clayfenderstrat
Well-Known Member
Ok.....I need some clarification on a principle of lift. During out last exam in an aviation class, we were posed this question:
In straight-and-level, unaccelerated flight, the Bernoulli Effect technically acts on the horizontal stabilizer as:
a) lift
b) drag
c) thrust
d) weight
I answered lift, but the correct answer was weight. His reasoning is that any downward force is considered weight. I disagree with this position, so I sent him the following email:
Professor XXXXXX:
After reviewing our first exam in XXXXX, I have an inquiry about a test question. The first question pertained to the Bernoulli Effect and lift on the horizontal stabilizer. During straight-and-level, unaccelerated flight, the horizontal stabilizer produces a downward force to oppose the weight of the front of the plane. This force is lift acting in a negative direction, not weight. Three dimensional physics dictates that weight is the vector produced by multiplying mass by gravity. As an example, consider an airplane during inverted flight. When the airplane is rolled upside down, the wings are producing lift in the negative direction, while the tail is producing lift in the positive direction. You have a negative AOA as well as negative lift. These two concepts negate each other, simply producing lift in the downward direction. The notions of up and down can be disregarded, as they are negligible in three dimensional physics. To conclude, I feel as though the first question was incorrect, as the tail experiences lift in the negative direction. Weight is a term that is unrelated to lift in this aspect. While the component may be in the same direction as weight, it is not weight. It is still lift.
His response was:
In straight-and-level, unaccelerated flight, the Bernoulli Effect technically acts on the horizontal stabilizer as:
a) lift
b) drag
c) thrust
d) weight
I answered lift, but the correct answer was weight. His reasoning is that any downward force is considered weight. I disagree with this position, so I sent him the following email:
Professor XXXXXX:
After reviewing our first exam in XXXXX, I have an inquiry about a test question. The first question pertained to the Bernoulli Effect and lift on the horizontal stabilizer. During straight-and-level, unaccelerated flight, the horizontal stabilizer produces a downward force to oppose the weight of the front of the plane. This force is lift acting in a negative direction, not weight. Three dimensional physics dictates that weight is the vector produced by multiplying mass by gravity. As an example, consider an airplane during inverted flight. When the airplane is rolled upside down, the wings are producing lift in the negative direction, while the tail is producing lift in the positive direction. You have a negative AOA as well as negative lift. These two concepts negate each other, simply producing lift in the downward direction. The notions of up and down can be disregarded, as they are negligible in three dimensional physics. To conclude, I feel as though the first question was incorrect, as the tail experiences lift in the negative direction. Weight is a term that is unrelated to lift in this aspect. While the component may be in the same direction as weight, it is not weight. It is still lift.
His response was:
As the judge said in the movie, "My Cousin Vinny", "...that is a lucid, well thought out argument...over ruled."
OK here is the deal. Any downward force technically is weight. This was shown on the slide in lecture regarding the four forces acting on an aircraft, and is shown in the text book as well. So again ANY downward force is considered weight.
When aircraft is upside down the wings do NOT produce lift in a downward direction. IF they did the aircraft would get very close to the earth, very fast. When the aircraft is upside down the "lift" is still being produced in an UPWARD direction.
These are the terms we use in the realm of aerodynamics.
I have to stand by my answer.
I can sort of see where he is coming from, but how would you answer the question? What is the technically correct answer? What do you think?
I can sort of see where he is coming from, but how would you answer the question? What is the technically correct answer? What do you think?