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May 13, 2017, 02:39 PM
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EdSoars's Avatar
Discussion

Thermal Plank


A new challenge: build a thermal plank. I have a conventional (tailed) fuselage and motor sitting in the corner, and though it would be useful under a thermal plank wing. This calls for a different airfoil than what I'm used to for slopers. I don't need much inverted performance; only a good cruise speed with a good L/D. In keeping with the quest for maximum flying fun with minimum time/money invested, the first prototype will be a 2-servo wing with the selected airfoil: the MH-22. Carolynne was building her beautiful Calliope with this section ("A Moderately High AR Plank Flying Wing").

The prototype"PsiClone" is ready to install servos and join the panels. It has an aspect ratio of 10:1. Not high, but higher than most planks in the USA.
Root chord 8", tip 6", span 72". Two-servo wing with elevons on the outer 45% of the half-span.

The weight should be around 30 oz., for a wing loading of 8 1/2 oz/ sq ft. Cruise speed should be about 30 mph; enough to cover some sky hunting for thermals.

What is still undecided is whether to go with a straight LE, a straight TE, or a straight 25% line. Would there be any difference in tip stall or other handling characteristics within this range of sweep angles?

Ed
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May 13, 2017, 02:57 PM
Herk
HerkS's Avatar
If I was building this design, I would use a straight TE and elevons. I suspect - though have no science to confirm it - that control surfaces would be more effective and the wing more efficient.
May 13, 2017, 06:58 PM
Registered User
I agree with Herk,

If you try a straight leading edge, the elevons will be pretty close to the CG. Thus when you're needing a nose up or nose down change you won't get much of a pitch change due to the elevator being so close to the CG. You'll be mostly relying on the pitching moment of the airfoil to become more positive with an elevator "up" input.

On the other hand, the fact that moving your control surfaces is going to have less effect on pitch makes that the perfect location for the ailerons. Now a roll input won't need to be countered by a corrective pitch input as much.

Still you need pitch control. With a straight leading edge that gives you the largest moment arm for an elevator if its located in the center at the root. Thus you'll need to add another control surface. You could still function with two servos if you connected the ailerons mechanically and drove them with one servo. Or you could still fly it with a two channel radio by wiring the two aileron servos together for the first channel and having the elevator be the second with a third servo.

All of that may require a redesign and it sounds like you're ready to try elevons only. So again if that's the case, I agree with Herk and have a straight trailing edge or even some sweep. Then you'll have the elevators some distance behind the CG to increase their authority. Just remember that you're probably going to get some nose up or down with aileron input because of that. Playing with mechanical differential can compensate to some extent if you can't mix it out at the transmitter.

.....Also I was curious about the MH22 so I ran it in Xflr. It looks like a great pick for a plank. However it doesn't appear to have quite enough reflex to be pitch stable as designed. You may need a little "up" elevon or "up" elevator trim.

Good Luck !
Mike
Last edited by ThermalSeeker; May 13, 2017 at 07:40 PM. Reason: added the end bit
May 13, 2017, 08:16 PM
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Knoll53's Avatar

go for bird like


I like the straight LE, that is square to the direction of flight, like my avatar. This provides some forward sweep of the 1/4 chord line. Inasmuch as there are a long list of disadvantages to rearward swept wings, it seem a golden opportunity with no significant downside. There will certainly be no spanwise flow. The nose of the pod can be shorter, which looks more birdlike. For what it is worth, I have never had a tip stall problem on a forward swept plank.

A central elevator should be effective due to the larger local chord. Regardless of whether the LE or the 1/4 chord is straight, they are very close and should perform about the same.

The real question is, how big of a ballast box are you going to build? You know, so you can show your mastery of thermal flying by flying at a high wing loading.

4 servos would allow for crow and a more sensitive elevator, which is a good thing for a plank. With super accurate elevator control you can inch the CG back just a bit more and reduce drag and increase lift. 60% aileron and 40% elevator would be a plenty big enough elevator. 70%/30% would probably be the sweet spot for a thermal ship.
May 14, 2017, 12:13 AM
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Chuck Clemans's Avatar
Just a thought: A flying wing with 25 degrees sweep is not a plank.

I've designed and flown planks with elevons and I prefer the feel and performance separate elevon and aileron.

Check out Jim Marske's Pioneer high performance full scale designs. Swept forward trailing edge works well. Trimming is done by shifting the cg with a moving weight.

You don't need a lot of reflex if you select an airfoil with a pitching moment near zero. If you need sources, check with Bill Kuhlman at B2Streamlines.com .
May 14, 2017, 11:54 AM
Herk
HerkS's Avatar
Quote:
Originally Posted by Chuck Clemans
Just a thought: A flying wing with 25 degrees sweep is not a plank.
Just a note -- this design as he has proposed it, and having a straight trailing edge, would have a quarter chord sweep of something between 7 and 8 degrees. Can't figure where 25 degrees came from.
May 14, 2017, 12:02 PM
Donate Platelets
Quote:
Originally Posted by EdSoars
[...]

What is still undecided is whether to go with a straight LE, a straight TE, or a straight 25% line. Would there be any difference in tip stall or other handling characteristics within this range of sweep angles?

Ed
Perhaps Chuck misread the above.


Jim
May 14, 2017, 12:08 PM
Donate Platelets
Quote:
Originally Posted by HerkS
Just a note -- this design as he has proposed it, and having a straight trailing edge, would have a quarter chord sweep of something between 7 and 8 degrees. Can't figure where 25 degrees came from.
At what sweep angle would twist begin to be needed?


Jim
May 14, 2017, 12:56 PM
Herk
HerkS's Avatar
Quote:
Originally Posted by JimHSoars
At what sweep angle would twist begin to be needed?


Jim
On a model that I was creating, that would depend on mission first and airfoil choices second. A model designed to go fast and/or for acrobatics and having near zero Cm airfoils would probably not need any twist at any sweep. One that gets some aerodynamic twist from airfoil changes along the span might not need much twist or might need more depending on the airfoils used. A floater model designed for thermal flight and having moderate sweep would usually trim out well at slow speed with moderate (combined) twist along the span. For this kind of thermal plane, the short answer for me would be somewhere between ten and fifteen degrees of sweep. Less than that amount of sweep gets into my idea of plank territory. IMHO of course
May 14, 2017, 02:24 PM
Donate Platelets
I see I need to educate myself about flying wings! It's obviously not simple.

Jim
May 14, 2017, 11:11 PM
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EdSoars's Avatar
Jim: No, wing-only design is not simple. Or it's simple but subtle. But a well-designed wing can be very simple to build, which is appealing.

So, taking the numerical average of all these helpful replies, I've elected to have a straight 25% chord line. Zero sweep.

And it will have only 2 servos for 2 elevons. That's part of the design challenge: simplicity is a high priority. I hope that having a fixed center section will lower the stall speed from the full-span elevon situation. This way, putting in up-trim does not reduce the camber of the center, and effectively washes out the tips, which should improve low-speed handling.

Where I fly, even thermal flying involves decent speed; there is no need to hang around at super-low sink, as with a thermal competition design. All our thermals eventually climb up a slope!

I decided to use the MH-22, but also noted the somewhat high negative pitching moment, so I mixed it 50-50 with the PW-1211. The result has a near-zero pitching moment of -.005 (CompuFoil's calculation). It's 7.1% thick, with 1.8% camber. Nice.

The half-span is 36". The elevons extend from 16" from center to 2" from the tips. It will be one-piece. The leading edge is 1/8" FG rod, except at the curved tips, which are 1/8" nylon rod. Only the battery is in the fuselage. Servos are KST D25MA: good resolution and centering, and these two are TIGHT. Receiver is the Minima 4-ch. Hitec compatible. Tiny, reliable, fits in the wing, and I haven't lost any control at extreme ranges even with its tiny 1" antenna.

I made some rapid progress over the last 3 days, so here's the first bench maiden. Looks like it should fly. I'll post construction details soon.
Last edited by EdSoars; May 14, 2017 at 11:26 PM.
May 15, 2017, 08:33 AM
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EdSoars's Avatar
The wing base is a little different than the usual plank design. Since we our landing zone is mostly rocks and trees, damage control is important. The fuselage FG wraps over a 1/8" flat plywood wing mounting base. The wing attaches to a separate 1/4" ply seat with two nylon bolts; 8-32 in back, 10-32 in front. The 1/4" ply seat is strapped to the fuselage with wraparound FG tape.

The back bolt is usually the only thing that breaks on impact. On harder landings, both bolts might break, or the back FG strap might break. Having the plywood seat be removable makes it easy to extract the nylon bolts from their T-nuts.

The wings' LE is 1/8" FG rod, except for the curved tips, which are 1/8" nylon rod. They are glued onto the foam, then wrapped over with Gorilla clear tape, which heat-shrinks for a smooth, tough LE.

The CF wrap over the tail boom is cut out on top for a slot to accept the fin. The fin is potted in the slot, and held in place with tape. Solid, removable and usually breaks away without damage on those impacts, er, landings.
May 15, 2017, 05:57 PM
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Knoll53's Avatar
Quote:
Originally Posted by EdSoars
I hope that having a fixed center section will lower the stall speed from the full-span elevon situation. This way, putting in up-trim does not reduce the camber of the center, and effectively washes out the tips, which should improve low-speed handling.
This was my thought as well with the Plank 101. At a 50" wing span, having full span elevons seemed like a bit of over kill. At 72" wingspan, full span elevons may be worth while.

My impression of the Plank 101 is that it has adequate elevator control. Not lots of elevator control. It's short elevons have to work harder to achieve nose up, inasmuch as they not only must alter the airfoil directly in front of them, but also the stock airfoil at the root. The wing tips have so much reflex, with nose up, that they wiped out their lift completely......mostly..........well a lot. Not a problem for a slope ship that has it's nose down 99% of the time.

Maybe full span elevons would be more efficient because less reflex is needed to create nose up.
Last edited by Knoll53; May 15, 2017 at 06:28 PM.
May 16, 2017, 12:45 PM
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EdSoars's Avatar
Thanks Kent. Elevator authority point well taken. The PsiClone's (Yep, that's her name now.)
elevons were originally drawn as extending inward to 16" from the center line, or 2" inboard of the half span. It will be easy to increase the inward extent if she suffers from lack of elevator.

I'm reluctant to go to the full-span elevon because the aileron travel, having larger deflection, is draggy and less efficient near the center line.
May 16, 2017, 02:45 PM
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Knoll53's Avatar
Considering that this project is a thermal plank, both elevator and aileron could justifiably be quite minimal. Super fine control of the elevator is handy when dancing around minimum sink in a thermal. The central elevator of the Plank 368 appears to be about 25% of the half span and they are quite effective. You could argue that they are way too big. I wonder what the practical outer limits of elevon sizing might be for a thermal plank? Could 10% of the half span be workable? If you have some dihedral and a rudder, turns can be accomplished with rudder alone.



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