project pattern
T
Redux
hanks to Brian Clemmons for shar- ing some invaluable tidbits last month on the selection of compo- nents for those choosing to power their plane with the YS option. I will en- deavor to have Brian return in a future col- umn with more information regarding the operation and maintenance of YS engines. This month, we return to Yuri for the last bits of construction; the installation of the ESC, motor Li-Pos, and ductwork to keep everything running cool.
With the majority of construction items completed and major components installed, completing a rough center of gravity check is in order. With the cowl taped in place, wings installed, and spinner/prop assembly loosely assembled, Yuri should be quite tail heavy. The absence of the canopy and the landing gear have little impact on the c.g., as they are quite close to the c.g. The key element for obtaining the correct c.g. (approximately 205 mm from the leading edge of the wing) is the location of the motor Li-Pos. As produced by RC-Composit, Yuri is fit- ted with a large 4-inch wide, 10-inch long slotted plywood tray that allows a variety of Li-Po sizes to easily be mounted with hook and loop material. As the Li-Pos are approx- imately 25% of the flying weight, the c.g. is easily adjusted by small shifts fore and aft of the Li-Pos. With the Li-Pos in place to ob- tain the proper c.g., the remaining task is to evaluate a convenient location for the ESC, and the length(s) of wiring needed between the ESC and motor, and ESC and Li-Pos. By my definition, a “convenient” location for the ESC means that it will be attached to
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
existing structure (or new structure for min- imal weight), wiring paths will be short and direct, and the ESC will receive good cool- ing. With the weight and form factor of the Thunder Power G6 ProLite 5s5000 (x2) Li- Pos, the front section of the Li-Po tray was not occupied, making it an obvious first choice for mounting the ESC. Attaching the ESC with hook and loop fasteners would be quite easy, and wiring from the ESC to the motor would be very short and direct. However, this is not the direction I chose
for Yuri, for several reasons; (1) hook and loop material can be heavy, (2) hook and loop material can wear out, (3) having expe- rienced a “Li-Po launch” in the past, I never want to relive that experience. Yes, the bad news is that I have had the experience of watching a set of Li-Pos launch from the plane (with the canopy) after a negative 45 downline snap. The good news is that the airplane stopped safely (minus the canopy and Li-Pos) about 9 feet from the outside edge of the runway, after rolling about 3 feet.
While a 2M Pattern plane at 7.5 pounds with a 3-inch aft c.g. will “glide” safely to a landing, it is not something I recommend trying. Should you ever be in such a situa- tion, figure on a glide ratio something worse than the Space Shuttle, using about three- quarters of the down elevator during the glide, and only slightly less down elevator for the flare. If you are lucky (as I was), the canopy fix was easy, but don’t plan on using the Li-Pos again.
At this time, I need to thank Earl Haury for one of his many innovations related to
Pattern aircraft construction. For those who do not know Earl, he is highly respected in Pattern circles at the national level for his flying, building, coaching, and leadership for many decades. In this instance, Earl figured out “the better mousetrap” with respect to retaining Li-Pos in the plane.
Essentially, Earl uses a clamp to hold the Li-Pos in the plane. Since learning of Earl’s method approximately 5 years ago, my own application of it has morphed a bit, and I have seen others adopt similar methods. In addition to being a 100% positive method of retaining the Li-Pos, it is also lighter than the various schemes using hook and loop. Lest the benefits of hook and loop be over- looked, for those interested in trying/using a wide variety of Li-Pos of varied sizes, weights, and form factors, it is a very versa- tile and accommodating method of retaining Li-Pos.
Preparing Yuri for the Earl system, the first step is removing the plywood Li-Po tray. I did not remove the balsa support blocks underneath the front and rear of the tray. The tray itself weighs in at 35 grams, and the amount of hook and loop I would feel comfortable using with the tray weighs in at least 15 grams, making the total weight of a conventional hook and loop retention sys- tem approximately 50 grams.
The key element of the Earl system is a threaded 4–40 stud (approximately 4 inches long) anchored in the landing gear plate. A “T-nut” (blindnut) is installed in the bottom of the landing gear plate to accept the 4–40 stud, which is secured with a combination of a nut and lock washer, or self-locking nut,
PHOTOGRAPHY: DAVE LOCKHART
While allowing many motor Li-Po mounting configurations, removing the stock Li-Po mounting tray (above left) saves a substantial amount of weight. Construction of a battery retention system that allows for simple battery removal as well as c.g. adjustments is quite simple. Motor Li-Pos rest on either side of the 4–40 retention bolt (above center), separated by ¾-inch balsa spacers. Ahead of
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the Li-Pos the ESC is mounted on a stringer attached to the “F3” former. 4–40 retention stud threads into a blindnut mounted in 1⁄8-inch plywood reinforced floor. Motor Li-Pos in place (above right), with the retaining clamp resting on top. A simple nylon retaining nut made from 5⁄8-inch nylon hex stock provides ample clamping force and is resistant to loosening from vibration and stress.
MARCH 2013
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