Active Flow Control

[Sefco]

I don't personally fly RC planes, but I have raced RC cars for years. I'm currently a graduate student performing research in the area of active flow control. There have been numerous publications demonstrating the effectiveness of leading edge synthetic jet actuators delaying stall at high angle of attack. I'm curious as to whether this new technology has trickled down into the modeling community.

[jkettu]

I've never seen such a system in a model. Our club has a full-size sailplane with pneumatic turbulator and it is a major pita to keep all those hundreds (thousands?) of microscopic air vents clean. Not sure if this has any relevance to the system you are talking about.

[Stef]

Have you seen the research and experimental work being done with sound for active flow control?

[AndrewBurns]

I did a bit of research into active boundary layer control as part of an aeronautical eng project last year and I have to say it was pretty interesting. We were trying to design a super short takeoff light aircraft, had to have a stupidly slow stall speed, so we were looking at technologies to get us lift coefficients of 4+. In particular I looked at boundary layer suction, basically the wing surface is either made of or includes a porous layer, some system (eg a compressor) sucks air in through the porous wing skins, removing the stagnant boundary layer around the wing. This has two effects, firstly it reduces drag significantly at high angles of attack and secondly it drastically increases the angle at which the wing will stall.

We decided it wasn't feasible as we roughly calculated it would require 600 horsepower of energy to reach a Cl of ~4 and our main propulsion system would only be around 100 horsepower. However I was thinking about using something like ducted fan suction tapped off from the primary propulsion system of an RC jet to perform the boundary layer suction, something to think about.

[Jim Petro]

In the late '70s, my son had a successful science fair project about electrostatic air control. The demo wing was an Ace foam wing with a coping saw blade inbeded in the leading edge and a thin copper ribbon glued at the high point of the airfoil. A high-voltage neon light transformer (about 18,000vdc) pushed electrons out to the points of the blade teeth and then they flew off to the opposite charged ribbon and carried some of the boundary layer air with them. The smoke stream in a wind-tunnel showed it effectively increased lift "in flight" and also worked "standing still".
Last week I saw an air purifier (hi-voltage source) the size of a lipstick and it plugs into the cigarette lighter in the car. Made me think about trying this on a rc plane.
The rest of the story......A water filled container was used in the test appuratus for vibration damping. I received a phone call at work from my son excitedly telling about how he bumped the pingpong table and splashed out water, and now there were really weird blue glowing coronas flowing on the table. I quietly told him to pull the power plug......wipe up the spilled water....and DO NOT mention any of this to his mother!

[Accu157]

There is one model out there I know of, a large R/C sailplane, that has... quasi-flow control (it's not active). I don't know how much of it was actually engineered. It's not really relevant what model it was, basically it had about 200 holes drilled in the top of the wing along a line about 1/3 chord with a tube running to the holes. Then an itty bitty air intake at each side of the wing functioned as the "pump" (probably a false description) to allow air to flow out the pinholes.

You'll find that modelers have a lot of relevant knowledge in areas you are not applying your science, but for the most part... a great deal of our models are not engineered. Most of the research you are looking into will not make it into the modeling world. However, our models may make it into your world. We're more about the joy of the flight than the science. If you're doing aero modeling right, it's parallel to fishing. We fly to get away from that sometimes horrible chaos math. ;)

A bit of rule of thumb advice... work with the biggest size model you can in the wind tunnel, stay away from models our size if you're working with good old fashioned air. If you do try to make a proof of concept model, make the wing big, and get as deep of a wing chord you can... Bring that reynold number up, and increase your allowed error bounds with the airfoil you use. Predicted performance versus real world performance (wind tunnel vs. outside) difference is pretty substantial. The error bounds are uncomfortably high. If you're using Oil as the fluid, do whatever your math tells you to do. If you try to go with a high aspect model, it better have a deep chord. People have found a stronger, quantitative difference in the way a model flies with deep chord (same deep chord, different airfoil) more than by just going to a wider wing. This is especially felt in large scale sailplanes, where 4 meter span models are startlingly different from 6 to 10 meter models. The extra span just doesn't do the same without the chord (surface area of the wing).

Talk to the old crusty sailplane guys too. Especially the ones that have designed some fancy fiberglass models. They deal with science a bit more deeply than the rest. Starting here is good too.