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Old Apr 29, 2002, 10:30 PM
OK, Who's Flying The Jet?
Talon Driver's Avatar
Albuquerque, NM
Joined Apr 2002
312 Posts
Why Aren't RC Flying Wings Negative Camber?

I fortunately have an undergraduate degree in aerospace vehicles (accredited). I am not bragging just providing to you my viewpoint.

I know that the B-2 (the real jet) has a negative cambered airfoil. The reason for this is because a neutral or positive cambered wing w/o horz stab or canard is not as aerodynamically efficient as one that is negatively cambered.

The mechanics of this are that a negative camber wing's pitching moment is countebalanced by the moment arm of the CG to the aerodynamic center (AC). Using a neutral camber or even worse a positive cambered airfoil for a flying wing requires the elevon to have deflection up to provide the necessary counteraction of the CG to AC moment arm and the upward pitching moment inherent of the airfoil.

Anyone know why none of the RC flying wings do not have a negative cambered airfoil (at least the projeti and the zagi that I have seen) for greater efficiency?

Cheers,

Chuck
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Old Apr 29, 2002, 10:59 PM
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Palo Alto, CA
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Reflexed or negative camber airfoils primarily effect the zero-lift pitching moment (must be positive to be trimmable). Without sweep this can only by accomplished with reflex, but with the addition of sweep, a +Cmo can be acheived with twist.

The downside of reflex: poor Clmax, possibility of aft lower surface separation, typically some drag penalty. Also many flying wings exhibit low inboard Cl's, uniformily applied reflex exacerbates this and can hurt performance

Downside of twist: planform can become a strong point design, with poor performance (particularly induced drag) away from the design trim point (CL or speed)

Typically sweep and twist is combined with low(but still negative) or slightly positive( reflexed) pitching moment sections

The zagi is sorta in the middle, the combination of sweep and the reverse tapered elevons unload the tips more than the root for a given flap deflection. A negative (up) flap defelction effectively reflexes the airfoil progressively from root to tip, allowing the root to work harder and the tips to unload.

-Pete Kunz
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Old Apr 29, 2002, 11:27 PM
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Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
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A modestly cambered with reflex airfoil is just more efficient than a purely negatively cambered airfoil. Lower drag at useable Cls..
MOF, a normal airfoil, such as a 2415, is stable inverted with no tail, for the reasons stated... the nose down pitching moment (which tries to raise the front when inverted) can be counterbalanced by a forward c.g.
I have seen at least two planes lose their horizontals in flight, and being high enough were able to complete the tuck under and maintain level but inverted flight.. the loss of weight of the horizontal moving the c.g. forward enough in those events. (Don't count on it though.)
A reflexed airfoil gives the designer a bit more latitude in positioning the c.g. as well.
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Old Apr 30, 2002, 09:40 AM
Visitor from Reality
United States, VA, Arlington
Joined Dec 1996
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My aerodynamics training involves being able to spell the word and remaining awake while reading some of Lennon's book!

But I did build a half dozen tailless models using NACA 0012 and 0014, with wide-ish elevons 'reflexed' up a tad and a CG that I read someplace should be at 16%. Whether they should or not have, all but one flew pretty well.

Of course, my development budget was a little lower than the B2, so I was forced to use a little less computer enhancement of flight characteristics Still, I didn't need stealth either - it's embarrassing when you have to admit you can't find your model in the pits

"Efficiency" needs defining. My last but one, with a low wing, had much of the aerobatic abilities of a sports pattern model, with passable knife edge capabilities and was easily managed in snap / spin areas, if a little fast thereabouts. I figured that made it 'efficient'. If you mean the ability to stay up ages - an OS 25 and a 6oz tank took care of that pretty well too.

It also has been flown since as an electric - time to lose that pesky tailplane thing again . On CG placement, one of my designs was published as a freeby in a UK mag - and they left the CG off the plan. One guy actually built one, balanced it at the trad 1/3rd chord, got it up, around the sky some and back down again to a runway landing under some kind of control. It was hairy, but the model survived. Turned out we were in neighbouring clubs and he was quiet happy with his model after he talked to me and took the ballast off the tail.
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Old Apr 30, 2002, 11:48 AM
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Northern New Mexico
Joined May 2001
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here is my guess

Mechanical engineer here, and I've done some reading on the caveats of flying wings. First, it doesn't seem quite right to me that a negatively reflexed wing would be more "efficient". It's more like a requirement for some degree of stability. With a positive cambered wing, the CG must be placed in front of the center of lift for stability. On an R/C plane this is no problem. You simply cram all of your radio equipment near the nose of the plane. The B2 has a different mission than to just fly around and do loops, though. It has to carry a big payload, which can't all be crammed in the nose. Just to make things clear, if you look at the geometry of a modestly swept, tapered flying wing, like the B2, the Center of lift is very far forward. To get the CG in front of that you have to cram all of the weight near the nose, which isn't really practical with a big payload. If you make the reflex negative, though, the lifting moment is in the oposite direction, and you can now place the CG behind the center of lift. I can't see how this would be more efficient, but more likely a necessary compromise in the design. That's how I see it.

Nathan
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Old Apr 30, 2002, 11:59 AM
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frankenfoamy's Avatar
Poway (near San Diego), CA US
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The PROJETI has such an animal
It uses little to no reflex
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Old Apr 30, 2002, 06:25 PM
OK, Who's Flying The Jet?
Talon Driver's Avatar
Albuquerque, NM
Joined Apr 2002
312 Posts
CL, CG and +Cmo I understand. However, what is relfex? I think this is something that is unique to RC flying. Anyone? What is the difference between this and Crow and Differential?

Cheers,

Chuck
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Old Apr 30, 2002, 06:59 PM
ὅπερ ἔδει δεῖξαι
Gerald's Avatar
Joined May 2000
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Reflex is the up-turning of the trailing edge (or elevons) from the downward slope at aft portion of the airfoil. In some ways it has the same effect as negative camber. It is not limited to R/C flight, gliders often use it as a trim setting to neutralize lift in order to reduce drag and increase speed. Slow speed flying wings such as hang gliders and some ultralights rely on it for positive stability. One advantage of positive camber with reflex over negative camber is improved low speed stall performance.
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Old Apr 30, 2002, 07:03 PM
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Sparky Paul's Avatar
Palmdale, CA
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"reflex" is where the trailing edge is raised.
Either by rigging the surface so the trailing edge is up, or building the upward curve into the airfoil profile.
Either works, but the purpose-designed shapes are a tad more efficient..
The reflex creates a nose-up pitching moment about the 1/4 chord point. This is counteracted by locating the c.g. ahead of the 25% chord line. As with a tailed airplane, the sensitivity of the wing will vary with the c.g. relative to the 1/4 chord point.
Here's some reflexed airfoils...
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Old Apr 30, 2002, 09:46 PM
OK, Who's Flying The Jet?
Talon Driver's Avatar
Albuquerque, NM
Joined Apr 2002
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Quote:
Originally posted by Gerald
One advantage of positive camber with reflex over negative camber is improved low speed stall performance.
Please elaborate.

Cheers,

Chuck
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Old Apr 30, 2002, 09:52 PM
OK, Who's Flying The Jet?
Talon Driver's Avatar
Albuquerque, NM
Joined Apr 2002
312 Posts
Paul,

Thanks for the graphics...sharp! I wanted to respond to another part of your entry.

Quote:
Originally posted by Sparky Paul
[BThe reflex creates a nose-up pitching moment about the 1/4 chord point. [/B]
This is true. However, all positive cambered airfoils have an upward pitching moment at 1/4 chord provided they are subsonic. Which moves aft starting in the transonic region to about 1/2 chord supersonic (not a factor for E-flight).

Cheers,

Chuck
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Old Apr 30, 2002, 11:04 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
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" all positive cambered airfoils have an upward pitching moment at 1/4 chord provided they
are subsonic."
.
No.
The usual cambered airfoil's pitching moment is nose down. The horizontal tail provides a down-force to counter act this, which is why c.g.s can go aft of the 1/4 chord point.
When the usual cambered airfoil (less horizontal) is inverted as in your 1st message, the nose down Cm is now trying to force the nose up. If the c.g. is forward of the 1/4 chord point pulling the nose down, the plane can be stable.
.
Here's a site which shows a normal plane that had the entire horizontal sawed off at the root.. reflex added to the ailerons which were mixed as elevons...removal of the servo and surfaces moved the c.g. forward enough..
http://home.earthlink.net/~pjburke1/Tailless.htm
.
and there's the profile Somethin' Extra I crashed the tail off, repaired and flew as tailless, with only a slight amount of reflex and rebalancing needed on that page.
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Old May 01, 2002, 06:51 AM
got any foam to bash?
Tom Hunt's Avatar
Lake Grove,LI, NY
Joined Aug 2000
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Quote:
Originally posted by Sparky Paul
I have seen at least two planes lose their horizontals in flight, and being high enough were able to complete the tuck under and maintain level but inverted flight..
Yup.... lost my Horiz in a midair on my Terry with another Terry during a streamer combat session (we usually get 5-6 in the air at once). She pitch inverted, stayed there and I flew it down to a safe landing using rudder and throttle! What a hoot! wish the video had been running then! I found the tail in the grass (two pieces) and she lives to fly again!

Tom

BTW the other guy survived too!
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Old May 01, 2002, 08:42 AM
Visitor from Reality
United States, VA, Arlington
Joined Dec 1996
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SNIP
and there's the profile Somethin' Extra I crashed the tail off, repaired and flew as tailless, with only a slight amount of reflex and rebalancing needed on that page.
SNIPEND

Hi Paul
My first tailless used the wing out of a low AR conventional I designed. It was mostly lost when someone who couldn't fly real good sawed her tail off in a pylon race, the subsequent "firm landing" taking the nose off. I removed the rest of the fuselage - not that much was left - from a wing that had somehow survived. Doubled the aileron area, called them elevons, added an aesthetically matching fuselage with a short rear and big fin/rudder.

Attempt 1 suggested I needed to know more about UC positioning on tailless. Attempt 2 proved it flew.

The only wing I've had published in two different models! It started in L'il Special", Radio Modeller in England, c 1990 and went on to feature in "Bubbles", RCM around 1993

With a root chord of around 11", the elevons were about 1/75" wide. "Reflex" amounted to a tad under 3/32" up from neutral with a NACA 0012 section (originally!) and 16% MAC CG.

Anyone still think designing model aircraft is hard ?
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Old May 01, 2002, 02:29 PM
Registered User
Canada
Joined Nov 2000
7,265 Posts
Well the question remains mostly unanswered.
The Zagi airfoil is frankly barely an airfoil by any normal standard.. in fact building a Zagi with a 'proper' , say a MH foil, results in a markedly superior abilities.
Simple fact is Toy Airplanes often have the crudest of Airfoils.. or facsimilies of..even flat plates work fine.
Am surprised by the lack of knowledge re Reflex.. it's prominent the few textbooks I' ve browsed.
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