Tethered R/C - A Solution to the FAA's Draconian sUAS Rules?

Even to a casual observer, the U.S. Federal Aviation Administration (FAA) seems determined to deal a death blow to the radio controlled (R/C) model aircraft sport and competition flying community and industry. Its draconian rules being implemented for onboard electronic identification broadcasting and severe limits imposed on flying sites have already caused great inconvenience to the way aeromodelers have safely and legally conducted themselves for a century, and threatens to kill even more activity. The proposed (and inevitable) electronic ID and pilot licensing requirements will further discourage pilots and equipment manufacturers due to increased operational costs. The root of these unreasonable laws can be traced back directly to the September 11, 2001, attacks on American soil by extremist Muslim terrorists. Our freedoms have been continually eroded since that time. Remember which group of people needs to be held accountable for your having had that inherited pocket knife of your grandfather's or a simple pair of nail clippers confiscated at the airport screening station, and for all the surveillance cameras and listening in to your telephone conversations over the last two decades.

For a long time I have been kicking around the concept of tethered R/C (TRC), where the airplane would be completely under remote control, with its inboard wing being attached to a tether that is in turn anchored to a pivot point in the center of the circle. My first effort was to convert an electric-powered control line stunt model to have R/C control of the elevator and motor speed. After doing the conversion, I decided that it would be safer to start out with a slow-flying, inherently stable model, so since I was in the process of building an electric-powered, three-channel Carl Goldberg ½A Skylane, it was used as the Guiney pig. The steerable nose gear was pegged in the center, and the rudder pushrod was secured with a screw in the servo mount rail so that it has permanent right rudder. A tether attachment point was epoxied into the left wingtip. It weights 25.3 ounces ready to fly.

One of the drawbacks and potential disaster points with having the airplane staked to a fixed point is that if a gust of wind pushes the model toward the center of the circle, there is no ability to just step back and tighten the lines. On the other hand, there is no requirement to have the line continuously tight if control of the elevator is not dependent on the line(s). In fact, the Stanzel brothers used that very feature to help pitch their monoline control line system. Elevator control was affected by twisting the single metal line, not exerting differential tension on two separate lines.

Just to be safe for starters, I used an exaggerated amount of right rudder. There winds were at about 10 mph with gust to 15-18 mph during the maiden flight of the tethered R/C Carl Goldberg ½A Skylane, and at no time did the line go completely slack, even though the model was flying at a fairly low speed. A faster model should have no problem under those or even more unfriendly conditions.

A simple pivot post was made using ball bearing wheel assembly (bought at Harbor Freight) with the wheel removed, and screwing it to a 4x4 with a plywood base plate. A standard 8x8x16" concrete block provided enough ballast for this setup. In order to estimate the tension on the line at flying speed, I used the following centrifugal force equation:

F = W * v² / (g * R) in pounds of force (lbf^†), where

W = weight in pounds of force (lbf): 25.3 ozf = (25.3 ozf) * (1 lbf / 16 ozf) = 1.58 lbf

v = speed in ft/s: 40 m/h = 40 mph * (5280 ft / mi) * (hr / 3600 s) =  58.7 ft/s

g = acceleration of gravity = 32.175 ft/s²

Hence:

Tension = [1.58 lbf * (58.7 fps)² ] / (32.175 ft/s² * 36.5 ft) = 4.64 lbf - which is not much!

A standard 8" x 8" x 16" concrete block weighs a little over 35 pounds, so it was plenty of ballast for the pivot post.

Here is a video of the tethered R/C Carl Goldberg ½A Skylane's maiden flight.

^† Be careful when using equations which use units of weight that you know whether to use values for pound mass (lbm) or pound force (lbf).

I have intended for a long time to try my hand at tethered radio control (TRC), especially as my aging body (now 62) does not tolerate turning in circles as well as it used to. There is a not a lot available about it on the WWW, so I'm on my own. I outfitted my e-powered Carl Goldberg 1/2A Skylane accordingly by setting right rudder and using the throttle and elevator controls with a Futaba radio system (see article on my AirplanesAndRockets.com website). The single tether line if about 35' long and it attached at the wingtip. A pivot post was made using a ball bearing caster wheel frame on a 4x4 timber, weighted down with a concrete block. The test flights went well under light winds, but got scary when gusts came along. Flying a faster model would reduce the effect. Future plans include converting an e-powered C/L stunt model to TRC on a longer line to see whether I can pull off aerobatics with it. Time constraints will probably not see that happening for at least a year from now.

Posted August 22, 2020