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NOTE:(PWS Aug 00)

The following paper appeared in " 1951-52 Model Aeronautic Yearbook" by Frank Ziac.

I have made 3 corrections to obvious typographical errors. The figures referenced are reproduced here as a GIF file.

DRAG OF CONTROL LINES AND MODELS

Henry Jex et. al.


During the summer of 1950, several members of the Tech Model Aircrafters at M.I.T. near Boston, Mass., started on a project to get reliable data on the drag of control-lines and speed models. So far, about 200 man-hours of testing and computing have resulted in the following report.

Through the help of Assistant Prof. E. E. Larrabee of the Aero Department (an active modeler in T.M.A.), the experiments were run in the modern 4.5'x 6V-100 m.p.h. Student Wind Tunnel which has very laminar (smooth-air) flow. To keep the Reynolds Number to that encountered on fast speed jogs at 150 mi/hr, the mahogany replica was accurately made to 1.5 model size (see Figure 1). After proper corrections for support interference and drag and tunnel wall effects had been applied, final results have been found for lift, drag, and pitching moment (coefficients) over a wide range of angles of attack for three configurations: basic model without cylinder, model with exposed cylinder, and model with a simple helmet cowl. (See Figure 2). The important point is that addition of the exposed cylinder doubles the basic drag, but a good cowling only increases it by one third more; that is, a cowled engine has one, third the drag of an exposed engine. This work was done by Henry Jex, Howie Wing, Gene Larrabee, Johnny Gionfriddo, Jack Stewart, and Myron Hoffman.

However, it was known that the majority of engine thrust went into line drag, so measuring equipment sensitive to +/- 0.04oz. was devised by Dick Baxter of Ruge de Forest, Consulting Engineers, making use of a cantilever beam and strain-gauges. (See Figure 3). Runs were made at different speeds from 50 to 110 mph to determine the change of drag coefficients with Reynolds Number due to separation differences. (See Figure 4). Note that the line drag coefficient is based in the frontal area of the line (length x diameter).

The addition of a light, rectangular 1/64" x 3/64" balsa fairing behind the lines reduced their drag by about 11%; but streamlining this fairing resulted in line vibration, probably due to separation phenomena. To prevent flutter, it is important that the C.G. of the line-plus-fairing be in front of the quarter-chord of the combination. These tests were performed by Dick Baxter, Bill O'Neill, Gene Larrabee, and Johnny Gionfriddo.

Now the values can be combined to check with control-line speed records. The overwhelming amount of power necessary to haul the lines around shows up well in fig. 5. The formula allows for drag variation along lines and the portion of drag taken out at the handle. It is apparent that more than 5/6 of the power goes into line drag near 150 mph., which means that drastic changes of model drag won't affect the speed appreciably. Evidently, even allowing for air density changes with temperature, the big engines must be putting out over 1 1/2 bhp with 80% prop efficiency in order to achieve 160 mph. Addition of line fairings would probably increase the top speed by 10 to 15 mph. Also, use of a single .024 line would up the speed 10 to 15 mph, and if this could be successfully streamlined, the speed might go up to 180 mph.