Flying wings - stability
 

Thanks for all the links & tips people gave me in my last question about flying wings. Its a bit more complicated than glueing 2 wings together and strapping your radio gear to it, isnt it ? :eek:

I've done a lot of reading and think that the following statements are true concerning stability:


Yaw stability - this is aided by sweep. It is not aided by dihedral but a small amount of anhedral can help.

Pitch stability - a well calculated and executed C/G is the key.

Roll stability - washout or reflex in the elevons provide stability here.


Is that right ? I'm particularly unsure about the roll stability as I've read a statement attributed to Bruce Tharpe (Vortex designer) who says that reflex isnt needed as long as a flying wing isnt nose or tail heavy. (whether or not you need washout instead - I dunno).


Any opinions would be great.

RE: Flying wings - stability
 

It really depends on what purpose the airplane serves.
A sloper doesn't need as much "stability" as a TD, which can get to distances and altitudes where orientation isn't immediately obvious.
Anhedral versus dihedral, again a purpose-driven requirement.
No dihedral is immensely easier to build.
Sweep back provides both the dihedral effect and yaw stability.
Reflex depends on the wing profile. Symmetricals use less to almost none, while cambered surfaces need more.
Depends on the requirements... there's no one right way to do it.
Washout isn't needed for a sloper.
A TD might appreciate some.

RE: Flying wings - stability
 

You've got a lot of those ideas mixed up in your assumptions.

.......Yaw stability - this is aided by sweep. It is not aided by dihedral but a small amount of anhedral can help. ........

Nope, yaw stability is determined by the fin area or EFFICTIVE fin area. Granted the sweep angle can induce some effective fin area but it's minimal and depends on the sweep angle, thickness of the wing section and probably a couple of other things. Zagis or Zagi like clones have been flown without the tip fins but they are really in the ragged edge from what I've read. And this ONLY works for swept wings. Plank style flying wings require fins. No way out of it for these.

.......Pitch stability - a well calculated and executed C/G is the key.

Roll stability - washout or reflex in the elevons provide stability here. .....

Both the CG position and the washout and/or reflex are pitch related only. The washout or reflex won't have any effect on the roll stability. Washout in a plank style wing is neither needed nor desireable. Planks depend solely on reflex. Swept wings can use washout or reflex or a combination of both. This is because the sweep places the tips behind the rest of the wing so that it sort of acts like a conventional planform with the "stabilizer" being located to the rear of the majority of the wing area.

.....Yaw stability - this is aided by sweep. It is not aided by dihedral but a small amount of anhedral can help.....

What? We did this one already, right? Well sort of. We've closed the circle of confusion. It's not YAW stability that dihderal affects but rather the ROLL stability. Dihedral works for swept wings just like it does for regular wings. Wing sweep acts like dihedral. I've heard that a 30 degree sweep is equivalent to a 5 degree dihedral angle. So why do you see so many swept wings with anhedral? Simple. The sweep acts like dihedral which makes it too stable for the desires of the flyer. So they add ANhedral to reduce the stability to what they desire. It doesn't help that the droop in the leading edge looks worse thanks to the strong washout angles towards the tips making it look like there is more anhedral than there is. But the long and short of it is that sweep back and dihedral add roll stability and anhedral takes it away.

If you make a swept wing with no anhedral it will tend to return to level flight thanks to the stabilizing effects of the sweep. So in that way it has roll stability. If you want to have the same stability with a plank style wing you need to use dihedral.

.....Bruce Tharpe (Vortex designer) who says that reflex isnt needed as long as a flying wing isnt nose or tail heavy.....

If it's a swept wing with a symetrical airfoil and no washout then you may be able to get by without any reflex. However most folks would find it hard to control as they would need to ride the elevator constantly. In the end you would probably start with a flat symetrical wing and trim the balance slightly ahead of the true neutral point and compensate by adding some "up" trim to the elevons. That up trim IS reflex. Most likely he uses some wing twist washout to provide the stability that most folks value. With that in place and the CG in the correct place it should trim out with no added elevon reflex.

RE: Flying wings - stability
 

Blimey - did I get any of that right ? :eek:

Is it true to say that cambered airfoils (like clark-y) dont fare to well on flying wings ? and that a semi-symmetrical or symmetrical airfoil would be a better choice ?

I'd like to design a flying wing that can fly slowly in 0-20ish mph winds (pipe dream ?)

RE: Flying wings - stability
 

.
It's been done. :)
Here's my ZAGI 3C in a 20 mph gust 35 wind today... That's about about as fast a wind as it can handle.. any more speed and it goes downwind. (Froze my butt off! Note the white fungus on the mountain tops.. that's as close as I care to see THAT stiuff! )
As for airfoils, a Clark YH is the starting point for many flying wings, but there are better more modern profiles.. The MH series by Martin Heppler are intended for wings of various configurations..

RE: Flying wings - stability
 

Don't sweat it. When you're trying to learn about and juggle 47 facts it's easy to get mixed up. You're interested and you're learning. That's what it's all about.

On sharply swept wings a Clark Y at the root blending along the span to a symetrical at the tip is a fine choice. You COULD use the ClarkY all the way but the Panknin formulas will tell you that you need to twist the wing totally out of shape. Another option is what Paoli did with his flying wing. It's ClarkY to the tips and flat but he cuts the controls off in such a way and flips them upside down and side to side and then hinges them onto the wing so that he ends up with an upside down airfoil that includes some washout all at once at the tips where it's needed. Check it out...

http://perso.wanadoo.fr/scherrer/mat...sh/paolie.html

If you want to fly slowly you just use the same methods with a flying wing as with a regular airplane. I've been dancing with the plank bug off and on over the past year and recently drew up the plans shown below. It's 48 inch span, flat with elevons, tip fins to act as vortex control tiplets as well as yaw control fins and a light weight structure. Area is 480 sq inches but because of the reflex the effective area for wing loading calculations is more like 300 to 330. The rest is there to act like the stabilizer. I'm hoping to keep the 049 glow powered verson down to about 16 to 18 oz. The neat thing is that all the radio is in the wing. So with three fuselage pods I can have a towline glider, glow power sport model that sill thermals or a quiet Speed 400 electric sailplane wing. This model should be able to skip around quite quickly but it'll easily slow down to about 8 or 10 mph for slow flying. It's just a big square wing that uses some laminated leading edge sections and 3 extra different sized ribs per side. With no dihedral it'll build on the board in one piece. Tiplets or tip fins are also known to provide a measure of dihedral effect so the model should be self leveling in normal flight. The tip fins will be canted outwards at about 10 degrees from vertical to enhance this effect and for looks.

It's not a proven design yet but if you have CAD to look at a DXF file I'll email you the file so you can look it over for ideas for your own model.

I've also seen folks that made up Zagi patterned wings that use a higher lift section and built up balsa construction. This produces a lightweight model that can fly like a Zagi but slows down much better.

RE: Flying wings - stability
 

Thanks for the advice - could you email the dxf file to [email protected] ? :D

RE: Flying wings - stability
 

I'm thinking about 15 degrees of sweep - is that "sharply swept" ?

(thats based on 5 degrees of dihedral x 3 = 15 degrees of sweep)

Assuming my 15 degrees is a correct calculation, could I still use Clark-Y at the root blending to a symmetrical airfoil at the tip - as you suggest ? Or is there a better solution ?


I've seen Paolis page and was very interested - he's using the cut-off wing tips as washout ? Am I right in thinking his solution could be used for any kind of flying wing planform ?


Thanks :D

RE: Flying wings - stability
 

Digsy,
I'm no expert. But, I'll tell you about my attempts to make a slow flying trainer delta. My small one has 360 sq inches and used a clark yh (not flat-bottomed) blended into a blunt symmetrical on the tips. Some say that I did it in reverse, others say it was right. What I can tell you is that it has displayed no bad habits yet. In the past I have used .15la's and made it to heavy to slow down very well. My current effort is to put a norvel .061 on the same thing and I expect to be able to go as slow as I like. But, none of them take the wind like Tall Paul's Zagi does in the pic. I think that a wide wing flies better than a delta in the wind due to the reduced need for a vertical stab. (I said I think!)

ps. 47 facts is an understatement of what goes on in your mind when your learning this stuff. The guys in this forum have been a big help to me in the past. (and present)

If you want to build a scale wing check out www.luft46.com for some great 3-views of german warbirds that never made it! Lotsa flying wings!