Simviation Forums

 Why does an aircraft usually go nose-down at low to medium airspeed, but goes nose up when the pilot performs a high-speed dive? It would appear that the Horizontal stabilizer's pitching torque would somehow grow faster with airspeed than the main wing's pitching torque. Why would that be? Could it be that the flow becomes more turbulent with high-speed and that it would affect the surfaces with the greatest area more than the ones with less area? Aircraft usually have their CG ahead of the Center of Lift, don't they?

Maybe it's just the air providing so much lift to the wings, that it forces the nose up?

More airspeed= more lift.  The plane will "want" to make the dive more shallow, trying to rise above your intended glidepath. I don't think the nose is pitching up so much as it takes more forward pressure to keep the nose ponted at the target point below the horizon.
 This is one of the reasons that the preferred method of extreme diving in fighters is to roll over, then drop almost straight down, nose low but inverted. Any additional force required to keep the dive steep (and much less should be required) will result in a positive load, whereas if you're diving "rightside up", as you push the nose down, you're adding a negative load. Most planes tolerate much higher positive loads than negative ones.

Thanks guys. I'm grappling with the physics of the problem. Can you tell me something about control reversal and its causes?

 

Thanks guys. I'm grappling with the physics of the problem. Can you tell me something about control reversal and its causes?

 

Other than damage, ice,  or failure of control surfaces, there's really only the kind of "reversal" that sometimes happens with ailerons. I... need to look that up; it's hard to explain. Hmmm...
Let's say you're flying at a high angle of attack for your desired airspeed, and although the wing hasn't stalled yet, it's very close. You decide to bank without letting the nose down for some reason, and as the aileron comes down on the wing you're trying to raise, it changes the angle of attack  (remember that's relative to the airflow over the wing) of that area of the wing, sending it past that critical number. That wingtip stalls, and no matter how much you try to lift that wing, down it will go. The effect is that you, say, turn the yoke to the right, and the plane rolls left. Probably into a spin if you're really not paying attention, as the other wing comes up and that tip stalls, then the whole wing, with the plane in a knife-edge attitude.
Most planes nowadays have sufficient "washout" (change in chord angle from root to tip; makes the wing look "twisted") to compensate for that effect, but it's still possible.... especially at high speeds. I've heard of it happening while inverted, such as in an outside loop.  Unless the wing has a symmetrical cross-section and almost no dihedral, the angle of attack upside-down is going to be quite different.
The other, more common cause of control reversal is mechanics rigging the cables wrong... that's why "controls free and correct" is a very important checklist item.

A couple of things nobody's mentioned yet. It's a long time since I studied aerodynamics so some of you students can correct me if I'm wrong. If I remember correctly the centre of pressure can be affected by airspeed. This would obviously depend on the type of wing but I think it moves aft with increased speed.* This can be quite marked with high-performance aircraft & I think it moved something like 6 feet on Concorde.

Control reversal was a big problem with the first jets when they approached Mach 1. This mainly affected the elevators & also caused flutter which could & did shake the aircraft to pieces. It was eventually overcome by using the all-moving tailplane (featured on the Miles M.52 & copied on the Bell X1) instead of conventional hinged elevators.

*PS. This would have the opposite effect by increasing the dive angle proportionally with airspeed (not nosing up). What's known as a "tuck-under" in the high-performance model sailplane fraternity.