Setting the Idle Speed

Drzook

Drzook

Charter Member
Hello again,
As some of you know I've been experimenting with different engine setups for my Lockheed Vega. So far the latest attempt with a BMW 132 (a German copy of the P&W R-1690 Hornet) has been nothing short of fantastic; it's a true 200mph plane, and climbs like a scalded monkey (somewhere around 3000+ FPM). So far I have noticed only one real hangup (besides the fact it goes thru the virtual gas like it's going out of style): the idle speed seems a bit high, at around 1150 rpm. I don't think this was a problem when it only had somewhere around 450-500 hp but at over 700 hp 1150 rpm translates to around 15 mph on the ground and landing runs that make a fully loaded 747 look like an STOL bird. How does one adjust the idle speed so the prop doesn't provide any thrust? I cannot find the idle set screw in the carb anywhere.:isadizzy: Any help would be appreciated; thanks in advance.
(BTW, the min_throttle_limit is already at 0.00)
 
Assuming you have a variable pitch prop, make sure that the "min_gov_rpm=" in the propeller section is set high enough to allow the prop to go fine at low rpms. Maybe 900-1200 rpms.

Also, keep air file table 511 minimum pitch angle at 15 degrees while in the cfg file, make "beta_min=9". (You can vary this up to 12 with same effect)

These are just starting points for testing. With this, you can raise your rpm's quite a bit without adding thrust and when you pull off the throttle, rpm's will decrease slowly at min_gov_rpm, but when it breaks through, your thrust should go to near zero.

Make sure you have a good test gauge to show all the variables. Try Jerry Beckwith's test panel setup or Herve Hors' AFSD.
 
Okay, tried changing min_gov_rpm from 1400 to 900 and changed beta_min from 14 to 9. I don't think there was any change in behavior however. Upon landing I came to a stop near the end of the runway and let off the brakes but then it started putting forward slowly (and this is in an 8-knot headwind). I also checked the airfile at table 511 and I think the air file table already has a minimum pitch angle at 15 degrees (see attached). After setting up these new values which way should I go to decrease thrust?
 
What is your prop pitch at idle? What are your RPM's?

Did you pick up a test panel or gauge?

Herve's test gauge is easy to use, shows all the engine parameters you need to evaluate your situation.

You can watch prop pitch change as you vary rpm's, along with change in torque, thrust etc.

You need a tool like this to promote understanding.

http://pagesperso-orange.fr/hsors/index.html

It's easy to use, runs along side of FS, not part of it. Requires freeware version or better of FSUIPC.
 
I just picked up Jerry Beckwith's test panel and took it for a spin. Nice. Answering your questions...
I believe the prop pitch is at 9 degrees at idle, which was around 1130 rpm. I did notice the thrust was registering at 516 pounds which seems excessive considering that 75% throttle and lean mixture gets 934 pounds and I'm doing 203 mph at that point. 350 pounds is enough to get the Vega moving so if there is some way to bring the thrust below that we should be good.
I tried bringing down the prop rpms but for some reason that doesn't do anything; the prop pitch staying at 24 degrees and when bringing back to fine pitch (ctrl-f4) it goes to 22.9 degress only.
 
What is the idle RPM (as read) and min MP values you have specified? Many FS engines have a base idle that is way too high, up around 1000 RPM. These engines should be able to lope along at about 600 RPM idle. If this a contributing cause you might start by reducing the min MP value in the .cfg file. If further reduction is needed, try increasing the idle RPM friction scalar in the .cfg file. The elgant way to do this is in the .Air file where the friction vrs. RPM curve can be adjusted. These factors will also affect the spindown of the prop and the required stsrter torque.

Cheers: T.
 
Fliger,
The normal idle rpm was around 1150 rpm with a min MP of 10. Changing the min MP to 5 was just what the doctor ordered. Idles at 600 rpm or so with almost no thrust (maybe 100 pounds or so--not enough to make 'er even twitch).:ernae:
Milton,
I want to thank you again for suggesting I get a test panel. So many things to tweak, so little time...:ernae:
I'm pretty sure I'll have more questions as I continue this adventure in flight dynamics. Thanks to all...:icon29::guinness::icon29:
 
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