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Old Nov 11, 2010, 08:33 PM
Fly, Fish or F#@%
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Quote:
Originally Posted by mlachow View Post
I'm looking forward to flying a Fosa.
Mike,
Did you order one or were you able to get Bob to sell you his??
Cheers,
T
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Old Nov 11, 2010, 10:03 PM
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United States, CA, San Luis Obispo
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Quote:
Originally Posted by OZPAF View Post
Reto,
While no expert I would have to disagree with this common misconception that profile or form drag increases with AR for the same WING AREA. By definition form drag is only related to the area and the operating Cd of the airfoil in question, thus the actual span has no effect(on form drag). Frontal area is misleading and its the actual Cd of the airfoil that is important not the actual thickness.
Hi John,

There are numerous aerodynamics texts that would disagree with your assessment. Hoerner would be a start. My previous post on the subject is right on the money. Nobody said that AR was driving the frontal area increase, just the fact that you are adding span.

We are talking about flying a plane at zero lift, so the minimum drag of the aircraft is mostly related to the frontal area..all other things being equal (both planes with V-tails, similar fuselages...etc).

-Aaron

PS- I should probably be more specific. The "operating CD" you mentioned is a translation of the force placed on the airplane to the reference area of the wing. What I have stated is that the frontal area has a direct effect on that force, and will have an inherently different "operating CD" because of that.
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Old Nov 11, 2010, 11:53 PM
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PS- I should probably be more specific. The "operating CD" you mentioned is a translation of the force placed on the airplane to the reference area of the wing. What I have stated is that the frontal area has a direct effect on that force, and will have an inherently different "operating CD" because of that.[/QUOTE]

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.
Cheers
John
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Old Nov 12, 2010, 07:57 AM
F3B and F3K
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Quote:
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.
Cheers
John
Hi John

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

Quote:
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).

Cheers,
Reto
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Last edited by RetoF3X; Nov 12, 2010 at 09:37 AM. Reason: servo added
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Old Nov 12, 2010, 11:44 AM
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Reto wins for these perfectly distilled sentences...there is a reason why he has a PhD:

"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."

-Aaron
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Old Nov 12, 2010, 11:57 AM
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Loosely translated, let's fly the thing and see how it does......



Quote:
Originally Posted by Avaldes View Post
Reto wins for these perfectly distilled sentences...there is a reason why he has a PhD:

"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."

-Aaron
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Old Nov 12, 2010, 12:01 PM
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Yes, let's stop guessing and get some empirical data!

Mike and I are comparable speed pilots (if we don't cut): when there was not too much air variation (morning, evening), we got several consecutive speed runs with similar times in our last few practice sessions (FS3 vs FS3, FS3 vs Shooter, FS3 vs Cyril).

Thus I hope with logging launch altitude, wing loading and speed times we will see a trend when we average enough while playing FS3 vs Fosa.

Paging MikeL: when are you done finishing the Fosa!

Reto
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Old Nov 12, 2010, 05:22 PM
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Hi Reto,Aaron,Mike

Yes I think that settles it - thanks for the discussion. I'd love to be in the position to do some of that empirical testing- sounds great Reto and Mike. Its stirring the old F3B memory cells.

I've looked at some old Dirk Phlug foils - the DP18 7.83 - 1.8%camber and 7.83% thick and was very impressed. Dirk must be one of the top airfoil designers around.

Rooting for the Fosa.


Cheers
John
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Old Nov 12, 2010, 08:38 PM
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And more pics while your at it
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Old Nov 13, 2010, 09:14 PM
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Originally Posted by AlpineDSacro View Post
And more pics while your at it
OK, some pics.....

I have to wait until Monday when I'll have my Shooter, Cryil and FS3 back to compare things.

Unfortunately it's hard to take pictures of some of the interesting details inside the wing.
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Old Nov 13, 2010, 10:09 PM
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hmmm.....

didn't know Bob had these in stock...!

B
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Old Nov 13, 2010, 10:45 PM
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When you are on the US team you get some benefits
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Old Nov 13, 2010, 11:23 PM
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hhhmmm nice pics
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Old Nov 13, 2010, 11:52 PM
Challenge is rewarding
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San Diego, CA
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Quote:
Originally Posted by Brandon View Post
hmmm.....

didn't know Bob had these in stock...!

B
As I understand it, that was Bob's personal plane that Mike sneaked up on. I can't wait to hear Mike's thoughts on the model. I hope to play with one myself later in the year.
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Old Nov 13, 2010, 11:55 PM
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Quote:
Originally Posted by djklein21 View Post
As I understand it, that was Bob's personal plane that Mike sneaked up on. I can't wait to hear Mike's thoughts on the model. I hope to play with one myself later in the year.
Yes....flight report would be good!

B
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