Manifold Pressure And Throttle Management

[CAP1]

ok.....here's probably what is an easy question to answer....but as i've not yet learned to fly complex(i do fly singel engine land in r/l)..but anyway......
in AH, my throttle controls only the manifold pressure..if i for some reason want to lower my rpm, then i need to use the + and - keys on the numeric pad.


so...would there be any advantage to switching them around? or possibly mapping them both onto the throttle control? if it can be done, how would i map 2 things to the throttle?
 i know this might sound stupid to some, but its been buggin me for awhile now, so it's time to ask.


thanks!!!!

john

[SgtPappy]

The bulk of your speed if you catchmy drift, is controlled by throttle. You could lower your RPM to decelerate even better, but I'd leave the throttle in hand.

From what I understand, RPM should decrease with throttle decrease, it only makes sense. I don't know why they don't show the change on the tachometer whenever you lower throttle.

[CAP1]

Originally posted by SgtPappy
The bulk of your speed if you catchmy drift, is controlled by throttle. You could lower your RPM to decelerate even better, but I'd leave the throttle in hand.

From what I understand, RPM should decrease with throttle decrease, it only makes sense. I don't know why they don't show the change on the tachometer whenever you lower throttle.

ya...thats why i'm asking this..i see my manifold pressure go down, when i reduce, but rpm stays the same...was wondering if it should be adjusted with the pressure....

[SkyGnome]

So here's the practical lowdown on RPM adjustment:

If you are trying to save fuel, lower RPM before manifold.  I've never found a situation where this wasn't equal to or better than lowering manifold, measuring in MPG at a given speed.

If your engine is dead, lower RPM to reduce the drag created by your prop.  This will allow you to glide a much longer distance.

Otherwise, having your prop at full RPM is generall beneficial.  It gives you the greatest power for accellerating, and also creates the most drag for when you want to slow down quickly (by reducing manifold using the throttle.)  Reducing RPM would _not_ help you slow down quicker - it's entirely the opposite.

One little tidbit is that you can just pop WEP on for a second to restore your prop to full RPM when you need to in a hurry, so there's no real need to map RPM keys to a controller.  (Only works at full throttle, when wep will actually turn on, though.)

[dtango]

Ah, engine management!  Yet another not so intuitive topic.  Directly to the question, keep your throttle mapped, well... to the throttle .  That's manifold pressure which controls the amount of air you let flow into the engine.

So what's the RPM there for?  Well for constant-speed props like the ones we have for the WW2 birds in AH RPM actually is a means of adjusting the pitch angle of the propeller blades.  Avoiding all the math that makes my head hurt, prop pitch is one of several key variables in determining how much thrust you can actually produce from the engine.  Prop pitch is a key factor in determining the efficiency of the propeller in converting engine horsepower to thrust horsepower.

Again skipping all the explanations why (and why RPM control is also separate from your throttle), here are a few key things that you want to use the +/- RPM settings for:
[list=1]

• Reduce your drag for improved glide ratio.  You know when you're out of assorted liquids, you know stuff like out of gas, out of oil, out of radiator fluid ...sometimes you have to glide with your engine out to get home.  For some of you getting home is overrated, I know!  For the others however, reducing your RPM to the lowest setting comes in REALLY handy because it reduces the propeller drag of your windmilling propeller with your engine dead.  This gives you an improved glide ratio for those glides home.
  
• Reduce your fuel consumption.  Yes, has to do with liquids again.  Ever pull up your E6B and you notice that for best cruise settings (most fuel efficient) shows an RPM less than the max setting?  Because you actually save fuel when you put the prop into a finer pitch setting so that the engine works less harder.
  

Tango, XO
412th FS Braunco Mustangs

[kvuo75]

manifold pressure is the air pressure in the intake manifold, "past" the throttle of the engine. so, at sea level, with the engine off, it will read around 29.92 inches of mercury (standard atmospheric air pressure).  a running engine at wide open throttle at sea level (assuming naturally aspirated -- not turbo/supercharged) it will read the same 29.92. any throttle setting below wide-open, the engine is "sucking" against the throttle valve, so the MP will be somewhere below 29.92. in our ah rides, i think pretty much they're all turbo/supercharged, so you will see MP wayyyyyy over 29.92


we use MP, because it shows how much air is going into the engine, a good indicator of how much POWER is being produced, since we cannot rely on prop/engine rpm since the prop is spinnning at the same rpm no matter how much power is actually being produced.


the prop rpm is basically controlled by a governor. powered by oil pressure fed into the hub of the prop where there is a piston, gears, etc. to change pitch of the blades. when you set rpm, you are controlling the governor, not the pitch of the blades directly.

you set prop rpm for a certain rpm, say 2000. At low power (manifold pressure), the prop needs to stay pretty fine to maintain 2000 rpm. the engine is doing 2000 rpm and has very little load upon it. as you add power, the prop and engine would quickly go over 2000 rpm being at such a fine pitch, so the prop automagically and smoothly goes to coarser pitch (bigger bite) to maintain the selected rpm. the engine is doing more work now at 2000 rpm.

the reason this is the way it is, is because props are really only efficient thru only a small rpm range.


i hope that makes sense.

[CAP1]

you guys are AWSOME!!!!!!!!!!!
all of you explained this VERY well, IMHO............i actually didn't realize that i could lower my prop rpm when i lose an engine though......i'm gonna try this when next i lose one of em in my 38.......wonder how much or if that'll help me in fight...seein as that by the time i lose one, i'm in no position to run:rofl

now the downside..........when i come up with more questions like this, i'm commin to you guys!!!!<<<>>>


thanks again!!!!

john

[Krusty]

The 38 auto-feathers if I recall. You don't have to worry about most multi-engine planes. They are usually designed to do this when an engine is turned off.

[LongCut]

If your a pilot, read the Airplane Flying Handbook chapter on complex A/C. There is a copy on the FAA website library (if you dont already have one). It does a good job or explaining the "how" and "why".