The 20cc racing class for
the Midwest AT-6 kit raises interesting design questions for the
optimum design of propeller to suit a radial engine/cowling. A propeller
was designed for use on the Webra 120, based on a design criteria
specifying minimum diameter of 14", and a maximum pitch of 12".
The performance of the test airplane with the Webra turning an APC
15X8 at 9700RPM was 98MPH, which latter figure was an average of
radar-measured speeds both upwind and downwind. This propeller was
analysed to determine the experimental pitch, as a guide for design
of the new propeller. It was clear by inspection that the APC propeller
was not optimised for the inflow field induced by the 8" diameter
radial cowl of the AT-6. Losses were expected to be considerable,
as the pitch of the APC blade component influenced by the cowl was
The effect of the cowl, being a very bluff body,
is to slow the axial inflow velocity forward of the cowl: this reduction
of velocity caused the APC blade elements to operate at too high
an angle of attack, a serious condition. Not only is the L/D poor
under such conditions, but more importantly, the 3-dimensional flow
so induced is very unfavourable. Essentially, the inner blade section,
operating at high lift, attempts to draw in air from the outer sections
of the blade disc, thereby increasing induced losses.
Analysis of the spinner/cowl configuration by the
Jones 2-source method, which reveals the flow field, failed as the
flow is so very awful. It was therefore necessary to guess the flow
field, with the following further design considerations:
1. The pitch of the blade within the influence of the cowl was reduced
2. The experimental pitch of the blade outboard of the cowl was
increased by .5".
It was expected that a speed gain would result with the new prop,
hence the need to increase experimental pitch.
3. The blade area within the influence of the cowl was reduced,
this area being considered as solely structural, to retain the working
tip section outside the cowl.
4. The airfoil in the working section was changed to yield a better
L/D than the APC, and also to delay the stall at high angles of
attack during turns.
5. The diameter was increased to 16", as an insurance policy.
The propeller could then be trimmed in diameter to increase the
engine RPM if the prop proved to be too much load and thus drag
the RPM down too far below below the power peak of the engine. Initial
testing with the full 16" diameter of the Supercool AT-6 propeller
yielded a rewarding 105 MPH at 9200 RPM, a gain of 7 MPH at somewhat
reduced RPM. The prop was then cut down to 15.3", bringing the RPM
up to 9500 and airspeed to 111 MPH, a total gain of 13 MPH. This
corresponds to an increase in both efficiency and thrust of 25%,
a striking validation of the need to allow for the modification
of the free-field inflow which results from the presence of the
fuselage behind the propeller.
APC 15X8 RPM 9700 Airspeed 98MPH
Supercool 15.3X8.5 RPM 9500 Airspeed 111MPH
(Radar measured speeds, results are average of up and downwind
legs, tests 5 minutes apart)