I think it was James Deck who asked: > A young friend of mine, impressed by viewing some of my videos, has > decided to make a presentation on DS for extra credit in her high school > honors physics class. I'm providing her with good footage from the videos > and she has gathered good background material for her talk. She likes John > Roe's explanation (even the non-honors students can understand it) and is > almost ready but has one question that I couldn't answer. She'd like to > tell her class the maximum velocity theoretically attainable by a sailplane > specifically designed for DS (she thinks the "tech-heads" would really like > that). Can anyone give me some educated prognostications she can use? I saved an earlier message on RCSE that may help answer her question. Regards, Dick ======================== >Reply-To: <[EMAIL PROTECTED]> >From: "Bill Swingle" <[EMAIL PROTECTED]> >To: "RCSE Soaring" <[EMAIL PROTECTED]> >Subject: [RCSE] Dynamic Soaring anthology >Date: Tue, 23 Nov 1999 13:07:33 -0800 > >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 >[EMAIL PROTECTED] >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). > >Joe Wurts > >--------------------------------------------- >From: Joe Wurts <[EMAIL PROTECTED]> >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?). > >Joe Wurts > >--------------------------------------------------- >From: [EMAIL PROTECTED] (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: > >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. > >Blaine Beron-Rawdon >Envision Design >Rancho Palos Verdes, California > > >RCSE-List facilities provided by Model Airplane News. 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