posted 02-18- 07:17 PM               

The planes I've looked at in SDOE use symmetrical airfoils for the stabilizers.

posted 02-18- 09:10 PM               

If the center of gravity is moved far enough aft (behind the center of lift), the tail surfaces would then produce UP lift, but the aircraft would also then become NEGATIVELY stable in pitch (i.e. if the airplane pitches up, it will tend to increase that pitch if the CG is aft of the center of lift, instead of tending to return to level flight). I don't think many aircraft prior to modern-day fighters are designed to be negatively stable.

So, yes, the stab is an airfoil, but it doesn't really provide lift (unless you deflect the elevator down, in which case it produces a tail-up moment, which you could consider to be a form of lift).

posted 02-18- 10:21 PM               

I also understood the moment of a wing to be an aerodynamic force having nothing to do with the centre of gravity of the aircraft.

I frequently come across references of certain types of aircraft being tail heavy, but not nose heavy. I would think the latter would be most unpleasant to fly. (I really don't have any expert knowledge of this; this is just my possibly out to lunch understanding of it.)

posted 02-19- 12:01 AM               

Look at this site if you're into it, it may be like finding a needle in a haystack, but it's great reading anyhow.
http://www.monmouth.com/~jsd/fly/how/htm/title.html#mytoc

posted 02-19- 01:38 AM               

I think they are there for more of a weather vane effect. This is why they now have stabilators on fighter jets. the weather vane effect hinders perfromance.

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Cheers!
Jaguar
The FS Hangar

posted 02-19- 03:45 AM               

I rememeber seeing in a book from the library on a cutaway drawing or a 3 way that the tail on the hurrican was made to creat lift (might not be called lift but ya know what i mean )to one side. and help compensate for the pull of the engine torque.

maybe this is why we have problems? cause our planes don't have this feature?

posted 02-19- 04:21 AM               

The real life spitfire hand one angled up and the other angled down to compensate for P-factor.

TS

posted 02-19- 11:06 AM               

The wings produce lift force. For a relatively straight-winged aircraft, like a Cessna 150 or B-17 or P-51, the lift force is generally accepted to act through a "point" approximately 25% of the way back on the wing surface (although of course it is really producing lift everywhere on the surface, the sum of all these little "lifties" is equivalent to one giant "lifty" at the "center of pressure" (CL).

So now we have a lift force pushing the plane up. The plane, however, has weight. The distribution of fuel, ammo, airplane parts, pilot, $100 in dollars, $100 in rubles, 4 pairs nylon stockins, etc etc all adds up to an effective aircraft weight which acts through the plane's center of gravity (CG). True, the airplane doesn't care where the CL is, and the wing doesn't care where the CG is, but simple physics says if I put a force on a body ANYWHERE but the CG, it WILL rotate from the moment created.

So, the CG will be either forward, aft, or even with the CL. If its even, fine. In this case, the tail would produce NO lift (i.e. the stab/elevator would be symmetric and aligned with the slipstream) because otherwise, the moment from the tail would rotate the aircraft nose-down, causing a dive unless you pull back the elevator, eliminating the tail lift.

If the CG is forward of the CL, then the tail MUST produce DOWN force in level flight. The wing lift, acting through the CL produces a nose-down moment. The ONLY way to counteract this is with tail-down moment from the stab/elevator combo. If the tail does not produce this down force, the nose will pitch down, and again the airplane will not maintain level flight until you force the tail DOWN, again using DOWN force.

OK, what if the CG is AFT of the CL? Well, now, finally, the tail MUST produce UP lift, because the wing lift is producing nose-UP moment, the tail must have UP force to counteract it and give us level flight. So the tail CAN produce UP lift, right? Well, not if we want a stable aircraft...

You're flying along in your AFT-CG plane, with your tail producing up lift. A gust of wind catches the wing, and gives you a momentary increase in angle of attack. As the plane rotates nose-up, this angle of attack increases, and the lift on the wing increases. Since the CL is forward of the CG, the aircraft will then tend to INCREASE it's pitch-up moment with increased lift, i.e. the tendency is for the plane to then nose up until it stalls, and you'd have to actually fight this tendency with the elevator to maintain level flight. In fact, any time YOU pulled the nose up, the airplane would try to get away from you, and you'd actually need to force the nose back down.

On the other hand, if the CG is forward of the CL, and the gust occurs, yes, the nose still pitches up, yes, the angle-of-attack increases, yes the wing gets more lift now, BUT, the CL is AFT of the CG, so the tendency is for the lift to actually RETURN the aircraft to level flight, forcing the nose back down. This is a "positively stable" aircraft, and, up to the introduction of the F-16, pretty much all aircraft were designed to be stable, since it takes sophisticated computers to manage the flight controls of an unstable aircraft.

Now, that said, there ARE "tweaks" made to counter bad flying tendencies, such as giving one stab a smaller angle of incidence than the other, mounting the vertical stab at an angle (Cessnas have this) to counteract torque and p-factor yaw, and even placing a small spoiler on the right wing so that it stalls before the left to prevent a snap roll as on the Corsair. But the basic principle is that the tail, in general, produces down force in level flight in a positively stable aircraft.

posted 02-21- 06:13 AM               

To check CG/CL relationship when I fly my R/C gliders I was told to use this trick:

Trim the glider for level flight. Now from alt, go into a 45 degree dive - build up some good speed. Now take your hand off the stick. Does the glider pull out of the dive, or tuck in? If it pulls out, that means the CG is behind the CL - this is the "stable" set up. The tail in this case must produce an upward lifting force (rotates plane downward) to keep the plane balanced. If the plane tucks into the dive, this means that the CG is in front of the CL - in this case the tail would need to produce a downward lifting force (rotates plane upwards) to keep the plane balanced.

I was told to set the glider to have the CG dead even or slightly in front of the CL for most effeciant performance. This is the non-stable set up. In this mode I need to keep slight down elevator in normal flight.

To tell the real truth, I'm not sure why this is considered better - I hear it allows the plane better manueverability. I guess the wing is on the elevator's side, so you can get better pitch control at slower speeds?

I can see how a perfect neatral setting (CG=CL) would be more efficiant; less drag, right? If you need a constant force from the elevator to offset the wing lift, wouldn't this cause more drag?

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-Sv =FC=

WWI in SDOE!

posted 02-21- 10:39 AM               

The "advantage" to having lower stability is that the slipstream and wing lift will then "assist" manuevering by adding force to the manuever input, rather than "opposing" it by producing a "return to level flight" force. So you get a faster "instantaneous" pitch up or roll for a given control movement. Useful for acrobatics or combat manuevers, but dangerous for inexperienced flyers.

F-18 and F16 are unstable in various portions of their flight regimes (presumably what gives them superior roll and turn rates) but require computers to translate the pilot's inputs into "stable" manuevers. Total failure of the computer (supposedly almost impossible due to the backup systems) would make the plane very difficult to control. I believe the F-15 is designed to be positively stable, and can be flown without computer assist.