How to convert thermal lift to airspeed.

[MikeSt]

Given a sailplane that is crusing at 100 knots, and descending at ~ 200 f.p.m. (or 2 knots on the variometer). (Just numbers made up to be easy).

You hit a thermal that gives +400 fpm ( + 4knot).

So you can immediately use 200 fpm (2 knots) to fly straight/level at 100 knots. That leaves you an "extra" 200 fpm (2 knots)

Instead of maintaining 100 knot / 200 fpm climb, you push the nose down to maintain altitude, and get the increased airpseed.

The question :

What does the arspeed increase to ? (Assume an "infinite thermal"

"Roller Coaster" answer

I was lookng at Denker's web site, and his "law of the roller coaster". He says that conversion factor = 9 feet per knot, per hundred knots.

A 200 fpm (2 not) gain would ~ 3 ft sec, so using the "roller coaster" logic, the speed gain would be ~ 1knot / 3 sec, i.e. ~ 20 knot speed increase. Does that make sense ?

Would the final speed by 120 knots? If so why would it stop at 120 ?



3 Angle of Attack - Lift Formual


I thought that somehow the Lift formula could be used to calculate the new speed.

How does a VSI rate relate to the L in the lift formual though?

Mike

[Ollie]

Every sailplane has its own polar graph of sinking speed versus airspeed. Even for the same plane, the polar curve shifts to higher airspeeds with increases in gross weight and wing loading. In your example, assuming you were flying at the best L/D in no sink you would draw a tangent from the origin of the graph to the polar curve to represent that flight condition. After entering the thermal and flying level, the new flight condition would be represented by a line from the point two knots above the origin, tangent to the polar curve. The difference in air speed between the two tangent intersections with the polar curve would be the airspeed gain in the thermal.

BTW, on thermal days the air between thermals is subsiding. This general subsidance replaces air at the surface that is feeding thermals. So, on the average, there is weak sink between thermals.

The slope of the coefficient of lift versus angle of attack characteristic for a wing depends on the zero lift angle of attack of the airfoil and the induced angle of attack which is a function for the wing's aspect ratio.

To use the lift formula to calculate airspeed, you need to know the coefficient of lift, the density altitude of the air, the wing area and the gross weight of the plane.

[MikeSt]

Thx Ollie.

[Stregone]

Translated: Its complicated.