Seat Build

[jeff54il]

Ron

Here's a curiosity on units: Most (if not all) of those creatures in the world called pilots measure atmospheric pressure in milibars - 1 atmosphere = aprox 1013 mbar = 101.3 KPascals = aprox 1 Kg.force/sq.cm = 10 metric tons/sq.m. But in imperial land they convert it to much easier conversion factors: 1 atmos = aprox. 14.7 lb/sq.ft = aprox. 2116 lb/sq.ft = aprox. 0.96 tonne/sq.ft = aprox. 1.058 short tons/sq.ft.
Sounds more like a problem of square heads rather than square feet.

On to much less serious matters - the subject of convex or concave molds:
We have basically 4 alternatives:
1. Convex mold - Lower surface Layup
2. Convex mold - Upper surface layup
3. Concave mold - Lower surface layup
4. Concave mold - Upper surface layup

Advantages/Disadvantages of each.

#1 & #4. Convex lower & concave upper - see the following sketch:

The sketch shows #4 concave - upper layup - has a major problem of wrinkling of the layup. #1 suffers from the same disadvantage but in both cases the problem can probably be solved using pultruded fiberglass sheet that won't wrinkle. But pultruded sheets have a weight penalty because in addition to the fully-filled pultrusion we have to add epoxy to bond the pultrusion to the foam.

#3 Concave lower- see the following sketch:

The sketch shows #3 concave lower layup that has a major disadvantage - you can't see the layup after you close the vacuum bag, and you'll only find out about problems with the layup when it's too late, after the epoxy has set, at which time you'll have to discard the whole casting. But it also has a major advantage - If we at some stage decide to change the thickness of the blue-foam core, we can still use the same mold. The disadvantage can be practically eliminated by using pultruded material instead of hand layup.

#2 Convex upper layup - see the following sketch:

The sketch shows #2 convex upper layup. Big advantage: you can see and inspect the whole hand layup before the epoxy has set. Disadvantage - the mold can only be used for one specific foam core thickness.

Personally I would choose #3 - A concave mold with the hand layup on the lower side. My preference is based on the fact that the mold geometry represents the finished geometry of the airframe, and in the worst case we can use pultruded glass bonded onto the lower side of the foam - a small weight penalty but a much better reduction of risk.
BTW - it's not such a big deal to make our own precast sheets of glass instead of the purchased pultrusions. A bit more work but a considerable cost saving and it will probably save some weight.

Regards

[wdyasq]

Jeffery,

I do believe building 'sheets' of cured fiberglass may be the way to go on the curved forms. As I have stated, one could even use polyurethane glue instead of epoxy to bond said sheets to the foam.

The Gougeon brothers would 'flow coat' sheets of plywood to coat them before using so they would not need to fill after construction. This method leaves a very smooth surface and, by 'flow coating' several sheets at one time, a very large smooth, blemish free surface could be created. Waxed, and coated with PVA, several square meters of 'fiberglass sheet' could be made at one time. And yes, I would want to vacuum the process. Believe it or not, such a sheet rolls up well.

Various thickness would be made while this was in production. It would be a good idea to build a bit extra while 'in production'. A couple of square meters extra of each thickness could prevent strategic cursing near the end of the project.

Hopefully, John will be the 'beta boy' and figure all of this out.

While we are dreaming, why don't we dream about getting some aluminum sheet the thickness of our desired 'jogs' in the fiberglass and use them in our molding process. If fact, if thin UHMW sheet is available, it won't bond to epoxy and, I think, can be had in many thicknesses. Worst case, it can be milled down or a proper depth recess could be milled in to make the "Joggle'.

I can see how a female aluminum leading edge mold might have a joggle molded in. In fact, the molds could actually be made of the galvanized sheet steel available from the 'box stores' . It takes a different set of skills to shape metal, and tools. Fortunately in my one man shop one of the workers can do metal shaping.

As you can tell, I like to think through a project before I commit too many resources. I think, by building one 'body' different wing plans and tail plans could be incorporated. I can see this 'body' with shorter wings and a smaller tail volume and be similar to the Wittman Tailwind. Again, with a different wing and tail ... it could be a real STOL plane similar in performance to the CH750. I think this ideal should be incorporated in the initial design. I know there will be SOME compromise to work this idea in. I suggest the compromises be made to adapt second/third platforms rather than our(your) initial design.

Ron

[Katharina]

An inexperienced builder will have to eat his $100 hamburger closer to home.