project pattern
Redux PHOTOGRAPHY: DAVE LOCKHART
Comparison of weight with two prop adapters. The stock prop adapter (above left) is “upgraded” to a double nut assembly (above center) while saving 6 grams. The barely visible clearance at the front face of the prop (at right) is the result of tapered reamer used on the front face. Text explains benefits.
W
ith all the construction fin- ished, and initial equipment in- stallation completed, it is time to work on final setup details,
starting with the power system. The stock Neu prop adapter consists of a tapered collet, thrust washer, large prop washer, and steel prop nut. On larger IC (in- ternal combustion) engines, it is a common practice to use a double prop nut assembly to avoid spinning the prop loose on the occa- sional backfire. While backfires are not an issue with electrics, occasional “hard starts” can occur, and substantial torque forces can be generated when electronic braking is en- gaged/disengaged.
Sensorless ESCs are blind to the motor when it is stopped or under braking, and can mis-time the motor until it is spinning pre- dictably. Therefore, I prefer to use a double prop nut on electrics, and this actually pro- vides for a small weight savings opportunity (6 grams). The stock aluminum washer can be replaced with a Harry Higley “Lite Lock” (HIGLOC010), essentially a large alu- minum hex nut, which is then backed up by the stock prop nut.
Most large electric propellers are bored for
8mm (5⁄16 inch) prop shafts, which is suitable for a number of common electric power- plants; however, the Neu requires 10mm. The first thing I do is check the balance of the prop. I bring the prop into balance by lightly sanding the back side of the heavy
blade near the tip, or adding a small piece of clear tape on the back side of the light blade. Some composite props are hollow and have small “breather” holes to allow injection of glue into the light blade.
After balancing the prop, I use my trusty
Fox stepped reamer (FOX7207) to open up the borehole to 10mm, boring from the back face of the prop. Next, I bore the front face of the prop using a tapered reamer until only the back face of the borehole contacts the prop shaft. Doing this prevents any side loading of the prop shaft that could occur if the 10mm boring process wandered off cen- ter. At this point, it should also be clear why the prop balancing is done prior to the 10mm boring. Balancing a prop with an off- center bore hole is a near pointless exercise as the dynamic balance will not be correct. It is certainly worth noting that APC elec- tric props include multiple bushing rings which should be used to accurately center the prop. The bore hole in APC electric props is not a precision hole, only a pilot hole; the precision surface is the molded mating face for the bushings. Using a tapered reamer from the front face of APC electric props is recommended by APC to ensure the only contact area (between the prop and prop shaft) is with the precision bushing. While already quite light at 40 grams, the backplate of the RC-Composite spinner is quite robust. Before final installation, I drilled six ½-inch holes and saved 5 grams.
As with any carbon fiber cutting, be sure to use sharp bits (I used a Forstner bit), cut slowly, and avoid building heat that could damage the epoxy of the backplate. Moving on to the installation of the pro- peller, it is worth noting that the 77-gram carbon fiber PT Models 22–12 is now also available (from F3A Unlimited) in an extra light version, weighing in at only 42 grams. Many of the electric props (including APC) have a rather thin hub, and many installa- tions require the use of a spacer to prevent the prop nuts from bottoming on the prop shaft threads. I have made spacers from ¾- inch round nylon stock, but in this case I used a nylon spacer left over from a Tru- Turn electric spinner adapter kit. In addition to the extra light PT 22–12, a couple of other noteworthy new/updated items are available that will further enhance the performance of Yuri. Firstly, for the pi- lots that prefer geared motor options, such as the Neu Motors F3A-1 selected for Yuri, the P42 6.7:1 gearbox is now available with helical gears. The use of helical gears in- creases the contact angle of the gears and the results are smoother running and less noise. Back to back testing of the standard P42 against the helical gear P42 in my Bravo showed a reduction of about 1.5 decibels dur- ing a static noise test. I’ve now logged about 100 helical flights in the Bravo, and without a doubt the noise in the air is not only less, but smoother and more mellow sounding.
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
Lightened backplate, PT 22–12, nylon spacer bushing, and double nut assembly (above) are normally hidden from view by the RC-Composit carbon fiber spinner. The familiar P42 gearbox (at right) used on the Neu Motors F3A- 1 is now slightly lighter with refinements to the outside of the case, and the inside now sports helical cut gears for smoother operation and reduced noise.
30 JULY 2013
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