High-lift, ultra-low RE wing design. Should I do an experiment?

[Flyingwingbat1]

I experimented with serrated LEs a while back, taking it to the extreme and making my plank flying wing gliders with a few large points(they don't look much like plank gliders anymore!) I noted a worse L/D, but much gentler stall characteristics and slower stall speeds compared to my flat and reflexed plank wings. A high-camber reflex airfoil shape w/ 3 points (serrations) works well at speeds of 5 ft/ sec, but loses its advantage at slower speeds versus no camber. Keep in mind, the flying speeds are roughly 3 or 4 ft/ second, and the chords are only 2" to 2.5"; very very low RE. Any ideas on getting higher Cl without rotating wings? (I already tried that, but then it becomes an entirely different type of aircraft!)

Any suggestions on further aerodynamic variations would be great, the responses I've gotten so far are wonderful, thanks everyone!

[mlbco]

What is the application for this wing? Can't you just increase the area to lower the flight speed?

It will be hard to get a high CL at low Re, but there are some tricks that can give you a high CD at low Re. If you want to design something that falls slowly, increasing CD might be beneficial. It turns out that a solid surface has lower drag at very low Re then an appropriately designed porous one. A dandelion seed is a good example of this, it has higher drag than a circular plate of the same x-sectional area at the Re at which it operates. A wing made of fine fibers which forces the air through many very small gaps will have a higher Cd than a solid wing (when falling). The Cd achievable can exceed the value of CLmax achieveable at the same Re. Maybe this will help for your design.

Steve Morris

[Flyingwingbat1]

I'm looking into high-lift designs for my indoor slope gliders, aka walkalong gliders. I started plenty of threads on them in other forums, so I'll spare you the spiel in here. Basically, my dream would be an 8" (re: SMALL) span glider made from 1/16" balsa that's easy to build, flies at a normal walk, with good handling so a novice pilot could at least learn to control it after several attempts (these things are harder to fly than you think). A high Cd is definitely interesting, though it would have to be accompanied by an especially high Cl to allow for forward flight. Thanks for the idea! Multiple wings are an option, though they complicate the building process. Due to the non-uniform lift from an angled air deflector, they might complicate the flying process, too :eek: I'm wondering whether a 2nd degree fractal serration on the LE would help or not, maybe I'll experiment with that this weekend...

[mlbco]

I don't know if you'll see much gain from serrating the LE further but it's worth trying. Have you tried a simple delta-wing design? If the LE angle is steep enough you should still be able to get a decent LE vortex and the associated non-linear lift that it creates. This may be an alternate way to boost CL.

Another avenue to investigate is placing very tall turbulators 90deg to the flight direction to create pockets of trapped vorticity, spaced approx. 10% of the chord apart. Some people believe that this is how dragonfly wings work (they have a very "jagged" cross-section).

If you try multi-element wings, the best approach may be to place the wings close to each other like a slotted flap system.

I haven't thought much about walk along gliders, but there was a mention of a small foam design in Flying Models September, 2006 (pg 27) that had a serrated LE.

Steve

[Ollie]

As the reynolds number goes lower the airfoil drag along with a thicker boundary layer including changes thicker with turbulent and even more thickness with separation bubbles and even with total separation flow. Under some conditions the total separation the lift increases even more with flapping wings. Humming birds and bees flap with special conditions that don't follow our assumpions applied to aeroenigeering math.

See:
http://www.ipfrontline.com/depts/article.asp?id=4775&deptid=5

So, if you want wings with low reynolds number and high lift then study how to flap with the right kind of flap. Nature knows the way.

[Flyingwingbat1]

Hi mlbco, I tried a few moderate-chord delta wing designs. The chord cannot be too long or else the glider suffers from a severe nose-down trim due to the lift from the air deflector (remember, walkalong gliders here). A staggered multi-element wing would work, with the aft elements placed higher than the forward ones, to better align with the lift gradient. Hence I went to a shorter overall chord with multiple delta points out front, to try & get the advantages of a delta wing while still keeping a short chord.

FWIW, that foam model in the september Flying Models is mine! I sent it to David Aronstein since his group at work was doing a glider evaluation of sorts during their lunch break. He mentioned the magazine article a while later; quite a surprise! I found that the large serrations DO work quite nicely for lowering the stall speed and softening the stall, but I don't want to "stand pat", it's time to kick it up a notch! The turbulators sound interesting btw.

Ollie; I don't know how to make a flapping-wing glider, but if I figure it out I'll let you and everyone else know!

[reinking]

Re: flapping wing glider-
A geared down electric motor could supply the motion through a crankpin/pull-pushrod. Easy to say but where does the wing bend, twist and remain stiff? This would not be an easy problem to solve.

Re: bird flight-
See http://www.palemale.com/ for an almost daily record of Red Tail Hawk activity that spans four years! Someone should take the time to look at these images and the arrangment of feathers, undercamber, dihedreal (sp), sweepback and other features that I don't even know how to describe.

Reinking

[Flyingwingbat1]

Thanks for your ideas everyone; I'm thinking about a biplane "jagwing" glider, with 3 or 5 serrations on the bottom wing, and 4 or 6 serrations on the top wing. The top one would be set back somewhat to help it maintain trim.

[Ollie]

For a plank flying wing increase the chord!!! For simple design with good airfoils increase the reynolds number four times compared to your idea.
See:
http://www.liftworx.com/pages/redherring.html
http://www.dream-flight.com/alula.html

If you want the cake then you can't eat it. Too bad.

[Flyingwingbat1]

Those are cool little DLGs, though they're a little too heavy and fast to be walkalong gliders. You got it right, I want to have my cake and eat it, lose fat, and gain muscle, all at the same time! :p Will somebody hurry up and invent an anti-gravity machine?

[BMatthews]

In the end the easiest way to achieve slow flight is lighter weight. Switch to built up designs. Or at least go for thinner balsa wood. There's no reason why your push along gliders cannot be made from 1/32 sheet. I'd start with this thinner wood and then sand it even thinner towards the tips and towards the leading and trailing edges. Sand it down to about 1/64 at the tips and perhaps even a trifle thinner at the leading and trailing edges. For an 8 inch span glider that isn't intended to be thrown for the climb this will easily prove to be strong enough.

The only reason to go with the serrations in the leading edge shape is to delay the stall by turbulating or invigorating the airflow. But doing that doesn't really move the stall point by more than a few %. To really make a difference in the flying speed you need to cut down the weight.

If the thin all sheet wing isn't light enough then your next step is fully built up like an indoor model such as an EZ-Bee or other dedicated lightweight indoor duration structure.

[nmasters]

For indoor slope gliders the only thing that matters is the minimum sink rate. I believe the serration's that Flyingwingbat1 is using are mimicking the tubercles on some aquatic mammals' fins. While it's true that CLmax is only increased by 6-8% the decrease in CD near the stall is on the order of 30%. This results in a big improvement in L/D at high CL and that, along with light span loading, is exactly what you need for minimum sinking speed. I think he's on the right track.

Lots of articles about tubercles on the net. Here's one:
http://jeb.biologists.org/cgi/content/full/207/21/iv
Sorry it's not technical, I haven't been able to find any actual data

[Ollie]

Flyingwingbat1 wants a chord 2" and flying speed of 4 FPS. That means Re= 4,000. Tubercles on flipers, with Re= 500,000, don't do the job.

[Flyingwingbat1]

For indoor slope gliders the only thing that matters is the minimum sink rate. I believe the serration's that Flyingwingbat1 is using are mimicking the tubercles on some aquatic mammals' fins. While it's true that CLmax is only increased by 6-8% the decrease in CD near the stall is on the order of 30%. This results in a big improvement in L/D at high CL and that, along with light span loading, is exactly what you need for minimum sinking speed. I think he's on the right track.

Lots of articles about tubercles on the net. Here's one:
http://jeb.biologists.org/cgi/content/full/207/21/iv
Sorry it's not technical, I haven't been able to find any actual data

Actually, the serrations I use (on my "jagwing" walkalong gliders) are much larger; 3 large points form the front half of my flying wing designs. Right now, I believe the increased lift and gentler stall I experience are b/c of vortices, much like how the Concorde's lift is generated by large vortices over the top of its wing. My designs actually have a worse L/D versus my conventional plank walkalong gliders, but the increased maneuverability, stability in turbulence, and slower flight speeds are worth it in my case.

For walkalong gliders ("indoor slope gliders") I've found that there is no one best design (sound familiar?) They can be optimized for high L/D, allowing for board-free flying (I can use my hands or even my head for the most efficient designs). Even if the walkalong glider is fairly heavy, it can still be flown hands-free IF the L/D is high enough for the given situation and you're able to keep up and control it.

For maneuverability, weight, wingloading, rough-air ability, and overall size become the determining factors in a minimum-radius walkalong turn. "Rough air ability"? Remember when turning around sharply, you'll be flying the aircraft back through your own turbulence, it needs to handle that.

Optimizing for minimum flight speed is simple enough in theory; lower the wingloading as much as possible, and increase max Cl. I found that condenser paper tumblewings are about as slow as they come. Note; the spans on these are only 1.5" or so, and there's NO separate frame, just 100% condenser paper folded to spin. Flight speeds are 1 mph at the most, with .9 mph pretty typical. The jagwing serrations in conjunction with increased camber help lower the stall speed quite notably. I have some 1g models with 5" spans that fly at a slow jog. My conventional plank designs with similar wingloading make me run to keep up!

If anyone is interested I could do some experiments and post the results here. I'd get the most notable info on the tested gliders, such as span, chord, wing area, weight, "airfoil" specs, wingloading, minimum flight speed, L/D (if possible, it's only valid if the glider flies dead straight, and the vagaries of thin foam often prevent that!), and RE to boot. Let me know and I'll see what I can do over the next few weeks! FWIW, all this info would be applicable to low-RE flight.

[nmasters]

Flyingwingbat1 wants a chord 2" and flying speed of 4 FPS. That means Re= 4,000. Tubercles on flipers, with Re= 500,000, don't do the job.

Vortex formation is dependent on the Strouhal number, not Re. There's no size limit. The cilia of bacteria generate vortices that exert pressure to move them around in a fluid

[Flyingwingbat1]

Weird stuff. From what I just read on it, if the strouhal # is greater than .2, vortices shed, and if it's less than that, I'd assume they stay attached? Or they don't form in the first place? Let me try to get it straight, the strouhal number gives an idea of the type of large-scale vortices that will form around an object? (I saw pics of vortex streets in the leeward side of islands on one website)

[Ollie]

Thanks, nmaster!

"For dolphins, sharks, and bony fish moving at their preferred cruising speed, the ratio of tail frequency and amplitude to forward speed is constrained to a narrow but efficient range of values. This dimensionless ratio is the Strouhal number, and evolution seems to favor efficient swimming motion with a Strouhal number in the range of 0.2–0.4. In the October 16th issue of Nature, Graham K. Taylor et al. show that Strouhal numbers for birds, bats, and insects flying at cruising speed also seem to be constrained to a similar range of values."

So, you tune the flapping frequency and airspeed. It might be using the energy of gliding and efficiently coupling flap to vortex energy.

[nmasters]

Weird stuff. From what I just read on it, if the strouhal # is greater than .2,

The only reason I mentioned St was to point out that scale isn't an important consideration when you're generating vortices because viscosity is not part of the formula. That data regarding a range between 0.2 and 0.4 only applies to flapping flight and propulsion. Since this walkalong glider doesn't flap the shedding frequency probably doesn't play a roll so you can ignore that (or at least I don't have any idea how to apply it :o ). What I do know is important is the leading edge angle of the points. If the LE sweep of a delta is less than 55 degrees the vortices will shed and reform every few seconds (at the Strouhal frequency). This would cause a variation in both CL and Cm, resulting in the nose bobbing up and down (also at the Strouhal frequency). To get a leading edge vortex that stays attached a point on the LE must have sweep of 55 degrees or more.

p. s. I looked around for some data on tubercles, here are two papers that are repeated on a lot of sites:

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PHFLE6000016000005000L39000001&idtype=cvips&gifs=yes
http://www.people.fas.harvard.edu/~glauder/reprints_unzipped/FishLauder2006.pdf#search=%22Passive%20and%20Activ e%20Flow%20Control%20by%0ASwimming%20Fishes%20and% 20Mammals%22

--Norm

[nmasters]

So, you tune the flapping frequency and airspeed.

As I understand it, yes. That's why big animals flap slower than small animals. Compare the irritating buzz of mosquitos to the soft hum of dragonflies and the flapping frequency of large birds is so low that you can count it, much like your heart beat (which is a turbulent flow too)

--Norm

[Flyingwingbat1]

[QUOTE=nmasters]The only reason I mentioned St was to point out that scale isn't an important consideration when you're generating vortices because viscosity is not part of the formula. That data regarding a range between 0.2 and 0.4 only applies to flapping flight and propulsion. Since this walkalong glider doesn't flap the shedding frequency probably doesn't play a roll so you can ignore that (or at least I don't have any idea how to apply it :o ). What I do know is important is the leading edge angle of the points. If the LE sweep of a delta is less than 55 degrees the vortices will shed and reform every few seconds (at the Strouhal frequency). This would cause a variation in both CL and Cm, resulting in the nose bobbing up and down (also at the Strouhal frequency). To get a leading edge vortex that stays attached a point on the LE must have sweep of 55 degrees or more.


Interesting info on those sites. I didn't read everything in the big one. The LE angle you mention is interesting, most of my jagwing designs have vertex angles of around 60 degrees, (equals 60 degrees of sweep), but a few have vertex angles greater than 70 degrees, corresponding to < 55 degrees sweep. I didn't notice any nose bobbing, this deserves some investigation...

[nmasters]

Are you sure about that? 60+60+60=180 70+55+55=180 I can't find my protractor but it looks to me that you're right in the sweet spot.

BTW an obscure fact about leading edge vortices. The strength of the LEV is inversely proportional to the sweep angle. However most deltas have sweep of 60-65 degrees because if the LE sweep is 55 any yaw will cause an immediate collapse of the LEV on the upwind wing and subsequent roll away from the direction of yaw

[Flyingwingbat1]

Well, it's been a little while since I've messed around w/ my gliders, I'll take them out of storage and get some data on them as mentioned in my previous posts. I will definitely measure the sweep angles. I think the lack of nose bobbing on my low-sweep designs may just be because they're not flying at a high enough Cl/ aoa to cause vortex formation. Who knows, I could always reduce the nose weight a bit and see what happens...

[Ollie]

http://www.aerodyn.org/Design/design.html
"Low Reynolds Numbers

Design methods at low Reynolds number must be able to take into account the strong viscous effects that lead to laminar separation bubbles, extensive boundary layer effects, turbulence transition, hysteresis in the force coefficients, non-linear behavior. The range of Reynolds numbers is roughly 50,000 to 500,000 (lower Reynolds numbers are not yet fully investigated)."

That means nobody knows exactly!

You are on your own!!!

[Flyingwingbat1]

I like a good challenge :)

[b36roxs]

what? :) ahaha to many long words that i have no idea what they mean (im an aeronuatics noob but passionate flyer) can someone explain what they mean :confused:

[MCarlton]

BTW an obscure fact about leading edge vortices. The strength of the LEV is inversely proportional to the sweep angle. However most deltas have sweep of 60-65 degrees because if the LE sweep is 55 any yaw will cause an immediate collapse of the LEV on the upwind wing and subsequent roll away from the direction of yaw

Interesting. Does that account for yaw instability that one sees in a conventional (highly swept) paper dart? Presumably, that is why mine always fly better with the wingtips turned down at right angles? Because I am thus (unwittingly) preventing the dart from yawing sufficiently to cause the LEV to collapse?

Also, how does that affect variable geometry aircraft? I am assuming, from what you have said,that this offers some explanation for the seemingly inordinately large fin on a Tornado, and the Sub-Fin on such as the MiG23/27, because they NEED that massive yaw stability to prevent the situation you describe from occurring?

Does that therefore have an implication for yaw control in those aircraft? Meaning that in order to avoid a significant enough yaw to collapse the LEV, with the wings swept, the amount of rudder input available is reduced so that it is impossible to impart that much yaw?

Does this also apply to much more highly swept surfaces eg LERX? Is it theoretically possible that aircraft with those features, such as the F18 et al, could lose the high AOA capability which LERX provide IF a significant enough yaw rate were imposed at high AOA to cause the LEV to collapse?