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The Reason Vx And Vy Are Different Is Really Simple, And Here's Why...

A while back, we ran a vote on Vx and Vy: which one gets you to altitude in the least amount of time. Most of you answered Vy, which is right. But why? And why aren't Vy and Vx the same?

It all comes down to force and power - and how they're different.

  1. So, POP QUIZ - What's the difference between force and power?

    Awesome!

    Power is force per time

    1. Power is work over time
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    2. Force is work over time
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    1. Power and force are the same
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    2. Who cares? Push the throttle up and climb faster!
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How did you do? Power is the amount of work you can do in a unit of time. You're familiar with both of these - "thrust" is force, and "horsepower" is power.

What does this have to do with Vx and Vy? An aircraft climbs because of excess thrust or excess power. Vx is your best angle of climb speed, and Vy is your best rate of climb speed.

  1. POP QUIZ number two - Vx is the speed where you have the most:

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    Excess Force

    1. Excess Force
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    2. Excess Power
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Did you get it right? At Vx, you have the most excess force, which is the same as the most excess thrust.

The Drag Curve (Thrust Required)

An aircraft has two types of drag, right? Induced drag, which is created by lift, and parasite drag, which is caused by air friction and some other things. (We'll do a post about that later.)

Check out this diagram below - it shows induced drag, parasite drag and total drag (the sum of the two):

Drag Curve

So, at slow speeds, you have more induced drag. (You fly at a higher angle of attack, right?) At high speeds, you have more parasite drag. (You're at a lower angle of attack.) If you add the two drag amounts together, you get total drag.

Total drag is also known as thrust required - the amount of thrust you need to stay level at a specific airspeed. The very bottom of this bowl shaped line is where the aircraft is the most efficient - you get the least drag. But, that's not Vx. Why?

Thrust Available

Your propellor can generate the most thrust at a slow speed. Why? Because it's at a high angle of attack. As your airspeed increases, the propellor's angle of attack decreases, and it can generate less thrust.

Look at the diagram below - it shows your total drag (the thrust required for level flight), and it shows your maximum thrust available.

Thrust Available Chart

The amount of power you have available to climb is the difference between the thrust line (blue) and the drag curve (black). But, if you measure it, this doesn't happen at the lowest point of the curve!

The largest space between thrust available and thrust required happens a little to the left of that point. Why? Because of the slope of the thrust available line. That's your Vx speed. Check out the animated GIF below:

Thrust Available Chart - Animation

Vy - It's All About Power

Vx is all about excess force (thrust) - but Vy is all about excess power (horsepower).

To figure out where Vy is, you need to draw a power required curve.

What's that? Power is work done per unit of time. So, you can compute power required by multiplying your total drag (force required) by your airspeed (distance over time). Power Required = Thrust Required X Airspeed.

In America, we use some wonky units for both force and power. As pilots, we generally think of thrust in terms of pounds. On a reciprocating engine aircraft, we measure power in "horsepower."

If you multiply the force required (in pounds) against your true airspeed in knots, you'll get an accurate, but unrecognizable, number for power required. To convert it to horsepower, you'll need to multiply it by roughly .003. (Don't bother - we'll do it for you.)

Take a look at the diagram below - this shows power required for level flight at different airspeeds:

Power Required Curve

See how the curve is skewed? It gets steep as it climbs to the right.

Now let's add power available. To draw this line, do the same thing - multiply thrust available by airspeed by (about) .003. Take a look:

Power Available Curve

Vy is the speed where you have the biggest difference between power required and power available.

But, again, it's not at the lowest point of the power required curve - it's a little bit to the right. Why? Because of the shape of the power available curve.

Power Available Curve - Animation

Not So Confusing, Huh?!?

Now you know! Vx is the speed where you have the most excess force (thrust), and Vy is the speed where you have the most excess power (horsepower).

Did you know that Vx and Vy change with altitude? And, they're the same at your maximum ceiling? We'll save that for another post, because too many charts all at once is never a good thing.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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