Takeoff
Factors Influencing Ground Roll
For propeller-driven aircraft, takeoff distance has been shown to be nearly proportional to wing loading (aircraft weight divided by wing area) and inversely proportional to the density ratio (actual density divided by sea level density), power-to-weight ratio, and the maximum takeoff lift coefficient.
Effect of the High Compression STC, PowerFlow Exhaust, and Sensenich Propeller
Note: The fundamentals of the discussion below are correct. However, after researching actual propeller data, it may appear that the propeller efficiency decreases with increasing RPM for the same airspeed. If this is the case, more analysis needs to be done to understand why some are claiming the effect of the extra 7.5 HP is disproportionate in favor of more performance. Please be patient while this issue is being researched.
The High Compression STC for the O-320 engines adds approximately 7.5 brake horsepower. Because the additional brake power allows the propeller to be turned faster and most Grummans have cruise propellers, the effect of the higher rotation speed is that the propeller now operates more efficiently. This means that more of the original 150 HP from the stock engine makes it into the air as thrust. It is also believed that the Sensenich propeller is more efficient by design than the original McCauley propeller. Therefore the combination of the High Compression STC and Sensenich propeller allows significantly more power to be converted to thrust than the stock configuration.
It can be shown that the reduction in takeoff distance is a combination of two factors. The first is due to raw propeller efficiency improvement. More efficiency means more of the original 150HP makes it into the air as thrust. The reduction in takeoff distance due to the first factor is approximately the percent increase in propeller efficiency. The second factor also contains the propeller efficiency improvement but also the effect of the additional brake power from the High Compression STC. More specifically, the second factor is the product of the ratio of propeller efficiencies and the ratio of the extra brake power (7.5 HP) to the original brake power (150 HP) of the engine.
Although actual propeller efficiencies numbers are not available, assume an overall 10% improvement. Then the first factor gives an approximate reduction in takeoff distance of 10%, and the second factor gives a reduction in takeoff distance of about 5% ( (1 + 0.1)*7.5/150 ). The effect that the High Compression STC has on propeller efficiency is two (2) times the effect of the additional power from the High Compression STC. For a 20% improvement in efficiency, these factors are 20% and 6% respectively. So the effect that the High Compression STC has on propeller efficiency is now three (3) times the effect of the additional power from the High Compression STC! For takeoff distance, the main impact of the High Compression STC is to allow more of the original 150HP to be converted into thrust. The secondary impact is the additional 7.5 HP.
The figure on the right illustrates the percent reduction in takeoff distance for any combination of propeller efficiency. Shown on the plot are the lines where the propeller efficiency improvement is 5% and 10%.
