Strijkplank. 60 inch tailless glider model.
Quote: "Literal translation: Ironing Board. To the wide range of different types of model aeroplanes, a new addition has been made, namely the flying plank tailless glider. This is interesting for its extreme simplicity and for the fact that it does net look like an orthodox tailless.
In order to compensate for the wing having neither wash-out at the tips nor sweepback, it is necessary to use an airfoil section with reflex trailing edge to obtain stability. The Stamm section used is Swiss and has proved its efficiency.
To obtain maximum efficiency with this section, an aspect ratio of between 5 and 8 must be used, although good resufts are obtained by the Swiss, using 9:1. The CG position is well forward, at 15% to 17% chord from the leading edge, this being necessary to obtain a long enough recovery moment to prevent stalling.
Very little dihedral is necessary in a plank design, 1:8 giving good results. Generally, the fins should be not too tali, but long and low, the greater vertical area being behind the CG. By making the fuselage of a rather high narrow section, it can serve the double purpose of ballast carrier and extra fin area.
The successful tow launching of a plank tailless depends mainly on the correct fin area having been found. However, once this is discovered, it will be as steady on the line as any well trimmed orthodox design. It is very important to allow the plank to fly itself off the line, as a jerk will cause it to stall and a lot of height will be lost in recovery.
To date, Swiss modellers have been almost alone in the building ai d flying of this type of glider, which, considering its simplicity and efficiency, is quite surprising. There would seem to be many possibilities for both rubber and power enthusiasts to experiment with the tailless plank. Who will be the first to have a diesel-powered Strijkplank airborne?
Building Instructions. Before commencing construction, study the wing section, from which it will be seen that both the under-surface spars are above the level of the building board. It will be necessary, therefore, to pack up the front spar 1/8 and the rear spar 1/32 in. Cover the plan with greaseproof paper, then pin down the front spar, with an extra piece of 1/8 sq balsa under it. The 1/8 by 1/4 in rear spar is similarly fixed in position, on top of a strip of 1/32 sheet.
The ribs can now he notched onto these spars, checking each one carefully with regard to its fore and aft and upright positions. When the cement has set, add the 1/8 sq top spar, followed by the leading edge. The angle at which the trailing edge sits makes it necessary to pin it to each rib as it is attached. Note that it tapers on the lower surface only, and should be sanded to this shape first.
The tip; cut to rough outline, can now be cemented to the tip rib. After addition of the 1/8 balsa fillets, the panel can be removed from the board. Now add 1/16 sheet covering and finish wingtip.
Repeat with other outer panel; the centre section is built in the same way, the front under-surface spar of 1/8 by 1/4 also being packed up 1/32 in. With the centre section flat on the board, the outer wing-panels are cemented strongly in position, the tips being raised 2-3/4 in. The 1 mm ply dihedral keepers complete the joint and should be pre-cemented for extra strength.
Before the 1/16 sheet is added, the fuselage pod is built into the centre section, commencing by cementing F.5 and F.6 to the centre-section spars (see side elevation). Next, notch the four side stringers into F.1, 2, 3, and 4, and when these have set, slide them into place in F.5 and 6. The two upper side stringers make a butt joint with the top 1/8 sq spar. Add the bottom stringer, attaching it to the trailing edge at the rear, after which the top stringer and the lower side stringers can be joined to it. The addition of the hollow, lead-filled noseblock, the sheet ballast box (leaving off one top panel of this for insertion of additional weight) and the tow hook, completes the construction. "
Strijkplank, Aeromodeller, December 1949.
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