Fiberglass truss boom, Ollie?

[gjs]

I'd like to mould a fiberglass truss type structure using glass tow, designed to be minimal in cross section, built in flight pack mount points. The main part in a two dimension mold, adding truss depth latter.
I've seen tool guys casting epoxy parts kind of like sandcast method. I'd like to add glass fiber to that. I love the finished look. Cast!

Could this be done by say pressing shapes into modeling clay rolled out on plate glass and filling with resin? Done that anyone? Ollie? Or what would you use to get a crisp male mold?

Know any links to do this inexpensively. I have epoxy, Box of chopgun tow,and other glass cloth to pull from, some kevlar and carbon fiber, no mold makeing supplies. Plaster of paris maybe. Please gjs

[Ollie]

If you want to build a structure with better performance, it should be no heavier than necessary, stiff enough and strong enough. This begins by estimating the forces involved so that it is not overbuilt.

Form follows function. The function of the structure is to carry flight loads, landing loads, etc. I think that you may have settled on the form before you have determined its function by quantifying the loads involved.

Another approach, where loads are hard to estimate, is to evolve a design by trial and error. The parts that break need to be strengthened and the parts that don't break need to be lightened. Where it is too flexible it needs to be stiffened. It takes a lot of trial and error with possibly a new mold for each trial. You may even find that your designs evolve toward another configuration than a truss.

The engineering approach involves a knowledge of mechanics of materials. The trial and error approach takes a lot of time and effort of another kind.

Maybe that is why less than one percent of modelers do any serious design work.

[gjs]

I am looking toward building electric powered craft. Most of my focus has been HLG's, built using blue foam and wood laminations. What I learned there is that the inertias tear planes apart. I've hammer the books to grasp the mechanics of materials and am now hopeing I'm adequately prepared to break all the rules.

Jack Lambe's book on HomeBuilt Aircraft covers a method of building I think they call "foam on frame" that intrigued me. Foam can be added to a frame for "shape". The frame carrying the loads, the foam is open to interpretation of "form". The same frame can be made to look like P-51, FW-190 ect. by different applications of foam (chunks of foam mounted to frame, shaped and glassed "lightly").

I failed to mention in my post I've been experimenting with Poly resin I've had laying around for years and the shiny blue glassy material I think would be perfect for doing a minimal structure that I've been evolving in my head and while talking to others, which is basically this. As you pointed out flight loads, landing loads ect. "rule". If I am going electric, Battery inertia's are Big. Toward Lightness It make no sense to me to put a pound battery into a structure weighing half that and having the high load areas joined with Pith. I realize I may be "way out" here because in Hand launch Gliders I'm used to using any excess weight as an energy source. But I want to use the battery as structure (somehow) and all the high load areas "joined" in a hard fashion, mount flight pack to it, then lightly build aerodynamic shape on to this with the"foam on frame" method ( Hatches where needed). Fiberglass a line from wing mount to motor mount and you have a place to mount a canopy. Glass a line from motor mount to battery and you have a place to mount under carriage and belly pan etc. This would be durable, strong, appearance changable, efficient, and Beautiful from where I'm sitting. I need to post some drawings. This is futile.

On another thead, talk of carbon rod building seamed heavy, unless it is use efficiently! Carbon rod is more efficient than Glass but glass is CHEAP. HOW about Glass nose frame work , carbon boom.

A Drawing is worth a thousands of words. Can someone explain to me how to send attachments. I'm using a MAC, careful, don't offend.
Thanks for the Heads Up Ollie! GJS

[gjs]

I am looking toward building electric powered craft. Most of my focus has been HLG's, built using blue foam and wood laminations. What I learned there is that the inertias tear planes apart. I've hammer the books to grasp the mechanics of materials and am now hopeing I'm adequately prepared to break all the rules.

Jack Lambe's book on HomeBuilt Aircraft covers a method of building I think they call "foam on frame" that intrigued me. Foam can be added to a frame for "shape". The frame carrying the loads, the foam is open to interpretation of "form". The same frame can be made to look like P-51, FW-190 ect. by different applications of foam (chunks of foam mounted to frame, shaped and glassed "lightly").

I failed to mention in my post I've been experimenting with Poly resin I've had laying around for years and the shiny blue glassy material I think would be perfect for doing a minimal structure that I've been evolving in my head and while talking to others, which is basically this. As you pointed out flight loads, landing loads ect. "rule". If I am going electric, Battery inertia's are Big. Toward Lightness It make no sense to me to put a pound battery into a structure weighing half that and having the high load areas joined with Pith. I realize I may be "way out" here because in Hand launch Gliders I'm used to using any excess weight as an energy source. But I want to use the battery as structure (somehow) and all the high load areas "joined" in a hard fashion, mount flight pack to it, then lightly build aerodynamic shape on to this with the"foam on frame" method ( Hatches where needed). Fiberglass a line from wing mount to motor mount and you have a place to mount a canopy. Glass a line from motor mount to battery and you have a place to mount under carriage and belly pan etc. This would be durable, strong, appearance changable, efficient, and Beautiful from where I'm sitting. I need to post some drawings. This is futile.

On another thead, talk of carbon rod building seamed heavy, unless it is use efficiently! Carbon rod is more efficient than Glass but glass is CHEAP. HOW about Glass nose frame work , carbon boom.

A Drawing is worth a thousands of words. Can someone explain to me how to send attachments. I'm using a MAC, careful, don't offend.
Thanks for the Heads Up Ollie! GJS

[Ollie]

Here are some things to consider:

Closed cell polystyrene foam may be strong enough to carry shear loads.

Stressed skin, glass or carbon over foam, is an inherently efficient structural configuration.

Strength of a given weight of fiber in a layup depends very much on how closely the fibers are packed, how parallel they are and how little resin is needed to encapsulate the fibers. The difference in strength can vary by a factor of three or four to one depending on how little epoxy, how well oriented the fibers and how closely they are packed.

The failure mode is usually buckling under compression long before the maximum compression load is reached.

Woven cloth has the fibers kinked by the weaving process resulting in poor fiber orientation, buckling under compression and poor fiber to resin ratio.

In a high wind the strong oak breaks before the flexible willow. Don't make the structure stiffer than necessary to maintain alignment under flight loads.

Mount the heavy battery so that its inertial load is not transmitted to the structure in a hard landing (crash).

"The bitterness of poor quality lasts long after the sweetness of low price is forgotten."

[gjs]

Fiber fraction in a composite is something I've wrestled with, particularly important in this application. Maybe if I built a male mold of soldered together bar stock of some sort, in the config. I want, of a deaper section than needed, I could also use it to compress the layup into the female mold. And being metal (with release agent on it) I could heat it while pressing it and give myself the illusion of making an autoclave quality part.

The reason I'm leaning toward this frame style structure is my belief the foam I'm so accustomed to working with is not of the right proportion, density, for a pound of battery. Wood is like foam, much denser. Endgrain balsa, glassed inside and out, that sounds proprotionally matched to 1+ # load.

However! What would the glass weigh to do that? I think i can take the same glass, or less and 40% resin and come up with a tougher structure with less drag (smaller wetted area) Because most fuses are at least 20% hollow. This is going "Denser". Glass rod mounting places triangulated to a stiff whole is tougher than "the egg shell" and as I mentioned in my last post, the skin is'nt structure and can be modified, shaped, molded, manipulated to any end, like experimenting with the effects, of changing the shape, has on trim,lift ect.
I ought to bounce that off Mike James for his HighLift Project.

My thinking is to remove the unecessary. I'd go so far as to solder the motor on top of the battery, landing gear on bottom of the battery and a wing rod on back of the battery, three sided glass truss epoxied to battery going aft to fins. Then you hollow out a 1 pound per cubic foot foam shell, Nasa air scoops carved and finished in, panel lines and any other details. "Killer" versatility. What do you think?

[Ollie]

I think you want me to agree with you but I have already told you what I think. I think you should go ahead and try out your ideas to see how they work out.