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rtfm

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Hi,
No worries at all. A digital HM293 is indeed an enticing idea, and certainly doable. And potentially profitable, given the popularity of the design. Well, "popularity" is relative, of course - the Flea segment is tiny within a very small segment after all.

Everything from "The outer wing panels" is more or less exactly what I'm doing, anyway. The real challenge is finding the best way to cnc the airframe. There would seem a number of ways to do this...

Duncan
 

rtfm

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OK, I need some advice.
Here is the supporting structure to ensure alignment as well as the 1 deg dihedral for the centre section and the 3 deg dihedral for the outer panels. All this was produced by DevWing CAM (very nice software, by the way).
But my problem is this:
The ribs have a 16mm circular cut-out at the LE to accommodate a 16mm dowel. So far, so good. But how do I attach the D-cell? The software assumes I'll be building the D-cell in two parts (top and bottom) bonded at the leading edge.
1618355287932.png
While that would be convenient because I can then lay the aft section of the skin easily enough, but I'm not happy with such a small area for the T88 to bond the skin to the dowel at the leading edge.

On the other hand, if I use my upright jig which has two holes through which I can thread 25mm SS tubes and thread the ribs onto these (holes already cut out of the ribs for this purpose)
1618356025556.png
it will be easy to wrap the 1.5mm ply skin around the leading edge as a continuous piece and bond it to the ribs and the spar. But once bonded in place, the DevWing CAM supporting structure won't fit unless I cut out a 1.5mm recess in front of the spar - something the software does not do.

So, how do you experienced builders do it?

Regards,
Duncan
 

rtfm

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This is what I'm thinking also. I'd use Sketchup's offset function, which easily would do this, except that Sketchup doesn't do curves - it approximates curves with multiple straight lines - some of them quite long. I'll see if one of the other 2D CAD applications might help...

Cheers
Duncan
 

rtfm

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Aaargh! This CAD stuff is doing my head in! I think what I'm going to do is to cut the formers as drawn by DevWing, and then overlay a strip of 1.5mm ply on the aft section when I need it. Easy Peasy. I've just wasted a pertfectly good morning slaving over a hat laptop when I should have been in the shed building.
 

rtfm

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I think I've nailed it. I built a D-tube bending jig yesterday, and bent my first D-tube this morning. Worked like a charm! LOTS of boiling water painted on (mainly on the outside surface) with a broad paintbrush. Then gently pulled the ply into the former, while painting on hot water. In very short order, the skin was ready to be pulled all the way in using a SS pipe as you can see. Now I just have to wait for everything to dry.
1618620915976.png
 

rtfm

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Sonja Englert seems to prefer 12% thick...I am for the 10% thick as it could be supersonic.


View attachment 107947

I can get pretty sturdy yet lite spar inside there.
Hi,
Nice looking airfoil. What is it?
I plan (on the Fleabike 3A - three axis) to fit Junkers ailerons on the inner half of the rear wing. Not the ideal position for ailerons, but SO much easier to build. The prototype (to keep things as simple as possible) will be as standard as possible. But once I'm happy with the flying characteristics of the plane, I *may* add the Junkers ailerons. So I'm looking for a nice airfoil. The one you have posted above looks perfect - I particularly like the max thickness position.

Regards,
Duncan
 

WonderousMountain

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Duncan, that's not an airfoil, it's an Ellipse, with a triangle flap.

Might try Eppler ~ E836 HYDROFOIL AIRFOIL ✓

Regards,
CK LuPii
 
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rtfm

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OK, some progress...

First, I built a vertical jig which has some 25mm SS pipes threaded between two uprights.
1620269700556.png
The ribs have holes cut through them aligned with these pipes, so that the ribs can be threaded, and (hopefully) will be perfectly aligned.
The next job was to cut the ribs. The inner panels have a number of ply ribs to act as hard points for the pivot fixture and the hinges.
1620270194421.png

Next, threading all ribs onto the tubes in the correct order.
1620269598107.png
Spread out...
1620270309133.png
I'm using a 16mm Tasmanian Oak dowel for the leading edge. Tassie oak is great stuff - very light and strong.

It's a bit difficult to show, but there are two 19mm x 19mm Hoop Pine spar caps, separated by a 3mm Gaboon ply shear web. At this point, a work of explanation is probably necessary. I was faced with a choice: A continuous shear web running the length of the spar - which would mean cutting the ribs in half and bonding them to the shear web again; or keeping the ribs intact, and inserting mini shear webs between them. I opted for the latter. A bonus was that the mini shear webs helped align the ribs. Neat.

Here you can see the two Hoop Pine spar caps, separated by the 3mm ply shear web. The additional hole in the right-most foam rib is not a mistake - it's for the 16mm Tassie oak dowel acting as an anti-drag member.
1620270765758.png

The outer wing pivot is anchored to its own mini spar and shear web.
1620270879966.png

And that's about as far as I've got. Most of the time spent on the wing has been in (1) cutting the ribs FOUR times to get all the holes, recesses etc right. (2) trying to figure out how best the BUILD the bloody thing. With no builder's manual to guide me, it's a matter of give it my best shot, scrap it, and try again. Someone building a Fleabike wing would be able to do so in a morning, I imagine. With the help of my Builder's Manual, of course...

Some weights:
Foam ribs: 18g
Entire set of ribs (foam and ply): 1.1kg

Next step:
Commit myself to bonding the spar caps, leading edge and shear webs.

And then start again, because I forgot something...

Bugger...

Regards,
Duncan
 
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Tiger Tim

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keeping the ribs intact, and inserting mini shear webs between them. I opted for the latter. A bonus was that the mini shear webs helped align the ribs.
You’ll want to give that one a really good think before committing to glue.
 

Victor Bravo

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Make absolutely certain you have as much bonding area as possible between the shear webs and the spars, on both sides of the shear webs. As many square inches of glue area as possible, which means the top of the shear webs will be flush with the top of the spar caps. And make sure you use the very best glue and gluing techniques... pre-soak and clamping pressure and glue viscosity and all that.

I also agree with Tiger Tim about having someone check your spar design. The mini-shear webs between the ribs is a very tried and true method on model airplanes, we've all built many models using that technique. But on a full sized aircraft there may be a difference, the small segments of un-supported spar cap could be a failure point when the far far larger loads of a "real" airplane are applied.

Have someone who does spars for a living run the numbers, no matter whether it takes a pint at the local pub or a few hundred dollars in hard cash. This alone will create a primary yes/no decision for anyone who wants to buy one of your kits or build form your plans later.
 

Tiger Tim

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the small segments of un-supported spar cap could be a failure point when the far far larger loads of a "real" airplane are applied.
To me it just calls back to that video with the little single seater that had a fatal crash due to its spar having an ultimate load of something like 1.2G. It had a very similar wing construction to what’s being designed in this thread.
 

Victor Bravo

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Engineers please feel free to correct me, but I do believe the vast majority of cantilever wood wings are built with solid spar webs and front/rear ribs, as opposed to solid ribs and segmented shear webs.

For safety and sanity, the question of why this is (or is not) the case, must be answered before Duncan finalizes his wing.

The Flea-Bike is a very viable, worthwhile, desirable thing to pursue. Because several others besides Duncan would certainly be interested in one, making sure that the design details are done right is important here.
 

rtfm

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Hi,
If I remember correctly, that spar failed because the spars themselves were discontinuous. Nothing to do with the shear webs.

I'll be doing a full load stress test.

According to Greenberg's plans for the HM293, his main spar has two15 mm x 15mm strips of Spruce (225mm^2 cross section), sided by two sheets of 1.5mm ply. My wing (considerably deeper than the airfoil he recommends) has two strips of 19mm x 19mm Hoop Pine (361mm^2 cross section) which sandwich a single 3mm sheet of ply.

By comparison, the Aeromax has a single 15mm x 15mm Hoop Pine spar cap, with what looks like a 2mm sheet of ply for the shear web.

Until the wing loading, at least, I have no fears about my wing's strength.
 
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