ABC Toothpicks (oz14767)
About this Plan
ABC Toothpicks. Free flight power model. Wing area (flat) 520 sq in.
Direct submission to Outerzone.
Update 16/2/2024: Added article, thanks to RFJ.
Quote: "ABC Toothpicks, by Gil Morris. Big brother (sister?) of the 1980 NFFS Model of the Year-1/2A Toothpicks - is designed for AMA Power with .19 to .36 engines. Well thought out construction gives maximum strength with minimum weight. Author's observations on when to fly are important to FF competition fliers of all persuasions.
PROBABLY the least explored but potentially the most rewarding area of Free Flight Gas is the combination of lightweight models matched to the power of Schnuerle-ported engines. This challenging area forces as to use the best in flight character and structural strength. A-B-C Tooth-picks was designed with that in mind.
This model is an enlarged version of the 1/2A Toothpicks (published in the June 1979 MA) but with the wingspan and tail moment arm stretched. The longer wingspan and fuselage produces sweeping power turns, instead of sharp angular ones, and this permits greater engine power without sensitive trim adjustments. AB-C Toothpicks is the most spirited and easy-to-fly plane I have ever had.
The technique. of building strong and light requires placing structural members where they will count, omitting or minimizing them otherwise. As a general rule. the greatest bending strength comes from concentrating materials at the outer edges. A good example of this is the I-beam used extensively as major support members of buildings and bridges. When the I-beam is bent in the direction of its web, one flange is in tension while the other is in compression; the web between serves only to hold the two flanges parallel so that the flange in compression doesn't buckle. The greater the separation of the flanges, the greater the strength and stiffness of the beam.
In the same manner, the box spar in Toothpick's wing places the top member in compression, with the bottom member in tension (reverse this at the instant of a dethermalised landing). The side shear webs simply hold the top and bottom members rigidly fixed to one another, so neither can buckle. Note that the box spar is at the thickest part of the airfoil for maximum separation and strength: the top member is thicker than the bottom one, since balsa is stronger in tension than in compression.
Although I didn't take the time to do it, the ribs (therefore, the wing) can be further strengthened with little added: weight by cap-stripping. This adds material to their outer edges, strengthening the ribs against bending. The same line of reasoning can be extended to the fuselage, where the material is concentrated at the outer edges (the corners in this case) with struts in between to maintain a fixed relationship, one corner to the other. Diagonals also resist twisting.
Wing torsional twisting is reduced significantly by covering the wing with silkspan, because the random filament arrangement allows little stretch in arty direction. It becomes the outermost edge of strength. This covering is followed by a protective cover of 1-2-mil Mylar.
Strips of Mylar tape are criss-crossed over the silkspan on the top and bottom of the wing, and the Mylar covering is then cemented to the tape. This is to break up the Mylar covering into small patches, rather than large sheets. The shrinking force, corner-to-corner, of large Mylar sheets would have much more leverage..."
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