Propellers - General

Question 1

What’s rotational velocity?

Answer 1

By cutting a blade, we can see that it has the same shape as wing. The speed over this section depends on the prop RPM and where it is along the length of the blade. The tip of the section travels much faster than a section near the root.

The rotational speed is the speed at any point of that blade section.

Question 2

What happens with effective alpha as a propeller rotates?

Answer 2

It produces an induced flow, which will change the relative airflow over the aerofoil, moving it forward from the propeller’s plane of rotation, REDUCING the effective angle of attack

Question 3

When is the propeller most efficient?

Answer 3

At about 4º alpha, where it delivers the Best L/D ratio

Question 4

What are the forces acting on a propeller?

Answer 4

Thrust: it is the component acting in line with the propeller shaft

Drag or torque drag; component acting in line with the plane of rotation

Question 5

What must be balanced in order to maintain propeller rpm?

Answer 5

drag must be balanced by the engine’s toque

Question 6

What determines the efficiency of a propeller?

Answer 6

The Thrust/Torque Drag ratio

Question 7

What happens when our propeller speed changes?

Answer 7

Alpha changes

Question 8

What happens on a propeller when IAS changes?

Answer 8

Both Thrust and Torque changes, as there is a change in TAS

Question 9

What the Geometric Pitch?

Answer 9

Theoretical distance a propeller would advance in one revolution at zero blade angle of attack

Question 10

What’s the Effective Pitch?

Answer 10

It’s the distance the blade actually moves forward relative to the airflow. It’s less than geometric pitch because, in reality, we have an alpha

Question 11

What’s slip in a propeller?

Answer 11

Difference between geometric and effective pitch

Question 12

What the Helix Angle?

Answer 12

It is the angle between the relative airflow over the blade and the plane of rotatoin

Question 13

What’s Pitch Angle?

Answer 13

Or called Blade Angle, is the angle between the blade chord line and the plane of rotation. It is fixed on simple propellers, but variable on constsant speed propellers

Pitch angle = Helix angle + alpha

Question 14

What’s the stated Pitch Angle of a complete blade?

Answer 14

It is the angle measured at a point three quarters of the way along the blade. “Reference section”

Question 15

What’s Thrust Horsepower?

Answer 15

THP = Thrust x TAS

Question 16

What’s the power delivered by the engine to drive the propeller called?

Answer 16

Engine Shaft Horsepower

Question 17

What’s the propeller efficiency? How high can it be?

Answer 17

Ratio of the output of the system, THP/EHP

Can be as high as 0.8, or 80%

If EHP is constant, efficiency reaches to a maximum where THP is a maximum, (+4º alpha) and decline as speed increases further, evntually to zrero

Question 18

What’s the torque of a propeller?

Answer 18

IT is the force from the engine that is applied to the propeller shaft to turn the propeller

Question 19

What’s the power output formula?

Answer 19

Engine Power Output (EHP) = Torque x prepeller RPM (a velocity)

Question 20

If propeller efficiency is THP/EHP, then?

Answer 20

Propeller efficiency = (Thrust x TAS) / (Torque x RPM)

Question 21

What’s fine pitch? When does it give its best performance?

Answer 21

Climb propeller, it has a relatively small pitch angle. Give best performance at low speeds, in the take-off and climb

Question 22

What’s a cruise propeller?

Answer 22

Fitted if aircraft is required to have a high cruising speed.

Question 23

What’s a windmilling propeller?

Answer 23

It’s a situation where pitch angle becomes so small (as alpha becomes negative) that the direction of total reaction acts in the direction of drag. Also, a component of total raction acts in the same direction as the propeller rotation, not as before, when alpha was positive, where that component was resisting the propeller’s action, now it is helping to drive it round.