View Full Version : Manifold pressure vs RPM
07-02-2005, 04:07 AM
I've noticed with some prop aircraft (especially WW2 fighters) some sources mention setting the manifold pressure to such-and-such and set the RPM to so-and-so. I thought the throttle was the only control to have any bearing on both of these but obviously I'm wrong.
Can the two be set seperately, and if so, how?
07-02-2005, 06:30 AM
Inititial takeoff power and prop settings will be full (max) power and RPM with throttle and prop control, or as per individual aircraft specs.
Documentation is needed for individual aircraft to know the specified envelope and limits for tweaking throttle/prop - Power/RPM settings.
In general, for many light aircraft with relatively small engines max power/throttle and Prop/RPM (full forward) is the setting
for takeoff. On climbout both MP and RPM are reduced to climb power
(i.e. 24.5"/2450 RPM for some engines).
At cruise altitude, both may be reduced again (i.e. 22.5"/2250 RPM).
Minor tweaks may be made with either within the specified limits of the individual aircraft, however the rule of thumb for all major increases or decreases is as follows:
For all major power reductions:
1. First reduce MP with throttle.
2. Secondly reduce RPM with Prop control.
(Note: for APPOACHES. prop control may be specified to remain at
max setting in the event of a go around on most light aircraft
when climb power may be expected. Obviously, the throttle (MP)
will be reduced during descent.
For all power increases:
1. First increase RPM with the Prop control.
2. Secondly increase MP with the throttle.
The reverse of this procedure outside of the power specifications may damage or reduce engine life severely.
Unfortunately, the result of reducing and increasing RPM (Prop Pitch Control) is not modeled correctly on almost all FS applications. Results will be exactly the reverse of real world implementation.
I am told there are exceptions but I am not sure which there are.
(Edit note: The above only applies to constant speed props. For fixed prop applications you are correct. Only the throttle is used and there is no individual prop/RPM control.)
07-02-2005, 10:14 AM
As always, your posts are clear, explicit and accurate.
You must have been a good instructor.
a fellow former CFII
07-02-2005, 10:56 AM
Thanks for the information which, as stated above, was very clear. I hadn't even considered using the prop RPM to effect the engine RPM. Most aircraft I fly (sim) with constant speed propellors are turbo props!
07-02-2005, 07:32 PM
Thanks loads Seamus for the kind words.
Something I loved dearly and miss terribly.
There is no fraternal group on earth quite like the aviation community both small and large.
One piece of advice: Don't get old. (Of course the alternative leaves something to be desired too.)
Maybe in the next age we will be provided with wings.
Thank goodness for flightsim and another of the best communities on earth.
07-02-2005, 07:48 PM
Glad to have been of any help MadCow.
Now the next step will be for you to have an engine fail on a twin and feather the prop. Real world, you will practice this many times in twin training. (I regret to say that I never flew anything with more than two engines).
Everything goes to the firewall (Throttles, Props, and Mixture)
One of the old cliches for identifying the failed engine is: Dead foot, dead engine. (Of course you want get this in FS).
Now the test question for any of you sharp youngsters out there who don't have senior moments (I sure envy you): What is the cliche pet word with the initials for shutdown of the failed engine (Leaned mixture, feathered prop, closed throttle)?
One adder that interested me way back when. I remember reading an accident report years ago of a twin turbo running away creating excessive power on one engine. The pilot identified the other good engine as having failed and shut it down, therefore creating an even greater problem. (Crash resulted).
07-02-2005, 07:49 PM
Since in a piston aircraft the prop and engine are directly connected (through gears on some aircraft), prop RPM and engine RPM change together. In fact there is only one RPM indicator per engine.
Turbo props, of course are different, and there are both connected and free running turboprops.
07-02-2005, 08:09 PM
"Don't get old"
That advice came too late, I'm afraid. I turn 74 next week.
07-02-2005, 09:12 PM
Good show Seamus:
You beat me by four years.
Happy birthday and many more to come.
07-03-2005, 06:57 AM
One thing that was profoundly :) pointed out to me by my flight instructor was to maintain MP on climbout or descent as the change of altitude air pressureeffects MP. Therefore roughly as I recall without adjustment you would lose one inch of MP per thousand feet as you climb out so you have to increase power to maintain MP. Of course the reverse is true on descent.
This is not as critical on turbo-charged pistons until you reach the upper altitudes of really low air pressures.
Your explanations are right on.
07-03-2005, 11:23 AM
Hey guys, just doing my instrument rating in a C182 (235 hp) and it's fun! I understand everything you guys have been talking about, except one thing. I understand when the engine is shut off, the MP gauge indicated pressure at the field. And I also know from physics that as air goes through the manifold it's velocity is increased, therefore pressure decreases and that's why our engine sometimes runs at 15", 22.3" MP, etc. But how come at full power (full MP) it shoes 25"? Is this because the reduction in pressure is approx. equal to crusing at 5,000 ft MSL? Likewise, how come if we reduce manifold pressure all the way it doesn't actually show the OPPOSITE, or higher pressure?
I hope my question is clear, thanks in advance!
07-03-2005, 12:18 PM
If you're only getting 25" in a C-182 at sea level under standard conditions, there's something wrong with the engine. It should show about 29". At 5,000 ft. MSL you'll typically see 23-24" on a standard day. In other words, there'll typically be an inch or so less on the MP gauge at full throttle than the ambient pressure.
And you don't mention where you are (field elevation) when seeing 25" at full throttle, nor the temperature, barometer, etc. And, do you also have the prop full forward?
07-04-2005, 03:34 AM
A little late, but I thought I'd throw this in.
One thing that can help to remember the relationship between prop RPM and MP is to think of the throttle (assuming its an aircraft with a variable pitch prop) like the gas pedal on a car with a standard transmission in that it just affects how much power the engine makes. The prop contol is like the transmission, it gives the ability to use the engines power to have lots pulling power (climb, takeoff, high RPM), or to go fast (cruise, low RPM
07-04-2005, 01:25 PM
Um, no this was at a field of elevation 246 ft MSL, air temp was around 26 deg. C that day, pressure was a bit higher, 29.98 in. Hg. And yes, as per the before taxi checklist props were full forward, mixture rich, etc. I guess I just imagined that when the engine slows down, so does the air going through the venturi, which would mean an increase in pressure, and vise-versa...
07-04-2005, 06:28 PM
So you were about 11º above standard with pressure slightly high, but elevation (and probably humidity) also above standard. And who knows how much the MP gauge is miscalibrated -- add it all together and...
07-05-2005, 09:55 AM
You do have to check the specs for the plane, as the original poster said some WWII planes (Spitfire for example) tell you to use a certain pressure, you would never ever use full manifold pressure in a Spit, you'd probably blow up the engine and at very least the supercharger.
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