A propulsion system turns fuel into thrust through an engine and a propeller: the engine delivers shaft power, and the propeller converts only part of that into thrust power, which is thrust times speed. The carrier’s 50-80 hp engine delivers about 37-60 kW at the shaft, but a salvaged engine may not give its rated power in the air, cannot be assumed to run reliably, and will at some point stop, so every flight must be planned so that an engine failure is survivable.
Part of Aerodynamics and Flight
The engine, propeller and the installation around them that turn fuel into forward force.
Power In, Thrust Out
1 horsepower (mechanical) is 745.7 W. The carrier’s 50-80 hp is therefore 50 × 745.7 = 37,285 W and 80 × 745.7 = 59,656 W, about 37-60 kW. Thrust power equals thrust × speed, and it is the shaft power times the propeller’s efficiency. A propeller is least efficient when the aircraft is slow (take-off) and most efficient near its design speed (Propeller Design). The corpus supplies no efficiency figure and none is given here; a static thrust test with a spring balance measures what you actually get (Ground Testing).
What The Engine Must Do That A Ground Engine Does Not
- Run at full power for long periods. Climb is sustained high power with little cooling air (Cooling Systems).
- Run in any attitude. Fuel and oil must keep flowing in a climb or a descent (Fuel Systems).
- Be light. A stationary-engine conversion such as Converting a Vehicle Engine to Stationary Use heads the other way; an aircraft engine is judged on power per kilogram.
- Be mounted so its vibration does not destroy the airframe (Engine Mounting).
- Drive a propeller directly, at a speed the propeller can use. A car engine that reaches its power at a higher speed than the propeller can use needs a reduction drive, which adds weight and a point of failure.
Engine building and its fuels are covered in Internal Combustion and Engine Construction.
Plan For The Engine To Stop
An engine that stops in flight leaves a heavy glider whose glide angle you may not know. Practical consequences: take off only from fields with a clear landing area ahead, keep to the carrier’s rule never to fly beyond gliding distance of a suitable landing area, and never take off with a rough-running engine.
Common Mistakes
- Trusting the label power — a worn salvaged engine gives less, and a hot day gives less again.
- Ignoring the propeller — the best engine with a poor propeller wastes its power.
- Skipping ground running — the most common engine problems show on the ground.
Safety
Propeller, fuel and fire
A rotating propeller is nearly invisible and cuts to the bone; no one stands in its plane, and the aircraft is chocked and tied during runs. Fuel and exhaust beside doped fabric are a fire hazard. An engine that quits just after take-off leaves almost no time to choose a landing place, and the answer is a clear landing area ahead, not hope.
What’s Next
- Learn next: Propeller Design, where the engine’s power becomes thrust
- Combine with: Engine Mounting, carrying the thrust into the airframe