Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Ex-jazz on March 28, 2009, 04:43:24 PM
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Hi
I still try to understand the airplane aerodynamic forces and now I'm curious how much lift the wing and tail must generate on level flight with certain plane weight at certain speed.
I coded a calculator to test a different settings, but I am not sure if my wing & tail lift results are totally off or not.
(http://fdm4bge.1g.fi/Files/10001/apics/airplane_forces.jpg)
The Cl, Cm, & Cd figures are generated with linear interpolation, between -5 - 11 aoa degrees. The static margin is ~10%MAC.
Is there available in public a real plane data, from where I could see/calculate the forces on cruising condition?
Thanks
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If your question is only how much lift does a plane in level flight produce, you are making this more complicated than it needs to be.
A plane in level flight generates exactly as much lift as it weighs. Any more lift (from airspeed or angle of attack) will cause a climb, and less lift will cause a decent. Your math appears to be right as your total lift is within an acceptable margin of error of your total weight.
Also, outside of very few specialized designs like canards, the tail surfaces of most aircraft do not produce lift, but instead actually produce a downward force opposite of lift. It is one of the reasons airplanes are aerodynamically stable, and can be trimmed with the elevator to fly at a constant speed and return to that speed if upset.
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Hi AKHog
Thank for your feedback.
I wasn't clear with my opening post.
I know the plane lift must match with plane weight. What I'm after is that longitudinal stability and involved linear & rotational forces.
I would like to know with what kind of wing lift(+) and tail lift(-) forces the known real life plane is flying on level with certain weight & speed. This could be a good reference for later on testing.
This is a short animation from calculator in action. NOTE! The plane figure is just a symbolic:
http://fdm4bge.1g.fi/Files/10001/apics/Force_cal_demo.avi
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What I'm after is that longitudinal stability and involved linear & rotational forces.
I'm not sure what you are getting at. Your question involves a lot more variables than your model takes into account.
These forces would also be very different from plane to plane.
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Actually if you take the supercalifragalisticexpialado cious and compound it with the fractional momentum of the hypotinuse of the rubicks cube, you would come to the fairly obvious conclusion that I had no idea what I was talking about!
Math is cool. :cool:
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Ex-Jazz, if I understand your intention correctly, I am afraid this is going to be a little complicated.
The force that the tail must apply on the wing (lets call it elevation torque) needs to balance the torque that the airflow applies on the wing. For this you need the pressure distribution around the wing surface. Once you have that, you can integrate the forces and calculate the force moment. Perhaps there is a way to estimate this from the averaged air speed difference above and below the wing and apply some vorticity/angular momentum argument, but I'll have to think about this.
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The numbers are going to vary considerably with changes in loading as well. Any CG movement will have an effect.
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Actually if you take the supercalifragalisticexpialado cious and compound it with the fractional momentum of the hypotinuse of the rubicks cube, you would come to the fairly obvious conclusion that I had no idea what I was talking about!
Math is cool. :cool:
Yo Boner,
Let me simplify it for you:
The limit, as time approaches infinity, that you will ever understand this thread, equals zero.
:D :salute
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Hi
Thank you for your feedback.
Yes, the aerodynamics is a bottomless swamp of mind bending equations :rolleyes:
I try to understand how this diagram is working and model it with python language.
(http://fdm4bge.1g.fi/Files/10001/apics/Forces_diagram01.jpg)
Current problem
The wing and tail are generating the lift based moment and I try to figure out how these moments are added to the final calculation.
# t = moment ( 0.5 * rho * v**2 * area * Cm)
# r = moment arm
# F = force
# F = wing moment(t) / moment arm(D1?)
Because the F will be negative, should I subtract it from the wing lift or add it to the weight or something totally else?
Thanks
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I'm not sure what you're looking for here, but:
I have static stability equations, trim drag equations, etc.
It looks like you're trying to find the neutral point?
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I, too, am confused as to what you are really asking about here.
You are asking a very complicated question that requires a lot of math and several variables, but seemingly either don't understand or are ignoring some very basic underlying concepts.
It sounds like you have an interest in the math behind flight. I would suggesting getting a basic aerodynamics text book, it would likely answer most of your questions and more.
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Hi
You are right. The subject seems to be far more complex than I though... (sigh).
It's a time to buy a book about the aerodynamics and take a good time with it. Few times...
Here is a last ' calculator demo' video(1.8M) , which most likely have just a 'wtf?!' entertainment value from aerodynamics point of view.
http://fdm4bge.1g.fi/Files/10001/apics/Force_cal_demo2.avi
'I'll be back' :)
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At least when I have been designing RC-planes, I start from wing profile, which has it's own Cm characters and then I try to extend that to entire wing and tail surfaces. In other words I don't see much point to calculatte tail moments without considering first the profile Cm.
I have a nice little program called Profili2 which contains a large profile library and which can generate NACA profiles so basic analysis rather easy with it (there is Xfoil built in). However, I have not designed anything from scratch with it yet but have been trying to improve existing designs.
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Hi Gripen,
I'm not trying to design the new plane from clean table. No. I'm trying to 'read' the real life planes in some level and calculate/estimate/guess the acting forces to get in same ball park with figures.
Lets see if I every will get there.
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Aerodynamics related question... I've seen it a couple times now... what is moment?
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Moment is actually a physics term I believe, but it basically refers to the force imparted on an object from a certain direction (vector) that results from the force times the arm. The arm being the distance between where the force is applied and where it is imparted.
A simple example if I got my terminology incorrect above is a box wrench. Your hand turns the wrench (force). The distance between your hand and the bolt you're turning is the arm, and the moment would be the resulting force you impart on the bolt.
(Physics majors be gentle... :confused:)
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Thanks stoney :)
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Moment is actually a physics term I believe, but it basically refers to the force imparted on an object from a certain direction (vector) that results from the force times the arm. The arm being the distance between where the force is applied and where it is imparted.
A simple example if I got my terminology incorrect above is a box wrench. Your hand turns the wrench (force). The distance between your hand and the bolt you're turning is the arm, and the moment would be the resulting force you impart on the bolt.
(Physics majors be gentle... :confused:)
That is exactly right but a little over complicated. Moment is just another word for torque.
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Physics related question... I've seen it a couple times now... what is moment?
Fixed.
Look up moment arm. Basically, its a component in the equation to calculate the force of a lever.
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Opps I didn't see the second page before I replied.
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That is exactly right but a little over complicated. Moment is just another word for torque.
Actually in typical engineering usage, moment & torque are used in different contexts. We'd never refer to how much moment an engine produces, or how much the peak torque would be in the middle of a bridge span. Moments (at a point) are considered to be produced by force(s) some distance (the moment "arm") from that point. So a moment is generally associated with bending, hence the term bending moment. Torque is usually associated with a pure twisting "force" (although in purist's terms, the word "force" is incorrect). A good example of torque would be that transmitted by a drive shaft.
Oh, and ignore the guy from Carolina. I think he just makes stuff up. :D
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I understand perfectly what you are saying but for what he is trying to do the plain meaning of each is interchangeable and expressed in the same units.
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I understand perfectly what you are saying but for what he is trying to do the plain meaning of each is interchangeable and expressed in the same units.
I wasn't my goal to correct you, but to simply standardize the terminology for future communication. It should also help the OP to better understand any technical references he comes across in his search for info.
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Don't know if the following NACA report will help, but it does provide calculations and some direct data about tail surface forces.
Flight measurements of aerodynamic loads on the horizontal tail surface of a fighter-type airplane
John B. Garvin
naca-tn-1483
Nov 1947
A comprehensive investigation was conducted to determine the loads applied to the horizontal tail surface of a fighter-type airplane in maneuvering flight. Differential -pressure-distribution methods were employed to obtain the values of load. Loads were measured at equivalent airspeeds ranging from 125 to 380 miles per hour and at accelerations up to 6g.
http://aerade.cranfield.ac.uk/ara/dl.php?filename=1947/naca-tn-1483.pdf
Enjoy!
Note:
Actually this particular report was mentioned several years ago here in a long discussion about how planes should react after their horizontal stabs and elevators are shot away i.e. nose up or nose down.
:aok
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:salute Old sport
The page 9 give an answer. I assume the main wing airfoil moment is also effecting at 'Xt' I.E. Neutral Point.