What happens to vx and vy at altitude?

Dan George
1 day ago
5 min read

When it comes to understanding aircraft climb performance, the speeds known as Vx - the best angle of climb speed and Vy - the best rate of climb speed are crucial for pilots to master. Most of us know what these speeds are for our aircraft at sea level and max gross weight, but what happens to these speeds as we climb? The answer may be surprising.

In this article, we'll break down the aerodynamic principles that affect the best rate and best angle of climb speeds with altitude changes.

The Basics: Understanding Drag and Its Role in Climb Performance

To understand Vx and Vy, we first need to grasp the nature of aircraft drag. Drag is the aerodynamic force that opposes an aircraft's motion through the air, and it consists of two main components:

Parasite Drag: This type of drag increases as the aircraft's speed increases. It results from the friction of air flowing over the aircraft's surfaces and the pressure differences around it.
Induced Drag: This drag is generated by the production of lift and decreases as speed increases. At slower speeds, induced drag is more significant because the aircraft needs to generate more lift by flying at a higher angle of attack.

When combined, these two drag components create a U-shaped total drag curve. At the bottom of this curve, where drag is minimized, lies the aircraft's most efficient speed in terms of drag. This point shifts with altitude, which is key to understanding climb speeds.

Thrust Available and Thrust Required Curves — Vx occurs where the thrust available curve is highest above the total drag, or thrust required curve.

How Altitude Changes the Drag Curve

At sea level under standard atmospheric conditions, the drag curve has a specific shape. But as you climb to higher altitudes, several factors change:

Air Density Decreases: The air gets thinner, meaning fewer air molecules are present to collide with the aircraft surfaces.
Parasite Drag Decreases: Because there are fewer air molecules, parasite drag reduces at the same true airspeed.
Lift Production Requires Higher Angle of Attack: Thinner air produces less lift, so the aircraft must fly at a higher angle of attack to maintain the same true airspeed.
Induced Drag Increases: Flying at a higher angle of attack causes more induced drag.

These changes mean that at altitude, parasite drag goes down, but induced drag goes up for a given airspeed. This shifts the total drag curve to the right, pushing the minimum drag point to higher true airspeeds. This shift has a direct impact on Vx.

Thrust Available and Thrust Required Curves at Altitude — As altitude increases, the drag curve is pushed right, which brings Vx speeds higher.

How Altitude Affects Vy

Vy is related to maximum excess power, which also decreases with altitude. In a propeller-driven aircraft, the power available curve looks different from the thrust available curve, but the principle remains the same—power diminishes as you climb because the engine gets less oxygen.

Power Available and Power Required Curves — Vy is the point where the power available curve is the highest above the power required curve.

As altitude increases, Vy also increases in terms of true airspeed, but not as dramatically as Vx does. This is because the power required curve shifts down as well, but the gap between power available and power required, which determines Vy, narrows.

Power Available and Power Required Curves at Altitude — Vy also increases with altitude, but not as dramatically as Vx does.

When plotted on a chart with altitude on the vertical axis and true airspeed on the horizontal axis, Vx and Vy curves start apart at sea level but get closer with altitude. At a certain altitude the two speeds converge and become the same true airspeed.

Vx and Vy speeds plotted with changes in altitude — Because Vy doesn't increase as fast as Vx does, their curves intersect as altitude increases.

Indicated Airspeed vs. True Airspeed: Why Vy Appears to Decrease

So, if both Vx and Vy increase in true airspeed with altitude, why does the POH show Vy decreasing in indicated airspeed as altitude goes up?

Best rate of climb speed from Cessna 172 POH — The Cessna 172 POH shows a decrease in Vy speed as altitude increases.

The answer lies in the difference between indicated airspeed (IAS) and true airspeed (TAS). The airspeed indicator measures ram air pressure, which depends on air density. At higher altitudes, the air is less dense, so fewer air molecules enter the pitot tube, leading to a lower indicated airspeed for the same true airspeed.

This means that even though Vy is increasing in terms of actual speed through the air (TAS), the airspeed indicator shows a decreasing value because it is calibrated for sea level conditions.

As a rule of thumb, Vy's true airspeed increases by about 1% for every 1,000 feet of altitude gained. However, when converted back to indicated airspeed, Vy decreases by roughly 2% per 1,000 feet. This mismatch causes the downward trend in indicated Vy speeds seen in the POH tables.

The Absolute Ceiling: Where Vx and Vy Meet

The altitude at which the indicated airspeeds for Vx and Vy become equal is known as the aircraft's absolute ceiling. At this point, the aircraft has no excess power left to climb, and the rate of climb drops to zero.

Understanding this ceiling is crucial for safe flight planning and performance expectations, especially when operating at high-density altitudes or in mountainous terrain.

Putting It All Together: Practical Implications for Pilots

Knowing how Vx and Vy behave with altitude helps pilots make better decisions during climb, especially when obstacles or terrain require a steep climb angle or the fastest climb rate.

Best Angle of Climb (Vx): Use when you need to clear an obstacle after takeoff, as it gives the greatest altitude gain over the shortest horizontal distance.
Best Rate of Climb (Vy): Use when you want to reach cruising altitude as quickly as possible, maximizing altitude gain per unit time.

As you climb higher, expect your true airspeeds for Vx and Vy to increase, even though your airspeed indicator will show lower values. This means you must rely on performance charts and an understanding of atmospheric effects rather than just the instruments to fly safely and efficiently.

Further Learning and Resources

Climb performance involves complex aerodynamic and engine performance principles, but grasping the basics of drag, thrust, and power curves is a great start. For those eager to dive deeper, FlightInsight offers comprehensive training on climb performance, private pilot ground school, instrument ground school, and more.

Check out resources and courses at:

Conclusion

In summary, both Vx and Vy increase in true airspeed as altitude rises due to changes in drag and engine power availability. However, the indicated airspeed for Vy appears to decrease because of lower air density affecting the airspeed indicator readings. This phenomenon explains the apparent contradiction seen in POH climb performance tables. Understanding these relationships is essential for pilots to optimize climb performance and ensure safety throughout the flight.

By mastering these concepts, pilots can confidently interpret performance charts, adjust climb speeds appropriately, and make informed decisions in the cockpit under varying atmospheric conditions.