Vy and Vx Vary with Altitude - Why?

[majorityof1]

So, I know Vx and Vy vary with altitude, but why? I understand the power required vs. Power available and the engine is less effecient at altitude. Why would the speed itself change?

Thanks,
Rachel

 

[tgrayson]

So, I know Vx and Vy vary with altitude, but why? I understand the power required vs. Power available and the engine is less effecient at altitude. Why would the speed itself change?

Thanks,
Rachel

Why Vy Varies with Altitude

 

[aloft]

Woo! I didn't have the complete answer at my lips, but I knew air density had to be a primary factor.