Barry and Sam make some good points.

For best power the engine must be as rigidly mounted as possible. If you have a test stand run, and compare it to on the airplane, the test stand will be slightly better. With our engine, there are two modes of vibration. The first is due to the motion of the piston going up and down. This motion is opposed by the polar moments of the airplane. With the engine mounted horizontally, the wing’s polar moments are helping us oppose the vibration (this is the only time that heavy wing tips do us any favors).

The other motion is torsional vibration where the prop is twisting the fuselage. This would not be much of a problem if we were flying electric motors with low cogging motors. But with a single cylinder engines with a single power pulse, the propeller is always accelerating, either speeding up on the power stroke, or slowing down during the compression stroke. So the fuselage has to oppose this twisting action without absorbing energy. Fiberglass fuselages don’t do this too well, especially when they are only 2 ¼” or so at the nose. Poorly designed balsa fuselages are not much better. But a well designed balsa and plywood fuselage with fiberglass comes close to the test stand performance.

Design of the fuselage is where the rubber leaves the road. When I developed the fuselage with the width located at the trailing edge of the wing, I found quite an advantage over what was then conventional design. That was around 93 or so, and I published the technique in an Model Aviation article in 98. The Shotgun is rather interesting in that a double bump is used, where I have always done either a straight section from the firewall back to the wing, and then the width expansion, or a straight taper from the firewall back. Which method is best? Time will tell, but just the expansion is good for about 5 mph.

But the plan view is just part of drag reduction. As Barry notes, some have a curved side view while others have straight lines. But what is not usually considered is the up-wash and down-wash from the wing. What this means is that the streamlines may not be parallel with the centerline or thrustline. Also complicating the situation is that in the turns, the up-wash and down-wash is different that when the airplane is in the straight-aways. So simply using an airfoil shape for the side view of the fuselage really doesn’t make much sense when you include the discontinuity that the wing contributes to airflow.