September 2000
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The next assumption requires an estimate of the power required of the
motor/battery system to provide the flight performance envelope that
is desired. R/C electric scale pioneer, Dr. Keith Shaw, has developed,
through empirical observation, a "power loading formula"
that suggests:
50 watts (volts x amps) of motor input power per pound of aircraft weight is required for safe takeoff from normal model fields, safe cruising flight at reduced power and at least modest aerobatics. 60 watts of motor input power per pound of aircraft weight should allow more aggressive aerobatics including horizontal rolls, clean loops from level flight and short duration vertical flight. 70 watts of motor input power per pound of aircraft weight and above starts to put your aircraft into the fighter performance category. Since I wanted the Scout to fly like an agile sport flyer and be capable of the original Scout's maneuvers (i.e.: inside loops, Immelman turns, split-S tums, and horizontal rolls), my motor input power assumption would have to be 60 watts per pound of aircraft weight or 675 watts total (note: 746 watts equals 1 horsepower). It must be noted that 675 watts is the maximum power required for short 10-second bursts during certain maneuvers, and the normal cruising power should be approximately 60% of maximum power, or approximately 400 watts. Now that we have a feel for the Scout's final weight and motor input power requirements, how many cells should be used for the battery pack and which motor should be chosen? Because of previous excellent results, my choice of motor was an Aveox brushless motor. I contacted the always helpful Matt Orme at Aveox to assist me with the selection of the motor/battery combination. Matt suggested that I use one cell per 70 square inches for a biplane ("Orme's Law" also states that a shoulder wing model should have one cell per 50 square inches of wing area), so the Scout's 1400 square inches of wing area would require 20 cells. This result is corroborated by the maximum motor input power requirement of675 watts, as 20 cells (at 1.1 volts per cell under load) will require the motor to draw only 31 amps (A) maximum. As for the proper size motor, the Scout would need a motor that would be most efficient at cruise (31A x 60% = 19A), yet is capable of operating for short periods at 31A. Matt suggested the Aveox 1409-4Y, with the 3.7:1 Planeta gearbox. This motor runs continuously at 19A on 20 cells and can be run for 30 seconds at up to 45A, and still provide 90% efficiency at 19A. As for propeller selection, he noted that with this motor/battery combination, a 15-8 prop would draw 20A for 440 watts of input power (20 cells x 1.1 V per cell x 20A), and a 16-8 prop would draw 23A for 506 watts of input power. This level of power was less than what I wanted for agile maneuverability, but it did provide 45 watts of power per pound of expected flying weight. However, this was a place to start for the first flight, as I would not know until then what amperage was needed for cruise, and I didn't want to go over the motor's 19A continuous power capacity. |
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Motor installation The standard Scout kit comes with aluminum wet-power "T" motor mounts that sit on a wedge attached to the firewall. This wedge provides 2 degrees of right thrust and 2 degrees of down thrust. At the same time, the dummy motor is mounted to the ends of the "T" mounts using two 6-32 screws. To mount the Aveox 1409-4Y/Planeta motor I replaced the heavy aluminum "T" mounts with a Aero Vee, model MM-1 sheet aluminum mount, made by Stitzer Model Design. This mount had to be recessed 5/s inch behind the front face of the firewall to allow the prop to clear the cowl by V4 inch. The mount was located vertically to center the prop hub in the center of the cowl, because, in my opinion, down thrust should never be used in an E-con, as it wastes power. The mount was also moved Vs inch to the left of center to allow for the prop shaft displacement to the right with the 3 degrees of right thrust that was installed (a 5/64-inch shim on the left side of the motor mount provided the 3-degree offset). I have found that 3 degrees of right thrust works in most cases, and is easily adjusted if required. After the mount is located, a cutout is made in the firewall to provide clearance for the recessed mount. The mount is then recessed 5/s inch. By gluing (epoxy) a "/4-inch plywood plate to the back of the firewall after shimming with V4inch square spruce strips. Four 4-40 socket head cap screws and blind nuts are used to attach the Aero Vee mount. A medium strength liquid threadlock is essential to secure the motor mount screws. |
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To mount the dummy engine I drilled three 3/s-inch holes through the
firewall and installed "Is-inch dowels that protruded 2 2V32
inches from the firewall (this also allowed clearance for the 3000CR
cells to be installed between the dummy engine and the firewall.) The
dowels were equally spaced around a 23/4-inch bolt circle with the
first hole located 7/s inch to the left, and 1 V16 inch below the
prop/cowl centerline. Plastic inserts were installed in each dowel end
to accept the 6-32 socket head, flush head screws that secure the
dummy engine.
The inside diameter of the dummy engine was enlarged to 2"116 inches to allow for adequate motor and battery cooling. Care should be taken that no metal part of the dummy engine touches the inside of the aluminum cowl, as radio frequency (RF) noise could be generated when the motor is running and this could cause radio interference. To prevent this problem from occurring, I removed the outer V4 inch and wraparound of each dummy cylinder's "pushrod" wires and glued the remaining wire length to each cylinder. This was done for all cylinders except the two cylinders that protrude from the bottom, through a slot in the cowl. The modification to the seven cylinders is hidden behind the cowl. |
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