help with composite construction design

I am working on an airplane design that is, for the most part, standard aluminum construction. The general description is a 2 seat tandem configuration with a rear mounted 220 HP 4 cylinder engine and CS prop. The airplane will have a GW of 1800 lbs and planned cruise speed of 200 kts plus. The wing mounted booms are located 4' from center line and extend rearward about 8'. The horizontal tail will have an 8' span and 2' chord'. 2 vertical tails will be mounted behind each boom. Because the booms taper and have a complex form, I have decided it would be best to construct them from composite materials. My first estimate of the bending force required would be about 1000 lbs per boom at maneuvering speed ( 6G airplane ). Looking for help with the overall design, especially the attachment points to the aluminum tail and wing structure. Any help would be greatly appreciated. Mike

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Originally Posted by mjmulia

...The wing mounted booms are located 4' from center line and extend rearward about 8' ... My first estimate of the bending force required would be about 1000 lbs per boom at maneuvering speed ( 6G airplane )...

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[Full disclosure: I do a lot of design, and a little bit of engineering under supervision, but I am not an engineer. Fools rush in, and all that...]

As a sanity check, I generally look for the maximum horizontal tail load to be somewhere around the gross weight of the airplane, so that appears to check out.

Given the boom length of 8 feet, that works out to a maximum bending moment in the neighborhood of 4000 ft-lbs at the forward end of each boom, or 48000 in-lbs or 5430 newton-meters depending on your units of choice. Next what you need to do is figure out how deep your booms are going to be, and from that find the sectional moment of inertia (I) required to react the moment. Most likely you'll interate the boom cross-section a few times to figure out the outside shape and the layup schedule that gives you a reasonable deflection. The taller and wider your booms get, the less material it takes to react the bending.

Basically what you're juggling is bending moment, height, and layup. Some notes and comments on this from an amateur-designer perspective:

* Design to deflection. It's pretty easy to encounter the pitfall of designing something that is strong enough, but flexes so much that it affects stability or control while still within the expected flight envelope. To get a fiberglass or even carbon fiber part that is stiff enough, you will probably end up with about twice the strength you need. However, especially with carbon fiber, it will still probably be much lighter and simpler than its aluminum equivalent.

* Calculations are great, but you really should count on testing a representative article to at least limit load, and evaluate its deflection.

* Machinery's Handbook has some great sections on calculating the moment of inertia of the cross section for circular, elliptical, and oval tubes.

* In composites, the limiting factor is likely to be compression strength of the material. In your position I would strongly consider using strips of pultruded carbon fiber along the tops and bottoms (and perhaps also the sides) of the booms.

--Bob K.

Bob, first of all, thanks for the very helpful reply. I could not find a simple equation for surface load based on tail surface area, deflection angle and airspeed. So I went to the lift equation of L=(.5)( rho)(V*2)(S)( Cl). Figuring a maneuvering speed of about 125 mph and Cl of about 3.0. This is the one part I was going to test prior to actually flying the airplane-possibly to failure. Being a RV guy I just am not comfortable with composites. The plan view of the boom is a constant 5" width. The profile view has a depth of about 12" at the wing and tapers to 5" at the tail attach point. I will look into the moment of inertia calculations. Do you have any suggestions on which type of material to use ( fiberglass, carbon fiber, kevlar etc )?

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Originally Posted by mjmulia

...I went to the lift equation of L=(.5)( rho)(V*2)(S)( Cl)...

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First off, doing the pv^2 on this puts you ahead of about 95% of those who dream of doing their own design. That's a very good sign. I suspect you are being too conservative about the Cl, but that's splitting hairs.

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Originally Posted by mjmulia

...Do you have any suggestions on which type of material to use ( fiberglass, carbon fiber, kevlar etc )?

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I would go carbon all the way. Or at least most of the way. In terms of $ per unit strength and $ per unit stiffness, carbon is pretty much the best stuff there is. Carbon prices are holding steady and even declining a little with inflation. Fiberglass is a lot heavier, and by and large you only use Kevlar where you care what the part does after it starts breaking. I use Kevlar in control surfaces for toughness, but I wouldn't use it anywhere in a bending beam.

If we were doing that in my shop, I'd probably make a simple mold out of .016" aluminum rolled into a curve and forced down into a bunch of MDF cradles that enforce a tapering half-elipse profile. I'd use tooling wax and some other tricks to give the finished part a joggle along one edge. The layup schedule would probably be 12oz carbon mostly +/- 45, with pultruded strips embedded in the layers to react tensile and compressive loads like the flanges of an I-beam. I'd make four identical vacuum-bagged pulls out of the mold, then join pairs of the parts into the two booms. There'd be a substantial bulkhead at the forward end of each boom, with large Garolite inclusions as hardpoints for the fasteners that react shear loads, and a similar bulkhead with hardpoints about 24" aft of that. Structure in each wing would react the bending moment applied by the tail out of the boom at the two bulkheads.

You might consider signing up for one of our Akaflieg sessions in 2018. You'd learn a lot about composite design and construction, and you'd see from our tapered carbon fiber racing wings for RV-6, -7, and -8 how we mix metal and carbon structures while avoiding the usual "black aluminum" pitfalls.

Thanks, Bob K.

OK Bob sounds good. Can you send me some info on your classes.

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Originally Posted by mjmulia

OK Bob sounds good. Can you send me some info on your classes.

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Here's the page for our Akafliegs. We haven't posted the 2018 schedule yet, but it looks like we'll be running three, with one in February. I'll get the schedule up as soon as I can:

https://hpaircraftblog.wordpress.com/akafliegs/

Thanks, Bob K.

February class sounds like a real possibility for me. I will check your website periodically.