Aerodynamics: Center of Pressure
- Thread starter Adrock
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fish314
Well-Known Member
Adrock,
Pm'd you, but I don't know if it was my best work. I'm a little drunk right now (hurray Baltika 9!! A Russian beer, 8.5% alcohol by volume, woohoo!!)... but I'll give it another shot if you're still up for it.
Basically, the explanation boils down to any force applied to a body that ISN'T through the center of gravity (center of mass... same dif), produces a moment about the center of mass.
The reason you may be confused is because you are thinking of an airplane on a string with the string going through the CP as a perfect model of the airplane in flight. But in your example with a string acting as your kind of pseudo-lift force in your model I think there will be some side to side forces that you aren't considering.
For example: imagine a plane suspended by TWO strings. One string goes through the center of pressure and one string goes through the nose... maybe the airplane is on a crane or something. Right now, the airplane is not moving. Some of the weight is carried by the front string, and some is carried by the back string.
Now imagine you cut the first string. Once you cut this string, you are in the exact same position that you would have been in for the model or example you mentioned: String through the CP, weight acting at the CG. Well, you are correct that the plane would pitch down, and you are correct that the plane would rotate about the CP rather than the CG.
But I would submit that this model of the situation in flight is imperfect and doesn't model the flying airplane very well. Why? Well look at what happens to the airplane. The airplane pitch down and rotate, but then it would also begin to swing on the string... Try it out, or picture it in your mind. As the nose swings down and reaches the bottom, our airplane on a string model would basically become a pendulum... The airplane would hit the bottom of the string and begin swinging on the string backwards.
So there must be some side to side forces on the airplane, or else, how do you explain why the plane begins to swing on the string back and forth? It is these side to side forces that cause the "airplane on the string" model to behave differently than the airplane in flight.
Anyway... that's my best drunk attempt at this question.
Pm'd you, but I don't know if it was my best work. I'm a little drunk right now (hurray Baltika 9!! A Russian beer, 8.5% alcohol by volume, woohoo!!)... but I'll give it another shot if you're still up for it.
Basically, the explanation boils down to any force applied to a body that ISN'T through the center of gravity (center of mass... same dif), produces a moment about the center of mass.
The reason you may be confused is because you are thinking of an airplane on a string with the string going through the CP as a perfect model of the airplane in flight. But in your example with a string acting as your kind of pseudo-lift force in your model I think there will be some side to side forces that you aren't considering.
For example: imagine a plane suspended by TWO strings. One string goes through the center of pressure and one string goes through the nose... maybe the airplane is on a crane or something. Right now, the airplane is not moving. Some of the weight is carried by the front string, and some is carried by the back string.
Now imagine you cut the first string. Once you cut this string, you are in the exact same position that you would have been in for the model or example you mentioned: String through the CP, weight acting at the CG. Well, you are correct that the plane would pitch down, and you are correct that the plane would rotate about the CP rather than the CG.
But I would submit that this model of the situation in flight is imperfect and doesn't model the flying airplane very well. Why? Well look at what happens to the airplane. The airplane pitch down and rotate, but then it would also begin to swing on the string... Try it out, or picture it in your mind. As the nose swings down and reaches the bottom, our airplane on a string model would basically become a pendulum... The airplane would hit the bottom of the string and begin swinging on the string backwards.
So there must be some side to side forces on the airplane, or else, how do you explain why the plane begins to swing on the string back and forth? It is these side to side forces that cause the "airplane on the string" model to behave differently than the airplane in flight.
Anyway... that's my best drunk attempt at this question.
Adrock
Well-Known Member
Fish you’re awesome I appreciate it. I was away from the airport, and therefore all my books all weekend when this problem entered my mind. I have just finished reading the section in "Understanding Flight" that I think clears up a false premise. The premise (my premise) that was incorrect is that: the system’s center of lift is behind the center of gravity in trimmed flight-- it is not. Because the tail creates a down-force that moves the center of pressure forward to coincide with the center of gravity, there is no theoretical string at the center of lift of the wing but the theoretical supportive string is now at the center of gravity. If the tail creates less down-force the average point of lift would move back behind the CG and there would be a nose down moment and because the center of pressure of the system would increase in magnitude due to the loss of down-force ( as well as move back) the wing moves up and around the center of gravity as the nose falls.
inventor
Well-Known Member
Yay, Adrock! Captain Luddite here, last night and this morning worked on a Word Doc explaining what you just said. I can PM it and have some more to add, if you'd like. My explanation has a couple folks I know do the 'Ooohhhh! I get it!' moment. Congrats, thought for a second there, we might have to hide the keys.
:beer:
:beer:
Adrock
Well-Known Member
Thanks invetor! I would very much like to see what you prepared on it. Its amazing how hard it is to come up with a correct answer when the facts you are using are incorrect! Thanks for the help.
Ziggyfuzz
Well-Known Member
Again a misconception....the tail down force in no way changes the center of lift! Positive stability can only be achieved when the CG is ahead of the CP. Unless the aircraft has a canard in some cases. If the CP and CG were the same there would be neutral stability and most pilots couldn't control the aircraft. The notion that there is an average CP is misguided. Each control surface has a CP, however the one that is most important in flight mechanics and stability is the center of pressure on the wing. See an earlier post of mine in this thread exactly about the phenomenon of a nose down moment that lift ALWAYS generates, thus the need for a tail to produce the good ole "tail down force". Check out William K Kirschner's Advanced Pilots Flight Manual; also Aerodynamics For Naval Aviator's. These both have great diagrams to go along with the physics and math, however Kirschner makes it really easy and less dry to read!
fish314
Well-Known Member
True. I think Adrock's concept was basically like an average center of lift for the whole airplane, and a center of pressure for the whole airplane. As a concept, this could work. We certainly COULD define a term which means the same thing as the average Cp for the entire aircraft, but it changes the terminology that we normally use and the ways in which we normally understand them. After all, it is true that in an aircraft in equilibrium flight if you sum the lift from the wing and the tail down force from the tail (and their respective moments) you do come up with a Force vector that is equal to weight and located directly over the Cg with zero net moment. After all, if you didn't, the airplane wouldn't be in equilibrium flight. It would either be accelerating up or down, and/or pitching.
But there is a problem in combining all of these terms. You need to separate them back out again to look at aircraft stability, or to look more precisely at what is really happening at the wing, or to look at downwash on the tail, or to look at structural stresses in the aircraft structure, or to look at several other things. And we don't typically reference them combined up this way using this same terminology... which means if you try to explain it to someone else you need to be very specific about the fact that you are using the words in a different way than other people typically use the words, or you are likely to confuse the audience.
Adrock
Well-Known Member
Yes stability requires the CG ahead of the CP but that is the CP of the wing not the entire system. If you include the downforce of the tail it brings the CP for the entire system forward. I was talking about the center of lift for the entire system not just the wing. Besides it must be true because I read it in a book! *sarcasm*
Adrock
Well-Known Member
Seriously I did read it yesterday in "Understanding Flight," and although unorthodox to think of CP for the entire system at once, it helped me understand. Using the negative lift of the tail to lower/move the average CP is really no different than using the negative lift created by upwash on the wing or the small pockets of HIGHER pressure that exist above a wing to influence the conventional average of lift for a wing. You all are correct that I will have to define my terminology when talking to a student.
Ziggyfuzz
Well-Known Member
....especially when trying to explain stability and control to a commercial applicant whom is expected to be able to understand these concepts. Take the building block approach, putting together step-by-step, the moments and forces that bring an aircraft into balance. As an examiner on the side, I can tell you that I have had many a Commercial applicant in front of me who really don't understand the concepts of stability and control. A leading question I use to break the "stability ice" is to ask why an aircraft will be more efficient with an aft CG as opposed to forward Tho thats a performance question, it directly involves the concepts here. Most don't really know why, beyond the answer their CFI told them to memorize, which shows the CFI doesn't really know it either. Ever wonder why we teach a lazy 8? It is a manuever that demonstrates a pilot's understanding of the concepts of stability and control, and putting those into practice. THe other indicator of someone who doesn't understand this is to watch them do a lazy 8 and totally over control the aircraft. Whats lazy about that?! With a very small amount of coaxing, the aircraft's diherdral (lateral stability), tail down force (longitudinal stability), along with minimal yet proper rudder coordination, an aircraft will fly a lazy 8 all on it's own! Hence the term lazy (not just a visual of an 8 on it's side as most manuals describe it). A well flown lazy 8 takes understanding and precision and is impressive when properly demonstrated, not a series of wing-overs!