A/1 Jetstream Towline Glider
November 1960 American Modeler

November 1960 American Modeler 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.

Airplanes and Rockets website visitor L. Ross wrote to request that this article featuring Warren Kurth's Jetstream A-1 towline glider be posted. I recently purchased the November 1960 issue of American Modeler magazine, where it appeared, so I scanned and processed the images and text. Detailed building, covering, and flying instructions are provided by Mr. Kurth. The Jetstream's projected wingspan is given on the plans as 47", with a wing area of 269 square inches. The fuselage is 31" long with a balsa box construction, while the wing an tail surfaces are sticks and sheet ribs. The wing airfoil is undercambered, which makes covering with Jap tissue a little tricky, but the horizontal stabilizer uses a flat bottom lifting airfoil. Instructions for making the regulation A-1 towline is even given. The model is built so light that it requires more than 1.5 ounces of ballast to bring it up to the A-1 class minimum of 5.08 oz (144 grams). Hopefully, Mr. Ross will provide a photo of his completed Jetstream for posting here.

A/1 Jetstream Towline Glider

By Warren Kurth

Now and again there appears on the model scene a design that looks every inch a contest winner-such as Goldberg's immortal Zipper, Hansen's '53 Nordic Winner or King's '54 Wakefield winner. Sometimes they sport slinky Bardot curves; sometimes no-nonsense lines like a Volkswagen. But almost without exception the world's top contest craft seem to share an unmistakable functional beauty. Such a model is Warren Kurth's 1960 and 1959 Nats-Winning and AMA Record-Holding A-1 towline glider-one of the finest examples of towline design to ride the thermals for many a season.

Warren's "Jetstream" is simple and rugged. Its proportions were developed after careful study of dozens of top glider designs-plus extensive flight testing and contest experience. The original model was built early in '59, taking 4th in the first contest entered (combined A-1 and A-2 event at Cedar Rapids, Iowa) and 1st in its second contest (combined A-1 and A-2 at Chicago Midwestern Championships). It is particularly easy for a beginner to adjust and fly.

Originally dubbed "Spike" (3-view appeared in May 1960 "A.M."), the new name came about following the suggestion that a less vulnerable upper fin be substituted, forward of the stab. The subsequent Mark Two version with the jet-age swept fin is reported to give slightly better flight characteristics. It won with 11 minutes, 26 seconds at this year's Dallas Nats. The plans give both types, you take your pick! The plans show cut-out parts arranged on 3" x 36" balsa sheets. Parts are numbered in correct building sequence; you'll sidestep snags and finish in the shortest possible time if you follow this procedure...

Pick out firm (but not heavy) balsa stock. Sand off any fuzz before marking patterns on to the sheet balsa (with carbon paper or by pinpricks). Warren's original models have no rear wing spar this is added on the magazine plans as a safety factor for the average modeler. If you use the best wood and have already built a few lightweight gliders, the rear spar may be omitted (but then remember not to cut the rear spar notches in the ribs). It's important that wing tips and tail surfaces be light so quick recoveries occur from stalls or rough air disturbance. Leading edge sheet for flying surfaces should bend easily to follow airfoil curves.

Rub over your working drawings with a bar of soap at all rib/L.E. and rib/T.E. joints to prevent parts from sticking to plans. Wax paper over plans isn't such a good idea since it retards cement drying.

WING. Build the three panels separately. Start by sanding trailing edge strips to shape and slotting them to take the ribs. Pin down leading edges (1) and trailing edges (2) directly over plan blocking up T.E. with scrap 1/32" sheet. Pin lower 3/32" square spars (3) over plan, packing up with scrap 3/32" sheet balsa. Cement ribs (4) in place, using dihedral-angle template (X) to slant four dihedral-break ribs correctly. Add top 1/8" square spars (5); avoid distortion of ribs by not using excessive down pressure. Install center section gussets (6). Give all joints two more coats of cement (use cement sparingly). Allow to dry for several hours before unpinning from building board.

Next, attach top wing leading edge sheeting (7 and 8) first cementing to leading edges and front 1/8" of each rib. When dry, cement sheeting to remaining portion of each rib and to main spars. Give sheeting joints an extra coat of cement, except at leading edge/sheeting seams and spar/sheeting seams (which could pull sheeting down and out of shape). Add wing tip pieces (9 and- 10); carve them to shape when dry.

Carefully sand the three wing panels; check for weak or broken joints. Clear dope and lightly sand with #400 paper, at leading edges, trailing edges, wing tips and end (dihedral-break) ribs. Also dope undercambered lower rib edges (two coats). Now line up the three wing panels, pinning center panel to building board and packing up each tip panel 3-7/8" under the tip rib at front and rear. If facing ribs of the three panels do not fit flush, carefully sand them to correct this.

Give each facing rib two coats of cement and let dry before finally cementing panels together. When quite dry, cut away front and back ends of ribs, add dihedral braces (11 and 12). Give all dihedral break joints another coat of cement, then cement center section wire brace (A) to trailing edge.

Your wing is now ready for covering with Jap tissue (cover dry), using clear dope as adhesive. Make sure that tissue is firmly doped to undercambered edges of ribs. Water spray or gently dab covering with sponge of cotton to tighten it. Constantly check for warps (twist panels in opposite direction if any should appear as tissue dries). When dry, apply four coats of clear dope (thinned 50% with thinner); gently sand any rough spots with #400 paper after the first coat. Dope only one panel at a time; watch carefully for warps as the dope dries. It's good practice to pack up each panel with scrap 1/8" sheet at leading and trailing edges and hold assembly in place on building board with small weights while dope dries. Any trim should be cut from colored tissue. Dyes such as "Rit" or "Tintex" can be dissolved in the thinner if a translucent coloring is desired.

STABILIZER. Built much the same as the wing panels, except its airfoil section is flat so no trailing edge packing is required. Start by sanding trailing edge to shape; slot it for ribs. Pin leading edge (13) and trailing edge (14) over plan. Add ribs (15) and 3/32" square upper spar (16), followed by gussets. (17). Give all joints another coat of cement, allow to dry for several hours before unpinning from building board. Add leading edge sheeting (18) and tips (19 and 20). Finish off, cover and dope in the same way as wing; finally add the two wire hooks (B and C) cementing them in place and covering with small patches of thin silk.

FIN AND RUDDER. For either fin hinges and auto-rudder details are the same. Join two fin pieces (21 and 22) flat on plan. Sand this assembly and fin (23) to sections. shown in top fuselage view-then cover each separately with tissue. Join fin and rudder with small silk hinges (D), give four coats of dope, add wire hooks (E) and rudder stop (24).

FUSELAGE. Cut fuselage sides; be sure they match exactly and bottom edges are flat. The grain ends of formers and lower doubler (for tow hook) are given two coats of cement. Bevel inside faces of fuselage sides at rear (25); join with two formers at wing position (26 and 27). Check that sides are correctly aligned and set flat along their entire length when placed on building board.

Now pull together and join at nose and tail (adding 28): checking fuselage top view by placing it over plan. Add remaining formers (29 to 32), bottom doubler (33) and stab mount (34-36 assembled flat over plan). Add top (37 to 40) and bottom sheeting (41). Cover fuselage with tissue, apply three coats of clear dope. Cement rudder in place, checking its alignment in both top and rear views. Add wing dowels (42), nose skid (F), rear hook (G). Make balsa auto-rudder wedge (43). Add .096" dia. steel wire "D/T limit."

Fasten wing and stabilizer to fuselage with rubber bands. Add lead weight to nose compartment until model balances 2-5/8" behind leading edge of wing. If total weight is still less than 5.08 ounces (the minimum required under A-1 rules), cut away small piece of top fuselage sheeting under wing, fasten required weight to inner face of this sheet and cement it back in place. Warren's original model needed 1.6-oz ballast weight under wing to bring it up to the required minimum. Carrying ballast this way is preferable to "building up" to the 5.08 oz. A light model with ballast has a low moment of inertia and recovers quickly from a bad launch or a stall off the towline.

Now add towhook (H) so vertical towing portion is 1-7/8" behind wing leading edge. This is a safe towhook location. With experience, you may like another hook up 1/4" behind first one (see "Flying and Adjusting"). Recheck all-up weight after several days. The dope continues to "cure" making the model slightly lighter. Add your AMA license number to wing (minimum height 1.2"), fuselage and tail (minimum height .4").

FLYING AND ADJUSTING. Make 164 foot towline from DuPont "Stren" (# 12 test monofilament nylon spinning line) using the improved clinch knot shown on instructions provided with this product. (Any braided fish line of at least 8# test can substitute.) A quick winding reel will add a lot to your flying pleasure and convenience. Remember to attach the 39 sq. in. flag (required in A-1 rules). Tie the flag around the line and not the line around the flag, since the latter method will eventually cause the line to break.

Check the model for correct balance. Steam out any flying surface warps. As a starting point; the rudder should be offset about 3/16" to the left at its trailing edge. Pick a calm day for the first flights. Hand glide the model over long grass. Launching on a slightly downward flight path from shoulder level should result in a flat, shallow glide with very slight left turn. Confine all trimming adjustments to the tail surfaces only; do not change the balance point after this has been correctly positioned 2-1/8" behind the leading edge. Correct diving tendencies by shimming up the stab trailing edge 1/64" at a time. Correct stalling tendencies by shimming up the stab leading edge in a similar manner.

Once satisfactory hand glides have been obtained, the first tow-launch may be made. Check that the rudder is straight for towing-with the towline and wedge (43) attached. The model is fairly sensitive to the rudder setting under tow; any offset will cause the model to pull to one side.

Have your helper hold the model at about a 15 degree angle of attack, with the auto-rudder line draped over the top of his hand (between thumb and index finger). Tow directly into wind. Your helper should run a short distance until the wings begin to lift-at which point he gently releases the model. As soon as the model "catches the wind" and starts to "kite" upwards, slow down and ease the model to the top of the tow. In windy weather, be prepared to run towards the model. When the model approaches the top of the tow, slow down so that the flying speed is normal at the moment of release (to avoid stalling). To release, run back towards the model and throw your towing hand towards model, but do not let go of the line as this results in a disqualified flight during a contest.

Carefully observe the flight pattern from the tow-launch. Re-adjust the stab and rudder setting if necessary to obtain the optimum settings. For best duration, adjust the rudder to give circles of about 100 yards diameter in calm weather conditions. The model will turn more tightly as it enters a thermal. In windy conditions, give a little more left rudder, to keep the model turning steadily and prevent stalling. This is easily done by adding masking tape shims to the rudder wedge.

For flying in areas where windy weather conditions prevail, it is desirable to move the towhook forward about 1/8" at a time, to obtain a slower (less steep) climb. Under very calm conditions, a slightly more rearward towhook position will give maximum possible tow altitude, by steepening the towing angle. Experienced glider modelers may make slight balance point changes-moving it back slightly for calm-air or forward for windy weather. The tow hook and balance locations as shown on the plans are best for average flying conditions.

A-1 Jetstream Towline Glider Plans

(AMA Plans Service link)

Posted December 19, 2020