Why does Vx increase, and Vy decrease with altitude? | Climb speeds explained

You may know about Vx and Vy, and how they are derived, but did you know that while Vx, your best rate of climb speed, increases with altitude, your Vy, best rate of climb speed, actually goes down? Here's an illustrated guide to understanding why.

[michaelhoffmann]

This was one of those "just memorise it for the exam" things and I always wondered why! Thanks!

[alinajmaldin]

This is something I always wondered and never was able to find a clear concise explanation! Thanks for explaining :)

[benjaminbarwick]

After many hours of reading, this is what made it all make sense. Big Thanks!

[philipmensah]

These videos are hitting what I think about each week… I think of something and that’s what you solve .. it’s awesome

[aviatortrucker]

Most POH’s have a cruise climb that is 10 to 15 kn higher than Vy. This is because the manufacturer recommends for visibility, engine cooling and fuel consumption savings. If you are not concerned with getting to altitude fast, and you have a long cross country, you’ll gain more distance from your departure end before you reach cruise altitude. This always puzzled me when it came to flightplan because the book gave you time, fuel, distance based on a specific indicated airspeed, and as long it was zero wind. If you went, climbed via a cruise climb airspeed, your calculations were messed up because you burned more fuel being at power settings to get you to altitude, but traded off distance to destination. One other thing to consider, especially with constant speed propellers. Is the recommended climb setting which is about 90% power. Big planes like airliners have their engines reduce after they reach a safe altitude. Technically there aren’t too many airplanes that climb to altitude at full power, except for those with a fixed pitch prop.

[noidea]

Vid didn't help me. Didn't really explan WHY Vx is increasing faster than Vy in terms of TAS.

[rupunnb]

@flightinsight9111 Great video. I suppose when Vx and Vy are being referenced, these are Indicated (IAS) and not TAS?

[jimmydulin]

Good explanation of the math of Vx and Vy. The practicality, from an old crop duster's point of view, is that neither Vx nor Vy are generally appropriate. Watch an modern Ag pilot take off using the basic level in low ground effect takeoff. He has a much more powerful engine than the 235 hp engine on my Pawnee, but he is hauling eight times the load. Yet, both of us value the free energy acceleration of level in low ground effect so long as runway (or open desert) is available until cruise airspeed before pitch up to just over (not well over) the obstruction. Either Vx or Vy is a very dangerous low energy state, low airspeed state, when not absolutely necessary to miss things.

[the_rustam]

And not a word that these are Zhukovsky curves

[williamk]

I seem to remember in my early training that the curves intersect (regarding IAS) at the service ceiling. Apparently it is lower than that.

[rmadridista]

So the altitude that Vx and Vy intersect in KIAS is the Service Ceiling of the aircraft?

[edbenelli]

This really comes into play for jump pilots, who need to climb up to 12, 000 feet or higher in the least amount of time. Both the jumpers and the drop zone owner will really start barking at you if they think you could be climbing faster-

[jimallen]

Only comment, the characteristics are Complicated not Complex. Complex is not synonymous with Complicated. Complicated is ordered, deterministic, predictable, repeatable, regular. It requires analysis but it always gives the same thing. In Complexity, you may have patterns, but results vary. In Complexity, analysis fails you as the situation changes hence what you’ve previously learned no longer applies. You’re analyzing here, you’re working in Complication.

[dullgeek]

Instrument rated commercial pilot and TIL.