Grnd-47 Theory of Flight 3 Propulsion Systems

Thrust

Is a force generated by the engine through some kind of a propulsion system, used to overcome drag and propel the aircraft

What is required to make a propulsion system work?

Physical contact with a working fluid.

How is thrust generated?

Through the reaction of accelerating a mass of gas to the rear of the engine producing thrust in the opposite direction of the accelerated gas.

What does the magnitude of thrust depend on?

The amount of gas accelerated and difference in velocity of the gas through the propulsion system.

Drag

The component of aerodynamic force parallel to the relative airflow which tends to slow aircraft motion and decrease efficiency and perforemance.

Two types of drag

Profile drag, induced drag

Profile Drag

Created by the displacement of air by the aircraft and the hindering of airflow over the surface of the aircraft

Induced Drag

Portion of total drag that is due to the production of lift

General categories of propulsion systems

Gas turbine engines, propeller & powerplant combo

Difference between gas turbines and propeller systems

gas turbine: produces thrust directly, thrust varies with fuel flow, rated in pounds of thrust

propeller: require a propeller to convert power to thrust, thrust varies inversely with speed, rated in shaft horsepower

Jet engine mass vs velocity of air

Provide a large change in velocity to a relatively smaller mass of air and fuel

Three types of jet engines

Turbojet, turbofan, turboprop

Turbojet

Largest and simplest gas turbine engine. Large amounts of surrounding air are continuously brought into the inlet, is compressed, heated, and accelerated leaving the exhaust nozzle at a much greater velocity

Turbofan

More energy is taken from the turbine to run the fan resulting in less exhaust velocity than the turbojet but more than turboprop.

Turbofan efficiency

High efficiency at high subsonic speeds, 450-650kt range

Turboprop

Utilizes a gas turbine with a propeller drive. The turbine takes more energy than is required to run the compressor with the excess used to run the propellor.

Turboprop efficiency

Most efficient up to 425kt, peak at 350kt

Propeller mass vs velocity of air

Develops thrust by producing a relatively small change in velocity to a relatively large mass of air

Propeller principle of operation

As a propeller moves through air it has forward and rotational velocity, following a helical path. The relative airflow is determined by prop RPM and forward velocity of aircraft

Propeller efficiency

Normally large diameter props favour high efficiency due to large air flow, however adverse effects are produced by high tip speeds and compressibility effects.

Prop Torque

As thrust is produced, torque (a rotational force) is also produced. Excess Q can cause undesirable flight characteristics and be extremely dangerous especially at low speeds close to the ground

Three types of propellers

Fixed pitch, variable pitch, constant speed

Feathering

Ability to streamline the blades and reduce drag and stop rotaion

Reverse thrust

Ability to rotate the blade angle to very small positive or negative angles with applied power producing large drag or reverse thrust

Propeller shapes

Low speed aircraft use larger propellers, higher speed aircraft will use smaller diameter blades to reduce blade tip speed avoiding transonic conditions.