Propeller Operating Principles

Propeller Lecture Topics

• The lecture covers propeller operating principles, types, construction, and designation.

Operating Principles

• During takeoff, maximum engine power is needed, so the throttle and propeller control are fully advanced.
• In cruise flight, the throttle can be reduced for better fuel economy, and the propeller blade angle can be increased for higher propeller efficiency.
• On constant-speed propeller aircraft, engine power is controlled by the throttle and indicated by a manifold pressure gauge.
• The propeller blade angle is controlled by a propeller control lever, with engine RPM changes shown on the tachometer.
• Controlling both engine power and propeller angle allows for efficient combinations in various flight conditions.
• Once a specific RPM is selected for a constant-speed propeller, if the aircraft speed decreases enough, the propeller blades will contact the low pitch stop. Further decreases in airspeed will cause engine RPM to decrease, like a fixed-pitch propeller.
• With a constant-speed propeller, aircraft acceleration increases the propeller blade angle to maintain selected RPM until the high pitch stop is reached; RPM then increases.
• During cruising flight with a selected RPM, changes in airspeed or propeller load are compensated by corresponding propeller blade angle adjustments to maintain constant RPM.
• The range of possible blade angles for a constant-speed propeller is the propeller's constant-speed range between high and low pitch stops; within this range, constant engine RPM is maintained.

Constant-Speed Propellers

• Constant-speed propellers are also called variable-pitch or controllable-pitch propellers.
• Constant-speed propellers are the most common adjustable-pitch propeller type on aircraft today.
• The main advantage is the ability to convert a high percentage of engine power into thrust across a wide range of RPM and airspeed.
• Constant-speed propellers allow the operator to select the most efficient engine RPM for the conditions.
• A governor automatically adjusts the propeller blade angle to maintain the chosen RPM.

Multiple-Position Propellers

• Multiple-position propellers help maintain engine/propeller efficiency over a wide range of settings.
• During takeoff, the propeller blade angle is set low for maximum engine power.
• The blade angle is then increased slightly during climb for optimal climb performance.
• In cruise, the blade angle is increased further to achieve the best cruise performance.
• To achieve a higher pitch, the propeller control lever is moved aft. The selector valve releases oil pressure in the propeller hub.
• Centrifugal force acts on counterweights causing them to move outward and rotate the blades to their high-pitch position
• As the blades rotate, oil is forced out of the propeller cylinder and into the engine. The blades contact their high pitch stops in the counterweight assembly and stop rotating.
• Pitch stops on two-position propellers can be adjusted via a pitch stop adjusting nut.
• When low pitch is selected, engine oil pressure forces the cylinder forward moving the counterweight and blades to the low pitch position.
• Oil flow is managed by a three-way selector valve controlled from the cockpit.
• Piston outward movement is linked to counterweights which decreases the propeller blade angle.
• Blades remain in a position based on low pitch stop in the counterweight assembly and oil pressure.
• The Hamilton-Standard propeller includes a two-part hub and spider that holds the blades.
• Roller bearings allow the propeller blades to rotate between pitch stops.
• Counterweight brackets are installed at the base of each blade.
• Hydraulic and centrifugal forces change the blade angle within a Hamilton-Standard propeller.
• Decreased blade angles results from hydraulic force, and increased blade angles result from centrifugal force.
• Engine oil flows out of the crankshaft and acts on a piston assembly on the hub.
• Within the two-position Hamilton-Standard propeller hub, the spider is centrally located and consists of two or three arms where blades attach.
• Blades are made from aluminum alloy and have hollow ends that fit over spider arms. Counterweight brackets are attached to the base of each blade once inserted on the hub.
• The low-pitch setting is used for takeoff and climb, maximizing RPM and horsepower.
• The high-pitch setting is used for efficient high-speed flight at cruise, increasing fuel economy.
• Key components of a two-position propeller include the propeller hub, blades, and piston assembly.

Controllable-Pitch Propellers

• Controllable-pitch propellers have the advantage that the blade angle can be changed during operation, providing optimal performance.
• The Hamilton-Standard counterweight propeller became popular as one of the first controllable-pitch propellers. The propeller was developed in the 1930’s and permitted one of two positions: low or high pitch..

Adjustable-Pitch Propellers

• Adjustable-pitch propellers allow the aircraft operator to change the propeller blade angle.
• Blade angle settings maximize efficiency for a specific propeller/engine combination.
• Ground adjustments by technicians were required for some older adjustable-pitch propellers, but modern adjustable-pitch propellers allow pilots to change pitch during flight.

Ground-Adjustable Propellers

• Ground-adjustable propellers allow the blade angle to be changed only when the aircraft is on the ground and the engine is off.
• A ground adjustable propeller consists of two aluminum or steel halves that make a matched pair.
• Each hub half is machined to hold the shank of two propeller blades.

Propeller Designation

• Federal Aviation Regulations (FAR) require propellers to be identified with builder's name, model, serial number, type certificate number, and production certificate number.
• Most manufacturers stamp fixed-pitch propellers with the required information on the hub.
• A McCauley propeller model number such as 1A90/DM 7651 conveys specific information.
• "1A90" = basic design
• "DM" = crankshaft type, blade tip contour, specific aircraft installation info.
• "7651" = 76-inch diameter, 51-inch pitch at 75% station.

Propeller Balance

• Propellers are horizontally balanced via lead wool in holes near the boss.
• Vertical balance is obtained by adding balance weights to the side of the propeller hub.
• Once balanced, propellers are anodized and painted.
• New propellers are balanced to prevent excessive vibration.
• Horizontal balance adjusts for blade tip imbalances
• Vertical balance adjusts for imbalances in the blade's leading and trailing edges.

Wooden Propeller Construction

• Wooden propellers have been laminated to a propeller blank with at least five layers and waterproof resin.
• The layers are generally the same thickness and wood, however can be different.
• The reason laminated wood is used allows for greater protection from warping vs. a solid core made of wood.
• Once the layers of wood has been laminated, it forms the propeller blank.
• Then a metal bore is drilled through the bore and a metal hub is inserted.
• Some propellors use a cotton fabric glued in the range from 12 - 15 inches.
• The fabric helps to reinforce more thin sections.
• The fabric prevents deterioration from weater and suns rays.
• The wood is coated in clear varnish to protect wood.
• Some propellers will be coated in gray or black to prevent chipping. This is armor coating.
• Monel, brass, or stainless-steel is applied to leading edge to provide protection and prevent damages. These are then notched.
• Countersunk screws are used at locations where blades are thick. Copper rivets attach where blades are thin.
• They ensure a good hold by using solder to secure screws and rivets.
• To protect wood moisture, the hole near the tip allows for it to breath.

Propeller Materials

• Wood
• Steel
• Aluminum
• Composite

Wood for Propellers

• Offers flexibility and strength on small aircraft with low horsepower.
• Resists resonant vibrations because its the structure of wood is molecular.
• They must coated to be protected from damage from things like gravel.

Aluminum Alloy

• Thin allows to happen and is more efficient vs wood.
• Better sections closer to the hub for cooling the engine.
• Maintenance is less expensive vs wood.

Steel

• Used mostly on older transport aircraft.
• Sheets are normally consists and form rib and metal structure to attach. The center includes material to absorb from the structure.

Composite

• Increasingly used more in popularity.
• Durable and light.
• The part is Resilient and absorbs well.

Fixed-Pitch Propellers

• Fixed-pitch propellers - are limited to one blade angle.

Fixed-Pitch Classifications

• A typical fixed-pitch propeller on a light aircraft is 67-76 inches in diameter and 53-68 inches in pitch, based on aircraft manufacturer specifications.

Climb Propeller Classification

• A climb propeller - has a lower blade angle for the best performance in takeoff and climb.
• The engines maximum RPM is reached with airspeeds associated with climb-out. Once a aircraft reaches the top of its altitude and starts speeding up it ends up less effective at a lower blade angle.

Cruise Propeller Classification

• A cruise propeller - is a fixed pitch with a slightly higher blade angles.
• This type is efficient with a high cruising altitude. Because it has a higher pitch they perform less than optimal for takeoff.

Standard Propeller Classifications

• A standard propeller - Is a blend between a climb propeller and a cruise propeller.
• In normal circumstances each has their own performance depending on the aircraft it is installed.

Aircraft Performance and Propellers

• Aircraft frequently using short runways or high-altitude fields benefit from a climb propeller.
• Aircraft operating at sea level from long runways may use a cruise propeller.