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Old Jan 02, 2016, 11:09 AM
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High Aspect Ratio Plank

On a different thread a couple of weeks ago, there was a very brief discussion about the concept of a high aspect ratio plank. It was disparaged and there was not much more about it on that thread.

Since then I have been thinking a bit about the idea - kind of got under my skin. I almost set about building something, but I have so many models in my shop - probably any one of 20 that I could fly pretty quickly if I wanted to. I just can't see putting anything more in there. I need to have a model yard sale.

Anyway in my reflections I've been thinking about a plank that has an aspect ratio in the range similar to a high performance thermal model sailplane. I think that would put it into the area of 12+.

Then I got wondering if anyone has tried it or maybe has any ideas about the concept. ?????????????
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Old Jan 02, 2016, 12:11 PM
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12:1 doesn't seem all that high to me. All of Marske's Pioneers are 12:1 or above. The only aditional problem for a model is the Reynolds numbers and you understand that so I'm sure you can do it
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Old Jan 02, 2016, 12:21 PM
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That's right Norm.
Drela's Supra wing has an AR of about 17. A plank laid out like that would make an interesting model. Similar size would provide similar RN. Of course the airfoils would have to be plank-suitable. Yaw damping and directional stability issues would have to be attended to, but somehow it seems like it could make an attractive model.
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Old Jan 02, 2016, 02:02 PM
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Old Jan 02, 2016, 02:14 PM
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Our buddy Flying Dennis has a gaggle of planks, some being high aspect ratio.

You can improve your Re by just flying fast, which I would think would the starting point for such a plank. My rule of thumb is that the lower the Re number, the lighter you must build, which is right up your alley.

Quote:
Originally Posted by HerkS View Post
I almost set about building something, but I have so many models in my shop - probably any one of 20 that I could fly pretty quickly if I wanted to. I just can't see putting anything more in there.
I can. Here's why. If you already had half a dozen Zagis, then it would be easy to say "I got enought already". But each one of your models is unique and indeed you do not have a high aspect ratio plank. And besides, you're only young once..............I say go for it. The bungee launch would be memorable.

Maybe a sketch or two to show us what direction you are thinking?
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Old Jan 02, 2016, 03:06 PM
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Quote:
Originally Posted by HerkS View Post
Then I got wondering if anyone has tried it or maybe has any ideas about the concept. ?????????????
High aspect plank from Christian, span 2 meter, AR ~ 14.5

VoT-P 2m Erstflug am Hang (3 min 1 sec)


And this my "Strong Mini Slim DS" under construction, span 2 meter, AR ~ 15. If it flies useful, you will see videos and get more informations

Regards, Uwe.
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Old Jan 02, 2016, 03:40 PM
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Really nice Uwe.
Just what I was thinking. Please fill in the details when you have it all worked out, or even use this thread as a build record.
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Old Jan 02, 2016, 05:46 PM
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The elevator response on a high AR plank is ultra-sensitive.

Ten years ago I posted a build thread on RCG of a 9:1 AR plank that flew well with a separate elevator, but it doesn't show up on a search, and it's too low in aspect ratio anyway. But I had the impression that it benefited from a higher taper ratio than other planks I'd built. The high taper, like the VoT-P, provides a high AR with a generous chord in the center, which should reduce the elevator sensitivity a little. The Pioneer definitely is an example of this.

Christian, that VoT-P would break me away from my swept wings for a while (it is beautiful!) except there is no flat grass where I fly. I am envious!

ed
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Old Jan 02, 2016, 06:19 PM
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Hi Ed,
Herk's theory of planks is that the elevator control function should be located on the outer part of the wing. Outboard elevons would be an example. An elevator at the middle of the wing always seemed to make the handling just a bit strange. My guess is that the effect on the lift distribution and the pitching moments is just not quite in the right place.

If the pitch control is too sensitive I guess a good dose of expo on the elevator function would be desirable.
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Old Jan 03, 2016, 02:16 AM
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A higher aspect ratio plank seems like it would be a likely candidate for a shifting weight to alter the CG for trim too. Since the range of safe CG movement would be slight it would take very little mass to give the required shift. Just moving the flight battery pack around would likely do the trick. And that way the tips don't need to shift their shape from the ideal for trim.

You'd still use elevons for momentary shifts but set up the flap control or maybe the throttle control for the trim shifting. This would obviously work best if the Tx has a lever or dial knob for this function. Although a 3 position "flap" switch could work pretty well too. Set for minimum sink at the slow end, Something that give a near best L/D in the middle and a fast ground covering cruise for beating back upwind or getting through zones of sink.

Whatch'a think?
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Old Jan 03, 2016, 09:20 AM
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Quote:
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A higher aspect ratio plank seems like it would be a likely candidate for a shifting weight to alter the CG
Since an aft CG can make a plane unflyable (by human control), I'd be more comfortable making screw device to move the weight. That way, the weight would be secure during a hard landing and very fine control would be possible. Also, space permitting, you could move the weight 3" if you wanted. Knowing exactly where the weight was, during flight, would be the hard part.
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Old Jan 03, 2016, 09:28 AM
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Elevator System

You would be hard pressed to design a plane that is more sensitive to elevator input. The elevator system should be a solid as you have every built. For me that would be a $$$ servo, potted to a big carbon fiber patch, aluminum servo arm, ball links to heavy linkage and a big G10 elevon horn.
Also very tight hinges hung off of a good sub spar.
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Old Jan 03, 2016, 12:24 PM
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Quote:
Originally Posted by Knoll53 View Post
You would be hard pressed to design a plane that is more sensitive to elevator input. The elevator system should be a solid as you have every built. For me that would be a $$$ servo, potted to a big carbon fiber patch, aluminum servo arm, ball links to heavy linkage and a big G10 elevon horn.
Also very tight hinges hung off of a good sub spar.
You forgot the reinforcing steel lattice filled in with high yield concrete....

I can just imagine that you're 110% right on the need for a very tight control system. Yet one with no sticktion either. Too tight with sticktion to fight would be just as bad as a setup with some slop.

I don't know if there are many screw jack style setups around but I do agree that either a screw jack or a closed loop sail winch style setup would be pretty well mandatory from a standpoint of durability.

By the way Herk, in your first post you said "like a thermal sailplane". I took this to mean that you mean it for thermal flying and not slope flying. Is this the case?

Something to watch for that I've seen in writeups on plank style wings is pitch oscillation damping issues that occur on examples with higher aspect ratios. But I don't recall if they were heavy slope flying models or if this occurred with lighter designs intended for thermal flying. I'd GUESS that it would be more of an issue on a heavy all sheeted slope model. But I can't recall. I do recall that the models that seemed to have this issue and wrote about it all had rather long fuselages. It's something that would be worth looking into though.
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Old Jan 03, 2016, 01:54 PM
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Quote:
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had rather long fuselages.
It's a given that a plank with a skinny wing will be twitchy in pitch. If there were only some way to dampened that behavior. You've got it Bruce. Inertia dampened. I like the idea of a small trim ballast box at the nose and at the tail.

The only sticktion could be in the hinges. I like a live Kevlar hinge.

I remember with my Alula, I'd adjust CG with just 3 BBs in the short nose.
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Old Jan 03, 2016, 02:06 PM
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Is it just me? Does the idea of flying a high AR plank slowly seem highly unlikely. I have all sorts of problems when trying to fly slow and always just feed in more speed to solve whatever is happening. It is the universal fix and I use it all the time.

Maybe a bottom flap at the spar line would be just the ticket for landing.

Here's an offer for you Herk. I'll give you a dollar, that's a whole dollar, if you can float this thing in to final flare and hand catch.
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