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Tether Line Drag for Aeromodels : Background

The largest contributor to drag in racing and speed events are the control lines in some instances comprising three fourths of the total aerodynamic drag. The only significant improvement in this situation since Jim Walker began selling kits is monoline. While not used in racing, or much used in any other event except speed, where it is permitted in speed, it is de rigeur.

The quantification of the drag contribution to total drag and the relation of line drag to total drag was not understood by the great majority of modelers in the 40's and 50's. Some, however, did understand there was a problem and set about to quantify it. In 1950 under the direction of professor Gene Larabee, the MIT model airplane club using the Aero Engineering department tunnel conducted a series of tests on a speed model and lines. MIT PAPER

These folks were concerned with line drag because they were aware of the great amount of work put in to wind tunnel testing and tunnel improvements during the late 1920's and early 1930's. The boundary layer phenomenon had profound changes as speed increased and cylinders and spheres were very sensitive to these changes. CYLINDER DRAG Testing these configurations was a way to insure your tunnel had the right turbulence correction and got cleaned up as well as producing useful engineering data for flying wires. This work was part of an effort world wide to get consistent results among tunnels using some basic shapes. The MIT people know that in the Reynolds number regime of the model the drag coefficient was very high.

I read their paper while on an all-expenses paid tour of the Arctic during the Korean War. I had previously worked for McDonnel Aircraft in the aero department and this was a fascinating use of real engineering applied to models.

Back after the war I eventually had daily access to a (early 60's) supercomputer and decided to do the calculations 'right' with fewer approximations. The results and my analysis of the F2C times over the years, corrected for number of lines and line thickness, was published in the Aeromodeller Annual in 1972. At the end of the article I stated that 'streamlining' the lines would be 'the speed secret of the century'. Within 24 months of the time the article was published tied lines were: (1) Developed and flown in Europe on F2C models. (2) Proven so successful in improving performance that they were adopted by many competitors that had the time, skill, and patience to build them. (3) Proven very tough to fly in competition. (4) Finally outlawed by the FAI. This is an extreme degree of alacrity for the cautious FAI Aeromodels Commission that normally must proceed at a glacial pace.

At this point F2C went back to the status quo and F2A models developed the maximum inner wing span permitted by rules. This state of affairs has lasted over 20 years.

I have written a technical paper on the estimation of the drag and shape of control lines that includes the effect of Reynolds number, model weight, wind, and flying technique. The note is significantly enhanced over the original work I published in The Aeromodeller Annual and is available from me in PDF format. In addition I have written a windows program (Win95/98/Me and Win Nt/2000) which implements all of these calculations in any atmospheric conditions you choose. If you want either e-mail me a request at muaero@gte.net. The PDF document and code are each about 200kb.

Today about three fourths of the F2A system drag and about two thirds of the F2C system drag is in the lines. In the performance area these events have become a contest between engine propeller and line drag. Of course there is still an operational aspect that has grown much harder over the years.