"rev-up, throttle back" principle,

[Dreamlinerwannabe]

Hi fellow aviators,

there is a principle about constant speed propeller, "rev-up, throttle back".

I googled all day long and find that besides my book, no one really explained online.

The reason is that you want to avoid over boosting the engine. May I ask if I don't follow this principle, how would I over boost it? And by selecting RPM, when do we mean increase pitch, when do we mean decrease pitch? how would I select the RPM then how would I move the throttle to avoid over boosting it, if I want to increase power or decrease power?

Thanks!

It makes me headache all day long!

[JustJoinedToSearch]

The phrase 'rev-up, throttle back' is just an easy way to remember the order you should increase/reduce power with a constant speed prop.

I.e. when increasing power 'rev up' meaning increase the pitch (revs) first before increasing the throttle (mp) and when reducing power reduce the throttle (throttle back) before reducing revs.

The phrase 'overboosting' generally is referring running a power setting that has the throttle too high and prop lever too low but realistically if you look at the cruise power settings tables from the engine manufacturers you will see 'oversquare' (i.e. >25MP/2500RPM) settings like 24 inches MP/2100 rpm etc as perfectly acceptable for a particular pressure altitude.

From what I have learned researching the matter/talking to engineers etc it is actually best for the engine to run the highest MP/lowest RPM combination approved by the manufacturer for the particular in flight conditions. This is more or less because you are running the actual engine components at a lower speed (albeit with increased torque demands due to the increased pitch).

Hope this has cleared it up.

[MX Trainer]

Dreamlinerwannabe


Didn't we do a good enough job with this thread????


http://www.pprune.org/tech-log/521484-paradox-constant-speed-propeller.html

[Another_CFI]

It is the same as your car. If you wish to accelerate (significantly) then you change to a lower gear before flooring the accelerator.

[snakepit]

Hi fellow aviators,

there is a principle about constant speed propeller, "rev-up, throttle back".

The reason is that you want to avoid over boosting the engine. May I ask if I don't follow this principle, how would I over boost it? And by selecting RPM, when do we mean increase pitch, when do we mean decrease pitch? how would I select the RPM then how would I move the throttle to avoid over boosting it, if I want to increase power or decrease power?

Thanks!

It makes me headache all day long!

You can over boost the engine by doing the opposite of 'REV UP, THROTTLE BACK" !
Normally in flight at a given TAS the propeller is maintaining the RPM selected by balancing engine TQ against propeller thrust and drag.

If you want to accelerate the aircraft to a higher TAS then the current propeller pitch will be too fine for the new TAS so you must select a new higher RPM first by "rev up" ("rev up" will initially reduce propeller pitch which is how the increased RPM is achieved) then follow with throttle to increase MP and thereby thrust (which can now be absorbed by the prop coarsening pitch but maintaining new RPM) with the correct RPM for the new TAS.
If when accelerating the aircraft you tried to increase throttle first and therefore MP you will likely find that the prop cannot absorb the resulting TQ due its inefficency at that TAS. I.E it cannot coarsen off enough at that TAS and RPM to achieve a RAF (Relative Air Flow) that would create sufficient propeller thrust to absorb the TQ. At this point I would imagine that you would be over boosting the engine leading to either detonation and more likely prop overspeed.

When decelerating to a lower TAS the opposite applies. In the current high TAS the propeller is maintaining the RPM selected by balancing engine TQ against propeller thrust and drag (at a relatively coarse pitch angle).
The correct method of "throttle back" first will result in less TQ and therefore a finer prop pitch to maintain current RPM by the prop governor. It will then be safe to reduce prop RPM (by coarsening prop pitch) to a value that will suit the new TAS. As the ac decelerates the prop pitch will fine off (RPM constant) and then have the range to increase as you throttle up again at the new speed.
If you were to "rev back" first the engine will still be producing the same TQ but the due to the high RAF the prop will again not be able to coarsen off enough to prevent overspeed and over boosting the engine, probably resulting in detonation.

I realise that you can only measure IAS but prop thrust is more correctly related to TAS so these effects will of course vary with altitude as well.

No wonder your head hurts. Mine does now as well