kts in trim and elevator close to neutral. I want to go 140kts so I push the stick forward, the elevator goes down, which pushes the tail up, which pushes the nose goes down, I go faster. And all this is because the elevator is producing more lift in the downward direction? For a fixed stab with moving elevator don't we have to consider the forces on both components separately to predict the failure mode? I think we are confusing transient and steady forces here. The overspeed problem occurs with the controls almost in neutral, but the plane in a dive, where the speed builds up steadily. In this case the tailplane will be generating an increasing downward force in relation to the longitudinal axis of the aircraft. This downward force is to counteract the forward rotation force generated by the wing. At a high enough speed these forces will increase beyond the capacity of the structure to support them. The transient case is when a large control excursion is input at high speed, and in this case the force on the tailplane could be in either direction, depending on the direction of control input. However downward total force is likely to be more severe in a pull-up than an upward force in a push-over, since the contribution of the elevator adds to the existing downward force in the first case and subtracts from it in the push-over case. That's my 2c worth... Cheers, John G. |
#26
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Andy Durbin wrote:
The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). Ok, so let me see if I've got this straight now. I cruising along at 60 kts in trim and elevator close to neutral. I want to go 140kts so I push the stick forward, the elevator goes down, which pushes the tail up, which pushes the nose goes down, I go faster. And all this is because the elevator is producing more lift in the downward direction? It is confusing! Here's what happens, simplified: *The horizontal stabilizer (with the flap we call the "elevator") is pushing down (at least at "higher" speeds - maybe not at 60 knots - dependes on the glider) *You push the stick forward *the elevator flap goes down *this _reduces_ the downward force of the horizontal stabilizer, but doesn't elimanate it *this allows the tail to rise There is more to it than that, of course. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#27
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Bert,
I'm sorry, you are not correct or not expressing yourself correctly. I'm not expressing an oppinion here, I'm telling you how it is. The CG is by design located ahead of the Neutral Point of the glider (otherwise the glider would be uncontrollable), so, it has the overall tendency to push the nose of the glider down, since the Neutral Point is the place where all the Lift is acting. To counteract this force, the tail planes are always pushing the tail DOWN, thus keeping the forces balanced. We vary the amount of down force produced by the tail planes by moving the elevator with the stick. When we move the CG aft, we bring it closer to the Neutral Point, which reduced the required down force produced by the elevator. In extremeley aft CG situations, the tailplanes MAY IN FACT produce an overall UP force on the tail, but this is the exception, rather than the rule. The more forward the CG position, however, the more DOWN force is necessary on the tail. This is the very reason pilots try to place the CG aft in competition gliders : so that the elevator doesn't have to produce quite so much DOWN force on the tail. The result is improved climb because of this. "Bert Willing" wrote in message ... Not quite correct. At high angles of attack, the elevator produces lift and at of angle of attack, it produces negative lift. The crossover (i.e. zero lift, minimum drag) is a design criterium and is usually placed at the max L/D angle of attack. But then, this will of course be influenced by a large variation of the CG. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de . com... Bert, It has little to do with airspeed. The position of the CG will determine the force on the elevator. "Bert Willing" wrote in message ... You're right - the elevator produces lift (same direction as the wings) at low speeds, not at high speeds. Got mixed up. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de om... "Bert Willing" wrote in message ... That doesn't make sense to me. At high speeds, the elevator produces lift so in case of structural failure, the bits would go upwards. -- Bert Willing ASW20 "TW" Bert, The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). |
#28
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On Wed, 16 Jun 2004 09:14:13 GMT, "Gldcomp"
wrote: Bert, I'm sorry, you are not correct or not expressing yourself correctly. I'm not expressing an oppinion here, I'm telling you how it is. Sorry to intrude, but Bert is correct. The induced drag of a low aspect-ratio horizontal stab is considerable, therefore the designer tries to minimize it at the speed of max L/D - since L/D is still the main number to characterize the performanc eof a glider, this is the number that needs to be maximized. The only case where induced drag is 0 is when the tail does not create any Cl at all. Situation at low speeds: Don't forget that the center of pressure (CP) moves forward with rising AoA, creating a nose-up momentum - and this needs to be encountered by the tail (wich is therefore creating lift at speeds below the speed of max. L/D). And vice versa. The more forward the CG position, however, the more DOWN force is necessary on the tail. This is the very reason pilots try to place the CG aft in competition gliders : so that the elevator doesn't have to produce quite so much DOWN force on the tail. The result is improved climb because of this. The aerodynamical benefit of an aft CG is the fact that the tail airfoil with upwards deflected elevator has got an extremely bad L/D due to its negative camber. Less upwards elevator deflection (due to aft CG) drastically improves the L/D of the tail. Bye Andreas |
#29
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If you had made your statements for powered aircrafts, you would be
completely right. However, the design criterons for sailplanes are slightly different because the airfoils used are different (well, today's airfoils anyways). The forces produced by the tailplane are fixed by the pitching coefficient of the airfoil in the first place. At high angles of attack, the airfoil pitches up which has to be compensated by a lift vector on the tailplane. As you need to have the CG in front of the neutral point for stability reasons, it reduces the required lift on the tailplane as the CG moves forward. To use your words, this is not an opinion. A good textbook wood be "Concept and design of Sailplanes" by Fred Thomas (the wording of the title may vary as I just know the German title). -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de . com... Bert, I'm sorry, you are not correct or not expressing yourself correctly. I'm not expressing an oppinion here, I'm telling you how it is. The CG is by design located ahead of the Neutral Point of the glider (otherwise the glider would be uncontrollable), so, it has the overall tendency to push the nose of the glider down, since the Neutral Point is the place where all the Lift is acting. To counteract this force, the tail planes are always pushing the tail DOWN, thus keeping the forces balanced. We vary the amount of down force produced by the tail planes by moving the elevator with the stick. When we move the CG aft, we bring it closer to the Neutral Point, which reduced the required down force produced by the elevator. In extremeley aft CG situations, the tailplanes MAY IN FACT produce an overall UP force on the tail, but this is the exception, rather than the rule. The more forward the CG position, however, the more DOWN force is necessary on the tail. This is the very reason pilots try to place the CG aft in competition gliders : so that the elevator doesn't have to produce quite so much DOWN force on the tail. The result is improved climb because of this. "Bert Willing" wrote in message ... Not quite correct. At high angles of attack, the elevator produces lift and at of angle of attack, it produces negative lift. The crossover (i.e. zero lift, minimum drag) is a design criterium and is usually placed at the max L/D angle of attack. But then, this will of course be influenced by a large variation of the CG. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de . com... Bert, It has little to do with airspeed. The position of the CG will determine the force on the elevator. "Bert Willing" wrote in message ... You're right - the elevator produces lift (same direction as the wings) at low speeds, not at high speeds. Got mixed up. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de om... "Bert Willing" wrote in message ... That doesn't make sense to me. At high speeds, the elevator produces lift so in case of structural failure, the bits would go upwards. -- Bert Willing ASW20 "TW" Bert, The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). |
#30
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Had a look at Amazon: The Fundamentals of Sailplane Design, Fred Thomas &
Judah Milgram -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de . com... Bert, I'm sorry, you are not correct or not expressing yourself correctly. I'm not expressing an oppinion here, I'm telling you how it is. The CG is by design located ahead of the Neutral Point of the glider (otherwise the glider would be uncontrollable), so, it has the overall tendency to push the nose of the glider down, since the Neutral Point is the place where all the Lift is acting. To counteract this force, the tail planes are always pushing the tail DOWN, thus keeping the forces balanced. We vary the amount of down force produced by the tail planes by moving the elevator with the stick. When we move the CG aft, we bring it closer to the Neutral Point, which reduced the required down force produced by the elevator. In extremeley aft CG situations, the tailplanes MAY IN FACT produce an overall UP force on the tail, but this is the exception, rather than the rule. The more forward the CG position, however, the more DOWN force is necessary on the tail. This is the very reason pilots try to place the CG aft in competition gliders : so that the elevator doesn't have to produce quite so much DOWN force on the tail. The result is improved climb because of this. "Bert Willing" wrote in message ... Not quite correct. At high angles of attack, the elevator produces lift and at of angle of attack, it produces negative lift. The crossover (i.e. zero lift, minimum drag) is a design criterium and is usually placed at the max L/D angle of attack. But then, this will of course be influenced by a large variation of the CG. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de . com... Bert, It has little to do with airspeed. The position of the CG will determine the force on the elevator. "Bert Willing" wrote in message ... You're right - the elevator produces lift (same direction as the wings) at low speeds, not at high speeds. Got mixed up. -- Bert Willing ASW20 "TW" "Gldcomp" a écrit dans le message de om... "Bert Willing" wrote in message ... That doesn't make sense to me. At high speeds, the elevator produces lift so in case of structural failure, the bits would go upwards. -- Bert Willing ASW20 "TW" Bert, The elevator does produce lift, but in the opposite direction as the wings (most of the time anyway). |
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