# If you know anything about aerodynamics.....

#### 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:

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?

Money. Money makes airplanes fly.

All kidding aside if I was the professor I'd give you your points back. You clearly understand the principle that is being questioned so why knock you for it? Sometimes I really hate standardized tests.

The wings produces lift (upward force) while the horizontal stab; be it an airfoil, still creates downward force (weight).

Your point is a good one, but your professor is correct (IMO).

I guess I would agree with you. This is how I see it.

Center of gravity acts down

Center of lift acts up

Horizontal stab acts down, the opposite of this would be up, which in this case is lift produced by bernoulis principle to "simulate" weight.

It is a poorly worded question and it seems like either answer could be right if you argue in the correct way.

P.S. Don't piss a professor this early into the semester, especially if you have them for later more advanced classes.

Money. Money makes airplanes fly.

This is not entirely correct. In my situation it is: Money and a PATIENT wife.

The Bernoulli effect can only act on a surface as lift. What do you call the Bernoulli effect when you displace the rudder? Lift to the right or left.

In this case the force acts in conjunction with the weight of the airplane, but the force is still created using all the Bernoulli effect lift principals.

Now if the question was reworded as: "In straight-and-level, unaccelerated flight, the Bernoulli Effect technically acts on the horizontal stabilizer in conjunction with:" the correct answer would in fact be weight, but in its original form it does create lift, because the bernoulli effect can only create lift (and as a byproduct induced drag). There is no way that the bernoulli effect can act as weight.

Now this is coming from an engineer that has specific definitions for the word "act" "lift" and "weight". Now for common english, it "behaves" like weight, and using that "act like weight" on the airplane, and by replacing the non-definitive word "like" as a comparison with the definitive word "as" which many people do, I see where the issue arises.

Any downward force technically is weight.

That right there is soooooo very wrong

That right there is soooooo very wrong

As an engineer, this is making my skin crawl. Your professor is wrong. Lift is a vector quantity, it has magnitude and direction. The direction may well be toward the earth (or not).

The question assumes straight and level flight. Even then, the lift vector from the horizontal stabilizer points down and behind the aircraft (not the at the earth), as it has induced drag too.

Now this is coming from an engineer that has specific definitions for the word "act" "lift" and "weight".

The FAA has specific definitions for those things too. If yours happen to be a little different from theirs, go with theirs because arguing is going to get you nowhere

And for the record, when it somes to flight, Downward Force = Weight. Period

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 agree with you, however these are the keys as to why your answer was incorrect.

Ask your prof this. Does the "weight/mass" of the tail cause the downward force he so calls weight? Some of it does, but I would venture to guess 90% is caused by the airfoil.

Oh and another thing. his example of the upsidedown airplane falling out of the sky when it's upside down would fall to the earth fast. According to his theory of "weight" it would only fall around 32fps^2. However, with the additional LIFT it would accelerate faster!!!! It gets a little hairy with the thrust vector added, but lets say that didn't exist.

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.

I would have to disagree here to. The aerodynamics don't change inverted, the AoA is what is changing. Inverted, the "top" surface of the wing is now hitting the relative wind. That is what then produces lift (albeit not as efficiently). The bernoulli effect would be negligable here on a GA a/c wing.

Most transport/ga aircraft wings are not designed to generate lift efficiently inverted (negative AoA). If the trim and AoA don't change when inverted, the aircraft most certainly will accelerate into the ground.

I think that's a lousy question to ask in the first place. A better question might have been something like: "What kind of pitching moment does the horizontal stabilizer produce in blah, blah, blah flight."

Or, you could be a wiseass and say you were thinking of a canard configuration, or that the aircraft had an extremely aft CG.

To me, "weight" implies a force associated with accelerating a mass, which the force from a horizontal stabilizer is not (unless it has a trim tank filled with ballast in it or something).

If you think about this as a weight & balance problem, you'll understand why the downward force of the airfoil is, indeed, weight.

An aft CG gives the entire airplane a higher airspeed because the total weight supported by the wing is less, and therefore allows a smaller angle of attack to produce the necessary lift. Lower angle of attack means less induced drag, which translates into higher airspeed and a lower stall speed because the airplane has to go slower to exceed the critical angle of attack. However, with the lever/arm of the elevator being shorter than with a forward CG, if the airplane stalls, you have both the weight of the tail, plus the reduced leverage of the elevator having less authority to push the nose down, reduce the angle of attack and get the wing flying again. That's why if the CG exceeds the aft limit, it may be impossible to recover from a stall

Conversely, with a forward CG, you can't fly as fast because the total weight the wing has to support is greater, which means the angle of attack has to be greater, making induced drag greater, etc, etc. As the airplane approaches a stall with a forward CG, the critical angle of attack will be reached at a higher airspeed (because of the total weight or load factor), yet recovery will be easier because of the weight of the nose, and the greater leverage of the elevator to lift the tail- in many cases, simply relaxing the back pressure (and thus the load factor) is all it takes to reduce the AoA enough to recover

If you think about this as a weight & balance problem, you'll understand why the downward force of the airfoil is, indeed, weight.

Weight is a force which equals mass times the acceleration of gravity. How do aerodynamic forces have anything to do with that? That is the definition of weight.

That's like saying if I throw a ball at the ground, the ball is heavier because I through it (added a downward force). That wouldn't fly in any freshman physics course I can think of.

I agree with you and believe the professor is wrong. If the force on a horizontal stabilizer were due to gravity then how would a tail stall ever occur? Someone turn off gravity on the tail? Personally, I think that the prof is trying to save face and not admit that he's incorrect. Perhaps a quote from A Few Good Men "...You want the truth? You can't handle the truth."

Ok.....I need some clarification on a principle of lift. During out last exam in an aviation class, we were posed this question:

I think this is where the divergence is coming from. In a pure aviation sense, there are 4 forces acting on an airplane. We've all seen the drawing of a cute little with an arrow in each direction and the labels of lift, weight, thrust and drag. In this (very simplistic) view, any upward force is lift and any downward force is weight. Any forward force is thrust and any rearward force is drag. Simple as that.

However, he did throw in the Bernoulli Effect wording which was pointed out on here can ONLY generate lift. The gray area on your exam is that the lift is operating in a downward direction (which according to the simplistic diagram should be called "weight").

You argued your point well (and the important thing is that you actually understand the underlying principle and not just the regurgitated test answer).

It's a poorly worded question, especially when read with any sort of engineering lens.

Beagle, you're forgetting the balance part of weight & balance

Throw a ball at the top end a see-saw with a kid on the bottom end. If you want to raise the end with the kid on it, you either have to throw the ball REALLY, REALLY hard, or you have to move the fulcrum to the point where the weight of the ball itself will pick the kid up.

Maybe his slide shows that, but his slide and the book used need some revision. Here are a few sources, IMO all credible aerodynamic sources that disagree with your professor:

First, NASA: http://quest.nasa.gov/aero/planetary/atmospheric/s+c1.html

Note a picture near the bottom that notes "tail lift down," not tail weight.

Flight Theory for Pilots by Charles E. Dole Page 234 and 235 figure 5.18

"The contribution of the horizontal stabilizer to the pitch stability of the aircraft can be seen in figure 15.18. Figure 15.18(a) shows that if an up-gust causes the aircraft to pitch up, then an upward lift is developed by the horizontal tail. This creates a nose-down moment, which is stabilizing. In figure 15.18(b) the opposite effect is achieved when the aircraft is pitched downward by a down-gust. A nose-up moment is developed in this case." If you look at the figure Dole calls it a "negative tail lift."

Aerodynamics for Naval Aviators page 63

Development of lift: "If a wing is producing lift, a pressure differential will exist between the upper and lower surfaces, i.e., for positive lift, the static pressure on the upper surface will be less than on the lower surface." I cannot find it in here but notice this defines positive lift, meaning upward lift. The other definition is negative lift, the same as Dole mentioned it, and any other aerodynamic source that is credible for which I have read.

So there, you now have a definition of lift and two sources pointing to lift on the tail as being lift in the negative direction. Lift is an aerodynamic force that operates in a 3 demential environment, there is no up or down related to it. It acts in which ever direction for which positive pressure results. You obviously understand this and IMO your teacher sounds like a prick. It does not sound like you will change his mind and as others have said here, it is clear you understand the content. After all, he is in the minority and holding you accountable for it, you should give him this: Page 6-5 of AIH

Types of questions to avoid, "puzzling, oversized, toss-up, bewilderment, and trick questions." He covered puzzling, toss-up, bewilderment, and trick all in one shot, kuddos to him.

Beagle, you're forgetting the balance part of weight & balance

Throw a ball at the top end a see-saw with a kid on the bottom end. If you want to raise the end with the kid on it, you either have to throw the ball REALLY, REALLY hard, or you have to move the fulcrum to the point where the weight of the ball itself will pick the kid up.

My point is the weight of the ball is not a function of all of the accelerations acting upon it, only the acceleration due to gravity. The "weight" of the ball is constant at any given point on earth.

Long see-saw is just changing the moment of the ball (by lengthening the arm) Ball still weighs the same.

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