Author Topic: RPM Versus Throttle (Read 407 times)

AKWarp · « **Reply #15 on:** June 13, 2002, 04:17:54 PM »

Ok, so I was wrong on the manifold itself, but the basic operation of the system is correct. It is designed to maintain a specific engine RPM.

(and yes, I re-read my post and it didn't look right either....scratches head......)

Either way, when you take off in AH, look at your engine RPM, vary your throttle and watch what it does....it stays the same

agosling · « **Reply #16 on:** June 16, 2002, 04:46:25 AM »

The best way to see this effect is to cut your engine and go into a shallow dive. Leave RPM at default setting until speed settles down. Now reduce RPM with the keypad "-" key. Watch the speed go up. This is the difference in drag between the RPM settings. Increase RPM again, and you'll see the slight braking effect. Imagine what it is doing to the shape of the propeller. Fine pitch (max RPM) puts the propeller blades flatter against the direction of flight, exposing more surface area to the wind. Coarse pitch turns them so that there is less area exposed.

I use this regularly for engine out approaches to maximize glide time. I have also noticed a very slight increase in speed at altitude for the same manifold setting by reducing RPM.

Sparks · « **Reply #17 on:** June 17, 2002, 07:48:01 PM »

Funked - if what you say is true then does this suggest that HTC is dumbing down the flight models ??

I regularly reduce RPM by prop pitch and bring back MP for cruise - especially in the Tiffie as it's range is so short - am I actually wasting my time if only P51 models fuel consumption properly ??

I have the Mosquito pilots notes book and cruise performance in terms of fuel usage is drastically different at different RPM settings - if the FE isn't modelling it then I'll be disappointed.

Sparks

funkedup · « **Reply #18 on:** June 18, 2002, 03:24:48 PM »

Sparks my best guess is that HTC had plenty of time to get the P-51 fuel consumption to be close to the manual values, and after that they decided it wasn't worth the effort to do it for every plane. It's a huge P.I.T.A. to test it and I imagine it's an even bigger pain to tune the model, because there are so many variables.

I've only tested about a dozen planes though.
If you want to test planes you can just use this procedure:

Quote

1. Go to the help pages and look in the planes section to get internal fuel load in gallons.
2. Load the plane offline with 25% fuel.
3. Set fuel burn rate multiplier (in arena setup) to the minimum value. This means you will not burn a significant amount of fuel while climbing.
4. Takeoff and climb to the desired altitude and trim for level flight.
5. Set desired manifold pressure and allow airspeed to reach equilibrium value. This will take a few minutes. Record True and Indicated Airspeeds.
6. Change the fuel multiplier to 10.0 and simultaneously start a timer. The timing must be precise.
7. When the engine(s) die stop the timer and record the time.
8. Perform calculations as follows:
A. Gallons is the max internal fuel load divided by 4.
B. Hours is the timer value (in seconds) divided by 360. This corrects for the fuel multiplier and converts the result to hours.
C. Gallons per Hour is Gallons divided by Hours.
D. Miles per Gallon is True Airspeed divided by Gallons per Hour.

You can repeat the tests for various power settings to find the best endurance (Gallons per Hour or GPH) and range (Miles per Gallon or MPG) settings and altitudes. The range will be a little optimistic because the fuel load is so light, but this procedure will give you a good estimate of the best settings and altitudes.