FREE FLIGHT DURATION
Louis Joyner, 6 Saturday Rd., Mt. Pleasant SC 29464
TIP TIPS:
A model's wing planform often owes as much to fashion as to aerodynamics. One year elliptical tips are all the rage, only to be supplanted by square tips. But is it all a matter of style?
Certainly some shapes offer an advantage over others by reducing the size of the tip vortex and decreasing drag. But often structure, not aerodynamics, is the determining factor in wing shape.
An elliptical planform is easy to make when building a solid-balsa Hand-Launched Glider wing. But for a built-up, tissue-covered wing the curving tip requires a good bit more work. And for a sheeted wing, or even one with a sheet-balsa D-box, the elliptical planform can get you into some compound curves where sheet balsa just doesn't want to go. You could make it work, but it involves a great deal of cutting and piecing, much like planking a round fuselage.
The introduction of high-tech materials didn't make building an elliptical-planform wing any easier. Ever greater use of aluminum-skinned wings from the late 1970s had aluminum skins only on the constant-chord main panels, with sheeted tips in the then-fashionable elliptical outline. With the widening use of aluminum for F1C wings, the elliptical shape went away almost completely, to be replaced by a variety of tapered tip planforms.
The more recent use of molded carbon-fiber D-boxes also put an end to rounded tips for F1A Towline Glider and F1B Wakefield. To build a wing with elliptical tips required separate molds for the right and left tip panels, as well as a straight mold for the main panels. That's a lot of work. On the other hand, a tapered wing only required one straight mold; the D-box skins made on that mold could be trimmed from the back to accommodate a taper in either direction.
But a tapered wing of any sort does require a bit more work than does a constant-chord wing. The ribs are not all the same, reducing in length and height gradually toward the tip. Rib blanks for a tapered wing can be stacked between a root and tip template then sanded to shape. Or the tip panel can be built using slightly oversize ribs, which are sanded to shape using a long sanding block. For a tapered wing, there are some aerodynamic advantages to reducing the airfoil camber and thickness percentages slightly from root to tip.
One method is to reduce thickness and camber 0.5% from root to dihedral break, then to reduce the airfoil thickness and camber from break to tip another 0.5%. Note that this is a percentage reduction in thickness and camber, in addition to the reduction in absolute thickness that you get from shortening the rib. You can't do this by simply photocopying the root rib to a smaller size.
In practice, most people simply build a constant-chord wing fixture to match the shape of the underside of the wing root rib. Then all the ribs are cut to the same lower curvature as the root rib. The wing's thickness is determined by the taper of the full-depth main spar and carbon-fiber D-box.
After construction, the upper side of the aft ribs are sanded to shape. The result is a wing that tapers in planform with decreasing airfoil camber and thickness percentages toward the tip, all built on a constant-chord fixture.
Until recently, most tapered wings consisted of four panels: two inboard and two tip. The inboard panels were slightly tapered or constant chord. As spans have increased, more and more modelers have gone to the six-panel wing. This planform allows a closer approximation to an elliptical planform, but without the need for specially molded curved tip pieces.
The six-panel shape also allows the dihedral angles between panels to be reduced, approximating elliptical dihedral. On a more practical note, it also allows a longer wing to be built using available D-box skins.
A constant-chord wing is easiest of all to build. Since the ribs are all the same, you just need to cut one template and start slicing away on your stash of quarter-sawn 1/16 balsa. But remember that one big problem with a constant-chord wing is that it can also be constant weight. Unlike a wing with an elliptical or tapered planform, the tip of a constant-chord wing will be just as heavy as the root. That can make the model less stable and less sensitive to finding thermals.
You can lighten up the tips of a constant-chord wing by using softer balsa for the tip ribs, spars, leading edge, and trailing edge. (Good ideas for a wing of any shape.) Since the bending load on a wing is greatest at the center and the least at the tips, taper the spar in width and/or thickness from root to tip. Consider using a lighter covering material on the tips.
For events in which the span is restricted, such as P-30, there is a strong temptation to make as wide a wing as possible and keep it wide all the way to the tip. Tapering the tip panels slightly won't cost that much in area and will probably result in reduced drag and better performance.
If you want to round things off, keep the trailing edge straight all the way to the end and curve the leading edge. This seems to be the best shape for reducing the tip vortex and its attendant drag. Thinning the wing at the tips also helps.
One practical problem that often presents itself, especially on lightweight wings and tails, is the end rib being bowed in by the covering. Using a thick end rib, a soft balsa block tip, or a sheet balsa "rib" angled out are ways to strengthen the tip and prevent bowing. Another approach is to use a balsa block tip that is wider at the rear, where the airfoil is the thickest.
The soft-balsa block tips I use for constant-chord stabilizers with a chord of approximately 3½ inches taper in plan view from ¾ inch at the trailing edge to ½ inch at the leading edge. This puts balsa where it can do the most good resisting the inward pull of the covering and keeps weight to a minimum.
Screws. Not hooks:
I never did like using wire hooks on stabilizers (stabs). In the days before epoxy, the hooks had a habit of popping off at the most inopportune time. Even after the advent of epoxy and Kevlar-thread tarps anchored with a belt-and-suspenders approach, the hooks stayed in place but kept puncturing everything else in the model box.
Some years back I gave up on wire hooks and switched to either plywood or dowel hooks. At least they wouldn't fall off, and they posed less of a risk to nearby parts in the box.
With the advent of the lightweight plastic films, such as quarter-mil Mylar™, a new problem arose: the somewhat fragile covering sometimes has to be re-covered on a regular basis — often a couple times a year. Trying to recover around a permanently pinned hook was next to impossible.
Luckily, at roughly that time Danish modeler Jørgen Korsgaard suggested a simple solution: a removable hook. This consisted of a pin-head plastic screw located on the stab spar and extending down through a thick balsa corner rib. The screw head sat up above the top of the stab enough to allow room for the stab hold-down line and the rubber bands.
It was an elegantly simple solution. The screw could be installed after covering, and easily removed for recovering. The wide screw head prevented the rubber bands and hold-down line from slipping off.
Installation was simple. You would drill a hole through the stab center rib, run a tap all the way through, remove the tap, and add a drop of thin cyanoacrylate glue (CA) to the hole. Repeat a few times until the tapped hole was hardened, then cover the model and run in the screw. For my F1B models I used readily available #2-56 nylon screws, which require a #40 drill.
The center rib was typically 1/8-inch balsa. If too soft a wood was used, there could be some movement of the screw in time; that could easily be fixed with a drop of CA and another run with the tap.
In an effort to develop a stronger screw hole, Rex Hinson improved on the idea by first drilling a 3/32-inch hole through the stab, then gluing in a piece of hardwood, trimmed flush with the top and bottom of the stab. Then he drilled and tapped the dowel to receive the screw.
A few months back I suggested the removable-nylon-screw idea to Thurman Bowls for use on his new P-30 model with a pop-up wing dethermalizer. Thurman further improved the idea by using a piece of 1/8-inch-diameter, thick-walled plastic tubing inside the dowel. The tubing is the white Plastruct stuff available at most hobby shops in 15-inch lengths.
I gave it a try on a new stab, and it worked well, but you do need to tap the tubing before gluing it in place. I did it as follows.
I cut a piece of tubing slightly longer than the thickness of the stab. Gripping the tubing with a pair of slip-joint pliers, I ran a 2-56 tap in a half turn, backed it out a quarter turn, and repeated until the tap was all the way through the tubing. I trimmed the tubing to the exact length, and glued into the 5/16-inch hole in the stab using CA. After covering, the nylon screw was run in from the top.
The result seems even better than the dowel method and far superior to using a balsa alone.
The Plastruct tubing should be easy to find. I can't recall ever visiting a hobby shop that didn't stock the stuff. It's usually in a countertop display in the model-train section, but if you can't find it locally, visit the Web site at www.plastruct.com.
Transcribed from original scans by AI. Minor OCR errors may remain.
