[Ref. Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 5-28, 7-4]
Propeller consists of two or more blade(s) and a central hub. Each blade of an aircraft propeller is essentially a rotating wing.
3 facts about propeller:
- WHAT’s a Propeller? – It is a rotating airfoil – subject to induced drag, stalls, and other aerodynamic principles
- Function of Propeller? – It is powered by the engine and the rotation of propeller provides the necessary thrust to pull, or in some cases push, the aircraft through the air
- WHAT else? – It is twisted.
In what way (HOW can) a propeller creates thrust?
When propeller blade moves downward and forward, the angle at which this air (relative wind) strikes the propeller blade is its AOA. As the air flows past the propeller, the air deflection produced by this angle causes the dynamic pressure – the area of decreased pressure is in front of the propeller, and the force (thrust) is in a forward direction.
Actually, amount of thrust produced by the propeller depends on:
- AoA of the blade
- RPM of the engine (as well as propeller)
- shape of the propeller baldes
Notes: AoA of the blade ≠ Blade angle
WHY propeller is twisted?
The propeller itself is twisted which means the blade angle changes from hub to tip.
- Answer: to produce uniform lift from the hub to the tip.
– As the blade rotates, there is a difference in the actual speed of the various portions of the blade. In the same length of time, tip of the blade travels faster than the part near the hub (because the tip travels a greater distance than the hub)
p.S – Hub has the greatest angle of incidence (AoI) [i.e the highest pitch] while Tip has the smallest angle of incidence [i.e smallest pitch]
Pitch v.s Blade angle
- Angle of Incidence (AoI) of blade = pitch (≠ blade angle)
“Pitch” is the distance in inches, which the propeller would screw through the air in one revolution if there were no slippage
“Blade angle“, usually measured in degrees, is the angle between the chord of the blade and the plane of rotation
- However, pitch is largely determined by blade angle and so two terms are often used interchangeably – again, “Pitch” is not exactly the “blade angle”!
Types of propeller
- Fixed-Pitch Propeller
- Adjustable-Pitch Propeller
- Fixed-Pitch Propeller has fixed blade angle, and
- The rpm of propeller maybe vary during operation
- Pitch of a propeller is set by the manufacturer and cannot be changed – Climb (low pitch) v.s Cruise (high pitch)
- Climb propeller – low pitch, less drag, higher rpm, more horsepower capability
- Cruise propeller – high pitch, more drag, but lower rpm – fuel consumption reduced
Operation (control) of fixed-pitch propeller
- achieves the best efficiency only at (one) a given combination of airspeed AND rpm, the pitch setting is ideal for neither cruise nor climb
Engine RPM = Crankshaft RPM = Propeller RPM
- Should a propeller is mounted on a shaft, which is an extension of the engine crankshaft, the rpm of the propeller would be the same as the crankshaft rpm.
- Tachometer is the indicator of engine power *, it also represents the rpm of a propeller.
- having to say that, anything about real operation, please refer to AFM/POH!
- * When operating altitude increases, the tachometer may not show correct power output- or in other words, same rpm at different altitude may denote distinguish engine power output! – for more read #(3) Three Factors significantly Affecting Engine Performance – that post explained “why fly high less power?”
- Pitch of adjustable-pitch propeller, as its name tells, the pitch can be adjusted. In the past, the pitch was only able to be adjusted on the ground with the engine not running – cannot be adjusted in flight!
However, by the 1930s, new technology introduced and the pitch of modern constant-speed propellers are adjustable in flight.
- aircraft with a constant-speed propeller (i.e constant rpm in certain correct setting) AND blade angles could be changed within a range (by utilizing a ‘Constant-Speed Unit’ (CSU) or governor as the pitch-changing mechanism.)
– The propeller’s constant-speed range, defined by the high and low pitch stops, is the range of possible blade angles for a constant-speed propeller.
– As long as the propeller blade angle is within the constant-speed range and not against either pitch stop, a constant engine rpm is maintained.For example, once a specific rpm has been selected, if aircraft speed decreases enough to rotate the propeller blades until they contact the low pitch stop, any further decrease in airspeed will cause engine rpm to decrease the same way as if a fixed-pitch propeller were installed.
Significance of blade angle?
Thrust = (mass of air handled x slipstream velocity) – velocity of the aircraft
For propeller aircraft, thrust equals mass of air handled multiplied by slipstream velocity minus velocity of the aircraft. The power expended in producing thrust depends on the rate of air mass movement.
Lower blade angle → decrease mass of air handled per revolution → light load →Higher rpm (Take-off setting);
Higher blade angle → increase mass of air handled per revolution → high load →Lower rpm (Cruise mode);
Amount of air handled depends on the blade angle, which determines how big a “bite” of air the propeller takes. Thus, the blade angle is an excellent means of adjusting the load on the propeller to control the engine rpm.
During take-off, propeller tends to adjust to low blade angle. Although it decrease the mass of air handled per revolution, the slipstream velocity is quite significant during low airspeed. The higher rpm somehow can be helpful to create maximum thrust for climbing;
At cruising level is similar, propeller tends to adjust to higher blade angle, the lower rpm can reduce fuel consumption. Meanwhile, as the slipstream is reduced while increase mass of air handled per revolution can be more efficient.
AOA is small no matter during Take-off or Cruising level
One thing should be notice is that the AOA is small in the above two cases. During take-off, lower blade angle is definitely has a small AOA. Indeed, at cruising level, the higher blade angle can also have a small AoA, it is because the airspeed is increasing.
Advantage of adjustable-Pitch Propeller (constant-speed propeller)
- converts a high percentage of brake horsepower (BHP) into thrust horsepower (THP)
- constant-speed propeller is more efficient than other propellers because it allows selection of the most efficient engine rpm for the given conditions – more than one combination of airspeed AND rpm
Operation (Control) of constant-speed propeller
- constant-speed propeller has two controls: the throttle and the propeller control. The throttle controls power output, and the propeller control regulates engine rpm.
- Manifold Absolute Pressure (MAP) gauge measures the absolute pressure of the fuel-air mixture inside the intake manifold and is more correctly
- Cautions :
- When the engine is not running, the manifold pressure gauge indicates ambient air pressure [i.e., 29.92 inches mercury (29.92 “Hg)]. When the engine is started, the manifold pressure indication decreases to a value less than ambient pressure [i.e., idle at 12 “Hg].
- pilot can avoid conditions that overstress the cylinders:
- increase rpm first, then increase manifold pressure
– push forward propeller control before throttle
if operating of the engine with extremely high MAP (manifold pressures) in conjunction with low rpm, it could cause detonation
- power settings are being decreased, reduce manifold pressure first, then reduce rpm
– pull back throttle before propeller control;
- minimize operating time at maximum rpm and manifold pressure – most forward propeller control and throttle,
AND avoid operation at maximum rpm and low manifold pressure – most forward propeller control with aft throttle
- increase rpm first, then increase manifold pressure
#Fixed #-Pitch #Adjustable #-Pitch #fix #variable