From P.L. Mcilmoyle
I was very interested in the description of the Pathfinder solar-powered aircraft (“Sunshine Flights”, 17 June). However, I am puzzled by one aspect of the design. Both the text and the photograph (p 32) suggest that this “flying wing” aircraft has “tractor” propellers, on the leading edge of the wing. Yet where maximum lift for minimum power is needed, the aerodynamics favour the “pusher” configuration – with the propellers on the trailing edge.
This is because the airflow over the wing is much less disturbed with this arrangement. With the propellers in front of the wing the airflow over the section behind them is relatively turbulent, thus increasing drag. Note that the US Air Force’s long-range strategic bomber (B70, I think) had pusher propellers, range being very important.
With propellers along almost the whole of the wing of Pathfinder it would seem that a useful gain might well be made by adopting the pusher arrangement. I wonder whether there is some structural reason for this not being used.
AeroVironment, which built Pathfinder, says the propellers do create turbulence, but because they are mounted well in front of the wing, the airflow has time to smooth out before passing over the surface. It also points out that tractor propellers produce two advantages. First, they increase the airflow over the wing which increases the lift artificially. Secondly, they help to stabilise Pathfinder which has no tail – Ed.
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