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Old Oct 07, 2010, 05:01 AM
Biting ankles since '53
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Quote:
Originally Posted by RetoF3X View Post
Hi Daryl

I agree on the turns, but I was not aware that so much energy is being dissipated in the three turns.
How much the hit in straight line speed really is might also strongly depend how thin they could make the wing/airfoil while keeping it stiff and enabling a clean servo install (which they did). Where the crossover point in the speed task lies (with todays building technology) will be seen by the results in the future (and thanks to the Fosa we might get a clue if it is more to higher AR).

The plane did deliver in the launch department! In speed it did not yet show an advantage. But it is correct, maybe one has to use its launch potential to carry more ballast, maybe one should bang the turns harder (they were pretty hard though).

But don't get my post wrong, that bit that might have lacked in speed is not that much (and it is also just an impression and not a long term observation). And Jiri's sub 14s was already promising.

Cool that there is again something going on in F3b designs!

Reto
I think this is the route that Thuro has been going down with the Needle 100 and 124. My 100 certainly turns well. I'm eagerly awaiting the 124 to be released.
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Old Oct 07, 2010, 05:34 AM
F3B and F3K
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While checking f3b.de:

1st place in Dresden
2nd place in Lünen
Podium missed in Belgium only due to penalty
6th place overall in Eurotour
1st place swiss NATS

Not a bad start for a new plane and it must somehow work in speed as well to perform on that level.

Jiri has so many cool planes.
Reto
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Old Oct 07, 2010, 07:27 AM
slope'n the Colombian Andes
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Originally Posted by RetoF3X View Post
Not a bad start for a new plane and it must somehow work in speed as well to perform on that level.
About the high AR wing: judging solely from my experience with flying the Lesky Pace F4 (wing span 3.15 m, AR > 20) on the slope, I conclude that these slender wings accelerate well and have a high top speed. I don't know the theory why they should be slower than lower AR wings.

Dieter Mahlein, ShredAir
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Old Oct 07, 2010, 07:48 AM
Deniable plausibility
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Mmmm... like it, a lot but can't help the feeling that once again the 'F3B wingspan see-saw' flops the other way
Does anyone know what the root chord is? Just wondering about the packaging issues. What I would say is that there is no need to go below 8% thickness [apart from, maybe near the tips] and I'm sure that with more work, sections of 8.5% thickness can be made fast enough thus enabling the higher A/Rs
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Old Oct 07, 2010, 09:36 AM
F3B and F3K
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Dieter

Here is the assumption (not theory):

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. And even with more drag in straight line speed, the plane can compensate with more ballast and tighter turns owing to high AR wing.

Reto
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Old Oct 07, 2010, 10:41 AM
slope'n the Colombian Andes
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FWIW, and sorry for the slight hijacking of Vic's thread:
My Pace F4 has 190 mm root chord and 76 mm chord at the aileron tip. According to the website, Daniel uses his own airfoil "DLf4 1.75-8.4" transitioning to 1.71-7.4 at the tip. I interpret this to mean 1.75% camber and 8.4% thickness transitioning to 1.71 and 7.4 respectively.
I have flown many high-performance moldies on the slope, so that's what I have to compare the Pace F4 to, and it surprises me. It is fast!, yet climbs eagery in lift, and it is docile in thermals and during landing. I never thought it would work so well, particularly when thinking about the Re numbers at the narrow wing tips.
Servo wells can accept 12 mm (maybe 13?) thickness at the flaps, and 11 at the ailerons. I use AIR 94761s all around, and I sanded the aileron ones down to 11 mm.

Dieter Mahlein, ShredAir
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Old Oct 07, 2010, 05:44 PM
Detail Freak
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I'd think that the higher AR plane would definitely thermal better, especially if the profile percentage thickness is greater.
R,
Target
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Old Oct 07, 2010, 10:14 PM
Build A Rocket Boys!
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Dieter et al. Keep the information flowing. I'm getting educated here.
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Old Oct 08, 2010, 03:18 AM
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Quote:
Originally Posted by RetoF3X View Post
While checking f3b.de:

1st place in Dresden
2nd place in Lünen
Podium missed in Belgium only due to penalty
6th place overall in Eurotour
1st place swiss NATS

Not a bad start for a new plane and it must somehow work in speed as well to perform on that level.

Jiri has so many cool planes.
Reto
It doesn't hurt that 2XWC Andreas Bohlen was flying it either

Lots of speculation here. Reto is mostly correct in his descriptions. The parasitic drag (drag at CL=0) of the airplane is what drives the top speed in a straight line (dive). It is mostly dependent on frontal area..if we are comparing two similarly shaped airplanes like a Fossa and a FS3. So if Baudis got away with a thin airfoil on the wing to keep the frontal area the same as the FS3, then the parasitic drag (at CL=0) would be the same.

The benefit being that the induced drag, or the drag that is a function of lift, or the drag that happens at high lift coefficients (hi CL) would be lower.

The cost would be designing airfoils that will work at the lower Reynolds numbers that would naturally be present on a very high aspect ratio wing. These airfoils *may* not be capable of producing the high CL's that the wing is capable of generating. And the result *might* be a wing that is tricky to fly.

I will leave this to the model designers to sort out. Again, I think it is a very cool looking airplane and I would certainly add one to my quiver just to try it out.

On a related note, there are several things going on in F3B right now that have driven the recent designs. First is that the Herrigs figured out that a small airplane has a lower frontal area so a lower minimum drag...the Schocker. I believe this plane is so revolutionary that we should really appreciate it. I remember reading about MH flying a 13.68 and AH flying a 16.X with a cut! And that was back in 2004. To me this changed the paradigm for F3B models, whether we knew it or not back then.

They also figured out the balance between small for speed and large for launch/distance/duration. Along with MW we got...Radical and then FS3/FS4. Last, they figured out that you can fly a faster speed run by turning near the best L/D CL, rather than the max CL of the airplane. In english that means that rounder smoother turns lose less energy and results in a faster overall speed run than if you bang the turns.

I remember watching Martin and Andreas fly speed in Switzerland. They got good but not killer launches. Then flew a very smooth and short course. Never banged a turn the entire contest. The model seemed to fly at the same speed through the entire run. Nothing spectacular to look at, but the times were amazing for the conditions! 15 second runs in 17 second air...and we were flying the same model. I learned a lot!
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Old Oct 08, 2010, 03:33 AM
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One more speed run anecdote. After my last run of the contest, I moved up three spots in the standings with a reasonable time. I stopped to talk with AH about the Radical. He said "nice run" and I was pleased to get the complement, but I also knew that I was 2 seconds slower than them with the same plane. I said something to that effect and he said "Well we have been flying them for 1.5 years."

At the time I kind of knew what he meant, but the more I thought about it, the more I understood. What I wrote above has a lot to do with that short conversation with AH.
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Old Oct 08, 2010, 07:56 AM
F3B and F3K
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Hi Aaron

Thanks for throwing in some of your knowledge and the anecdotes. From my simple AR calculation above, the Fosa has about 12% less mean chord than a Freestyler and to keep the frontal area the same the absolute thickness of the airfoil would need to decrease by the same factor. From looking on the Fosa wing it did not look like it.
But again this is too simple, we don't know the airfoil and how it performs at the RE's it is operated. It will be interesting to see if the high AR wings can take the full advantage in launch and turns and compensate for the hypothetical higher drag at cl=0 (which is assumed, we don't know).

A funny anecdote from chatting with Dirk Pflug: They always wanted to make the high AR wing and so they milled the wing moulds and Jiri built a prototype (extended Cyril Fuselage). That experiment worked so well that Jiri flew with it at Kirchheim where he also aced a sub 14 time. They then decided to make a production mould.

I think this is great that the duo Jiri/Pflug just try out things when they believe they can have a shot.

Reto

PS: Some germans still fly with the Schocker at Eurotour. It is a rather small modell, but they always pull out blistering fast runs!
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Old Oct 09, 2010, 09:43 PM
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More pics ..... so sleek.
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Old Oct 10, 2010, 04:04 AM
Thermal Wrangler
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Launceston Tasmania
Joined Mar 2004
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Can anyone get onto Jiri's website?

I get a malware infected iframe message and I'm hesitant to go further.

http://www.baudismodel.com

Chris.

ps. got in via the news page:

http://www.baudismodel.com/news.php
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Old Nov 11, 2010, 06:57 PM
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Sydney
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Quote:
Originally Posted by RetoF3X View Post
Hi Tom
Please correct me:
In speed you fly close to cl=0 (except in the turns) so induced drag is not high in that flight regime ( induced drag scales to the power of two to the lift and reciprocally to AR). Thus a high AR plane has no advantage in straight line speed but has a higher form drag.
The lower induced drag makes the biggest effect when you fly at high cl like the launch.

Reto
Reto,
Thanks for the info on the Fosa. It is refreshing to see the high AR approach being looked at again.

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.

Deformation or twisting of the wing due to structural weakness could definitely make a difference and slow down the model. The higher AR wings also require more care in airfoil selection/design. If these areas were not handled well it would certainly be slower in a straight line and lose some advantage in the turns as well.

It appears the Fosa is strong enough - time will tell.

John
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Old Nov 11, 2010, 07:47 PM
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I got a look at a Fosa at Soaring USA tuesday. Jiri knows how to build a stiff model. Just take a look at a Cyril which is the stiffest stock F3b model that I have. The Fosa has two wing joiners to take care of the loads at the center and joiner boxes. Looking inside the wing, there are good things to keep the structure stiff. The fuselage is similar to the Cyril but he has gone to a square joiner for the V-tail which should make that stiffer too.

And I love the aspect ratio. I flew a modified Caracho that was similar AR in 2003 but the wing construction was not as light towards the tips and the V-tail was not sized/angled correctly on that model. Still launched well and cruised nicely in distance.

I'm looking forward to flying a Fosa.
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