Author Topic: Using RPM (Read 206 times)

HomeBoy · « **on:** November 21, 2002, 12:25:49 PM »

I can't figure this out. What does the RPM setting on the planes in AH actually do? Does anyone use this feature? As best I can tell, maximum cruising speed is achieved when you run with maximum RPM. This is not the way it's supposed to work. Am I missing something? Please explain this to me.

On a real aircraft that has a constant speed (or variable blade pitch) prop, engine performance is controlled by the throttle (manifold pressure) and by adjusting the angle of the prop blade (RPM). To achieve maximum cruise speed you would of course run with more throttle and drop RPMs (i.e. increase blade pitch to get more "bite") to a certain setting (published by the aircraft manufacturer) to achieve maximum cruising speed. The plane I am familiar with is the Piper Malibu. The manufacturer's published max cruising speed is 220 KIAS @ 20,000 ft. This is achieved by setting throttle at 24 in-mg (max is 32) and 2100 RPM (max is 2500). [I'm sort of guessing at these numbers but they are close] Trying to reduce RPM below that crosses over into diminishing returns and throttling up more than that overstresses the engine.

I have tried to use the same logic while flying, say the P51D in AH. I make an attack run then extend out and level off at 12,000 ft. I check my airspeed and I'm doing 400 MPH let's say. I go full throttle, kick off WEP, and begin backing off on RPM searching for that optimal setting. It appears to me that I have to max RPM in order to get max speed. Is this correct? The RPM seems to behave like a throttle rather than a "gear shift".

I probably should have experimented more before posting but I'm hoping someone already has figured this out and can explain it to me.

Thanks for any thoughts

swoopy · « **Reply #1 on:** November 23, 2002, 08:33:09 AM »

I use RPM's to save fuel on long trips thats the only thing i use it for.

214thCavalier · « **Reply #2 on:** November 23, 2002, 09:45:59 AM »

1st the pipers max cruising speed is NOT a max speed its the speed and conditions to fly to balance fuel economy and speed.

2nd if your motor produces its best power at say 2,700 rpm and your flat out, now you adjust prop pitch making it coarser to get a bigger bite and thereby reducing engine speed you fall out of the best power range of the motor.
Less power = cannot maintain same speed but better economy due to slower rpm.

HomeBoy · « **Reply #3 on:** November 25, 2002, 09:21:55 AM »

Quote

Originally posted by 214thCavalier
1st the pipers max cruising speed is NOT a max speed its the speed and conditions to fly to balance fuel economy and speed.

Don't mean to argue with you here but this is just wrong. Maybe this makes some sense in a simulation but in real life, you will never reach maximum speed with the prop at the max setting. Think of trying to outrun someone in a car by staying in first gear! The principle is very similar.

My conclusion with AH is the constant speed prop modeling simply does not work and I have deprogrammed the buttons I had assigned to that function. Too bad as once you fly a plane with a constant speed prop you never want to go back. I find it disappointing that I can't control mixture either. I suppose simplifying these functions makes the game more fun though overall. It would be nice however to have an "auto" switch that allows you to do these things yourself or let "auto" do it.

funkedup · « **Reply #4 on:** November 25, 2002, 11:48:42 AM »

The only sure thing with RPM in AH is to set it to minimum when you are gliding with the engine out. This gives you the best glide ratio.

For fuel economy, P-51 and probably F6F have enhanced range with reduced RPM. But for the rest of the plane set the jury is still out.

So unless you are gliding or flying a Mustang or Hellcat a long distance, keep RPM maxed out.

HomeBoy · « **Reply #5 on:** November 25, 2002, 02:16:54 PM »

Thank you swoopy and funkedup. You make very good points. I can see that I was being a bit hasty in abandoning my RPM buttons. Since AH models feathering the prop on engine-out and fuel flow control through power management, I will certainly begin to use these features. That's good to know.

Thanks again.