CG problems after scaling

[suvansh]

I want to know that will the scaling of any Airplane's plan will affect the CG point.
I did made a 50% old fogey and flew it and it didn't flew well. I am working on a 145% FT DUSTER SO I WANT TO CONFORM That the CG POINTS WILL REMAIN THE SAME, or they will shift a bit.

Thanks for the help!!

 

[xuzme720]

Sometimes scaling down doesn't work. Weight is already working against you trying to go smaller and more so since air doesn't scale. That being said, the CG should not need to change when scaling, especially when going up. As long as the airfoil remains the same, and all the other factors scale along, the center of lift and other aerodynamic forces should stay in line, and the center of gravity should not need to change. If it does need to change, then usually it means that some other change has been made in addition to the scaling up.

 

[rcspaceflight]

I thought Bixler said in a podcast, or somewhere, that smaller planes have a tighter CG allowance. So the smaller the plane, the more of a pin point the CG is. It has to be dead on. Where a bigger plane, like a real plane, flies perfectly fine without the CG having to be exact.

And sometimes when you scale you change the center of lift and other factors that xuzme720 pointed out.

 

[xuzme720]

I guess I should have said "Sometimes scaling down doesn't work well". But even if everything else is equal, making it smaller is automatically going to make the cg range smaller, too. Additional factors can add to that but if you make a 50% smaller plane, that is identical in every way to it's big brother, the only difference being that it is half the size, you have essentially given it more drag to contend with due the relative size of the air molecules it "sees". The smaller version is going to be flying in essentially a denser fluid than the larger (yes, air is a fluid). Also, if big brother has a comfortable CG range of a 10 or 20mm, little bro is down to 5mm or 10mm so you can see how it can get out of hand very quickly. And this doesn't even take into account materials thickness and mass distribution, which unless you can find a half size foamboard, IS going to be different.
Now, these are reasons why it is harder to scale down, but the center of lift on the plane and it's relationship with the center of gravity and where the CG needs to be doesn't change with scale.
Sorry, I didn't mean to ramble on. Aerodynamics is one of those things that just isn't easy to explain, even when you keep it simple...

 

[earthsciteach]

I agree with Xuzme. Where the cg location SHOULD be will not change. But, you may have to distribute weight differently as compared to a different scale to achieve that location. I don't know if this would cause any issues, but the weight of the plane will not scale linearly.

 

[rcspaceflight]

Maybe I should clarify my point. The CG should be the same, but maybe not exactly. I think Bixler tends to put the CG mark on plans a little tail heavy which is why he always says "set the CG a little nose heavy". Because a smaller plane needs to be even more exact, if you set the CG exactly where the plans say, it may end up too tail heavy. Or simply that it's possible that the CG mark is a little off and that little off gets exaggerated when you scale down.

So the actual CG should be exactly the same. But it's hard to tell just where the CG is exactly with a bigger plane. It becomes noticeable when you scale down.

But every scratch build is a little different. You may have accidentally made the elevator a little bigger or smaller. So you should always fine turn the CG with how it flies/glides. But the starting point should be exactly the same when you scale up or down.

 

[xuzme720]

Sorry RCSF. My long winded post might have sounded like I was disagreeing with you, I was only trying to 'splain why the cg is more finicky on a smaller scaled plane. You had it right and I too remember Bix mentioning it in one of the videos.

 

[rcspaceflight]

Well, I just wanted to explain my logic behind it better.

I think it was a podcast that Bixler admitted that he's kind of bad with marking the CG on his plans. But I'm sure everyone has noticed that in every build video he says to make it a touch nose heavy. Which actually means his CG mark on the plans is wrong. lol.

 

[xuzme720]

Exactly!

 

[suvansh]

So that means my 145% duster's CG Points will be the same? Right?
Can't I just measure 30%-33% from the leading edge for the CG point? On this plane?

 

[xuzme720]

So that means my 145% duster's CG Points will be the same? Right?
Can't I just measure 30%-33% from the leading edge for the CG point? On this plane?

That is correct and you can use the same percentage as the smaller one.