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Building Tips and technical articles. => Building techniques => Topic started by: Larry Wong on May 21, 2011, 04:28:57 PM
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Just had a thought, If the say elevator is tapered or air foiled would the movement have to move more to make it work? Because of the taper is 1/8", if moved 1/16th the top would be level with stab. ???
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Good question Larry. It is true that your elevator when moved 1/16" an inch, would only now be even with the stab surface, but, the airfoil has changed, and therefore the lift generated by the combined surface of the stab and elevator. It's because the airfoil formed is more like a lifting airfoil rather than the symetrical arfoil we started with.
It would appear that near, and around nuetral, the lift generated would be smaller than when the elevator is deployed more, acting like a greater and greater undercambered airfoil. If this is so, then the effect may be quite desirable, acting somewhat exponentially.
It's similar to, using a thicker stab so the elevator is less effective around nuetral, though not as drastic.
The largest draw back to the thick stab method of trimming, is that when the elevator is blanked out by the thicker stab, there is often a hesitation that is very apparent at the intersections of manuvers such as the Eights.
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YES i thought about the lift, and the expo. effect, so how about a none taper elevator ? like on smaller planes just a slab, ( plank) ~^ ~^
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Well Larry, I believe the surfaces will work fine as a plank. In fac, I've seen planes finished that way, with no taper at all on the movable surface.
Aside from not looking finished to me, I would assume that the reaction to control input would be minimally faster than tapered surfaces.
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Igor likes flat flaps and can probably tell you why. There are some old NACA reports that discuss this. Google NACA Flap Shape. No doubt John has read them. One interesting shape is tapered just at the flap TE, sorta like a cold chisel. That reduces hinge moment, which is a virtue, but it might do perverted things if the flow separates abruptly.
I make tapered flaps because they look better. The last set I made took a lot of work to get the leading edge piece chamfered right. Then the epoxy I used to assemble them never set. That made me think favorably of flat flaps.
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In my experience, the flat/plank elevator (same thickness as the stab) may have a problem opposite to the hesitation problem--oversensitivity around neutral with difficulty tracking in level flight, especially if the controls are set up on the sensitive side, and the pushrod hole in the horn is not a bit oversize.
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I remember using the steel rod meathod for tapering flaps. Imagine the state I was in when I put built up flaps between the two rods and sanded away. upon flipping the flap over to taper the other side, only to find that I sanded the entire taper into only one side only. Rather than having equal taper on both sides. HB~>
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One of my 1/2A stunt airplanes had a built up 1/32 sheet covered 1/4 in thick plank stab and elevator. It flew to suit me. Perhaps a more important question is whether the trailing edge should be square or rounded.
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In my experience, the flat/plank elevator (same thickness as the stab) may have a problem opposite to the hesitation problem--oversensitivity around neutral with difficulty tracking in level flight, especially if the controls are set up on the sensitive side, and the pushrod hole in the horn is not a bit oversize.
Kim, are you talking about a flat plate elevator, that is the same thickness at the hingeline, and trailing edge, , or an elevator that is the same thickness as the stab, at the hingeline, but tapers from there to the trailing edge?
Both of these will react differently from each other.