propellers


fig 6 - 4 cross section of a propeller. An a (alpha) denotes angle of attack of airfoil sections

Propellers may be classified as to whether the blade pitch is fixed or variable. The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise.

fixed pitch propellers

The propeller is made in one piece. Only one pitch setting is possible and is usually two blades propeller and is often made of wood or metal.

Wooden Propellers:
Wooden propellers were used almost exclusively on personal and business aircraft prior to World War II .A wood propeller is not cut from a solid block but is built up of a number of separate layers of carefully selected .any types of wood have been used in making propellers, but the most satisfactory are yellow birch, sugar maple, black cherry, and black walnut. The use of lamination of wood will reduce the tendency for propeller to warp. For standard one-piece wood propellers, from five to nine separate wood laminations about 3/4 in. thick are used.

Metal Propellers: During 1940, solid steel propellers were made for military use. Modern propellers are fabricated from high-strength , heat-treated, aluminium alloy by forging a single bar of aluminium alloy to the required shape. Metal propellers is now extensively used in the construction of propellers for all type of aircraft. The general appearance of the metal propeller is similar to the wood propeller, except that the sections are generally thinner.

variable pitch propellers

There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power.

The variable pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means.

Two-position: A propeller which can have its pitch changed from one position to one other angle by the pilot while in flight.

Controllable pitch: The pilot can change the pitch of the propeller in flight or while operating the engine by mean of a pitch changing mechanism that may be operated by hydraulically.

Constant speed propellers

In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed.

Constant-speed propellers may have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is wind-milling, a situation that can damage the engine and increase drag on the aircraft.

Some controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.

Beta Control: A propeller which allows the manual repositioning of the propeller blade angle beyond the normal low pitch stop. Used most often in taxiing, where thrust is manually controlled by adjusting blade angle with the power lever.

Propeller theory

The forces. Propeller blades are constructed using aerofoil sections to produce an aerodynamic force, in a similar manner to a wing. Consequently the blades are subject to the same aerodynamics induced drag, parasite drag, wingtip vortices, lift/drag ratios at varying aoa, pressure distribution changing with aoa etc. There is a difference in application because, in flight, the propeller has rotational velocity added to the translational [forward] velocity thus the flight path of any blade section is a spiral a helical flight path.

The diagram at left represents a blade section in flight and rotating around the shaft axis. Because of the different application it doesn't serve much purpose to express the resultant aerodynamic force as we would for a wing, with the components acting perpendicular (lift) and parallel (drag) to that flight path, as in the upper figure. So we represent the aerodynamic force component acting forward and aligned with the aircraft's longitudinal axis as the thrust force, and that component acting parallel to the direction of rotation as the propeller torque force.

As you see in the lower figure the component of the lift acting in the rotational plane has now been added to the drag to produce the 'propeller torque force' vector. The remaining forward acting portion of lift is then the thrust. That is why propeller efficiency is usually no greater than 80 85%, not all the lift can be used as thrust and the propeller torque force consumes quite a bit of the shaft horse power. The propeller torque and the engine torque will be in balance when the engine is operating at constant rpm in flight.

There are other forces acting on the blades during flight, turning moments that tend to twist the blades and centrifugal force for example. The air inflow at the face of the propeller disc also affects propeller dynamics.

Blade angle and pitch

Although all parts of the propeller, from the hub to the blade tips, have the same forward velocity, the rotational velocity and thus the helical path of any blade station will depend on its distance from the hub centre. Consequently, unless adjusted, the angle of attack, will vary along the length of the blade. Propellers operate most efficiently when the aoa at each blade station is consistent (and, for propeller efficiency, that giving the best lift drag ratio) over most of the blade, so a twist is built into the blades to achieve a more or less uniform aoa.

The blade angle is the angle the chord line of the aerofoil makes with the propeller's rotational plane and is expressed in degrees. Because of the twist the blade angle will vary throughout its length so normally the standard blade angle is measured at the blade station 75% of the distance from the hub centre to the blade tip. The angle between the aerofoil chord line and the helical flight path (the relative airflow) at the blade station is, of course, the angle of attack and the angle between the helical flight path and the rotational plane is the angle of advance or helix angle. The aoa and helix angle vary with rotational and forward velocity.

The basic dimensions of propellers for light aircraft are usually stated in the form of number of blades, diameter and pitch with the latter values given in inches. e.g. 3 blade 64" 38". The pitch referred to is the geometric pitch which is calculated, for any blade station but usually the 75% radius position, thus:

Geometric pitch = the circumference (2 π r) of the propeller disc at the blade station multiplied by the tangent of the blade angle. Thus it is the distance the propeller and aircraft would advance during one revolution of the propeller if the blade section followed a path extrapolated along the blade angle.
e.g. For a blade station 24 inches from the hub centre [0.75r] and a 14 blade angle, the circumference = 2 3.14 24 = 150 inches and tangent 14 = 0.25. Thus the geometric pitch is 150 0.25 = 38 inches. Propellers are usually designed so that all blade stations have much the same geometric pitch.

Designers may establish the ideal pitch of a propeller which is the theoretical advance per revolution which would cause the blade aerofoil to be at the zero lift aoa; thus it would generate no thrust and, ignoring drag, is the theoretical maximum achievable aircraft speed.

The velocity that the propeller imparts to the air flowing through its disc is the slipstream and slip used to be described as the difference between the velocity of the air behind the propeller ( i.e. accelerated by the propeller) and that of the aircraft. Nowadays slip has several interpretations, most being aerodynamically unsatisfactory, but you might consider it to be the difference, expressed as a percentage, between the ideal pitch and the advance per revolution when the the propeller is working at maximum efficiency in conversion of engine power to thrust power. Slip in itself is not a measure of propeller efficiency; as stated previously propeller efficiency is the ratio of the thrust power (thrust aircraft velocity) output to the engine power input.

Pitch and velocity

The performance of aircraft fitted with fixed pitch or ground adjustable propellers is very much dependent on the chosen blade angle. Fixed pitch propellers limit the rpm developed by the engine at low forward velocity, such as occurs during the take-off ground roll and may also allow the engine rpm to exceed red-line maximum when the load on the engine is reduced, such as occurs in a shallow dive. Fixed pitch propellers operate at best efficiency at one combination of shaft power and airspeed. Blade angle is usually chosen to produce maximum performance at a particular flight condition, for example:
        Vy climb i.e. a climb propeller
        Vc cruise i.e. a cruise propeller
        High speed.

The climb propeller is usually chosen when the aircraft normally operates from a restricted airfield or in high density altitude conditions. The climb propeller will produce maximum efficiency at full throttle around the best rate of climb airspeed and will perform fairly well at take-off, but during the initial take-off acceleration even the climb propeller may restrict the engine rpm to less than 75% power. The cruise propeller will achieve maximum efficiency at 75% power at airspeeds around the design cruising speed but aircraft take-off and climb performance will not be the optimum. The cruise propeller usually has a little more pitch than the standard propeller fitted to the aircraft. A high speed propeller might be fitted when the aircraft is intended to be operating at, or above, rated power for short periods in speed competition for example.

A constant speed propeller allows the engine to develop maximum rated power and rpm during the ground roll and to develop full power throughout its normal rpm range. With a constant speed propeller the pilot controls inlet manifold air pressure [MAP] with the throttle lever and the engine rpm with the rpm control lever or knob/switches. The pilot has several combinations of rpm/MAP to achieve a particular power setting. For example, in one particular aircraft, the recommended combinations for 65% power at sea level are 2100 rpm + 26 inches Hg MAP or 2200 + 25 inches or 2300 + 24 inches or 2400 + 23 inches. So you can use low rpm and high MAP or high rpm and low MAP to achieve exactly the same power output. The low rpm / high MAP combination probably gives more efficient cylinder charging and better combustion plus less friction. The high MAP also acts as a cushion in the cylinders, reducing engine stress. MAP is usually measured in inches of mercury [Hg] rather than hectopascals. Standard sea level barometric pressure is 29.92 inches Hg or 1013.2 hPa.

The windmilling propeller

The angle of attack of a fixed pitch propeller, and thus its thrust, depends on the forward speed of the aircraft and the rotational velocity. Following a non catastrophic engine failure the pilot tends to lower the nose so that forward airspeed is maintained while at the same time the rotational velocity of the engine/propeller is winding down. As the forward velocity remains more or less unchanged while the rotational velocity is decreasing the angle of attack must be continually decreasing and at some particular rpm the angle of attack will become negative to the point where the lift component becomes negative ([reverses) and the propeller autorotates, driving the engine. This acts as greatly increased aerodynamic drag which seriously affects the aircraft's L/D ratio and thus glide angles. The drag (including the negative lift) is much greater than that of a stationary propeller, also the engine rotation may cause additional mechanical problems if oil supply is affected.

If the forward speed is increased windmilling will increase, if forward speed is decreased windmilling will decrease, thus the windmilling might be stopped by temporarily reducing airspeed, probably to near stall, so that the negative lift is decreased to the point where internal engine friction will stop rotation. This is not something which should be attempted without ample height.

In the diagram the upper figure shows the forces associated with a section of a propeller blade operating normally. The lower figure shows the forces and the negative angle of attack (aoa) associated with the propeller now windmilling at the same forward velocity.
A variable pitch propeller may have a feathering facility which turns the blades to the minimum drag position (i.e. the blades are more or less aligned fore and aft) and halts windmilling when the engine is stopped.

The runaway propeller

As a propeller system increases in complexity then the possibilities for malfunction increase. A problem associated with constant speed propellers is governor failure during flight which, in most installations, will cause the propeller blades to default to a fine pitch limit. This greatly reduces the load on the power plant, and the engine will immediately overspeed, particularly if in a shallow dive. The rpm of an overspeeding engine sometimes referred to as a 'runaway prop' will quickly go way past red-line rpm and, unless immediate corrective action is taken, the engine is likely to self destruct and/or the propeller blades depart the hub due to the increased centrifugal force.

The corrective action is to immediately close the throttle and reduce to minimum flight speed by pulling the nose up. . Once everything is settled down fly slowly, consistent with the fine pitch setting, to a suitable airfield using minimum throttle movements. (The constant speed propeller fitted to a competition aerobatic aircraft usually defaults to the coarse pitch limit to prevent overspeeding but an immediate landing is required.)