A nice thread.
After considering the foregoing posts, in my non-specialist eye I can see occasions where a tail-less bird noses up and other occasions where it would nose down.
First, aircraft CG does shift forward slightly due to weight lost from the tail. But how much does a duralumin and sometimes fabric-covered structure weigh? If only the horizontal stabs and elevators go missing, but the rest of the aft fuselage in tact, then weight loss is likely much less than
? 300 lbs? and that from planes ranging from three to seven tons. Even if you include the aft fuselage, there certainly isn't a major loss to the total mass of the aircraft. So the resulting CG shift is not great. It would not seem to be enough to instantly drive the nose down.
Other points worth mentioning related to this question: Thrust line and wing location on the airframe, i.e. high-wing, mid-wing (Wildcat), or low-wing like most of WWII fighters. The moment produced by wing-drag and thrust line on a high-wing plane at high power is a moment to lift the nose. A low-wing plane at high power settings produces a moment that drops the nose. For a mid-wing, like the Wildcat, where thrust line coincides with wing placement, there is no moment induced by high power settings. I believe the thrust induced moment must be considered in relation to all the other aerodynamic factors when the tail is lost. For a low-wing airframe this moment contributes to nosing down.
Nevertheless, it seems reasonable that a tail-less bird would "nose up" in the following conditions: High power setting, lower speed range, nose-up attitude (high AoA) as in a climb or turn, for air combat maneuvering.
In this case, the stick is pulled back to cause the tail surface to produce negative lift (relative to the total lift vector on the plane), forcing an increase in AoA, increasing the lift of the main wing. If at this point the tail surface is lost, the negative force aft is lost, resulting in a momentary drop of the nose and AoA. However, with this immediate decrease in AoA, much lift is also lost from the main wing. At lower speeds the plane ceases controlled "flying" and shortly resumes nosing up, due to high engine/propeller thrust downward, pulling the plane upward, with the weight of the aircraft being pulled down under the propeller by gravity. This was pointed out by Badboy and we see it modeled in AH II. Bravo. And if you immediately shut off the engine you'd presume the plane would eventually assume a nose-down attitude.
Nevertheless, I don't see any reason why a plane in high-speed level flight, or especially in a dive (how about straight down), that loses its tail, would pitch up and start a descending hover.
Comments have been made that the tail-plane provides lift, in conjunction with the main wing, and that its removal causes the tail to drop. Others are saying the tail provides "negative lift" to stabilize the plane, and keep the nose, at the other end of the "teeter totter," level. The diagram posted by Badboy supports the idea of some amount of positive lift of tail surfaces for certain aircraft. However the text indicates that this effect occurs in conditions of "intermediate" AoA. So when do WWII fighter aircraft fly at "intermediate" AoA? Economy cruise? I doubt if it is when you are yanking the stick around in ACM. Moreover that diagram shows a negative lift vector for the tail, and the camber of the elevator is inverted from that of the main wing.
The F-4 Phantoms I used to work on definitely had a tail surface designed for highest aerodynamic efficiency producing negative lift. But that plane also traversed the regions of mach 2, so you can't make direct comparisons between planes.
My personal opinion (purely that) is that tail surfaces of WWII fighters were designed for something close to neutral lift in level flight for most loads and speeds. Here's why. The vertical stabilizer and rudder do not contribute a force vector until you mash the rudder pedal. Until then, the two opposite and equal vectors on the vert stab and rudder result in a net-zero force vector. Nevertheless, the vert stab and rudder provide an aerodynamically stabilizing force that prevents the tail from wallowing around. So with the horizontal stabilizers. They primarily provide a stabilizing force that keeps the AoA where you want it. While the horizontal tail could contribute to total lift at intermediate AoA in steady flight, it seems obvious to me that during combat maneuvers the tail surface provides either excess lift (driving the nose down) or negative lift (driving the nose up).
So is there any further evidence from which to draw conclusions?
I conducted an interesting experiment to discover AH II 's model of the force generated by the tail, relative to the main wing, during high-speed level flight. I wanted to know whether the tail produces:
Positive lift, like the main wing, lifting the weight of the tail
Or negative lift, opposite that of the main wing, pushing down on the tail and forcing the nose up at the other end of the "teeter totter"
I used:
P-38, 40, 47, 51D, the F4U, F6F, FM2, Tempest, Spitfire Mk IX and FW 190D
100 percent fuel load
no drop tanks
initial trim set for level flight at low power
I found that as speed increases beyond 200 IAS, each of these planes noses up . So the net-effect of all aerodynamic forces is a net negative lift tail-plane, pushing the tail down, lifting the nose, and this force increases with speed. This also seems to indicate the tail-plane is not bearing any (or much) of the weight as speed increases. The pilot must change the tail's trim to counter this effect as speed increases, to raise the tail and lower the nose. (Probably most people in AH use auto trim and never noticed.)
In RL this effect would doubtless be due to longitudinal dihedral designed into the airframe (also mentioned in Badboy's diagram) to contribute to aerodynamic stability. And this nosing up occurs despite the center of lift creeping aft as speed increases.
So, think about it. Based on the FM in AH, as speed increases in level flight, the tail generates increasing negative lift in relation to the main wing, raising the nose. So much so that you have to seriously trim against it. What will happen if in high-speed level flight you suddenly remove that aerodynamic force pushing down on the tail? Seems to me that the aft of the plane will rise and the nose will drop. Several posts above we see diagrams of the disintegrating Mooney. They didn't come out of nowhere. Add to that the nose-down moment produced because the thrust-line is above the main wing drag vector in low-wing airframes and it seems nose down is inevitable. Moreover, in level flight or in a dive, the line of thrust is not far enough above the center of mass to let gravity pull the plane below the propeller into a state of descending hover.
So it seems to me that at higher speeds in level flight, or a dive, the plane is prone to nose down if the tail-plane suddenly ceases to exist. It seems unlikely that it will quickly pitch up and begin hovering.
That said, AH is tops! Thanks Hitech and all your crew for the fun.
Regards,
Cement
