Dear Topaz and Cluttonfred,
I have sketched a dozen variations of Richard Voss's asymmetric airplanes.
One of my first design criteria is separating the propeller arc from the cockpit. I insisted on installing rudder-pedals 8 to 18 inches (50 cm) aft of the propeller disc to prevent thrown propeller blades from amputating the pilot's feet. Note that FAA certification standards forbid installing crew seats in line with prop discs.
So my design concept starts with fixing the distance between the prop disc and rudder pedals, then sliding components fore and aft until they balance.
In the case of a BV 141 replica, I started by leaning the pilot's shoulders against the main spar, then positioning the propeller a bit forward of his feet (SAE 95 the percentile male), then doing preliminary balance calculations.
Secondly, single-engined pushers are always difficult to balance. By the time you position the propeller aft of the wings' trailing edge, your centre-of-gravity is a long way aft of optimal. You could regain balance by moving the pilot farther forward, but that creates a long and heavy cockpit structure.
The other problem with pushers is streamlining the aft end of the passenger cabin.
For smoothest airflow, fuselage walls are ideally parallel across the wing root, then taper at 2 or 3 to 1 until they converge at the propeller. This need for streamlining clashes with engine balance.
Seabee solved that problem with an extension shaft, but we all know that the longer the drive train, the more likely it is to suffer torsional vibration problems.
David Thruston's first light flying boat was the Grumman Tadpole with the engine (on a short pylon) raised a bit above the mid-wing. The aft fuselage was cutaway to allow the bottom propeller blade to spin close to the waterline. Note Thurston only designed one plane with that configuration. All his later flying boats (Colonial Skimmer, Lake Buccanneer, Teal and Seafire) used pylons to mount engines well above the wing, where they were easier to balance (fore and aft) because they did not have to worry about prop blades hitting the wing, or wing wake confusing airflow into the propeller. The disadvantage is that Thurston's airplanes suffer major trim changes when you change thrust.
In the short run (please pardon my pun) the simplest drive train is short.
In the long run, light-weght electric motors will be positioned anywhere it is convenient to mount propellers, with few balance problems.
I have sketched a dozen variations of Richard Voss's asymmetric airplanes.
One of my first design criteria is separating the propeller arc from the cockpit. I insisted on installing rudder-pedals 8 to 18 inches (50 cm) aft of the propeller disc to prevent thrown propeller blades from amputating the pilot's feet. Note that FAA certification standards forbid installing crew seats in line with prop discs.
So my design concept starts with fixing the distance between the prop disc and rudder pedals, then sliding components fore and aft until they balance.
In the case of a BV 141 replica, I started by leaning the pilot's shoulders against the main spar, then positioning the propeller a bit forward of his feet (SAE 95 the percentile male), then doing preliminary balance calculations.
Secondly, single-engined pushers are always difficult to balance. By the time you position the propeller aft of the wings' trailing edge, your centre-of-gravity is a long way aft of optimal. You could regain balance by moving the pilot farther forward, but that creates a long and heavy cockpit structure.
The other problem with pushers is streamlining the aft end of the passenger cabin.
For smoothest airflow, fuselage walls are ideally parallel across the wing root, then taper at 2 or 3 to 1 until they converge at the propeller. This need for streamlining clashes with engine balance.
Seabee solved that problem with an extension shaft, but we all know that the longer the drive train, the more likely it is to suffer torsional vibration problems.
David Thruston's first light flying boat was the Grumman Tadpole with the engine (on a short pylon) raised a bit above the mid-wing. The aft fuselage was cutaway to allow the bottom propeller blade to spin close to the waterline. Note Thurston only designed one plane with that configuration. All his later flying boats (Colonial Skimmer, Lake Buccanneer, Teal and Seafire) used pylons to mount engines well above the wing, where they were easier to balance (fore and aft) because they did not have to worry about prop blades hitting the wing, or wing wake confusing airflow into the propeller. The disadvantage is that Thurston's airplanes suffer major trim changes when you change thrust.
In the short run (please pardon my pun) the simplest drive train is short.
In the long run, light-weght electric motors will be positioned anywhere it is convenient to mount propellers, with few balance problems.
