Elevator-Downward or upward force?
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Elevator-Downward or upward force?
Suppose we have an elevator.Lets say of a C172.In the manual I read that there is a negative angle of incidence for the horizontal stabilizer.Probably in order to have a downward force to counteract the upward moment of lift.
I tried to take a close look at the elevator of the cessna that I fly,and noticed that it is a symmetrical wing.Or it looks like.
In the theory,I read that with the downward deflection of the elevator,the downward force will be reduced,and we may have an upward force.
My question:is it possible the downward force exerted by the elevator to be inverted and become an upward force?Even though the deflection is big enough.
Consider an horizontal stabilizer.
I tried to take a close look at the elevator of the cessna that I fly,and noticed that it is a symmetrical wing.Or it looks like.
In the theory,I read that with the downward deflection of the elevator,the downward force will be reduced,and we may have an upward force.
My question:is it possible the downward force exerted by the elevator to be inverted and become an upward force?Even though the deflection is big enough.
Consider an horizontal stabilizer.
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yes it can be. Its just that in cruise it is the counter to the lift moment so is negative.
Transitory application of control input may get it being positive lift. Wouldn't have thought though it would happen very often in a C172. Aerobatic aircraft more often .
Transitory application of control input may get it being positive lift. Wouldn't have thought though it would happen very often in a C172. Aerobatic aircraft more often .
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So,in a cessna the more we can get is to reduce the downward force cause by the downward deflection of elevator?Or it can be inverted if we deflect it too much?
Thanks for your answer!
Thanks for your answer!
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Depends on what your doing.
Fully forward on the stick and it will produce lift to the limit that the designers deem to be the max the airframe will take or they think you will require. They may stipulate a max air speed which you may use the full range of the controls.
When you do your full and free look at the tail and see the range of movement. If the chord is pointing down it will producing negative lift and if its pointing up its positive.
Fully forward on the stick and it will produce lift to the limit that the designers deem to be the max the airframe will take or they think you will require. They may stipulate a max air speed which you may use the full range of the controls.
When you do your full and free look at the tail and see the range of movement. If the chord is pointing down it will producing negative lift and if its pointing up its positive.
There was an interesting discussion here concerning tailplane lift (downforce) and stability.
Should answer most questions about this.
Basically, there is a couple between lift and c of g, with c of g being forward of the c of p. The down-force provided by the tail balances the couple. This provides longitudinal stability. As the airspeed changes, the c of p (for practical purposes) moves only very little* vs the amount of lift - or negative lift - provided by the tailplane, which (like the wing) increases/decreases as a function of the square of the speed (among other things).
What that means is that airspeed is reduced, the amount of negative lift provided by the tail decreases, and the nose will try and pitch down. (C of g doesn't change, so can assumed to be constant.) Vice versa if the nose drops, or is pushed down, and the airspeed increases.
You've heard of phugoid oscillations? The more longitudinal stability the more these will be self- regulating; the aircraft returning to its' trimmed airspeed within only a few oscillations.
When the c of g is at the aft limit, longitudinal stability is reduced, but still present. If c of g is sufficiently aft of the aft limit, a situation arises where there is no longitudinal stability, and the aircraft, left uncorrected will, basically, crash.
I've read a flying report written by someone who flew a Baron severely aft loaded. It was an extreme handful. He calculated that the c of g was way outside the aft limit afterwards. I forget the figure, but it was something around 12 or more inches.
* The C of P moves forward on the main-plane as angle of attack is increased, to the point of the stall, when it moves aft. The value is of the order of inches. This also tends to be pro-longitudinal stability. It's a significant value, but still quite minor in comparison to the changing down-force produced by the tailplane in a correctly loaded aircraft. The tail has a large moment to work through, being way back there, so even if the actual lift/negative lift value is small, it's effective.
Should answer most questions about this.
Basically, there is a couple between lift and c of g, with c of g being forward of the c of p. The down-force provided by the tail balances the couple. This provides longitudinal stability. As the airspeed changes, the c of p (for practical purposes) moves only very little* vs the amount of lift - or negative lift - provided by the tailplane, which (like the wing) increases/decreases as a function of the square of the speed (among other things).
What that means is that airspeed is reduced, the amount of negative lift provided by the tail decreases, and the nose will try and pitch down. (C of g doesn't change, so can assumed to be constant.) Vice versa if the nose drops, or is pushed down, and the airspeed increases.
You've heard of phugoid oscillations? The more longitudinal stability the more these will be self- regulating; the aircraft returning to its' trimmed airspeed within only a few oscillations.
When the c of g is at the aft limit, longitudinal stability is reduced, but still present. If c of g is sufficiently aft of the aft limit, a situation arises where there is no longitudinal stability, and the aircraft, left uncorrected will, basically, crash.
I've read a flying report written by someone who flew a Baron severely aft loaded. It was an extreme handful. He calculated that the c of g was way outside the aft limit afterwards. I forget the figure, but it was something around 12 or more inches.
* The C of P moves forward on the main-plane as angle of attack is increased, to the point of the stall, when it moves aft. The value is of the order of inches. This also tends to be pro-longitudinal stability. It's a significant value, but still quite minor in comparison to the changing down-force produced by the tailplane in a correctly loaded aircraft. The tail has a large moment to work through, being way back there, so even if the actual lift/negative lift value is small, it's effective.
Last edited by Tarq57; 7th May 2013 at 19:37. Reason: mistake. correction in bold.
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Couple of points to add.
The downwards lift generated by the tailplane obviously also generates induced drag. Aircraft that are loaded towards the aft limit will be less stable (as stated) but will also have less induced drag and are thus more fuel efficient. Aircraft such as the A380 (I think - not quite sure) automatically transfer fuel aft when the onboard 'puters sense that the aircraft is in a stable cruise and on autopilot, and automatically transfer fuel forward when the aircraft starts its descent to land. All in an effort to reduce induced drag.
Furthermore, it is possible to have the tailplane providing positive lift instead of negative, but in order to maintain the stabilizing effect, this only works if the angle of incidence is lower than the angle of incidence of the main wings. Or something along those lines. It's a very tricky balance and obviously the CofG has to be just right to do so.
The downwards lift generated by the tailplane obviously also generates induced drag. Aircraft that are loaded towards the aft limit will be less stable (as stated) but will also have less induced drag and are thus more fuel efficient. Aircraft such as the A380 (I think - not quite sure) automatically transfer fuel aft when the onboard 'puters sense that the aircraft is in a stable cruise and on autopilot, and automatically transfer fuel forward when the aircraft starts its descent to land. All in an effort to reduce induced drag.
Furthermore, it is possible to have the tailplane providing positive lift instead of negative, but in order to maintain the stabilizing effect, this only works if the angle of incidence is lower than the angle of incidence of the main wings. Or something along those lines. It's a very tricky balance and obviously the CofG has to be just right to do so.
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Rear stabilisers produce lift not downforce
I heard a story that I'm still not sure I believe despite the fact I want to.
if you look at a stabiliser on most ac it is rigged at a positive angle of attack. It produces lift not downforce. The story instructors tell about downforce on the tail plane is a convenient way to explain longitudinal stability.
When we draw the lift weight couple it shows the net lift from both surfaces ( main and tail) together not separately. If you draw them separately the main lift is forward of the weight and the lift from the tail stabilises it.
I've never seen a diagram of this and it was so contrary to what I was taught I was in serious denial at the time. However I can't look at aircraft and see negative stabiliser pitch.
This came to me from David Schouler so I'm thinking there's some truth in it. However if I heard it wrong in a fug of classroom overload I apologise for bringing it up.
Anyone else heard this theory?
if you look at a stabiliser on most ac it is rigged at a positive angle of attack. It produces lift not downforce. The story instructors tell about downforce on the tail plane is a convenient way to explain longitudinal stability.
When we draw the lift weight couple it shows the net lift from both surfaces ( main and tail) together not separately. If you draw them separately the main lift is forward of the weight and the lift from the tail stabilises it.
I've never seen a diagram of this and it was so contrary to what I was taught I was in serious denial at the time. However I can't look at aircraft and see negative stabiliser pitch.
This came to me from David Schouler so I'm thinking there's some truth in it. However if I heard it wrong in a fug of classroom overload I apologise for bringing it up.
Anyone else heard this theory?
I've heard it, but don't necessarily believe it.
When you look at tailplane incidence, it might indeed appear positive. Add to that the downwash from the mainplane, it might then become negative.
Look at the tail/stabilizer of almost any jet airliner. The side with the most curvature is always the underside.
Look at a Cherokees' all moving tailplane. Note the incidence at full down vs full up. That should tell you a lot about where the lift vs weight moment is.
When you look at tailplane incidence, it might indeed appear positive. Add to that the downwash from the mainplane, it might then become negative.
Look at the tail/stabilizer of almost any jet airliner. The side with the most curvature is always the underside.
Look at a Cherokees' all moving tailplane. Note the incidence at full down vs full up. That should tell you a lot about where the lift vs weight moment is.
The story instructors tell about downforce on the tail plane is a convenient way
to explain longitudinal stability.
to explain longitudinal stability.
Go to AH&N and look at the thread "A26 Upkeep Crash" and see what occurred when the tail came off.
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Two things worth mentioning, one is that the tail is indeed a net downforce (In a straight and level situation), Backpackers point is dead right, an aft c of g means the tail needs todo less work, less induced drag giving better fuel consumption. The other thing to say, more generally, is to not think about elevator deflection, but relative wind, as that is the true cause of lift, or otherwise, and what is actually happening. The static angle of incidence is only of note if the relative wind is from dead ahead.
The stability point doesnt really relate to downforce, it would be true if it were an upforce too, the stability explanation is more about moments produced by the tail to oppose the disturbance, and where the planes c of g is relative to the tail. An aft c og g means the tail is less effective stability wise as the distance betewwen te tailforce and pivot (cofg) is reduced, so the restorative moment is smaller
The stability point doesnt really relate to downforce, it would be true if it were an upforce too, the stability explanation is more about moments produced by the tail to oppose the disturbance, and where the planes c of g is relative to the tail. An aft c og g means the tail is less effective stability wise as the distance betewwen te tailforce and pivot (cofg) is reduced, so the restorative moment is smaller
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[IMG]
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Longitudinal stability. With the Centre of Pressure behind the Centre of gravity, the force from the tailplane must act downwards to counteract the pitch down moment. If the aircraft then pitches down, the (negative) angle of attack of the tailplane increases, so the downward force increases causing the nose to pitch back up. Converse is true for an initial pitch up. Thus the aircraft is longitudinally stable. If it deviates from a given pitch attitude, the tendency will be for it to return to that attitude.
Put the CoP ahead of the CoG and the force from the tailplane now has to act upwards. Sounds good - you now need a smaller wing. But if the aircraft pitches down, angle of attack of the tailplane increases, upward force on tailplane increases, aircraft pitches further down. So aircraft is longitudinally [U]unstable[U].
Unstable configuration can be made to work, but you need a very clever computer between the stick and the elevator!
[/IMG]Longitudinal stability. With the Centre of Pressure behind the Centre of gravity, the force from the tailplane must act downwards to counteract the pitch down moment. If the aircraft then pitches down, the (negative) angle of attack of the tailplane increases, so the downward force increases causing the nose to pitch back up. Converse is true for an initial pitch up. Thus the aircraft is longitudinally stable. If it deviates from a given pitch attitude, the tendency will be for it to return to that attitude.
Put the CoP ahead of the CoG and the force from the tailplane now has to act upwards. Sounds good - you now need a smaller wing. But if the aircraft pitches down, angle of attack of the tailplane increases, upward force on tailplane increases, aircraft pitches further down. So aircraft is longitudinally [U]unstable[U].
Unstable configuration can be made to work, but you need a very clever computer between the stick and the elevator!
Last edited by Tigger_Too; 2nd May 2013 at 11:40.
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Hi, if you are an aero designer (or easier still an aero-modeller.) you can put your front wing on the airplane in any position you want.
You can get the Centre of Pressure to coincide with the CofG, and the tailplane would need no angle of attack at all.
You could put the CofP in front of the CofG, and you would then need a lifting tailplane.
However for best Stability it has been proved, since the days of Wilbur and Orvile, that CofP behind CofG is best. This results in the tailplane having about 2-3 degrees less angle of attack that the Mainplane.
It also means that the Tailplane does not normally stall before the Mainplane, which could result in something like a tail-slide stall.
You can get the Centre of Pressure to coincide with the CofG, and the tailplane would need no angle of attack at all.
You could put the CofP in front of the CofG, and you would then need a lifting tailplane.
However for best Stability it has been proved, since the days of Wilbur and Orvile, that CofP behind CofG is best. This results in the tailplane having about 2-3 degrees less angle of attack that the Mainplane.
It also means that the Tailplane does not normally stall before the Mainplane, which could result in something like a tail-slide stall.
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And if you do not believe the explanations above, here is simple proof.
Get a pilot and pax in your (nose-wheeled aircraft).
Chop off the nosewheel --> nose goes down and hits the ground. Why else would we need a nosewheel?
Put a fat bloke on the tailplane --> voila, straight and level attitude.
Do not forget to put the nosewheel back on afterwards... :-)
Get a pilot and pax in your (nose-wheeled aircraft).
Chop off the nosewheel --> nose goes down and hits the ground. Why else would we need a nosewheel?
Put a fat bloke on the tailplane --> voila, straight and level attitude.
Do not forget to put the nosewheel back on afterwards... :-)
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This topic came up on a different forum recently, with some really persuasive writing and math supporting the possibility of the tail providing lift [nose down direction] in some C of G configurations. I'm not persuaded yet.
To support the theme that Cobalt and others have presented, if there were a configuration where in stable flight the tail lifted up, rather than down, as convention has it, how would you rotate for takeoff in a tricycle aircraft? The tail would already be on the ground! A Cessna Caravan, as other types has a "pogo stick" for loading when parked, but not 'cause you need it for taxi takeoff and landing!
I remain convinced that in sable flight, in a conventional aircraft, the tail always provides a downforce to some degree. I have flown Cessnas a bit behind the aft C of G limit, and they still required a nose up control input to rotate on takeoff, because they did not rotate themselves - happily!
To support the theme that Cobalt and others have presented, if there were a configuration where in stable flight the tail lifted up, rather than down, as convention has it, how would you rotate for takeoff in a tricycle aircraft? The tail would already be on the ground! A Cessna Caravan, as other types has a "pogo stick" for loading when parked, but not 'cause you need it for taxi takeoff and landing!
I remain convinced that in sable flight, in a conventional aircraft, the tail always provides a downforce to some degree. I have flown Cessnas a bit behind the aft C of G limit, and they still required a nose up control input to rotate on takeoff, because they did not rotate themselves - happily!
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I would imagine that if you have an aircraft with a permanent positive-lift tailplane, the main wheels (assuming a nosewheel configuration, and not a tailwheel aircraft) would be rather further backwards and not roughly below the center of pressure (CEP) of the main wings. After all, a positive-lift tailplane means the CofG is behind the CEP of the main wings, and if you put the main wheels roughly at the CEP of the main wings, the aircraft will fall over backwards.
But I don't think airplanes exist that have their CofG permanently behind the CEP of the main wings. For starters, that would mean they would have an extremely narrow CofG range, so they would not be exactly practical.
To the best of my knowledge, the A380 is loaded on the ground, takes off and performs the initial climb just like any other aircraft, with a tailplane providing negative lift. So the CofG is in front of the CEP of the main wings, and in front of the main wheels. Only once it's in stable cruise, or maybe at the last stages of the cruise climb, is the fuel transferred aft and will the tailplane reach some measure of positive incidence/lift - with the CofG slightly aft of the CEP of the main wings. My gut feeling is actually that the fuel transfer is done until a precise positive incidence of the tailplane is reached. After all, if the positive incidence of the tailplane would exceed a certain value (which is more or less the incidence of the main wings), the inherent dynamic stability would be lost.
Well before landing (I think once the cruise altitude is left), the fuel will be transferred forward until a normal situation, with negative lift of the tailplane, is reached again.
(And if that final fuel transfer fails, or is forgotten, I would imagine the landing and rollout would be rather spectacular - imagine an A380 sitting on its tail...)
But I don't think airplanes exist that have their CofG permanently behind the CEP of the main wings. For starters, that would mean they would have an extremely narrow CofG range, so they would not be exactly practical.
To the best of my knowledge, the A380 is loaded on the ground, takes off and performs the initial climb just like any other aircraft, with a tailplane providing negative lift. So the CofG is in front of the CEP of the main wings, and in front of the main wheels. Only once it's in stable cruise, or maybe at the last stages of the cruise climb, is the fuel transferred aft and will the tailplane reach some measure of positive incidence/lift - with the CofG slightly aft of the CEP of the main wings. My gut feeling is actually that the fuel transfer is done until a precise positive incidence of the tailplane is reached. After all, if the positive incidence of the tailplane would exceed a certain value (which is more or less the incidence of the main wings), the inherent dynamic stability would be lost.
Well before landing (I think once the cruise altitude is left), the fuel will be transferred forward until a normal situation, with negative lift of the tailplane, is reached again.
(And if that final fuel transfer fails, or is forgotten, I would imagine the landing and rollout would be rather spectacular - imagine an A380 sitting on its tail...)
Last edited by BackPacker; 7th May 2013 at 10:19.