In a pretend world, where friction and drag weren't factors, could you drag the inverted fin of your aircraft along the ground (assuming all incidences were set at zero) and then increase the throttle to gain altitude?

From : Don Stackhouse

Since you were so gracious to answer my 1/4 scale question, I have another one for you. Some of us modelers were debating an issue the other day that raised some eyebrows. I thought it would be interesting to get your take on it since we are doing nothing more than speculating.

The debate started when one of our more daring members flew his plane inverted and tried to drag the vertical fin along the grass. After his plane (trainer type) buried its nose into the ground, a discussion of "why" followed. One theory, (not mine) was that once the fin was on the ground the aircraft could no longer "rotate" and thus change the angle of attack to gain altitude. My argument was that the sudden drag created by the fin scraping the ground, and being so far behind the CG, together with the forward inertia caused the plane to nose in. (no different than tying a cable to the rear axle of your car and then driving until the slack ran out, the nose of the car would initially drop)

So the question........In a pretend world, where friction and drag weren't factors, could you drag the inverted fin of your aircraft along the ground (assuming all incidences were set at zero) and then increase the throttle to gain altitude? We are on the verge of trying this by raising the tail wheel on a symetrical winged airplane until the wing is zeroed and then try to take off. Your input may save us alot of time and money. (Although you might suspect we have too much time anyway) Thank you in advance for your help.

An airplane in flight acts as if it was mounted on a ball joint exactly at the C/G. Any time it changes its yaw, roll and/or pitch attitude it does so by pivoting around the C/G. If we assume its flight path is exactly parallel to the ground with the fin touching the ground (a HUGE assumption, since its flight path had to be originally downward towards the ground in order to get into this position), in order to raise its nose away from the ground it must also lower the tail. This is impossible of course, since the ground is in the way.

Now let's change our point of view a little. The assumption about all motions occurring around the C/G actually applies to a model that isn't in contact with anything else (except air). When the fin touches the ground, the tip of the fin becomes the pivot point for pitch change. Since the elevator is pushing almost directly at this new pivot point, it has virtually no leverage to cause a pitch change anymore. This is why it can't raise the nose. If you have something else well in front of the fin, such as wing flaps that can go to negative deflections (or that were set positive BEFORE the model touched the ground and can now be retracted), or a canard with an elevator on it, you can use those to raise the nose and fly out of contact with the ground.

In a real world situation, the friction you mentioned above is indeed trying to pull the model into the ground exactly as you describe, and it's also stealing away the model's airspeed and lift, worsening the situation. This means that if you want to have any hope of recovery it probably must be done very quickly.

There are other ways to change the lift of the model and get the nose moving away from the ground. If you can increase airspeed, you increase the lift of the wing (as long as the angle of attack is not already at what's called the "zero lift angle", which is zero for a symmetrical airfoils and slightly negative for cambered ones). Adding enough power soon enough is one way to accomplish this, just as you suggested in your moly-disulphide grass and Teflon dirt scenario.

In addition, if your model has a significant amount of downthrust built into the engine mount (so that the engine is pointed up relative to the ground when inverted), or if the nose is still pointed sufficiently up, some of the thrust from an increase in power will be pulling the nose up away from the ground.

If the propeller disk is angled slightly upward as I just described, it will also be making some additional nose-up forces. A propeller in an angled flowfield makes sideways forces, just like a wing or tail surface. This is why forward mounted props (like most tractor propeller installations) are slightly de-stabilizing in pitch and yaw, while aft mounted props tend to help stability. You can see this on the old Northrop flying wing bombers. The original XB-35 had pusher props and no vertical fins. When they converted these to the YB-49 jet version, they had to add small vertical fins to make up for the loss of the yaw stability contribution of the props.

The downward moving blade in your slightly nose-up inverted pass situation sees a slightly higher airspeed and a slightly higher angle of attack than the upward moving blade, so the aerodynamic drag of the downward moving blade is slightly greater. This creates an upward force parallel to the plane of the prop disk, which tends to pull the nose up if the prop is ahead of the pitch axis (usually the C/G, or in this case the tip of the fin).This force is probably negligible in this situation (although every little bit helps!), but can sometimes be very significant. For example, on the V-22 Osprey tilt-rotor aircraft, this sideways force is supporting a large portion of the aircraft's weight while it is transitioning between helicopter and airplane mode.

So in summary, yes, the contact with the ground makes it virtually impossible to raise the nose with elevator. Yes, the friction with the ground tends to pull the nose toward the ground and the loss of airspeed robs wing lift, aggravating this situation. Yes, the addition of enough power soon enough (while the wing is still at an angle of attack that produces upward lift) MIGHT cause an increase in airspeed and increase wing lift enough to get the model to lift off again.

A historical footnote:
This super-low inverted pass was a specialty of Germany's Graf von Hagenburg and his Bucker Jungmeister in the late 1930's. During an airshow at the 1937 Cleveland Air Races he didn't push forward quite enough on the stick, and got into exactly this same situation. He was extremely lucky, ending up with just a cut on his head. His Jungmeister wasn't nearly as lucky. After sacrificing itself to protect its master, its mangled remains went home to Germany in a box.

Don Stackhouse @ DJ Aerotech