View Full Version : Best Glide Speed?

4th Feb 2010, 12:16
Any technically minded pilots out there who can help me with this?

I've been playing around with Airfoil diagrams trying to understand lift, drag, stall speeds etc.

Using the lift equation, I can plug in the CL and the weight of the aircraft and come up with the stall speed. This works out pretty close to actuals on a couple of sample aircraft.

However, when I shift to glide speeds it all goes pear shaped.

Best glide is for max L/D (so I was taught). If I take the corresponding CL from the airfoil curve, plug that into the lift equation for a known weight, I get ridiculous numbers. eg a Cessna 172 with best glide of 94 knots. Or a Piper Cub with best glide of 85 knots.

Can anyone tell me what I am missing? Gotta be something obvious that is staring me in the face, but I just can't see it.

4th Feb 2010, 12:37
Don't confuse the Cl/Cd diagram of an airfoil with the CL/CD diagram of an airplane.

The airfoil is only a 2D representation of a section of the wing. For the real airplane, you have to account for the fact that the air flow is 3D (finite wing length, induced drag) and other parts of the airplane that contribute more to drag than to lift, e.g. the fuselage, gear, tail planes etc.

Take these into account and you will end up with a higher CD, and hence a lower speed for optimum glide.

The stall speed is not affected by the extra drag, since the maximum CL remains largely unchanged.