Now you have a good general idea of what a propeller is, let’s go over it in more detail.
Maybe you’ve never seen a propeller or haven’t noticed, but you have to know they’re not straight though they may seem so at first glance.
Propellers are always changing their incidence angle, the biggest angle being in the middle, at its root. This is done to increase its efficiency as the end goes round faster than the centre.
But hang on, how does it go faster in the middle? Yes, that’s right, it may sound strange, but the ends of the propeller rotate a lot faster than the centre does and this is due to having to cover a longer distance within the same time.
The answer for propellers to produce a regular force and it not being noticed in terms of speed, is to reduce the attack angle gradually. In this way, on the one hand, the root has a high attack angle with low speed but the edge has a low angle of attack with high speed. In the end, speed is compensated, being the same in the whole edge of the blade.
One of the most important aspects in the design of propellers is to calculate the speed in the ends of the blades as, if speed is near to that of sound, a decrease in performance as result of the increase in resistance will be produced.
This is the reason diameter and propeller revolutions are limited. To compensate for this, some aircraft are fitted with a gearbox between the engine shaft and the propeller.
Let’s talk about one of the concepts that most commonly gives pilots a headache at the start of their training: propeller pitch.
As explained above, the angle formed between the plane of rotation of the propeller and the propeller chord is called pitch.
We will now go into this further, the quickest way to understand pitch is to relate it to the speed of cars.
On the one hand, a low pitch gives a lower speed but high traction; like the first few gears when driving a car. On the other hand, a high pitch provides a higher speed and lower traction; like long speeds.
In some models of aeroplanes, the pitch of a propeller can be negative, aiding the aeroplane in braking.
This system is widely used in parachute drop aircraft, as once the jumpers are in the air, it is most cost-effective to get the aircraft back to the ground as quickly as possible.
This system can be seen in Pilatus Porter, single engine light aeroplane specially designed for short take-offs and landings.
During take-off and landing it is necessary to have a low pitch as the most important thing is having a good traction.
According to the type of aeroplane, the control of the propeller´s pitch can be automatic or manual. Their difference can be seen by just looking at them because the manual ones have a blue lever.
In manually adjusted aeroplanes, a low pitch is obtained when the lever is in its most forward position.
In aircraft with variable pitch adjustment by the engine, the system uses the engine oil to move the propeller.
When we are in flight, it is necessary to set the pitch and throttle appropriately to avoid overloading the engine. Therefore, when we want to increase power, it is necessary to adjust first the pitch and then the throttle; and, to reduce power, first the throttle and then the pitch.
Since our origins, at One Air we have been committed to giving our students the most complete training possible, and that is why we have several models of aircraft.
The Diamond DA20 are fixed pitch aircraft. The Cirrus SR20 and the Diamond DA42 are variable pitch aircraft automatically adjusted by the engine itself. And the Diamond DA40 are manually adjustable variable pitch planes.
This allows our students to see, first hand, the differences between the different types of propellers. Something very enriching as a pilot, don’t you think?
