Is there a more accurate way to calculate lift force in aerodynamics?

In summary, the conversation discusses a student's extended essay on aerodynamics and their experimentation with a "wind tunnel" to calculate lift. The student has concerns about the accuracy of their results due to assumptions made and possible sources of friction. Other students offer suggestions and advice for improving the experiment.
  • #1
F1Guille
11
0
Hi, I'm right now coursing IB (equivalent to A-levels) 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
 
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  • #2
F1Guille said:
Now, I calculated the lift assuming the acceleration was uniform to then use the uniformly accelerated equations, which were then displayed in force diagrams. H

You say you calculated lift assuming the acceleration was uniform. What acceleration are you talking about?

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
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
F1Guille said:
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.

I'm doing my extended essay also on lift and i thought also about doing your experiment but isn't there to much friction in order to get accurate results ?
 
  • #5
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
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
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
F1Guille said:
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?

Guille,
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
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
F1Guille said:
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 :/

Even with a little space between the wing and the platform, you will have significant wall effects. The majority of the airflow is likely going to just pass under the platform as opposed to going between it and the wing except for the case where the distance between the wing and the platform is significant. That is why in a real wind tunnel, you don't see the test articles being installed too close to the ceiling or floor. You end up with a large amount of blockage and it will significantly affect your results.
 
  • #11
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
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
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
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
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
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
Travis_King said:
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?

No. It can get you a pressure from a velocity field. That is about it. If you already know the velocity field, then yes, Bernoulli can get you the correct (or nearly correct) value for lift (or lift coefficient). However, it still does not explain lift. It also will not provide a complete understanding.

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
boneh3ad said:
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.

Do you think for IB (similar to A levels) Bernoulli is enough ? What does completely explain the lift theory then to explain the velocities? Thank you
 
  • #19
I agree that Bernoulli is insufficient to fully explain the value of lift. And I also agree that in order to properly explain velocity fields you need your viscosity. But to explain the theory of lift, ie why a wing creates lift, it will serve as a pretty good description.

What I mean is that if you make the assumption or observation that air is traveling faster over the wing than beneath, you can adequately explain lift with Bernoulli.

Think of the spoon and faucet water experiment. Run water from a faucet and hold a plastic spoon neer the water. You will feel the spoon tend toward the water. You don't need to know ANY values to demonstrate theory.

That being said. I would still say no. It can't correctly explain lift in a complete fashion.
 
  • #20
ykobe23 said:
Do you think for IB (similar to A levels) Bernoulli is enough ? What does completely explain the lift theory then to explain the velocities? Thank you

I don't know anything about IB, so I couldn't tell you what is and isn't sufficient.

I can tell you where lift actually comes from though. In an inviscid flow over a lifting body with a sharp trailing edge (as is the case with nearly all airfoils), the flow would start from the forward stagnation point and travel around the wing and leave from a rear stagnation point that depends on angle of attack. If the stagnation point just happens to not be at the trailing edge, the flow will just wrap around and find the rear stagnation point. This results in no lift and is predicted by inviscid theory.

However, in real life with a viscous fluid, for the fluid to wrap around the sharp edge like that would require the velocity to go to infinity. Clearly this can't happen. The incredibly fast speeds around that trailing edge lead to the formation of the starting vortex, which is soon shed from the airfoil, which then has an equivalent circulation around it since vorticity must be conserved. This bound vortex, as it is called, is what enforces the sharp trailing edge as the rear stagnation point and what leads to lift. The bound vortex circulates in such a way that the upper surface of the airfoil sees faster moving air than the lower surface, leading to lift.

This is often called the Kutta condition, and the value of this bound circulation can be used with the Kutta-Joukowski Theorem to directly calculate lift.
 
  • #21
I don't know much about IB either. But any respectable aerodynamics program will require an understanding of the K-J equations. You can UNDERSTAND lift with Bernoulli, but you can't explain it without vorticity, the K-J equations.

Have you done any thin airfoil theory?
 
  • #22
F1Guille,

I suggest you fix the foil and measure the force with strain gauges, which are not expensive. It is important to do a good signal conditioning (Weathstone bridge, amplification, ...) and calbration, e.g. with know weights.
 
  • #23
The Kutta condition does not explain lift. It is an observation that, as boneh3ad said, the fluid can not flow at an infinite speed around the sharp trailing edge. Without viscosity the Kutta condition would not occur and there would be no lift, the primary use of the Kutta condition is in potential flow theory so that this important effect of viscosity can be included without having to incorporate any of the more difficult effects. If the Kutta condition was not enforced the pressure would still change along the surface of the airfoil. You would still have a stagnation point at the leading edge and then the pressure would decrease as the flow accelerates around the leading edge. But the stagnation point would be on the upper surface and if you integrated the pressure distribution the net force would be zero, similar to a cylinder.

Bernoulli does not explain lift because it simply relates pressure and velocity ALONG a streamline. I also don't think it can help you understand lift because it does not explain why the velocity increases (pressure decreases).

Lift is a result of streamline curvature. The streamlines curve because the body (airfoil) forces them to curve. When streamlines curve a pressure gradient results ACROSS streamlines. This result is not very difficult to derive from the Euler equations. The result of this streamline curvature is that the pressure on the upper surface is lowest near the surface and increases as you move away from the surface into the freestream. The reverse is true on the lower surface. The pressure is highest on the surface and increases as you move down into the freestream.
 
  • #24
Thanks all of you guys, managed to fix my experiment! good discussion I'm reading here, but my question has been solved. Thanks to all of you once again, very appreciated.
 
  • #25
Wow...I feel so lucky to have found this forum.
I'm going to kick butt in college.
There are some very helpful threads by very helpful users in this site.
 

1. What is lift force in aerodynamics?

Lift force in aerodynamics is the upward force that is generated when an object, such as an airplane wing, moves through a fluid, such as air. It is the force that allows an object to stay in the air and is essential for flight.

2. Why is it important to accurately calculate lift force in aerodynamics?

Accurately calculating lift force is important because it allows engineers to design and build more efficient and safe aircraft. It also helps pilots to understand how their aircraft will perform in different conditions, such as during takeoff and landing.

3. What is the current method for calculating lift force in aerodynamics?

The current method for calculating lift force in aerodynamics is based on the principles of Bernoulli's equation, which states that as the speed of a fluid increases, its pressure decreases. This is used in conjunction with the concept of airfoil shape to determine the lift force generated by an object moving through a fluid.

4. What are some limitations of the current method for calculating lift force?

One limitation of the current method is that it does not take into account the effects of turbulence, which can significantly impact lift force. Another limitation is that it assumes an idealized airfoil shape, whereas in reality, most airfoils have imperfections that can affect the accuracy of the calculation.

5. Is there a more accurate way to calculate lift force in aerodynamics?

Yes, there are other methods for calculating lift force in aerodynamics, such as computational fluid dynamics (CFD) simulations, wind tunnel testing, and flight testing. These methods take into account factors like turbulence and airfoil imperfections, making them more accurate than the current method. However, they also require more resources and time to conduct.

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