Variable pitch propeller: principle, history and uses

A variable pitch propeller is a propeller whose blades pivot about their axis, which changes the pitch angle of all their sections by the same amount. A fixed-pitch propeller only gives its best efficiency at one operating point; by varying the pitch, the blade stays close to its best angle of attack when speed or load change. Aircraft gain at take-off as well as in cruise, boats when manoeuvring, and wind turbines find in it their main way of limiting their power.

On this page:

• Fixed pitch, variable pitch: what are we talking about?
• Why vary the pitch
• Aircraft propellers: the first solutions
• Aircraft propellers: pitch change mechanisms
• Governing: the constant-speed propeller
• Variable pitch boat propellers
• Wind and tidal turbines: pitch to regulate power

1: Fixed pitch, variable pitch: what are we talking about?

Before reading this page, a reminder of the difference between geometric and effective pitch may be useful.

In everyday language, "fixed pitch" means a propeller whose pitch angle cannot be changed: the blades are one with the hub. Its geometric pitch may be constant or vary along the blade, depending on the chosen twist.

"Variable pitch" means a propeller whose blades are each mounted on an axis. By pivoting a blade, the pitch angle of all its sections changes by the same amount; the twist itself does not change.

2: Why vary the pitch

A propeller turns the rotation of the engine into thrust, as a screw turns rotation into advance. But a screw bears on a rigid nut, whereas a propeller bears on a fluid that gives way. This "elasticity" is an advantage: the propeller can turn at a given rpm whatever the speed of the vehicle, from the static point to top speed. It comes at a price: part of the energy goes into the jet accelerated by the propeller, and that share varies with the operating point.

A fixed-pitch propeller is cut for a single operating point, defined by its rotational speed and the speed of the fluid. Elsewhere, its sections work far from their best angle of attack. The problem is the same as the gearbox problem of land vehicles: getting the highest possible power from the engine, with the best propulsor efficiency, over the whole operating range. One can act on the transmission between engine and propulsor, or on the propulsor itself. No wonder a whole generation of engineers set out to build the variable pitch propeller.

3: Aircraft propellers: the first solutions

In aviation, a gearbox is ruled out: it is too heavy, and the engine torque would have to be cut to change gear. What remains is to act on the propeller. Four levers exist to match the propeller to flight: blade area, section shape, section camber and pitch angle.

The early inventors tried each of these levers:

• Varying the area, either through the diameter (blades whose root screws in or out) or through the blade chord. In the United States, the Clarence Holman propeller carried a leading-edge slat, like the slotted wing of the Potez 36.
• Doubling the propeller. The French Salaün propeller mounted two similar propellers one behind the other on the same shaft. The front propeller slid along the shaft, so the gap between the two varied automatically in width.
• Changing the flow around the section. Around 1930, Bréguet built a propeller with hollow blades: holes drilled along the lower surface produced nozzle effects that altered the flow.

The slotted wing of the Potez 36: the same leading-edge slat principle was tried on the Holman propeller.

All these solutions proved hard to build. The ideal propeller would have a diameter, section, area and pitch all variable in flight. Variable pitch meets only one of these four conditions: that is why a propeller's efficiency remains lower at take-off than in cruise.

The idea of two propellers on one axis has not gone away: it returns today in unducted contra-rotating propeller projects.

4: Aircraft propellers: pitch change mechanisms

The solution that prevailed is rotating the blades about their axis. To get an idea of how complex such a system is:

Diagram of a hydraulically operated variable pitch system.

Cross-section of a Hamilton variable pitch propeller.

Variable pitch propellers are classified by four criteria:

Mechanical control takes its energy from the engine. A clutch, placed between the engine case and the blade control system, drives the pitch change through a reduction gear. It is operated by hand, or by electromagnetic or pneumatic couplers.

Electric control uses an electric motor, either mounted at the end of the shaft and rotating with the propeller, or fixed. A reduction gear turns its rotation into blade rotation. The energy source is independent of the aircraft engine, which is both an advantage and a drawback. Governing is easy to achieve, but the system is fragile, especially with a rotating motor, and a fast pitch change draws a lot of current.

Hydraulic control uses the oil of the engine lubrication circuit, with no other energy source. There are three variants: moving cylinder and fixed piston (Rotor), fixed cylinder and single-acting moving piston (Hamilton), fixed cylinder and double-acting moving piston (Hamilton). With a single-acting piston, the propeller usually has two pitches: oil pressure gives one, centrifugal force returns the propeller to the other. The double-acting piston makes feathering easy.

5: Governing: the constant-speed propeller

From the start, manual pitch control was seen as temporary: the pitch had to change automatically, continuously or almost instantly.

Each operating point has an optimum blade angle of attack. The governor therefore links the blade pitch angle to the engine rpm at all times. Classically, a centrifugal device acts through a relay on the pitch control: when the aircraft speeds up, rpm tends to rise and the governor increases the pitch; when it slows down, the governor reduces the pitch. The engine thus runs at constant rpm: this is the so-called "constant-speed" propeller.

6: Variable pitch boat propellers

Despite their complexity, variable pitch propellers offer real advantages to boats. There are many systems; we look at two families.

Self-pitching propellers are fitted to small craft and sailing boats. They derive from the work of Griffith (1849). The blades pivot freely about their axis: going ahead, lift and a slight offset bring them against a stop at their working pitch angle. When the shaft stops, they feather and drag less while the boat is under sail. Going astern, they come against a second stop, set for astern running.

Exploded view of a self-pitching boat propeller.

Both stop angles are set in advance and do not change under way. The angle of attack is therefore optimal only if the stop angle matches the boat's actual operating point.

Controllable pitch is fitted to large vessels. It brings several advantages:

• adjusting the pitch angle so the propeller gives its best efficiency for each load and speed;
• manoeuvring without changing engine rpm, and setting speed precisely in narrow passages or in a line of ships;
• going astern without reversing the shaft, with better efficiency than a fixed propeller turning backwards;
• feathering a stopped propeller on a multi-propeller ship, to limit its drag.

Controllable pitch ship propeller.

Controllable pitch has taken a large place in shipbuilding. Among ships above 2,000 bhp classed with Lloyd's Register, the share of controllable pitch propellers rose from about 5 % in 1960-1964 to about 35 % since the 1980s. It exceeds 60 % on passenger ships and ferries, tugs and fishing vessels.

Share of controllable pitch propellers among ships above 2,000 bhp classed with Lloyd's Register, 1960 to 2004, overall and by ship type.

7: Wind and tidal turbines: pitch to regulate power

On a wind turbine, variable pitch is first of all a way to limit power. Strong winds are rich in energy but rare; sizing the machine for them would be costly, because the price of some components rises quickly with the power transmitted. Wind turbines therefore reach their rated power well before the strongest winds, and shut down automatically at around 25 m/s. In between, wind power grows with the cube of wind speed: it must be limited. There are two main methods:

• Fixed pitch, stall regulated (stall control). The blades are rigidly mounted, and their geometry is designed to stall above rated wind: aerodynamic efficiency drops and power stays almost constant. This system is simple, hence reliable and cheaper, at the cost of slightly lower performance.
• Variable pitch (pitch control). The blades pivot about their long axis. When the wind becomes too strong, a mechanism increases the pitch to reduce lift, hence torque. More complex, this system regulates more finely and gives better efficiency. A control system works out the best blade position for the wind and drives the actuators.

Variable pitch brings further advantages:

• it reduces loads on the structure in strong wind, by turning the blades so they catch little wind;
• it eases start-up and ensures rated power is never exceeded;
• it protects the turbine against overspeed;
• it matches output to grid demand and compensates for changes in air density;
• feathering brakes the turbine aerodynamically and relieves the mechanical brake.

Wind turbine pitch change mechanism.

On large machines, the pitch angle is often driven by electric motors: either one motor per blade, synchronised with each other, or a single motor for all blades. Geared stepper motors drive pinions that mesh with a toothed ring fixed to the root of each blade.

Whatever the method, the system controls the pitch at all times:

1. at start-up, it gives the blades a pitch angle that helps them start turning;
2. between cut-in and rated wind, it keeps the machine at its maximum aerodynamic efficiency;
3. between rated and maximum wind, it increases the pitch to limit the power captured;
4. at maximum wind, it stops the turbine by feathering its blades.

The same principle applies to tidal turbines with pitchable blades.

To go further, we can come back to blade twist and section pitch angle, or see how the propeller rotation speed sets its operating point.