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Nov 29, 2019, 11:45 PM
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Multi element airfoil at low Reynolds number?


Has anyone seen a good discussion of multi-element airfoils at low Reynolds numbers? Like, maybe as low as 10,000? Thinking of building a footy rc sailboat with a wing sail. Like a tiny, much simpler version of what you might see on a racing catamaran. Besides the possible performance advantages, maybe a light footy with a wing sail would talk more reliably without the drag of a flapping sail holding it back.

Thanks
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Nov 30, 2019, 01:03 AM
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I would think the mechanical effectiveness/efficiency (i.e., real-world response to subtle wind change) of sealed-gap hinges would be a major factor to consider, especially since low-Re likes sharp, discrete breaks, eh? .... in BOTH directions
Nov 30, 2019, 04:00 AM
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Originally Posted by xlcrlee
I would think the mechanical effectiveness/efficiency (i.e., real-world response to subtle wind change) of sealed-gap hinges would be a major factor to consider, especially since low-Re likes sharp, discrete breaks, eh? .... in BOTH directions
I'm not sure I understand your point. We're not talking about a random gap by mistake. Rather, we're talking about a gap that's designed to be there. The axis for the flap is forward of the trailing edge of the wing. Certainly this works very well at higher Reynolds numbers. If not, why so many airplanes with slotted flaps, slats, etc. and why all these very expensive racing catamaran using expensive and awkward wing sails? What I'm wondering is how well it works for lower Reynolds numbers.
Nov 30, 2019, 10:45 AM
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Here is an interesting work which shows that slotted flaps seem to work fine at Re=200k:
https://www.longdom.org/open-access/...92.1000125.pdf

But much lower than that, I don't know. At very low RE, any change that lowers Reynolds number even further is very detrimental to performance. With slotted flap, you effectively trade one wing with two lower chord ones.
Nov 30, 2019, 12:14 PM
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JimZinVT's Avatar
Had to look up "footy".....a 12" sailboat.

Sounds like a fun project. Do it!
Nov 30, 2019, 12:27 PM
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Quote:
Originally Posted by Davidz90
Here is an interesting work which shows that slotted flaps seem to work fine at Re=200k:
https://www.longdom.org/open-access/...92.1000125.pdf

But much lower than that, I don't know. At very low RE, any change that lowers Reynolds number even further is very detrimental to performance. With slotted flap, you effectively trade one wing with two lower chord ones.
Yup, and exactly my point! I based my post above on my own and others' experience with excellent-flying low-Re models. And that usually involves sharp discrete breaks in often non-curved surface-segments with no gaps, but slight crease bends instead.

This flies GREAT, slow and very efficient even with bulbous safety nose.

It is specifically a room-circling glider and as such was not designed for max L/D but for optimal tightly banked centripetal lift in an under 3m circle with no lost altitude, rahter usually slightly climbing .... at fairly slow speed, leveling the bank to nose-up just before plopping onto¨my hand with ca. zero fwd. speed (I made it for my birds).

Such maximum lift (optimized w.r.t. aerodynamic sail vs hull water drag for its use) is exactly what is needed as thrust for a small sailboat likely going much slower than the wind (full-size iceboats are quite different in that regard).



notice discrete creases between connected flat sections







Same airfoil as many raptors plus washout and simple conical camber by having the creases expand chordwise along the tip. Lifting stab.

Flies REALLY super and float-drops onto my hand; easily thrown in either circling direction.



Specs

span: 230 mm
length: 182 mm
weight: 1.9 g

Steady State (straight ahead)
airspeed ~ 3 m/s
sink rate ~ 0.7 m/s

Average speed in circle, from and to hand ~ 6 m/s (fast launch, landing ~ 0.13 m/s)


CG ca. 60% back from root L.E.
Last edited by xlcrlee; Nov 30, 2019 at 12:57 PM.
Nov 30, 2019, 12:40 PM
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JimZinVT's Avatar
Just seal the gaps.
Last edited by JimZinVT; Dec 05, 2019 at 06:21 PM.
Nov 30, 2019, 12:47 PM
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Quote:
Originally Posted by JimZinVT
Just seal the gaps.
That was what I addressed in Post #2:
Quote:
Originally Posted by xlcrlee
I would think the mechanical effectiveness/efficiency (i.e., real-world response to subtle wind change) of sealed-gap hinges would be a major factor to consider, especially since low-Re likes sharp, discrete breaks, eh? .... in BOTH directions
Nov 30, 2019, 02:33 PM
An itch?. Scratch build.
eflightray's Avatar
Quote:
Originally Posted by lincoln
Has anyone seen a good discussion of multi-element airfoils at low Reynolds numbers? Like, maybe as low as 10,000? Thinking of building a footy rc sailboat with a wing sail. Like a tiny, much simpler version of what you might see on a racing catamaran. Besides the possible performance advantages, maybe a light footy with a wing sail would talk more reliably without the drag of a flapping sail holding it back.

Thanks
Are you thinking one of these --



?

.
Nov 30, 2019, 03:00 PM
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Thanks, Ray! What I had in mind when I wrote Post #2 and detailed in Post #6.

Specifically, the large full-size sail, with much more force on it then a "footy", can distort along a twisted hinge line, but having an almost totally resistance-free hinge was my first concern for a small model. Likely vertical carbon-rods along or at the hinge could help.
Nov 30, 2019, 03:15 PM
An itch?. Scratch build.
eflightray's Avatar
There is also a 'solid' winged wingsail, (used on some iceboats/landyachts), but I would have though they would be too heavy for a small sailboat.

Going by some videos I watched, they may also be a little too complex to control with just a two channel RC, (Footy sailboat requirement).

.
Nov 30, 2019, 04:56 PM
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Now here's a thought (sorry, my mind will do that once in a while): namely to approximate a joined, segmented sail with a flexible plastic sail membrane (as in Kiki's hang gliders: pic below) by using spanwise stiff vertical carbon tubes with straight horizontal "ribs" to create relatively flat sections between the vertical carbon tubes.

That should work ....

of course the thick bottom "root" rib would also be segmented and controllable, the planform closer to that in the pic Ray posted, with broader chord and thus lower AR for low-Re considerations

Dec 01, 2019, 12:17 AM
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I've had some experience with indoor, rubber powered models. No one is using creases in their wings with pennyplanes, but they fly amazingly well.

I've attached a picture showing the kind of wing wail I mean. It's the one on the bottom. Mine would probably have a thinner main section. Years ago, guy named Francis Reynolds had success with an RC sailboat that had two flaps.

I suspect I can make a wing sail that's pretty light. Remember that volume, and therefore weight, goes down with the cube of the size. I've also figured out how to control it without using a lot of servo power. An entire pennyplane weighs 3.12 grams and has a bit more area than I have in mind. I realize that my wing will be more complicated, but the pennyplane includes a large prop and a fuselage.
Dec 01, 2019, 06:34 AM
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I sincerely beg to differ! My background is in experimental physics and engineering (come from an engineering family: engineer/inventor-grandfather plus engineer-dad promoted to engineering chief at a major aircraft mfg. before quitting to start own business). Was talking to engineers at 9 months, reading his books so soon as I could hold them. Making own-design models from 3 yrs. old on. Worked at GD as "in-house consultant" for special weird or unusual problems.

Although the above can be dismissed as "Genetic Fallacy" in the following (no worries), and I am not always right, nonetheless in fact I have been correct in "outside-the-box" solutions way more than one might expect. The reason is that I rely on real-world tests and actual evidence, from my own and others' experiences. And the tests can be simple: I won an international contest to explain the details of what's going on with horizontally rotating kites ("UFO Sam") by doing shadow flow studies in my bathtub.

OK, here is what I found years ago using a fan (ca. 10 mph measured flow speed) with a large rectifying flow-grid and incense smoke streams, studying why an underside "spoiler" on an Olympic II (chord in the range of a small sailboat model) acted like a flap: the flow basically just kept on going with minimal (max. ca. 5%) circulation. So to eliminate the "flap" effect I had to add a top-side spoiler and then I wound up with highly effective spoiler/dive-brakes (could safely dive vertically from altitude with no issues) >



The original KFm glider also showed this. We know that low-Re flows have some similarities to supersonic flows (has to do with boundary layers and the factors that comprise Re).

You use Pennyplanes as an example to "prove" your conception/contention, but that ignores the fact (as I pointed out before) that aerodynamic and hydrodynamic forces are quite different in magnitude for a "footy", yet the basic Pennyplane (and similar application) structure usually creates the same effect between spars and L.E.-T.E. with flat segments. That is, light weight is the over-riding factor for those indoor models, why I used the fairly heavy, highly cambered, high-lift but relatively low L/D room-circling glider as an example not so different for your intended "footy" use!

Significantly noting that the "Figure 1 airfoil" posted totally disregards the well-known fact that low Re requires THIN airfoils, in any case I strongly advise you to make a simple model to make your own flow study and see for yourself what happens; you can even have the wing-sail horizontal on a balance arm to measure "lift" force (plus drag, independently if you then change the balance axis 90°) .... but this is what I would expect to happen >




Wish you luck, whether your approach is scientific or not: "The Proof Is In The Pudding".


Have fun!
Lee
Last edited by xlcrlee; Dec 01, 2019 at 07:47 AM.
Dec 01, 2019, 12:25 PM
It's time for me to fly
JimZinVT's Avatar
Quote:
I've attached a picture showing the kind of wing sail I mean. It's the one on the bottom. Mine would probably have a thinner main section.
I like this idea. How would you control the shape (camber) formed by the multiple elements?

My aerodynamic knowledge is limited to what I've learned on RCG, and I didn't associate with any scientists as an infant. But wouldn't the flow through the slots act as a turbulator, keeping the flow attached better at high camber/AOA settings? Or is that affected by the low Re #s we're working with? If not, then taping the gaps would approximate the kinky airfoil that Lee is promoting.



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