Does VMD change with altitude?

VMD remains the same in terms of IAS, but increases in TAS as altitude increases.

Is VMD the same as the speed for minimum sink rate?

No, the minimum sink rate occurs at a lower speed than VMD.

Why is VMD important for gliding?

VMD gives the shallowest glide angle, maximizing glide range.

What is the relationship between VMD and L/D ratio?

VMD coincides with the maximum lift-to-drag ratio, indicating peak aerodynamic efficiency.

How do speed brakes affect VMD?

They increase parasite drag, raising both VMD and the minimum total drag value.

Minimum Drag Speed (VMD)

Easy4 min readPrinciple of Flight (A)

Moderately Examined

Minimum Drag Speed (VMD) is the airspeed at which an aircraft experiences the lowest possible total drag in straight and level flight. At this speed, induced drag and parasite drag are equal, and the aircraft achieves its best lift-to-drag (L/D) ratio, resulting in the most efficient glide angle.

In depth

Explanation

What is VMD?

VMD, or Minimum Drag Speed, is the speed at which the total drag acting on an aircraft is at its lowest. This occurs where the increasing curve of parasite drag meets the decreasing curve of induced drag. At VMD, both drag types are equal, and the aircraft operates at its maximum lift-to-drag ratio.

Relationship with Angle of Attack and L/D Ratio

The angle of attack (alpha) at VMD is typically around 4 degrees for most aircraft. At this angle, the wing produces just enough lift to balance weight while minimizing drag. The maximum L/D ratio at VMD means the aircraft is gliding at its shallowest angle, which is crucial for maximizing range in a glide.

Operational Importance

VMD is a reference speed for performance calculations, particularly for best glide and range. In straight and level flight, flying slower or faster than VMD increases total drag. For jets, VMD is not the same as the speed for minimum sink rate, which is lower. VMD is also unaffected by altitude when expressed as indicated airspeed (IAS), though true airspeed (TAS) at VMD increases with altitude.

Speed Brakes and VMD

Deploying speed brakes increases parasite drag, shifting the total drag curve upward and moving VMD to a higher speed. This means the minimum drag now occurs at a faster airspeed, but with a higher total drag value.

The essentials

Key Points

VMD is the speed where total drag is minimized in straight and level flight.

At VMD, induced drag equals parasite drag.

VMD coincides with the maximum lift-to-drag (L/D) ratio.

The best glide angle is achieved at VMD (minimum descent angle).

VMD occurs at the same IAS regardless of altitude, but TAS increases with altitude.

Speed brakes increase VMD and total drag.

Flying below VMD can lead to speed instability in a glide.

Watch out

Common Exam Traps

Confusing VMD with minimum sink rate speed (VMP); VMP is lower.

Assuming VMD is at minimum angle of attack; it's at the angle for max L/D, not minimum alpha.

Believing induced drag is greater than parasite drag at VMD; they are equal.

Thinking VMD changes with altitude when expressed as IAS; only TAS changes.

Assuming VMD is the speed for maximum endurance in all aircraft types.

Test yourself

Example Exam Questions

Question 1Easy

At the minimum drag speed (VMD) in straight and level flight, which of the following is true?

Question 2Medium

What happens to VMD when speed brakes are deployed?

Question 3Easy

At what point on the drag curve does the best lift-to-drag ratio occur?

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