Author Topic: Prop pitch and constant speed handling  (Read 1085 times)

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Offline Adrian Chitan

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Prop pitch and constant speed handling
« on: May 06, 2015, 10:19:33 AM »
Hi guys,

just a quick post on propeller pitch and the theory behind constant speed propellers (variable pitch).

Propeller pitch


The pitch of a propeller is defined as being the distance that the propeller (aircraft) will travel during one revolution if the blade followed a path extrapolated along the blade angle. To make this more intuitive I made a drawing.


As you can see in the drawing (sorry for the rather poor text positioning, Paint in Win XP sucks), the pitch is the movement of the propeller after one revolution with the propeller being in parallel air stream on the blades, where a is the blade angle. If you remember a little geometry and trigonometry, you will realise that the formula for the pitch is
pitch = tan(a)*cos(a)*2*Pi*r,
but because the blade angle is usually small (less than 16o), y can be approximated to 2*Pi*r (the tangent of the angle equals the angle in radians) and thus cos(a) ~ 1. So, a propeller's pitch is:
pitch = tan(a)*2*Pi*r.

I guess we've all seen that propellers are constructed with a twist from root to tip. The twist is usually done so that the pitch of the propeller is constant throughout the blade's length (the speed of the propeller varies from root to tip because the rpm remains the same but the radius gets larger).

Also, the pitch of a propeller can be translated into a speed if we know the blade angle and the rpm of that propeller. For example, a Cessna C180 has a propeller diameter of 2.08 meters (R = 1.04 m; r = 75% of R = 0.78 m). The rpm of the engine/propeller is 2'400 rpm while we'll consider a blade angle of 14o (don't know if this is normal for the C180 but it doesn't matter). The pitch of the propeller in this case is 1.22 meters. Because we have an rpm of 2'400, in one minute the propeller will have to travel a distance of 2'932 meters. This translates into a speed of 95 knots. If we go to a higher angle like 18o, the pitch is now 1.592 meters which translates into a propeller pitch airspeed of 124 knots. This can be called a propeller terminal velocity because this is the speed when the air stream will actually fall parallel to the blades.

What happens when the speed of the air around the propeller is smaller than this propeller pitch speed (slipping spropeller)? Well, the blades will bite the air and accelerate it backwards, creating thrust. If the air speed is larger than the propeller pitch speed, than the propeller will decelerate the air, acting as an aerodynamic brake.

As propeller pitch increases (blade angle increases), one can define the smaller angles as fine propeller and the higher angles as coarse propeller. A fine propeller is better suited for take-offs and climbs where you need the highest thrust at low aircraft speeds. Coarse propellers are better at higher speeds during cruise.

Fixed pitch propellers

When you're flying a fixed pitch propeller, you do not care about the angle of the blades because it is constant. But, that doesn't mean that the propeller manufacturer's also don't care. For example, a C172 has a fixed propeller. But there are multiple propeller manufacturers who create a plethora of propellers with different blade angles to fit different flying techniques. For example, if you use your C172 for training around an airport, you may equip it with a fine propeller to have the best performance for low speed flying. If you use your C172 for cross country flying during the weekend, you may buy a more coarse propeller to be optimized for those long cruise segments.

Unfortunately, the only add-on that lets you change fixed pitched propellers is the C172 from A2A simulation. BUT, if one knows X-Plane, through its plane maker module, it lets you modify every aspect of your plane including the pitch of the propeller. So you can make your own propeller, if you know what you're doing (increase the pitch of the default one for the C172 from Carenado, etc. :D). Thus one can access a propeller manufacturer's site, get some specs for a certain type of prop and put it in the sim.

Constant speed propellers

Constant speed propellers are variable pitch propellers. So the pilot can modify the angle of the blade to better suite the actual flight condition he/she's in. They are called constant speed because modifying the angle makes the propeller change speed of rotation. BUT, there is no constant speed propeller airplane that lets the pilot directly control the blade angle. The prop control is linked to a governor which moves a piston in the hub of the propeller that actually changes the blade angle, taking into account the throttle setting. The pilot really controls the speed of the prop/engine, but not the actual blade angle. And for different settings of the throttle there are different blade angles that can achieve the same prop rpm. But you can be sure that for a particular throttle setting, when you decrease the rpm the governor increases the blade angle. The purpose of the whole thing is to optimize fuel consumption (lower rpm = lower fuel consumption, higher cruise speed = less time = less fuel, etc.).

So how does it do it? Well, it uses a centrifugal flywheel. This is composed, usually, of two metal balls that spin at the same rpm the prop is. If the balls suddenly spin faster, the centrifugal force will pull them farther apart, while if the balls spin slower, there will be less centrifugal force and the gravitational force will get them closer. That is actually how the governor knows if the engine is over-speeding or under-speeding for a certain prop lever setting.

The other main part of the governor is a two way oil valve. When the two balls move apart, the valve is pulled/pushed to insert oil into the circuit and increase the blade angle (over-speed situation). When the balls come together, the valve is pulled/pushed to get oil out of the circuit and decrease the blade angle (under-speed situation). The oil is not a special oil circuit made specially for this feature. It is oil drawn directly from the engine, with the same temperature and viscosity.


The oil controlled by the flywheel and valve thus goes into a cylinder with a piston that actually moves the blades. The pressure of the oil is provided also by the engine.

As I said above, changing from a fine pitch to a coarser one actually increases the propeller's terminal velocity. The best comparison to this would be changing the gear in a car to a higher one. And that is also what pilots have to do in flight. As they move from take-off to cruise and back to landing, they have to change speeds. And just as in a car where you don't change to the fifth gear at 10 km/h but wait to reach the minimum idle speed of that certain gear or otherwise you could decelerate, you should change propeller pitch when the airspeed needle stops increasing (or at least close to that moment). The pilot even has the ability to engine-braking if they are flying fast and put the prop in a higher rpm setting (finer pitch, lower terminal velocity). But because a plane moves in air which is very dynamic, micro and macro, what setting worked today may not reach the same performance tomorrow.

The power problem

While the governor adjusts the rpm of the prop/engine, it is limited by the amount of power an engine produces at any one time. So, even if you set the rpm at max with the prop lever the engine still remains at a idle rpm because there isn't enough power produced to actually take even the fine pitch to 2'400 rpm. So a constant speed propeller lever works only if there is enough power coming from the engine.

Most constant speed prop planes have manifold pressure gauges that have green arcs painted on them. Usually, if you keep the engine throttle within the green band or above you will have a working and responsive prop speed governor.

Procedural usage

Because there is a lag between any prop lever or throttle lever change and the governor response (it takes time for the balls to move, oil to get out or enter, etc.). So there is a certain way to do stuff.

Normally you can manipulate the prop lever by its own without changing the throttle setting but usually you want to change prop and throttle to reach known and optimized settings. For example, when you move from take-off to climb or from climb to cruise you normally have to manipulate both levers to reach the published performance envelope. The rule is "throttle first", ALWAYS. Manipulating throttle first on all mode changes assures the engine against over torque which can dismember an engine and break a prop. The "throttle first" policy also protects the engine against stalling (to low an rpm with low throttle setting).

But, as in the case with any airplane engine (even on some older fighter planes), you have to do it slowly. Engines like constant regimes and fast disturbances can break or stall an engine. So be smooth on the controls.

Prop feathering

Prop feathering is a special setting of the blade angles where the angle is actually 90o (parallel to the airstream around the plane). This mode is also called "ground stop" configuration because the props create the most drag against their rotation, stopping them. Also, prop feathering is used when gliding on engine(s) out because it exercises the least drag on the forward movement of the plane so increases the gliding range. The prop pitch in this case is infinity so even if the rpm is 0, the terminal speed doesn't exist (= infinity).

Beta regime

The beta regime of a turbo-prop or conventional constant speed prop is the throttle regime of a engine-prop aggregate where the rpm is controlled by the throttle level and not the prop governor. In a conventional engine setting this is achieved in low power settings (bellow the green arc of the manifold pressure). In turbo-props, this is a special regime controlled by computers when the throttle levers are moved into the lower portions. It can sometimes be called "ground idle" or "beta reverse".

It should be noted that in the case of turbo-props the prop rpm cannot be the engine rpm, so that's why you have computers and/or reduction gears managing the prop rpm.

Prop reverse

The are some prop planes that actually employ reverse. The logic is simple, a prop spinning usually wants to take the plane at the terminal velocity so to stop a plane using the prop a 0 terminal velocity needs to be reached. This is done by moving the blade angles to 0 (0 pitch = 0 terminal velocity). While to increase the braking power it would be logical to put the blade angle into a negative setting, the forces on the prop would be against the normal way a prop is constructed and could break it. But there are some props that go negative and push air forward on landing.

No prop lever

There are some constant speed propeller planes that have no prop levers (i.e. JS-32). These are usually turbo-props that use a computer to maintain the highest possible prop speed while outside the beta regime. That doesn't mean that the prop pitch doesn't change. Because the pilot changes the output power of the engine, the computer adjusts the blade angle to maintain the set rpm.

Conclusion

So now you know (if you didn't already) about the tweaks behind a constant speed propeller. Hope you liked this and it will make flying more immersive.
« Last Edit: May 08, 2015, 06:43:01 AM by Adrian Chitan »

 

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