In order to develop the propeller design for a MK IV with retractable undercarriage and an auto engine conversion it is necessary to understand the performance of the stock Cozy MK IV. Once these data are calculated it becomes possible to develop the design so that one is not working in the dark with nothing more than educated guesses.

My first calculations are to bound the equivalent flap plate drag for the stock MK IV. Since the Pilots Operating Handbook (POH) gives the maximum MSL speed of 216 mph at an engine power of 180 HP it is possible to calculate the equivalent flap plate drag. This in simplistic terms is derived from the relationship:

Thrust Power = 1/2 r V³ A_fp

Where the thrust power is the engine power multiplied by the propeller efficiency (180 hp x 85%),

r is the air density (1.2 kg m^-3), and;

V is the velocity of the aircraft (216 mph).

Re-arranging this equation yields a figure 2.273 ft² for the equivalent flat plate area. This figure for flat plate area is the worst case scenario since the actual drag is composed of three elements:

1. Parasitic Drag,
2. Induced Drag, and;
3. Slipstream Drag.

When one considers these other drag elements the value of Afp is reduced further. So how does one calculate these figures? I found it possible to calculate the first two elements in terms of the original Afp value. but I could not calculate the value of slipstream drag, at least until I came across Don Bates' "PROPOPT" software.

Using the Bates program it was possible to calculate many of the aircraft parameters using data from the POH. The following data was used for the input data. The five data values with a large number of significant figures were the parameters computed with the IOPT set to 1,3,6,7,10. Notice the ADRAG figure of 2.025. This is the equivalent flat plate drag sought above.

Output data, from the program is given below. Notice how the estimate for climb rate is very close to that indicated by the POH and the estimate for propeller pitch is again close to that indicated by the POH. Clearly the model is achieving good agreement with the practical data.

The flat plate drag area is now defined as 2.025 ft2 which is a little less than the rudimentary calculation predicted. This was expected since the rudimentary calculation did not take into account the slipstream or induced drag.

Last Updated:    Thursday August 31, 2006