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 for a Cheetah, the rate-of-climb at a gross weight of 2200 lbs is 660 ft/min. By definition, the excess power of a Cheetah in the original factory configuration is 44 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

The effect of these performance enhancers is a common topic on the Grumman Gang. Until estimated performance data is available for the PowerFlow exhaust and Sensenich propeller, only the improvement due to the High Compression STC is discussed. Data approximating the performance impact due to the High Compression STC is discussed in Basic Engine and Propeller Performance and will be used here.

The improved rate-of-climb can be computed from the original rate-of-climb as follows: new rate-of-climb = old rate-of-climb + (new - old thrust power) / aircraft weight. Recognizing the data in Basic Engine and Propeller Performance is somewhat off due to the generic propeller curves and handpicking points off of those curves, at 80 KIAS the old and new thrust is 458 lbs and 498 lbs (an increase of 8%), respectively. When converted to thrust power at 80 KIAS, this results in a net excess horsepower increase of 9.7 HP, which translates into a net increase in rate-of-climb of approximately 22%, from 660 ft/min to 805 ft/min at Cheetah gross weight. Note that this is from the High Compression STC alone without any propeller or exhaust improvements. But the shocker to most is that a 22% increase in rate-of-climb can be obtained from a 6.7% increase in maximum rate engine power.

The story does not really end there. Considering the higher power output of the engine-propeller combination and the relative location of the minimum of the drag power curve (the difference of these curves is the excess power), the maximum rate-of-climb will usually occur at a slightly higher speed than with the old engine. Because most 4-seat Grumman pilots climb the old engine at speeds higher than best rate-of-climb, flying the new engine at the same speed will result in more than the 22% increase calculated here. This is because the original excess power will be lower than 44HP and the thrust power increase from flying at the higher speed will be greater. However, no attempt is made to estimate these effects.

Climb Gradients

Climb performance can also be thought of in terms of altitude gained per distance traveled. The formula to derive climb gradient is (vertical speed x 60) / groundspeed and the units are typically feet per nautical mile. Note that standard IFR departure gradients require minimum 200 ft/nm. This table Climb Gradient Table and graphs Climb Gradient Graphs show climb gradients for the AA-5B Tiger for zero wind and 10-knot headwind conditions with gradients below 200 ft/nm highlighted in red and gradients between 200 and 300 ft/nm highlighted in yellow. These are based on POH values with estimated TAS calculations; they require maximum available power (leaned as necessary), no flaps. Your specific numbers will vary.