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Power Loading

Scan a spec sheet for that latest aircraft you crave to fly and you’ll see a numerical reference, the significance of which could pass you by. It’s the power loading and, like the wing loading value with which it shares significant importance vis-a-vis the aircraft’s performance parameters, it will surely tell you something as to how your aerial steed will fly.

The power loading of an aeroplane is the gross weight of the aeroplane divided by the rated horsepower of the engine. It’s expressed in terms of pounds per horsepower and relates, in no small way, to aircraft performance factors such as takeoff distance and time-to-climb, as well as to general flight manoeuvring conditions and aircraft responsiveness.

Power loadings typically range from 10 to 15 lbs per horsepower for most aircraft. A Cessna 150, for example, at 1,600 pounds will have a power loading of 16 lbs/hp given its 100 hp Continental engine. A higher single-engine performer, such as a Pilatus PC-12, will break the bonds of earth at a power loading of 8.2 lbs/hp developed through its 1,200 hp Pratt & Whitney turboprop, lifting its 9,920 lbs of maximum takeoff load into the beautiful blue yonder.

Esoteric though the term may be, the information supplied by power loading ties directly to another term of which people may be more familiar: power-to-weight. The latter – the power the engine develops divided by the aircraft’s weight – is simply the reciprocal of the numbers used to determine the figure that is the power loading. Thus, using the example of the Cessna 150, its power loading of 16 lbs/hp found from dividing its 1,600 lb weight into its power output of 100 hp can also be looked upon to provide you with a power-to-weight of 0.06 hp/lb found from dividing its 100 hp figure into its 1,600 lb gross takeoff weight. Though the verbiage may be confusing, the data derived from power loading numbers – or, alternatively, power-to-weight figures – render clear information about the aircraft from which respective horsepower numbers are derived.

An aircraft with a higher power-to-weight will accelerate more quickly, climb faster, reach a higher maximum speed, and sustain higher rates of turn. On the downside, a more powerful engine will consume more fuel throughout its operation which will raise the aircraft’s takeoff gross weight, owing to the requirement that it carry more fuel to accomplish its mission.

Any additional weight on an airframe requires that more lift be generated by the wings to get it into the air. However, more lift translates into more induced drag which slows the aircraft down. Therefore, for any two aeroplanes powered by the same engine type, the lighter of the two should always be the faster. Thus it is that power-to-weight is also a factor in an aircraft’s top speed.

Low performance aircraft tend to have higher values for their power loadings. For example, the Piper J3 Cub has a power loading of 18.7 lb/hp. (Reciprocally, its power-to-weight is 0.05 hp/lb, barely above the figure for a standard powered sailplane.) Fighter aircraft occupy the other end of the scale, typically having power loadings of 5 to 6 lb/hp which, in power-to-weight terms, brings some modern fighters to a power-to-weight ratio of 1.0. Low power loading values for advanced jet fighters are necessitated by their requirement for exceptional rate of climb and manoeuvring characteristics. Indeed, in combat conditions, as fuel is burned off, such aircraft have power-to-weight values that can exceed 1.0 which is what makes them capable of accelerating while pointing straight up.

In a nutshell, high power loading figures (or low powers-to-weight) relate to modest aerial performers – think J3 Cub and Cessna 150. Low power loading figures (or higher powers-to-weight) might rip the shingles off your roof.