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        Discussion Competitive potential for flying wings?

#1 JoeCube Oct 06, 2012 12:30 PM

Competitive potential for flying wings?
 
If it weren't for the disadvantage in launch height, do you think that a flying wing DLG could have the potential for being seriously competitive with the current traditional DLG designs? I hope the question is not too ambiguous.

I have searched the forum with no success and apologize in advance if this is a totally ridiculous question. I was just wondering what people's thoughts were on the subject.

Joseph

#2 GaryO Oct 06, 2012 08:02 PM

Joe,

No experience with dlg wings but some experience with free flight wings. Don't glide really well.

Gary

#3 RCPC Oct 06, 2012 10:17 PM

Gary hit the main point of a flying wing: they dont glider very well compared to the traditional DLG design.

a flying wing has the advantage when it comes to parasitic drag because of the lack of the empennage and fuse but the airfoil requires a cubic camber line requiring it to fly at a higher AOA to create the same lift coefficient, thus creating "excessive" drag. since our planes fly predominantly in a high lift coefficient/low parasitic drag regime, a flying wing is ill-suited. a flying wing may have an advantage in launch due to this transient low lift coefficient/high parasitic drag regime, but in order to dampen the yaw excursions on launch, a large vertical stabilizer and/or boom is needed, again destroying the advantages of a flying wing design in DLG. many advances in design come from unlikely places, so dont let this stop you, but keep it all in mind.

lastly, i must balance my post with a disclaimer that i am not a top authority on aero design and it has been several years since i have read up on all this stuff, so i may be mistaken about some of my comments. maybe someone can confirm or deny my statements. doing a quick search of flying wings on the internet and you are sure to find some info that can help you decide for yourself. do a search here on RCG as well in the HL forum. there has been talk of this design for a while. some guy in Europe also made a flying wing DLG, but i dont know about the performance.
cheers
paul

#4 RubSon Oct 07, 2012 07:38 AM

Just for info.

Search: Christian Behrens, Nurflügel on this forum. And you´ll end up with something like this...

http://www.youtube.com/watch?feature...&v=11BzyS5CcMs
And
SAL Nuri erster Thermikflug (2 min 14 sec)


Ruben

#5 lincoln Oct 07, 2012 08:59 PM

I don't think flying wing design has been thoroughly enough explored to be sure that one can't be competitive, though it's hard to see around the launch problem.

#6 discostu956 Oct 07, 2012 09:18 PM

Is it just me, or does that DLG wing fly really nose high?

#7 Mr. Wiz Oct 07, 2012 09:35 PM

Quote:

Originally Posted by discostu956 (Post 22940758)
Is it just me, or does that DLG wing fly really nose high?

Well, if it is then it backs up what Paul was saying above about the AoA of a flying wing.

Mike

#8 kcaldwel Oct 08, 2012 12:15 AM

I believe Christian Behren's flying wing DLG uses AG airfoils. The airfoils will fly at the same AoA as they do at the same wing loading and speed on any other glider. The difference is that you are used to using the fuselage as the reference on a conventional glider, and the wing is already at a positive incidence angle to the fuse. On a flying wing you have nothing but the wing to relate to, and the sweep makes the AoA of the wing more evident.

With a swept flying wing, you can basically use any airfoils you want. They do not have to have a "cubic camber line" whatever that means. Some airfoils will work better than others, as with any wing design.

It is difficult to maintain a good lift distribution over the required speed range on a flying wing, and have some pitch stability and control at the same time. Because the stability, pitch trim, and adverse yaw are all tied up with the lift distribution of the wing, it is very difficult to optimize those as well as the lift distribution at a variety of AoA. With a conventional design you can optimize the lift distribution separately from stability, and adverse yaw can be tuned out without affecting stability or trim drag at different speeds. The wing sweep also effects the lift distribution, and can hurt the low speed handling.

The shorter length that fins act on, rather than on the end of a nice long boom, means the area has to be bigger and therefore likely has more drag than the entire tail of a conventional layout.

There are lots of challenges on a flying wing, and the imagined advantages are hard to actually obtain. The torsion loads are much higher due to the wing sweep, which even makes them harder to build light enough. Flutter is more of an issue. They look very cool, but it isn't easy coming close to the performance of a conventional design.

Kevin

#9 RCPC Oct 08, 2012 12:51 PM

Quote:

Originally Posted by kcaldwel (Post 22941778)
They do not have to have a "cubic camber line" whatever that means.

Sorry, I thought cubic camber line was a common term. Martin Simons uses the term cubic camber line in his book Model Aircraft Aerodynamics but Wikipedia uses the term reflexed camber line, with which you may be more familiar. As you can see, a reflexed camber line is defined with a third order polynomial, hence the term cubic camber line.

Wikipedia has some nice articles on tailless airplanes. This one has a good summary of how longitudinal stability is attained.
Paul

#10 kcaldwel Oct 08, 2012 03:52 PM

Reflexed airfoils are only required for plank flying wings - well, actually airfoils with a positive pitching coefficient. That can be obtained by methods other than reflex, and the camber line does not have to follow a cubic equation even for reflex. All DLG airfoils can be easily made to have a positive pitching moment by reflexing the flaperons, and if you invert any cambered airfoil it will have a positive pitching moment. Neither case is likely to have a cubic equation camber line.

Swept flying wings can use unreflexed airfoils with negative pitching moments. They can even use flaps, which have a large negative pitching moment.

There are lots of websites dealing with the design and optimization of flying wing RC models:

http://www.b2streamlines.com/winglinks.html

It is very difficult on a swept flying wing to meet all the lift distribution and handling requirements at the same time as having pitch stability and pitch trim, in order to approach the performance and handling of a conventional design. DLGs add the additional problem of the throw with a huge yawing moment to deal with at release.

Kevin

#11 wyowindworks Oct 08, 2012 08:12 PM

I think one of the biggest disadvantages is that you cannot use flaperons on a flying wing to shift Cl/Cd curve for drag reduction.

#12 lincoln Oct 08, 2012 08:59 PM

Not quite right. Some swept flying wings have a substantial amount of the trailing edge where you can adjust the camber and the trim in the same direction.

#13 wyowindworks Oct 08, 2012 09:11 PM

Quote:

Originally Posted by lincoln (Post 22949774)
Not quite right. Some swept flying wings have a substantial amount of the trailing edge where you can adjust the camber and the trim in the same direction.

Unfortunately, you need the camber and the trim to go in opposite directions.

Changing the camber on a flying wing changes the pitch. With a conventional tail you can compensate for the pitching moment change efficiently due to the long moment arm of the tailplane. This allows one to alter the camber so the wing can more efficiently operate at varied CL's.

You can split the flaperons of a flying wind so one part alters the camber and one part alters the trim. This is much less efficient though due to the short moment arm of the trim change and loss of camber change in that portion of the wing.

#14 kcaldwel Oct 08, 2012 11:36 PM

1 Attachment(s)
Camber changing is difficult on a flying wing while maintaining trim, stability and an efficient lift distribution. If stability could be done electronically rather than aerodynamically, then it is potentially possible. Negative stability margins could help the efficiency of the a flying wing as well.

Planks with negative stability margins have been flown in the past. The advances in electronics and microprocessors, and the high speed digital servos developed for helicopters, should make it easier to do now. I think that would be the best approach to perhaps capture some of the potential of flying wing gliders. The yaw with the discus launch is a problem though, since gyros are banned on the yaw axis for competitions.

It does seem a lot of work to replace a little stabilizer, especially if you end up with a vertical stab on a boom anyway:

http://www.charlesriverrc.org/articl...lke_asfwpp.htm

http://www.rcsoaringdigest.com/OTW/o...orris_wing.pdf

Kevin

#15 barryplem Oct 09, 2012 09:42 AM

I was thinking about this idea some more, as I have been toying with a handlaunch Zagi 60, and came on the idea that Kevin just mentioned above with the electronic stability assistance. I expect that these methods won't be allowed in competition, which I'm philosophically okay with, but it could be fun to try. The Zagi I have now has super-fast heli tail rotor digitals, so if I can source a good stabilizer, it may be up to the task. I'll let you know how it goes when I get around to it.


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