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Jul 15, 2017, 07:08 PM
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EdSoars's Avatar
Building is going VERY slowly this summer. Still, there is a plank under construction on my bench: 72" span, 8" root chord, 6" tip chord for 10:1 AR, and a straight 25% chord-line. This one will be for active thermal/slope flying, not DS or hot aerobatics.

I'm thinking about the controls and the suggestions made in this thread. So, I intend to have a central "elevator", but only used for down-trim, and crow. Three servos only. The elevator function will be on the elevons only. Up trim will be through the elevon function, not the central control surface. This way, there will always be effective wash-out on the elevons.

Any thoughts?
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Jul 15, 2017, 10:46 PM
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BMatthews's Avatar
The problem with getting your washout from the elevons being even slightly reflexed upwards is that you also de-camber the airfoil at the same time. So it isn't as effective. Better to use some slight washout twist that is built in so you can keep the airfoil in its stock shape most of the time. The washout will affect the lift at the tips but at least it's the same airfoil just operating at some slightly lower lift coefficient instead of being deformed by the reflex deflection in the elevons.

This idea that the controls should be normally kept close to the stock airfoil shape is what led me to suggest that the trim function for speeding up and slowing down the flying speed be done using a movable weight to adjust the CG location in flight by some small amount. And then only displace the control surfaces for momentary maneuvers or during turns while in a thermal. By shifting the CG to alter the trim speed we keep the wing as efficient as it can be.
Jul 16, 2017, 07:24 AM
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EdSoars's Avatar
Yes, but: altering the CG not only changes the trim speed, it affects the stability. It works pretty well on swept wings or conventional airframes, but it REALLY changes the handling of a plank. Maybe elevator trim could be coupled to the moveable weight. The up-trim on this wing would only be for slowing down, not cruising. And this planform won't need any washout. And moveable weights make for structural complexities that aren't ideal for flying over our raggedy, no-level surface, tree-infested areas.

I'm hoping that by not decambering the whole wing, as with full-span elevons, the stall speed will be lower, since the center section will not be decambered, and camber could be added with the central surface... which grades seamlessly into crow, I guess, with enough fiddling. I don't mind losing a little efficiency during slow flight, anyway.

With my previous plank, I used the central surface for elevator and crow, which worked pretty well, but using it for up-trim caused a noticeable increase in tip-stall tendency, as the tips were effectively washed in at lower speeds. So I reprogrammed it to act as described here, but on its first, very enjoyable, flight, after proving that it worked, a passing cyclist and dog distracted me long enough to lose sight of the wing. Add bikes and critters to the above list of hazards.
Last edited by EdSoars; Jul 16, 2017 at 07:31 AM.
Jul 24, 2017, 07:13 AM
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This is a response to Don Stackhouse's information about the effect of props, tractor and pusher, on the Res-Dart thread. His point is that a tractor has the effect of destabilizing the central portion of the wing, which increases the tip stall effect, and requires a larger vertical tail volume. On the other hand, a tractor cutting clean air is 10-20% more efficient than a pusher working in the wing's turbulence.

Trade-offs. The design I'm currently building has no central elevator, or one that would only be used for crow on landing. Central elevators on slow landings effectively de-camber or wash out the central portion of the wing, opposite of what's desired. So a powered plank would benefit even more from outboard elevons.

Now the question is: how to size the elevons to provide good roll rates without being so large that they need very small, critical pitch control movement? And how to balance them against the down elevator (actually central flap) for crow on landing? I foresee lots of calculations that fly over my head.

Back to seat-of-the-pants calculation, TLAR, and cautious flight tests. It's easy to modify the span dimension of elevons and flap after testing.
Jul 24, 2017, 08:05 AM
Herk
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Thread OP
Hi Ed, Your models are primarily gliders. Unless you have a different purpose in mind------.

Given that, propulsion is used only during climbout and recovery. So fine points of propulsion efficiency and stability effects are not really significant. That efficiency is significant if range and endurance are important. I doubt that either of those is important for your concept.

My plank gliders have all been tractors and I have had no stability issues, but these models don't use power during the phases of flight when that effect might be noticeable. I have put together a couple of swept wings that are pushers and one that's a tractor but they are gliders too so power is used only at relatively low AOA flight conditions where that effect is pretty much insignificant.

Your issues are a good example of the reason that any design should begin with a clear idea of the mission/purpose of the aircraft and what it's typical flight profile would look like. The design would then be optimized for the most important part of the flight profile. For example, a thermal sailplane does not need the same amount of agile maneuverability that an acrobatic or racing design might require. It does however need excellent stability and efficiency while gliding at relatively high lift coefficients.
Aug 18, 2019, 07:56 PM
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EdSoars's Avatar
Herk et al,

A few planks have flown over the bridge and under the dam since last posting on this excellent thread. Since then, three planks with moderate aspect ratios (10 to 12) have been built and flown under a wide range of slope/thermal conditions. A few conclusions float to the top:

1) Aspect ratio dominates other parameters, at least in the 10 to 14 range. A clean design with a wing loading of 10 to 14 ounces/square foot will thermal nicely except when there is no ambient wind. Being able to explore large volumes of air quickly more than makes up for not having a low sink rate. It's a matter of being able to apply a decent L/D in a dynamic airspace to find the lift. I will eventually work up to aspect ratios over 15 someday, but they will be bigger than normal wingspans for me... and my landing zones.

2) Herk, you're right about mission/purpose and designing for the most important part of the flight profile. It takes a little thought. Not flying in contests or on flat fields eliminates the need for low sink rates in poor conditions. I can avoid low-lift conditions by doing something else! Here in the Rockies at elevations from 7000 to 10,000 feet, thermals are almost always quite turbulent, so stability is needed at the same time as high L/D at speeds of 30 to 50 mph. Planks excel at this, since they have inherently good spiral stability and performance at low lift coefficients.

A thermal sailplane does need good stability at high lift coefficients when flying on flat fields, when staying in a thermal over that field is more important than covering a lot of airspace. In our high-altitude, breezy slope-thermal conditions, it isn't possible to linger for more than 2-3 turns in a single thermal before bailing out and hunting up the next one, which could be 500 yards upwind.

Onward and upward...
ed
Aug 20, 2019, 08:03 PM
Herk
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Thread OP
Hi Ed - been following and enjoying your builds. For your flying the more moderate spans are probably preferable. Also high AR can amplify the adverse yaw effects of roll control. I think if you do build a really long wing model, you will probably want to include an active rudder control. And keep in mind that at those altitudes Rn is 20-25% lower than at sea level where I fly.
Aug 20, 2019, 11:01 PM
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Herk, yes to moderate spans. It's interesting how different considerations lead to the same outcomes: high aspect ratios need long spans and make landings in rough terrain difficult. We could learn from watching how different birds' lifestyles are matched to and matched by different birds' physiologies. For me to try landing a 20:1 aspect ratio, 12' span plank would be like a duck trying to perch on a clothesline. Dignity would be lost. Repairs would follow.


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