Wind Tunnels

[andrew lorino]

Would wind tunnel testing of the model produce beneficial data? Or would it be best to tweak the model based on test flights?

[bernard]

Would wind tunnel testing of the model produce beneficial data? Or would it be best to tweak the model based on test flights?

Can't say exactly since I haven't worked with a wind tunnel, but I'd imagine most wind tunnels are made for testing larger planes that can take a larger breeze. A Wright Stuff plane made close to the minimum weight would likely be made of material that would warp or snap under the breeze of even a hair dryer.

What data would you want to collect and how might you use it? Many teams that have been successful with this event have done it through lots of flying and adjusting.

[jander14indoor]

Not lots and hard to do right.

These things fly VERY slow and a wind tunnel with such slow air speeds is amazingly difficult to do right. A key thing to get good data in a wind tunnel is uniform flow across the tunnel. At higher speeds, not too hard to do with simple flow straighteners where the turbulence is a small part of the flow. At these low speeds, small turbulence becomes a much bigger piece of the picture and makes the data hard to interpret.

Second, the aerodynamics is pretty well understood already, not much to be gained over what is available in published materials.

Finally, understand why wind tunnels are used. Its often to dangerous or expensive or time consuming to take data on full size airplanes. So to gather data a scaled down model is used in the wind tunnel. Or some part is studied in isolation (airfoils mostly). Then the data has to be extrapolated correctly to a full size plane, not a trivial exercise. But these planes aren't large, nor dangerous, nor expensive. You can test the plane directly to get the data you need and skip all those steps.

My suggestion (especially for a beginning team) would be to focus on building fast to get flying, and then spend a lot of organized time flying the models themselves and learning to trim and wind.

There is one area that might be interesting for tunnel testing if you can get the tunnel right. Airfoil design. Maybe. A lot of designs have been tried over the years and competition has driven to pretty good designs that we use now. They may not be the best because we can't quantify them well without wind tunnel data (which is hard to get right on these things). Before you go down that path though, consider cost vs benefit. With no wind tunnel data it is possible to build VERY good airplanes, flight times over three minutes. But few teams achieve this. I suspect (no data mind you) that there isn't more than maybe a few to 10 or 20% at most to be gained by a better wing. That means at most 20 to 40 seconds on a three minute flight. If you can't fly three minutes already, there are faster ways to improve than messing with airfoil design! If you can fly that long, you can do pretty good comparative testing of alternative airfoils without a wind tunnel. So again, cost vs benefit...

Jeff Anderson
Livonia, MI

[andrew lorino]

...What data would you want to collect and how might you use it? Many teams that have been successful with this event have done it through lots of flying and adjusting.

I would use it mostly for lift/drag ratios, with maybe some instruments to test pitch/yaw deflections or oscillations.

[Less_Incidence]

I don't think wind tunnel testing would be of very much merit for Wright Stuff. The main reason is that this event (and indoor aeromodeling in general) is 90% matching prop and rubber combination. In the words of Bill Gowen, 4-category F1M world record holder, a mediocre model with a great prop and rubber will absolutely beat out a great model with a mediocre prop and rubber combo any day of the week. As long as you have an airframe that is stiff, stable, and adjustable, you can achieve very high times.

Last year, my top times with a very mediocre model having somewhere around 85 field repairs on it were in the 3:35-3:40 range in a 35' ceiling. With a much more well-built, newer model with the same prop/rubber and similar, optimized trim settings, my times only went up by 5-10 seconds.

All that being said, the one crucial element of an airframe is weight. Any weight over 7 grams is just lost efficiency.

L/D ratio isn't a major unknown in Wright Stuff or indoor aircraft in general. It's well known that a wing with an elliptical planform has a higher L/D than a more rectangular wing. The trade off is the higher wing loading on an ellipse, and, of course, the fact that elliptical wings at very low aspect ratios (like Wright Stuff models) are very difficult to construct, especially with relatively thick, heavy materials. I've never seen a Wright Stuff plane with an elliptical wing, and I don't think it would have a whole lot of benefit. F1D flyers haven't proved it one way or the other, but I would think that such a heavy model might result in lower wing loading being of greater benefit than high L/D.

In all my experience (which really isn't very extensive in comparison to someone like Jeff Anderson, I've only flown 6 or so classes of indoor for a little over a year), I have never once encountered or had a problem with aerodynamic oscillation or flutter in an indoor model. I did watch one of Joshua Finn's F1D props go into flutter in a dive at Lakehurst, but that was an exception and the one and only instance that I know of.

Pitch and yaw deflection could be interesting to measure in a wind tunnel, but actually extrapolating what sort of effect this will have on the model from a bench test seems quite difficult.

Overall, it's much better just to experiment by doing lots of flying and testing and observing the practical rather than the theoretical.