Surface Area.--Ditto.
Aspect Ratio.--Ditto.
Camber.--Ditto.
In addition to the above factors there are, when it comes
to actually designing a propeller, mechanical difficulties to
consider. For instance, the blades must be of a certain
strength and consequent thickness. That, in itself, limits
the aspect ratio, for it will necessitate a chord long enough
in proportion to the thickness to make a good camber possible.
Again, the diameter of the propeller must be limited, having
regard to the fact that greater diameters than those used
to-day would not only result in excessive weight of construction,
but would also necessitate a very high undercarriage
to keep the propeller off the ground, and such undercarriage
would not only produce excessive drift, but would also tend
to make the aeroplane stand on its nose when alighting.
The latter difficulty cannot be overcome by mounting the
propeller higher, as the centre of its thrust must be approximately
coincident with the centre of aeroplane drift.
MAINTENANCE OF EFFICIENCY.
The following conditions must be observed:
1. PITCH ANGLE.--The angle, at any given point on the
propeller, at which the blade is set is known as the pitch
angle, and it must be correct to half a degree if reasonable
efficiency is to be maintained.
This angle secures the ``pitch,'' which is the distance the
propeller advances during one revolution, supposing the air
to be solid.
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