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I haven't been able to understand.

If propeller control controls the pitch of the blades in a constant speed engine, what do the throttle control do?

I've seen (in some normal operation manuals for some planes) that you have a Torque limit and control the Torque with the throttle control, and the RPM with the Propeller Control, but, if the propeller is spinning always at the same speed and the blades are positioned at the same pitch how can the throttle reduce or increase thrust?

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In a constant speed prop, the prop control lever basically changes the speed of the prop (it is not always running at the same speed as you indicate in your question, but rather once the speed is changed by the prop control lever, the governor keeps it at that speed). If power (or airspeed) is changed, the blade angle will automatically change in order to keep the prop turning at the same speed (within the limits that it is designed for).

Since adding power would normally increase the speed of the prop, the governor automatically changes the blade angle in order to keep it at the same speed. The same is true when power is reduced or airspeed is changed.

Thrust is changed by engine power because, even though the prop is turning at the same speed, the angle of attack of the prop has been changed, which changes the amount of thrust.

For more general information about how a constant speed prop works, see this question and answers:
How does a Constant Speed Propeller Work?

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First and foremost, speed of the propeller does not govern the thrust it is producing. It only limits it from above. The faster the propeller turns the more thrust it can provide, but it can produce less depending on power the engine supplies. With engine at idle, the propeller produces drag only and at higher rpm it produces more drag.

Next engine speed (and aircraft have fixed gear, so engine speed is proportional to propeller speed) does not govern the power it provides either, but also only limits it from above. The engine can only produce this amount of power per revolution, but if you restrict the fuel or fuel-air mixture flow into the engine, it will produce less. Consider coasting at high speed in a car with gear on. The engine turns at high rpm, because it is connected to the turning wheels, but with accelerator released it does not produce any power and the car is slowing down. The same happens in aircraft. At idle the propeller windmills by oncoming air and cranks the engine, but because that produces no power, the net effect is decelerating.

The throttle restricts flow of fuel-air mixture in the engine and is therefore the main control of engine power. "Throttle" really means the restriction flap in carburettor, so in diesel and turbine engines where fuel flow is controlled instead the lever is called a "power" or "thrust" lever.

So why don't you always run the propeller at the maximum rpm? Higher rpm increases wear on the engine. So usually high rpm is used for take-off where maximum power is needed, then rpm is reduced to recommended cruise value to reduce wear of the engine and for landing high rpm is selected again to increase drag of windmilling to slow down and to have full power available in case of go around.

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The throttle controls the engine power. In most engines (car, lawnmower etc.) this also controls the RPM. The prop control adjusts the pitch of the prop blades and thus the load on the engine, so the RPM stays the same (hence: "constant speed") but it will fall off speed if there isn't enough power.

And it's a "constant speed PROP", not a "constant speed engine"

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  • $\begingroup$ So, if I understood, you mean the the drag created by the blades being in a higher pitch makes the engine need more power (throttle) to keep the speed. Then, why you can have 50% of throttle or more with the same amount of RPM? $\endgroup$ – JuanBonnett Oct 27 '14 at 4:10
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    $\begingroup$ @JuanBonnett Let's say you have the throttle at 50% of the available power with the prop control set to 2500 rpm and that is giving you 130 knots of speed in level flight. Now you increase the throttle to 75% of power but you leave the prop control at 2500 rpm. The additional power will tend to make the prop speed up, but the mechanism will keep the rpm at 2500 by increasing the pitch of the prop, which will generate more thrust and the airplane will speed up. Hope this helps. $\endgroup$ – Terry Oct 27 '14 at 5:32
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    $\begingroup$ @Terry loved your explanation, it's simple and made me understand it! $\endgroup$ – JuanBonnett Oct 28 '14 at 17:02
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When you run up the engine before takeoff, you exercise the prop control. At a fixed throttle setting, move the control from "high rpm" to "low rpm". The prop blades will take a bigger "bite" on the air, and even though the engine is still producing about the same HP, the rpm will slow way down. It's a case of torque increasing due to the aero loading, and rpm decreasing because of the torque.

Remember: high rpm = low pitch, and low rpm = high pitch.

The Brits say it a little differently: high rpm = fine pitch, and low rpm = coarse pitch. (This is analogous to a fine or coarse pitch on a screw)

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