Thermal Plank - Page 2 - RC Groups
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May 17, 2017, 08:44 AM
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EdSoars's Avatar
Yes, a central elevator offers more options for trim, and it's a good idea to separate the roll and pitch control. I did this with the most recent Tiger Moth, and it worked well, until an unfortunate out-sight landing, well, impact. The best trim setup was with up-trim (slow) done with the ailerons to provide washout, and down trim (fast) done with the elevator, also providing washout. I would have used both controls for speed trim, but the aileron servos (Dymond) had far too much gear slop.

It should be possible to calculate the necessary size of the elevator from the planform. A strong taper with a wide root section should need far less than a slight taper, with a root chord nearly the same as the tip chord. But my aerodynamic math isn't up to it.

I may leave space at the wing center for an elevator servo, but I want to see how well the wing performs with just two. One servo isn't much, but it's a 50% increase!

ed
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May 17, 2017, 11:09 AM
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Knoll53's Avatar
My comment is not about the location of the elevator, but rather the sizing. The question I am proposing is, with conventional elevons and a 2 servo wing, could elevon sizing as small as 10% of the half span be adequate for a thermal ship? The purpose of this reduction is for finer pitch control with inexpensive servos. Those small elevons would need to deflect a lot in order to provide a pitch change, thus a little linkage slop would be more tolerable.

just thinking out loud...

Looking forward to your reports on the airfoil.
May 18, 2017, 03:16 PM
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EdSoars's Avatar
Ah. Now I get it. A flying friend built a conventional E-power thermaler last summer, with pretty darn small ailerons. He claims it handles very well. I'll check the percent of span with him.

That doesn't address all the elevon questions of course...

So, I'll start with elevons of a little less than half of the half span. Should be easy to make the changes up or down, since the TE is solid balsa.

Another focus going forward with this is to make it easier to produce a more accurate wing, especially around the TE, which is so important to pitch on these planks. Your out-loud thoughts are welcome!

ed


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