Dynamic Soaring has gotten a lot of interest lately, and has now been refined to a well-developed flying style. If you haven't seen it, the video "Lift Ticket" (distributed by Radio Carbon Art, and available from "TuffPuppy" himself Dave Reese tuffpuppy "at" aol.com, or Steve Cooper steve "at" scOOp.com for international PAL-only tapes ) has some of the most eye-popping footage of extreme sailplane flying ever documented, with lots of footage to get the feel of the "DS groove" and "the sound". ("the sound" is that indescribable sound of a sailplane going over 150 mph powered by nothing but a moderate breeze...!)
For a while I've been curious about why dynamic soaring works, having been treated to a number of "interesting explanations". Mark Drela offered to write up an explanation of how DS works, so I took him up on it!
This page has both Mark's description of DS (How and Why Does Dynamic Soaring Work?) and a great writeup of DS tips by Scott Hewett (Some Dynamic Soaring Tips - Ideas for Getting Started).
How and Why Does Dynamic Soaring (DS) Work?
A DS'ing glider gets its energy the same way as a sailboat -- by using forces generated in two media moving past each other. Except a DS'ing glider does this cyclically rather than continuously.
In DS, the fast air above the shear layer is "the wind", and the still air under the shear layer is "the water". When the glider does the sharp turn above the layer its wing is acting as a sail. When it does a sharp turn below the layer its wing is acting as a "keel".
It's better to look at how DS works via energy rather than forces. Each time the shear layer is penetrated at a shallow angle, there is an increase in airspeed of delta(V) and hence the glider's kinetic energy relative to the new airmass also increases. So the glider gets a K.E. kick every time the shear layer is pierced, and this energy is dissipated more or less continually by drag.
The maximum speed is reached when the average K.E. input rate equals the average dissipation rate. It can be estimated as
Vmax = K * delta(V) * L/D
where delta(V) is the velocity jump across the interface (wind speed for a boat), and K is some efficiency factor which depends on how well the dynamic turns are done.
This estimate also works for sailboats, where L is the sideways sail force (= keel force), and D is the total sail+hull drag. The L/D of an iceboat is much better than a water boat, so it can go many times faster. The L/D of a glider is better still, but K is lower because of the intermittent forces. But the K*L/D product can still be a lot larger than 1, so Vmax can easily exceed delta(V).
An unbreakable full-size sailplane with L/D=50 could probably reach 500 mph in 50 mph DS conditions. Not for the faint of heart.
-- - Mark
Diagram of Dynamic Soaring loop in a vertical plane (8 kB Acrobat PDF file)
Please note that folks are most often these days are flying loops in a plane tilted to one side, but the vertical-plane loop is easier to see in a diagram. Watch Lift Ticket for how this looks "in real life".
Some Dynamic Soaring Tips - Ideas for Getting Started
> Another quick question... when exploring something that looks like
Boomerang's are a good choice. Watch the movies to "get it" locked into your head as to what the flight path should be. Be aggressive with the entry and "drive it" around the bottom turn.
When at the top, the most common mistake is to "loiter". Hit the boundary of onrushing air, get the energy it offers and turn. Too many guys will stay at the top too long or go too far forward and kill the cumulative effect of picking up speed with each turn. They'll simply have the same speed (mediocre) each turn. Hit it and PULL.
If you get it working, keep your brain tuned ahead of the plane because stuff happens quick. I never used dual rates in 10 years of flying, UNTIL DS. If the DS is working good, I'll flip to low rates. It just really smooths everything out since you're flying at twice the normal speeds. Don't hesitate to ask me for any advise.
By the way, the advise on the exchange about dams as a place to look for DS sites is good. The more narrow at the top the better. Oh, also, regarding foamies and DS, they tend to work best in the smaller circles. Hot composite planes can loiter and get away with it, but foamies just tend to "hit the wall".
Really, if the wind is right and the shape good, a 50' high slope would work just fine. Believe it or not, the backside drop is much more important than the front slope. I've seen Dave Reese launch on little humps that would barely support lift. IMMEDIATELY upon launch, he'll dive for the backside and wind it up. Of course, you have to get some kind of seperation of the air masses, so the steeper backside will sometimes provide that, as well as the "gravity drop" you'll want to start the process off.
After you've experienced DS for the first time, you'll know right away if it is working or not. The quality of DS can vary so much from minute to minute. Some places, like Parker, is is usually good most of the time. But, even the great places shut down, and you know right away just after the bottom turn. It is pretty exciting when you hit great DS for the first time, especially that first time it comes back up the hill at you going faster than it went down.