Dowty Aerospace: Propeller Blades
Dowty Aerospace commenced manufacture of propeller composite blades during the mid 1960's. Initially, such blades were made of glass fibre reinforced resin and developed on hovercraft which operate in severe air/sea conditions. By 1970, the company was producing composite blades for a range of aircraft and air cushion vehicles. These blades incorporate carbon fibre spars, polyurethane foam filling and a glass fibre reinforced resin aerofoil shell covered with a tough polyurethane coating for erosion protection. Further protection is provided by replaceable leading edge strips. Other features include braid type lightning conductors and blade root retention, de-icing operation and safety counterweight advantages.
A propeller blade is a primary component therefore no fatigue or other structural failure is permissible at any time or in any circumstances. The component is regarded as coming in the same safety and reliability category as an aircraft wing.
In order to be granted certification by the Aviation Authorities in the UK, USA and Europe, rigorous testing of the complete composite blade is undertaken to evaluate its structural integrity.
All stages of the manufacturing process have to be closely controlled. The method of manufacture utilises special tooling and a resin injection process specifically developed to produce composites with high fibre volume fractions and correspondingly optimum structural properties.
The Ciba-Geigy epoxy resin is heated, mixed, degassed and then fed at a precisely controlled rate into the mould.
In keeping with the overall manufacturing process, the resin has to be heated in a controlled and precise manner. To facilitate this, Thermosafe Induction Heaters were selected for the heating operation. Energy transfer by an a.c. magnetic field provides uniform heating without hot spots. The absence of heating elements, and as a result, lack of retained heat, allows for very close temperature control.
Dowty Aerospace have engineered advanced composite materials to produce propellers which have good reliability and are easily maintained in the field. Although sophisticated, manufacturing methods have been developed to enable blade production in large quantities and in a wide range of sizes.
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