½A Arrowplane

Here are some construction pictures for a couple of foam-style combat planes of (what the Americans call) ½A class (.049 cu.in. engines). There should be an image of the plans on the Download page.

Here is a photo of the first version (frog) of the ½A Arrowplane. Since I was learning and expected to crash it a lot, I built it rather “strongly” (read as “heavy”) with quite a bit of fibreglass cloth and thread. It was also a little tail heavy, but it seems that with a stabilator on a boom, it isn’t that sensitive to where the CG is. It weighed in at 230grams or 8oz, including engine. The design uses a bladder tank, so I had to get used to using these, but I found they were a dream compared to feed problems associated with the hard metal tank of my trainer. Overall, the plane was very nice to fly (and prang). The only signs of the dozen or so ground hits it has received is a slight crinckle in the solarfilm covering (and probably the underlying foam) on one wing where I suspect the wing-tip has hit the ground first. It has done its job well in allowing me to learn to fly a combat plane, although I was still flying it on 42 foot lines compared to the 35 foot lines it should be flown in in American ½A competition.

After the frog, I had a few design ideas in mind so next came the banana. This time I wanted to document some of the building techniques that went into it, so I took a few more photos this time. The changes I made, compared to the frog are:

less FG cloth and thread, less glue (epoxy) and more use of superglue
a simpler through-the-wing cutout in the foam for the bladder instead of an internal constructed bladder tube
moving some of the pushrod guides and bellcrank around to prevent tension build-up when at extremes of control movement
tapered the leading edge / wing tip curve to move the centre of drag further back so the CG could be further back (so the motor can be back near the LE).

In the end, the banana came out to more like 170 grams (6 oz) which made a huge difference to its flying performance. At this stage, I was also realising that the engines produce much better performance with a 5x3 propellor than the 5.5x3 I had been using before that. So this plane was now flying very fast and was very manoeuvrable

	The "skeleton" of the plane, which includes the engine mounts and the carbon fibre tube boom.
	Skeleton with bellcrank and pushrod installed.
	Next, some leadouts on the bellcrank.
	After the foam wings and spars are installed. The wing tips are sanded to give a nice curve which improves the stability of the plane. Notice the simple cutout for the bladder tank. I was worried that the cutout would be too far back and affect the CG as the fuel was used up, but I didn't notice any significant effect.
	Some covering (solarfilm) over the foam (careful with that temperature!)
	The tail, loosely sewn up, ready to be glued onto the boom.
	The finished, installed tail.
	The final banana.
	A close-up showing the bladder cutout and clear plastic covering top and bottom (reinforced where the fuel tubing comes out).
	Here are the two planes complete with bladder tanks installed. You may notice that the fuel line passes through a line-clip bolted to the engine mount - this is the fuel shut-off. The banana uses a bladder tank made from a single length of ~3/32" I.D. latex fuel tubing, which is weakened at the bladder end (by inflating tightly for some minutes). At the needle-valve end, I roll the tubing back on itself to make sure it is a tight fit on the fuel connector. The props are 6x3" cut down to 5x3". This, unfortunately creates a prop with a wide blade which unloads a great deal in the air, requiring tuning to be reasonably rich at launch.

PeterMills 2007-01-17 CategoryComplete