Thrust vs. Prop Pitch

[mattag08]

Hmm...Yeah I guess now that I think about it, in a controllable pitch design it would be possible to run a higher than useful RPM. Constant speed is so much nicer...

Would you agree there should be an RPM where you get best power though (so adjust blade angle to maintain that RPM throughout all phases of flight)?

[5-in-50]

It all comes down to the Lift : Drag, or also called, Thrust:Torque Ratio.

There is a particular ratio that offers the largest amount of Thrust (push forward) to the least amount of Torque (resistance/effort in turning the propeller).

Three things determine the T:T ratio. Aircraft forward speed, RPM and Blade Angle of Attack (the size of the 'bite', as mentioned in the previous post).

By increasing the RPM past the redline, you're setting a very small bite angle and causing the engine to work extra hard (more RPM) to actually produce less thrust (forward push), which is a poor T:T ratio.

For engine management (in the real world), it's better to operate at lower RPM settings because it means less engine wear and cooler operating temps.

[Madfish]

Lower RPM also means highest torque. Just because the engine spins fast doesn't mean it has the power to pull through load. In other words: once that initial inertia is gone the engine suffocates.

It's like with a car - most of them have the highest torque around 2400-3400RPM - beyond that you might have a spike but only for a brief moment and after that the engine loses power rapidly.

[TheEditor]

yeah but a transmission changes the dynamics. You saying prop pitch has the same effect as a tranmission?

I've been a pilot since 1976 and flown fixed, two-speed and constant speed props. You should always get best performance at max map and max allowable rpm.

[Blackdog_kt]

Sure, but unless the aircraft has a constant speed prop the max allowable RPM does not correspond to setting the pitch controls full forward and keeping them there.
For example, a 109 with the pitch set at the 11:30 or 12:00 (full fine) position will easily go above maximum RPM in case of a power on dive at somewhat high speeds

I guess it's little things like that, subtle but important details, which confuse most people along with the habits carried over from the previous IL2 series.

[Peril]

Sorry, miss read this top be constant speed prop.

[Crumpp]

You should always get best performance at max map and max allowable rpm.

Yes, Vmax is achieved with max manifold pressure and rpm.

[Sternjaeger II]

Originally Posted by zipper

I've been a pilot since 1976 and flown fixed, two-speed and constant speed props. You should always get best performance at max map and max allowable rpm.

that's true for constant speed propellers.