Vy and Vx with altitude

This is the least technical explanation I know. See if it helps.

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.
 
MattXJ7 said:
Any cfis have a good explanation of how Vy and Vx vary with altitude?
Click to expand...

I suspect you got lost at the end when the article shifts from a discussion of true airspeed (what is actually happening) to indicated airspeed (what the pilot sees).

You know true airspeed increases with altitude, right? If you know this then reread the article with this question in mind: Will indicated airspeed decrease faster than true airspeed increases? The following analysis will help you realize this answer:


For comparison we will use a True Airspeed Calculator (located at the bottom of the linked page) with Vy speeds from a Cessna 172R POH. You'll notice they agree with Taylor's graphical depictions, particularly graph 4 & 5:

Alt --- IAS --- TAS

S.L. -- 79 -- 79

2000 -- 77 -- 79.292

4000 -- 76 -- 80.625

6000 -- 74 -- 80.909

8000 -- 72 -- 81.170

10000 -- 71 -- 82.567


Note: The calculator requires CAS to convert to TAS. However, since CAS for our example aircraft is within 1 knot of IAS at climb speeds we can ignore CAS and just use IAS. Accepting this small error as negligible.

Interesting isn't it? TAS is actually going up for Vy, but indicated airspeed is going down. You're probably wondering now, "well how does Vx, then, increase to meet Vy?" The answer is Vx increases in both indicated and true. Vx's true airspeed increase is at a faster rate than that of Vy and it eventually catches up to Vy. You can experiment with the calculator for this as well.

For simplicity, however, just focus on what he says about Vy in graph 5. A pilot need only worry about what he/she needs to do with indicated airspeed. For all intensive purposes the rest can be assumed white noise. Hence, when presenting this to a student graphically you need only focus on the following:

1) Power required curve moves up as we increase altitude.
2) Power available curve moves down and left as altitude increases.

And just keep in mind that you're presenting this in terms of what the pilot is flying, INDICATED speeds, not actual.

Hope that clears it up for you.
 
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