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Kirt Blattenberger
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Even during the busiest times of my life I have endeavored to maintain some form of model building activity. This site has been created to help me chronicle my journey through a lifelong involvement in model aviation, which all began in Mayo, MD ...

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Ready to Try Towline Gliding?
April 1960 American Modeler

April 1960 American Modeler

April 1960 American Modeler Cover - Airplanes and Rockets Table of Contents

These pages from vintage modeling magazines like Flying Aces, Air Trails, American Modeler, American Aircraft Modeler, Young Men, Flying Models, Model Airplane News, R/C Modeler, captured the era. All copyrights acknowledged.

In 1960 when this article appeared in American Modeler magazine, radio control (R/C) was still in its infancy and was a rich man's sport (mostly). R/C also required an amateur radio operator's license in order to use the transmitters. Control line and free flight constituted the purview of the vast majority of aeromodelers. Towline gliders were very popular in areas where enough open area was available - and back then there was a whole lot more open area than there is today. Take a look at the backgrounds of photos and movies in the 60's and before and notice how relatively undeveloped the land was, even around larger metro regions. Anyway, this article offers sage advice to modelers considering getting into towline gliders. Many kits were available in the day.

Ready to Try Towline Gliding?

The graceful soaring flight of gulls and hawks has long been the inspiration for model glider and people-sized sailplane flyers the world over. Towline glider flying comes closer than any other phase of modeling to duplicating the grace and beauty of nature's sky creatures. A good model glider is really one of the simplest types of model to build and fly. There is no noisy engine and messy fuel to contend with, no flying wires to get tangled, no radio to get out of tune. The model is reduced to its bare essentials, wing, tails, simple fuselage, nose weight to balance properly and a tow hook for launching ... that's all.

Flying, too, is in its purest form; the glider is towed aloft on a length of string, released and then glides slowly back to the ground. Because of light and efficient design, a good glider will soar and gain altitude on the slightest rising air current. Out-of-sight flights are not unusual on a warm summer day when thermals (rising air currents) are popping over the landscape.

Ready to Try Towline Gliding?, April 1960 American Modeler - Airplanes and Rockets

Towline glider flying is one of the best ways for a model fan to learn the basic techniques of flying and proper adjustment. Initial cost is low, construction is simple and the inevitable crash hazard is reduced because of light weight and slow flying speeds. The one requirement for glider flying that can be a handicap for city dwellers is space. Towline glider flying needs large open fields to give the model room to circle and drift downward. Even an area the size of a football field could be considered on the small side. Today's model glider competition is conducted in the "Nordic" class, but don't let the name scare you off. Nordic model designs are simple and serve equally well for just plain Sunday afternoon fun flying. Nordic class glider regulations are international and there are two divisions, A-1 and A-2. The A-1 class is for smaller gliders and specifications require: maximum surface area 279 sq. in. and minimum total weight 5.08 oz. There are no restrictions on design, towing must be done from the ground with an inextensible line (although 15% stretch is permitted) with maximum length of 164 feet. The larger A-2 gliders are permitted a surface area of 495.9 to 526.9 sq. in. and minimum weight of 14.46 oz. Complete regulations governing Nordic glider competition can be found in the A.M.A. official rule book.

The glider described here is designed to fit the Nordic A-1 specifications. Due to its simple construction it can be easily and quickly built. It will turn in fine flights and show you all the tricks encountered with the larger A-2 designs. Since full size plans are not absolutely necessary for this model dimensions are given on the plan for easy enlarging. Full size patterns are presented for the tricky parts such as wing and stab ribs.

Begin by drawing full size wing and stab outlines and rib positions on sheet of shelf or wrapping paper. The most important part of any glider is the wing, so let's tackle it first. Select medium stock rather than soft or very hard. Balsa that is a bit on the hard side can be used since it is better to have a little extra weight and strength rather than too weak a structure because of soft wood.

Ribs should be cut from quarter-grained stock if available. This type stock gives the ribs rigidity and helps prevent warping of wing structure. Cut all the ribs, stack them together, pin or clamp and sand so that they all have same contour. With ribs still stacked cut the spar notches using two hacksaw blades held together. Set rib stack aside and when building wing use ribs as they come off the stack so that any notch misalignment will not twist or bend the spars.

Pin down the main wing panel leading and trailing edges over the plan and cement ribs in place. Next cement top spars only into the rib notches. Assemble tip panels in the same way, flat over plan drawings, but omit top spars. When dry, take up tip panels and cut leading and trailing edge ends at an angle for dihedral. Join tip panels to main panel, block up to proper dihedral angle and then add top spars to tip panels.

Check alignment from front, back, side and top. Surfaces should be as warp-free as possible.

Gliders with lifting tail surfaces should balance between 50% and 75% of chord back from leading edge.

Towline is "monofilament" fishing line, maximum 164 feet for contests. Model end fitted with ring and nylon strip to pull off for release. Store on small fishing reel.

Towing technique: Hook up towline, reel out line into wind, take up slack. Helper holds glider and releases as launcher starts moving into wind. Auto-rudder is adjusted to maintain straight climb. Launcher continues moving up-wind, glider climbs toward maximum altitude overhead.

When maximum altitude is reached, give slack on towline to release glider. Auto-rudder disengages permitting gliding turns back to ground.

When dry, take up wing assembly from plan and add bottom spars first to main panel and then to tips. Coat dihedral breaks with three or four layers of cement to strengthen. Cement 1/16" sheet to bottom of wing at center to form mount.

Short dowel "keys" are cemented under trailing edge to position wing on fuselage. Add these after covering wing and space to match fuselage width. Cut away tissue covering where keys are located so that good strong cement joint can be made. If keys are cemented onto tissue the first hard landing bump will peel tissue and keys off wing bottom.

The stabilizer is built flat over the plan in the same manner as the wing. Select lighter wood stock than used for wing for the stab. A lightweight stab will require less weight in nose to balance model properly. Add sheet planking to stab center section, note that both top and bottom planking is set into structure flush with rib outlines. Bend wire parts to shape and double cement in place.

Sand exposed surfaces of wing and stab smooth before covering. Apply light weight tissue dry and then wet to shrink tight. Apply about four or five coats of dope to wing and two or three coats to stabilizer. Thin dope 50% and add 10 drops of castor oil per ounce of dope to plasticize. This keeps covering from drying out and prevents excessive shrinking that causes warps.

The glider fuselage is made from a single piece of 1/2" sq. tapered on sides only towards rear. Select very hard stock for fuselage. Piece should be as warp free and straight as possible with grain running parallel to sides along greatest length possible. Obtain a piece of flat lead sheet from plumber supply shop, cut to nose top view shape and cement to bottom of fuselage nose. Add piece of 1/2" sq. to top of fuselage nose. Carve and sand to shape shown on full size parts drawing.

Wing is keyed to fuselage with short dowels to maintain alignment

Pop-up stab de-thermalizer necessary. See Starting Free-Flight in March 1959 American Modeler

Towline glider construction must be strong and light. Clean design enables soaring on slightest thermal. Finish with high visibility coloring to keep in sight. (Red, orange, silver.)

Raising stab trailing edge gives negative incidence; allowing tighter turns. If negative is decreased turn must be opened up or glider will spin.

Hand launch with nose down slightly to check glide and turn trim. Correct for stall or dive (dotted lines).

"Hi-start" launch is combination of catapult and towline method. Good fun in calm weather. Use single strand of 1/2" flat rubber 25 ft. long tied to 75 ft. of kite string. Anchor end of rubber ta stake in ground. Hook towline to glider, draw back stretching rubber about three lengths. Release and glider will climb catapulted by rubber tension. Vary rubber size for more power when glider trimmed well.

Cut out plywood wing and stab mount pieces and cement in place. Double cement because a strong joint is needed here to support wing and stab. Cut out fin and rudder, cement to fuselage top. When dry, sand edges round. Cement wire hinge to front edge of rudder. Bend eye to hold hinge to fin top. Push hinge wire into fuselage top at rear of fin then cement eye to top of fin over hinge wire on rudder. Next bend wire rudder horn to shape and cement in place over trailing edge of rudder. Bend hook for rubber band and cement to fin near leading edge. Cement triangular balsa horn stops to sides of fin level with wire horn ends on rudder. These triangular blocks will be trimmed later to make rudder travel adjustments for trim on tow and in glide. Sand fuselage smooth and apply three or four coats of dope.

Bend tow hook to shape and cement to bottom of fuselage. Bind with thread and double cement since the tow hook takes a lot of strain. Cut balsa strip and cement into open part of tow hook. The rear end of this balsa strip is trimmed to change location of tow ring to achieve best towing position. Bend other wire parts to shape and cement in place. These include wing hold down, auto rudder line guide eye at fuselage center, auto rudder release pin guide and lower de-thermalizer hook at tail.

The auto rudder linkage is a simple push-pull affair. Left turn rudder trim for glide is held by rubber band on left side of rudder horn. Nylon thread and rubber band connects the right rudder horn to release pin under fuselage behind tow hook. The tow line has length of slack behind tow ring connected to release pin. In operation the thread to right rudder horn is held by the release pin at the eyelet. This swings rudder to right while glider is on tow. When tow rig drops off tow hook, the release pin is also pulled letting rudder swing over to left turn glide position.

As shown on the drawings the end of the towline connects to the tow hook with a 3/8" dia. ring or loop. An additional 4" length of line connects the tow ring to the auto rudder release pin. Tie a 6" x 6" piece of nylon onto the towline about a foot ahead of the tow ring. The nylon will drag the tow-ring off the tow hook when tow pressure is released. The tow-line can be stored on a small inexpensive fishing reel for ease of handling in the field. Monofilament fish line can be used for towline.

With glider completed and towline prepared we are ready for test gliding and practice towing. Try and pick calm weather for your first test hops. A wide open site with high grass and few obstacles is desirable. To test glide properly hold the model above your head and run slowly into any breeze that may be blowing. Hold glider slightly nose down. When enough speed is gained the glider will lift out of your hand and glide back to the ground. Do not throw hard with nose up or wings banked. Resulting stall or steep turn will not give a true indication of glider trim.

Make repeated hand-launched test glides noting tendencies to stall or dive and degree of turn. With small amount of left rudder as set up by horn and triangular block stop shown on plans, glider should turn to the left. Too steep a turn can be corrected by reducing left rudder angle or increasing negative stab incidence. If the model stalls, correct by steepening turn or by reducing negative stab incidence. A bit of modeling clay may also be added to the nose. Turn trim both in glide and on tow is adjusted by trimming stop blocks and bending rudder horns to get desired turn in glide and straight tow.

When glide trim looks good it's time to try a tow. Reel out about 25 or 35 feet of line for first tows. Put tow ring on tow hook and engage auto rudder. Have your helper hold the model lightly with nose up about 30° at shoulder height. Model and towline should make a straight line directly into any breeze blowing. Take up slack in towline then start to move slowly directly into wind watching model over your shoulder.

A bit of teamwork is required here between you and your helper. He must release model just as you start moving and as he feels a firm tug on the model. He may find it is helpful to move a few steps forward holding glider when you do. Your launching helper should never throw glider because towline will go slack and tow ring will drop off, releasing model too low.

It is a good idea to work out hand signals between launcher and towline handler. You can't always hear each other clearly when wind is blowing. Procedure can go something like this:

Reel out towline, launcher connects tow ring, checks hook-up and signals when he has glider ready in position. Towline handler then takes up towline slack and checks alignment into wind. When he is ready he raises arm to let launcher get set, then drops arm and starts moving upwind. Launcher can take few steps and release model. As model starts climb towline handler must watch model and continue moving upwind. If glider swerves sharply, stop and release tow immediately. Model should recover to normal glide. Note turn tendency and make rudder adjustment to correct. Then try another tow. Repeat procedure until glider climbs straightaway on tow.

With good towing technique the model can climb almost directly overhead. At this point stop moving, give slack to pull towline of tow hook. Now you can try tows with longer line. Increase length about 20' on each successive tow until maximum length of 164' is reached. You will probably have to continue adjusting rudder for straight tow as you increase line length since short line tows are easier than those with long line.

One important point that should not be overlooked is the use of the de-thermalizer even on test flights. Towline gliders have been known to catch a thermal from 50' tows and soar out of sight. The de-thermalizer is not a fancy word we use to impress you, but rather a tried and true gadget to bring your model gently back to earth if it should encounter a thermal. The type used on our A-1 design is the pop-up stab. When the de-thermalizer releases, the stab tilts up "spoiling" the fore and aft trim of the model. Forward speed is arrested and the model sinks in a level attitude.

The stab is held down on the tail by rubber bands over fuselage and hook at leading edge. Another rubber band holds the trailing edge down by passing over wire at trailing edge and wire on fuselage bottom at rear. A short length of untreated (not waxed) venetian blind cord is inserted between rubber band strands at trailing edge. Just before launching the cord is lit and burns slowly until it burns through rubber band. Stab tilts up from tension of rubber bands at leading edge. Stab is provided with string or wire on underside to limit tilt angle to about 35° or 45°. Cord burning rate should be checked before using on model. Use one minute or less length cord for test flights and three minutes for contest flights.

Practice your towing technique until you can get maximum altitude before release. The higher the model climbs the longer the glide duration which after all is the main idea in towline glider flying. As you become more proficient you will be able to "play" the glider like a kite on tow, releasing when you feel a thermal. This is what wins contests. With a good working de-thermalizer you will have your glider back at end of 3 minute flight limit ready for a repeat performance. Try to get in the required number of flights when thermal conditions are good. Don't waste a lot of time with with practice hops at a contest, come prepared with model adjusted and come to win.

Even if you never enter a contest you will find towline glider flying one of the most satisfying phases of modeling. So get out the balsa and cement and tackle the bird accompanying this article. Happy cross-country running!

Kits recommended to start out with include Sinbad, Towline-Terror, Thermic Series, Jetco Nordic.

 

 

Posted May 29, 2021

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