How to convert thermal lift to airspeed
Thread Starter
Joined: Mar 2001
Posts: 223
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From: Toronto, Ont, Canada
How to convert thermal lift to airspeed
Ive been doing some reading about soaring.
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
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
Joined: Nov 2002
Posts: 149
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From: the pub
It would depend entirely on the polar curve of the glider you were flying at the time.match the thermal lift to r.o.d.for given airspeed and there's your answer.Gliders with poorer performance will not gain as much airspeed(obviously).
Joined: May 2000
Posts: 3,204
Likes: 2
From: Seattle
For a moment, assume no thermal. If you're flying at your minimum sink speed, to what speed will you accelerate when you push your nose over to get an additional 200 (or 400) fpm sink? That is the approximate speed you will attain to convert the thermal lift into forward speed.
Joined: Jan 2003
Posts: 811
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From: The Heart
Yes you could use the lift formula but bear in mind that the increase in lift produced by the thermal must first be calculated and subtracted from the Lift on the one side of the equation and then all the other factors removed from the other side to leave the the square route of vsquared, ie. speed.
But why?
But why?
Joined: Mar 2000
Posts: 111
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From: UK
One dot right has it spot on - just start at the the point on the polar you choose (100kts in this case) and just move right along the polar until you reach a point where rod is 200fpm greater, that's your new speed.
Of course, this is all entirely contrary to good soaring practise, whereby one sould pull up immediately on encountering said updraught to decelerate to an appropriate speed or to start circling.
Of course, this is all entirely contrary to good soaring practise, whereby one sould pull up immediately on encountering said updraught to decelerate to an appropriate speed or to start circling.
