Aircraft propellers, types and operation

Aircraft propellers, types and operation

There’s no question about it, one of the main differences between commercial aircraft and general aviation is their means of propulsion.

Commercial aeroplanes are usually equipped with large powerful turbines whereas general aviation uses propellers.

In today’s post we discuss their characteristics and the different types of propellers there are…So, fasten your seatbelts for take-off!

Propellers, a nautical contribution

The history of propellers in aviation goes back to the first flight in history; Flyer I, designed by the Wright brothers, already had two great propellers. But, if we’d like to know about the origins of propellers, we need to swap our means of transport and go nautical.

Jossef Ressel was the first inventor who was able to successfully develop a propeller by requesting the Austrian patent on 28th November 1826, although it wasn’t approved until 1827.

Ressel’s intention was to improve steam boats, which, up until then, used wheels as a means of propulsion.

But the efficiency of the propellers was such that years later, they were incorporated directly as means of propulsion for the first aeroplanes.

As you can imagine, propellers used by aeroplanes at present are nothing like those used in 1903. These days, we can find propellers made from complex materials, metal and wood and engineers try to design more and more efficient designs.

But what is an aircraft propeller and how do you operate one?

A propeller is a device made up of several blades that are placed around a shaft operated by an engine.

When it goes round, thanks to its aerodynamic edges, propellers generate a change in the pressure of air and its speed which generates a thrust force.

In our post about Bernouilli’s principle, we talked about pressure distribution on the airfoil; the same is true of the propellers, except that they also rotate.

But before we go further with this topic, it is very important to review some definitions:

Chord: An imaginary straight line joining the leading and trailing edge of an airfoil. In this case, we will consider the propeller blade as an airfoil.
Angle of attack: Angle formed between the relative air (free air) and the chord.
Pitch of the propeller: Angle formed between the plane of rotation of the propeller and the chord.

Estructure of a propeller

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.

Did you know…?

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.

Propeller pitch

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.

Attention! Here’s an interesting fact!

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.

Adjusting pitch of a propeller, an essential manoeuvre

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.

Propeller pitches in our fleet

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?

Types of aircraft propellers

Fixed pitch propeller

In this type of propeller, the pitch is given by the manufacturer and cannot be changed by the pilot. Thus, a good propeller for climbing and descending will not be as good for cruising and vice versa.

Generally these propellers are manufactured according to the revolutions and characteristics of the aircraft.

Although it is not possible to adjust them in flight, on the ground, with the engine stopped, with specific tools and the corresponding authorisation, it is possible to make some slight modifications.

Variable pitch propeller

As the name suggests, it is possible to adjust the pitch of this type of propeller in flight. There are three types:

The older ones only allow adjustment between a low and high pitch position.
Other models allow the pilot to freely adjust the pitch between the low and high pitch position; this is the case with our Diamond DA40.
The most modern are the constant speed propellers. In this type of propeller, the pitch is automatically adjusted to maintain a fixed and constant rotational speed set by the pilot, regardless of the power setting. This system is fitted to the Cirrus SR20 and Diamond DA42.

Propeller configuration

According to their function

Depending on their function, there are two types of propeller configurations: traction or propulsive.

Tractor configuration: This is the most common. The propeller is located at the front of the aircraft, “pulling” it.
Propulsive: When the propeller is at the rear, “pushing” the aircraft. This system is used in very few models.

There may be aircraft that combine traction and propulsion propellers; this is the case of the Cessna 337.

According to the direction of rotation

Depending on the direction of rotation, there are also two types of propellers:

Dexotropic: These are the most common; they are the ones that rotate clockwise (seen from the pilot’s seat).
Levogyrotor: These are the ones that rotate counter-clockwise (always seen from the pilot’s seat).

Because of this, some aircraft have, what we call, a critical engine; that is, an engine that, in the event of failure, will make the handling of the aircraft more complicated. To avoid this, the propellers of some multi-engine aircraft rotate in opposite directions.

A very special case in the assembly of propellers is the counter-rotating propeller. In this system, one propeller rotates in one direction and the air it expels backwards is used by another propeller that rotates in the opposite direction, increasing efficiency and eliminating torque. However, due to its complexity, this system is no longer used, although it can still be seen on the Antonov An22.

Torque

We could not finish this article without talking about torque, because, as we explained above, propellers generate a force when they rotate.

According to Newton’s third law, every force corresponds to an equal and opposite force, so if the propeller turns to the right, it will generate a force to the left. This is known as torque.

This effect is most noticeable when we make large power adjustments, which is why, if you are going to be an aeroplane pilot, during take-offs and landings, you will always hear your instructor telling you: “Right foot, right foot… More right foot”.

Keep learning aviation concepts

We hope that this article has helped you to learn a little more about the propellers used in aircraft.

We are aware that it is a dense subject and that, at first, so much information seems impossible, but as you go deeper it all makes sense.

And as we told you before, this topic is present in the syllabus for the private pilot and commercial pilot license, so we recommend you keep browsing our aviation blog, where we explain many other curiosities related to aviation.

Aircraft propellers, types and operation

Propellers, a nautical contribution

But what is an aircraft propeller and how do you operate one?