Electric Yogi - Radio control model for electric power.
Quote: "Another old-timer adapted for electric power and R/C by Don Srull. This time its the1944 offbeat contest winning design by Jerry Stollof - the Yogi.
In 1944 Jerry Stollof published one of his many popular free flight contest gas model designs. The Yogi's unconventional layout and distinctive look has a purpose; winning contests. The high thrust line pusher engine and the tall pylon wing arrangement were intended to help prevent looping under high power and to prevent spiraling-in when turning in tight circles. There were two problems that plagued early power models. It must have worked - the Yogi racked up an impressive win record in the 1940s. The design was also kitted for a short period by the Eagle Model Aircraft Company.
For our purpose the Yogi makes an ideal R/C electric sport machine, in addition to capturing some of the feeling of an old classic free flight design. Electric power also eliminates the problems that went along with starting an awkwardly positioned pusher engine; and it does spare us from that oily mess such an engine placement would create. We can even use the fairly heavy electric motor flight batteries to our advantage by locating them in the solid balsa nose,
thereby helping to move the Center of Gravity to a respectable location. In the original Yogi all of the ignition system components - the coil, batteries, and timer - had to be located as far forward as possible to help offset the weight of the Bantam .19 engine located behind the wing.
For any of you out there who would enjoy flying the old Yogi design, but don't have an antique ignition engine or a giant model field suitable for power free flight, why not try the electric R/C version? If you want to have a go at it, read on.
Model Design: The model was adapted to use any of the popular 05 electric motors currently available, either geared or direct drive. To provide a suitable wing area for such a model, the original Yogi was enlarged 20%. This gives our R/C version a span of 50 in and a wing area of 430 square inches. The total weight can easily be kept down to under 45 ounces, which will result in a wing loading of less than 15 ounces per square foot. That's in the right range for an easy to fly, good performing sport model. The only other intentional deviations from "scale" include a slight decrease in dihedral, an increase in the size of the fins, and a flat bottom airfoil instead of the original undercambered airfoil section.
In our prototype model we used the most powerful of the 05 motors, a geared Astro Cobalt 05. With seven 800 mAh cells, this powerhouse gives sizzling performance to the Yogi —about all that it can handle. With this motor set-up I recommend the use of a good motor speed control device, such as the controllers available from Jomar Products, 2028 Knightsbridge Dr., Cincinnati, Ohio 45244, (513) 474-0985, or Astro Flight, Inc., 13311 Beach Ave., Marina Del Rey, California 90291, (213) 821-6242. Fully adjustable power settings makes for much more pleasant flying with any model, but on a high powered model like the Yogi and Cobalt 05 combination, hand launches and landing approaches will be much safer and easier than they would be with a simple off, full on, switching arrangement. If you are not going to incorporate a motor speed controller, then I recommend using only six cells with the Cobalt 05; or use one of the less expensive Ferrite 05 motors, several of which are available from both Leisure Electronics, 22971-B Triton Way, Laguna Hills, California 92653, (714) 581-1198, and Astro. These motors will provide plenty of performance for a sport model the size of Yogi.
Our model, carrying a medium size radio and sporting a silk and dope covering, grossed out at exactly 40 ounces. With a six cell battery pack instead of the seven cell pack, the total weight came down to about 38 ounces. The resulting wing loading of around 13 ounces per square foot is not much higher than the original free flight model, which carried about 11 ounces per square foot. Not surprisingly, our model has a very nice glide indeed; plenty good enough for thermal hunting and soaring in modest lift. In fact, the old free flight spirit of the original model still seems to reside in this R/C version. While repeatedly launching and landing the model one day for photographer Tom Schmitt, the fuselage hatch came unlatched, going unnoticed. At the next launch the hatch fell off, followed by the receiver battery pack! Straight out into the teeth of a good breeze the Yogi climbed at a 20° angle like an arrow. Nothing to do but turn off the transmitter and watch. In 4 or 5 minutes it was out of sight upwind. After another 15 minutes a tiny speck appeared.
With the motor now run down, the Yogi transitioned to wide gliding circles to the left, slowly drifting back toward the field. Over the next hour or so we chased and watched the shiny orange model as it soared over and beyond the launch point, going in and out of thermals as if it would never come down. Tom Schmitt finally found the model a few days later in a cow pasture a couple of miles from the R/C field, unharmed. It was partially filled with water, having sat through a couple of rain storms. After drying out for a week or so, and steaming out a warp in the stab, the model and all equipment, including the motor, flight batteries, and radio seem to be perfectly okay and are still in use. Let's build.
Construction. Very lightweight balsa should be used for construction throughout, except for the wing and stab spars, wing leading edge, and the 1/4 x 1/2 fuselage crutch pieces. These should all be very firm, straight grained balsa. Keep the tail surfaces and rear end of the fuselage as light as possible to help get the CG in the right place. Before starting the fuselage, you will have to select the motor and radio servos you intend to use since their installation is customized to the particular equipment used. The crutch-type fuselage construction is typical of many old-timer designs. It's simple, quick, and assures a straight fuselage. Lay down the hard 1/4 x 1/2 balsa crutch pieces over the top view on the plan..."
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