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Nov 30, 2015, 02:11 AM
Balsa&Tissue
payne9999's Avatar
Thread OP
Help!

Building lighter and stronger


I just maidened a scratch built 28% Yak 55m. It was a total failure due to a design flaw in the wing spar area. It was easily identified by looking at the failed air frame. I should have caught the issue while building it so my mistake.

The fuse is mostly 1/8" ply originally specifiied to be lite ply (mixed feelings about light ply). I used birch and the plane came out to heavy for a variety of reasons.

I want to explore using lite ply in a proper way or balsa in a way that could replace some of the ply. Maybe even make my own balsa ply and use carbon fiber in some areas.

The air frame was not designed by me but I would like to use it as a platform for a modified version.

It flew well in the maiden until one wing just blew apart and took out the whole air frame. some shear webs between the fiberglass wing tube receiver and spars would have prevented that but still she was too heavy.

I love the plane and want to build it better, I have a CAD system and the current design is in SolidWorks format. I also have a 3 axis router so I can cut the kit again myself. I can buy Russian birch ply at the local woodworking specialty shop but it is heavy.

Looking for a way to redesign in a more efficient manner and still have the strength to do traditional aerobatics.

Attached is a picture of the current airframe (fuse almost all 1/8" ply, wing ribs almost all 1/8" balsa, spars spruce, wings and tail surfaces sheeted.

Dave
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Nov 30, 2015, 10:12 AM
Registered User
builderdude's Avatar
I'm sorry for your loss, but I do admire your design abilities and your desire to figure out the problem and try again. I often design on the ragged edge of my abilities and often end up with planes pushing the weirdness meter into the red. Sometimes they work and sometimes they don't. If I stick with conventional planforms, they tend to do pretty well though.

Anyway, yours is conventional, but the aerobatics is what pushes your design to the edge, I suppose. I don't see anything wrong with your wing design. All you need is more strength.

So I'd suggest a couple of things. I would add a carbon fiber reinforcement to your spar, along the length of your wing. If you're not using dihedral, that should be easy.
Then I would sheet the leading and trailing edges in balsa.
That wouldn't add much weight to your design, and would probably handle the aerobatics better.

Just my 2 cents...
Nov 30, 2015, 01:08 PM
Neophyte hacker
portablevcb's Avatar
Lite ply.

It is great stuff when used in the proper place. But.....

If designing a plane just for me...I'd use a balsa ply sandwich. 1/32 ply and light 1/8 balsa as a core. And it is easier to laminate the stuff first and then machine.

I'd also get rid of all the lightening holes in ribs and use full shear webs on all the spars. CF reinforcement is not a bad idea either. Use it as a cap strip top and bottom.

I'd also pick the wing sheeting carefully. Good straight grain stuff. Might even want a 1" or so strip of it to be hard balsa instead of the 'normal' stuff. Align this strip along the main spar.

charlie
Nov 30, 2015, 02:57 PM
Balsa&Tissue
payne9999's Avatar
Thread OP
Quote:
Originally Posted by builderdude
I'm sorry for your loss, but I do admire your design abilities and your desire to figure out the problem and try again. I often design on the ragged edge of my abilities and often end up with planes pushing the weirdness meter into the red. Sometimes they work and sometimes they don't. If I stick with conventional planforms, they tend to do pretty well though.

Anyway, yours is conventional, but the aerobatics is what pushes your design to the edge, I suppose. I don't see anything wrong with your wing design. All you need is more strength.

So I'd suggest a couple of things. I would add a carbon fiber reinforcement to your spar, along the length of your wing. If you're not using dihedral, that should be easy.
Then I would sheet the leading and trailing edges in balsa.
That wouldn't add much weight to your design, and would probably handle the aerobatics better.

Just my 2 cents...
The CAD picture in my originalpost is minus sheeting. Everything was sheeted and glassed except for the belly of the fuse.

3/32" on the wing and 1/16" sheet on the tail feathers.

Dave
Nov 30, 2015, 04:35 PM
AndyKunz's Avatar
I would go with fewer ribs, and skip the holes in them. The wing halves separated because there wasn't anything trying to keep the top with the bottom. Any amount of flutter or vibration stress will snap those ribs top and bottom, and it'll just explode as Bernoulli and Coanda suck the top and bottom surfaces apart.

The "too heavy" wasn't a result of the structure. You don't have enough there, in my opinion. Unless you were using cast iron instead of balsa.

To paraphrase Einstein, Build as light as possible, but no lighter.

Andy
Nov 30, 2015, 06:31 PM
Balsa&Tissue
payne9999's Avatar
Thread OP
Quote:
Originally Posted by AndyKunz
I would go with fewer ribs, and skip the holes in them. The wing halves separated because there wasn't anything trying to keep the top with the bottom. Any amount of flutter or vibration stress will snap those ribs top and bottom, and it'll just explode as Bernoulli and Coanda suck the top and bottom surfaces apart.

The "too heavy" wasn't a result of the structure. You don't have enough there, in my opinion. Unless you were using cast iron instead of balsa.

To paraphrase Einstein, Build as light as possible, but no lighter.

Andy

Andy,

The seperation was the same theory we came up with. The design defect was obvious looking at the failed parts. I did not design this plane but built it from CAD files.

The needed to be material between the wing rod receiver and the spars.....diagrams below.
Nov 30, 2015, 07:19 PM
Registered User
Quote:
Originally Posted by AndyKunz
The "too heavy" wasn't a result of the structure. You don't have enough there, in my opinion. Unless you were using cast iron instead of balsa.

Andy
Agreed. I am curious as to what "too heavy" means. I am an engineer, but not aeronautical. I don't design aircraft structures, full size or model. But looking at the view of the 3D model I see a very lean structure. Maybe as Andy says too lean. Would be interesting to know the weight and size.
Nov 30, 2015, 07:47 PM
Balsa&Tissue
payne9999's Avatar
Thread OP
Quote:
Originally Posted by Yoda466
Agreed. I am curious as to what "too heavy" means. I am an engineer, but not aeronautical. I don't design aircraft structures, full size or model. But looking at the view of the 3D model I see a very lean structure. Maybe as Andy says too lean. Would be interesting to know the weight and size.
Plane is 28% scale, 89" wingspan, ideal all up weight would be 18 lbs. It may look lean but the sheeting is not shown in the cad screen shot. It came out 22 lbs.


Dave
Nov 30, 2015, 07:54 PM
Registered User
Quote:
Originally Posted by payne9999
Plane is 28% scale, 89" wingspan, ideal all up weight would be 18 lbs. It may look lean but the sheeting is not shown in the cad screen shot. It came out 22 lbs.


Dave
Wow, that is heavy. What sheeting and where?
Nov 30, 2015, 08:14 PM
Balsa&Tissue
payne9999's Avatar
Thread OP
Quote:
Originally Posted by Yoda466
Wow, that is heavy. What sheeting and where?
3/32 on the wings, 1/16 on the tails with one layer of 2 ounce glass.
Nov 30, 2015, 08:18 PM
A man with a plan
Balsaworkbench's Avatar
Based on your aftermath photos, I agree that you didn't have the top of the wing tied to the bottom. Your new plane can be lighter than the old one. Focus your attention on the hot spots. Put full webbing between the wing spars, and perhaps add an aft spar, top and bottom, also with webbing. I also like what you said about adding filler between the joiner tubes and the spars.

If you are going to glass the new airplane, the strength of the sheeting becomes less of a factor. Just make sure you have the strength you need in the spars, at the wing connection points, and between the firewall and landing gear.
Nov 30, 2015, 08:24 PM
Balsa&Tissue
payne9999's Avatar
Thread OP
Quote:
Originally Posted by Balsaworkbench
Based on your aftermath photos, I agree that you didn't have the top of the wing tied to the bottom. Your new plane can be lighter than the old one. Focus your attention on the hot spots. Put full webbing between the wing spars, and perhaps add an aft spar, top and bottom, also with webbing. I also like what you said about adding filler between the joiner tubes and the spars.

If you are going to glass the new airplane, the strength of the sheeting becomes less of a factor. Just make sure you have the strength you need in the spars, at the wing connection points, and between the firewall and landing gear.
Agreed but I still need to find 3-4 lbs to take out.

A list of things I have been considering experimenting with:

1. Making my own plywood from select sheeets of balsa
2. Using light ply appropriately (if that is possible)
3. Using some composite methods
4. Using Russian/Baltic ply only where absolutely needed.
5. Covering in fabric (Solertex) with paint instead of glass
6. leaving some open areas in the tail with cap strips to reduce the need for nose weight.
7. Lighter tailwheel assembly
8 Use balsa ply in aft portion of fuse to eliminate nose weight.

She flew majestically but somewhat sluggish so I think she needs to be much lighter.

Dave
Nov 30, 2015, 08:30 PM
Balsa&Tissue
payne9999's Avatar
Thread OP
BTW, the failure of my plane looked a awful lot like this poor fellows Decathlon:

Brad Wursten's 56% Bill Hempel Decathlon Crash (2 min 51 sec)
Nov 30, 2015, 08:38 PM
Balsa&Tissue
payne9999's Avatar
Thread OP
It is interesting to look at some sequential photos of the Decathlon failure. Not sure if it is the same failure mode......
Nov 30, 2015, 08:58 PM
Registered User
aymodeler's Avatar
I think you are on the right track with the shear webs. It's hard to tell from the drawing, but it does not look like there is much of spar at all as designed. You may also want to double the top and bottom of the main spar for about the first third of the length. Lastly, I agree with portablevcb that you could eliminate some of the ribs, but make the remaining ones solid.


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