Author Topic: MAN&RPM  (Read 3052 times)

Offline dtango

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Re: MAN&RPM
« Reply #15 on: July 10, 2010, 03:30:16 PM »
i ahve one guys saying i changes prop picth you saying its just fuel thing

The statements are all part of a whole.  Let's see if we can pull it all together here.

For constant-speed propellers another way of describing manifold pressure and RPM control is this:  manifold pressure controls the amount of power that is "available" to spin the prop while RPM/prop pitch controls the amount of power "required" to spin the prop.  The throttle (MAP) controls the amount of air that can be sucked in by the pistons and thus the amount of air available to the engine.  RPM controls the speed at which the crankshaft can spin and thus how fast the pistons are allowed to pump.  Along with fuel mixture (which is automatic in AH) these three controls determine the power output of the engine.

For a prop in constant speed operation there isn't a direct RPM control for the actual engine shaft.  RPM's are controlled via the propeller governor that senses how fast the shaft is spinning and then adjusts the propeller pitch that either creates more force or less force for the engine to oppose.  The force created by the prop varies with the amount of "lift" (thrust) that it is producing.  The higher the aoa of the propeller blade, the more lift created.  The more the lift, the greater the induced drag thus the more force needed to spin the prop.  The opposite is also true.  Using these forces the prop governor then maintains the engine RPM at the set speed by adjusting blade aoa/pitch as the shaft overspeeds or underspeeds the desired RPM.

What does have to do with aircraft performance?  It's related to the amount of thrust and power available to propel the airplane.  In flight, the efficiency of the propeller to convert engine power into usable power will vary by the propeller advance ratio, J.

J = airspeed / prop_rotational_speed * prop_diameter

For a fixed pitch propeller max efficiency of a propeller occurs at a specific value of J (specific airspeed & rpm).  Outside of these values the efficiency drops.  With constant speed propellers we're trying to maximize propeller efficiency over a range of airspeeds thus the maximum amount of power converted from the engine to flight.  The way this is done is setting the RPM constant and then automatically varying the propeller aoa with varying airspeed to maintain a J value that maximizes propeller efficiency.

How all this is related to fuel consumption is that the fuel consumption of the engine can be maximized by adjusting MAP (power available) and RPM (power required) so that power output of the engine is at a setting most economic for fuel consumption.

For a fixed pitch propeller, RPM is completely tied to MAP/throttle setting of the engine.  All AH aircraft are constant-speed props which means adjusting the RPM means adjustment of the blade pitch (aoa).

Hope that enlightens the topic!

Tango
« Last Edit: July 10, 2010, 03:32:51 PM by dtango »
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Offline olds442

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Re: MAN&RPM
« Reply #16 on: July 10, 2010, 03:54:09 PM »
The statements are all part of a whole.  Let's see if we can pull it all together here.

For constant-speed propellers another way of describing manifold pressure and RPM control is this:  manifold pressure controls the amount of power that is "available" to spin the prop while RPM/prop pitch controls the amount of power "required" to spin the prop.  The throttle (MAP) controls the amount of air that can be sucked in by the pistons and thus the amount of air available to the engine.  RPM controls the speed at which the crankshaft can spin and thus how fast the pistons are allowed to pump.  Along with fuel mixture (which is automatic in AH) these three controls determine the power output of the engine.

For a prop in constant speed operation there isn't a direct RPM control for the actual engine shaft.  RPM's are controlled via the propeller governor that senses how fast the shaft is spinning and then adjusts the propeller pitch that either creates more force or less force for the engine to oppose.  The force created by the prop varies with the amount of "lift" (thrust) that it is producing.  The higher the aoa of the propeller blade, the more lift created.  The more the lift, the greater the induced drag thus the more force needed to spin the prop.  The opposite is also true.  Using these forces the prop governor then maintains the engine RPM at the set speed by adjusting blade aoa/pitch as the shaft overspeeds or underspeeds the desired RPM.

What does have to do with aircraft performance?  It's related to the amount of thrust and power available to propel the airplane.  In flight, the efficiency of the propeller to convert engine power into usable power will vary by the propeller advance ratio, J.

J = airspeed / prop_rotational_speed * prop_diameter

For a fixed pitch propeller max efficiency of a propeller occurs at a specific value of J (specific airspeed & rpm).  Outside of these values the efficiency drops.  With constant speed propellers we're trying to maximize propeller efficiency over a range of airspeeds thus the maximum amount of power converted from the engine to flight.  The way this is done is setting the RPM constant and then automatically varying the propeller aoa with varying airspeed to maintain a J value that maximizes propeller efficiency.

How all this is related to fuel consumption is that the fuel consumption of the engine can be maximized by adjusting MAP (power available) and RPM (power required) so that power output of the engine is at a setting most economic for fuel consumption.

For a fixed pitch propeller, RPM is completely tied to MAP/throttle setting of the engine.  All AH aircraft are constant-speed props which means adjusting the RPM means adjustment of the blade pitch (aoa).

Hope that enlightens the topic!

Tango
this is what i was looking for thanks
only a moron would use Dolby positioning in a game.
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Offline morfiend

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Re: MAN&RPM
« Reply #17 on: July 10, 2010, 05:10:07 PM »
Nice writeup Tango :aok,but you forgot we have fixed props on the WW1 birds.

 Turn off the govener and watch the prop over spin the engine in a dive.

   :salute

Offline TequilaChaser

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Re: MAN&RPM
« Reply #18 on: July 10, 2010, 05:20:49 PM »
Manifold ( ie... Throttle ) increase :  use the  = key

Manifold ( ie... Throttle ) decrease:  use the  - key

RPM ( prop Pitch ) increase  : use the NumPad  + key

RPM ( prop Pitch ) decrease : use the NumPad  - key

unless you want to go in and custom keymap these features to your HOTAS system.......

also, hitting the P key ( WEP ) will over ride the RPM setting and put it back to Full RPM



Cha Ching! Then perhaps you can explain. ACP know's a crap ton of info about flight, help a aeronautical engineer in training out. 

I never meant to degrade anyones answer.

Oh and by the way,  a lil info about yourself may help in your sig. I have no idea that OldMan  has a HP endorsement...is he also a CFI? Dichotomy you can STFU since you are "just returning".



dud, very surprised to see that great post by you about 10 Things For Noob's to practice, then to read this thread and see your reply to Dichotomy

you might not of known about Oldman, but same can be said for ACP and what you say he knows.... he doesn't have it in his sig either   :headscratch:  just don't understand why the abbreviated comment.......

nice write up dtango  :cheers:
"When one considers just what they should say to a new pilot who is logging in Aces High, the mind becomes confused in the complex maze of info it is necessary for the new player to know. All of it is important; most of it vital; and all of it just too much for one brain to absorb in 1-2 lessons" TC

Offline dtango

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Re: MAN&RPM
« Reply #19 on: July 10, 2010, 05:21:14 PM »
Nice writeup Tango :aok,but you forgot we have fixed props on the WW1 birds.

 Turn off the govener and watch the prop over spin the engine in a dive.

   :salute

Very true.  I never fly in WW1 so it's out of sight, out of mind ;).

Tango
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Offline Big Rat

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Re: MAN&RPM
« Reply #20 on: July 10, 2010, 09:11:40 PM »
dtango,

Nice write up :aok  I was trying to put it in automotive terms since that's what I know best (doesn't transfer exactly), but you explained it great :D

 :salute
BigRat
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Offline FLS

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Re: MAN&RPM
« Reply #21 on: July 11, 2010, 05:44:42 AM »
Manifold pressure is changed by using the + - keys(default) on numbers pad.

                                                                       <S> Oz
                                         

I think you meant to say that KP +/-  adjusts RPM and throttle adjusts manifold pressure.

Offline ozrocker

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Re: MAN&RPM
« Reply #22 on: July 11, 2010, 06:21:56 AM »
Yes, my bad
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Offline FLS

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Re: MAN&RPM
« Reply #23 on: July 11, 2010, 08:08:29 AM »
Yes, my bad
 <S> Oz

I missed page 2 when I replied.  :D
I see TC already covered that.  :aok

Offline bj229r

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Re: MAN&RPM
« Reply #24 on: July 11, 2010, 10:02:58 AM »
Real planes would have 3 controls...manifold, rpm, and pitch. In AH, rpm's/pitch are tied together? Is that right?
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Offline Oldman731

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Re: MAN&RPM
« Reply #25 on: July 11, 2010, 10:04:39 AM »
Real planes would have 3 controls...manifold, rpm, and pitch. In AH, rpm's/pitch are tied together? Is that right?

That's right.

First I've heard that there might be separate controls for rpm and pitch on a constant speed prop.  How does that work?

- oldman

Offline olds442

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Re: MAN&RPM
« Reply #26 on: July 11, 2010, 10:44:01 AM »
Real planes would have 3 controls...manifold, rpm, and pitch. In AH, rpm's/pitch are tied together? Is that right?
you for got fuel mixture :neener:
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Offline Oldman731

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Re: MAN&RPM
« Reply #27 on: July 11, 2010, 12:27:52 PM »
you for got fuel mixture :neener:

Right.  From left to right, throttle (MAP), prop (RPM) and mixture (lean the sucker out).

- oldman

Offline olds442

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Re: MAN&RPM
« Reply #28 on: July 11, 2010, 04:29:18 PM »
just wondering why dont we have to change fuel mixtuer in AH it would be nice if this is added :pray
only a moron would use Dolby positioning in a game.
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Offline bj229r

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Re: MAN&RPM
« Reply #29 on: July 11, 2010, 04:53:44 PM »
just wondering why dont we have to change fuel mixtuer in AH it would be nice if this is added :pray
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