MJD

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Kind of a half ellipse. Those three-panel wings get pretty close to the crescent shape in terms of efficiency don't they - without the compound curves.

The airfoil in question is designed for low drag at Rn of 1.5x10^6 and up, and a 10.5" chord (tip) reaches that value by 175mph. Of course you gotta get there, but it's pretty clean below that too.

For my first speed dedicated design I'll stick to a constant taper, and see how things go. Maybe niftier wing shapes later.

MJD

HighPlains

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Almost a pure elliptical wing planform. If you look closely you will notice the the leading edge is straight, and the cores were tapered, parallel to the ailerons. The maximum deviation from a true ellipse was 1/4", or zero to 2 1/2% over the entire span.

This design would run 175 mph with a good X-40 and over 180 with a great motor while doing 180 degree turns about every 3 seconds. This design is the first to use a high aspect ratio wing in RC pylon racing. Aspect ratio on this was 7.4:1 while most designs of that era were in the 5.6:1 range. I started building the first of these in the fall of 1986.

MJD

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Cool. I'm guessing the straight leading edge helped in maintaining profile in the first 1/3 or so of the chord? This is kind of a slightly forward swept crescent ain't it?

MJD

HighPlains

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Yes, it made it possible to sheet the wing without compound curves (though once in a while you find that potato chip sheet of balsa in your stash). I built 7 or 8 models of this design over the last years of F1 racing. Some with washout, some straight, some with warps. All ran extremely well with the exception of one.

One was a complete dog, it never was fast, all because of a sanding error at the leading edge. I had raised the leading edge about 1/8" on the airfoil by mistake. The difference between the bad wing and the good wings of the others was about 10 seconds on the course.

The wing has a 1/3, 2/3 ratio. So I guess it depends on the airfoil, but the intent was just a wing centered over the high point straight out. This was about 10 years before I experimented with forward swept wings in quickie with good success.

But it is interesting to note that designs like the Cosmic Wind 'Little Toni" have straight leading edges and with the 50% taper do in effect have forward swept wings. I think this helps keeps the spanwise flow to a minimum, and lowers drag somewhat.

I would use wing like the Cosmic Wind series for a speed ship with a straight leading edge. As you have guessed by now, I believe that higher aspect wings are the way to go. This is because with an RC model you have to either turn it or do some sort of climbing split S at each end of the pass with a high G pullup back to level flight. Reading some of the German posts, they indicate that they may lose as much as 100 mph from the peak speeds of the dive to the end of the traps. Simply pathetic.

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MJD

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As I've been thinking about it, it seems to me one of the important design balancing acts for speed applications is the Rn of the wing coupled with the aspect ratio and speed losses. High aspect ratio is not as much a driving force as it is for pylon as I see it, but it is still important with regards to the speed losses. I guess the trick is to find the optimum balance. For this design, I used the KISS principle on the wing, choosing a minumum chord that stays up in the desireable Rn regime and applying a taper ratio that gave suitable area. I may try to work out some tradeoffs in this respect, but really, that might be design #2. I like to stick to frist instincts on #1 sometimes.


MJD

HighPlains

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You have to remember that wind tunnel data on airfoils assumes that the wing is infinitely long. In the real world, you have so many variables. Interference from the fuselage, prop wash, dust and bugs, and spanwise flow.

But regardless what you build, the second one is usually the one you wish you did first. I always hated the lab prototypes that got sent to customers before they were ready.

MJD

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I do not believe it has been through wind tunnel testing, it's one of Martin Hepperle's new sections. Could be wrong but at this point I think it strictly theoretical and I have to supply the wind. But yeah, real world is different and complex, although one is left with little choice but to use wind tunnel and theoretical data unless using proven stuff, and add in the bugger factors as you see fit from experience I guess.

MJD

I-Love-Jets

@ Highplains

Such top speed losses (from let's say 280 mph down to 200 mph from the beginning to the end of the 300 m long straight and level sequence of the FAI flight pattern) are not related to wing or fuse design at all in case of the EURO Speed Cup nitro participants!

In fact it is not the aerodynamic quality of the mostly very refined F3S Speedcup airframes (CAD & molded wings) but the super critical choice of the best suited speed propeller.

Yes - the speed prop itself as a big windmill can also slow-down extremely! But in F3S we need highly oversquared props in order to gain maximum average speed (this is what counts eventually).

On the other hand all pylon type props (“squared”) work quite inefficiently in F3S, be it electric or nitro.


BTW, here we will find more F3S videos (“vintage”):

http://www.rcgroups.com/forums/showthread.php?t=984610

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HighPlains

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You might as well leave off the prop and engine.

MJD

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Wow - this one impresses me:

- The small mega fast (268 kph) Red Devil was powered by a highly modded WEBRA 2 cc Speedy.

160mph on a .12? Seems to me that is pretty decent.

I-L-J, are these engine/pipe setups completely off-resonance at launch? From the descriptions, it seems that the pipe jumps into full resonance only in the dive and starts to fall off as the engine loads up during the straight section - is that how you are doing it? I can see how this would result in very high terminal dive speeds wih the resultant slowing down through the traps. I guess an analogy would be driving down a long hill in a tall gear that the engine will not pull at top speed on level ground, then driving through the traps as it slows down - with a higher average speed than the vehicle could ever manage in level flight.

Do you have an idea of the typical level flight speed of, for example, a well built 10cc model, without diving? If you mentioned this before, sorry I missed it or forgot.

I notice the model I have drawn so far closely resembles the red model in the link. I guess there are only so many ways to draw a skinny airplane that fits the engine and pipe and holds enough componentry to do the job!

MJD

I-Love-Jets

Well a 10 cc sized speed plane optimised for F3S requires a quite long fuselage stabilising for the straight line part of the flight pattern after diving.

If going for a non-diving 10 cc plane route, some changes are required affecting prop size and airframe design mainly:
[ul][*] shorter fuselage between wing and elevator unit[*] pylon-type wing airfoil and more wing aspect ratio allowing decent horizontal cornering without much speed loss[*] a carbon fiber propeller that is sized squared or slightly oversquared (e.g. 9x9, 8.5x10)
[/ul]
So if diving is not desired, a pylon-type layout would be the optimum.


Maybe in this case the most promising solution would be an adoption of a current F3D plane design modified to fit a 10 cc RIRE engine with pipe. A problem still will be the visibility because of the approach to the 300 m long straight and level section. The low altitude horizontal turns will be located quite far away from the pilot then... [X(]


Of course the achievable speeds within the FAI speed trap will be slower with that kind of pylon-derived setup versus an identically powered speed plane following the regular F3S philosophy (highly oversquared prop + diving). For instance an up-to-date stock F3D pylon plane (MB 40 powered) will never be faster than about 225 mph applying the FAI rules, be it with or without diving.

I-Love-Jets

For F3S the pipe length is set perfectly if the engine jumps into resonance during the first dive and remains "on pipe" throughout the entire 300 meters long (100 m approach + 200 m speed trap) straight and level part of the FAI pattern.

However, most F3S speed ships have their pipe length slightly longer to achieve resonance (stage 1) during take-off already. This is done to manage the critical take-off sequence better because of the lack of static thrust of most of the highly oversquared props.





During the diving a well running engine typically jumps from stage 1 to 2 or even stage 3 resonance, meaning an OPS .60 VAE engine for example increases its rpm from about 20000 at the beginning of the dive to approximately 26000 ( > 27000 if highly modified) at stage 2 or 3 resonance.

The amount of rpm increase of course mainly depends on the used propeller size and the engine timings and equivalent pipe length...



But we can predict that the more oversquared the prop size is
[ul][*] the more difficult the take off can be (most of the 10 cc plane have a take-off weight of 2 to 3 kg)
[*] the higher the plane's speed becomes at the end of the dive (where the 100 m approach of the horizontal sequence starts). This is the pattern sector where the F3S prop works most efficiently.
[*] the more critical it is to achieve constant speed through the 300 m horizontal sequence (lack of torque to maintain the top rpm level!)
[*] the more difficult it is to find the best suited pipe length/nitro % content/compression ratio in general.
[/ul]




If the engine falls off resonance during the climbing, that really doesn't matter...





Reasons for potential speed loss during the horizontal sequence:

The main reason for losing a considerable amount of speed from the beginning to the end of the speed trap is the combination of a very narrow engine torque peak (determined by the engine's timing setup) and the application of highly oversquared props.

Such highly oversquared props preferably require a wide high-level torque band, the wider the better! [:@]

So this sometimes quite tricky engine tuning only affects the nitro driven speed planes since the electric motors of the F3S-F class deliver a broad torque band.





The next German generation of F3S-F planes most probably will feature lower rpm (below 25000) but much more prop pitch such like 8x13 to 8x15 or 9x15 to 9x17. The often custom made motors/controllers used for the electric F3S class provide so much torque at any rpm level, that even these "mega" oversquared props will be able to work efficiently. So future FAI compliant top speeds of at least 280 mph will be achievable, making pilot skills becoming more and more important...






Some nitro freaks in Europe however don't want to leave the “battlefield” to the electrics without a struggle. There's ongoing research and development for supercharged nitro engines!

Current calculations and simulations are promising:
Depending on the setup plus about 50% of peak power increase should be gainable. That on the other hand would also mean a much better (wider) torque delivery making the application of the F3S typical highly oversquared props easier for the nitro driven speed planes. [8D]

MJD

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Thank you ILJ! As usual, a pretty detailed explanation.

It seems logical that the climbing/diving style of speed flying, whether for FAI competition or sport, is perhaps more friendly to the eyesight and reflexes than making high speed turns 1/2 kilometre down range.

MJD