Composite/Glassfibre vs. Rag & bone

[gimballock]

Can anyone tell me which is more likely to give more strength per kg - carbon/glass fibre monocoque (like, say a sailplane or a Jabiru) or a tube steel construction with doped fabric or perhaps even a light fibreglass skin (like a nose cone pod on, say, a Drifter)?

 

Just curious.

 

Thanks.

 

Gimballock.

 

 

[Bob Llewellyn]

Can anyone tell me which is more likely to give more strength per kg - carbon/glass fibre monocoque (like, say a sailplane or a Jabiru) or a tube steel construction with doped fabric or perhaps even a light fibreglass skin (like a nose cone pod on, say, a Drifter)?Just curious.

 

Thanks.

 

Gimballock.

Good question. There are two separate (but overlapping) issues here; strength, and stiffness. Lurking in the background is longevity, or ongoing airworthiness, or fatigue - take your pick!

Because air is thin stuff, most of an aeroplane's structure is also thin, gathering dispersed loads (air pressures!) and feeding them into more major structural members. Apart from the most major of loaded elements (e.g. wingspar root attachment), the maximum strength of aeroplane structures is generally set by buckling; and buckling is a function of stiffness. For example, alloy steel has about three times the working strength of PH aluminium, and is about three times as dense - so, why not use very thin steel instead of thin aluminium? Buckling - because the aluminium bit will be three times thicker than the steel equivalent, it'll have ~27 times the buckling resistance (up to yield - but the working stresses are such that, in sheet and thin extrusions, an aluminium structure works out lighter than Chrome Moly).

 

The other stiffness issue is aeroelasticity, or "flutter" - if the airframe (or any bit of it in the breeze) is springy enough, it can interact with the airstream and resonate - the first sign of, say, aileron flutter is generally the instrument panel blurring because the shaking is so violent that you can't see; rapidly followed by either the aileron (if you're lucky), or the wing falling off.

 

Getting back to your specific question, rag does nothing in compression; which is to say, it contributes naff all to the overall strength/stiffness of a structure, though it's a lightweight way to catch small airloads and feed them into a skeletal structure. A steel tube skeleton - i.e. a fully triangulated spaceframe - is itself a very effective structure, and can be designed to be pretty efficient. However, combining the two gives less than optimal material usage. Welded 4130 (chrome-moly) spaceframes are ideal for engine mount frames and cabin structures (that have to feed lift truss loads, landing loads, and powerplant loads around the crew); they are less ideal for such things as tailplanes.

 

Glass/Epoxy is less efficient in sheet form than aluminium; and it's less efficient in beam form than steel. However, it's much much easier to arrange to have just the right amount where needed - whereas variable thickness aluminium skins are impracticable on light aeroplanes! Also, judicious usage of core materials (coremat, various foams) can make "sheet" structural elements in glass/epoxy look very competitive in stiffness/weight.

 

Compare the airframe weights, if you can get them, of the Lightwing and the early Jabiru (pre- Light Sports Aircraf category Jab LSA); the Jab structure is a tad smaller % of the MTOW, for an aircraft with much the same payload (i.e. 2 seats), a faster cruise, a similar stall, and more endurance. Both aeroplanes have enviable accident survivability records.

 

Carbon/epoxy is a clear winner in the stiffness/weight stakes for things like wingspars and D-boxes, but has low toughness and low lug effectiveness - it's very hard to get a light but reliable bolted or pinned connection, such as to an engine mount. Compare the airframe weights of unlimited sailplanes of today, to Blaniks and the wood, steel and rag generation (with the same payload).

 

If you're aiming at LSA, then a welded steel cabin structure, ditto engine mount, aluminium rear fuselage, glass/epoxy/foam tail surfaces, and a glass/epoxy/coremat/foam wing with aluminium struts is about the optimum for structural efficiency - but the joins between the sections better be well designed!

 

 

[gimballock]

Thanks, Bob.

 

That's about as comprehensive a response as I could have hoped for!

 

You raise some very good and valid points that I will need to digest and ponder!

 

Stay tuned for further questions.

 

At this time, designing and building my own aircraft is pure science fiction/fantasy, but I'm always mulling over ideas for when the time is right.

 

Until then, I will struggle to get my piddley posterior off the ground by conventional means!

 

Thanks again,

 

Gimballock.

 

 

[Bob Llewellyn]

Thanks, Bob.That's about as comprehensive a response as I could have hoped for!

You raise some very good and valid points that I will need to digest and ponder!

 

Stay tuned for further questions.

 

At this time, designing and building my own aircraft is pure science fiction/fantasy, but I'm always mulling over ideas for when the time is right.

 

Until then, I will struggle to get my piddley posterior off the ground by conventional means!

 

Thanks again,

 

Gimballock.

No worries!