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Oct 18, 2019, 04:44 PM
the occasional flyer
rothlisburger's Avatar
I think you'll find upon flying this thing that the wing has some serious structural problems. Looks like the only bending strength will be the D-tube (leading edge).

The two CF flats will do nothing in bending. The horizontal one has almost no bending stiffness, and the vertical one, while being stiffer, will probably buckle at fairly low load because the only supports are occasional ribs.

Lastly, the joiner rods aren't connected to either of the CF flats.

If you look at your Ridge Rat plans, the spar for the wing is an assembly of two ľ" spruce strips at the top and bottom of the wing, joined along the full length with vertical grain balsa shear web. This acts like an I-beam. The center joint between the two wing halves is a pair of ply doublers glued directly to the spruce spar caps. If you want a robust wing, you're going to have to duplicate this somehow. If you want a joined wing, you'll need to work the joiner into the spar as well.

You could test your current wing. Suspend the wing between two supports at the wingtips, and place a load at the center. The wing of an aerobatic plane of this size should easily be able to support 10-15 lb (5-7 kg) without failing.

Google "balsa glider wing joiner". You'll get stuff like the photo below. Note that all the bending is taken through the main spar, and the second joiner that ends after three ribs is for alignment only.

But test your wing, rather than believe me..

-David
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Oct 18, 2019, 05:44 PM
new to the sport
Thread OP
Quote:
Originally Posted by rothlisburger
I think you'll find upon flying this thing that the wing has some serious structural problems. Looks like the only bending strength will be the D-tube (leading edge).

The two CF flats will do nothing in bending. The horizontal one has almost no bending stiffness, and the vertical one, while being stiffer, will probably buckle at fairly low load because the only supports are occasional ribs.

Lastly, the joiner rods aren't connected to either of the CF flats.

If you look at your Ridge Rat plans, the spar for the wing is an assembly of two ľ" spruce strips at the top and bottom of the wing, joined along the full length with vertical grain balsa shear web. This acts like an I-beam. The center joint between the two wing halves is a pair of ply doublers glued directly to the spruce spar caps. If you want a robust wing, you're going to have to duplicate this somehow. If you want a joined wing, you'll need to work the joiner into the spar as well.

You could test your current wing. Suspend the wing between two supports at the wingtips, and place a load at the center. The wing of an aerobatic plane of this size should easily be able to support 10-15 lb (5-7 kg) without failing.

Google "balsa glider wing joiner". You'll get stuff like the photo below. Note that all the bending is taken through the main spar, and the second joiner that ends after three ribs is for alignment only.

But test your wing, rather than believe me..

-David
Hi David, thanks for the feedback, I will definitely try the weight test - each former is glued to the CF flats

Phil
Oct 18, 2019, 06:11 PM
F3Foamie Pilot
Larger diameter carbon tube spars would probably be your best bet. Might be something to consider in the future. I've used that carbon ribbon material and my experience was it just helped in one direction. I use it to stiffen up my EPP fuselage noses.

Matin at NCFM uses large diameter tapered carbon tubes for his kits. I believe they are from golf club shafts. That would be the ticket to tie your ribs together.

Anyway, I'm quite impressed with your innovations.

Oct 18, 2019, 07:10 PM
new to the sport
Thread OP
Quote:
Originally Posted by Slopemaster
Larger diameter carbon tube spars would probably be your best bet. Might be something to consider in the future. I've used that carbon ribbon material and my experience was it just helped in one direction. I use it to stiffen up my EPP fuselage noses.

Matin at NCFM uses large diameter tapered carbon tubes for his kits. I believe they are from golf club shafts. That would be the ticket to tie your ribs together.

Anyway, I'm quite impressed with your innovations.

They feel quite strong, but I have no real experience to know! If I can get a bit more practice in while finishing this build we will find out one way or another!
Oct 18, 2019, 09:34 PM
Registered User
Ward Hagaman's Avatar
I’m STILL waiting for a failure from flight loads rather than pilot error...

Will you be using balsa ailerons?
Oct 18, 2019, 11:50 PM
new to the sport
Thread OP
Quote:
Originally Posted by Ward Hagaman
Iím STILL waiting for a failure from flight loads rather than pilot error...

Will you be using balsa ailerons?
Hi Ward,

I am planning to use foam covered in newsprint which I did that for my current DLG build and it turned out light and strong. There is no reason not to use Balsa, but I thought to use the materials I have in stock!
Oct 19, 2019, 11:31 AM
Registered User
Ward Hagaman's Avatar
Foam is good, but I suggest sanding the trailing edge down a bit.
Oct 19, 2019, 11:51 AM
Registered User
chip.greely's Avatar
Quote:
Originally Posted by bracesport
Hi Chip,

In looking at the original RR plan, yes my fuse is a little slimmer. Also, the original wing was 9" and my MH32 is 6" which makes the nose of my ship longer from the LE and leaves the TE to horizontal stabiliser roughly the same - let's hope it will work!
Hi Phil,

I was thinking slimmer in Y as opposed to Z, but I think you've achieved both, with your fuselage.

Decreasing the wing chord had 2 affects on my Black Jart, reduced total wing area for creating Lift and reduced Drag to increase speed. Now it gets flown on windier days

Assume your Ridgie will do the same
Oct 19, 2019, 10:40 PM
new to the sport
Thread OP
Quote:
Originally Posted by chip.greely
Hi Phil,

I was thinking slimmer in Y as opposed to Z, but I think you've achieved both, with your fuselage.

Decreasing the wing chord had 2 affects on my Black Jart, reduced total wing area for creating Lift and reduced Drag to increase speed. Now it gets flown on windier days

Assume your Ridgie will do the same
Now I am scared! I am a complete nube to sloping, so I am hoping to get some air time with some other slower ships before the RR is finished!

Not knowing the original RR intimately, I think the new fuse is reasonably good proportions - either way, its is done now!
Oct 21, 2019, 04:31 PM
the occasional flyer
rothlisburger's Avatar
Quote:
Originally Posted by bracesport
Hi David, thanks for the feedback, I will definitely try the weight test - each former is glued to the CF flats

Phil
Hey, what I'm saying is that the CF flats are doing you basically nothing. They are in the wrong place. The wing is like a beam. Pulling positive Gs bends the wing up. That puts the top "fibers" in the beam in compression, and the bottom fibers in tension. But inside, in the center, the fibers are not loaded at all. This is called the neutral axis of the beam. You've put your CF flats right there, where they will do nothing.

If you put CF flats on the top and bottom surfaces of your wing, you'll have a strong material taking tensile and compressive loads. Of course, you'll need something connecting them, like the shear web on an I-beam. But since you put the CF flats down the center of your airfoil, they add basically zero strength. The wing would be just as strong and stiff if you left them out.

So the only thing causing any structural resistance at all are the sheeting on the leading and trailing edges. I assume you're glueing them to each other along the span. But even so, the PLA simply is not that strong. It may make your wing feel stiff, but it won't be strong.

But again, test the wing. Assemble the whole thing, including joiner rods. Put a block under each wingtip, and start loading it in the center (where the fuselage is). You should get almost zero deflection (sag), and it should take some serious weight (like 20 lb or more) before it explodes.

Good luck.

-David
Oct 21, 2019, 04:44 PM
new to the sport
Thread OP
Thanks for the input - this all makes sense - If it all turns to custard I will remake the wing with this great info

Quote:
Originally Posted by rothlisburger
Hey, what I'm saying is that the CF flats are doing you basically nothing. They are in the wrong place. The wing is like a beam. Pulling positive Gs bends the wing up. That puts the top "fibers" in the beam in compression, and the bottom fibers in tension. But inside, in the center, the fibers are not loaded at all. This is called the neutral axis of the beam. You've put your CF flats right there, where they will do nothing.

If you put CF flats on the top and bottom surfaces of your wing, you'll have a strong material taking tensile and compressive loads. Of course, you'll need something connecting them, like the shear web on an I-beam. But since you put the CF flats down the center of your airfoil, they add basically zero strength. The wing would be just as strong and stiff if you left them out.

So the only thing causing any structural resistance at all are the sheeting on the leading and trailing edges. I assume you're glueing them to each other along the span. But even so, the PLA simply is not that strong. It may make your wing feel stiff, but it won't be strong.

But again, test the wing. Assemble the whole thing, including joiner rods. Put a block under each wingtip, and start loading it in the center (where the fuselage is). You should get almost zero deflection (sag), and it should take some serious weight (like 20 lb or more) before it explodes.

Good luck.

-David


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