I've got a question about flight characteristics and size.

[Air95]

I'v been experimenting with various scales of the same aircraft. They all had about the same weight per sq in. Of coarse they were different sizes and weights. The first plane was about 30 in WS with a 7 oz weight; the second was 21 in and 4.5 oz. The larger was tame and easy to fly but it's smaller counterpart was much more touchy and at certain times unstable. Any ideas? :confused:

[nmasters]

Area and mass don't scale the same. cubic wing loading (http://www.rcgroups.com/forums/showthread.php?t=604935&highlight=wcl) scales better. This spreadsheet (http://www.rcgroups.com/forums/showpost.php?p=11728258&postcount=3) can calculate the lifting ability of a wing at a given speed. Most of the stuff is flying wing specific but the 2D lift part is generic. Just put in your wing geometry, coefficient of lift (CL) and speed. If the result is different than the weight of your plane adjust the CL until it's right. You'll find that if the planes cruise at the same CL and power loading they'll handle about the same.

--Norm

[JetPlaneFlyer]

The smaller model has a 28% higher wing loading than the larger one, so it's not surprising that it handles different. That's because a 30" span model will have twice the wing area as 21" span (assuming same planform shape), so if the big model was 7oz then the little one would have to be 3.5 oz to keep the same loading.

On top of this you have Reynolds (Re) number considerations. As the wing chord gets less then Re decreases. Wings generally work less well at lower re numbers.

Steve

[fnev]

M = 1/2ρSV^2Cl

M = aircraft mass

ρ = air density

V = air speed

Cl = coefficient of lift

The coefficient of lift is linked to the Reynolds number. It means that air velocity and density with the chord of the wing section (length) have an influence on the capacity of the wing section to generate lift.

If we stay in a simple case and consider the air density to be constant, the coefficient of lift is related to the geometry (size of the chord) and the air velocity.

To have SIMILAR aerodynamic characteristics, a homothetic smaller aircraft will have to fly faster than its bigger brother. This is why when doing wind tunnel testing on a smaller scale model the air velocity is increased. You ONLY have to adjust to get the SAME Reynolds number.

This means that once you have determined what is the required Reynolds number to get the same Cl at the SAME angle of attack, you will know at what speed you need to fly. With the equation above you will then get at what mass your aircraft has to fly to have similar flying characteristics.

This is a basic approach as things are not as simple in a 3D flow because the wing plan form (and other more complex surfaces interactions) has (have) an influence. It is ALWAYS easier to scale up (within reason) than scale down.

I hope it helps, and for the sharp crayons: I tried to make it as simple as possible. With Profilli and a calculator our friend should be able to figure the solution with ease.

[nmasters]

It is ALWAYS easier to scale up (within reason) than scale down.

That is pretty much the heart of the issue. That spreadsheet I pointed to yesterday calculates Reynolds number for 3 places on tapered wings. There's a discussion of Reynolds number here (http://www.rcgroups.com/forums/showthread.php?p=9009659#post9009659)

I hope it helps, and for the sharp crayons: I tried to make it as simple as possible. With Profilli and a calculator our friend should be able to figure the solution with ease.
I agree, Profili is great for its ease of use. However it's nice to have the polars but you don't really need all that information. Just remember that the smaller plane will not be able to work at as high an angle of attack as the bigger plane.

--Norm

[fnev]

Just remember that the smaller plane will not be able to work at as high an angle of attack as the bigger plane.

--Norm[/QUOTE]

Can you elaborate on that statement? As long as the scaled down model flies at the SAME Reynolds number (therefore faster) it will have (without entering into complicated details and explanations) a SIMILAR flight envelop…

The Cl curve of the wing section will be the EXACT same.

[nmasters]

A1/4 scale plane would have to fly 4 times as fast as the full scale to have the same Re. Have you ever seen a big Cub fly 400 MPH?

--Norm

[fnev]

A1/4 scale plane would have to fly 4 times as fast as the full scale to have the same Re. Have you ever seen a big Cub fly 400 MPH?

--Norm

BINGO!!!

This is why we change wing loadings and wing sections, therefore the flying characteristics are changed. This was the question, if our friend now understands better how all these things are linked together, personally, I am satisfied. Thank you for your support.