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## Vx and Vy and L/D ratio

• Posts: 57

### Flightsurgeon created the topic: Vx and Vy and L/D ratio

There was a question that was asked where Vx and Vy where, Vx is less than Vy which is fine, however,
there was a question regarding L/D ratio's. Is the L/D ration better in Vy compared to Vx or how does L/D compare with Vx and Vy please

• Posts: 2441

### bobtait replied the topic: Vx and Vy and L/D ratio

Vx (best angle of climb) is decided by maximum surplus thrust.
Vy (best rate of climb) is decided by maximum surplus power.
Both of these in turn depend upon both engine performance and aerodynamic performance of the airframe.

Best L/D ratio on the other hand depends only on the aerodynamic performance of the airframe as in gliding.

It's quite possible that when the aerodynamic performance is at it's best, the engine/propeller combination may not be. There is no reason to assume that flying at the best L/D ratio will give best climb performance (angle or rate), because those items depend on the engine/airframe combination.
The following user(s) said Thank You: katherine2023

• Posts: 57

### Flightsurgeon replied the topic: Vx and Vy and L/D ratio

This is what I thought, however the question was very specific in relation to looking at L/D during Vx and Vy and that is what confused me a considerable degree. I have though that during a Vy climb, there is in general a better L/D compareed to a Vx climb.
Would this be the case? In both climbs, there is a L and a D, so in which scenario is it better. The RPL question was of that nature so even my flight instructors were confused when I raised the question with them

Kind regards

Ravi

• Posts: 2441

### bobtait replied the topic: Vx and Vy and L/D ratio

No matter what the aeroplane is doing, there is always an associated lift/drag ratio. It's difficult to say whether or not Vx or Vy is associated with the best lift/drag ratio for a particular aeroplane. What matters is the surplus power or surplus thrust available at any given speed. If you put a new engine in the aeroplane, Vx and Vy would change even though the airframe was the same.

If you can find the actual wording of the question you are referring to maybe that would help.