Propellers Lesson

Plane of rotation (POR): Is the rotational velocity of the blade section.

Advance per revolution (APR): Is the distance the propeller travels forward in one rotation.

Chord line: Is an imaginary line from the leading edge to the trailing edge.

Angle of attack: Is the angle between relative airflow and chord line.

Helix angle: Is the angle between the plane of rotation and relative airflow. The direction of motion of a propeller is a helix and not a straight line, as is the case of an aircraft wing. Every section of the blade travels on a different helix. This means that the relative airflow approaches each section of the blade from a different position. Each blade section moves forward the same distance; however, the spiral distance travelled by each blade section is different and depends on its radius. Because the outer sections need to move through a greater distance in the same time, they will have a much greater displacement in the POR than inner sections near the propeller hub. Thus, it can be said the helix angle decreases as the radius increases.

An analogy may be driving two different screws an equal distance into a piece of wood for the same period of time. If both screws are of the same length, but both have different threads, this can only be achieved by turning the screw that has the smaller (and therefore more) thread faster than the screw with the larger (and less) thread. What this means is that the outer sections of the propeller have to spin faster than the inner sections.

Blade angle: Is the angle between the plane of rotation and chord line. When an aircraft is in flight, each section of the propeller blade will have the same forward velocity component. However, the rotational velocity component (movement in the POR) will not be the same along the length of the blade. The closer to the tip, the faster it will be moving. Because the speed of the blade increases towards the tip, the helix angle near the root is large and gradually decreases towards the tip. If the blade angle were the same along the length of the propeller, then the angle of attack would progressively increase towards the tip. This would give us a very inefficient propeller! For this reason, the blade angle decreases towards the tip of the blade (known as helical twist). This is to ensure that all sections of the blade meet the relative airflow at the same angle of attack (i.e., 4°). As a result, an efficient angle of attack is maintained along the length of the propeller blade, producing the most amount of thrust for minimum torque. We will explain this further now.

You may have noticed that propeller blades aren’t flat plates; they actually twist. There is a method to this madness, and this is to keep each section of the blade at the optimum angle of attack: 4°. This is best illustrated by example.

Let’s look at what happens when there is no twist at all on a propeller. (Refer to sections A, B, C, and D labelled below.) With each section of the blade being at the same blade angle (16°), the only section at the optimum angle of attack is Section A, with the other sections being at differing angles of attack. This means that Section A is the most efficient section of the blade.

Now, let’s look at a blade with twist that puts each section at differing angles. You can see that all sections of the blade are at the optimum angle of attack: 4°. This is the reason for the helical twist.

Although the angle of attack is the same the full length of the blade, thrust and torque are most efficient at 75% of blade diameter due to loss of propeller efficiency at the propeller root and tips.

Normal Flight

The aerodynamic force produced by setting the propeller blade at a small positive angle of attack, i.e., the total reaction (TR), may be resolved with respect to the direction of motion of the aircraft.

The component that acts 90° to the plane of rotation and is parallel to the flight path is thrust force. Thrust is positive, as it acts in the direction of motion.

The component that acts parallel to the plane of rotation is propeller torque force. Note that the propeller torque force is the resistance to the motion of the POR, i.e., it acts in the opposite direction as the POR. Therefore, torque is positive.

An effort must be made to overcome the propeller torque in order to obtain thrust, in the same way that an aircraft wing must overcome drag to obtain lift.

Increasing the throttle increases engine power, which increases engine torque, causing the propeller to rotate faster until a point is reached where propeller torque is equal to engine torque.

If the propeller suffers a loss of positive torque (i.e., if an engine failure is experienced in an aircraft fitted with a constant speed propeller), the CSU will ‘fine’ the pitch in an attempt to maintain the RPM selected at the time. The relative airflow will now meet the cambered side of the blade (front section) at a negative angle of attack, which is large enough to produce a TR in the opposite direction to normal. The propeller blade torque is now acting in the same direction as the propeller rotation (POR). Effectively, the air is now driving the propeller around, just like a windmill. This will drive the engine, even if no power is being produced. A windmilling propeller can result in an engine over-speed condition, leading to possible engine damage. The thrust component now also acts in the same direction as aircraft drag. It is commonly referred to as windmilling drag.

A windmilling propeller creates drag equivalent to the drag that would be created if a stationary disc of the same diameter as the propeller were attached onto the front of the engine.

If the propeller blade is turned through the fine pitch stop to a negative angle of attack of about −20° and power applied, reverse thrust is obtained. The blade section is working inefficiently ‘upside down’ similar to an inverted aerofoil. Mechanical devices are used to prevent application of power during the potential over-speed situation as the blade passes beyond the fine pitch stop until safely in the braking range (i.e., when the blade angle is likely to be small and negative as in the windmilling position). This is, of course, only for propeller-driven aircraft, generally turboprops only (jet-turbine aircraft have entirely different systems involving air flow redirection). When the propeller control is moved into the reverse position, it is known as beta mode and power has to be applied due to the huge increase in propeller torque with the blade in the negative position.

Propeller pitch is the distance or advance a propeller makes in a revolution.

Practical Pitch
This is what occurs in flight. It is the actual distance the propeller moves forward during one revolution in flight. The practical pitch depends on the forward speed of the aircraft and the propeller RPM, i.e., if the forward movement of the aircraft is 100 metres per second and the propeller is rotating at 1200 RPM, the advance per revolution is 5 metres.

Geometric Pitch
This is the distance a propeller would advance in one revolution if it were moving along a helix angle equal to its blade angle (i.e., 0° AoA). If the propeller blade is rotated at 0° AoA for one revolution, it will advance a certain distance. This geometric pitch is dictated by the geometric dimensions of the blade and not by its performance.

Experimental Mean Pitch (Ideal Pitch)
This is the distance the propeller blade would move forward in one revolution when it is giving no thrust. When the forward speed (and thus the advance per revolution) reaches a certain value, the thrust becomes zero. Here, the blade has reached the AoA for zero lift (−2°). This only exists in theory and testing.

Propeller Slip
This is the difference between the experimental mean pitch and practical pitch. It is the difference between what the propeller can achieve and what the propeller actually achieves. Propeller slip is a ratio of distances and is usually expressed as a percentage of the experimental mean pitch, i.e., if the propeller’s experimental mean pitch is 10 m and its practical pitch is 7 m, then the propeller slip is 3 m.

An easy way to think of slip is to compare winding a screw through a block of wood with winding a screw through a block of butter. Clearly, the latter would not advance as far through the medium in question. Some of the butter would slip by as the screw fails to ‘grip’ onto the medium and advance through it. Exactly the same situation exists when concerning air, which is anything but a perfectly dense medium. Early propellers were referred to as airscrews for reasons now obvious.

The efficiency of a propeller is the ratio of the useful work done by the propeller in moving the aircraft to the work done by the engine in turning the propeller (i.e., Work done = Force × Distance). Propeller efficiency is the ratio of the propeller power output to the propeller power input.

Most propellers are capable of giving 80% efficiency, with the best efficiency being obtained when slip is around 30%. Note that some slip is actually required in order to produce thrust. Propeller slip is a ratio of distances whereas propeller efficiency is a ratio of power. If the propeller moves forward at such a speed that it produces no thrust (i.e., operating at its experimental pitch), propeller efficiency is zero. Also, when the aircraft is stationary on the ground, propeller efficiency is again zero because although thrust may be developed, no useful work is being done. Between these two extremes, propeller efficiency rises to a peak value in the range covered by normal conditions of flight.

Propeller efficiency can be measured by the ratio of thrust force to torque forces for a given forward airspeed and RPM. This ratio is influenced by two main factors:

The efficiency of the blade sections
The angle of advance

The first consideration is for the propeller blade to have a high lift-to-drag (L/D) ratio; the L/D ratio depends on the angle of attack the blade is operating (ideal 4°). Remember, the AoA depends on the RPM and the forward speed of the aircraft. The highest efficiency is obtained when the angle of attack is close to that for the max. L/D ratio (4°). Fixed pitch propellers, therefore, reach their highest efficiency only under a particular set of conditions.

A propeller with a fine pitch is efficient at low speeds.
A propeller with a large pitch is most efficient at high forward speeds.

As discussed earlier, the highest propeller efficiency is achieved at approximately 75% of the propeller diameter. At this portion of the propeller blade, the most thrust is produced for the minimum torque. A propeller will suffer a loss of efficiency near the boss, caused by the low velocity compared with that of the tip. The airflow is affected by the engine cowling and the thicker aerofoil section that is required for strength, thus increasing drag and propeller torque, and decreasing thrust and, therefore, propeller efficiency. A propeller will suffer a loss of efficiency near the tip, caused by a tip vortex similar to a wing producing lift. This produces a drag (propeller torque), decreasing thrust and propeller efficiency. Also, when the blade tip is about to approach or exceed the speed of sound (i.e., go supersonic), compressibility problems arise, causing an increase in propeller torque. Again, this slows down the propeller and losses in thrust and efficiency result.

A fixed-pitch propeller with a fixed blade angle, travelling at different speeds at a constant RPM, will have a change in the angle of attack with a change in forward speed of the aircraft. As speed increases, the angle of attack decreases, and with it, thrust decreases. The effect on propeller efficiency is at a high forward airspeed; the angle of attack of the blade will be close to the zero lift angle (−2°), and thrust will reduce to zero. Therefore, propeller efficiency will be zero. There will be only one speed at which the blade is operating at its most efficient angle of attack and, thus, where maximum propeller efficiency will be obtained. At low speeds, thrust will increase as angle of attack increases. Provided that the blade is not stalled, the thrust is large but the speed is low, and thus propeller efficiency is low. However, the constant-speed propeller is still basically a fixed-pitch propeller at the fully fine or fully coarse settings.

Fixed Pitch Propeller

A fixed pitch propeller is a propeller that does not have an adjustable blade angle. It can only reach its peak efficiency under one set of circumstances. A fine pitch propeller is most efficient at low speeds (i.e., take-off and landing), and a coarse pitch propeller is most efficient at high forward speeds (i.e., in the cruise). Therefore, a compromise is reached where the blade angle is set in such a way that the aircraft has both acceptable take-off and cruise performance. Simple aircraft with relatively low horsepower are normally fitted with a fixed pitch propeller.

Ground Variable Pitch Propellers

Variable pitch propellers were developed to partially overcome the limited efficiency of fixed pitch propellers. The pitch setting on this type of propeller can only be adjusted while the aircraft is on the ground and while the engine is not operating. A pilot can select two pitch positions:

1) A fine pitch for low speeds (take-off and landings)
2) A coarse pitch for high speeds (cruise)

The pitch will remain set until the next ground adjustment can be made.

Constant Speed Propellers (CSU)

Constant-speed propellers are a wonderful invention. They automatically adjust the pitch of the blade to maintain a certain RPM. As with anything, there are drawbacks: they are heavier and more expensive. However, these factors are more than offset by the ease of operation and ability to absorb power and operate over a wide speed range. The three types of propellers can be likened to vehicles:

Fixed-pitch is like a car with only one gear.
Variable-pitch is like a car with a manual gearbox.
Constant-speed is like a car with an automatic gearbox.

A constant speed unit (CSU) or speed governor automatically adjusts the propeller blade angle to maintain a preselected engine RPM; therefore, a selected propeller speed controls a constant-speed propeller. The governor combines pilot demands with engine performance to keep blade angle at its most efficient. The CSU is principally the balance between two forces:

The pressure of the speeder spring (controlled by the pilot through the pitch lever)
Pressure from the flyweights, which move due to centrifugal force (controlled by engine RPM)

When an unbalanced situation occurs, the pilot valve moves to a neutral position to correct the blade angle. This allows oil to flow under pressure to the propeller hub or return to drain back to the sump. Take-off pitch is fine; blade angle increases automatically as speed increases to maintain constant RPM. During descent, the blades will coarsen.

On-Speed

If the engine is operating at the speed selected by the pilot, then the governor will not allow oil to flow to or from the pitch-changing cylinder. A volume of oil is trapped in the cylinder, and the pitch is held constant.

Under-Speed

If the engine RPM decreases, the flyweights in the CSU slow down and move inwards. The speeder spring forces the pilot valve down, allowing the pilot valve to move downwards. Governor oil under pressure will now flow and force the propeller cylinder forward. This movement, along with the centrifugal twisting moment, takes the blades towards the fine-pitch position and allows the RPM to increase back to the on-speed position.

Over-Speed

If the engine RPM increases, the flyweights in the CSU move outwards under the effects of centrifugal force. This action lifts the pilot valve. Oil pressure is then released from the propeller cylinder, which moves rearwards, allowing the blades to move to the coarse position and reduce the engine RPM. When shutting the engine down, the pitch should be selected to fully coarse; this will cause the cylinder to move back and cover the exposed portion of the piston. Starting the engine in this position will assist in preventing temporary starvation of the engine lubricating oil.

Reduction gearbox: The typical reduction gearbox is used to allow engines that need a higher RPM to run faster, but it also enables the propeller to maintain an RPM that is still efficient. If the engine were directly linked to the propeller, the propeller tip speed would be supersonic—especially with turboprop engines that turn at high speeds (10,000 to 15,000 RPM). Therefore, a reduction gearbox is installed to drop the propeller RPM down to manageable speeds.

Blade angle: With this type of prop, take-offs are made in the fully fine setting (i.e., engine redline). After take-off, power is usually reduced, and the prop pitch is increased by the pilot to give a climb RPM. After levelling off, the blades can be coarsened even more to utilise the engine power more efficiently, thereby giving a higher cruise speed.

IMPORTANT NOTE:

Carb icing causes a decrease in manifold pressure not RPM.

Application of carb heat will cause a further drop in manifold pressure and then a rise if ice was present.

PRIOR TO START:

Press 29.92 mm Hg
RPM 0000

START-UP:

Propeller control set to full fine – Throttle set for start – Start the engine

Press 12 mm Hg
RPM 1000

The engine is not developing enough power to drive the propeller blades to the full fine position; so, increase the power by opening the throttle.

PRE TAKE-OFF:

Press 14 mm Hg
RPM 1500

The aircraft is positioned at the take-off end of the runway.

In preparation for take-off, check that the propeller control lever is positioned in the fine or take-off range, as the throttle is opened until the desired manifold pressure is obtained.

ADVANCING THROTTLE:

Press 20–24 mm Hg
RPM 2700

By advancing the throttle fully, the engine instruments should indicate as above.

As the throttle is advanced, RPM will increase (just like a fixed pitch propeller) until the redline is achieved (around 20–24 inches). As the throttle is opened further, the blades will coarsen to absorb the extra power and to maintain maximum redline RPM.

TAKE-OFF (Full Throttle):

Press 28 mm Hg
RPM 2700

Notes on the Propeller Checks

The propeller check is performed to ensure propeller operation of the pitch control and change mechanism. During run-up, the propeller should be in the fully fine position.

Set a high power setting (usually 1700–2200 RPM, depending on propeller and governor type). The propeller lever should then be exercised to allow a 300 RPM drop. This allows warm oil to circulate through the governor and propeller hub, clearing out any sludge while at the same time lubricating the walls of the hub—this confirms the satisfactory operation of both.

A specific check of RPM and manifold pressure relationship should be made during the ground check. This test will measure the performance of the engine against an established standard. The calibration tests will have determined that the engine can deliver a given power at a given RPM and manifold pressure.

At about the 2000 RPM position and the propeller in the fully coarse position, the manifold pressure should be approximately equal to the atmospheric pressure.

Some installations call for a power reduction after take-off (manifold pressure reduction).

CLIMB

After take-off, the power output of the engine is reduced to climb power by decreasing the manifold pressure and reducing the RPM. By increasing the blade angle, a greater mass of air is thus handled and an increase in airspeed is achieved.

The torque horsepower is reduced to match the reduced power of the engine.

LEVELLING OFF

Manifold pressure is reduced first, then RPM to maintain square / under square situation, to prevent over-boosting the engine.

25 mm manifold pressure 2500 RPM: Square
20 mm manifold pressure 2500 RPM: Under square
28 mm manifold pressure 2700 RPM: Over square

CLIMB FROM STRAIGHT AND LEVEL

Advance RPM first then manifold pressure. (Prevents overboost.)

In a prolonged climb, the throttle needs to be advanced to maintain required manifold pressure, as air density decreases.

In a prolonged descent, the throttle needs to be retarded to maintain required manifold pressure, as air density increases.

Considerable stresses are placed on the propeller and the pitch-change mechanism (CSU) in flight. They are the:

Centrifugal Force
Centrifugal Twisting Moment (CTM)
Aerodynamic Twisting Moment (ATM)

Centrifugal Twisting Moment

On a rotating propeller, all portions of the blade are subject to a centrifugal reaction acting outwards. The centrifugal force can be resolved into two components:

A tensile stress or ‘blade tension’ acting in the plane of rotation parallel to the centre line of the blade (i.e., a vertical force). As the propeller rotates, the blade tension tries to pull the blade outwards.
Force acting in the plane of rotation but at right angles to blade tension (i.e., a horizontal force).

The CTM tends to fine the pitch and the pitch change mechanism must be able to overcome this tendency. A decrease in blade angle could result in a propeller over-speed condition.

An aerodynamic twisting moment (ATM) arises whenever the centre of pressure does not act through the pitch-change-axis. If the centre of pressure is ahead of the pitch-change-axis, this force tends to increase the blade angle. During normal flight, the ATM tends to coarsen the pitch and partially offsets the CTM.

When the propeller is windmilling (e.g., an aircraft in a steep dive), the propeller is at a negative angle of attack and is, therefore, producing negative lift. The ATM is reversed and acts in the same direction as the CTM (both attempting to fine the blade angle). The combined effect may cause the pitch change mechanism to operate in an over-speed condition (i.e., the propeller drives the engine).

A propeller is designed to absorb the power developed by the engine within the range of RPM and blade angles of attack necessary for efficient operation (i.e., conversion into thrust). If the propeller is inadequate in this respect, propeller torque balance will only be reached at excessive RPM or at inefficient angles of attack. If the engine’s power exceeds the resisting torque produced by the propeller, the propeller will race and both propeller and engine become inefficient. If an engine of greater power is fitted to an aircraft, then the ability of the propeller to absorb the extra power may be increased by:

Increasing Blade Angle and Angle of Attack

This is not efficient, since the propeller blade is no longer operating at its best angle of attack. It will, however, absorb the power.

Increasing Propeller RPM

This increases the speed of the propeller tip. Although the greatest power is developed at high engine RPM, if the propeller rotates at the same speed as the engine crankshaft, the tip speed of the blades is likely to approach the speed of sound. At such speeds, compressibility problems arise, causing an increase in propeller torque and decrease in thrust. Compressibility effects can be offset by propeller design:

Slight washout of the blade angle or curved propeller tips (tip sweep)
Changing the blade section near the tip to a thin laminar-flow type

A reduction gear between the engine and propeller so that the engine RPM can be kept high to produce maximum engine power while the propeller tip is kept below the speed of sound.

Increasing Blade Camber

A thicker blade section with increased camber, as with a wing, is less efficient at high speeds. Thus, at high speeds, a thinner blade has the best L/D ratio.

Increasing Blade Length (Propeller Diameter)

If the diameter of the propeller is increased, propeller efficiency will increase, particularly when a large fuselage or engine is positioned behind the propeller and causes a blanketing effect on the slipstream. Increasing the blade length means the tips of the blades travel near the speed of sound where compressibility effects cause efficiency losses. Factors affecting the diameter of the propeller:

Ground clearance
Efficiency of the propeller
Strength of the blades
The centrifugal turning moment
Aerodynamic turning moment
Airframe clearance
Propeller clearance (prop to prop)
Large fuselage behind the prop

Note: The propeller diameter should be small enough to avoid the necessity for long undercarriage (for ground clearance), and also small enough to facilitate installation of the engines (of a multi-engine aircraft) as close to the fuselage as possible.

Increasing the Area of the Blade (Solidity)

The most common method of increasing the power absorption of the propeller involves increasing solidity. Solidity is the ratio between that part of the propeller disc covered by the blades and the total area of the disc. Solidity is measured by:

Solidity can be increased by:

Increasing the chord of the blade: When the blade chord is increased, the aspect ratio is decreased, and as with a wing, efficiency is decreased. In addition, extra strain is put on the pitch change mechanism (CSU) by the CTM, an effect that increases with blade width, which tends to turn the blades to a fine pitch position. For a given solidity, the CTM is less for four blades than it is for three, and the CTM decreases as the number of blades increases.
Increasing the number of blades: The number of blades on a hub is usually limited to five, since any greater number causes aerodynamic interference between the blades, therefore decreasing efficiency. The greater the solidity, the greater the power that can be absorbed. It is preferable to increase solidity by increasing the number of blades due to the lower efficiency of wide blades. Where it is not practicable to increase the number of blades due to blade interference, the same effect of increasing solidity can be achieved by using counter-rotating propellers.

Discuss the following questions in class now.

What is the Helix Angle?
What is the Blade Angle?
Is the Blade Angle the same on the Propeller Blade or does it change? – justify answer
What is Helical Twist?
The Centrifugal Twisting Moment (CTM) makes the propeller twist towards FINE PITCH or COARSE PITCH?
The Aerodynamic Twisting Moment (ATM) makes the propeller twist towards FINE PITCH or COARSE PITCH
Draw the forces acting on a propeller in Normal Flight
Draw the forces acting on a propeller in Windmilling Flight
Draw the forces acting on a propeller in Reverse Thrust.
In Normal Flight; THRUST is POSITIVE / NEGATIVE and TORQUE is POSITIVE / NEGATIVE?
In Windmilling Flight; THRUST is POSITIVE / NEGATIVE and TORQUE is POSITIVE / NEGATIVE
In Reverse Thrust; THRUST is POSITIVE / NEGATIVE and TORQUE is POSITIVE / NEGATIVE
Describe a Feathered Propeller
What is Solidity?
List four factors causing Swing on Takeoff
Describe Asymmetric Blade Effect
What is the difference between EXPERIMENTAL MEAN PITCH and PRACTICAL PITCH?

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