Community designed flying wing

[sloper steve]

Instead of having a secret lab for testing airfoils, sweep, twist, reflex in private I'd rather have opinions of others given along the way in development.

Slo Flying Wings (http://www.slofly.com) do tricks and glide well in light slope wind but maybe they can be improved?

4 comparison models are on the board which differ in root airfoil, tip airfoil and twist. The sweep is already chosen and can be modified if for good reason.

#1 reflexed root and tip with twist
(this is the current plane and it works great)

#2 regular root, regular tip, twist

#3 regular root, reflexed tip, less twist

#4 regular root, regular middle and a reflexed and twisted tip.
The tip of x" would be made on a different cut and attached to the wing.
This gives an efficient wing all the way to the 'tail' as opposed to an interpolated twist in a wing which loses spanwise efficiency as you get closer to the tip.
It is like taking a regular tailed airplane, making the wing a V, then splitting the tail in two and gluing each half to one side of the V. The only loss of efficiency would be the sweep.

[Ollie]

There is the old joke that," A camel is a horse that was engineered by a committee." ;)

You probably want to use a linear taper distribution along the span to keep the production process simple. In that case you want to use the Panknin twist formula. See:
http://www.b2streamlines.com/Panknin.htm
For airfoil recommendations see:
http://www.aerodesign.de/english/profile/profile_s.htm

[Ollie]

Your present design with such a light wing loading will be great in light winds and thermals. The main limitation I see is that it will have a hard time penetrating when the wind picks up. That can be improved by just increasing the wing loading with ballast. The trick is to add ballast without increasing the wing bending load much. That can be acomplished by distributing the ballast spanwise so that it floats on the lift rather than being concentrated in the center where it increases bending during high positive G maneuvers like pulling out sharply from a long dive. One way of doing it is to include a tubular spar through the CG that can be loaded with pencil like lead slugs. This complicates the manufacture a little but greatly improves the range of flyable wind speeds.

[sloper steve]

I agree.
This model is made to be light and has a max of about 12mph wind.

Thin lead strips across the wing works pretty good but I will probably make an EPP version for higher winds down the road.

[BMatthews]

Twist is pretty much a set item. Look up the Panknin spreadsheet that tells you what the twist and CG location should be for a typical swept flying wing.

After that it's just what airfoil you want. The options you mention sound pretty good to me.

If you're doing this in foam with packing tape as a skin and spar system then there's not much point in picking some of the new and fancy critical airfoils. The tape will destroy any of the subtle differences. I'd say you want something simple. For the symetrical airfoil try the Selig 8020. It was designed as an all moving stabilator airfoil for RC sailplanes. It was designed to have lower drag over a wider range of AoA's than the typical NACA symetrical airfoils of the same thickness. It's also blunt enough that it should do well with the tape skins or if done in EPP.

[Ollie]

The URL for the Panknin twist formula in my first post doesn't work because it has been discontinued. However Google has cached it. Do a Google search for "Panknin Twist Formula" and choose the first item. The airfoils for the root and tip strongly affect the geometric twist through their pitching moment coefficients and their zero lift angles of attack. The airfoils should be reproduced fairly accurately for best results. Small errors in airfoil reproduction can be compensated with a small shift in the CG to correct for pitch trim but if the correction by CG shift puts the trimmed CG too far aft, pitch stability will be lost in the attempt to achieve pitch trim. The calculations also depend on the accuracy of the planform. The accumulation of too many inaccuracies may result in a plane that is uncontrollable. That's what makes flying wings so challenging. The interactions between various aspects of the design are so much greater than on a conventional configuration. You can regain some pitch trim adjustment capability by making the elevons outboard rather than strip ailerons. This will allow some adjustment of effective twist and effective pitching moment of the outer part of the wing which would otherwise be locked in by the use of strip elevons.

I had the good fortune of attending Dr Panknin's presentation at the National Sailplane Symposium in Madison. One of the points he made was that tip fins, ears as he called them, have the non aerodynamic function of providing improved visibility and pilot orientation when the wing is edge on to the pilots view. He suggested that the inside and outside surfaces of the "ears" be of contrasting colors to aid pilot orientation.

[Antonsoarer]

Steve, I recently designed and built a lightweight, didn't get to use the Panknin formula (well not consciously, but I have been designing and building for decades) so it may not be ideal but flies well.

I used Profili2 to select and modify sections and determine twist.

42" span
S8025-root, JW-tip
EPP/carbon spar and minimum tape
Ironed down edges of strapping tape so it’s an OK finish
Installed 750mah AAAs and micro radio.

Not much camber but this seems to help it handle high winds. Only 1 degree of twist combined with low sweepback and the elevons take care of the rest of the twist adjustment as they are wider outboard. To reduce drag the winglets start well back on the tip chord. I will post a photo soon.

This is a link to a good site that covers design for lightweight inspiration/construction. Videos and building/modifying instructions. Go to the laboratory section for construction techniques. Probably best to print off most of it and keep in the workshop.

Hand Launch Aerobatics (http://www.handlaunchaerobatics.com/HTML/HLAPages/HLA.html)

This site inspired me to build the S8025 based lightweight, it can't handle anything above about 15 mph but below this it really is fun. If you watch the videos they are able to fly their designs in low winds of small ridges, something I really enjoy. When watching the videos it looks strange because these things keep flying over flat ground including aerobatics! I have flown walls as low as 8 feet for hours with the model often inches off the ground in front of the wall. It's also good in the winter because I stand facing the wall with my back to the wind. This type of flying develops skills better than most big-air flying and is easy to do close to home.

Tony.

[Antonsoarer]

Some videos of the fun you can have even without a slope, I am sure the HLA guys want mind if I post some of their fun moments.


More flying wing stuff (http://www.glide.dyndns.org/on-the-wing3/159_Malone_Arrow_60.pdf)

and more (http://www.b2streamlines.com/Panknin.html)


Unzip and view with your media player.

[sloper steve]

Great stuff thanks so much!

[Antonsoarer]

Steve, Some food for design thought:

Update on the S8025/JW-tip sectioned lightweight wing. Now I have adjusted the CG (backwards) and tested it on a bigger slope it can cope with winds of 24mph and has an amazing turn of speed for such a low wing loading. At neutral trim the elevons now follow the bottom wing surface exactly which I think has helped reduce drag.

The stall is interesting; it manages about 2 a second with very fast recovery. This provides me with a nice indicator of when I am at the limit (pulling too hard in turns, etc).

I have used very lightweight elevons as per the HLA designs but they are flexible. I was going to replace them with stiffer designs, but not after this weekend's flight. The servos connect close to the centre of the model so at speed the elevons can twist or blow back. This turns out to be a great design feature, if I trim the model for a gentle climb and throw it hard, it just stays flat until it slows down. When its really moving it stays where it's put as the elevons blow back a little and this also gives a dynamic rates effect so the sensitivity on in pitch and roll stays constant independent of speed!

Tony.