I am building your B-17. I am an experienced RC model designer, builder and retired aero eng from McDonnell Douglas in St. Louis, MO.
So far I have had no problems but I have noticed that the aft fuselage is really wobbly compared to the rest of the kit. ...I wonder is this is normal? In pitch it is stiff enough of course.

From : Don Stackhouse

Yes. On slow models like this you can get away with quite a bit that would not work on faster, heavier models. The very first RK Series Lockheed Electra used a 1/16" balsa fuselage and about the same sized elevator joiner as the earlier RK models. Joe and I have a little adage: "Try it the easy way first, you can always make it heavier and more complicated later." The first Electra did not get the same elevator deflections on both elevator halves in flight because the air loads on one elevator half could twist the elevator joiner in the load path from the elevator to the control horn mounted on the other half. This created an aileron effect in the elevators. With that huge horizontal tail, that could put quite a bit of torsional load on the aft fuselage. It was not unusual to see the tail assembly banked 20 degrees or more relative to the rest of the airplane!

This was quite scary to watch, but it had absolutely no noticeable effect in the handling of the plane. No funny quirks, no noticeable coupling between pitch and yaw, nothing. Nada. Period. The airplane flew and handled absolutely magnificently. However, watching the tail doing aerobatics by itself while the rest of the plane was flying straight and level was more than a little disconcerting. In the production versions we stiffened up the entire tail assembly, drastically increased the size of the joiner and went to thicker balsa for the fuselage.

I am afraid that the fuselage will break in the area of the side gunners cutout when subjected to the stress of a less than perfect flight or landing.

The prototype B-17, the one in the box cover photos, has been officially retired due to wear and tear (although I have not yet removed anything, and it could be flown in its present condition). It's been flown head-on into concrete block walls at cruise speed and power, crashed by radio problems vertically into concrete-backed Astro Turf (some prankish spectator had partially unscrewed the antenna on the transmitter while we weren't looking!), been in the back of a car that was rear-ended on the interstate, and generally has been thoroughly abused. It's been repaired so many times that portions of it now qualify as a composite material composed of cellulose fibers in a cyanoacrylate matrix (which, BTW, is a very brittle material!). It has never broken at the waist gunner windows, not even any hints of incipient cracks. Where it can break is along the joint between the front and rear fuselage sections, just ahead of the trailing edge of the wing. A patch about 1" of 1 or 2 ounce fiberglass on both sides, from about 1" in front to about 1" aft of the joint, applied with laminating epoxy or water-based urethane varnish such as Varathane might be helpful in this area.

>I am considering laminating a 1/64 inch plywood strip that is about 1/2 wide on either side of the fuselage starting just forward of the trailing edge of the wing to just aft of the leading edge of the >horizontal tail to stiffen the fuselage. Another approach would be to put the strip at right angles to the fuse to get the higher moment of inertia of the section that would result. I would use carbon fiber but don't have any (typical engineering decision)!

We kept an eye on the flexibility issue when we designed and tested the big Boeing, and if it had needed to be stiffer, we would have used thicker balsa for the fuselage. Yes, if you pay attention you can see it move around a little on the ground, and to a lesser extent during large control inputs in flight. However (as someone like yourself, familiar with large aircraft stuctures such as wings for airliners, should certainly appreciate), "flexible" is definitely not the same thing as "weak" when it comes to aircraft structures. The big factor involving stiffness is of course flutter, and we have not had flutter problems with the Boeing. The loads in flight on the tail are really not all that high, and the flexibility of the aft fuselage has not been a problem. Adding a lot of unnecessary extra weight to the tail will hurt performance, control response and dynamic stability, but will not solve any problems that need solving.

Excellent question, definitely worth asking, but in this case we have considered (and tested) this whole issue, and the plane is fine as-is. Keep it light back there.

Don Stackhouse
DJ Aerotech