Author Topic: Setting correct propeller RPM/pitch  (Read 7940 times)

Offline 4X-NTY

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Setting correct propeller RPM/pitch
« on: January 05, 2010, 06:11:09 PM »
Hey all, I got a question i didn't find an answer for it.
I've started recently flying planes in the flight simulator that have variable propellor RPM or pitch,I don't know what it's really called, in israel it's called variable pace, if you still don't understand what I'm talking about,it's the thing that's controlled with the blue handle next to the throttle and the mixture handle.
So my question is-how do i set the right propeller RPM? I don't see any pattern like the mixture where the RPM rises as I get closer to the right mixture.
Nitay "Pitz" Ronen

Offline cotejy

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Re: Setting correct propeller RPM/pitch
« Reply #1 on: January 06, 2010, 06:01:32 AM »
Are you talking about constant speed props? If so, the idea is, the speed of props are controled with the blue thing. The throttle controls the manifold pressure. The blue thing control the speed of the props, the throttle controls the "force the engine is giving".

Use the blue control to setup the rpm you want. After this, the throttle won't change the rpm. The prop will just eat more or less air depending on how you want the engine to work.

Offline cotejy

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Re: Setting correct propeller RPM/pitch
« Reply #2 on: January 06, 2010, 06:07:40 AM »
I just read again your post and the last part about the right mixture... I'ts been a long time since I've sit in a constant speed prop airplane but my guess would be to look at fuel flow and manifold pressure for mixture. Will look for other's input on this.

Offline Fabo

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Re: Setting correct propeller RPM/pitch
« Reply #3 on: January 06, 2010, 04:44:30 PM »
Or regimes in the POH, as it would happen. Usually full for TO and LDG, take a bit for climb, take somewhat more for cruise. Works kinda like transmission in car, if you had one with continuous gears. More RPM - smaller gear.
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Offline 4X-NTY

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Re: Setting correct propeller RPM/pitch
« Reply #4 on: January 07, 2010, 04:51:11 PM »
Thanks guys.
Let me get this straight,highest RPM is somewhat like the first gear in a car,and as you decrease the RPM it's like going to second gear,third gear etc?
Nitay "Pitz" Ronen

Offline Fabo

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Re: Setting correct propeller RPM/pitch
« Reply #5 on: January 07, 2010, 07:40:26 PM »
Kinda.
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Offline cotejy

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Re: Setting correct propeller RPM/pitch
« Reply #6 on: January 07, 2010, 10:00:43 PM »
Exactly 4x-ntv. On TO, you want to gain speed as fast as possible so max rpm (smaller pitch) with full throttle will make it. But on this configuration, you will never get a good airspeed so you will eventually need to change the rpm for a better speed.

As when you enter the highway with a standard car and a short ramp. Keeping the third gear until 55 mph will make you go at the speed of the trafic quick but it would be stupid to continue crusing on third gear. You will eventually go on fifth gear to save on fuel and engine life.


Offline YawningMan

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Re: Setting correct propeller RPM/pitch
« Reply #7 on: January 22, 2010, 02:57:48 AM »
I learned about these in school. Naturally, that means I have no practical application with them; just book knowledge.

Having said that, Cotejy's first post is accurate for an actual airplane. At least with King Airs. I'm not sure how they work with the flight simulator. I would assume they programmed the King Air in the game to fly like the real King Air.

I'll be working on them soon enough, I'm sure.

Offline 4X-NTY

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Re: Setting correct propeller RPM/pitch
« Reply #8 on: January 22, 2010, 10:49:57 AM »
I don't know how it is with real planes,but in the sim when you change the RPM the plane is getting a little bump causing the nose to pitch up or down a bit and most of the times the speed is decreased by one or two knots and most of the times stays there,so I barely change the RPM in cruise,even if i feel that the setting isn't optimal.
And talking about Beechcrafts, I had a model of the B200 and the user guide said that you shouldn't mess with the power lever while cruising and I was pretty sure that they mean the throttle,does that means I'm supposed to control the speed with the RPM?
Nitay "Pitz" Ronen

Offline Baradium

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Re: Setting correct propeller RPM/pitch
« Reply #9 on: January 24, 2010, 02:55:45 AM »
On the beech 1900 we used 1700 (full) prop rpms on takeoff, then brought it back to 1550 for climb.   In cruise it was 1450.            torque was set initially at a minimum of 3,000 ft/lbs  on takeoff, it increases as you roll down the runway (not allowed to exceed 3400 ft/lbs).    We'd increase power to maintain torque until we got to 750 degrees C on the ITT and then take what we could get in the climb.   For cruise we'd run 730 on the ITT.

Prop rpms were to both be easier on the engines and reduce noise (book for the 1900C actually had you keep rpms at 1550 all the time after you pulled it back for climb for best power etc, but down to 1400 was authorized and 1450 was much quieter).    This is also fairly true with constant speed props on aircraft engines.  You don't want to lower rpms too much because then you're really lugging the engine down.  You also get your best POWER out of high rpms.   Full rpms will give you most power at any point.  You aren't setting a prop pitch, you are setting a governed speed on the engine, which is why the transmission analogy doesn't work.   The prop pitch will do what it needs to do to maintain that speed you set.  IE, it'll get more bite to lower it or go flatter to raise it.   That's the big advantage of constant speed props, you can have a flatter pitch on takeoff to get the engine straight into the power band for thrust, increasing pitch as you move to keep it there (and not overspeed) and then increase it more in cruise.  You can also keep a higher manifold pressure at a slightly lower rpms to maintain power output while reducing noise and engine RPMs.   Max prop rpms are usually paired to an engine to occur right around the torque peak of the engine (the max power it can supply you), but that's not neccessarily the best rpm to run an engine at for longevity or fuel economy.  However, high manifold pressure and too low rpms can also be detrimental. 
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Offline K^2

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Re: Setting correct propeller RPM/pitch
« Reply #10 on: September 21, 2010, 12:16:39 PM »
I don't know how it is with real planes,but in the sim when you change the RPM the plane is getting a little bump causing the nose to pitch up or down a bit and most of the times the speed is decreased by one or two knots and most of the times stays there,so I barely change the RPM in cruise,even if i feel that the setting isn't optimal.
And talking about Beechcrafts, I had a model of the B200 and the user guide said that you shouldn't mess with the power lever while cruising and I was pretty sure that they mean the throttle,does that means I'm supposed to control the speed with the RPM?
If the only thing you adjusted is the prop step, you will see a change in engine RPM, rate of climb, and a slight attitude change. Essentially the same response as adjustment of throttle, but less pronounced.

The air speed isn't supposed to change. Your aircraft is trimmed to maintain constant speed. (Tail produces negative lift. As air speed increases, it causes the plane to pitch up and slow down. If it slows down, heavier nose pulls the plane down, speeding it up.) If you want to change the air speed, you should change the attitude (nose down = faster) and then trim the plane to maintain that attitude once you're happy with it.

And if you aren't watching fuel consumption, it's not really worth paying attention to the prop step on a sim. Performance difference is small - nothing you can't correct for by giving it a bit more throttle - and you don't have to worry about straining the engine.

Offline chuckar101

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Re: Setting correct propeller RPM/pitch
« Reply #11 on: September 23, 2010, 12:01:17 AM »
Baradium expained it real well.  Just remember that the biggest thing a constant speend prop gives you is more efficiency throughout the entire flight regime.  Where as fixed pitch props have a set angle of attack that are designed either as a climb prop or cruise prop.  So full forward gives you max power while pulling it backs allows you to cruise more efficiently and quietly.  Smaller aircraft your not going to see huge changes in rpm and mp differences but you see a big difference in turboprops.  In the twin otter we take off full forward but in cruise we pull the props all the way back to the stops. So right before they go into feather.  The change in the noise level inside and outside the aircraft is significant.  Hope all these answers have helped.
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