Prop Pitch vs. TAS

[uwochris]

Hey guys,

I have heard contradictory statements on the relation between your prop pitch (fine vs. coarse) and TAS, and I would like some of your opinions.

I always thought that with a finer setting (high RPM), you will have a higher fuel consumption, higher temperatures for CHT, and a higher TAS. Is this the case?

Also, can someone explain when and why prop efficiency is greatest- low, or high RPM?

I understand that with a coarse setting (low RPM), the prop's AOA is greater and thus, the props take a "bigger" bite out of the air, implying greater distance travelled per revolution (i.e. pitch). I always thought this implied a higher TAS, but this is contradictory to the first statement I made. Am I misinterpreting something?

Any help is greatly appreciated.

Chris.

 

[bobbysamd]

TAS v. power setting

Maybe the following mnemonic regarding airspeed derivation will help a little.

ICET, where

I = Indicated airspeed; what you're reading off the instrument;

C = Calibrated airspeed; indicated airspeed corrected for installation and positional error, you have correction tables in your POH that address CAS;

E = Equivalent airspeed; calibrated airspeed corrected for packing of air into the pitot, a non-factor for slower aircraft; and, finally,

T = True airspeed; equivalent airspeed corrected for altitude and temperature.

Nothing mentioned in here about power settings, because pitch + power = performance. It doesn't matter how you have the prop set as long as you have it set so that the engine puts out the power you want.

It is true that a lower pitch setting lets the prop take bigger bites of air. But bigger bites of air do not equate to higher TAS. Your power chart will show you that you can set the controls several ways to obtain the same power.

Hope this helps.

 

[Timebuilder]

I always thought about the prop control as a kind of continuous drive transmission.

When you need a lot of power at a low speed, you set for a low pitch/high RPM, sort of like first gear.

When you are cruise climbing, you might reduce that prop pitch/RPM setting a little (like second gear) and when you are setting up for your cruise at altitude, reduce a little more (third gear).

Most cars wouldn't try for 65 mph in first gear, would they? Even though they get the best power in First, they get the most effcient operation in their top gear.

Most power in fine, most cruise efficiency in coarse.

Does that help?