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Drzook
June 12th, 2009, 20:49
On a lark I decided to swap out virtual engines on my Lockheed Vega from the default Wasp to a BMW-132 (same as that on the Junkers W34 and Ju-52; basically a license built P&W Hornet). It wasn't too much to find out weights on the engines in question and adjusting the cfg file to reflect the higher weight of the BMW was a snap.
I took my Vega on a couple of test flights to see what nearly 300 extra horsepower and only 400 extra pounds does to a plane. Climb rate was up by over 50%, to 3000fpm+. I had calculated top speed to be over 225mph; however at around 200mph it starts shaking like a paint mixer:isadizzy:.
I know there is some line in the cfg file that can stop this; anyone care to share? Thanks in advance.

fliger747
June 13th, 2009, 03:13
Dz:

If you are interested in getting into such things in a casual way I suggest getting Jerry (Sparks) Beckwith's Airwrench. It has grown into a fairly sophisticated tool, but allows you to adjust many parameters via simple sliders. Generally manual adjustments, especially ones involving tables in the .air file get to be a bit more complex.

Cheers: Tom

sparks
June 13th, 2009, 07:51
Since you're modifying the Vega, I'm assuming this is FS9. First, I think this tendency to shake at high speed is an artifact in FS9. If you port that flight model to FSX, there's a good possibility it won't shake at all.

As to curing it, I think you'll have to work on the air file. First you'll have to determine whether the shake is in pitch or yaw, but I'd guess it's pitch. I just took a look at the Vega air file, and I'd recommend changing CM_adot to 3000 or less.

Drzook
June 13th, 2009, 21:57
Yes, it is the default Vega for FS2004, with some mods (texture, sound, panel, and cfg have all been touched one way or another). What section in the air file is this CM_adot? I just took a gander at the air file using AirEd but I couldn't find it. Thanks in advance

sparks
June 14th, 2009, 06:17
That's the problem with AirEd. The definition of air file parameters depends on the aired.ini file you're using. In the FS9 Vega, CM_adot is in section 1101 at offset 8A.

Another approach that usually cures the shakes, but may also affect other handling characteristics more than you'd like is to increase the pitch and/or yaw MOI values in the aircraft.cfg file.

Drzook
June 14th, 2009, 13:03
Sparks,
I couldn't find that CM_adot thing in the air file but went with your advice about the pitch MOI value, changing it from 1788.8 to 2000, from 2000 to 2350, from 2350 to 2250 and finally settling on 2300 in a series of flights around 2000' ASL.
With the current power settings balls-to-the wall wide open throttle gets me around 230 mph indicated and no shakes. at that speed there might be the beginnings of the shakes but at higher MOI settings it feels...different. With the pitch MOI at 2300 it seemed a good compromise and it is unlikely I would have the Vega at that kind of speed for any length of time.
I think what will come next is that I will likely de-rate the engine from 789hp to around 750hp (the same as my Junkers W34) and add maybe an extra 100 pounds to the dry weight to simulate extra bracing. In any case thank you very much for your sage advice :medals: