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Old Nov 23, 2015, 10:02 PM
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BreakDancer 1.8m

In the video posted under the thread titled "Old Design Resurrected" (July 9, 2015, Report #14), I never realized the video included two planes until a few days ago, but at 24 seconds the trim colors change from chord-wise color stripes halfway out to the tips, to Vee stripes in the center of the wing. The plane in the first 24 seconds is a 1.7 m BreakDancer, not the constant-chord Talus. Check it out! The Talus in the remaining 3+ minutes flies very well, but is not as fast as the BreakDancer.

That BreakDancer was destroyed later in the day due to transmitter failure... a loose antenna wire.

The video reminded me how nicely that design flew, so I thought it was time to simplify the construction and build another one. The new version is stretched from 67" to 72", will use EPP wing cores with balsa trailing edge/elevons, covered in 5 or 7 mil mylar with a ribbon spar. The airfoil is the HQS1509.

I'll post construction pictures as I make progress. For now, here are the plans. If I'm patient, there will be flaps, but the design is so tough that they aren't really needed for landing, even on our rocky slopes.

Let me know what you think of the construction... improvements are always welcome.
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Last edited by EdSoars; Nov 23, 2015 at 10:18 PM. Reason: error in description
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Old Nov 25, 2015, 09:08 AM
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Looks cool. Hope to watch your progress...

-Dave
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Old Nov 25, 2015, 05:09 PM
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I cut the cores today. I use a weighted drop-arm hot wire (first photo) powered by a cheapie reostat: the portable heater's one-wire element (second photo) acts as the variable resistor. I used a voltmeter to test the various positions to avoid burning up my limited supply of Rene wire. The label (third photo) lets me quickly place the clip-on leads to provide voltage for either EPP or EPS, 24" or 50" bow.

I cover the bottom beds with cheap packing tape (fourth photo) to prevent gluing the cores to the beds during installation of the main spar, servos and joiners.

The cores are sanded to remove all strings and blobs, and to make the leading edge radius constant from root to tip. I constantly check that the panels are as close to identical as possible., because distortions and differences add up to increased drag later on.
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Old Nov 26, 2015, 03:14 AM
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Ingenious! I've never cut EPP Ed, if there much difference (other than wire temperature) when compared to cutting EPS?
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Old Nov 26, 2015, 10:52 PM
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Yes, EPP needs higher temperature than EPS. I haven't measured the. temperatures, but I cut EPS at about 33V as opposed to 50V for EPP.
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Old Nov 27, 2015, 08:50 PM
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Spar change: I wanted to make the spar curved, thinking it would resist twisting, something like the CF spars being put into combat wings, so I made a test on some scrap foam. The foam compressed under load, allowing the spar to deform, which made it useless.

Back to the drawing bench.

I made up a scrap test using a balsa web and plastic pallet strapping (first photo) from the local lumber yard trash pile. The qualitative results are in: not as stiff as carbon, but WAY cheaper (first photo).

The attached plan sheet (second photo) shows the straight spar and relocated servo positions.

The remaining photos show the wing tiplets: a flat-bottomed airfoil in 3/16 balsa, glued to a spruce stub. Exhaustive tests show that they work.
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Old Nov 28, 2015, 03:23 PM
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Laying up the spars: everything clamped in two directions to keep things straight. Plastic pallet strap is glued to balsa cores with contact cement.

Tiplets covered to be as visible as possible as far away as possible (if not as far away as sensible).
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Old Nov 30, 2015, 08:27 AM
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Quote:
Originally Posted by EdSoars View Post
I used a voltmeter to test the various positions to avoid burning up my limited supply of Rene wire.
Have you tried stainless steel fishing leader wire?
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Old Dec 03, 2015, 10:15 PM
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Haven't tried fishing leader as my Rene wire is holding up very well so far.

I suspended work on the wing because the pallet strapping didn't bond to the balsa spar cores. So,
I ordered more balsa sticks and a supply of CF and FG braided sleeves to make up some tubular spars.
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Old Dec 04, 2015, 09:41 AM
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Originally Posted by EdSoars View Post
I suspended work on the wing because the pallet strapping didn't bond to the balsa spar cores. So, I ordered more balsa sticks and a supply of CF and FG braided sleeves to make up some tubular spars.
I recall you mentioning the pallet straps in your post, and I wasn't sure if you were using them as spar caps or as a way to distribute clamping pressure on the actual spar caps.

In my experience pallet straps are made of polyethylene, which is extremely difficult - almost impossible - to glue.

I can't imaging they would have offered any compressive strength anyhow.

Any reason you didn't use spruce strips?

Cheers-
Dave
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Old Dec 05, 2015, 03:40 AM
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Spruce would work. It's a little heavy, and weight makes the wing more prone to flutter, especially when it is not distributed in a skin. Spar caps work for bending loads, and I want some torsional load resistance too. That's another reason to abandon the pallet straps.

I have now shaped the balsa spar cores by rounding off the corners. I stretched 1/2" FG braided sleeve over the cores and will wet them out, blot with paper towelling, and set them into place in the foam wing cores to cure in the core beds.

The spars are not quite full depth but I don't anticipate strong bending loads on this model.

I'm obviously designing on the fly here so I appreciate the comments!
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Old Dec 05, 2015, 07:44 AM
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Originally Posted by EdSoars View Post
Spruce would work. It's a little heavy, and weight makes the wing more prone to flutter, especially when it is not distributed in a skin. Spar caps work for bending loads, and I want some torsional load resistance too. That's another reason to abandon the pallet straps.

I have now shaped the balsa spar cores by rounding off the corners. I stretched 1/2" FG braided sleeve over the cores and will wet them out, blot with paper towelling, and set them into place in the foam wing cores to cure in the core beds.

The spars are not quite full depth but I don't anticipate strong bending loads on this model.

I'm obviously designing on the fly here so I appreciate the comments!
Flutter is complex enough that I don't know the analytical math behind it, but, my understanding is to have the mass distributed towards the LE, and also have the torsional stiffness towards the LE. Don Stackhouse (designer of the Chrysalis sailplanes) has talked about it quite a bit in another thread ... if I recall correctly he mentioned that the shear center and center of mass at any given wing station need to be coincident.

Without any math or engineering computations, what that tells me is to 1) put more structural mass towards the LE, and 2) put in some structure that adds stiffness.

I'd think spruce spars would fulfill #1.

Are you going to skin the wing with glass? If so that should/would fulfill #2. Wood sheeting would also do the same (although probably less efficient/effective than glass).

Now, with all of that said, I understand concern about overall mass. Resonant vibration frequencies of a structure are related to mass and stiffness. If I can reduce mass and increase stiffness, I increase the resonant frequencies ... what is done in a good engineering approach is manipulate the mass (make it lower) and stiffness (make it higher) to ensure any resonances are far away excitation frequencies (phenomena, like vortex shedding, that excite resonances). Another way to think of this is if you have an excitation that energizes the first or second resonant frequency, that's bad ... if the excitation has enough energy the odds are the structure will self-destruct.

I imagine Herk or someone else will contribute ... and also let me know if what I said is incorrect.

-Dave
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Old Dec 05, 2015, 11:23 AM
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You pretty much "got it". The local Aerodynamic Center is at about 25% chord. To avoid flutter, get the local C/G and the local "shear center" (the axis the structure twists around when you apply a pure torsion load to it) to coincide with the local Aerodynamic Center. If you can achieve that, the forces that drive flutter will tend to cancel each other out.

In addition, yes, the natural frequencies need to be as high as possible. The airspeed where flutter occurs corresponds to an airspeed, and the higher the natural frequency, the faster the airspeed before flutter raises its ugly head.

So, light and stiff (torsional stiffness in particular) rule the day.

For control surfaces it's the hinge line those rotate around, so use that in place of the local C/G.

The other thing to bear in mind is that control surfaces are typically at the aft edge of the flying surface. So, if you add mass balance to an aileron (or rudder, flap, elevator, whatever) to avoid control surface flutter, you just added a bunch of weight to the aft portion of the wing it's mounted on. That weight counts as far as getting the local C/G of the wing to coincide with its local Aerodynamic Center, so whatever you add to the aileron means even more than that you have to add to the wing. Be careful, obviously this is the sort of problem that can "snowball" on you. Trim tabs can be an even bigger problem, because they add another link to the chain. The balancing on the tab requires more balancing on the control surface it's attached to, and the weight on it affects the flying surface it is attached to. In addition these new elements give the system more "degrees of freedom" (i.e.: ways the system can vibrate), and that means additional natural frequencies you have to worry about.

Keep it light, keep it stiff, both of those in the right places, and keep it simple.
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Old Dec 05, 2015, 04:09 PM
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"Break Dancer". Sounds like it would be the perfect name for a 3D plane.

Don't mind me, just my mind wandering...
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Old Dec 08, 2015, 05:31 AM
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"BreakDancer" refers to my habit of flying with too much exuberance and too little altitude. Or too much speed and too little skill.

I was actually thinking about Mark Drela's beautiful Bubble Dancer, and the advanced building techniques he uses, versus my own slap-and-tickle methods.

How about"Slamdancer" for the next one?

Ed
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