VX & VY — CLIMB SPEEDS

What are Vx and Vy and why are they so important?

A relationship between force and power, and why our airplane climbs.

Vx

Vx is your best angle of climb speed.

Vx is all about EXCESS FORCE, or ‘thrust’. Referring to the Drag Curve we see that Parasite drag + Induced drag = Total drag, aka ‘thrust required’.

‘Thrust required’ is the amount of thrust you need to stay level at a specific airspeed.

‘Thrust available’;
We know that our propeller is able to generate the most thrust at low airspeeds, as we are at a higher angle of attack. If we plot thrust available as a line on our thrust required (drag) curve, we shall find the amount of power available to climb.

The largest space between the two lines happens left of the lowest point of the curve. This is so because of the slope of the thrust available line. Meaning, we have the most excess force (thrust) at a slightly lower airspeed than our minimum drag speed (Vmd). THAT IS OUR VX!

Vy

Vy is your best rate of climb speed.

Vy is all about EXCESS POWER, or ‘horsepower’. Here we refer to the ‘Power Required curve.

‘Power’ is ‘work done per unit of time’.

We calculate our power required by multiplying our total drag (force required) by our airspeed (distance over time).

Power required = Thrust Required x Airspeed

Let’s have a look at the above graph. On the power required curve, we plot the power available line. Vy is where we have the largest space between the power required and power available.

Note, just like Vx, Vy is not on the lowest point of the curve. This is again due to the slope of the HP available curve.

In future posts we will dive deeper into why and when we fly at Vx and Vy.