View Full Version : propeller_moi

August 25th, 2011, 14:11
propeller_moi =

Does propeller MOI actually affect engine performance?
I need to reduce my prop MOI to fix a minor animation bug.
The prop spins too long after shutdown.
I am concerned this could affect the engine which has been meticulously tuned.
Does anyone have experience with this in FSX.
My hunch is the FSX piston engine doesn't go so far as to consider prop mass.

I will test this and post back too ;)

August 25th, 2011, 14:41
The main issue that will affect your prop spin down rate is in the air file, Item 509, which affects internal friction vrs RPM. Where this value crosses the 0 rpm origin will determine whether or not the engine stops at all! Note that SP2 and Accel calculate this differently. I have had perfectly behaved props in Accel continue to spin in SP2. If you increase the Low RPM friction, it might be necessary to incresae the starter tourque value in the cfg file.

Cheers: T

August 25th, 2011, 16:05
Thanks T, this helps...
...the thing is, I want to avoid adjusting the airfile - at this late stage.

Prop MOI (.cfg) also affects how the prop stops.
But I'm not sure if it will affect the airfile (- the piston engine performance).

If it does not - then I will fix the bug now.
If it does - then I won't, the minor bug can stand until the airfile can be properly revisited.

BTW, I do think my Prop MOI is possibly high (by comparing configs).
I made this setting in the early days..before all the airfile work.

August 26th, 2011, 02:25
No, it doesn't affect the performance. I normally use lower values than 'usual' to get a more realistic response if you e.g. slam the throttle from idle to full.
The lower the MOI the faster the prop RPM increases with fast throttle changes.
In many planes the 'lag' is almost jet engine like.
Of course it's good practise to advance the throttle slowly and the prop will less likely overspeed as the prop governor doesn't need to catch up, but if you need the power now , the response should be realistic.

August 26th, 2011, 04:30
You probably don't want to change this value once you have your flight model tuned for handling.

- Ivan.

August 26th, 2011, 05:30
but he can compensate the lower torque etc in the [flight tuning] section

August 26th, 2011, 10:28

Thanks for the replies and help.
For piece of mind, I looked at how a change would affect the engine(using AFSD).
This is the engine responding to full throttle. (x=seconds)
I think it confirms what you guys have already said.
MOI changes the lag time to get to full power.
But it doesn’t seem to affect the final power.
So I think a change here won’t mean retesting the engine at all different alts’ and settings.
If MOI affects yaw I guess I’m less concerned...that will get tuned more yet.


The value of 8 (actually 8.518) was calculated by formula.
And I think it’s high, because a lot of weight is in the hub.
I don’t remember how I solved that problem...but maybe not well ;)
I have a note that says, “HC-C2YK-1BF/F7666A-4 72' 51.0+1.2lb”

I'm guessing it could be 6 or 7 for a 72' constant-speed prop?

But I see now why T recommends using the engine friction to adjust this.
The MOI comes from data and a formula...easier to produce with some confidence - not much mind you;).
And then the table-509 can get that low RPM friction just right - without influencing much else.


August 26th, 2011, 13:23
You may be able to compensate for TORQUE effects but I think Gyroscopic Procession is a bit harder.

- Ivan.

August 26th, 2011, 23:40
One, even eight, are pretty low MOI values! Perhaps suitable for a LSA?

Cheers: T

August 27th, 2011, 08:02
1 was used just for contrast...it’s not really a practical value.
But 8 is, from what I understand.
It's a 72' Hartzell constant speed...it's a typical GA prop for say a 4-cylinder 0-360.

I did find the formula I used...
moi rod: Jr=2/3m r^2
moi disc: Jd=1/2m r^2
moi avrg: J=(Jr+Jd)/2

I couldn’t find a published value at the time (i'm sure one's around).
But I figured the formula was close enough for FS.
It just doesn’t account for the hub being the heaviest of the parts.
So the final value should be less than the estimate…

T are you using a different method?

August 28th, 2011, 05:54
You may be able to compensate for TORQUE effects but I think Gyroscopic Procession is a bit harder.

Not really as you can tune that in the cfg file as well.
gyro_precession_on_yaw etc...

August 29th, 2011, 14:37
In most such GA applications the torque is pretty minor, compared to say a Warbird. Surprised that's all it weighs as those Const speed hubs are pretty heavy. I would guess that the Borer prop on my cub weighs almost that much.

Your calcs are in the ball park, but the MOI is probably not as important as the Per Cyl friction curve as far as the spin down is concerned, though a bit messier to deal with.

Cheers: T

August 30th, 2011, 13:22
Hi BStolle,

I am used to playing in CFS1 which doesn't have a CFG file that is tunable.

- Ivan.

August 31st, 2011, 14:24
Thanks, again for the conversation ... great information.

August 31st, 2011, 22:28

As you are discovering, much in FSX can be approached from more than one direction. There are ups and downs to the various paths. The reason I prefer andjusting the 509 table is I can see exactly what I am doing in AAM, and generally have to set values in the table anyway to get the available power setting tables to more or less agree with the output as observed in AFSD.

Cheers: Tom