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Old Mar 06, 2012, 12:07 AM
dwmumford is offline
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Bountiful, Utah
Joined Dec 2003
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Build Log
Ray Hayes Big Bird XL Nose conversion for electric

Big Bird Nose Design for Electric Conversion and misc detail notes:

I redesigned the nose of the sky Bench Big Bird XL electric conversion rather than using the drawing supplied in the plans because I wanted a nose that looked like it was designed to fit the spinner and something that look a little more finished. The plans supplied in the kit show the area below the spinner as simply angled back at a 45 degree angle. I wanted something tha FIT FLUSH right up to the spinner, so this is what I came up with: The photos show what the nose looks like before painting.

The nose is exactly 50 mm wide at the nose. This is the same width as the spinner and rest of the fuse until it starts to taper back toward the tail feathers. I cut off exactly 2 inches (what the plans recommend) as measured from what would be the most forward point of the nose if you were building the non-electric version.

The firewall is recessed back 1/16 inch back to provide a small epoxy fillet in front of the firewall for extra strength.

The motor mounting bracket that comes with the motor is not used. The Motor is mounted directly to the firewall rather than using the mounting bracket that came with the motor. I did this because I needed all the shaft length I could get since I used a 3/8 inch firewall that was recessed 1/16 inch back for strength. This gives enough shaft length for the spinner. The firewall/engine mount bolts are 3mm x 8 mm allen head bolts that I purchased in the car section of the hobby store. I liked these because they have a large head and no washer is needed. They also seem to stay put because of the extra friction, but I will probably still use thread locker to make sure they stay put.

The firewall is 3/16 inches thick. Made from bonding 1/16 and 1/8 ply, since I did not have 3/16 and only needed a small piece.

I used 3/8 square bass wood that I cut diagonally to make triangle corner blocks behind the firewall (see photos). You will need these corner blocks because you will sand right through the corners when shaping the nose to the full round shape. The spinner was used as a template to draw a round circle on the firewall. This is used as a guide to sand a perfectly round nose -360 degrees all the way around to be made flush with the spinner.

Used a BB 50 mm Turbo Spinner (from Esprit Models). These spinners have a hole in the middle for air to enter the motor and fuse area and have a built in yoke. A few strategically placed holes are drilled into the firewall for air to flow into the motor. I chose a turbo spinner to eliminate an air scoop. I do not think heat will be a problem anyway because I don’t plan on long engine runs…only 20 to 30 seconds max… just enough to get the sailplane up to about 600-700 feet, then the motor is off for thermal hunting and soaring and then landing. The engine is a Turnigy 3536-1100 and I will turn a 12-6 or 11-6 Aeronaught folding prop. I have bench run this set-up and there should be plenty of power. It has a strong pull, but I have not measured the thrust.

Since the spinner is flush with the bottom of the fuse, the thrust line is 25 mm up from the bottom of the fuse at the nose. I put in 2 degrees right and 2 degrees down thrust. This is what Ray Hayes recommended on his plans for the electric conversion. I won’t know exactly how this will balance out, but it should be OK because the entire area under the wing can be used to slide the battery forward or rearward back from the F-2 fuse former to F-3 fuse former. (inserted note August 1, 2012: Balance was perfect just by moving battery for and aft as mentioned above).

A few other misc notes on my build: I used .06 carbon fiber rods for push rods from CST Composites (they come as a kit) with sheaths, clevis, threaded push rod end and instructions. Nice setup. Very smooth action. Rods cross at back of fuse to avoid sharp bends (see photos). The rods actually start to taper inward at the last former (so new rod holes were drilled in the former), but they intersect behind the former. I used these rods in by 100 in Big Bird (non electric) to eliminate tail weight. I did not have to add any weight in the nose.

Hitec HS 45 HB servos are used on the spoilers. These beautiful little puppies are light (.28 oz.) and come with Karbonite gears. Street price is $16 each. Perfect for spoiler servos on this sailplane. Now these may not be strong enough if you deploy them in a high speed dive, but I will be using them only for landing purposes on final to get the sailplane down on the ground.

I hope this abbreviated build will help others that are building the Big Bird XL or similar kits that want to convert to electric. I build slow and have limited time, so the finished sailplane will take about 2 more months. I will post some final pictures when completed. (Final inserted note: OK it took longer that the 2 more months, but it turned out nice)

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Last edited by dwmumford; Aug 01, 2012 at 04:05 PM.
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