Prop pitch & RPM Vs IAS theory questions?

[MadTommy]

Been having a look through the manual and basic cockpit orientation training and have been left with a few questions regarding prop pitch & RPM Vs IAS.

 

From the manual:

High RPM settings are used when maximum performance is desired, such as takeoff or combat, while lower RPM settings are used to maximize fuel economy and engine wear during less demanding flight stages.

 

 

(caveat: my fixed speed prop experience comes from Cliffs of Dover and associated reading regarding Spitfires & Hurricanes)

 

Documentation for DCS P51 indicates you want high RPM for maximum performance. To be used in combat & takeoff. This seems like a very simplified statement. I can't argue with the takeoff element, but the combat sections does not match, my possibly flawed, understanding.

 

I thought coarse prop pitch i.e low rpm (assuming level flight) gave maximum IAS. Due to reduced torque drag. And fine pitch gave lower IAS due to increased torque but better acceleration equating to better climb rates.

 

 

Now i'm used to controlling the pitch angle that in turn controls the RPM (in the Spitfire IIa), but if i understand it correctly in P51 you control the RPM which in turn controls the pitch angle.

 

Bottom line is I'm confused how you achieve the best IAS by controlling the RPM. Am i just confusing myself, and in fact low(ish) RPM gives best IAS and when the manual refers to 'best performance' it really mean highest torque, which to me is quite different.

 

I hope this makes sense to someone! :music_whistling:

[sam777777]

Best performance could mean fastest variation in range. I.e. the rate at which you can go from max drag/little thrust to maximum thrust from the propeller. I tend to agree that maximum speed would come from a coarser setting, but we really need an expert:thumbup:

[Yo-Yo]

The directions to use max rpm (3000) to achieve maximal power is not simplification. Generally, the power curve for the engine increases with rpm. At least for Merlin. Brake horse power, though, is not a power that drag a plane. The prop power depends not only on the brake or shaft power but on prop efficiency as well. I do not want to duplicate the curves of prop efficiency vs prop advance ratio (complex ratio representing the distance that blade tip travels parallel prop axis per 1 revolution - it is proprtional to TAS) - if you want you can find it yourself. Prop efficiency for a certain pitch increases proprtionally this ratio then reaches its maximum (75-85% usually) and then decreases very fast to zero and negative values.

THe more is the pitch the more is advance ratio for efficiency maximum.

 

Thus, the prop can be considered as a special kind of variable ratio gearbox for a car. At constant engine rpm it provides maximal or near maximal efficiency depending on TAS changing prop pitch to maintain its constant speed (rpm).

 

 

Another way to understand the constant speed prop is to remember that maximum airfoil efficiency is at certain AoA (usually 7-8 deg). If the prop has constant rpm (to have maximal shaft power) and TAS is increasing actual blade AoA decreases so it is necessary to increase pitch to maintain this optimal AoA.

Edited October 7, 2012 by Yo-Yo

[sobek]

I thought coarse prop pitch i.e low rpm (assuming level flight) gave maximum IAS. Due to reduced torque drag. And fine pitch gave lower IAS due to increased torque but better acceleration equating to better climb rates.

 

You need to draw a line between the first variable pitch props in which you controlled pitch directly and the RPM governor control in which you tell the governor what RPM you want.

 

 

Now i'm used to controlling the pitch angle that in turn controls the RPM (in the Spitfire IIa), but if i understand it correctly in P51 you control the RPM which in turn controls the pitch angle.

 

Correct.

 

Bottom line is I'm confused how you achieve the best IAS by controlling the RPM. Am i just confusing myself, and in fact low(ish) RPM gives best IAS and when the manual refers to 'best performance' it really mean highest torque, which to me is quite different.

 

You are approaching this from the wrong angle. Basically, the constant speed prop does it (almost, later designs even controlled RPM themselves, sea fury, e.g.) all for you. All you need to think about is, how much power do i want (of course weighing in other factors like engine life and specific fuel consumption).

 

If you want a lot of power, you need high RPM, because the nature of a piston engine simply dictates that it cannot produce a lot of power at low RPM. Don't worry about pitch, because the constant speed prop will always adjust pitch to give you the RPM you set, regardless of speed.

 

So the way you approach setting power in a plane with constant speed prop is sort of like regard the RPM and throttle as connected controls, they need to always be set with regard to each other.

 

In the end it all comes down to high power->high RPM and manifold; low power->low RPM and manifold; the relation is not linear, however, so use the typical power settings given as guidelines and do not exceed the manifold settings given by much, or you risk engine damage due to overboosting.

 

Edit: Another way to think about it is: What is the lowest RPM that i need to allow me to get the power from the engine that i currently need without wrecking it.

Edited October 7, 2012 by sobek

[MadTommy]

Thanks Yo-Yo & sobek.. both very helpful to understand the principles going on here. :thumbup:

[Ratfink]

Is there an optimum rpm for dogfighting?

[louisv]

Is there an optimum rpm for dogfighting?

 

I guess "Military" in table (first post)...3000 RPM, but not indefinitely.

[sobek]

Is there an optimum rpm for dogfighting?

 

Is there an optimum power setting for dogfighting? ;)

[louisv]

Is there an optimum power setting for dogfighting? ;)

 

read first post, look-up "military"...

[sobek]

read first post, look-up "military"...

 

Look up "rhetoric question". :D

[louisv]

Look up "rhetoric question". :D

Oh ! Sorry...

[Chill31]

If math suits your understanding a little better, here is a little more info...

 

piston engines are rated in horse power due to the difficulty in defining the thrust produced by the propeller. Easier comparison is made engine to engine by using HP.

 

Power is a rate of energy transfer. A rate of work being done.

 

Work is force times a distance. In the case of an engine, torque times RPM, which also includes a time element to yield power. This means that the most torque at the highest rpm gets the most power to the propeller for providing thrust at the highest rate (thrust itself is only a force and decreases with increases in airspeed in the case of prop driven planes).

 

So now you can see that the highest torque (manifold pressure) applied over a distance (one rotation of the prop) at the fastest rate (rotations per minute) will transfer the most energy to the air around the propeller disk and subsequently give the highest speed. Thats a basic version that gets the point across perhaps?

[Yo-Yo]

If math suits your understanding a little better, here is a little more info...

 

piston engines are rated in horse power due to the difficulty in defining the thrust produced by the propeller. Easier comparison is made engine to engine by using HP.

 

Power is a rate of energy transfer. A rate of work being done.

 

Work is force times a distance. In the case of an engine, torque times RPM, which also includes a time element to yield power. This means that the most torque at the highest rpm gets the most power to the propeller for providing thrust at the highest rate (thrust itself is only a force and decreases with increases in airspeed in the case of prop driven planes).

 

So now you can see that the highest torque (manifold pressure) applied over a distance (one rotation of the prop) at the fastest rate (rotations per minute) will transfer the most energy to the air around the propeller disk and subsequently give the highest speed. Thats a basic version that gets the point across perhaps?

That's absolutely right but leaves for somebody a question - what about climb or acceleration? :)

 

Considering a constant speed prop as a transformer for the engine power into thrust (better to say - in propulsion POWER = thrust * speed) that provides the best efficiency at any speed we can say that the key point is engine power.

This power (not torque!) is maximal (for the majority of engines) at the highest rpm they can bear for a certain time without significant decay.

The second point is that having highest possible rpm we can maintain required MP for the highest altitude.