Boxer 74 swappable scratch build

Background

Having made a few of the planes in the swappable series, I had to try making my own design from scratch.  As I find unconventional aircraft fascinating, I had to pick a fairly obscure plane.  The Lockspeiser LDA-01 (later renamed to Boxer 500 -- see http://en.wikipedia.org/wiki/Lockspeiser_LDA-01) is about as weird as they come and seemed to be a perfect shape to hold a swappable pod.

The plane itself is a pusher, has dual rudders, elevons, two true wings (not just a canard) with the front wing containing only flaps and not an elevator.  There is an angle difference between the front and back wings so that they will not both stall at the same time.  The Boxer 500 variant differed from the LDA-01 with the rudders positioned at the wingtips.  This seemed like it would be much easier to build and potentially provide better rudder control with the control surfaces further away from the center of mass.

After measuring every picture I could find, I made a list of all the general dimensions and sketched out some initial plans for something that is approximately 1/7 scale (14.8%) with a 40 inch wingspan.

The resulting plans

The wing spars use a tab similar to what the FT spitfire has to join the two angled wings.  While these plans don't show a proper cutout for the tab to rest against the opposite spar, it was cut in there.

The fuselage

View from the front

Like most of the views, the servos are completely hidden.  All four servos are in the rear wing; one for each rudder, and one for each of the elevons.

Side view:

View from the back:

Wing attachment

Due to the wings being angled up 4 degrees from horizontal, a raised wing retainer needed to be built.  I made this version with tabs that are inserted into the fuselage, but after more thought, this seems like a bad idea in the case of a crash.  If I make another version, I would probably make a triangular shape that is permanently attached to the fuselage and a small v-shaped groove in this attachment so that the rubber bands would stretch and the wing could slide off without causing damage to the body.

View showing the front opening 

Elevon in action

From here you can see the skewers for the wing tie-downs.  I put these right below the pod so that any upward pull would pull against the pod and spread out the force of overly-tight rubber bands.  There are a total of seven skewers in this plane -- two for each set of landing gear, two for the rear wing, and the normal pod-retaining skewer.

Flight tests

I haven't successfully flown it yet, as I need to make another pod with some beefier components.  My initial hand-launched test flite didn't move more than 20 feet horizontally before the air speed dropped to the stall point.  The HK donkey 1550 just doesn't have enough power to make this thing go.  While this is supposed to have a very wide range of allowable CG, I had to mount my 3s 850 battery all the way up front to keep it in the region where I believed the proper CG should have been.  The under-powered first flight was at least mosttly flat, so I didn't mess up that much.

As a learning experiment, this was fairly useful.  I learned that trying  to judge a motor's thrust by holding it in my hand and giving it full throttle just doesn't work.  

What I would do differently 

I have some better ideas for wing mounting where the tabs won't tear into the fuselage, and have decided that the friction fit of the front wing isn't sufficient to withstand a crash (it pushes back into the plane and cuts into the fuselage as well), so it needs to be glued.

The area above the rear landing gear needs to be reinforced a bit more than the small strip of foam board I placed there.  The crash pushed the wire for the gear into the bottom of the plane.

I also think that I would make the wings a bit wider.  The lift to weight ratio of these 5 inch wings (which are close to scale) just seems a bit small for any kind of low-speed flying, so making them 7 inches would probably improve things, but it would still need a much beefier motor.