Climb

Factors Influencing Climb Performance

Rate-of-climb (ROC) is identically the specific excess power. Excess power is the difference between how much thrust power the propeller is putting into the air and how much power it takes to overcome drag. Specific excess power is the excess power divided by the aircraft's weight. Because ROC is related to excess power, doubling or halving the thrust power does not mean the rate-of-climb or descent is doubled or halved. A good example is a light twin such as an Apache. If an Apache loses an engine (thrust power reduced by half), the remaining engine usually does not have enough power to even sustain level flight.

Using the numbers quoted in the Pilot's Operating Handbook, the rate-of-climb at a gross weight of 2200 lbs is 600 ft/min. By definition, the excess power of a Cheetah in the original factory configuration is 40 HP. Therefore, even small amount of additional brake power or improvements in propeller efficiency will have a large impact on excess power and, therefore, rate-of-climb.

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.

This is a common topic on the Grumman Gang. Because each aircraft is a little different in weight, drag, engine, and prop, it is difficult to give a single estimate for the improvement in rate-of-climb. However, like the takeoff analysis, the High Compression STC not only adds additional brake power but also turns the propeller at a higher rotational speed thereby improving the efficiency.

Rate-of-Climb Improvement

The rate-of-climb improvement therefore has two factors. First is the additional 7.5 brake horsepower at the higher propeller efficiency. Second is the additional thrust power from the original 150 HP operating at the higher efficiency. This second term is just the product of the original 150HP and the difference between the original and new propeller efficiencies. Dividing by weight and the original rate-of-climb yields the improvement in rate-of-climb for both factors. Assuming the original propeller efficiency is about 0.7, the improved efficiency is 0.75, weight is 2200 lbs, and the original rate-of-climb is 600 ft/min, the effect of the improved propeller efficiency is approximately 18% and the effect of the additional 7.5 brake horsepower from the High Compression STC is approximately 15%. Unlike the takeoff analysis where the improved propeller efficiency was the main effect, both improved propeller efficiency and the additional brake power are roughly equal effects.