RPM Versus Throttle

[Kanth]

What is the difference between lowering your RPM and throttling back?

both  drop performance but does lowering the RPM conserve fuel?

Why would you use one over the other?


Different question entirely:

I need to manually trim my plane, I'm noticing that when I'm level and I pick up topspeed that my aircraft wants to climb.
If i autopilot level it'll still climb even after it settles.

This doesn't workout well when I'm being chased and need to keep all of the E that I can..holding my stick fwd can make my plane unstable which blows some E and can cost me the plane.

Can someone link me to a manual trim information type page so i can eliminate this?

DmdKanth

[ccvi]

I think reducing RPM is usually archived by increasing prop pitch, thus beeing able to fly at still high speed with less fuel consumption compared to reduced prop pitch (throttle) at high RPM. Just a guess though

I'd like to add a question about trim:

Is there a difference between pushing the stick forward or trimming down (e.g. more drag when pushing instead of using trim)? I think there shouldn't be a difference, because I don't believe it matters if the pilot pushs or the trim tab does.

[funkedup]

Kanth:  I have done a lot of testing and only the P-51 series shows any improvement in miles per gallon when reducing RPM.  For all other aircraft I have tested, best mpg is obtained at max RPM.

CCVI:  You are right.  No significant difference in drag.

[hitech]

No difference in AH.

In real life there might be a very very small  drag difference , would tend to be less drag with trim tab centered in the control surface and holding the stick.

[Pei]

With regards to rpm/prop-pitch:
In AH is there any point to reducing RPM?
Can you get a higher top speed by reducing RPM (at the cost of climb and acceleration performance)? Or does the lower rpm merely lead to lower all round performance (assuming it has no -effect on fule consumption in AH)?
Currently the only time I use the prop pitch control is during landing.

[Kanth]

whoop already answered, thanks answers =)

DmdKanth

[Turbot]

I didnt know we even *could* control prop pitch

any tips on how to use this feature to some advantage?

I have an extra slider on stick that I don't need because of throttle - be neat to be able to map prop pitch to the extra slider in JS setup.

[funkedup]

Pei - In my testing, reducing RPM benefits range only on the P-51.  I didn't test all the planes though.  And there might be some benefit in maximum endurance, like if you had to loiter on a CAP station or something.

One situation where RPM control can help you is in gliding with a dead engine.  If you set RPM to minimum it will reduce drag quite a bit on some planes.  First thing on my engine out checklist is to zero out the RPM.  I mapped the RPM control to the unused throttle wheel on my Fighterstick.

[AKWarp]

When you slide your throttle in AH, you are controlling the manifold pressure (the manifold of the hydraulic system that controls the pitch of the propeller).  In real world, this system has a governor in it that constantly tries to keep the engine RPM the same by varying the pitch of the blades and thus altering the load on the engine.  It's called a "constant speed propeller".  Th eintent is two-fold.  To maximize the efficiency of the propeller (and thus the performance of the aircraft) and to unburden the pilot with engine RPM management (to keep him from over revving the engine and blowing it).   The easiest way to think of this is that your engine is running 100% the entire time.  When you slide your throttle you vary the manifold pressure, which in turn varies pitch of the blades and that will vary the RPM of the engine depending on the load.  You can also adjust the power output (or RPM) of the engine directly and that's what everyone is talking about when they say "reducing the RPM".  

In actuality, your engine isn't straining itself 100% all the time without your intervention.  The actual power output varies right along with the manifold pressure, but it is controlled by the governor in the propeller sytsem and not by any direct "throttle" that you control in the normal sense.  It's not really a complicated thing, but it can be complicated to explain  

[Furious]

AKWarp, not sure where you got that info, but it is not correct as explained.

"Manifold pressure is an indication of engine power affected primarily by throttle setting. The instrument is calibrated in inches of mercury (Hg) and indicates the pressure in the induction air manifold."



F.

[Czpetr]

Originally posted by funkedup
First thing on my engine out checklist is to zero out the RPM.  

Can I set RPM to zero or do you mean it just to set it on lowest value?  On my RPM indicator it can be set only within 1 - 3 indicated value. Can I set 10? Or 0? Speaking about Spits.

czpetr

[Starbird]

Just put it at the lowest setting.

Some props in single engine aircraft will stop totally in flight, and some will still windmill. The slower the prop rotates the less drag it will produce giving you a bit longer glide times.

In multiengine aircraft a dead engine will automatically feather, stopping it totally so you dont need to worry about rpm control in them.

just remember to set the rpms back to full on your next sortie. Its not going to harm anything, but its a suprise when you need the entire runway to takeoff with.

[funkedup]

Warp you are close.  

Manifold pressure is the pressure of the charge (fuel-air mixture) in the intake manifold of the engine, downstream from the supercharger(s).  You control it with the throttle.

RPM is just the engine RPM.  The prop has a regulator (hydraulic or electric) which varies the blade pitch to maintain your desired RPM.

[EDO43]

Here we go again..................

Before I begin, let me state again, that I work on these engines/props as well as being a jet aircraft mechanic.  The museum is right down the street.  I've been doing this for over fifteen years now.  We will assume that the test aircraft has a supercharger, a turbosupercharger, and a constant speed propellor set in the constant speed range (ie, not at either stop, fwd or aft) and in level flight.

Having said that.......

As previously stated, Manifold pressure is an indication of how hard your powerplant is working (in general terms) The gauge in the cockpit is calibrated in inches of mercury (for US aircraft) and  directly indicates the pressure/vacuum in the intake manifold of the engine at any given moment.  The throttle in our test case scenario has absolutely nothing to do with engine RPM.  In our case, the throttle raises and lowers the manifold pressure for any given propellor setting.  The prop pitch lever sets the engine operating RPM.  This lever is connected to the prop governor by a cable or hard linkage.  For any given pitch lever setting, moving the throttle will cause the flyweights inside the prop governer to open or close the various oil ports in the governer to keep the engine RPM constant by either supplying high pressure oil to the prop (governor oil pressure 425 psi) to increase the prop pitch (overspeed condition; engine running too fast) or draining oil from the prop (engine oil pressure 60-90 psi) to decrease the prop pitch (flatter blade angle; underspeed condition; engine running too slow)  While the throttle has no effect on engine RPM, it does affect your airspeed.  Increasing the blade angle will make the prop take a bigger "bite" of air (theoretical pitch) an your airspeed will increase.  Conversely, lowering the blade angle will cause the prop to take less of a "bite" and will decrease your airspeed.  That is how a constant speed engine/prop combination works on real aircraft with hydromatic propellors.  Curtiss Electric hubs I have no experience with but in essence will perform the same things without the oil.

Remember, supercharging (gear driven) compacts the fuel-air mixture; turbosuperchargers (exhaust gas driven) only compress air.  Usually after running through a turbo, the air is routed through an intercooler to cool the air before adding the atomized fuel and heading for  the gear driven supercharger(s).

Feathering props?  simple........governor oil pressure is routed to the prop dome until the blades reach maximum travel, then the oil pressure is bypassed back to the engine........unfeather?  the unfeather pump builds governor oil pressure to 600 psi.  The distributor valve inside the prop shaft shifts, reversing the oil passages and the high pressure oil is routed to the front side of the prop dome, taking the blades out of feather.

That's how it works folks......

[funkedup]

WTG Edo