Get rid of tail heavy physics

[Stoney]

Now I am a bit confused... This physics make my head hurt.

I though the planes, with traditional wing-tail configuration, have a horizontal stabilizer with negative angle of incidence to provide a negative lift for the Longitudinal stability.

The H-stab creates positive lift.  And, to paraphrase dTango, its not physics, its aerodynamics...   

[PJ_Godzilla]

Ok, think about this.  Most horizontal stab airfoils are symmetrical.  They are also placed on the aircraft at a positive angle of incidence, relative the wing, typically at an angle that will minimize trim drag at the design lift coefficient for the wing.  So even when the elevator is not deflected at all, the horizontal stabilizer is producing lift.  Why, if the aircraft naturally has a nose down pitching tendency, do we decrease the amount of lift the H-stab produces in order to pitch the nose of the aircraft up?  If the H-stab provides a "down" force, we would need to increase the amount of lift it produces in order to pitch the nose up.  In actuality, we increase lift on the H-stab to pitch the nose down, and decrease the lift on the H-stab in order to pitch the nose up.  

[EDIT] Center of pressure is not the same thing as Aerodynamic center.  The static margin equation uses Aerodynamic Center, not Center of Pressure.

Static Margin uses the NP (neutral point) - which is the resultant of ALL aero forces, including, e.g., the effect of the fuselage. Agreed on the point - and I think you can say Aero center too, as long as it is of the entire assembly. I think I misstated it up front but correctly defined it as based on NP later.

As for the question, perhaps you're getting tangled in convention. Clearly, just looking at a free body diagram, you'd need to decrease (or increase negative) the force on the h-stab to pitch nose up .

[Ex-jazz]

The H-stab creates positive lift.  And, to paraphrase dTango, its not physics, its aerodynamics...   

With all respect, I still disagree

Please, see picture on below.



Picture is from
http://www.centennialofflight.gov/essay/Theories_of_Flight/Stability/TH26.htm

[Stoney]

As for the question, perhaps you're getting tangled in convention. Clearly, just looking at a free body diagram, you'd need to decrease (or increase negative) the force on the h-stab to pitch nose up .

Only if you consider the force acting on the H-stab is acting in a downward motion.  Its not.  Unless there's some down-wash or relative wind issue I'm not considering, the lift vector of the H-stab is up, in the same direction of the lift vector of the wing.

Again, answer why we de-camber the H-stab (by pulling back on the yoke/stick, which induces an upward deflection of the elevator and decambers the airfoil of the H-stab) and reduce lift to make the plane pitch up?  Why do we increase the lift vector of the H-stab to make the plane pitch down?

[Ex-jazz]

Only if you consider the force acting on the H-stab is acting in a downward motion.  Its not.  Unless there's some down-wash or relative wind issue I'm not considering, the lift vector of the H-stab is up, in the same direction of the lift vector of the wing.

Again, answer why we de-camber the H-stab (by pulling back on the yoke/stick, which induces an upward deflection of the elevator and decambers the airfoil of the H-stab) and reduce lift to make the plane pitch up?  Why do we increase the lift vector of the H-stab to make the plane pitch down?

Actually it increase a negative lift ( relatively to wing) by changing a H-stab effective aoa and therefore causing the plane pitch up, which increase a wing aoa, which causing a extra lift for the climb / turn.

Picture on below is made originally for the flaps, but it works basically same way, if you turn it up-side down for the H-stab.


Ok, this is just how I understand it.

[Baumer]

Well the guys as NASA explain it this way,

"The elevators work by changing the effective shape of the airfoil of the horizontal stabilizer. As described on the shape effects slide, changing the angle of deflection at the rear of an airfoil changes the amount of lift generated by the foil. With greater downward deflection of the trailing edge, lift increases. With greater upward deflection of the trailing edge, lift decreases and can even become negative as shown on this slide. The lift force (F) is applied at center of pressure of the horizontal stabilzer which is some distance (L) from the aircraft center of gravity. This creates a torque

T = F * L

on the aircraft and the aircraft rotates about its center of gravity. The pilot can use this ability to make the airplane loop. Or, since many aircraft loop naturally, the deflection can be used to trim or balance the aircraft, thus preventing a loop. If the pilot reverses the elevator deflection to down, the aircraft pitches in the opposite direction."

http://www.grc.nasa.gov/WWW/K-12/airplane/elv.html

[Dream Child]

Baumer: Nice diagram. What the OP fails to realize is that with pictures describing Center of lift and CG. The Center of lift is normally shown as only the net lift of both tail and wing.

Flying with an down force on the tail simply creates more drag, and hence is not normally desired characteristic for fighters .

HiTech

Airfoils have the least wind resistance when they are producing no lift, so any force up or down on the tail produces extra drag, the problem is that pushing up on the tail (providing lift) also produces instability, an undesirable trait in airplanes. While many modern fighters are unstable in design, they also have computers to deal with such a thing. Making a plane too stable also hampers it from maneuvering, so no, you don't want a bunch of down force on the tail to make it fly. Aircraft are also dynamic with respect to the center of gravity, something you already said you don't model, or do you only not model it with respect to losing parts?

[hitech]

Dream Child, producing an up force does not produce an unstable aircraft. As long as the net force of tail and wing is behind the CG the aircraft will be stable.

The drag difference is if you create a down force then the wing will have to produce the weight of the plane + the down force in lift, I.E. the extra drag comes from the wing do to needing more lift.

When the Horizontal stab is producing lift in the up direction, the wing will have to produce less lift.

HiTech

[Stoney]

the flaps, but it works basically same way

Absolutely the same way, just not the way you're understanding it.  When we drop flaps, we increase the Cl at the same AoA--i.e. we increase lift.  Obviously there are other results, but for illustrating what's happening at the elevator, will keep it simple.  Dropping flaps changes the camber of the airfoil on the wing, and therefore, a downward deflection of the elevator increases lift, since the elevator increases the camber of the h-stab. 

[Rino]

     I  can understand why this effect could bother someone, but seriously, if your
tail is gone, the fight's pretty much over. 

[Dream Child]

     I  can understand why this effect could bother someone, but seriously, if your
tail is gone, the fight's pretty much over. 

Actually, I'm really thinking big picture here. If the physics are done correctly, then the airplane characteristics will correctly fall in place, no tweaking necessary to get correct effects. The simple one to see is when someone loses both wings. They should spin like a drill bit due to the engine torque, but instead they "fly" in a nice arc until contacting the ground. Having said that, I'm not sure I want totally realistic physics in a simulator like this, as it will likely make the planes too hard to fly for many people here.

[Wedge1126]

In level flight, the net force of tail and wing should be at the same point as CG. Otherwise, there will be a pitching motion, right?

[Nemisis]

Well, you would also get some sideways push (yaw) that would have to be corrected with the rudder, due to the change in drag of the damaged wing. You would lose lift on that side due to reduced wing area at same angle of attack, so would need to correct with remaining aileron. All this would add drag back into the equation. How much extra drag is the question, so would depend on how much wing you lost, how jaged the edge is, and perhaps how much control is available for correction depending on how bad and what side the damage is, and engine torque (direction) will affect what side is easier to deal with too.

Yes, but I'm saying loosing part of a wing won't affect speed in and of itself with the exception of drag created by the jaged edges. The rest of that stuff will cause a reduce in airspeed, and is required due to the loss of part of a wing, but loosing part of a wing in and of itself won't have a negative effect on airspeed.

[Chalenge]

...we do need to add a CM curve also to modify the basic wing CM. This will allow us to fix a few planes that pitch incorrectly with flaps.

HiTech

You do that and I will be much happier!
Add the A-36 Apache and Uptown will send you a bottle of Whiskey.
Top that off with your wish "I wish AH had support for dual sound drivers so ear phones and mics could be split off from speakers" and I will send you two bottles of Whiskey myself!

[Dream Child]

In level flight, the net force of tail and wing should be at the same point as CG. Otherwise, there will be a pitching motion, right?

In short, no. To demonstrate this, pick up a broom by the end of the handle and hold it parallel to the ground. The center of gravity is somewhere in the middle of the broom. All the lift is at one end. You could do the same thing with the wings, though it would be a very innefficient system to be that far from the center of gravity.