17.2 - Propeller Construction Flashcards
(20 cards)
What is most common wood used for wooden props
Birch
What does 5 layers of wood called in prop construction
A blank
What is meant by a ‘white’ in prop construction.
When a blank is allowed to rest for a week, moisture comes out.
Why is cotton frisbee sheathing used on wooden props.
Applied to outer 12-15 inches to reinforce the tip and thin sections. Doped for protection.
Composite props with a metal spar
Titanium, aluminium or steel
Honey comb or high density foam filling.
Fibreglass, kevlar, or carbon fibre shell.
Spar absorbs centrifugal forces and bending loads.
Shell gives it torsional rigidity.
Heating element in leading edge.
Composite props without a metal spar
Has two carbon fibre components as main structural members.
Centre filled with polyurethane foam.
Can be balanced horizontally with lead wool, in a hollow balancing tube , retained by a rubber plug.
Aluminium props
Most widely used props in aviation.
Better engine cooling is achieved.
Anodising is most common used finishing process.
Steel props.
Not in common use.
Found on older a/c or some transport a/c.
Typically hollow to save weight
Very brittle so all repairs and damage are major.
Prop blade stations
6 inch increments from root to tip.
Used in damage assessment and maintenance.
Master reference station.
Area where most thrust is produced.
If blade angle is measured is measured correct here, it is assumed correct at all other stations
75% for fixed prop
50%-75% for variable pitch props.
Cons of fixed pitch propellers
Max efficiency for one rpm and airspeed
Power available at take-off is reduced due to lower rpm.
Continual changes in rpm with variation in altitude and airspeed is detrimental to the efficiency and life of the engine.
Cockpit controllable pitch propellers
Uses gears.
Pitch change is initiated by cockpit by a crank handle.
More modern designs use toggleable switch controlling a motor.
Controllable pitch propellers.
Pilot can adjust pitch.
Two position propeller - low or high.
Variable pitch propeller - everything inbetween.
Two controls :
- throttle control.
- propeller control
Pilot can set it up for the most efficient performance.
Most popular design is the beech-Roby design.
Constant speeding propeller
Keeps a constant rpm by automatically changing pitch.has a constant speed unit or governor.
Constant speed unit
Most work on the principal of centrifugal force, uses a speeder ring and fly weights.
What happens in a propeller governor during an overspeed
Fly weights force out by centrifugal force .
Moves pilot valve up
More oil pressure is ported into the mechanism.
Pitch angle becomes courser.
Tapered shaft.
Found on small, old aircraft. Low powered piston engines.
Has a key to transfer the torque.
Splined shaft.
Used in high output radial engines.
Flanged shaft.
Used on modern aircraft.
Some have index pins for synchronising.
Spinner installation
A fairing for better aerodynamics, protection of the hub, and aesthetic.
Usually made form aluminium or glass fibre composites.
Dynamic load on spinner is high.
Repairs are limited. Stop drilling a crack is allowed.
Must check there is no wobble.
