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Old Jul 23, 2012, 09:58 PM
Entropy is happening!
Jim.Thompson's Avatar
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Autostable (plank) airfoils - inverted flying theory?

I'm fascinated and unable to explain why reflexed, autostable airfoils are able to fly inverted with little or no down elevator.
I have a couple of planks that are capable of this, and I'm aware it's not uncommon if the CG and deflection rates etc. are optimised. Even a high cambered PW106 airfoiled plank that I have flies nearly neutral hands off when inverted.

I'd very much like to read an explanation of how and why this is possible. It seems to me, that the designers and builders go to all the trouble of engineering reflex into the profile, plus (often) some additional reflex set on the elevons to ensure pitch stable upright flight. Then we flyers invert the plane and it still flies!
My elementry understanding of the aerodynamics of these airfoils suggest this should be impossible without lots of down elevator to effect reverse reflex.

Why is it so?.........

Jim.
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Old Jul 23, 2012, 10:37 PM
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I think a lot of it has to do with the correlation between the CG point and the elevator trim. The farther back you push the CG, the elevons can be trimmed more level to the centerline of the airfoil to get a straight glide path (no diving or porpoising). When you get to the point where the elevons have barely any reflex (1/32"-1/16") then you only need about the same amount of reflex right-side up as you do upside-down.
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Old Jul 23, 2012, 10:56 PM
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Only one I've had that lived up to the hype is my M60. That thing's awesome. My other planks (fat albert, Zipper, Kaze, Atoms, Weasel) and combat wings, all need a bit of down when inverted, some more than others. Maybe I'm doing it wrong (or have unreasonable expectations) but I don't think I'm that bad and have the CG as good as I can get.
That question is a good one, Jim. I'd like to know too.
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Old Jul 23, 2012, 10:57 PM
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It's magic.
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Last edited by Hamburglar; Jul 23, 2012 at 11:14 PM. Reason: Cm is very close to zero. So basically the same inverted/upright.
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Old Jul 23, 2012, 11:03 PM
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AoA?
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Old Jul 23, 2012, 11:23 PM
Entropy is happening!
Jim.Thompson's Avatar
Australia, NSW, Bellingen
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Reflex.

[QUOTE=orni-freak;22248207..........When you get to the point where the elevons have barely any reflex (1/32"-1/16") then you only need about the same amount of reflex right-side up as you do upside-down.[/QUOTE]

This means, (I'm assuming here), reflex above what is designed into the original airfoil. Correct? Come to think of it, that's all it could mean as there is no other reference to compare it to.
If so, that is just about equivalent to the up elevator travel on one of my PW51 planks. Which of course is in addition to the set or timmed reflex.
Full down elevator will be barely less than the designed reflex. Inverted this will be more like zero or negative reflex.
So, based on my assumption and what I have set up on all my planks, that still does not seem to be anywhere near enough to similate upright flying trim reflex.
As I explained in my first post, two of my planks will fly hands off inverted!

I think Hamburglar's explanation is right.

Jim.
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Old Jul 24, 2012, 12:41 AM
I'd rather be out sloping...
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I have re-read your question Jim and have to agree with Orni-freak.

Planks are very pitch sensitive (short tail moment), therefor very little down elevator required for inverted flight (providing the CG is correct). The more forward the CG, the more down elevator required for inverted flight. So maybe your one plank's CG is a bit more forward than the other two? My theory... otherwise, different built-in reflex or Magic...

Jakes
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Last edited by Jak35; Jul 24, 2012 at 03:22 AM. Reason: who thinks he understands the question now.... ;)
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Old Jul 24, 2012, 05:05 AM
I'd rather be out sloping...
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Hi Jim,

It would be interesting to know the different chord lengths, elevon widths, AUWs and throws that you are using on both planks that fly hands-off inverted (PW106) as well as the one that needs a bit more down elevator (PW51) and also whether moving the CG a bit more aft on the PW51 makes a positive difference with inverted and level flight.

Have you used the "standard" airfoils or have you modified them for extra camber, reflex etc?

Maybe this comparison can help clear up the Magic part a bit for me.

Jakes
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Last edited by Jak35; Jul 24, 2012 at 05:07 AM. Reason: Who is very tempted to over design a plank. ;)
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Old Jul 24, 2012, 06:21 AM
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Old Jul 24, 2012, 05:18 PM
Entropy is happening!
Jim.Thompson's Avatar
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Chord/Datum line.

It seems the meaning I tried to convey in post #6 was not understood, I obviously did not explain well enough . This is the time that a diagram would be most helpful and I cannot use any of the computer drawing aids!
I will try and add some clarification to the above post.

When such terms as "added reflex" is used, it must be remembered that this is in addition to the designed reflex which is already positive compared to the chord line. (These airfoils are "cranked" or shaped into a slight "S" shape which gives them self pitch stable properties.) This needs to be negative to a large degree (with reference to the original airfoil) to approximate zero effective reflex. That degree would be more than what my set up provides even with full down elevator!!
And as I have stated more than once, both my planks fly inverted with no down elevator. I'm not talking little down elevator, I repeat here: No down elevator.

Jak35:
All the parameters that you ask about on my planks are on my blog (I think). I will check up and if necessay, update them. The main critical factor is CG. If this is set to the designers recommendation, the rest is comparatively easy and performance will be very impressive. However, the main question of this thread is independant of my gliders sizes and set up. Other models exhibit this phenomenon.

edit: Don't forget that the "throws" or deflection rates have no effect whatsoever in the "hands off" situation!

Jim.
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Last edited by Jim.Thompson; Jul 24, 2012 at 05:54 PM. Reason: see edit.
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Old Jul 24, 2012, 05:28 PM
Phil.T-tailer
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Quote:
Originally Posted by Hamburglar View Post
It's magic.
this one

OK - if you really want to know, you need to think in terms of what happens to the "stability" coefficients of the airfoil (CM). You've already said that the plank airfoils are "autostable" when upright. Now you've got to take into account the stability characteristics of an inverted airfoil - which may be somewhat different to upright - unless its a fully symmetrical airfoil.

try a quick experiment - take a wing or wing half from a conventional tailed plane - twizz around with it & try varying the angle of attack - (when theres no wife / significant other / large pets in the way) - tries to pitch up? - so its unstable. Now invert it & try the same thing - doesnt try to pitch up? - or at least, not as much?

yes, you need to get the CofG right back on that plank - but then its magic !

Phil.
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Old Jul 24, 2012, 06:19 PM
Just Toss It !!!
MATIN's Avatar
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I'm still contemplating all meanings of the term "autostable", and can't wrap my brain around it.....

Matin
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Old Jul 24, 2012, 06:30 PM
Alphabets make great soup
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Quote:
Originally Posted by MATIN View Post
I'm still contemplating all meanings of the term "autostable", and can't wrap my brain around it.....

Matin
Old people probably call them autostables. I call them garages.
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Old Jul 24, 2012, 06:49 PM
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United States, CA, Santa Barbara
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Quote:
Originally Posted by Jimbello View Post
I'm fascinated and unable to explain why reflexed, autostable airfoils are able to fly inverted with little or no down elevator.
...

Why is it so?.........
Remember that when flying slope we are almost always flying in an upward rising volume of air... meaning that planes trimmed to fly "hands off" (whether upright or inverted) are actually trimmed such that, if they were in a no-lift situation, they'd be in a descent.

So... per what everyone else has written... if the lift is strong enough (or the plane light enough) and the airfoil, balance and throws are amenable, then it is quite possible. But IMHO the inverted performance will never quite match the upright performance - you'll always have to make loops bigger (more gentle) than you would if upright.

Steve
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Old Jul 24, 2012, 09:19 PM
Just Toss It !!!
MATIN's Avatar
United States, CA, San Diego
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Quote:
Originally Posted by Hamburglar View Post
Old people probably call them autostables. I call them garages.
Humor......There's nothing better for digesting aerodynamics!

Matin
A Jew, a Christian and a aerodynamic professor are sitting at a bar.....
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