Scale aerodynamic characteristics

[FoxLeader]

Hi,

I made a 1:3 scale model of my self-designed concept to be tested in a small scale wind tunnel, however there's one thing I still can't sort out:
How can I do the proportion between the results (essentially lift & drag) I had with the 1:3 to the 1:1 model?

I've heard opposing things on this. Somebody told my I just have to do x3, but I'm quite sure that's wrong... It has to do with the Reynolds number, right?

The goal is to get estimated stall speed, cruise speed, etc.

Thanks for your help!
Fox

[JetPlaneFlyer]

Increase the airspeed by the scale factor (i.e. 3x). This should give you the same Re as full size, same drag and same lift.

This should work ok providing your tunnel can go fast enough and providing speeds remain well under sonic.

Steve

[fnev]

Been there, done that… for work. It is NOT simple. Effectively if you have a 1/3 scale by increasing the speed by a factor of 3 it will give you the EXACT same performances. BUT: the wind tunnel installation (pylon, scale, etc) gets in the way and can screw the results big time. We spent a lot of money on the model, its integration in the wind tunnel to find out after running the tests for a couple of days that we were NOT able to use the results…

So, if your initial model is large and the 1/3 scale big enough for the wind tunnel installation to be not too obstructive, fine. As long as you are getting your x3 speed. If your initial model is relatively small, maybe it fits in a wind tunnel with the proper velocity… much cheaper on the long run.

Be careful: The smaller the model the more difficult to build…

[FoxLeader]

Thanks for your answer fnev. Unfortunatly it's impossible for me to fit the full-scale (139 cm lenght, 95 cm wingspan) in the wind tunnel. I may however be able to have access the university's wind tunnel, but for now I'll do without it.

Sure, the smaller model makes it somehow less accurate since small details or imprefections in it would be 3x bigger on the full scale, while important details in the 1:1 are smaller and harder to build in the scale model... I've worked hard to reduce any drag or flow distortion with the pylon which is placed on the CG, under the fuselage. It's a single metal slide which makes it quite slim.

The point of those wind tunnel experiments are to get a relativly accurate idea of stall and cruising speed with everything known (instead of manually calculating weight vs theorical lift, since this doesn't take in account drag created by the fuselage and control surfaces, the flow following the fuselage, etc...).
I know these won't be the absolute correct data, but it will also be interesting to gather data with the full-scale model to build a database for comparison with future experiments.

The thing I'm still unsure is about the Reynolds number thing. I've been told to look with this, but very briefly and I didn't quite understood how it has to do with scaling the results...

Thanks again!

[fnev]

Re = VL/ν

V = air speed

L = reference length (in this case the mean chord)

ν = Cinematic viscosity

[nmasters]

Here's a table of standard atmosphere up to 65,000 ft: http://www.pdas.com/e2.htm Kinematic viscosity is the second column from the right.

[FoxLeader]

So if I'm not mistaking, viscosity should be the same in both cases so am I right saying that

Re = V ∙ L / v
<=> Re ∙ v = V ∙ L
<=> v = V ∙ L / Re
so
V1 ∙ L1 / Re1 = V2 ∙ L2 / Re2

V1 being the wind tunnel's speed
L1 being the mockup's ref. lenght
Re1 beingh the mockup's Reynolds number
and vice-versa for V1, V2, Re2

Does it somehow makes sense?

[FoxLeader]

So I've thought about it, and I'm wondering.. Is it possible that with the mockup beng 21.5% from the full scale*, the difference in Reynolds is not that important to justify overly complex equations?

As I said, while I want useful results, the mockup isn't 100% accurate anyways so if there is a small difference, I would live with it by the time it's not overly important. So if anybody actualy know how big a difference it would do versus just multiplying to the scale, it would be really appreciated.

*Sizes are 139 cm lenght, 95 cm wingspan for the full-scale and the mockup has a 29,9cm lenght and a 20.5 cm wingspan.

[JetPlaneFlyer]

I's hard to say how significant it would be. A lot depends on the airfoil you use. each airfoil has a critical Re below which it no longer works very well. If the smaller scale model puts the airfoil below it's critical Re then the results wont be anything like represemtative of full size.

What was wrong with my original suggestion to increase the air speed by the scale factor?

Steve

[lincoln]

Increasing the air speed by the inverse of the scale factor is the correct way to get the Reynold's number right, assuming the air is around the same pressure and temperature as that you'll be flying in. Since the forces will increase by the square of the airspeed and decrease by the square of the size (because there's less area), the forces will have the same magnitude. However, moments on the model will be reduced by the scale factor because the same forces are acting on a smaller object.

[fnev]

Increasing the air speed by the inverse of the scale factor is the correct way to get the Reynold's number right, assuming the air is around the same pressure and temperature as that you'll be flying in. Since the forces will increase by the square of the airspeed and decrease by the square of the size (because there's less area), the forces will have the same magnitude. However, moments on the model will be reduced by the scale factor because the same forces are acting on a smaller object.


Moment or Cm??? Don't confuse our friend anymore. He is on the right track while complicating a very simple issue: increase the darn wind tunnel velocity!!!

[fnev]

An other thought: how come that someone having access to a wind tunnel can’t get help for his project from the people running the facility???

[Bobber Bob]

Hi Folks...

I remember reading an article some years ago about just what I believe is being talked about here, and the below link should lead you to an RCGroups disscussion about it, and it's value...


http://www.rcgroups.com/forums/showthread.php?t=43751

And here is some more info on the same subject.... Ain' Google wonderful ????

http://members.cox.net/moorman1/WingLoading.htm

Bob Reynolds
. "ComeUpHere"

[bwalt822]

I thought when you match Re by increasing the velocity by the inverse of the scale factor that the wind tunnel results would just match your non dimensional coefficients like CL and CD. The actual forces wont match.

[JetPlaneFlyer]

I thought when you match Re by increasing the velocity by the inverse of the scale factor that the wind tunnel results would just match your non dimensional coefficients like CL and CD. The actual forces wont match.

The forces should match because lift and drag are proportional to velocity squared x area

A 3 fold increase in velocity will increase lift and drag by 9 times. The 1/3 scale model decreases area by the same margin.. So one exactly ballances the other.

[bwalt822]

I guess that is true for below compressible speeds.

[vintage1]

The forces should match because lift and drag are proportional to velocity squared x area

A 3 fold increase in velocity will increase lift and drag by 9 times. The 1/3 scale model decreases area by the same margin.. So one exactly ballances the other.


No. You dont WANT the same lift and drag as the full size.

You decrease speed by the square root of the scale. That gets the aero stuff right, and the right L/D . The lift will be the cube root of the scale factor, and so will the drag probably.

[JetPlaneFlyer]

No. You dont WANT the same lift and drag as the full size.

You decrease speed by the square root of the scale. That gets the aero stuff right, and the right L/D . The lift will be the cube root of the scale factor, and so will the drag probably.

Nope.. Your Re will be unrepresentative of the full size if you reduce the wind speed therefore the results will be useless. Increasing wind speed by the scale factor gives the same Re as the full size plus same drag and same lift. Increasing speed by the scale factor to simulate full size Re is standard practice in wind tunnels, but is limited by tunnel speed capability and mach number. Tunnels that can operate at multiples of atmos pressure are another option but they are expensive.

Root of the scale factor would be the speed the scaled down model should fly at.. but that's not what you want the wind tunnel to tell you. You would be looking to simulate the behavior of the full size aircraft, not simulate the flying conditions of the scaled down version.


Steve

[vintage1]

I am fairly sure that the wave patterns on boats dont work at high flow rates..to get the flows right you run at much less than full size speeds.
.

[bwalt822]

I just remember what we were told in my aerodynamics courses. If you want your non dimensional coefficients to match, you have to have dynamic similarity (google it). Dynamic similarity for an airplane in a wind tunnel are determined by reynolds and mach numbers. Mach number becomes irrelevant below compressible speeds.

Now if you are testing things like droplet or wave formation then there are many other non dimensional values to match as well such as Froude number.

[markschaffin]

Full scale Reynolds number testing is very expensive, which is why few companies do much of it. Sure they do it, but for airliners, there are only a couple of tunnels in the world that can do it. Most size the model to best work in the tunnel. As large as possible such that wind tunnel wall corrections can be trusted. Then the engineers use various techniques to correct the data to the flight Re.

For a model, I'd say build it and fly it. It will cost a lot less. Wind tunnel testing is not as simple as it looks, especially when you want good quality data.

[FoxLeader]

I just found a little software named RcCalc...
http://www.e-voo.com/downloads/rccalc.zip

Does anybody knows about its reliability (regarding its results)? Seems interesting and very helpful to me (don't forget to change the language from spanish to english in the toolbar).

[Renold]

Fox,

Depend what you what to measure. There are some thing called Dimensional Analysis.

For example, if drag is the subject. Then drag cofficient Cd defines as follow.

Cd = D/(v*s)

D = total drag force
V = wind speed
s = planform area of wing

Results from different wind speeds could be ploted for partical scale. When you have larger wing area and known flying speed, the total drag can be worked out with the corresponding Cd.

There are other test such as lift cofficient, pressure cofficient....etc

Hold you could share your test results.