propellors Flashcards
what are the two types of propellers?
fixed pitch and variable pitch.
what is a propellor blade?
an aerofoil which generates an aerodynamic force in the same way the wings generate an aerodynamic force but instead of lift it produces thrust.
as the propellor blade rotates what happens?
acceleration of the air over the front of the cambered surface causes a reduced static pressure ahead of the blade.
if the propellor blade has an AoA with the oncoming airflow, thrust is produced and this propels the aircraft forward.
what is the construction of the typical propellor?
the blade back is more cambered and blade face is the flatter side. the angle between the chord line of the propellor and the plane of rotation is called the blade angle.
the cambered shape of the blade propellor allows for what?
allows for reduced static air pressure to be produced ahead of the blade and there’s increased static pressure behind the blade creating a Total Reaction.
if an aircraft is stationary the motion of the propellor is purely what?
rotational and the speed of the blade section depends on radius and RPM. propellor with a high RPM will have a greater rotational velocity.
what is the forward velocity in a propellor?
as aircraft moves forward in flight there’s a forward velocity as well as a rotational velocity.
so the propellor experiences airflow from the direction of rotation but also from in the front meaning that the angle that the relative airflow strikes the propellor blade is made up from both the forward speed of the aircraft and the rotational speed of the propellor.
what are the three main angles to consider in propellors?
AoA - the angle between chord lien and relative airflow
Helix angle-angle between relative airflow and plane of rotation
Blade angle-angle between chord line and plane of rotation.
what is the helical path?
because propellor experiences a rotational velocity and a forward velocity, each propellor blade section follows a corkscrew path called a helix.
the aircraft moving forward experiences what?
velocity in the direction of flight, the propeller’s rotational velocity combined with its forward velocity contribute to the overall relative airflow.
what does relative airflow create?
a total reaction.
the total reaction can be broken down to what?
two components:
- lift which acts 90 degrees to the relative airflow
- drag which acts parallel to the relative airflow.
since it’s a propellor its more appropriate to use thrust and propellor torque.
on a propellor diagram how do thrust and torque work?
thrust acts perpendicular to the plane of rotation an propellor torque acts in the same direction as the plane of rotation.
when considering a wing drag must be overcome to provide lift but what is it for a propellor?
propellor torque must be balance by an opposing force which is engine torque.
how is engine torque increased?
opening the throttle and increasing engine power increases engine torque, causing the propellor to rotate faster up until a point where the propellor torque matches engine torque and the RPM then becomes constant.
the center of the hub is not shaped like an aerofoil for what reason?
as it needs to be thick and strong to absorb all of the forces on the blade.
having a fixed pitch propellor means what?
you cannot adjust the propellor pitch, so for a given RPM, this propellor will only operate at its most efficient AoA at one airspeed.
what is the relationship between airspeed and fixed pitch propellers?
with a lower RPM set, the aircraft is travelling with a low airspeed. the relative airflow will approach from a little way in front of the plane of rotation.
what happens if the aircraft has lowered the nose to increase their airspeed?
because of the increased airspeed, the relative airflow experienced by the propellor approaches from a more forward position reducing the AoA.
if lowered even more, it may reach such a high speed that the AoA of the blade will be so small that little to no thrust will be produced.
what happens if the aircraft maintains its airspeed bt has different RPM settings?
high RPM with a high rotational velocity and the same airspeed, the relative airflow experienced by the blade produces a relatively high AoA.
if pilot were to instead have a lower RPM, but travel the same airspeed, the RAF experience by the blade causes a lower AoA.
what is the difference in efficiency of different fixed pitch propellers across different airspeeds?
- fine pitch, propellor will achieve maximum efficiency at a relatively low airspeed.
- coarse pitch propellor, which achieves maximum efficiency at a relatively higher airspeed.
- note for a given RPM each of these propellers can only achieve maximum efficiency at one airspeed, indicated by the peak of the curves.
what is the difference in efficiency of a fixed pitch propellor to a constant speed propellor to a coarse pitch propellor?
constant speed can vary the pitch of the propellor blades in flight. a pilot governs the pitch using a constant speed unit.
a fine pitch propellor will achieve maximum efficiency (most efficient AoA) at one airspeed, indicated at the peak of its curve.
coarse pitch propellor achieves its max efficiency at just one airspeed. a constant speed can be operated at its most efficient AoA over a much wider range of airspeeds and RPM.
an engine with a constant speed system is referred as what?
as to having a constant speed unit (CSU)
with a fixed-pitch propeller, you only need how many throttles?
only one to control engine power.
