Question: Wood construction with fiberglass exterior

[Trevor Norman]

Good afternoon,
I have a Super Koala frame that I'm looking at completing. Simply put, if the frame is wood, what would I need to do in order to enforce the frame with carbon fiber fabric, or fiberglass instead of fabric covering?
I already have wings made up, and I plan on using a foam core with fiberglass on the exterior. This should give me a bit more strength, am I correct?
Weight issue aside (I am planning on adding a more powerful engine and adjusting a few more things to make it work), am I crazy or is this a viable option to creating a new aircraft?
Trevor

[rwanttaja]

Good afternoon,
I have a Super Koala frame that I'm looking at completing. Simply put, if the frame is wood, what would I need to do in order to enforce the frame with carbon fiber fabric, or fiberglass instead of fabric covering?
I already have wings made up, and I plan on using a foam core with fiberglass on the exterior. This should give me a bit more strength, am I correct?
Weight issue aside (I am planning on adding a more powerful engine and adjusting a few more things to make it work), am I crazy or is this a viable option to creating a new aircraft?
Trevor
The problem is, you don't know where the critical spots are in the structure... and whether your reinforcement is making the situation better or worse. You may strengthen one area, which puts more stress on another, which maybe wasn't reinforced to the same degree.

Plus, of course, you're adding weight to the airframe, a lot of which probably isn't contributing to actually making the structure stronger. If a given component is already good for ~10 Gs, making it good for 11 Gs doesn't really buy you anything.

Besides, the wood structure may not be the limiting factor, here. The issue may be metal fittings such as wing strut attachment points, and making the skins foam-cored fiberglass won't do a thing for the load-carrying ability.

In short, a shotgun approach is not necessarily going to get you a stronger aircraft. You need to analyze the structure to find where the weakest points are. If you aren't willing to do that, do it the brute-force way: Build the structure and ground-load it to destruction. Determine the failure point, then build the structure again with *that* component strengthened. Proof-test it again, find out what fails *that* time, lather, rinse, repeat.

Ron Wanttaja

[Dana]

As Ron said, adding strength to one point (like the covering) may well actually make the overall structure weaker by limiting deflection in one place and dumping the loads into another spot that may not be strong enough.

The Fisher designs have a good reputation. Why do you think you need to make the structure stronger?

Dana

[Bill Berson]

It won't add much bending strength unless you go with full sandwich skin. Laminate/foam/laminate.
It would add a huge amount of weight and work load. If anything, the spars would be the first to reinforce, not the skin.

A strut braced wing doesn't need a hard skin. Modern polyester fabric is ideal for this and other light airplanes.
The short answer is don't do it.

[cub builder]

As Ron said, adding strength to one point (like the covering) may well actually make the overall structure weaker by limiting deflection in one place and dumping the loads into another spot that may not be strong enough.

Dana

Dana and Ron both make very good points about stiffening one area moving the loads causing an unexpected failure at a different point. I've seen this in practice when an acquaintance decided to modify his wood prop then reinforced the prop tips with Carbon Fiber, thinking he would make the prop tips stronger, which it did. However, carbon fiber is much more rigid than wood, so where the prop tips used to flex at the tip, now was rigid. That transferred the bending loads to the point where the carbon fiber ended some 8" down the prop blades. Sure enough, he was cranking along in cruise flight when he lost 8" off one prop blade. Fortunately, the other wood tip had also fatigued in a like manner so the blade imbalance only lasted for a second or two before the other prop tip failed as well. The good news is that the prop was now reasonably balanced again, although now missing 16" of total blade length. The engine reved something north of 4000 rpm, but was able to produce enough thrust with the stubby remains of the prop to stay airborne and reached an airport for landing.

Be very careful about stiffening an airframe or wing assembly as there may be unintended consequences.

-Cub Builder

[WLIU]

As hinted at above, to change materials, you have to start over with your structural analysis. You can not just add foam and skin it with a new material. Likely adds weight without increasing strength. Reading post #1 the obvious question is whether any analysis has been done on the existing structure? Adding a heavier engine imposes what stress on the existing structure? Without doing some math it is unwise to start gluing more "stuff" onto the existing structure.

Example, is the landing gear rugged enough for the additional weight? In an airplane, changes have a snowball effect. Change one part of the airplane and other parts need to be changed too.

Best of luck,

Wes
N78PS

[Trevor Norman]

That's exactly what I needed to know. Thanks for all your help!

[Sam Buchanan]

Good afternoon,
I have a Super Koala frame that I'm looking at completing. Simply put, if the frame is wood, what would I need to do in order to enforce the frame with carbon fiber fabric, or fiberglass instead of fabric covering?
I already have wings made up, and I plan on using a foam core with fiberglass on the exterior. This should give me a bit more strength, am I correct?
Weight issue aside (I am planning on adding a more powerful engine and adjusting a few more things to make it work), am I crazy or is this a viable option to creating a new aircraft?
Trevor

I built a Super Koala twenty years ago. It is a strong, light structure as designed, the geodetic construction is amazingly rigid. Build it per plans and keep it as light as possible.