Prop Disc Loading

Prop disc loading is defined as the engine’s ‘BHP divided by the prop disc area’. If the propeller’s diameter is increased, it will lead to an increase in the prop disc area, which will reduce the prop disc loading and in turn, increase the propeller efficiency. The prop disc loading can be reduced by either increasing the prop diameter or by reducing the engine’s BHP. If the BHP is increased and a prop of the same diameter is used, it follows the prop disc loading will be increased. However, if the increase in prop disc loading is too great, a loss in efficiency will result. This is due to the increased air pressure at the rear of the prop disc leaking around the propeller tips causing an increase in prop tip vortices and induced drag. The same affect occurs on a wing that is too short for the aircraft. In fact, the prop disc loading has the same affect as the aircraft’s wing loading.

The value of the prop disc loading can be found given the engine’s BHP and the prop’s diameter, or better still, its prop disc area, which for a 74 inch diameter prop is found to be 29.86 square feet. The prop disc loading is then found from the formula:

„ Brake horsepower

Prop disc loading = ————– —– ——-

Prop disc area

The power absorbed by a fixed-pitch propeller will vary as the cube of the RPM change (RPM3) depending on the air density and RPM. A given engine power is produced by any one given RPM, air density being constant. The maximum RPM of a modern light aircraft piston-engine is usually limited to around 2700 RPM, mainly due to the noise and compressibility caused by the high propeller tip speed. Some relatively recent production models run even slower usually around 2500 RPM maximum. Most engines could run up to about 3600 RPM before destruction occurs, but it is the prop tip speed that determines the maximum allowable engine RPM, indicated by the red line on the engine’s tachometer. A propeller reduction gear with a fixed gear ratio will then be incorporated in the drive between the engine and propeller As far as the engine is concerned, running at higher RPM is advantageous because the greater number of power strokes per minutes produces greater power. The Lycoming TIO-540 engine is a good example here; this is a direct drive engine producing 380 BHP at a red line of 2900 RPM. The geared version of this engine, the TIGO – 541, produces its maximum power of 425 BHP at 3200 RPM. Both engines are identical except for the reduction gear on the TIGO-541 engine. The propeller on the geared engine can absorb the greater horsepower and therefore, produce greater thrust while maintaining the prop tip speed within acceptable limits

With the prop of a geared engine turning at a lower RPM than a direct drive prop, the blades will meet the airflow into the prop disc at a much lower speed. They would therefore do less work in producing thrust, resulting in a loss in efficiency. To overcome this loss, the prop’s solidity is increased by using more blades or wider chord blades, or by increasing the prop’s diameter. However, increasing the diameter too much results in
too a high tip speed, which was the reason for using reduction gear in the first instance. This emphasises the need to match the prop to the engine’s BHP and the aircraft design air speed. On a piston engine, the reduction gear will have a ratio of around 3:2, but for turboprop engines due to their inherent design, have an operating speed of 10,000-15,000 RPM (or even higher depending on the engine design) require a much greater reduction gear ratio. The Rolls Royce Dart engine for example, has ratio of 10.75:1. It must now be emphasised here, it is the engine that is geared, not the propeller. A geared piston engine can be recognised by its designation. For example, the letter ‘G’ in Lycoming’s TIGO-541 engine indicates the engine is geared

It has been determined thus far that the maximum propeller efficiency occurs when the prop produces the maximum thrust/torque ratio. Also considered were the factors, which determine how much engine power, or torque, the prop absorbs and transmits as thrust energy to the propwash.