Throttle and RPM's

[Christo]

Would someone explain the relationship between the Throttle and the RPM controls?

The Throttle Control adjusts the RPM's, so why have a separate control to lower or raise the RPM's?

Christo

[SPKmes]

one is for prop speed, the other is for prop pitch

[fuzeman]

Throttle is your main speed control device. Although it doesn't regulate prop speed per say RPMs  have an effect on prop drag and can increase range and lower fuel usage. Check the E6B on clipboard for power settings.

[Lusche]

Throttle does control manifold pressure. It's influence on RPMs is only indirectly. Example: In a P51D, you will have full RPMs (3000) at a wide range of throttle settings, from 67 MP down to about 30 MPs. Only when you go below that by further reducing throttle, RPMs will adjust.

RPM control is basically used in AH to give you more fuel economy at cruise speeds. See E6B on clipboard for various MP / RPM settings. Also reducing RPMs reduces your prop drag, important when you had to shut down engine (due to fuel starvation or damage) and are trying to glide back with your prop still spinning.


EDIT: slow snail...   

[PFactorDave]

You might google up "Constant Speed Prop", or maybe "Variable Speed Prop".  Those descriptions may (or may not) clarify things for you.

[dtango]

Would someone explain the relationship between the Throttle and the RPM controls?

The Throttle Control adjusts the RPM's, so why have a separate control to lower or raise the RPM's?

The throttle controls the amount of air that can be sucked in by the pistons and thus the amount of air available to the engine.  RPM controls the speed at which the crankshaft can spin and thus how fast the pistons are allowed to pump.  Along with fuel mixture (which is automatic in AH) these three controls determine the power output of the engine. 

Here's how throttle, RPM, and power are related:  The amount of fuel-air mixture in the piston determines how big of a bang we can combust in the piston.  Open the throttle, we're allowing more air to be sucked in by the pistons.  More fuel-air mixture, more bang in the piston.  More bang, more power.  But the suction of the air into the engine depends also on how fast pistons are pumping up and down which also is regulated by RPM.

So let's say we increase throttle but reduce RPM.  So we allow the engine to suck more air in but with the RPM control we have limited how fast the pistons can pump and thus limited the amount of air actually sucked in.  The end result is the engine produces less power.

Let's do the opposite: reduce throttle but increase RPM.  The pistons are now allowed to pump faster and suck more air in to make more power but now we've limited the amount of air the pistons can suck in because we've reduced the throttle.  Net result the engine produces less power and doesn't spin the crankshaft as fast as it can go.  In this case we've essentially starved the engine of oxygen.

The other dimension of all this is how the RPM is regulated for constant speed propellers is based upon the amount of power-required to turn the propeller which is varied by changing the blade pitch angle of the prop.  We can discuss the details of this another time.

Hope that helps!

Tango, XO
412th FS Braunco Mustangs

[Christo]

Thank You for all the great information.  Very helpful.

Christo