Anonymous
Anonymous asked in Cars & TransportationAircraft · 6 months ago

# Glide Ratio vs Lift: Drag ratio?

I've seen these terms used interchangeably on forums and the like. I've seen that one can divide aircraft weight by the ratio to determine the amount of lift needed to maintain level flight. But are these ratios really the same? It would not seem so. Glide ratio is self explanatory, but what then is the lift to drag ratio?

Relevance
• They're not the same. Glide ratio is a mathematical function of a planes lift to drag ratio.

• Anonymous
6 months ago

You seem to be mixing several notions.

First, if you take the weight of an aircraft, say 50000 lb, and have a ratio of 10 to 1, then dividing one by the other, you get 5000 lb. What does that give you? Not lift. That would be drag. So in essence, that would be the THRUST required to balance the drag.

Second: the lift to drag ratio of an aircraft is not a constant. If a plane flies slower, it would need to fly at a higher angle of attack to compensate -- but the lift it needs is STILL the same as the weight of the plane. If the plane flies at a speed that is less than optimal, then the lift to drag ratio will not be as good, meaning that you would have more induced drag than you could have gotten away with flying at a better speed. Likewise, if the plane flies very fast, it will be at a reduce angle of attack which may have a higher lift to drag than optimal.

Lift to drag ratio is something that is important when the plane is designed; we try to make the airplane cruise to the best lift to drag ratio because that will be the best range. The glide ratio is what the pilot sees, and is the result of the conditions, from an operational point of view. Lose all power, and the crew will consult the flight manual, and establish, for the current aircraft weight, what speed they should try to keep in order to have the longest glide distance for the altitude they have.

• The  glide ratio is indeed the same as the lift/drag ratio (L/D); the aerodynamic equations do indeed work out that way.  L/D is the aircraft's lift, which equals the aircraft's weight, divided by the aerodynamic drag (which equals engine thrust in level unaccelerated flight).