If you know anything about aerodynamics.....

[beasly]

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.

Agreed.

Unfortunately, you will run into these sorts of ill-worded questions throughout your college career.

You reasoned correctly, imho.

Keep your chin up and move on.


b.

 

[seagull]

What is the background of your professor?

Wondering the same thing.

 

[clayfenderstrat]

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.

 

[drunkenbeagle]

And the above statement is good science? Like, from an aeronautical engineering textbook?

Some component of lift will be due to the relative wind striking the bottom of the fuselage in many cases.

How about vectored thrust? Is that lift or thrust? If it is pointed up, does that make it "weight?" I think not. In any case, weight can only act on the CG. Anything else is a torque along some axis.

Just define lift as a vector, and the sum of all lift vectors will still equal weight in level flight. It isn't much harder to just do things correctly in the first place.

 

[fish314]

This whole issue boils down to definitions. I imagine that if you look back at the lecture slide and the textbook, you will probably see that they clearly define weight as being the sum of all forces, whatever their origin, acting on the airplane in the direction of the ground. Likewise, they probably define lift as being the sum of all forces, whatever their origin, acting on the airplane in the direction of the sky. These terms need to be defined this way in order for the statement "lift=weight in unaccelerated flight" to be true. (If we consider pure lift, i.e. the total aerodynamic force produced by the wing, it will be greater in unaccelerated flight than pure weight, i.e. the actual mass of the airplane x g.)

In essence these composite forces are being named for their biggest contributor, and your professor is taking advantage of that to write a tricky question.

You are correct, of course, that the aerodynamic forces produced by the horizontal stabilizer are not "weight" in the gravitational sense.

That's a good point and similar to the one Tgrayson made earlier. Provided you are allowed to define the words however you want, you can't really say that anything is "right" or "wrong".
For example I could define the word "cat" to mean that animal with a long waggy tail, a wet nose, that likes to bury bones, sniff the butts of other animals of the same species, dig out in the yard, chase cars, and bark. Based on that definition, I could say, "cats and dogs are exactly alike (provided you accept my definition of cat)." Of course words are only useful to the extent that we all agree on the definitions, and that is even more important in the sciences, where precision (in everything, including our language) is more important that it is in the humanities or our day to day life.
That is sort of what the professor has done with this question. His answer, therefore, makes sense provided you work within the boundaries of his definition. His definitions, however, are not particularly useful because they don't really agree with with the way almost anyone else would use the words, especially the aeronautical engineers and aerodynamicists who developed the concepts he is trying to teach in the first place.

If I were your teacher, I would have worded the problem the following way:

"Consider an airplane in straight-and-level flight. The aerodynamic force produced by the horizontal stabilizer contributes to which of the four fundamental forces acting on the airplane?"

Same question, more right answers.

Or even better:

"Consider an airplane in straight and level flight. The aerodynamic force produced by the horizontal stabilizer is in the same direction as which of the four fundamental forces acting on the airplane?"

(Of course, even my wording is inaccurate. The aerodyanmic force of the horizontal stab is both down and back, since it also contributes to drag. In the same way the aerodynamic force produced by the wing is up and back in straight and level flight, contributing to both lift and drag).

 

[tgrayson]

Some component of lift will be due to the relative wind striking the bottom of the fuselage in many cases.

Why do you think this is different from the Bernoulli effect?

An airflow "striking" anything comes to a stop and the pressure will rise according to the Bernoulli equation. This is all the object feels, so saying that lift is "part Bernoulli" and "part Newtonian" doesn't really make any sense. Moreover, the air flows around an airfoil, rather than bouncing off of it, so the imagery suggested by "Newtonian" lift is somewhat misleading.

Regardless, all lift is due to the pressure differences around the airfoil and you can determine those pressure differences if you know the flow velocity at each location.

 

[fish314]

Some component of lift will be due to the relative wind striking the bottom of the fuselage in many cases.

How about vectored thrust? Is that lift or thrust? If it is pointed up, does that make it "weight?" I think not. In any case, weight can only act on the CG. Anything else is a torque along some axis.

Just define lift as a vector, and the sum of all lift vectors will still equal weight in level flight. It isn't much harder to just do things correctly in the first place.

Beagle, Tgrayson's point was in response to a common fallacy or flaw in the way aerodynamics is taught. Many instructors teach (incorrectly) that there are two types of lift. The first type is "Bernoulli Lift", which is due to the change in pressure from the airflow over the top of the wing being faster, and hence having a lower pressure, than the airflow over the bottom of the wing. The other type of lift is "Newtonian lift", which is caused (in this incorrect theory) by the kinetic energy of the airflow hitting the bottom of the wing and being turned by it. Your response sounded like you were referencing that theory of Bernoulli versus Newtonian lift.

In fact, the theory itself is incorrect because these "two seperate effects" are not really two seperate effects at all, but rather two different descriptions of the same single effect. You don't need to "add the two different parts to come up with the total lift," because there really are not two different parts at all.
The problem arises because the motion of gases, lift, drag, and aerodynamics and fluid dynamics in general are all complex topics. They defy simplification, and when we do try to simplify them we usually come up with explanations that are at least partially incorrect.

Here's a reference from NASA. It's from their "beginner's guide to aerodynamics." Their description is quite a bit better than mine.

The phrase that you used about how "the relative wind strikes the bottom of the wing" is not correct, either. After all, you've heard about a boundary layer, right? Due to viscous effects, the airflow right next to the surface has no net velocity (although it still has the random motion of the molecules, i.e. pressure). If there is no net velocity at the surface the airflow can't really "strike" the bottom of the wing (at least not in the way that a baseball bat strikes a baseball).
http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

 

[fish314]

Dang it all, T, you always type faster than I do!

 

[moxiepilot]

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!

I think that you need to elaborate your statements a bit to be more clear to the professor as to why these are supporting arguments rather than just "you are wrong and I am right" statements.

Kudos to you for continuing the pursuit.

 

[texas_pilot]

I think you'll find proving the instructor wrong will cause you problems later-
My questions are directed more to the school's environment. Will you have to endure other classes by the same guy? Is he a 'flies once a month with his free hour from the school' or retired military? Sounds like a substantial ego to me. Could come back at ya later if you continue to bump heads.
Tis not a perfect world in which we live,...

 

[shdw]

Could come back at ya later if you continue to bump heads.
Tis not a perfect world in which we live,...

I wish I was alive in the 60's, too many people just roll over and take it to avoid conflict these days. Just because it isn't perfect doesn't mean you shouldn't verbally fight for a necessary and correct change. Especially on an intellectual level, stupidity and misinformation has run this world for far too long.


clayfenderstrat: Sounds good, as moxie said try to avoid direct claims of him being wrong. While he is wrong, those are typically taken as ego attacks and will not help your cause. You have a couple spelling/grammar issues in there you might want to correct before sending. Finally, capitalizing words doesn't improve their impact, IMO it is horrible educate for any argumentative letter and makes the sender look desperate to prove a point. I was quite disappointed reading your instructors reply in that regard.

Would you ever capitalize, bold, or underline words in an argument paper for your english professor?

 

[Minuteman]

And I wouldn't use the phrase "First off..." when addressing one of your higher-ups.

Go visit during office hours instead of email-slapfighting.

 

[clayfenderstrat]

Thanks for all the support. I have the utmost respect for the professor and the class. He is a great teacher, and his methods can really get the point across. I am not trying to personally attack the professor, I just don't want future classes to get the wrong idea about this concept. I am not, in any way, trying to throw this back at him. I just want to know how all of my studying of FAA publications and aerodynamics are incorrect due to a poorly worded question.

 

[ackeight]

If you have a paper for him this semester you should do it over this:rotfl:. He would fail you out of spite however.

 

[tgrayson]

Dang it all, T, you always type faster than I do!

I am a professional typer.

 

[African_Swallow]

Thanks for all the support. I have the utmost respect for the professor and the class. He is a great teacher, and his methods can really get the point across. I am not trying to personally attack the professor, I just don't want future classes to get the wrong idea about this concept. I am not, in any way, trying to throw this back at him. I just want to know how all of my studying of FAA publications and aerodynamics are incorrect due to a poorly worded question.

I'm guessing this is likely the only question you missed on this test. You seem quite conscientious and studious I bet you still got an "A."

In that case, I'd drop it then. I don't think you'll become a "hero" for subsequent classes; you'll just annoy your prof and it won't make you or anyone fly any better...although it might make someone build a better airplane.

I'm sure all of the guys on this thread had to endure the aerodynamics fallacies taught to them--but we're all hear to talk about it and become smarter, hopefully.

This brings me back to the idea behind "Stick and Rudder" (Langewische). Somewhere in the first few pages he makes it clear that what he is describing is what is useful for a pilot and would not, er, "fly" with an engineer.

IMHO, when you're in the cockpit it's all about cause and effect, not the whys behind them: "If I do X, Y will happen." You can get the theories straight when you're on the ground.

-A.S.

 

[drunkenbeagle]

Beagle, Tgrayson's point was in response to a common fallacy or flaw in the way aerodynamics is taught. Many instructors teach (incorrectly) that there are two types of lift. The first type is "Bernoulli Lift", which is due to the change in pressure from the airflow over the top of the wing being faster, and hence having a lower pressure, than the airflow over the bottom of the wing. The other type of lift is "Newtonian lift", which is caused (in this incorrect theory) by the kinetic energy of the airflow hitting the bottom of the wing and being turned by it.

Don't want to start a debate about "Newtonian" vs. "Bernoulli" lift. I am in the Bernoulli camp though, I agree. Let's just say that lift is a result of pressure differentials, which can be caused by several phenomena, principally Bernoulli's law when talking about airplanes. Even a fully stalled airfoil produces some lift.

Back to the original topic, the PHAK does define weight as "The force exerted by an aircraft from the pull of gravity." (PHAK pp. G-35) Further defined as "the combined load of the aircraft itself, the crew, the fuel, the cargo or baggage. Weight pulls the aircraft downward because of the force of gravity. It opposes lift, and acts vertically downward through the aircraft's center of gravity (CG)." (Ibid., pp. 4.2)

The FAA seems to think that weight is a result of gravity, and can only act through the CG. This agrees with the way the rest of the world sees things.

Further, "Therefore, in steady flight: - The sum of all upward forces (not just lift) equals the sum of all downward forces (not just weight). (Ibid. pp. 4-2).

Seems pretty clear to me that the FAA recognizes that weight is not the only downward force. On the next page, they point out that these all must be treated as vector quantities.

Seems to me the FAA can describe this correctly and succinctly in the FIRST TWO PAGES of the aerodynamics chapter of this book. They also manage to point out the problems of the gross simplification lift=weight and thrust=drag.

The score so far:
FAA: 1
Professor: 0

 

[drunkenbeagle]

In that case, I'd drop it then. I don't think you'll become a "hero" for subsequent classes; you'll just annoy your prof and it won't make you or anyone fly any better...although it might make someone build a better airplane.

Good advice...

I'm sure all of the guys on this thread had to endure the aerodynamics fallacies taught to them--but we're all hear to talk about it and become smarter, hopefully.

Hey, in Electrical Engineering, we used all kinds of "magic." We assume that electicity flows from positive to negative (even though we know it doesn't). We use theoretical devices like op amps in circuit analysis, even though we know they don't have those properties in real life. My first engineering professor reminded us that what we were doing was a mixture of "magic and lies." We still understood what the shortcuts we were taking were, and how things actually worked.

Shortcuts and models are great for simplifying things, but you can't adjust reality to make your model work.

 

[Mike H]

 

[drunkenbeagle]

pc load letter???