Hotwire tips to get a perfect tapered wing (pun intended)
Why make a tapered wing?
“Only design a tapered wing if you never intend to build it”.
-A Wiseman long ago
Tapered wings are beautiful to look at, but they can bring misery to the craftsman.
Tapered wings are hard. They are hard to transport. They are hard to build. They are hard to make last. Moreover, too much taper makes a wing worse. As the Reynolds number falls the wing feels the inertia of air - it’s harder to move through.
The only reason to build a tapered wing is when every ounce of aerodynamic performance is needed to complete the mission.
A recent challenge called for maximum efficiency.
Build a plane with the maximum flight time on a single 18650 cell
Designing the most efficient plane possible required a tapered wing with a high Aspect Ratio.
What are the stakes?
The calculations showed that a wing with a taper ratio of .55 (AR 13) would have 15% better performance than a rectangular wing (AR 7). Either you want to win or just compete.
Hotwire cutting the optimal wing
The problem with hotwire cutting a highly tapered wing is, that the tip profile is traced much slower than the root. Extra heat at the tip melts the foam and makes the surface uneven.
Fig 1 - Tapered wing with heat damage at the tip
Fig 2 - Close up of melted foam at the tip.
Trap #1: Increase the kerf at the tip. That doesn’t work, it just makes the airfoil fatter with a poor surface.
Fig 3 - Large kerf to account for melting.
The fix:
Design the wing with a maximum taper ratio of .65
The tip profile will experience more heat than the root, but the wing surface will still be useable. The original AR of 13 can still be maintained with some geometry adjustments, and the efficiency gains are achieved.
Fig 4 - Plane designed to fly on a single 18650 cell with a slightly tapered wing.
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