Thrust vs. Prop Pitch

[mattag08]

I've noticed that the Axis aircraft with controllable pitch propellers seem to generate maximum airspeed (which means max thrust) at around 2400 RPM and full throttle. What doesn't sit well with me is when you increase the prop control to max RPM (even past redline on the tach) you actually LOSE airspeed.

This doesn't make sense from a physics standpoint. As someone else wrote succinctly:

The reason you always want maximum allowable RPM for takeoff: All else being equal, each combustion event is going to deliver a certain amount of power, which is generally limited by the amount of air in the cylinder. The more combustion events you can have, the more power you’ll be developing. (More revolutions per minute = more combustion events per minute). In a 4-cylinder engine, you have 2 combustion events per revolution, and in a 6-cylinder you have 3 combustion events per revolution. So, in terms of RPM, the higher RPM will always give you more power (again, all else being equal).

The amount of power we can develop is limited mainly by airflow, which relates to all three engine controls. We want the most air molecules we can possibly get, and the right fuel mixture to develop the most power. Throttle controls airflow by changing the pressure of the air in the intake manifold. Higher manifold pressure = more air = more power. RPM controls airflow by allowing each cylinder’s volume to be filled with air a certain number of times per minute. Higher RPM = more volume = more air = more power. Finally, the mixture control allows us to balance that air with the right amount of fuel to develop the most power. Too lean = Not enough fuel to develop power (air cannot make power on its own, of course). Too rich = the volume of fuel in the cylinder is too high to combust properly and is simply displacing some of that air we need.

So, for takeoff – Always the maximum allowable manifold pressure (full throttle in a normally aspirated airplane), maximum RPM, and best power mixture.

What's the beef here? Full throttle and full prop should always be more power than some other prop setting.

[adonys]

prop pitch controls the angle made my the propeller blades with the propeller's shaft/knob.

this angle is the angle of attack at witch the propellers are "biting" the airflow, and is controlling the projection area of the propeller blades' sufrace: the smaller the angle, the less "biting" (the projction area tends to 0), the closer to 90 degrees the more "biting" (the projction area tends to 100% of propeller's blade area).

the more "biting" the propeller blades has, the bigger the obtained propulsion force and therefore the bigger the force (the more the power) which is spent on actually moving the propeller shaft/ blades, which means that at a constant motor power, your RPM will be lowered.

now, the propulsion force is the inverce of the force with which the iar is pushed to the back of the airplane by its rotating propellers. this force = the force generated by a propeller's blade 360 degree rotation x RPM. Thing is, this force generated by a propeller's blade 360 degree rotation and the RPM are inversely proportional (when one rises, the other lowers), so you need to find the balance (the point at which you can get the max propulsion).

to complicate the whole thing, inertia comes into play too: rise it too quickly (the bigger the difference between the current/intended propulsion force given by the propeller blades angle, RPM) the engine can't assure that much power, and it drowns.

if you also add in it the altitude, engine power variations (WEP), the airplanes' control surfaces drag variation, etc.. you will give yourself a nice headache

strictly to your point: "when you increase the prop control to max RPM (even past redline on the tach) you actually LOSE airspeed" -> when you "increase" the prop pitch you're actually lowering the propeller blades angle to the propeller's shaft, which translates in less biting, less air pushed to the back, less propulsion force, less force need to rotate the propeller's shaft, higher RPM (ie the propulsion force generated by a propeller's shaft rotation x RPM is actually lower because the force decrease is higher thant the RPM increase).

[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.

[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.

[Peril]

Sorry, miss read this top be constant speed prop.