A friend and I are working on a school project building an airplane to house a synthetic aperature radar. We want to use a composite for the fuselage but are both new to composites and are unsure of what works the best. Our main problem is that landing gear would interfere with the radar antennas so we have to belly land a 10 lb airplane at 15-20 mph. We considered fiberglass because that seems to be a strong and popular material for RC planes but we are afraid it would be too brittle in landing. We have also looked into kevlar because we have been told it has a little more give than glass. We really don't know if one or the other or even a combination of the two would be best or if there are other things out there we haven't considered. Carbon fiber is out because it is too conductive a material. Our fuselage is a trapezoid 26" long with a 6" top and 4" bottom (unusual I know but it the application calls for it). Our idea right now is to have some wooden structural members inside which support the airframe and house the radar, an autopilot and all other necessary flight components. Any advice or ideas would be very much appreciated. Thanks guys, we have been learning a lot from previously posted threads, it has been very helpful.

Is the RADAR looking through the fuselage bottom, ie. radome? Would retractable landing gear mounted mid wing be feasible? If the wave is passing through composites you still have to consider the density and type of resin regardless of the composite type...

The radar antennas have to be mounted at a 45 degree angle, hence the odd shape. We didn't build the radar but from what the people who did have told us, because it is a synthetic aperature radar and not a traditional radar having material in front shouldn't be a problem unless it has low electrical resistivity (e.g. carbon fiber). We have about a 6 foot wing span as is due to the weight and low speed. I don't know a whole lot about retractable landing gear on models. How much weight would be added? What all do they require in terms of hardware, extra servos, etc.?

I dont think fiberglass is all that brittle, if you end up with a brittle layup it is probably due to the type of resin or the amount of hardner you used. I have a fiberglass p47 that has suffered a few nose overs on landing and has even flipped over at the end of the runway a few times. It hasnt developed any cracks at all, the main problem is that every time it noses over on landing the fiberglass gets eaten away from friction with the runway. Your main advantage going with kevlar will be overall strenght, but you have to remember to get a fiberglass structure stiff enough for model use it normally ends up being stronger than necessary. Some type of internal structure will go a long way to making the overall structure strong and stiff.

Fiberglass is a viable option. Aerospace companies like INFINITY Aerospace, http://www.infinityaerospace.com/infgear.htm , are manufacturing fiberglass main gaer for full size aircraft. I have not seen these for RC applications yet but they may be out there, your budget might limit this option though... You could add multiple layers of glass in a quasi-isotropic layup to the belly of your aircraft but it will eventually fail as it gets warn away on multiple landings; especially on a 10+ pound aircraft. A grass field would help with short term erosion issues. Would a single wheel mounted to the belly, similar to those used on gliders, be possible? If so you would need a simple skid on the wing tips to protect them on landing.

Mechanical retracts would be the easiest retractable option. You would need to reinforce the wing to accommodate the gear and the will add about 10 oz with the wheels and retract servo installed. Another option might be to mount fixed gear on the wing outboard of the fuselage. A third option could be to mount a manufactured, non-metallic, gear such as DuBro's or Goldberg's, to the fuselage.

To make life easier for you. You could make a removable/replaceable landing skidplate out of something like HDPE plastic. That eliminates the retracts which are not very complicated but just another mechanical issue for you to overcome. I think the "KISS" principle will come in very handy here. Just my .02

NAES

Thanks for the suggestions guys we will explore those options. It's great to have insight from more experienced builders. I'm sure we will have more questions as we get deeper into the construction stage of this thing