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Lift on a wing

teflyer

Full Circle
#1
Hello,
I am not sure if this is really the right place to put this, but...

... if I was to test how much lift an airfoil has in a wind tunnel, how would I measure it? i.e. Are there any ideas on how to cleverly set up a system to measure lift without inhibiting the aerodynamic properties of the airfoil with the creation of any exterior turbulence.
 

colorex

Rotor Riot!
Mentor
#2
Hard to explain with words. You have a section of the airfoil inside the tunnel, this must slide up an down on four bars that keep it at the right angle of attack. Then a piece of string tied to the center of the airfoil goes through the floor of the tunnel to a weight that lies on a scale. As the airfoil lifts and pulls the string, the reading on the scale goes down. The difference between the highest and lowest reading will be the maximum lift of the airfoil (in a perfect world)

Hope this helps.
 

pgerts

Old age member
Mentor
#3
You will find a lot on google and youtube http://www.youtube.com/watch?v=6UlsArvbTeo.
Lift by itself is hardly interesting.
Lift to drag is what you are asking for.
A profile for low speed high lift (STOL plane) is not at all similar to a profile for high speed (AN 225).
There are wind tunnels for rent and you will probably find a lot of technical information if you search.
 
#4
Lift is produced by multiplying the lift coeficient of the airfoil x half the density of the air X surface of the wing X velocity x velocity

lift.JPG

or if you want to use it in a wind tunnel; design a Z shaped piece of wood; the top of the Z you concet to the top of the airfoil, and the bottom of the Z you conect to a weight scale .... blow some wind and read the sacle measurement !!
 

teflyer

Full Circle
#5
Thanks all!
Colorex:
I was thinking something similar as well.

pgerts:
Since you reccomend finding a lift to drag ratio, how do you find how much drag an airfoil has. I would think there would not be any relatively straightforward way to measure that.

KKArioKA:
Thanks, but I am not sure what you mean by the Z piece of wood.
Actually, reading over your post again, I think finding the lift coefficient would be interesting. Would it?

Note: I want to test some things with the KF2 airfoil.
Once I get this process done, I probably will need some suggestions if how to correctly make the KF2
i.e. should the leading edge be sharp or blunt?
 
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colorex

Rotor Riot!
Mentor
#8
Just make sure your wing slides easily up and down. Then you're good to go.

The problem with your design, KKArioKA, is that as the wing lifts, the angle of attack changes, and the lift increases as does drag.
 

teflyer

Full Circle
#9
What do you guys think of investigating the lift coefficient of the KFm2 airfoil at different step thicknesses to find the ideal step height? (I need to explain what I am going to do to the college before they let me use the wind tunnel)
 

pgerts

Old age member
Mentor
#10
pgerts:
Since you recomend finding a lift to drag ratio, how do you find how much drag an airfoil has. I would think there would not be any relatively straightforward way to measure that.
I am sorry - I am not specially interested in reinventing things that others have done.
If you find a wind tunnel there will probably be all equipment needed to measure and record all your data.

One thing that is a misunderstanding according to lots of professionals is that the profile makes the most lift.
It is more the angle towards the wind that makes the lift. The profile is to minimize the drag.
The wing needs some thickness to be strong but the thinner profile the better.
Flaps make the profile thicker against the wind (actually changes the angle) and gives more lift but a lot more drag. That is only needed to get the speed down for take off and landing.
The KF profiles are just easy to make from sheets of foam. The added thickness makes them more rigid and with slow flying planes the drag is not so important.
 
#11
this might help you; it has some useful data on it


one thing you should evalute on choosing the best airfoil is the size ... small planes need to be light ... cant use KFm8 on them as will be too heavy !!

 
#12
The wing needs some thickness to be strong but the thinner profile the better.
Flaps make the profile thicker against the wind (actually changes the angle) and gives more lift but a lot more drag. That is only needed to get the speed down for take off and landing.

take note in the formula i posted; since Lift is generated by x speed x speed ( speed at second power ? dunno the term in english )

much thinner - low CL (lift coeficient) on the wing - the wing can be because a small increase in speed will increase alot in lift... as in subsonic/supersonic flights, airliners etc .... they have low profile wings but they use flaps to increase the CL for takeoff and landing where the speed is low ....
 

colorex

Rotor Riot!
Mentor
#13
Now, if you want to roughly measure drag, you just need to do the following:

- First, the wing needs to roughly keep it's position and it's angle of attack.
- Drag is just the push of the wind on the wing, a force going parallel to the wing.
- You need to transfer that force to a scale, one of those hanging scales would be good.
- Tie a string to the leading edge, and to the scale.
- The airfoil sample should be able to move back and forth freely, very freely.
- The airfoil needs airflow over and under the wing.
- When the air passes along the wing, it should create drag, which is the force that wants to push it backwards. That is recorded by the scale as grams of drag of an airfoil.
 
#14
pgerts:
Thanks. So how does an airfoil work? From what I read, there is something called a circulation effect (that I still do not really understand) that has the air flowing from tail to nose on the bottom and nose to tail on the top. When the airplane starts moving, the air flow on the top speeds up while the airflow on the bottom slows down. With Bernoulli's principle, there is lower pressure on the top of the wing, then the wing moves up.

or are you just saying the pressure difference on the airfoil is just not enough to lift the whole weight of the airframe. Instead, the major part would be the ange of attack the plane has to redirect the air down.

Note: I am not sure if the wind tunnel that is available locally has any equipment in it to measure those variables since it was built as a gradueate project and the college is not exactly one that is focused on aerodynamis.

KKarioKa
I think I decided on the KFm2 since it is the simplest to make and most clear to change the step on since there is only one step.

Colorex:
Thanks! I will probably use that idea orsomething similar if I am goingto measure drag as well, depending on what pgerts says.
 
#16
Thanks, I have done some scratchbuilds with the KFm2 and KFm3 although they were hardly successful due to my own newbie mistakes. What I want to do is do a research project for school on the ideal step height on the Kfm2 and maybe on a more personal interest compare it with a clark Y. I would need for the data to be as accurate and reproducible as possible.
 

pgerts

Old age member
Mentor
#17
pgerts:
Thanks. So how does an airfoil work? From what I read, there is something called a circulation effect (that I still do not really understand) that has the air flowing from tail to nose on the bottom and nose to tail on the top. When the airplane starts moving, the air flow on the top speeds up while the airflow on the bottom slows down. With Bernoulli's principle, there is lower pressure on the top of the wing, then the wing moves up.
or are you just saying the pressure difference on the airfoil is just not enough to lift the whole weight of the airframe. Instead, the major part would be the ange of attack the plane has to redirect the air down.
Note: I am not sure if the wind tunnel that is available locally has any equipment in it to measure those variables since it was built as a gradueate project and the college is not exactly one that is focused on aerodynamis.
You are right.
An example - Piper CUB, a common plane, has a high lift wing. when it starts it looks like the tail is above the wing (funny).
Just adding speed gets the plane to lift. The wing normally has an angle of more than 3 degrees compared to the stabilizer. That is what makes the majority of the lift.

When you build a model you need the http://www.robart.com/products/model-incidence-meter if you are serious. This is one of the tools you need to your wind tunnel experiments. The angle (incidence) has to be 0 if you are to measure only the lift produced by the air foil. The angle is measured from the midpoint of the front edge to the mid point of the trailing edge.

You will find spring meters at the school physics lab - better than scales but in Newton (used to measure friction and other forces). You need two - lift and drag.

You will also need a wind speed (anemometer) - possible found in the school. Place the anemometer after the wing not to disturbe the air flow. Better is i pitot tube to measure the wind speed if it could be found in the school.
 
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#20
Thanks pgerts! I will probably buy that wing incidence meter later on. (if I am going to do this project, it will be during the summer)

And I will probably get an anemometer becase I think it is cheaper
EDIT: Actually I probably will go with a pitot tube. I looked on Hobby King and this one is as cheap as an anemometer. Also it probably will disrupt the air a lot less.
Hobby King Airspeed Expander V3

Colorex: The only problem I am worried about the method you suggested is that the wing may flutter from side to side like how a streamer does. Maybe if I put vertical sablizers on the wing, it would help some.
 
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