In regards to achieving equilibrium conditions, what is the relation between airspeed and the tailplane?
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1Did you perform basic research? if so, what didn't you understand? Your question must be quite more precise. see the help center for more information. – Manu H Apr 16 '20 at 06:33
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This is the third time this week I provide a link to the same section of how it flies. Thus I think you can find in aviation.SE parts of the information you seek and come back with a more precise question about aspect not yet discussed on this website. – Manu H Apr 16 '20 at 06:36
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The article you link is a teeny tiny bit off. The tail of a non-canard NEVER produces vertical lift in any steady state condition. Only in transient conditions, mainly those where an elevator input drives a ND pitch rate that exceeds exceeds the inherent nose down pitching moment. An airplane with a CofG that required a tail to lift up would be uncontrollable for any length of time. In steady state flight there is always downforce. – John K Apr 16 '20 at 13:11
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I think it may be a bit unclear what exactly you're asking. What do you mean by the tailplane? Do you mean the relationship between the horizontal stabilizer, center of gravity, and lift? – Ryan Mortensen Apr 16 '20 at 13:12
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@JohnK I think you will agree that a tandem wing can be stable and that the rear wing provides lift. If the rear wing is progressively shrunk to the size of a tail plane, at what point do you imagine it must reach the crossover point between lift and downforce, and why? – Guy Inchbald Apr 16 '20 at 16:53
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@JohnK Please stop spreading this nonsense. Of course is it entirely possible for a tail to produce lift in a steady state condition. Slow flight, no flaps and a rear center of gravity are all what is needed. – Peter Kämpf Apr 16 '20 at 19:05
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@PeterKämpf yes in theory you can have an airplane fly with the CG aft of the neutral point creating a net pitch up moment so the tail is forced to lift up to keep the nose from rising, but it will be very unpleasant, with forward elevator pressure or ND trim required to keep from slowing more, and you wouldn't get me flying it. Not aware of any normal airplanes designed to do that. – John K Apr 16 '20 at 19:57
1 Answers
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To be statically stable in pitch, the tail has to generate a downforce that opposes a continuous nose down pitching moment. Everything else being the same, more speed increases downforce, less speed decreases it. The equilibrium state is when the opposing forces are in balance in steady state flight, hands off the controls, and the airplane is said to be "in trim". This is achieved at a given airspeed for a given set of "configuration of forces" you might say (pitching moments, thrust lines, C of G positions etc) and is called "trim speed".
Trim speed is the speed at which tail downforce balances the nose down pitching moments to achieve equilibrium (not accelerating or decelerating) for a given set of physical conditions. FAR 25 static stability requirements require the plane to positively seek this speed by pitching uphill or downhill when disturbed from that speed in an attempt to regain equilibrium.
So the relation to the tailplane is that speed in excess of trim speed causes the nose to pitch up due to excessive downforce, which makes it go uphill and slow down, trying to get back to that equilibrium state. Vice versa when speed is less than trim speed; it pitches down trying to get to the equilibrium state. In an actual airplane it overshoots its equilibrium point several times trying to get there and eventually zeros in (the Phugoid oscillation).
Some other configuration change, a power change, a C of G change, a dragging item change, a flap sticking down, or elevator incidence change, will alter the speed at which equilibrium is achieved or trim speed. Once stabilized at a new trim speed, it will now seek the revised trim speed as before, if disturbed, as the tail responds to downforce changes with airspeed as described.
John K
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1For a different view see https://aviation.stackexchange.com/a/77188/34686 , https://aviation.stackexchange.com/a/30720/34686 , https://aviation.stackexchange.com/a/76837/34686 (including embedded links to See How It Flies website sections) , https://www.rcgroups.com/forums/showpost.php?p=26413333&postcount=2 -- note sentence "Some free flight airplanes actually balance behind the trailing edge of the wing." – quiet flyer Apr 16 '20 at 14:12
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You can balance anywhere you want as long as the net result is a ND pitching moment for the tail to oppose. A horizontal tail lifting up in steady state conditions would require ND trim to slow down and would be rather dangerous and unpleasant to fly. – John K Apr 16 '20 at 14:39
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Another good one-- https://aviation.stackexchange.com/a/22090/34686 – quiet flyer Apr 16 '20 at 14:41
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@quiet flyer Sorry, but the flying wing gets its pitch stability from downforce opposing a pitching moment also. It's just that the downforce is coming from the local area of the trailing edge where the elevator is. It's just the horizontal tail moved forward to the wing trailing edge. The arm it has to work on is very short, so the pitching moment has to be very low using airfoils designed for the purpose. For this reason, flying wings suffer from two problems - overly light and sensitive pitch control, and a very limited CG range. – John K Apr 16 '20 at 15:44
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@JohnK Downforce is quite unnecessary - we have known how to make a lifting tail stable since around 1918. What you are thinking of is the relative change in moments around the centre of lift, which is a function of the lift slope coefficients of the two surfaces. For stability, the tail must exert a lift moment change greater than change in wing lift moment. The absolute direction of the force is unimportant. – Guy Inchbald Apr 16 '20 at 16:48
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@GuyInchbald you're describing a canard or tandem wing where the lift slope of the forward wing has to be steeper than the aft one for it to work at all. The Rutan canards and tandems all use that. Rutan used the UofGlasgow airfoil on the ezes and the quickies because its unique lift slope characteristics gave strong static stability. Conventional tailed airplanes don't even need an airfoil. Anything to deflect the air will do. Like the flat plate elevator-stab you see on any fabric covered light plane. If a jet had an up lifting tail, the tail's LE would have to move up to slow down. – John K Apr 16 '20 at 17:28
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@JohnK You really do need to think harder about the borderline between a small rear tandem wing and a large tail plane. You have been downvoted at least twice for a reason. – Guy Inchbald Apr 16 '20 at 17:52
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@GuyInchbald the post is about how regular aircraft respond to speed changes. I explained how normal airplanes trim to a speed. Tandem wings planes are not relevant to the question. Perhaps your comment is related to the other post about tandems. – John K Apr 16 '20 at 21:04
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@JohnK I was commenting on your wholly misguided comment, not on the original question. meanwhile, I see that someone has indeed marked this question as a duplicate of one about tandems. – Guy Inchbald Apr 17 '20 at 06:59