Vx and Vy

[lev84j]

Could someone please explain why
Vx increases with altitude and
Vy decreases with altitude?
I think it may have something to do with TAS, but I am not sure.

Thanks.

[JEP]

[ QUOTE ]
Could someone please explain why
Vx increases with altitude and
Vy decreases with altitude?
I think it may have something to do with TAS, but I am not sure.


[/ QUOTE ]

Check Out The Search Engine....Plenty of Info...........

[UAL747400]

I also think its interesting that where Vx and Vy cross is as high as the plane will fly. It's either that or that is it's service ceiling, I dont remember which.

Tom

[MidlifeFlyer]

Personal FAQ. Will probably make the engineers wince, but here goes:

An airplane climbs because it has more power available than the amount of power it needs for straight flight at a particular airspeed. This simply means, for example, that if you are flying an airplane that has a normal cruise speed of 100 kts at full power and you keep it at 80 knots with full power, it must climb.

Grab you book and look for the power available vs. power required chart. Vy is the airspeed at which the difference between the two curves is the greatest.

With altitude, power available decreases. As you get higher, it decreases a lot. The point at which the greatest difference exists between the curves moves down the airspeed scale.

Vx is a little different (and takes some visualization). Once you have power available for a climb, it's how high can you bring the nose and convert that excess power into thrust pointing upwards, so that you climb steeper, not faster. An airplane with virtually unlimited thrust could fly straight up and the distance it can climb within a set horizontal distance be "infinite" (very loosely speaking). Think of our cartoon view of a rocket ship - straight up to outer space with no horizontal movement at all - forward airspeed is zero. (Of course, we don't fly airplanes that go straight up, but it helps visualize the concept.) Vx is the point at which you'll get the maximum excess thrust.

The same loss of power with altitude means that there isn't as much excess thrust available. So, you have the lower the nose. And that means airspeed has to increase.

As altitude increases and power and thrust both decrease, Vy will decrease because there's less power available. Vx will increase because there's less thrust available.

[FlyboyZR1]

Midlife you should really write a book, that's an excellent explanation on the topic I know this one has been asked many many times.

[img]/ubbthreads/images/graemlins/spin2.gif[/img]

[chris]

To add to this topic,

Although power available and thrust available decrease with increasing altitude, both power required and thrust required increase (more thrust is required to maintain any IAS because of the reduced air density, and more power is required for any IAS to produce this extra thrust required).

If you want to see exactly how the curves shift, look in the Advanced Pilot's Flight Manual. Also, look at this site:
http://142.26.194.131/aerodynamics1/...ce/Page10.html

So, if youre going to examine this issue from the curves, you have to consider the effects on both curves (required and available).

[MidlifeFlyer]

[ QUOTE ]
Midlife you should really write a book,

[/ QUOTE ]Thank you. That's very kind. I'm still working on a way to toss all of these "Personal FAQ" I've collected into an organized setting on my web site.