Thread: New Product Baudis Fosa F3B/F3F
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Old Nov 12, 2010, 07:57 AM
RetoF3X is offline
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F3B and F3K
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Originally Posted by OZPAF View Post

Hi Aaron,

Apologies to Reto if this is straying off his original thread, but I think this is an important design issue.

I think we are both a little guilty of not being specific , so I would like to explain myself a little better.

What has been labelled wing "form drag" in this thread would more correctly be called "wing zero lift drag" and includes both pressure drag of the foil and 'skin" or "friction drag" as I understand it.

If we compare wings of equal area and of similar airfoils (every thing else being equal) then the wing zero lift drags will be same for low and high AR, providing the airfoil performance is the same with the change in Re No's(assuming the same flight speed). Both wings will have similar pressure drag(airfoil characteristic) and the same wetted area(skin area).

I feel that what has held these low AR models back has been stuctural limitations and maintaining airfoil perfcormance(at the lower Re's) as others have mentioned.

However it appears that the Fosa may have overcome these limitations with a very stiff wing and competitive airfoils of a slightly higher thickness ratio.

Again apologies to Reto for the drift off the thread.
Hi John

Your input is very welcome! But I think I posted a similar conclusion in post #20, albeit very crudely and overly simplified:

Compare two planes with same area but with different aspect ratios (e.g. AR1=14 and AR2=18, so Freestyler vs Fosa)

wingspan increases by sqrt(AR2/AR1)
mean chord and mean RE decreases by sqrt(AR1/AR2)

If the same airfoil (same %thickness) is used and if it were invariant to the RE change (here about 12%), drag at cl=0 (no induced drag there) would be the same for both planes as the two ratios cancel out (resulting in the same frontal area).

This is unlikely and it was assumed that plane 2 has a thicker airfoil (for structural reasons), resulting in more drag at cl=0.

However we don't know the airfoil and how it performs at the corresponding RE's, so it is only guesswork.
Thus I agree that if one could use a similar airfoil on both wings (and if it would be invariant to smaller RE over that range), it would result in the same frontal and wetted skin area and presumably not add more drag at cl=0.

But due to structural reasons and the need to put in servos, I assumed that the high AR wing needs to have a thicker airfoil. But we anyway don't know what airfoils are used on the Fosa and how they work over the relevant RE range, so it is all speculative.

I also agree that advances in building technology and thinner servos will allow high AR wings with airfoils of suitable thickness. I am looking forward to this development. From what I could see, the Fosa launches really well and the results in distance task were very promising.

Mike and I will surely do some one to one practice in the near future and try to deliver some more insights (FS3 vs Fosa).

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Last edited by RetoF3X; Nov 12, 2010 at 09:37 AM. Reason: servo added
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