Tailplane lift
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Tailplane lift
Talking recently with someone at work about tailplane lift. He reckoned that all aircraft (conventional) had tailplanes that exerted a downforce/negative lift. I had thought that some did, some didn't i.e. C of G could be either ahead of or aft of the mainplane.
He also thought that the tailplane would stall first to provide a nose drop at the stall. however I thought this was because of the C of lift moving back as the stall progresses, tailplane will be unstalled and fully useable during the stall.
I would welcome informed opinions.
(I know I could go and look it up, but the discussions are normally quite illuminating on PPRuNe, and I often learn more).
He also thought that the tailplane would stall first to provide a nose drop at the stall. however I thought this was because of the C of lift moving back as the stall progresses, tailplane will be unstalled and fully useable during the stall.
I would welcome informed opinions.
(I know I could go and look it up, but the discussions are normally quite illuminating on PPRuNe, and I often learn more).
Dog Tired
I was told by a Typhoon test pilot (what does he know?) that only the Typhoon tail produces lift.
What I do know is that my wonderful A330 stuffs 5 tonnes of fuel into the tail for the cruise to force the tail to produce less down-force (I didn't say 'lift'). This reduces the 'apparent all-up weight'. That reduces drag.
So, aft CG reduces drag.
It's got nothing to do with stalling.
QED
What I do know is that my wonderful A330 stuffs 5 tonnes of fuel into the tail for the cruise to force the tail to produce less down-force (I didn't say 'lift'). This reduces the 'apparent all-up weight'. That reduces drag.
So, aft CG reduces drag.
It's got nothing to do with stalling.
QED
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I don't have any data to back this up but my understanding is that to provide positive static and/or dynamic stability, most aircraft incorporate a design that has a tail downforce. I think generally that it wouldn't matter where the CG was located (either at the aft or forward limit) there would still be downforce on the tail. With an aft CG, however, there will be less downforce required to balance the aircraft and therefore you will have better cruise performance. Basically, the tail downforce acts like weight and has to be compensated for by lift (or a vertical component of thrust). To do that, you need to increase your angle of attack to create more lift which also creates more drag and slows you down. It's generally better to have a forward CG for recovery from unusual attitudes, ie: stalls or spins, and better to have an aft CG for cruise performance.
As you increase your angle of attack on the wings, your angle of attack on the tailplane decreases. You'll notice that as you decrease your speed towards the stall, you'll require more back elevator - it will be essentially an exponential increase in elevator deflection. This is partly to do with the decrease in speed (lift being a function of velocity squared) and partly to do with the decrease in angle of attack of the tailplane.
It is true that if you stall the tailplane, the nose will pitch down. With any stall, there is generally a buffeting action that occurs immediately prior to the stall. In a normal wing stall, you will feel the wings buffeting the whole aircraft. In a tailplane stall, the buffeting will mostly be felt through the control column. I would have to disagree that the tailplane is stalling first, causing the nose to drop. Of all the planes that I've stalled, I've never gotten the tailplane to even buffet. As a note, the tailplane stall recovery is completely opposite. To recover, you must pull back on the elevator assertively to break the stall.
So, what causes the nose down pitching moment at the point of stall? A lot of stuff.
1) The center of pressure will move backwards at the point of stall.
2) The amount of lift sharply changes - some people believe that lift drops off completely at the point of stall and that's why what happens at a stall happens. But, looking at a Cl vs AoA graph, you can see that that is not the case.
Take a look at this graph: http://classicairshows.com/Education...Airfoil2CL.gif
You can see that at the Clmax, the lift drops off sharply - but it's still making TONS of lift! This airfoil stalls at what looks like 22 degrees AoA. Even at 8 degrees beyond the Clmax, it's creating lots of lift. But that's not analyzing the whole picture and that's why you initially don't think that lift still is high.
http://i.imgur.com/cKpPK.png
I've drawn a red line where, if you were able to continually increase your AoA and maintain the same lift, the Cl vs AoA line would follow. Essentially, about 1 degree beyond the critical AoA, the green line shows how much lift you've lost. So instead of being at 23 degrees AoA with a coefficient of lift of 1.8, you're stalled with a coefficient of lift of 1.15. At 1 degree beyond the critical AoA, you've lost 36% of the lift you should have at that AoA. The blue line shows roughly 6 degrees beyond the critical AoA. Not only are you losing a significant amount of lift, looking at the drag line you can see that at the critical AoA the drag rises sharply and continues rising at a high rate beyond the critical AoA. So, at the point of stall you're losing a significant amount of lift and, with the significant increase in drag, you're slowing down quickly - this is how you 'fall out of the sky'!
Going back to my points...
3) When you stall, your wings are essentially useless at this moment (not quite, but work with me here!) and you'd actually be better off without them. So, imagine that you're flying at a high angle of attack in level flight and your wings vanish. Just strictly based on the CG of the airplane, it will want to point the nose down to the ground. Not only that, but you've got that tailplane still there and if the CG was completely neutral, it would be the one to make sure that the nose pointed directly at the earth. It's now a missile heading directly for the earth. But before it can get to that point, the wings reappear and create lift effectively, ensuring that you don't nose dive the earth. Going back to the previous picture, you can see that a very small AoA increase at the Clmax point can have a drastic effect on the lift and drag. The same can be said for a very small AoA decrease at a point beyond the Clmax. So just as soon as your nose starts dropping down towards the earth like a missile, the wings come back to work and ensure that you don't dive straight down! You'll notice that the deeper the stall you enter, the more nose down the airplane will go and the above analogy should explain why.
Not all airplanes will handle exactly like that but for the majority of airplanes, that's pretty much the way she works - as I understand it.
Hopefully I answered your questions. If you have more let me know!
If you find some inaccuracies here also let me know.
As you increase your angle of attack on the wings, your angle of attack on the tailplane decreases. You'll notice that as you decrease your speed towards the stall, you'll require more back elevator - it will be essentially an exponential increase in elevator deflection. This is partly to do with the decrease in speed (lift being a function of velocity squared) and partly to do with the decrease in angle of attack of the tailplane.
It is true that if you stall the tailplane, the nose will pitch down. With any stall, there is generally a buffeting action that occurs immediately prior to the stall. In a normal wing stall, you will feel the wings buffeting the whole aircraft. In a tailplane stall, the buffeting will mostly be felt through the control column. I would have to disagree that the tailplane is stalling first, causing the nose to drop. Of all the planes that I've stalled, I've never gotten the tailplane to even buffet. As a note, the tailplane stall recovery is completely opposite. To recover, you must pull back on the elevator assertively to break the stall.
So, what causes the nose down pitching moment at the point of stall? A lot of stuff.
1) The center of pressure will move backwards at the point of stall.
2) The amount of lift sharply changes - some people believe that lift drops off completely at the point of stall and that's why what happens at a stall happens. But, looking at a Cl vs AoA graph, you can see that that is not the case.
Take a look at this graph: http://classicairshows.com/Education...Airfoil2CL.gif
You can see that at the Clmax, the lift drops off sharply - but it's still making TONS of lift! This airfoil stalls at what looks like 22 degrees AoA. Even at 8 degrees beyond the Clmax, it's creating lots of lift. But that's not analyzing the whole picture and that's why you initially don't think that lift still is high.
http://i.imgur.com/cKpPK.png
I've drawn a red line where, if you were able to continually increase your AoA and maintain the same lift, the Cl vs AoA line would follow. Essentially, about 1 degree beyond the critical AoA, the green line shows how much lift you've lost. So instead of being at 23 degrees AoA with a coefficient of lift of 1.8, you're stalled with a coefficient of lift of 1.15. At 1 degree beyond the critical AoA, you've lost 36% of the lift you should have at that AoA. The blue line shows roughly 6 degrees beyond the critical AoA. Not only are you losing a significant amount of lift, looking at the drag line you can see that at the critical AoA the drag rises sharply and continues rising at a high rate beyond the critical AoA. So, at the point of stall you're losing a significant amount of lift and, with the significant increase in drag, you're slowing down quickly - this is how you 'fall out of the sky'!
Going back to my points...
3) When you stall, your wings are essentially useless at this moment (not quite, but work with me here!) and you'd actually be better off without them. So, imagine that you're flying at a high angle of attack in level flight and your wings vanish. Just strictly based on the CG of the airplane, it will want to point the nose down to the ground. Not only that, but you've got that tailplane still there and if the CG was completely neutral, it would be the one to make sure that the nose pointed directly at the earth. It's now a missile heading directly for the earth. But before it can get to that point, the wings reappear and create lift effectively, ensuring that you don't nose dive the earth. Going back to the previous picture, you can see that a very small AoA increase at the Clmax point can have a drastic effect on the lift and drag. The same can be said for a very small AoA decrease at a point beyond the Clmax. So just as soon as your nose starts dropping down towards the earth like a missile, the wings come back to work and ensure that you don't dive straight down! You'll notice that the deeper the stall you enter, the more nose down the airplane will go and the above analogy should explain why.
Not all airplanes will handle exactly like that but for the majority of airplanes, that's pretty much the way she works - as I understand it.
Hopefully I answered your questions. If you have more let me know!
If you find some inaccuracies here also let me know.
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I was told by a Typhoon test pilot (what does he know?) that only the Typhoon tail produces lift.
There are some difficulties with creating upwards lift on the tail. I'd be interested to hear from anyone who's familiar with the aerodynamics of an airplane with that type of configuration.
Edit:
It's got nothing to do with stalling.
QED
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I spent a great deal of my youth making free flight balsa models. I can assure you that an aircraft can be perfectly stable producing lift from the tail.
It's all about the moments baby!
It's all about the moments baby!
Theorethicaly, it would be possible to have stable aeroplane with Cp ahead of Cg and tailplane generating upforce to balance wings' pitch-up momentum, provided every change of AoA leads to greater change of pitching momentum produced by tailplane than wings'. Whether such design can really be made (or was ever made), I have no idea.
On conventional design with tailplane producing downforce, its stall leads to rapid pitchdown with vertical dive easily and quickly attained, so it has to be designed to be immune to stall. On some specific designs, heavy ice accretion on tailplane's leading edge can compromise this. Non-FBW canards, such as VariEze, absolutely must have foreplanes that stall before mainplane, otherwise canards still producing lift combined with stalled wing would produce further pitchup - not quite desired effect when stalling.
Moving the CG aft by transferring fuel into tailplane reduces pitch-down momentum of the wings by reducing its arm. With lesser downforce from tail, less lift is required to balance it (decreasing a bit the stall speed), also trim drag is decreased. Downside is with shorter Cp to Cg arm, restoring pitch momentum with change of AoA is also smaller, making the aeroplane less stable - that's why it is desirable to empty trim tanks before approach.
On conventional design with tailplane producing downforce, its stall leads to rapid pitchdown with vertical dive easily and quickly attained, so it has to be designed to be immune to stall. On some specific designs, heavy ice accretion on tailplane's leading edge can compromise this. Non-FBW canards, such as VariEze, absolutely must have foreplanes that stall before mainplane, otherwise canards still producing lift combined with stalled wing would produce further pitchup - not quite desired effect when stalling.
Moving the CG aft by transferring fuel into tailplane reduces pitch-down momentum of the wings by reducing its arm. With lesser downforce from tail, less lift is required to balance it (decreasing a bit the stall speed), also trim drag is decreased. Downside is with shorter Cp to Cg arm, restoring pitch momentum with change of AoA is also smaller, making the aeroplane less stable - that's why it is desirable to empty trim tanks before approach.
Talking recently with someone at work about tailplane lift. He reckoned that all aircraft (conventional) had tailplanes that exerted a downforce/negative lift. I had thought that some did, some didn't i.e. C of G could be either ahead of or aft of the mainplane.
For the same loading, the difference in AoA will be smallest for low AoA (high speed) cruise -- after all, you pull back (increase the tailplane lift in a downwards direction) to increase the AoA. But that means that the mainplane AoA is already at its lowest, and if the tailplane AoA has to be even lower, it's likely to be only just positive. I don't know if that ever occurs in practice.
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I think only the cannard style planes can use positive lift on the tail. Stability is caused by the negative lift of the tail plane allowing the nose to drop when slowing down and up when speeding up. Yes, a rearward CG increases efficiency by requiring less negative lift of the tailplane. Aircraft use fuel transfer aft to accomplish this increasing fuel economy. I did it in the older Lear Jets 35 years ago. Fighter jets use computers to not require much or any tailplane negative lift.
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I spent a great deal of my youth making free flight balsa models. I can assure you that an aircraft can be perfectly stable producing lift from the tail.
It's all about the moments baby!
It's all about the moments baby!
FEHoppy is right. Several classes of free flight competition models routinely use lifting section tails and have their centre of gravity around 75% chord, or in the case of the F1D class microfilm model below somewhere between the front and rear surfaces.
When the wing loading is very low, the induced drag from a lifting tail is minimal too. The QinetiQ Zephyr solar powered UAV used this to good effect as well. It should be borne that this and most models that use a lifting tail are virtually single speed aircraft.
Tandem wing aircraft such as the Henri Mignet's Flying Flea also generate lift from both surfaces, and if you know anything about that, you will know it can cause problems, as the forward wing at low angles of attack will act as a slot for the rear wing.
When the wing loading is very low, the induced drag from a lifting tail is minimal too. The QinetiQ Zephyr solar powered UAV used this to good effect as well. It should be borne that this and most models that use a lifting tail are virtually single speed aircraft.
Tandem wing aircraft such as the Henri Mignet's Flying Flea also generate lift from both surfaces, and if you know anything about that, you will know it can cause problems, as the forward wing at low angles of attack will act as a slot for the rear wing.
Last edited by Mechta; 10th May 2012 at 19:54. Reason: Added picture
It's true to say that for a conventionally controlled aircraft (i.e. not FBW) the tailplane should generate proportionally less lift than the wing to maintain positive longitudinal stability. This can be acheived by a different aerofoil section, lower angle of attack or both. Often, the tailplane is designed to produce a downforce which makes for a very stable design.
Some interesting footage of what happens when you remove the tail plane of an aircraft in flight. This was during the test flights in the lead up to the Dambusters raids.
I think it's safe to say that the tailplane on that aircraft was generating a downforce!
That was the US trial of the "Highball" bouncing bomb desined for use against ships. It was smaller and more spherical than the "Upkeep" weapon used against the dams. Designed to be carried by the Mosquito -or similar size it was being trialled for use in the Pacific war. Some were sent out to the Pacific, but the war ended before they were used.
That was the US trial of the "Highball" bouncing bomb desined for use against ships. It was smaller and more spherical than the "Upkeep" weapon used against the dams. Designed to be carried by the Mosquito -or similar size it was being trialled for use in the Pacific war. Some were sent out to the Pacific, but the war ended before they were used.
What I do know is that my wonderful A330 stuffs 5 tonnes of fuel into the tail for the cruise to force the tail to produce less down-force (I didn't say 'lift').
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I'd have to agree with the A330 quote - the tail would be producing less downforce. The tail downforce acts the same way as weight does - down and needs to be compensated for by lift. So increasing the angle of attack on the wings will create more lift but also more drag which slows the plane down. This explains why more weight or a forward CG will decrease cruise performance.
I completely agree - but I'd also say that the CG has changed drastically! I would say both conditions contributed to the accident in this case.
I think it's safe to say that the tailplane on that aircraft was generating a downforce!
Last edited by italia458; 11th May 2012 at 14:48.
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Stability is one thing and equilibrium is another.
Negative lift at the tailplane is a requirement for the latter. It is due to the wing aerodinamic moment, which is nose down for positive lift flight. Tailplane provides nose up with negative lift, thus balancing moments.
Negative lift at the tailplane is a requirement for the latter. It is due to the wing aerodinamic moment, which is nose down for positive lift flight. Tailplane provides nose up with negative lift, thus balancing moments.
Martin Simons 'Model Aircraft Aerodynamics' 1994 edition has a good explanation on pages 163 to 165 of zero lift tails and lifting tails. If you can't be bothered to go and find where you've put your own copy, there is a pdf version on 4shared...
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Surely an aft CG helps improve cruise performance?
Stability is one thing and equilibrium is another.
Negative lift at the tailplane is a requirement for the latter. It is due to the wing aerodinamic moment, which is nose down for positive lift flight. Tailplane provides nose up with negative lift, thus balancing moments.
Negative lift at the tailplane is a requirement for the latter. It is due to the wing aerodinamic moment, which is nose down for positive lift flight. Tailplane provides nose up with negative lift, thus balancing moments.
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Aircraft stability is determined by the relative location of ac and cg, irrespective of wether tailplane lift is positive or negative.
For equilibrium, cp location with regard to cg is what matters. For an equilibrium condition, total airplane's lift moment about cg must balance any other pitching moments (due to drag an thrust).
Normally drag line of action is below cg, so it creates nose up moment. More often than not, so does thrust (good characeristic in case ong engine failure).
Wing lift acts somewhere behind the 25% chord, depending on the aoa (the less the angle, the further aft the cp is). It normally creates a pitch down moment, but it can be pitch up at high aoa, with an aft cg. Probably this is what they do in those models?
Tailplane lift must balance the net pitching moment of the other forces, plus the pitch down aerodynamic moment of the wing. For normal flight conditions in typical transport airplanes, it is a downwards force which must be balanced by wing lift just as if it was weight.
An aft cg means taht the wing lift nose down created moment will be less, so that the nose up moment required from the tail is less, so that less part of wing lift is wasted in balancing "apparent weight".
If wing aoa increases, so does tailplane's. Can't be otherwise.
For equilibrium, cp location with regard to cg is what matters. For an equilibrium condition, total airplane's lift moment about cg must balance any other pitching moments (due to drag an thrust).
Normally drag line of action is below cg, so it creates nose up moment. More often than not, so does thrust (good characeristic in case ong engine failure).
Wing lift acts somewhere behind the 25% chord, depending on the aoa (the less the angle, the further aft the cp is). It normally creates a pitch down moment, but it can be pitch up at high aoa, with an aft cg. Probably this is what they do in those models?
Tailplane lift must balance the net pitching moment of the other forces, plus the pitch down aerodynamic moment of the wing. For normal flight conditions in typical transport airplanes, it is a downwards force which must be balanced by wing lift just as if it was weight.
An aft cg means taht the wing lift nose down created moment will be less, so that the nose up moment required from the tail is less, so that less part of wing lift is wasted in balancing "apparent weight".
the wings will decrease AoA and the tail will increase AoA...
Last edited by Microburst2002; 12th May 2012 at 09:44.