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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Supercool 16 X 8.5 AT-6 Racing Propeller


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 excessive.

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 to 5".
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.

Results summary:

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) 

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