1/12 Scale Reiher Sailplane foam

[bcarter1234]

I wish I knew now what I'm likely to know in a few weeks but learning is half the fun right?

I've been having a great time with a trio of recently built 36" tow line gliders. A larger RC model that could be flown in the same small field, handle light wind and intentionally thermal has some appeal.

The first iteration is being kept light. Rudder and elevator using a receiver with built in linear servos and minimal wing reinforcement. If it works well and demonstrates that it could thermal with more weight I may coat he wings and/or add ailerons. A small motor is also a possibility in future depending on how it launches from a tow. The same receiver will accommodate both. Then again it may end up adorning the ceiling or the trash bin.

The fuselage is shaped, the wing saddle and stabilizer platform have been hot wire cut.
 
The second image shows the rectangular center block used to attach the wings. The block makes aligning the wings more straightforward.

 

[bcarter1234]

Current state of the 1:12 Reiher. 70 grams as shown. 62.5" wingspan, 200 square inches of wing area, E392 airfoil with 3 degrees of positive wing incidence at the root and 3 degrees of washout. Tip stall is a concern but it looks like turbulators could help if it does prove to be an issue. The wing stiffness seems marginal at best but I'll try a few test glides before I make any attempt to add weight and stiffness.
     

[OZPAF]

That's very neat - 70gms is very light as DLG's of that size are around 200gms. I guess though that is without RC?

I would personally consider using lightweight glass cloth(20gms/sq metre) on a foam wing of this size. If you still have the foam from which it was cut then it could be used as beds to press or vacuum the glass on.

Do a web search for info on weight or vacuum pressing.

John

[bcarter1234]

John,

Yes, the current weight is without RC gear. I'm curious how much that figure will climb once it is installed and ballasted to the proper CG. I hope to stay closer to 100 than 120 but that remains to be seen.

I initially planned to skin the wing but then thought I'll never know how light it could be without at least a few test glides using the wing as is. If, as we both suspect, it flexes too much I'll try to stiffen it. My current plan is to employ one or some combination of these: strapping tape "spars" top and bottom, carbon or balsa spars top and bottom, glass leading and/or trailing edge, cover the wing with film. A full height balsa spar would be pretty easy to do also. If anyone has thoughts on the potential effectiveness (or lack thereof) or other suggestions I'd like to hear them. This is learn as you go for me. 

There is a good chance I'll cut another set of cores and fully skin them. One of the many benefits of building with foam is that no single part takes very long to make.

I was actually ready to cut a second set of cores for skinning but decided to wait in case I find something I want to change about the wing, i.e. airfoil, washout, dihedral/anhedral angle etc. 

I hope to get the gear and ballast installed tonight so it may be possible to try some test glides in the next day or two. I'm frequently overestimate my ability to get things done so take that with a grain of salt.

I have two other questions.
1) Any suggestions for relatively easy to acquire tubing to run 1mm carbon pushrods through?
2) Is there a guide somewhere to making leading edge "tape" from fiberglass? I thought I had seen some stuff but can't find it now. Just wax paper and spray glue or is there a better way?       

[OZPAF]

Here is a good site on home built DLG's

https://www.rcgroups.com/forums/showthread.php?4699611-Scratch-Built-DLG-on-the-cheap%C2%85-redux

I initially planned to skin the wing but then thought I'll never know how light it could be without at least a few test glides using the wing as is. If, as we both suspect, it flexes too much I'll try to stiffen it. My current plan is to employ one or some combination of these: strapping tape "spars" top and bottom, carbon or balsa spars top and bottom, glass leading and/or trailing edge, cover the wing with film. A full height balsa spar would be pretty easy to do also. If anyone has thoughts on the potential effectiveness (or lack thereof) or other suggestions I'd like to hear them. This is learn as you go for me.

The answer I feel lies in exactly how you tend to use the glider. Light bungee launches would probably be ok using a simple system of spars to what you have mentioned. Torsional stiffness will probably be the main worry and I feel will need a skin similar to glass reinforced tape.
As a suggestion how about considering a system of chord wise "weblets' - say 3 top and bottom on edge of say 1/4"x 1/8" balsa under the tape  - at say 20%, 30% and 50% chord.
These should help with both torsion and bending.

Future wings for heavier DLG launches would need i feel, glass skins and carbon spars.

1) Any suggestions for relatively easy to acquire tubing to run 1mm carbon pushrods through?
2) Is there a guide somewhere to making leading edge "tape" from fiberglass? I thought I had seen some stuff but can't find it now. Just wax paper and spray glue or is there a better way?

I buy this from Hyperlink in the UK but check the site I referenced- it may provide more local sources.
That sounds a reasonable approach but test that the spray glue is compatible with your epoxy first. I have used Hair Spray as a binder for the glass cloth but it is not too strong.

Good luck.

John

[bcarter1234]

John,

Thanks for some excellent suggestions. Skins are probably the ultimate answer but I'll try some of the other techniques first just to test their efficacy. 

As expected I made no where near the progress I thought I would. I always forget how long it takes to do the "first one" while you are developing methods for each stage. Subsequent similar builds go much faster once those methods are established. 

On the plus side while searching for .020" music wire for control rod ends I found a 60% complete 18" wingspan HE-111 I started from blue foam years ago and completely forgot about. It should probably move up on the to-do list. I also found rolls of 1.5 and 3mil laminating film, a half finished Navy-Wright Racer...the list goes on.

The area for the RX is cut out. I'm going to try working with these grooves. A plastic soda straw fits perfectly in them as a liner. 

[SP250]

If you're looking for alternative bonding agents for skinning the foam, take a look at G4 Pond Liner.  Many of the outdoor large scale guys use it to glass skin wings and fuselages instead of 2 pack epoxy finising resin.  Its lighter, single pack and water clean up.  Which also means it can be diluted with water to flow more and for smaller models may be an option depending on what material you use for the skinning instead of glass.

John M 

[bcarter1234]

John M,

Interesting, I'll do some reading. After a quick look I'm guessing I'm after G4 Pond Sealer, it appears to be a something like water based polyurethane. 

[Prosper]

Yes, interesting. Never heard of it before now. Worth playing with IMO.

"Rustins Bonda G4 Pond Sealer is a moisture-cured polyurethane, which forms a non-porous seal on concrete, brick or porous stone. It uses the moisture in the air and substrate to cure or harden. It can be applied onto slightly damp substrates. However, it is important that the substrate is sufficiently dry to be porous so as to enable the first coat to bond and obtain a mechanical grip. It can be applied in poor weather conditions, high humidity and at temperatures down to 0 degrees C. G4 Pond Sealer seals out lime."

Thanks for the lead John M.

Stephen.

[OZPAF]

That's very interesting John. Thanks for the extra info Stephen.

John

[SP250]

Yes sorry chaps I called it Pond Liner and should have said Pond Sealer.  I used it in a 5ft deep concrete lined service pit in my garage which used to sweat profusely and I'd have 6 inches of water in the bottom after each winter.  One coat of G4 and it was dry as a bone for several years till I sold the property.
I skinned Wot 4 wing and a Scale Zlin with glass and G4 with the leftover.  Not tried it on very lightweight indoor models but worth some experimentation I would think.

John M

[bcarter1234]

Some progress on the RC gear installation. If the wind cooperates I may get a test glide or two in this weekend.

[lincoln]

That's a beautiful shape. Keep us updated.

If it was my project, I might inlay strips of thin, unbacked wood veneer top and bottom, at maybe 25 percent of chord. Alternatively, carbon fiber strips or tow. Or glass tow. Or fragments split off from pultruded driveway marker poles, which are very strong but can give you invisible splinters if the color doesn't go all the way through. You won't need much, but I think you'll need some.

If I was going to sheet the wing, I might consider light kraft paper with epoxy. The epoxy should be a SLOW type that cures hard. Relatively low viscosity is probably useful, too. On my project, I painted on the epoxy, let it soak in for a while, scraped off the excess, and then did the other side. I vacuum bagged it onto the wing cores, though perhaps evenly applied pressure would also do. I used 60 lb kraft, which is turned out a little heavy for an RC hlg. 30 or 40 lb might be better. This stuff is as stiff in bending, for its weight, as fiberglass. Not sure about tension or compression. It's easier to work with than fiberglass. I've used kraft paper with thinned Titebond to repair a foamy, which worked quite well. No need for pressure, and it drapes well on curvy surfaces. I think it's probably not quite as stiff or strong, though, and there may be warps. I wonder if fancy linen stationery would be good if used this way or with epoxy.

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Have you tried Highload 60 or Plazamate? Can be wet sanded very smooth, dings less than regular blue foam and is about 2.4 lbs per cubic foot. There are also Highlighted 40 and 100. I think Corning has similar products.

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I think you will need to turbulate that thick airfoil for best performance. Especially with that narrow chord, which makes for low Reynolds numbers. I think the AG03 might work better, especially if you ever have to get back upwind in a breeze. Apogee uses AG03 RC gliders benefit from airfoils with a wide speed range, though maybe your glider doesn't need quite as wide as a discus launch would. Maybe slightly more camber would be ok. Slightly thinner might be good, too, but in either case you will lose the convenience of the flat bottom. You could keep it if you reduce the camber while thinning, which I guess is like thinning about the chord line instead of the camber line. If you lose the flat bottom, you might as well start with the AG04.
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It's hard to find a lower aspect ratio sailplane that's as pretty as the Reiher, but some of them still look good. The DFS Habicht is one. DFS Habicht  Note that there were even stubbier versions of the Habicht. You can find some other attractive gliders with modest aspect ratios by selecting "voltige" in the "fonction" window here: https://www.j2mcl-planeurs.net/. The Castel-Mauboussin Jalon is one of my favorites: https://www.j2mcl-planeurs.net/dbj2mcl/planeurs-machines/planeur-fiche_0int.php?code=2757

A modest aspect ratio is structurally and aerodynamicly superior in small models. Look how low the aspect ratio of Drela's Apogee is.

Unless you are already an experienced RC glider pilot, your Reiher may be better for practicing repairing than flying.

[bcarter1234]

On my way out the door so this will be a quick note. I was able to do some test glides today. Here is the short version, I'll add more detail and respond to the comments above later. I do want to say thank you for the excellent information though.
 

1) It did fly and if I could leave it as is the wings would be stiff enough for careful towline launches and the very floaty glide it provides.
2) I don't have enough roll control. If everything goes well it is flyable but if it gets a gust from the side it won't turn back even with full opposite rudder.

All up weight as flown today was 87 grams.

Would a wing without anhedral, like the Nemere, give more roll control with rudder only?

I plan to add ailerons to the existing wing. The process of adding them will also add some stiffness.   

[OZPAF]

Even with ailerons I would try and increase the dihedral of the outer wing panels to around 3 - 4 degrees to help with roll stability.

The scale small fin/rudder area and short tail moment is also not helping recovery from gusts in roll and pitch probably.

Mixing some aileron into the rudder may also be necessary to control adverse yaw.

Good luck with it.

John