Is anyone familliar with what kind of airfoils are optimal for dynamic soaring? I suppose one would like a thin airfoil with low Cd0 values at higher reynolds numbers (>3.25e6). This seems to be a pretty hot topic in the aerodynamics world. Seems that the RC glider people nowadays know more about dynamic soaring than any aerodynamicists do.
Thanks,
Ian

What is dynamic soaring? I've never heard that term before? Are you talking about thermalling?

Dynamic soaring is high speed and aerobatic flight with unpowered aircraft. You thermal for altitude (so the airfoil needs to be good for lift) and you then dive for speed to do your aerobatics. (simplistic... but covers the basics)

I thought dynamic soaring was slope soaring in the rotar developed behind a cliff top?

a088008 is a lot closer to the true story than the rest of you guys.

It's where the model transitions from a high energy wind zone to a lower energy zone and gains speed by virtue of the inertia of the model reacting with the wind speed change. The model sets up a climbing and diving circle or oblique looping path that presents the wings to the wind shear area in a way such that it accelerates the model. The best spot to do this is at a sharp lip at the top of a slope where there is a severe wind direction and speed change right at the lip. The model dives into the back rotor and then shoots forward through the shear area and into the high energy area and changes it's flight attitude to gain the most speed energy out of the transition.

Done right the models easily exceed 100 mph and many flights have hit well over that up to, I seem to recall, 180 to 200 mph. This is obviously serious stuff and Gentle Lady owners need not apply....

Any of the slope racing airfoils would work well. Low camber airfoils in the 1% range combined with 7 to 8% thicknesses will really work at these speeds. I don't have my Profili at hand just now but I seem to recall that the HN or HS series of airfoils look like they would work very well at this task. There's some 1 to 1.5% camber RG and Selig airfoils that would probably work quite well too.

If no one chimes in with some more specific thoughts or choices I'll move this thread to Sailplanes.

BMatthews has it nailed.

The models can go very fast indeed. I recently saw a 2-meter Opus sailplane clocked at 232MPH with a radar gun. A few moments later the wing exploded. Seriously.

What is needed is a very low camber, very thin airfoil. What is also needed is a VERY strong wing. Not only does the spar (or spars) need to be very strong, but the skin must also be very stiff. Full carbon skins are almost a prerequisite.

The problem is that the models simply are not strong enough and something starts to flutter somewhere... and then you have carbon confetti. The thinner the airfoil, the stronger the wing needs to be to maintain bending and torsional stiffness. Most sailplanes, even those "made for DS," are not up to the task, IMHO.

-David

Perhaps a picture would help:

courtesy of speedfreeks.com

We also have a direct line to Santa so watch the implied insults. Everyone knows that RC glider fliers fly gliders because they can't keep a motor running:-)

That illustration is good. From a088008I and below are good data. Above that, welllllll.... they tried:-).

I had read some theories that had the increase in velocity coming from going between energy layers, but it did not take into account the airplane angle when it hits the air over the lip of the hill where energy is imparted at a high rate in the form of load factor. It is like swinging a weight around your head and only putting a little pull on the string in the same 30 degrees of arc. You can get it going really fast with the same pull each time until a aero drag limit is hit. In this case the gain in gravity on the back side of the loop would be lost in the climb so the only increase comes from the lip action (pun intended - OK not much of a pun, sorry). It would imply that a reallllly light airplane could not dynamic soar since it would always tend to revert to the local flow velocity too quickly. Does that sound reasonable?

It would be exciting to watch. Considering that the airplane rapidly reaches speeds that would keep a brick flying and the load factors involved are near awesome, the airfoil choice would be something that would have minimal drag and yet a thick enough section to hold together under the g loading. Solid carbon fiber might be in order! I would love to see these things in action. My mental simulator tells me it is a little frightening. The sounds must be great. I believe Joe Wurts designed a flying wing to use for training in this kind of flight.

I think that sounds VERY reasonable Ben. But I don't think it needs to be that heavy either. Just regular weight and CLEAN so it hit's the drag limit at a higher terminal velocity.

There was a video on the web a while back of Joe Wurts DS'ing. The speed was quite scary looking and I can see that it would take a lot of concentration to just breath on the sticks at just the right times to best work this stuff. There was also a vid of someone with a discus launched HLG DS'ing with wind coming over a line of wind break trees in a park so it doens't take a lot of difference I'd say if the model is up to taking advantage of it.

It doesn't have to be a geographical setup either. One of my most memorable model flying days was on a stormy and blustery day at the thermal field. I had a 2 meter aileron bird and put it up just as the guys were packing it in due to the weather. I played the model back and forth across the wind and whenever a gust picked up the windward wing I'd turn hard into the wind. The gust energy would pick the model up anywhere from 10 to 50 feet during these "tacks". I'd crab across the wind in the opposite direction and do it again. All in all I was in the air for about 20 minutes at altitudes ranging from 10 to 80 feet and it was up and down about 40 or 50 times between those extremes. Finally the pattern broke down long enough to put me on the ground. It was a very intense and gratifying 20 minutes that I'll never forget.

The mere idea of being within "range" of the debris from a 200+ mph model disintegrating in mid-air keeps me from being anywheres near a DS event!
Or a jet event!
Too much energy converted to ballistic projectiles coming down who knows where!
We did an analysis of the g-loads on another forum... using the radar speeds and the time for a loop..
Close to 20 g!
It seriously stretches model airplane construction techniques at the top end of the performance range.

Paul, if you saw the vid of Joe's plane I have no doubt the G loads in the upper and lower corners where the looping was tightest was even higher than 20 G's.

For myself I'd have no problem being there but I'd be sure to place myself in a spot outside of any likely trajectories fer sure! ! ! !

But NOTHING matches being near a control line Jet speed model or a trio of K&B 40 powered Fast Rat Racers for shear fear factor. It was always in the forefront of my mind that a broken set of lines on these would mean instant perforation for anything or anyone that was in the path. I never did stand within 3 circle diameters of any jet speed model without being behind something like a car.

Bruce, I watched a DS video where a short-lived record was made up north on a beach. The plane exploded on a onshore pass... the wreckage ended up in the surf!
Vincent Hill where the DS guys go really fast is only 2 miles from where I slope. Probably far enough, but I wouldn't be surprised to find a battery pack or servo land there...
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Many years ago I -heard- a control-line Proto speed plane hit a spectator. The fuselage/motor slid off the wing, which remained attached to the Monolineā„¢.
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As I was the AMA Control-Line Contest Board Chairman at the time, that and a couple incidents involving safety at the 1962 Chicago Nats led the AMA President Pete Sotich and I to make things a bit safer... the control-line pull-test was modifed to test for precisely the bad wing-fuselage joint, and spinners were added to R/c airplanes following a head-strike to a control-line flier by an out-of-control r/c plane doing a demo on the last Sunday of the event.
It seems some of the control-line speed fliers.. and these were the good ones, were cheating on the pull test, to save their modified Monolineā„¢ units. They'd attach the control wire to a hook they'd surreptitously hold under the wing, so the hook took the load.
With both hands around the fuselage, the wing-fuselage joint is fairly tested. No hands on the wingtip..
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The fastest model I've seen that scared me is one of the cars running on the wire at Whittier Narrows. I watched the pusher get it going, the guy in the center whip it up to speed, and get up on the pylon support.. the car -vanished-! All that was visible was a red streak on the -other- side of the circle! The catch barrier wasn't much taller than the car, and gave me ZERO confidence in its ability to actually contain a loose vehicle.
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As for jets, it's not the kinetic energy they represent so much as it is the lack of safety when flying at 200 mph. 200 yards out is where they should be, not 20 feet out at a foot above the runway!
I never went back.

Interesting speed stories there Paul. Had me smiling and shaking my head at the extremes we can reach in our pursuit for "fun".

Some folks at my club in Ontario years back explained the requirement for spinners as being to avoid having the exposed threads of the crankshaft grab the skin and bore into someone's leg or other body part. They were totally unshakable in that belief. I just shrugged and walked away....

The best DS'ing airfoil know to man today is the RG-14. Every DS ship has it or somthing very close to it. Very low camber is a must! These models are amazing as you can dive it 100 ft and pull straight up and be right at the 100ft you started at. So effecient.