[Courtesy of Bill Swingle, as posted on RCSE, November 1999]
I've saved several of the RCSE posts from when Joe Wurts first started mentioning DS. I found it fun to watch the topic evolve over the course of the discussions. I've included some of the posts below. If possible, try to get your hands on one of the video's John Roe sold for the F3B fund raiser. The video explains it in seconds.
Bill Swingle bill_swingle "at" adelphia.net Pleasanton, CA
From: Joe Wurts Date: 28 May 96 02:35:19 EDT Subject: Dynamic Soaring
For quite a long time I've heard about "dynamic soaring", but have almost never really used it in any operational sense while flying rc gliders. In fact, I've kind of filed it under the Holy Grail category. Just one of those things that you read about. But I've now had a bit of practical experience with it.
One of the slopes that I have been flying at has a very pronounced "razor back" to it (Parker Mountain near Acton CA). What is really neat about it is that the air behind the hill is completely separated. That is, it can be blowing 25 mph on the face, and behind the hill, it is almost calm and sometimes even blowing softly in the opposite direction. It turns out that this is an absolutely perfect set-up for dynamic soaring. All you have to do is fly straight down-wind over the hill into the calm air and turn around. If you want, when you come back over the upwind face, turn around and repeat. With each turn, you get an amazing boost in the energy of the glider. The first time I really played with this was with my Floyd, and on the second go-around I fluttered the wings. The plane will take an extended vertical dive without any possibility of flutter, so I was able to get it to above the terminal velocity of the glider in horizontal flight!!!
One thing that is especially wild is when the wind dies down a bit, and you can just stay up in the normal lift in minimum sink mode. Start doing the orbiting for the dynamic soaring and you can get up to about three times the speed that you can when you just fly in the normal slope lift. Wild stuff. What really gets entertaining is when you make a mistake behind the hill. The air is a bit turbulent, and occasionally I miss the air (read: smite the earth). This is where a good foamie comes in handy. I woulda never really investigated this phenomena without a crash-proof plane.
If your slope has separated air behind the hill, and you do not mind occasionally crashing while you learn a new trick, give this a try. Caution, I'd recommend trying this maneuver out sometime when you have the hill to yourself. It takes a little getting used to... And a hint, the lower you go on the downwind side, the better off you are (more delta-vee typically).
From: Joe Wurts <103610.3507 "at" CompuServe.COM> Date: 31 May 96 01:09:58 EDT Subject: RE: dynamic soaring
>> With each turn, you get an amazing boost in the energy of >the glider. >"Dynamic soaring"--- is this what seabirds do over ocean >chop/swell? Where is >the extra energy coming from (are you sure there is any?!)? >Using gravity to >pick up more ground speed while in the dead zone with less >headwind=lower drag?
The energy increase in dynamic soaring is due to flying into a airmass that gives you a change in airspeed "free" of charge. Lets go through an example here. Lets assume a 25 mph wind on a slope, with the backside completely calm (I've flown at slopes where the wind on the backside is blowing towards the top at 1/2-2/3 of windspeed, but we will use the worse case above). I turn downwind with 25mph airspeed, and with the windspeed, I get a 50 mph groundspeed. I then enter the calm air, and with the 50 mph gorundspeed, I now have a 50 mph airspeed as well. I turn around, and fly into the active wind on top/in front of the hill with this 50 mph groundspeed and the 25 mph wind speed I now have 75 mph airspeed. Without drag/turning losses, each turn adds 25 mph to the airspeed! Who says there ain't no such thing as a free lunch!
You can tell when flying in these dynamic turns that it is purely a relative wind change that gives you the energy boost. If I make a mistake when I go behind the hill, or try and fly back there without crossing the airmass boundary, I quickly prepare for a long hike, as the model is not going to be anywhere nearby for long. Also, you can really hear the airspeed do a quite sudden change when the model crosses the shear boundary between the airmasses, with an almost step function change in noise indicated airspeed. Just see it in operation once, and you will become a believer that it is not rotor induced lift on the backside, but a delta velocity thing.
Due to the practical limitations of the drag increasing with airspeed as well as the turn losses, it seems that the plane reaches an equilibrium after 3-5 turns. The foamies reach equilibrium in 2-3 turns due to a higher drag situation. Still, I quickly get the foamies to a faster speed doing this than I ever get in front of the slope. In fact, I've used it occasionally in combat for recovery. I get hit, tumble for a while before a recovery, and now I have the option of turning back into the wind with low speed and energy. Or, I can go downwind behind the hill, get a quick boost from a dynamic turn and reenter the combat zone with lotsa energy. A cool manuever to add to your repertoire (sp?).
From: evd "at" netcom.com (Blaine + Deborah Beron-Rawdon) Date: Sat, 1 Jun 1996 07:14:07 -0700 Subject: Dynamic Soaring - How It Works
Brad Hawley recently asked for an explanation of dynamic soaring. Here is my understanding of it.
In Still Air:
In still air, sailplanes glide at a given speed and sink rate, according to their glide polar. For a short period of time this sink rate may be altered by exchanging speed and altitude. Ignoring for the moment the underlying ongoing energy loss which is reflected by the sink rate, it can be shown that the sum of kinetic energy (from speed) and potential energy (altitude) must remain constant. If you pull up you loose speed but gain altitude.
This can be expressed in the formula:
mgh + 1/2mv^2 = c
where m equals the model's mass in slugs (= pounds / 32.17), g is gravitational acceleration (32.17 ft/sec^2), h is altitude in feet, v is airspeed in feet/sec, and c is a constant which is a function of the starting position, or reference altitude.
This equation can be messed around so that you can see the rate of change of altitude with a change in speed:
dh/dv = -v/g
Example: If a plane going 64 ft/sec pulls up so that it looses 1 ft/sec speed, it will gain 2 feet. Note that a plane going 16 ft/sec gains only 1/2 ft for a 1 ft/sec loss of speed. This is an issue with dynamic soaring.
This equation can be flipped:
dv/dh = -g/v
This shows that the loss of speed with height is less for faster planes. Let's call this value the model's "gradient".
Dynamic soaring requires air that is moving in a particular way. Specifically, what you need is a steady, strong wind moving along a large, relatively open, smooth surface. This results in a deep, relatively unmixed boundary layer in which the air near the surface is moving considerably slower than the air at higher heights. The wind "gradient" is the rate of change of wind speed with altitude. The gradient is strongest near the surface and diminishes gradually with altitude.
If the wind gradient is greater than the model's gradient an interesting phenomenon can occur. While flying directly into the wind, the model can be pulled up. As it gains altitude it looses speed, but this is compensated by the increased wind speed at the higher altitude, so the model continues to climb until the wind gradient is less than the model's gradient (minus a factor which is dependent on the model's sink rate).
At this point, you can turn 180 degrees to straight down wind. Now the airplane sinks due to its basic sink rate, but as it looses altitude the wind gradient causes it to increase airspeed! This is neat! This is an increase in energy which can be traded for a little altitude which in effect diminishes the sink rate of the plane. When the plane gets close to the ground, you can turn around again and climb back up, and repeat the cycle.
If you have a very strong gradient, or a very clean, fast plane you don't have to go directly up and down wind to get this to work. You can climb and descend at an angle to wind so that you can move across the wind as well as down wind.
I have seen gulls do this sawtooth cross wind pattern over the ocean on a strong day, as well as over large fields in England on a very strong day. No flapping, just a big zig-zag across the wind. Minimum altitudes were something like five feet. Max height was something like fifty feet, but my memory is not well calibrated here.
Note that this effect works best for low sink rate, very fast airplanes. Perhaps this explains some of the differences between sea birds and land thermal soarers. Sea birds tend to have high wing loadings and aspect ratios. Hawks and eagles tend to have much lower aspect ratios and lighter wing loadings in order to work thermal lift which favors low speeds and sinkrates.
It is somewhat difficult to exploit this phenomenon with an R/C sailplane since winds which generate sufficient gradients are likely to be considered too strong to fly in. Also, it may be difficult to dynamic soar from a fixed location!
We do see some gradient effects with our models. For instance, when landing in strong wind (heading upwind) a much greater sinkrate near the ground can be noticed. Also, when landing downwind in even a medium breeze, the plane seems to come down much more slowly - this is a gradient effect.
That's all for now.
Rancho Palos Verdes, California