Micro wing - RC Groups
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Sep 18, 2017, 08:27 AM
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EdSoars's Avatar
Discussion

Micro wing


My grandson has worn out the nose of the JR Cub trainer I gave him for Christmas. So now I get to design a REAL airplane around the power / guidance system. The all-up weight of the Cub was 2.0 oz / 58 gm. Design priorities: stable handling, damage-resistant, super-simple design, fast build (replacement!) and better glide and speed range than the Cub.

I think the best planform will be a constant-chord, 20 deg swept wing of 20-26 inch span. I haven't extracted the flight system from the Cub, so I'm guessing at 1 oz for the airframe and 1 oz for the flight system. The simplest build would be to hotwire one long wing panel that can be cut in half and joined at the sweep angle. Covering for stiffness: no idea yet. Twist can be provided by the elevons. An EPP stub fuselage can hold the motor, receiver and battery. 2mm depron for the tiplets.

But what about the airfoil? The only experience I have with really small Reynolds numbers comes from old handlaunch gliders (chuckies). They spanned 18", had a 4" max chord, but I don't remember ever weighing one. Maybe the Drela AG-03 would be a starting point. ??
Last edited by EdSoars; Sep 18, 2017 at 09:47 AM.
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Sep 18, 2017, 10:28 AM
Herk
HerkS's Avatar
Hi Ed, this sounds like fun.

This concept will have a wing loading of about 3 oz/ft^2 and fly at about ten mph or less. That means it's max RN will be about 30,000. For such low RN I think that thin and low curvature is more important than a specific airfoil. Viscosity is dominant in this RN range, so you can think of exaggerating it's effect by mentally designing a very small model to glide well in a vat of water or motor oil. Imagining that, I would want very little camber, low curvature on all surfaces and a fairly thin entry contour on all surfaces. I probably wouldn't worry a whole lot about too much weight either - within reasonable limits.

If it was going to build something like this I'd probably use a symmetrical section of 4-5 percent thickness and a bit of twist because it will fly so slowly - but also at low Cl --- so somewhere in the 3 degree range. An airfoil section even thinner would be ok, but some depth for structural reasons is probably worth it. I'd also put the max thickness back in the 40 - 50% chord range so there is not much curvature around the leading edge and to put the structural center fairly far back to reduce flutter.

Probably more than you needed here, but thanks for the distraction provided by a little thought experiment - I enjoyed it - Herk
Sep 18, 2017, 10:40 PM
Registered User
EdSoars's Avatar
Actually, I was thinking of a section like the old handlaunch gliders before discus launching was invented. The elevons will be tapered and reflexed to provide twist.

Rigidity is going to be an issue. Elapor, the plastic so many injection-molded RTFs are made of, is stiffer than white or blue foam.

Maybe a very thin FG skin laid up on glass. Or just 1 oz FG on bias adhered with polyurethane varnish. Or thin dacron cloth. Tissue applied with varnish?
Last edited by EdSoars; Sep 18, 2017 at 11:04 PM.
Sep 18, 2017, 11:19 PM
Registered User
EdSoars's Avatar
Herk, I'm surprised at such a rearward max thickness location. The AG-03 is up at 25%. That's the lowest Rn airfoil out there that isn't in the Handlaunch glider forum.

Here are the Nur.exe input and planform.
Sep 19, 2017, 12:38 AM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by EdSoars
Rigidity is going to be an issue. Elapor, the plastic so many injection-molded RTFs are made of, is stiffer than white or blue foam.

Maybe a very thin FG skin laid up on glass. Or just 1 oz FG on bias adhered with polyurethane varnish. Or thin dacron cloth. Tissue applied with varnish?
How about an A-frame spar system made from aluminum tape?
Sep 19, 2017, 08:15 AM
Herk
HerkS's Avatar
Quote:
Originally Posted by EdSoars
Herk, I'm surprised at such a rearward max thickness location. The AG-03 is up at 25%. That's the lowest Rn airfoil out there that isn't in the Handlaunch glider forum.

Here are the Nur.exe input and planform.
Well Ed, as I said I don't think the airfoil's exact contours are that important at such low RN. It might fly just as well with a flat sheet of depron. This model is a little larger - 36 inch span, 5 inch avg. chord. Weighs 4 ounces. It flys really well. I built it to explore bottom surface mid chord flaps. Its wing is 6mm depron sheet. After this photo was taken I added a 6mm cf tube leading edge. I flew it quite a bit while I was moving fixed flaps around taped to the bottom of the wing. It was fun to fly and quite agile. Since it's essentially a plank I wasn't concerned much about flutter.

There are two reasons for the aft thickness on my conceptual low RN airfoil. One is that more curvature near the LE causes the pressure to drop rapidly which gives little time for boundary layer transition to turbulent flow and so promotes separation of the flow from the surface. A gradual curvature drops the pressure more slowly so gives more time for transition and helps to delay separation. That's pretty theoretical and probably not very important in the region of RN=30K. I don't think the boundary layer is likely to transition under any circumstances in that RN range. The other is something many swept wing builders don't seem to recognize. Moving the center of torsional stiffness rearward helps to delay flutter. So forward spars and/or forward thickness on a wing with no spars tend to increase that tendency. IMHO of course
Sep 19, 2017, 08:43 AM
Registered User
EdSoars's Avatar
Norm: excellent suggestion! I'll try Al tape for sure.

Herk: I have some 2mm depron that will make good trailing edges. Thanks for the info on low Rn sections. A flat sheet is REALLY easy to cut. Are your ailerons driven by torque rods? I will probably put the servos out on the wing. They are tiny linear-drive units.

Maybe the whole wing should be 2mm depron wrapped over a center depron web to stiffen it up and add a little top curvature.

The JR Cub trainer wing was quite stiff: 24" span x 4" chord, injection molded of course, so it had faux ribs and curvature that helped the stiffness. Maybe a little more than 2mm thick.

More soon...
Sep 19, 2017, 10:26 AM
Herk
HerkS's Avatar
Quote:
Originally Posted by EdSoars
Norm: excellent suggestion! I'll try Al tape for sure.

Herk: I have some 2mm depron that will make good trailing edges. Thanks for the info on low Rn sections. A flat sheet is REALLY easy to cut. Are your ailerons driven by torque rods? I will probably put the servos out on the wing. They are tiny linear-drive units.

Maybe the whole wing should be 2mm depron wrapped over a center depron web to stiffen it up and add a little top curvature.

...
I considered the 2mm also, when I was thinking about building that model. I had plenty, but decided that it's just too fragile. It might be a bit fragile for yours too considering who the main pilot will be. This thing is really rugged and easy to repair. The ailerons were driven by torque rods, but I found out that the model handled great with just rudder, elevator and throttle. So I taped them off and added central elevator surfaces. All that was after the photo was taken. Since the mission of the thing was to explore the flap issue, the original elevon concept just wasn't needed. I did taper the bottom of those surfaces to create a bit of reflex but didn't use them for controls. I thought later about resetting them for use as elevons just to compare, but have not done it. When I first flew it, that wing was just too flexible in bending. That's why I added the LE rods. Yours is to be swept, so stiffness is even more important.
Sep 19, 2017, 12:48 PM
less is more
Knoll53's Avatar
It's a little known fact that "Herk" is derived from the base Latin meaning "low Reynolds number".

Barn owls have fluffly down like feathers that provide the most silent flight. I wonder if that contributes to performance at low Re?
Sep 19, 2017, 01:46 PM
Herk
HerkS's Avatar
Quote:
Originally Posted by Knoll53
It's a little known fact that "Herk" is derived from the base Latin meaning "low Reynolds number".

Barn owls have fluffly down like feathers that provide the most silent flight. I wonder if that contributes to performance at low Re?
I think that all kinds of bird feathers do that.
Sep 20, 2017, 02:05 AM
You know nothing....
Stuart A's Avatar
How about a simple KFM2wing?Simple construction and supposed to perform well.Note the 'step'is at 50%.
Sep 20, 2017, 09:49 AM
Registered User
EdSoars's Avatar
Great suggestions!

The K-F airfoil might be the ticket. I'll use a CF rod to stiffen things spanwise, and maybe on-bias aluminum tape strips for twist resistance. Using depron eliminates the fuss of setting up my hotwire cutter.

Looks like the layout has to be a plank; the transmitter in the trainer Cub package isn't programmable, so there will need to be a central elevator and ailerons, no elevons.

Micro-Moth it is!
Sep 20, 2017, 11:57 AM
You know nothing....
Stuart A's Avatar
Quote:
Originally Posted by EdSoars
Great suggestions!

The K-F airfoil might be the ticket. I'll use a CF rod to stiffen things spanwise, and maybe on-bias aluminum tape strips for twist resistance. Using depron eliminates the fuss of setting up my hotwire cutter.

Looks like the layout has to be a plank; the transmitter in the trainer Cub package isn't programmable, so there will need to be a central elevator and ailerons, no elevons.

Micro-Moth it is!
If possible could you let the g'son lend a hand on the build?kf's are about as easy as they come.
Sep 21, 2017, 09:04 AM
Registered User
EdSoars's Avatar
Stuart, I definitely want grandson Axel to help with the build, but the design has to be thought through first, so there aren't a lot of hitches and glitches to cause frustration. I have enough depron for two half-size Moths, ("Haffamoth").

So I have one wing cut out and folded over the LE. The K-F step is formed over a 1/16" dia CF rod. It doesn't look like it's thick enough, so I might figure something else out. Once it goes together and actually flies, I'll post photos and plans.

I love the current state of posts: from 24" to 4 meter spans; 6.5 to one!
Sep 21, 2017, 09:22 AM
You know nothing....
Stuart A's Avatar
Quote:
Originally Posted by EdSoars
Stuart, I definitely want grandson Axel to help with the build, but the design has to be thought through first, so there aren't a lot of hitches and glitches to cause frustration. I have enough depron for two half-size Moths, ("Haffamoth").

So I have one wing cut out and folded over the LE. The K-F step is formed over a 1/16" dia CF rod. It doesn't look like it's thick enough, so I might figure something else out. Once it goes together and actually flies, I'll post photos and plans.

I love the current state of posts: from 24" to 4 meter spans; 6.5 to one!
That's about the long and the short of it


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