Here is the message that I composed to send back to the professor. Please let me know if I can strengthen any of my arguements before I send it. Thanks!
Professor XXXXXX,
I have spent some additional time studying the problem in question. While the lecture slide and text book show the 4 basic forces acting on an airplane, I still disagree with the way the question is worded. I have accrued additional examples to support my view.
First off, the Bernoulli Effect can ONLY produce lift--per its definition. Additionally, the lift vector produced from the horizontal stabilizer acts in direction down and behind the aircraft (taking into account induced drag). Weight acts directly toward the Earth, while lift from the horizontal stabilizer falls down and slightly behind the aircraft.
Additionally, according to the notes, "when the wing stalls, the wing produces no useful lift." If this statement is true, then a tail stall could not occur. Since tail stalls are recognized and demonstrated during flight training, we know that the tail CAN stall, therefor the tail produces no useful lift. Even though this lift is in a downward direction, is has to be lift. If it was weight, a tail stall could no occur.
Even NASA agrees that the tail produces downward lift. See the illustration at
http://quest.nasa.gov/aero/<wbr>planetary...eric/s+c1.html The FAA's Pilot's Handbook of Aeroautical Knowledge explicitly states, "The downward force thus produced holds the tail down..." as well as "The faster the airplane is flying, the greater this downwash and the greater the downward force on the horizontal stabilizer.
Another solution can be found if the aircraft is put into a condition other than straight-and-level flight. During a climb, descent, or bank, the force acts perpendicular to the flight path, but no longer parallel with weight. This example proves that the force is not weight, but negative lift that is independent of the orientation of the aircraft.
Also, does the lift being produced by the horizontal stabilizer contribute to the induced drag on the airplane? If so, is weight creating this drag? Since we know that induced drag is a by product of lift, we can infer that weight cannot be the culprit of induced drag by the horizontal stabilizer.