ENGINES
We don’t often think about helicopter in-tank and in-line (auxillary and boost) fuel pumps until they fail. Fuel pumps do their job day in and day out – but they don’t last forever. This article will provide some tips to ensure you are properly maintaining and inspecting the fuel pump and fuel system to get the most out of your fuel pumps.
FUEL PUMPS PROPER CARE AND MAINTENANCE OF By Joe Escobar
Pump Design Many auxillary and boost type fuel pumps are direct current (DC) motors. 12V or 24V DC electrical power is applied to the pumps and this power transfers to carbon brushes that ride against a commutator. The electrical charge sent through the commutator to the armature causes the copper windings in the armature to act as electromagnets. The resulting attraction/repulsion of the magnetic fi elds between the armature and fi xed magnets in the pump housing causes the armature to turn.
There are two basic types of fuel pumps (plus a combination- type pump):
Constant displacement. Constant displacement pumps are in-line pumps such as boost pumps and transfer pumps, depending on the airframe application. “Constant
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HelicopterMaintenanceMagazine.com February | March 2014
displacement pumps have a metal rotor with four slots where carbon vanes sit in,” says Barrett (Bart) J. Briggs, vice president and general manager for Monroe, WI-based Fleet Support Services, Inc. (an FAA Repair Station that overhauls fuel pumps and is affi liated with Rapco, Inc.) “The vanes spin around in an elliptical cavity. Fuel comes in through the larger portion of the cavity, and as the carbon vane pushes it though the narrow portion of the cavity, it is pushing the large volume of fuel through the narrow portion under pressure. For every revolution, there is a constant displacement of fuel pushed through the pump.”
Centrifugal. Centrifugal pumps are what we would call cartridge pumps. They use a disk-like impeller to pump the fuel. The cavity where the impeller sits in is round. The impeller has openings towards its center where the fuel is picked up. As the impeller spins, the fuel is pushed by centrifugal action through channels inside the impeller, and exits through outlets on the impeller’s edges. “This is high- fl ow, but low-pressure pump,” explains Briggs. “Centrifugal pumps don’t generate a lot of pressure. They move a lot of volume, but at minimal pressure because there is not a lot of pressure pushing the fuel like the vanes in constant displacement pumps.”
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