Big Square 55cm F1D Construction Text
by Bernie Hunt

This model is full size (wing and tail as big as the rules allow) and built down to weight despite being unbraced and Y2K covered. I have flown it for most of last Summer at (150′ high) Cardington. It seems plenty strong enough and flies a beautiful pattern. With a fixed pitch prop and the optimum motor, it comfortably outclimbs the site. From 1/3 and 1/2 motor test flights, it looks like the full motor and no back off, the max height will be about 165′ for a just deadstick landing. Using a pretty conventional VP prop from my 65cm F1D (19.25″ dia, bottom stop 28″ ish with 5gcm = 0.07 oz in preload, top stop 36″ish) and a 2% back off, it managed one flight of 33:55 from about 135′ landing with 4% turns left. Clearly the VP needed some more movement ( slightly higher top pitch) to achieve the ideal no back-off flight. On this evidence, really high sites like Akron (180′) and Slanic (210′) will be fine with fixed pitch but the rest will need VP, VD or whatever.

I have avoided curved outlines so I can use springy wood (my best spar wood will not go round curves easily without kinking). I think the 19″ prop is a bit big for this class and 16-18″ diameter might make trimming easier and save weight. My prop is a scaled down version of Steve Brown’s 21.5″ prop with my own VP mechanism. I don’t give details as Steve’s is well described elsewhere (see IN&V #89 Oct 1996).

I suggest that builders new to F1D increase the wood sizes on the wing and tail by about 10%, use a 16-18″ dia fixed pitch prop (even an all sheet EZB prop scaled up to 16″ dia I tried was fine) and shorten the wing posts to 2″. It is always better to have a stiff, if slightly heavy, model that flies well rather than a floppy, down to weight, one that does not. Covering is with 0.3-0.5 micron Y2K plastic film or microfilm applied from frames (0.9 micron Polymicro plastic is too heavy). I have used 3Ms 777 in toluene as adhesive and also saliva which I now prefer.

I use my normal packing system where the model breaks down to wing (no posts),stick / posts, tail, prop and boom which can be fitted into a very compact (23″ x 9″ x 8″ for 3 models) box for hand carrying on planes. I found a sturdy computer box which was the exactly the right size when cut down on one side. Each model fits in a slot in the box. The wings and tail sit one on top of each other on posts on a slider on the bottom of each slot. The props, sticks and boom are held in foam blocks on a sliders at the top of each slot (props in wider top slot only). Increasing the depth of the bottom two slots by an inch would make the packing a whole lot easier. The box travels safely as carry on hand luggage or in the hold inside a plastic suitcase.

There is not much I can say about flying this class. The models are launched with very little back off and have a fast initial climb so they need the sidethrust and wing / tail washin to get a nice circling pattern. The washin on the wing comes entirely from stick twist from the motor torque. All my tall post designs like plenty of “down” pulled in at launch. I adjust the tension on the stick bracing (shorten post) to get the right amount. My prototype had the bracing slack except for the first few seconds of the flight.

Now for a few constructional details: Motor sticks for 55cm rules FlD are much less of a challenge than old rules. The 0.6g motor x ca 0.050″ motor between 9″ hooks means you can get away with small dia. sticks and quite thin wood. I use a 0.22″ (7/32″) mandrel and 0.013″ x 4.3 lb wood + 4 boron strips, but I have heard of thinner wood being used. Three boron strips are probably enough and some guys over here use 4 or 5 borons and no tungsten bracing. I make a 13″ long motor tube and let in the rear hook on its web into a slot 9.6″ from the nose. You could make a 9.6″ stick complete with boron, bearing, hook and bracing and then add a separate 3.4″ rear section before adding the wing posts and stub boom.

I have used plug in booms for years. You can manage with 1″ overlap but 1.5 2″ is better. It is really important to get the tapers the same and exactly the right tightness. Binding the end of the boom with a few strands of Kevlar or tissue helps stop slackness developing. If like me, you use a short stub boom directly at the rear of the stick then adding a 0.25″ long internal sleeve to the joint between the stick and the stub boom avoids a weak spot. Well engineered, plug in booms are trouble free and they have the advantage of easy adjustment of tail tilt. The 3 strips of 0.003″ boron keep the boom stiff and straight.

I use hobbyshop wood for all spars. I select the best feeling 1/8″or ¼” 5-6lb wood and strip it down to 1/8″ x 1/4″ x 18″ pieces with a Kavan stripper and test it for stiffness (see Don Slusarczyck’s web site for details). Finally I strip it to size / taper with a Jim Jones stripper. Remember the old adage: “if it weighs enough it is strong enough” so make the spars as least as heavy as I quote.