Crow mix on a flying wing?

[CactusJackSlade]

I was just wondering if you had split ailerons on a large flying wing and used them for crow mixing to slow it down. Would this work?

I would not use any differential and I would not move just the outer panel for turning, instead I was thinking of just toggling both inner and outer panel at the same time... in other words I would not set it up like flaps, but both inner and outer aileron pannels would move equally up and down in the same direction.

So I would come in for a landing and flip my butterfly/crow switch, pull back on the stick and the inner pannels would drop and the outer pannels would go up... all in equal proportion. When turning, the inner and outer panel would as I said, just move together.

I realize that the elevon is part of the flying wing surface so I imagine care must be taken not to disturb the center of lift without proper compensation (inner pannel angle down vs. outer pannel up).

Would this work?... or would it just plummit to the ground like an arrow? I know on real flying wings the have a double pannel, one on top of the other and they just split apart, one up, one down, but I'd like something simpler than that.

Thoughts anyone?

CJS

:confused:

[Ollie]

It's not impossible but it is very complicated to predetermine. The twist in a swept back flying wing is what provides longitudinal trim together with the camber of the inner and outer airfoils as modified by control throws. Ailerons and flaps when deflected change the airfoil function to that of a different airfoil with more or less camber and more or less incidence (twist). It doesn't matter whether you call the inner aileron an inner aileron or a flap. When both down going inner ailerons go down they function as plain flaps. it matters a great deal where the flaps and ailerons are placed spanwise and it matters a great deal how much the flaps go down and ailerons go up to increase the twist of the wing, the increase in camber of the inner panels and the decrease in camber of the outer panels. All the factors in the configuration and control throws have to be coordinated to result in workable longitudinal trim.

Swept back flying wings are stabilized by the wing tips lifting in the opposite direction from the rest of the wing. When the ailerons are located in the wing tips the shift in lift distribution is opposite from that of a conventional tailed
configuration and the ailerons produce proverse yaw rather than adverse yaw.

Conventional tailed configurations are far easier to design because the functions are more seperate and interact less. Swept back flying wings are very complicated to design because there are more interaction of functions where the wing itself has to do everything. There is wide experience with conventional tailed configurations and rules of thumb are well understood and apply well. The opposite is true of flying wings.

[CactusJackSlade]

Ollie,

I should have figured you would be the one to tackle this question! Thanks!

I understood what you said and you articulated it perfectly. I am going to be building a large 100"+ flying wing and since I love the way my full house ships can be slowed down with crow, I just started to ponder the same about a flying wing.

Yes, I can see it would be a delicate balancing act... and if I were to proceed in the endevour a foamie would certainly be an asset... I would foresee many "incidents" while trying to perfect a set-up like that...

Thanks Mr. Techno!

CJS

[Ollie]

Jack,

EPP foam flying wings work well for slope soaring. However they would not be suitable for towline launching without special structural changes. The reason you can get away with EPP swept back flying wings is that much of the mass is distributed similar to the lift distribution so that little bending is imposed on the wing even in high G maneuvers.

When tow line launching there is a stron central force from the tow line that causes wing bending. When a sweptback wing bends the wash out increases and pitch trim is lost. Also, aht high speeds, very strong twisting loads are imposed on the wing even when it isn't being bent. Structural norms for conventional planes don't work very well for swept back flying wings.

Because the aerodynamics and structures of swept back wings ar so unique you would be well advised to closely copy a successful design on your first attempt. See:
http://members.ozemail.com.au/~flyingwing/
http://www.flying-wing.de/index2.htm
http://www.b2streamlines.com/
http://perso.wanadoo.fr/scherrer/matthieu/english/fwairfe.html

[CactusJackSlade]

Ollie,

Thanks again!

CJS

[Sparky Paul]

The Northrop B-35 and B-49 wings had flaps!
Crow should work, but expect to work to get it.

[Aio_1]

Does the wing you're planning have a reasonably high aspect ratio and wing sweep?
If so you could use pitch neutral flaps acting in one direction only.
It is quite straightforward to calculate the position of the flaps so that they will be pitch neutral.

If the aspect ratio and sweep are sufficient then sizable flaps can be installed which (if deflected downwards) provide nose up pitching moment from the inboard section and an oppossing nose down pitching moment from the outboard section. Obviously the reverse is true if the flaps are deflected upwards. By calculating the length and position of these flaps it can be arranged that there is no net pitching effect.

I've done this with a 1.8m flying wing
Sweep was 20 degrees at the quarter chord line.
Aspect ratio was 9
As I remember the flaps covered about 2/3 of the span with the remainder of the trailing edge being elevons.

I don't know if this is of any interest to you as you're after crow braking but it's another simpler option. Again, this arrangement will not be possible for an unswept wing or a wing with a low aspect ratio.

If you want more details let me know and I'll try and dig out the technique I used for calculating the flap length.

Aio

[CactusJackSlade]

Aio_1,

The wing I am thinking of using is "OverKill" from Justin who makes the Bird of Prey so popular on the foamie forum.

It is a high aspect ratio 104" wing... but I will have to dig it out of it's box to determine the info you requested... I will PM you should I start on this endevor!

Thanks!

CJS

[CactusJackSlade]

Hummm...

I was just on the http://www.northcountyflyingmachines.com website and there was mention of the new 60" Moth with flaps and ailerons set up with crow for spot landings...

I will be looking into this!