Effect of CG on Vx/Vy

[KevinJH7]

I did a search but failed to find anything. Just wanted to make sure I wasn't crazy. Does anybody have a good explanation for how Vx/Vy change based on CG?

If I remember right, Vx/Vy decrease with an aft CG because of the lower wing loading (just like Vx/Vy decrease as weight decreases).

Thanks

 

[ppragman]

I did a search but failed to find anything. Just wanted to make sure I wasn't crazy. Does anybody have a good explanation for how Vx/Vy change based on CG?

If I remember right, Vx/Vy decrease with an aft CG because of the lower wing loading (just like Vx/Vy decrease as weight decreases).

Thanks

Regardless, you're better off just to fly the published speed if you need it, trying to calculate a 1kt difference because of an aft loading in a light airplane on the fly won't really help you that much. Come to think of it, I don't usually fly Vx/Vy in general because it doesn't give me the cooling I want. Usually, I drive around at top of the white arc in the climb unless I'm climbing for terrain, or for a tailwind.

Also, and I'm sure someone will correct me if I'm wrong, but I believe that your wingloading isn't any lower, its the induced drag by the required tail down force that's increasing the speeds when the CG is forward.

 

[tgrayson]

Does anybody have a good explanation for how Vx/Vy change based on CG? If I remember right, Vx/Vy decrease with an aft CG because of the lower wing loading (just like Vx/Vy decrease as weight decreases).

I don't recall seeing any numbers thrown at this, but it stands to reason that Vx/Vy would change, due to the reason you suggest. There will be increased induced drag both on the horizontal stabilizer and the main wing with a forward CG. You can see the resulting effect on Vx by drawing the induced and parasite drag curves, and then moving the induced drag curve up and down to reflect the changes in drag. Moving it up will move the point of minimum drag to higher airspeeds, reflecting a movement forward of the CG.

I'd be surprised if the changes were large.

 

[KevinJH7]

Thanks. I wasn't planning on actually flying a different Vx/Vy as the differences would be so small but more just looking into the details of how and why it changes.

Also, and I'm sure someone will correct me if I'm wrong, but I believe that your wingloading isn't any lower, its the induced drag by the required tail down force that's increasing the speeds when the CG is forward.

I agree. I guess I just misused the term wingloading to describe the increased "down force" (weight and tail downforce) lift has to counteract for straight and level flight.