Okay. I was teaching a ground class the other day at the flight school, and was amazed at the ignorance to a well known (I thought) aerodynamic/weight & balance related phenomenon. The fact basically that as you move CG aft the datum, cruise speed increases and "mileage" increases. The opposite is true when CG is moved forward the datum.Got me to thinking about TESTING this observable occurrence in the sim(s). As of now I'm with out an FS system while waiting on the parts to my new one, so I can't test (this laptop isn't going to cut it). Next week I plan on doing some long cross countries in smaller aircraft with varying CGs and constant weather. I doubt that MSFS will simulate this correctly, then again that's why I'm testing it.I could go into the technical reasons why this happens --parasite drag, less weight on tail, etc-- but this isn't the lessons forum so I'll save it for someone who ask.Anyone care to participate?

I'm confused about the CoG being forward or aft of the "datum". Do you mean reference datum, or empty CoG ? Most aircraft have reference datum where it would be impossiblr to have a CoG forward of it ?

I bet FSX doesn't simulate this. Not sure how the fuel burn or airspeed calculations actually work in FSX but I doubt it goes so far as to include the CoG in the equation. I could be wrong so a test would answer that.If you come up with some conditions for the test I could help you out. Are you thinking to test it on a Carenado Cessna or default aircraft?

All the High end add-ons simulate this, the PMDG MD11 Fuel system controller keeps the CG as far aft as possible in flight.

I suspect we would see this effect in FS.But we might only see a portion of it, and not the entire effect.First, as you’d expect, in FSX lift generates induced drag.One question is whether the empennage components calculate this lift drag.They do not have drag coefficients (as far as I know) which is a bad sign.But, this could just mean they’re not tunable, or their drag is insignificant.Or they've been ignored.Secondly, we would still see an effect because with aft CG the empennage’s 'negative' lift gets reduced.This means the wing needs to produce less lift. It's not working as hard to compensate for the negative effect of the empennage.Less wing lift mean less drag…so you’ll go slightly faster or you’ll reduce throttle to maintain speed.There’s no specialized calculation required…it is just a relationship ;)

I suspect we would see this effect in FS.But we might only see a portion of it, and not the entire effect.First, as you’d expect, in FSX lift generates induced drag.One question is whether the empennage components calculate this lift drag.They do not have drag coefficients (as far as I know) which is a bad sign.But, this could just mean they’re not tunable, or their drag is insignificant.Or they've been ignored.Secondly, we would still see an effect because with aft CG the empennage’s 'negative' lift gets reduced.This means the wing needs to produce less lift. It's not working as hard to compensate for the negative effect of the empennage.Less wing lift mean less drag…so you’ll go slightly faster or you’ll reduce throttle to maintain speed.There’s no specialized calculation required…it is just a relationship ;)

These were my thoughts on it, though I'm not sure if they're correct. I'll have the new rig fired up--no, on-- tonight. Maybe I'll have time for a little test.As for the question in the second or third reply after mine, I mean relative to each other, on the datum plane. I kinda figured it'd be common sense but things are different in writing! :(

I believe FSX is probably inconsistent.As discussed in another post, there is a lift coefficient due the elevator deflection (CL_de) but not one for trim deflection. So if an aircraft is flown manually with an elevator deflection to trim it then there should be a corresponding lift force. If the aircraft is flown on autopilot with a non-force feedback stick the elevator deflection becomes zero and is effectively replaced by a trim deflection so there would be no additional lift force.

Another member and I were having a heated argument on relatively the same subject. You can read about it here: http://forum.avsim.net/topic/326880-cg-before-the-wings/

Another member and I were having a heated argument on relatively the same subject. You can read about it here: http://forum.avsim.net/topic/326880-cg-before-the-wings/

Well with class and being at the flight school, I haven't had time to set up FS. I will take a look at that thread, Chris. Maybe I'll even get around to testing the original scenario...

Another member and I were having a heated argument on relatively the same subject. You can read about it here: http://forum.avsim.net/topic/326880-cg-before-the-wings/

You weren't having a heated argument, you were having a heated misunderstanding: You were both saying the same thing, except one of you was referring to "center of lift" only for the wing while the other one included empennage lift in the center of lift calculation.Cheers,- jahman.

You weren't having a heated argument, you were having a heated misunderstanding: You were both saying the same thing, except one of you was referring to "center of lift" only for the wing while the other one included empennage lift in the center of lift calculation.Cheers,- jahman.

True it was a misunderstanding but it stemmed from where the location of the CG is.