Prop windmilling..

[BobSeaman]

I've got a little problem I'd like to fix without screwing up the flight dynamics.

All I want to do is speed up how quickly it takes to shut down the engine on a Piper Cub. Its a very nice MDL that needed a little TLC on its flight dynamics. There was a very good update that fixed a lot of problems but when I shut down the engine the prop "spools down" like the main rotor on a Huey (something on the order of 20 seconds from shutting down to having the prop stop rotating).

 

Anyone know which variable in the aircraft.cfg or .air file controls the "shut down speed" of the prop? I just want to speed up the shut down animation without effecting flight performance. I'm thinking its one of the prop M.O.I. or torque settings?

[mrzippy]

I found this while searching for stopping the prop spin.

 

Two main things affect the spindown rate: The 509 table in the .Air file controls the low to RPM cylinder friction. Lowering this value causes the engine to spin down more slowly. The same value may vary with the brand of FSX installed; The prop MOI gives a bit more inertia for higher values.

 

Can one assume that increasing this value would shorten the spin of the prop?

[tiger1962]

A quick comparison with the default Piper Cub aircraft.cfg shows: propeller_moi=3.86478

Try this setting and see if it improves.

[BobSeaman]

So, ...

 

Yow, Mr. Zippy wins the rancid corndog !

 

:D

 

I tried changing the "prop MOI" value (both up and down) but when you have to say "OK, I think that did something.." -it probably didn't. :p

 

Checking sect 509 of the .air file in AirEd showed a graph that was different from some other freeware Pipers (well, most other small props) so I copied the 509 section from a small plane with a faster animation but for some reason (probably in Win10) had a devil of a time installing the new record in the .air file. AirEd was treating the Cub .air file like it was read-only when it wasn't and I had to snag a copy out of my trash to get it to accept the new 509 values. :confused:

 

But, it worked! The 135 HP engine now shuts down like it has piston rings and a light prop. :cool:

 

Its also a little "balky" on start up. Sometimes the engine quits unless you give it some throttle and mixture after it fires up. I'm treating that as a Win since its a Cub, after all. :pilot:

[BobSeaman]

Can one assume that increasing this value would shorten the spin of the prop?

 

I'm pretty sure the original .air file in the Cub was based off something from the FSfW95 or the FS98 days. It started out old and was tweaked along the way. At least, that's what it looked like to me. The older 509 section was MUCH simpler, picture a short ramp that then went level. The newer 509 sections start with a ramp up to a peak that then ramps back down to a steady level. From what I've looked into, the 509 section is based off some odd units (slugs per foot per unit of time??) to show momentum. :confused:

 

So, the short answer is to just replace the 509 section of the .air with something newer from a small prop plane. If I had to guess, its the slope of that ramp in the graph that controls how fast the prop spins down (I used to pal around with Sam Chin, so I know just enough to be dangerous :cool: ). How to change that slope in 509 would be tricky.

[PhantomTweak]

How to change that slope in 509 would be tricky.

Not at all. Notice that there are vertical "dividing lines" across the length of the graph. Once you have the graph showing, click on it once. Then use the regular PC Keyboard arrow keys to move back and forth to the different vertical lines.

For example, to lower the height of the graph's "peak" at the beginning of the graph, just move over 1click of the right pointing arrow. Once you are on it, and it will be slightly darker than the others to show which one you're on, use the down arrow to lower the peak without affecting any other setting. Down along the bottom of the graph, there are numbers, like x=2.34567 y=0.154463 or whatever. They give you an idea of how much each arrow click moves the graph, a lot or very little.

If, and I suspect you are right, lowering the first slope up to the first peak will allow the prop to spin longer, raising it will shorten the spin down time, then you can give it 1 or 2 clicks up. Click the little X in a box in the upper right corner of the graph's window, and save the file. Load the plane up and shut it down, observing the prop's behaviour to see if that has changed the way and the amount you want it to. If not, open up the graph again, make another SMALL change and save it off and try the plane again.

Small side notes: I suspect the problem you are having with the .air file with adding in another's section 509 is possibly due to your FS9 being located in the dreaded \Program Files folder. Just move the .air file you want to work with out to another folder, like say C:\Temp or even to your desktop, edit the heck out of it, and copy it back. Saves a lot of heartache. Second, then I'll shut up (ZIPPY!! :D ), When you go testing repeated changes to aircraft, you want to have ALL the AI traffic OFF, or it can retain previous .air file values, unless you switch to another plane entirely, then back. Even then....just turn all the AI traffic off. ALL of it, not just airplanes, or ships or whatever.

And of course, ALWAYS MAKE A BACK-UP BEFORE YOU MAKE ANY CHANGES TO ANY FILE!!!!!

Have fun!

Pat☺

[BobSeaman]

Not at all.

 

Well, what I meant was that the units in sect 509 are fairly odd ball (think hog's heads per furlong) so you can adjust them, but what "them" are is still a mystery. :cool:

 

And, yeah, Win10 can be a bit of a nanny at times. I had a lot of experience (ie, new gray hairs) thrashing on Vista so this is nothing new. Its just a little worse. :p

 

What I suspect happened is that the old 509 section was smaller than the new (at least it wasn't the same size) and Win10 started yelling "Whoa!!!". :rolleyes:

 

Actually, the first edit was going along just fine until I tried to save it. Win10 suddenly turned into Bill Mumy from the original Twilite Zone and sent the .air file "off to the corn field". :rolleyes: I'm sure its on my hard drive, somewhere....

[keefpee]

Table 509, It is recommended that the first value (normally x=-300 & the next value, normally x=+300 have the same y value but negative with neg X & pos with pos X.)

Now if one goes to the value that needs changing by using the keyboard arrow keys, if one then presses either your x or y key a box will appear top left, you can then modify that value directly using you keyboard then hitting your Enter key to save the change. Making the y value greater for the first two values will slow the prop quicker. If too great it can make engine starting difficult.

 

I think the values are stubborn horses per second i.e. friction in terms of horsepower for y & rpm for x

[mrzippy]

So, ...

 

Yow, Mr. Zippy wins the rancid corndog !

 

:D

 

Could I get a deep-fried Twinkie instead? ;)

[BobSeaman]

Could I get a deep-fried Twinkie instead? ;)

 

 

I guess so, as long as you brought the right relish..

 

:)

 

Sorry, I had some of the original comic books back in the day. Great stories, but printed on terrible paper (by Last Gasp?) that turned brown and started to (literally) fall apart after a couple of years.

 

BTW, thanks to all of you for the replies. This is one of the fun aspects of FS- figuring out what is actually going on. :cool: