An Introduction to Constant Speed Propellers

INTRODUCTION

The most common type of propeller in sport aviation is the fixed pitch propeller. You know the type; a lump of carved wood or forged aluminium, bolted onto the front of your engine. Although cheap, this is one of the crudest propulsion devices you could use, and has been long superseded by a variety of more advanced options, readily available on the market. But, how do you know how each type of propeller operates and what advantages the different types offer? How are you going to choose between the different types available for your aircraft, especially considering that a more capable propeller is also more expensive?

There are four common families of propeller, which I will introduce to you. They are fixed pitch, ground adjustable, in flight adjustable and constant speed. The last two are both examples of variable pitch propellers.

In order to appreciate the advantages which are characteristic of the different families of propeller, we must first consider the most fundamental characteristic of a propeller - the pitch. Pitch is important, as it is the manner in which pitch is controlled that allows us to differentiate between one family of propeller and another.

A useful analogy when considering the affect of pitch is that of an automobile gearbox. By comparing a propeller's pitch to a gear ratio, and considering the function of a gearbox, we will gain an appreciation of the different families of propellers.

WHAT IS PITCH?

Propeller theory includes a variety of concepts that may at times be called pitch. Pitch can refer to the blade angle with respect to a flat plane, the distance that a propeller will advance through the air for each rotation or the amount of ‘bite’ that the blade has on the air. Essentially these concepts all describe the same thing. To use our automobile analogy, pitch is like the gear ratio of the gearbox. The important thing to note with pitch, is that it is available in a wide variety of degrees, or 'amounts', much like different gear ratios. To demonstrate, consider the following examples:

• A fine pitch propeller has a low blade angle, will try to move forward a small distance through the air with each rotation, and will take a 'small' bite of the air. It requires relatively low power to rotate, allowing high propeller speed to be developed, but achieving only limited airspeed. This is like having a low gear in your automobile.
• A coarse pitch propeller has a high blade angle, will try to advance a long distance through the air with each rotation, and will take a big 'bite' of the air. It requires greater power to rotate, limiting the propeller speed that can be developed, but achieving high airspeeds. This is like having a high gear in your automobile.

PITCH AND THE DIFFERENT FAMILIES OF PROPELLERS

As we saw above, pitch is a key element in the description of propellers (along with other factors such as diameter and blade area). When considering the four families of propellers it is useful to start with the simple fixed pitch propeller, and look at the enhancements in pitch control that are gained as we progress through each family to the most advanced, the constant speed propeller.

Fixed Pitch Propeller

With a fixed pitch propeller, the pitch of the propeller is fixed from manufacture. The performance of your aircraft is determined on the day your propeller is fitted, and is going to be limited within the constraints of the propeller. An analogy with an automobile is as though you had only one gear. Often when choosing a fixed pitch propeller for your aircraft, manufacturers give you a choice of either a climb or a cruise prop. A climb propeller has a relatively fine pitch and a cruise propeller has a relatively coarse pitch. This is like a car manufacturer giving you a choice of a low or a high gear. Either you will be really slow off the mark, or your engine is going to have to be red-lined to get anywhere at a reasonable speed.

Ground Adjustable Propeller

Many propellers manufactured and sold for ultra-light and experimental aircraft are ground adjustable. These propellers have the advantage of being able to have their pitch set before each flight if required, taking into account the type of flying you intend to do. More usually however they are used as a low cost way to try out various pitches and settle on the propeller pitch that best suits your aircraft and your style of flying. This can be compared to having a gearbox in your car that you can only change before you set out on your journey.

Variable Pitch

With a variable pitch propeller, you really have choices. To use the automobile analogy again, your car now has a real gearbox that you can change gear with on the go. (I hope that your car can do this at least!) In addition, rather than being limited to 4 or 5 gears, you can utilise any pitch along the continuum from maximum to minimum. The pitch of the propeller may be controlled in flight to provide improved performance in each phase of flight. Typically you would take-off in a fine pitch (low gear) allowing your engine to develop reasonable revs, before increasing the pitch (change up gears) as you accelerated to your cruising speed. YouÕll end up with the propeller at a relatively coarse pitch, (high gear) allowing the miles to pass beneath you at a rapid rate, while your engine is gently ticking over at a comfortable speed.

This feature of a variable pitch propeller will provide you with performance advantages, including:

• Reduced Take-Off Roll and Improved Climb Performance. Fine pitch allows the engine to reach maximum speed and hence maximum power at low air speeds. Vital for take-off, climb, and for a go-around on landing.
• Improved Fuel Efficiency and Greater Range. Coarse pitch allows the desired aircraft speed to be maintained with a lower throttle setting and slower propeller speed, so maintaining efficiency and improving range.
• Higher Top Speed. Coarse pitch will ensure your engine does not overspeed while the propeller absorbs high power, producing a higher top speed.
• Steeper Descent and Shorter Landing Roll. With a fine pitch and low throttle setting, a slow turning propeller is able to add to the aircraftÕs drag, so slowing the aircraft quicker on landing.

Variable pitch propellers actually come in a variety of versions. These different versions refer to the different ways that they are controlled, and include:

• Two-Position propeller.
• In Flight Adjustable propeller.
• Automatic propeller.
• Constant Speed propeller.

A couple of these are now of historic interest only, so lets concentrate on the two most common options these days; the in flight adjustable operation and the constant speed propeller.

The in flight adjustable propeller allows the pilot to directly vary the pitch of the propeller to the desired setting. Combined with the throttle control, this control allows a wide variety of power settings to be achieved. A range of airspeeds can be maintained while keeping the engine speed within limits. While rare in larger aircraft, the in flight adjustable propeller is the most common type of variable pitch propeller that is encountered in sport aviation.

When operated in manual mode, the Airmaster propeller is an example of an In Flight Adjustable propeller.

Constant Speed Propeller

The constant speed propeller is a special case of variable pitch, which is considered in a family of its own, and offers particular operating benefits.

With constant speed control, the pitch of the variable pitch propeller is changed automatically by a governor. After the pilot sets the desired engine/propeller speed with the propeller speed control, the governor acts to keep the propeller speed at the same value. If the governor detects the propeller speed increasing, it increases the pitch a little to bring the speed back within limits. If the governor detects the propeller speed decreasing, it decreases the pitch a little to bring the speed again back within limits. This operation may be compared to an automatic gearbox in an automobile, where the gears are changed automatically to keep the engine operating at a reasonable speed.

A constant speed propeller will automatically deliver you the advantages outlined above for variable pitch propellers, with almost no control required from the pilot. Once a propeller/engine speed is selected, the pilot is able to control the power purely with the throttle (actually controlling manifold pressure, which then determines power output) and the controller will act to keep the propeller/engine speed at the selected setting.

While allowing the pilot to ignore the propeller for most of the time, the pilot must still choose the most appropriate engine/propeller speed for the different phases of flight.

• Take-Off, Go Around and Landing. A high speed setting is used when maximum power is needed for a short time such as on take-off. The high speed setting may also be used to keep the propeller pitch low during approach and landing, to provide the desired drag and be ready for a go-around should it be required.
• Climb and High Speed Cruise. A medium speed setting is used when high power is needed on a continuous basis, such as during an extended climb, or high-speed cruise.
• Economic Cruise. A low speed setting is used for a comfortable cruise with a low engine speed. This operation produces low fuel consumption and longer range, while the advantages of low noise and low engine wear are also enjoyed.

When operated in automatic mode, the Airmaster propeller is an example of a Constant Speed propeller.

Special Pitch Modes

As well as the ability to vary the pitch of the propeller to optimise the aircraft performance, some variable pitch propellers have some other special modes of operation that can be very useful in certain circumstances:

• Feather. A feathering propeller can alter the pitch of the blades up to almost 90 degrees. That is, the blade pitch is changed so that they have their leading edge pointing right into the direction of flight, offering minimum resistance to the airflow. This mode allows the propeller rotation to be stopped, without adding excessive drag to the aircraft. Feather may be used to improve the performance of the aircraft after the failure of an engine, but more usually in light aircraft it is used in motor glider applications. Here the engine is used to gain altitude, before the engine is switched off, the propeller feathered, and then gliding flight commenced.
• Reverse. A reversing pitch propeller can alter the pitch of the blades to a negative angle. That is, the blade pitch is changed so that they have their leading edge pointing slightly opposite to the direction of flight. This mode allows reverse thrust to be developed by the propeller. In larger commuter and transport aircraft this feature is often used to slow the aircraft rapidly after landing, but in sport aircraft it is more usually used to enhance manoeuvring on the ground. A popular application is in seaplanes, where the ability to manoeuvre backwards, and sometimes to reduce the thrust to nothing, is especially useful.

SUMMARY

This overview was designed to assist the understanding of how the ability to control propeller pitch is used to categorise the different families of propeller design. More importantly it has illustrated that as we progress from one design family to another, we realise significant improvements in performance, effectiveness and efficiency.

While a family of propellers that offers better performance is likely to be more expensive to purchase, you can expect that over time the efficiency of a higher performance propeller will produce savings that will offset the initial cost. In addition your flying will be a more relaxed and enjoyable experience!

When deciding what type of propeller to buy for your aircraft, you have to weigh up the relative advantages and costs. To help, we can summarise the most common families of propellers, and make a simple comparison of their respective advantages in cost and capability:

CONCLUSION

If performance, economy and enjoyment are your goals, Constant Speed is the choice you should make.

Marcus Graney
Aeronautical Engineer
November 2000