Powered Parachute Flying Handbook

Chapter 4 — Powerplants

Centrifugal Clutch

Some gearboxes come with a built-in centrifugal clutch, and others have allowances for installation. A centrifugal clutch is very useful in a two-stroke engine because it allows the engine to idle at a lower speed without the load of the propeller. Otherwise, twostrokes can generate a lot of vibration at low RPM when loaded. As the engine speeds up, the centrifugal clutch engages the rest of the gearbox and smoothly starts the propeller spinning. When the engine is brought back to idle, the clutch disengages and allows the engine to again idle smoothly; the propeller stops when on the ground and windmills when flying.


The propeller provides the necessary thrust to push the powered parachute through the air. The engine power is used to rotate the propeller, which in turn generates thrust very similar to the manner in which a wing produces lift. The amount of thrust produced depends on the airfoil shape, the propeller blade angle of attack, and the engine RPM. [Figure 4-5] Powered parachutes are equipped with either a fixed-pitch or ground adjustable pitch propeller.

Fixed-Pitch Propeller

The pitch of this propeller is set by the manufacturer and cannot be changed. Refer to Chapter 5 of the Pilot’s Handbook of Aeronautical Knowledge for basic propeller principles.

Ground Adjustable-Pitch Propeller

Adjustable-pitch propellers for PPCs can only be adjusted on the ground with hand tools. If an engine is over-revving, more pitch can be added to the propeller. If the engine is not developing the full recommended RPM during flight, then some pitch can be taken out of the blades. This should be done per the PPC’s POH and by a qualified technician.

Induction Systems

The induction system brings air in from the atmosphere, mixes it with fuel, and delivers the fuel-air mixture to the cylinder where combustion occurs. Outside air enters the induction system through an air filter on the engine. The air filter inhibits the entry of dust and other foreign objects. Two types of induction systems are used in powered parachute engines:

1. The carburetor system is most common; it mixes the fuel and air in the carburetor before this mixture enters the engine intake, and
2. The fuel injection system, which injects the fuel into the air just before entry into each cylinder.

Carburetor Systems

PPCs use float-type carburetors. Reference the Pilot’s Operating Handbook of Aeronautical Knowledge for basics on float carburetor operation.

Modern two- and four-stroke carburetors operate with one of three jetting systems, depending on engine power. [Figure 4-6]

When the throttle is closed, for engine idling, the throttle valve is closed and the fuel is supplied through the idle (pilot) jet and idle (pilot) air passage. The fuel/air/oil mixture is supplied to the cylinders through the bypass hole. [Figure 4-7]

As the throttle is advanced and the throttle valve is raised, the fuel is sucked up through the main jet but is controlled by the opening and taper of the jet needle and needle jet. This is effective throughout most of the mid range operation. About half throttle, the main jet size starts to influence the amount of fuel mixed with the air and this effect continues until it is the main influence at the highest throttle settings. [Figure 4-8]

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