CF flying wing - Page 4 - RC Groups
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Jan 19, 2008, 01:03 PM
internet gadfly
nmasters's Avatar
Sweep and speed are not connected until you get well over 500mph. I've seen them from 13 degrees to 60. The 13 degree one tumbles but is recoverable. The 60 degree one didn't tumble but it had a nasty tip stall problem. Sorry I don't have time for the other questions today. I'll work on it tonight. See if these links are helpful in understanding induced drag of swept wings

https://www.rcgroups.com/forums/show...kb#post6047175
http://www.homebuiltairplanes.com/fo...pwash#post2702

http://www.homebuiltairplanes.com/fo...pwash#post2723

http://www.homebuiltairplanes.com/fo...wash#post17555

--Norm
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Jan 19, 2008, 06:00 PM
Registered User
"Tumbles"? Please to be defining ..... "tumbles". We've been working flying wings in the field for the past 7 (no theory here, just cold, hard flying!) years and I don't believe I've EVER seen anything that sounds like a "tumble".
Jan 19, 2008, 08:42 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by CenTexFlyer
"Tumbles"? Please to be defining ..... "tumbles".
A tumble is where the plane flips over backward. In the only video I've seen of one it looks like very small radius (<3 chord length) loops. In fact the 1:1 AV-36 was known to occasionally do one tumble at the top of a loop. The video was of the 1:1 BKB-1. I've heard that Laddie Mikulasko's model BKB also tumbles and recovers. In the descriptions Ive read is sounds like the aftermath of an inverted stall. Why some 'wings do it and others don't I couldn't say but there was a very long paper on it a few years ago. I'll see if I can find it.

--Norm
Jan 19, 2008, 10:31 PM
Registered User
..... and what would it take to precipitate this condition?

Flying straight and level and it just.... "tumbles"? Full elevon deflection? Inverted stalls we've done.... they'll break over quicker when inverted, but if the thing is not designed for such, it's to be expected. Positive loading on the foil is recommended in all manuvers. Recovery from any unusual attitude has been pretty much a "non-event" for a properly loaded/designed flying wing. We have never entered into a manuver that ended in a terminal situation. I do hope you find the paper, because it appears I'm ignorant of many conditions my wings can get into!

Gene
Jan 20, 2008, 12:45 AM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by CenTexFlyer
..... and what would it take to precipitate this condition?
Here ya go: NASA-TM-111858

The long paper I was thinking of was Guy Graton's PhD thesis. However tumbling of rigid wing tailless aircraft is just a couple of paragraphs in a very long document. This NASA TM is more to the point
Jan 20, 2008, 09:18 AM
Registered User
Thanks!

But........ it's not real world because they used flat plate airfoils. We can easily duplicate the "free to pitch" mode by cutting a wing from bluecore and test gliding it without any further modifications. You will get the "tumble" every time. The paper was some great mental gymnastics, but that's about it.

But again, thanks for the read.

Gene
Jan 20, 2008, 12:56 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by CenTexFlyer
The paper was some great mental gymnastics, but that's about it.
Oh... shoot. I just assumed that I could count on scientific documents that were recommended to me by one of the engineers on the X-48. Guess I should be more critical of my sources.

BTW the full size planes I mentioned had better airfoil sections than the test models.
Jan 20, 2008, 02:10 PM
Registered User
Joel K. Scholz's Avatar
Norm thats a nice looking airfoil you have used to illustrate the yahudi. Can you tell me a bit more about it. As far as tumbling goes I have been developing some RC Rogallo wings and to my surprise I had one do just that. It occurred when I added too much power and the high drag of the wing and the low thrust moment forced a stall and the plane flipped on its back. It continued to do very short radius loops (about 10) with absolutely no input by me. I finally in desperation added some power and it recovered. I believe I read something about this phenomena in Dr. Gale's "Tailless Tale". Apparently the wing gets trapped in It's own vortex, if I remember right.I believe this may be what happened to the YB49 that killed Col. Edwards.
Last edited by Joel K. Scholz; Jan 20, 2008 at 02:48 PM.
Jan 20, 2008, 06:50 PM
internet gadfly
nmasters's Avatar
Hi, Joel--

Quote:
Originally Posted by Joel K. Scholz
Norm thats a nice looking airfoil you have used to illustrate the yahudi. Can you tell me a bit more about it.
It's an MH108 11.98% I just picked it at random.

Quote:
I believe I read something about this phenomena in Dr. Gale's "Tailless Tale". Apparently the wing gets trapped in It's own vortex, if I remember right.
Yeah, he said that. I don't have any idea what he meant by that. As I understand it (now that I've reread that tech report) it's an inertial coupling and aft CG problem. Interesting that your model could be started and stopped with the throttle.


Quote:
I believe this may be what happened to the YB49 that killed Col. Edwards.
The YB-49 was scheduled to do stall and CG tests that day. The other B-49 attempted to finish that test but after the first spin with the CG back the test pilot refused to do it again. It was just like yours did but that giant bomber took about 20,000ft to recover and you couldn't pull out hard because the airframe was only stressed to 2 Gs. The second aft CG stall was started 5,000ft higher than the one that crashed. Although there's no official cause of the breakup I would bet money that he exceeded the safety margin during pullout from the spin recovery.

--Norm
Last edited by nmasters; Jan 20, 2008 at 07:32 PM.
Jan 20, 2008, 08:38 PM
internet gadfly
nmasters's Avatar

Bell shaped lift distribution


Quote:
Originally Posted by Idriveaboxster
So, wou[l]d the additonal area and possible yaw stability make up for the increased drag?
I think so but I'm biased

Quote:
or should i go with the regular design how most rc wings are?
OK, this is going to be long and rambling but it does lead into yaw control I promise. The drawing will be referred to often so you might want to open it in another tab or window now so you can go back and forth between it and the text without losing your place. Or skip right to the end to see my recommendation.

Say you have a swing wing airplane, the wing has a taper ratio of about .35 and is equipped with tapered elevons on the outboard half of each semispan. When fully forward it has an elliptical lift distribution illustrated by the green dashed line in the attached drawing. As you sweep the wing back the lift distribution changes even though nothing else has changed , illustrated by the orange line. The natural lift distribution (actually CL distribution) of this swept wing has two humps that we'll call “lift mountains” and the area between the mountains is, naturally, the "lift valley”. This wavy line has several consequences, all bad but mostly easily fixed.

The first negative is that the humps and valley cause increased induced drag.

Second is that because the wing stalls first at the highest point of the curve any uncoordinated stall can turn into a spin .

Third is that by taking lift away from the nose and putting it near the tips sweep generates a nose down pitching moment.

Forth is that a roll input makes the mountain on the up going wing bigger and the opposite wing gets it's mountain trimmed of. This is the source of adverse yaw

You could trim off the lift mountains by reflexing the elevons but that creates its own drag and doesn't do anything for the valley

Washout can correct the first three effects but only for one speed. When you fly slower than the design speed the mountains and valley come back. Fortunately the elevons are trimming the peaks off the mountains but you still have a somewhat distorted lift distribution that creates drag and cuts down on your lift to drag ratio.

So what can you do to get rid of the lift valley? Well a root chord extension like I described in the yehudi posts would provide more lift there over the whole operating envelope. That's basically what the Hortens did but the extension was often much bigger and, as you've said, it makes balancing a bit awkward. Another way to fill in the valley is to use inboard flaps for pitch and leave the outboard surfaces plain ailerons like Biber's Multibumm. But again that moves the engine farther aft because the flaps and prop interfere if they're very close. It appears that his only yaw control is the passive influence of the fixed tip fins where the drag builds up faster on the forward wing. Notice that the correcting force of fins develops after the plane has started to side slip. The lag isn't very much and flying wings can actually slip quite a lot without losing efficiency so it's OK but your turns aren't quite as coordinated and graceful as they could be.

OK so it's drag from the tip fins that makes the plane point in the direction you want it to go in spite of the wing's own drag trying to point the nose away from the center of the turn. Is there some way to stop the adverse yaw moment from happening in the first place? Well, yes, there is. Look at the gray curve in the middle drawing. This is the so called “bell shaped lift distribution”. I say so called because any swept wing will have this lift distribution when operated faster than its design CL. The curve looks like it deviates from an ellipse by a wide margin but notice that if you ignore the revers curves at the tips the central hump is basically just a lower aspect ratio ellipse.
There are some papers showing that this lift distribution actually has lower induced drag for a given amount of lift than elliptical but the non lifting area still has form drag so there's probably no significant difference in L/D during cruising flight. What's interesting about these three curves is what happens when you give it a roll command. Although all three of the wings may have a small net drag increase. Only the one that had a BSLD yaws in the right direction without needing tip fins. Since the tip fin is producing at least enough drag to counter the adverse yawing moment the wing with the elliptical lift distribution builds up more total drag than the bell. At least that's the way it would work if you fly with the CG really far forward. Since a lot of pilots are dissatisfied with the aerobatic ability I don't suggest that anybody twist the wing as much as Dr Horten did.

So here's my recommendation. Use as generous a yehudi as you can to fill in the lift valley. The span of the yehudi should be about 1/3 of the wing span. A taper ratio of .3 to .4 is good, ignore the yehudi when figuring the root chord. Twist the wing according to the Panknin formula but start the twist at the panel break outboard of the yehudi, don't twist the center section. Tapered elevons on the outboard 30 or 40% of the wing. As high an AR as your span and weight constraints allow. An unfinished tip so you can stick a small fin on if it doesn't work quite like I described it There's more but I wouldn't do it to a plane without landing gear

--Norm
Last edited by nmasters; May 06, 2008 at 12:51 PM. Reason: Updated annotations
Jan 20, 2008, 11:59 PM
Speed King
Idriveaboxster's Avatar
Norm- thanks so much for all the advice.
I think i get most of what u wrote.
I dont know that much about aerodynamics and they dont play that big of role in models because anything can fly wiht a big enough motor and lipo. this yehudi thing seems good and i'll start to put one on the normal wings i do. the sweep is essential in flying wings. the first one i made had very little sweep and the TE was almost straight. it just did flips like u said.
I was a bit confused on the bottom graph. which one is which type of plane?
With yaw stability i was thinking it would keep it from sliding by directing airflow over certain areas. fins are the most reliable way to keep it sraight, but a plane without fins looks realy cool. anyone know how to make blended winglets?



With yehudi
Jan 21, 2008, 10:53 AM
internet gadfly
nmasters's Avatar
You do like that kinky trailing edge don't you That should fly fine under power. The kinks in the TE won't help your glide any but unless you're planning to compete with other purpose built gliders it should be OK
Jan 21, 2008, 11:43 AM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by Idriveaboxster
I was a bit confused on the bottom graph. which one is which type of plane?
Okay I added some more annotation to the drawing.
The blue curve is the un-swept wing.
The orange curve is the the swept wing without twist or with insufficient twist.
The gray curve is the swept wing with extra twist.

--Norm
Jan 21, 2008, 01:06 PM
Yes, yes I am.
cjbucher's Avatar
Not to throw a monkey wrench in this but doesn't carbon fiber interfere (or block) radio waves? If your whole plane is carbon fiber, would that cause problems with your radio reception (even with the antena on the outside?).

The reason I ask is because I have a small RC heli with a carbon fiber tail boom and they warned (hobby shop and other posts I've run across) me never to wrap the antena there or it could cause loss of signal.
Jan 21, 2008, 03:53 PM
Speed King
Idriveaboxster's Avatar
ive seen it run out the back in a little plastic tube so its about a foot away from the trailing edge. does it sitll interfere with 2.4 ?


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