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Lift Force Calculations 
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#1
Jul2811, 07:17 AM

P: 11

Hi, I'm right now coursing IB (equivalent to Alevels) and one of the compulsory essays we must do is called Extended Essay. It's an essay you do on a topic you choose. I choose aerodynamics. Created a "wind tunnel" out of plastic and made it rigid with hard plastic pieces which were partially flexible, and I also created a wing out of balsa wood. Now, I calculated the lift assuming the acceleration was uniform to then use the uniformly accelerated equations, which were then displayed in force diagrams. However my results when plotting lift generated vs angle of attack it gave me the optimum angle 45º, and the shape of the graph was an inverse x^2, clearly it is not the correct graph.
I know the results must be innacurate due to my assumptions, but does anyone know how to calculate the lift with another methodology? I also use the modern lift equation but I want experimental datas :) Thankss 


#2
Jul2811, 06:50 PM

P: 365

In a typical wind tunnel test the wing should be fixed as the air rushes past it at a constant velocity. The forces acting on the wing are then usually measured with a force balance. The air accelerates locally over the wing but I do not believe that is the acceleration you are referring to. The calculation of lift for an arbitrary body usually requires the use of panel methods or more sophisticated techniques. 


#3
Jul2911, 03:18 AM

P: 11

I haven't said much about the actual experiment... my bad, I'll explain. I made two holes on each side of the wind tunnel, where I put two wooden cylinders. Then there is a squared platform which attaches both cylinders at both ends of the platform where the wing will lay. Then in that platform there are two vertical cylinders. At the side of the wing there are attached to it two small circles which go through the vertical cylinders. So to calculate the force I time how long it takes the wing to go from the platform (resting point) to the top of the vertical cylinders. Therefore I have displacement, time, assuming acceleration is uniform, I can calculate the acceleration using one of the uniform accelerated formulas. Then newton second law nad I find the resultant force upwards, which is the lift. Of course there is friction but I can't get into that much detail due to the fact I do not have available any cool apparatus, no sensors, nothing. I built an anemometer to calculate the velocity of the fan, to then compare my experimental results with theoreticla results (Lift equation).
The whole set up is hard to explain, tell me if you understood. THank you very much. 


#4
Aug1511, 03:49 AM

P: 6

Lift Force Calculations



#5
Aug1511, 05:08 AM

P: 11

YKobe23, yes there is a lot of friction I must say, however I think it's the same in all AoA's, so therefore the only problem is I get less lift than there really is. However I might be wrong, because my results right now are wrong, so...
Guille. P.S What are you doing it on now then your EE? 


#6
Aug1511, 09:00 AM

PF Gold
P: 1,503

One problem I foresee is that your wing is so close to this platform that the platform will affect your results. If it starts out resting on the platform, you won't even get the thing to lift off.
You would be better off just attaching the wing to the horizontal cylinders that leave the tunnel and Gavin those cylinders on a vertical track instead of just a hole. Attach some springs or rubber bands of known (or measurable) spring constant to cylinders to hold it at the bottom of this track and measure how far it moves against that spring force. 


#7
Aug1511, 09:04 AM

P: 11

Boneh3ad, You are right in that the wing is too close to the platform, however I did introduce some wood pieces so some air could get underneath the wing, making it lift, however I tried already the idea of a spring, using a newtonmeter, the smallest I found, and the force upwards is too small to distinguish the force produced at different angles :/



#8
Aug1511, 09:33 AM

P: 6

is the friction the same whatever the AoA ? I think if you increase the angle of attack the amount of air passing under and over the wing increases, and therefore more molecules hit the wing and doesn't that create more friction ? or am i wrong ? Well i started with Daniel Bernouilli's principle in order to explain lift and now i wanted to find mathematically and experimentally the lift formula. And then to finish explain drag, and all that stuff... Yannick 


#9
Aug1511, 09:37 AM

P: 11

Normally yes, however in my experiment due to the fact the resting point of my wing is on a wooden platform, this restricts the air flowing under the wing, therefore no matter what AoA the air molecules under will be nearly consistent, which is what I think causes my data to be wrong. However the friction your talking about is tiny, air molecules cause friction, but the worst is the friciton caused by the poles when the wing is lifted.



#10
Aug1511, 10:44 AM

PF Gold
P: 1,503




#11
Aug2111, 07:22 AM

P: 1

Can someone help me on how to calculate lift distribution function of linearly tapered wing. I have to design a composite UAV wing.my email is cloudiuschuene@gmail.com



#12
Aug2911, 12:02 PM

P: 841

If you don't allow air to freely flow beneath the wing, then you don't have a wing. You have an air trap. If you don't have a chord length of space between the wing and the "bottom" of the wind tunnel (or really, a lower surface) then you are ruining your experiment. I would go much further than a chord length, in fact.
You are creating a mini wind tunnel beneath the wing which is not only causing a TON of drag (screwing up your results) but is also speeding up the air flowing under the wing, which causes a less significant pressure drop, which again, screws up your data. 


#13
Aug2911, 07:28 PM

P: 11

Okay I get the idea my data is screwed, so if I fix the airflow below I should be okay? I'll try that then and see how it goes.
Another question: Anyone used javafoil? Thanks to everyonee. 


#14
Aug3011, 05:05 AM

P: 6

Can someone tell me if Bernouilli's principle ( 1/2p(v1)^2+ P1= 1/2p(v2)^2+ P2 ) correctly explain the theory of lift ?



#15
Aug3011, 08:07 AM

PF Gold
P: 1,503

No. It is impossible to fully explain lift without viscosity. Bernoulli can explain why the pressures are different as a result of the differing velocities, but it can't explain why the velocities are what they are.



#16
Aug3011, 08:39 AM

P: 841

Bonehead, I agree that Bernoulli can't FULLY explain lift. But it can explain the pressure differential that defines it. It can provide a correct understanding of lift even if it doesn't accurately or fully describe it, no?



#17
Aug3011, 09:41 AM

PF Gold
P: 1,503

Try and explain lift using only Bernoulli. The first thing you will say is there is a pressure differential. My question then would be "Why is there a pressure differential?" The answer is that from Bernoulli's equation, the air speed over the top, which is smaller than that over the bottom surface, leads to a lower pressure on top. "Alright, so why is the velocity on top faster?" At this point, you can't answer the question using Bernoulli. I invite you to try, but I am telling you it can't be done. Lift cannot be properly described without viscosity. That is why in potential flow around an airfoil, to get the correct lift you have to add a vortex at the trailing edge. 


#18
Aug3011, 09:48 AM

P: 6




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