Propellers Flashcards
geometric pitch =
the theoretical distance a prop would advance in one revolution at zero blade alpha (as if the blade were being screwed in to something solid)
Prop effective pitch =
slip =
the distance the blade actually moves forward relative to the airflow. Less than geometric pitch because in reality we have an angle of attack
the difference between geometric and effective pitch
what is prop helix angle / advance angle?
the angle between the relative airflow angle over the blade and the plane of rotation
what is prop pitch angle or blade angle?
the angle between the blade chord line and the plane of rotation
helix angle + alpha =
pitch angle
why is a prop blade twisted along its length?
The change in relative airflow caused by airflow through the disc will be much less at the tip, and alpha will remain high due to the higher speed of the tip
The slow rotational speed of the root will mean that the induced flow will give a much larger reduction in effective alpha
The blade is twisted a lot at the root to compensate for this, with very little twist at the tip
If the twist is geometrically correct all sections of the blade will have the same affective pitch as well as the same alpha
what and where is the reference station of a prop blade?
The stated pitch angle of the complete blade is the angle measured at a point three quarters of the way along the blade – reference station of the blade
the power delivered to the aircraft by the engine and propeller is called …
and the formula is …
thrust horsepower (THP)
THP = thrust x TAS
power delivered to the propeller by the engine is called …
engine shaft horsepower (ESHP)
prop efficiency =
THP/EHP
(output to input)
Engine power output can be calculated by multiplying … by …
so prop efficiency can also be written as …
torque (a force) by prop RPM (a velocity)
(thrust x TAS) / (torque x RPM)
which is
(THP / EHP) or (output / input)
a climb propeller has a … pitch angle to give best performance at low speeds, in the take off and climb
what happens to thrust at higher airspeeds?
small (fine pitch)
thrust will quickly be lost as speed increases and so cruising speeds will be low
a cruise propeller has a … pitch which will provide more thrust at higher cruising speeds
larger
On selection of a lower RPM, the CSU will … pitch to … drag and slow the prop
However, the effect of reduced rotational speed will be reduced … so a further … will need to be applied by the CSU to maintain the new level of drag
increase pitch
increase
alpha
increase in pitch
in a high speed dive, what happens to prop alpha and the subsequent forces on the prop?
If the pilot enters a dive from the max level flight cruising speed the increasing TAS will quickly reduce effective alpha. With a fixed pitch prop the alpha may reduce to zero and then turn negative, and the same will happen with a variable pitch prop once the blades have reached the limit of their coarse pitch movement
The prop is now producing very high levels of drag. The low pitch angle means that surface area of the blades is almost presenting a flat plate to the airflow
The total reaction is now acting in the direction of drag, not thrust
A component of the total reaction is also acting in the same direction as the propeller rotation. Before, when alpha was positive, that component was resisting the rotation of the prop, now it is acting to assist the rotation and speeds it up – this is windmilling propeller
what is the difference of prop torque between a windmilling prop and one in reverse thrust?
the total reaction is still acting in the reverse direction but because of the negative pitch when in reverse thrust it has tilted further round to return the torque drag component to act against the engine, instead of assisting engine rotation as it was when windmilling
