BMT (oz14642)

About this Plan

BMT (Better Mouse Trap). Radio control slope aerobatic model. Wingspan 60 in, wing area 450 sq in.

Quote: "Have fun with this aerobatic 60 in span two-channel slope soarer, subject of our full-size plan this month. BMT, by Dave Parsons.

If you've got one of those two-function, bottom-of-the-range radios, then that is all you need to fly this model - plus of course, the necessary skills or some help from a more experienced flyer. Perhaps you have got some time in on a rudder/elevator model and fancy trying your hand at aileron control with a view to learning some more advanced aerobatics.

Choice of wings: The foam wing for the more aerobatic version could be bought in, made to order from one of the various firms that advertise this service. Or you could cut your own, just a constant cord and thickness rectangle, over-length to allow for the sweep-back angle. Cover with 0.5 obechi veneer using Copydex as an adhesive. Use a template to mark and cut the root and tip sweep-back angle.

Or if you prefer you can do a bit of balsa-bashing and build your own, using the flat bottom section, which is very easy to construct.

The aerofoil section shown for the foam/veneer wing is biconvex for aerobatics and the wing is built with no dihedral, the flat bottom built-up version has 1 in per tip dihedral and has more stability. The ailerons are only 1 in wide and may look a bit small, but roll response is very good, especially with the wing with no dihedral - just stuff the control stick fully over to one side and watch it rotate. The rate is a bit slower for the flat bottom wing but gives you time to decide what to do with the elevator stick. Push a bit of 'down' in and watch how much is required during the inverted part of the roll; centralise the sticks when the manoeuvre is complete - it doesn't matter which way the model is pointing as long as it is in level flight - and you have rolled it! Give yourself a pat on the back, the next one you do will be easier and even better.

The other benefit of small control surfaces for the ailerons is a nice soft neutral, allowing for poor servo centering, and nothing twitchy. The fuselage has a fairly long moment arm, giving smooth but powerful pitch control. The elevator response is particularly good, giving nice round loops whilst aiding stable level flight. Another benefit of swept wings is that the centre of lift is further back, thus the CG is further back down the fuselage, and you haven't got to jam all the radio gear in the nose to get the balance right.

Some lead in the nose was required so, with the knowledge gained when building the first one, it was installed in the battery compartment when constructing the second model, before the top nose block was glued in place. Lead flashing, cut to the required size, was wrapped in masking tape and glued to the ply sides using Evo-stick, one piece to each side - a small price to pay for ease of installation. The only other items in the nose are the receiver and the on/off switch, so there's plenty of room for some foam protection.

Talking about lead in the nose, remember the longish fuselage moment arm, and the penalty of more lead in the nose if the back end is too heavy, so go easy on the paint job; no epoxy, nor glass cloth with two coats of enamel. Keep it simple - lightweight coloured tissue and dope and a final coat of Tufcoat fuel proofer is recommended; iron-on plastic film is acceptable though not so durable.

Linkages and things: Servo installation next. The elevator servo is in the fuselage under the wing behind the main bulkhead - a nautical term. I seem to remember. It has the space completely to itself with plenty of room to be moved either side to suit the servo output response. There is nothing that makes flying easier than a precise, no slop, good neutral, elevator control. The zero friction hard balsa pushrod, connecting the servo arm to the elevator horn, takes some beating! There is a slight bend in the rod at the front to pass the aileron servo but at the back it is dead straight!

The servo is installed vertically in the wing centre behind the main spar, operating the control surfaces via torque rods - short ones with no backlash. The linkage is on top for a change, in plain view for instant inspection, not down inside the fuselage taking up valuable space and liable to damage when the wings come off unplanned! Thin ply sheet top and bottom is used to reinforce the ailerons at the root area over the torque rod slot and first hinge. Take care when cutting slots for the hinges - there isn't a lot of 'meat' in the 1 inch ailerons. I cut about 1/8 off the end of the hinge that goes into the aileron, and I like pinned hinges for freedom of movement.

A one-piece built-up canopy and wing fairing gives access to the receiver and on/off switch in the nose and streamlines the airflow around the aileron servo output disc. For retention I use a small flat magnet glued to the hatch, in contact with a small ferrous (iron) metal plate glued to the top of the wing. It hasn't come off yet, even during downward bunts in strong wind turbulent conditions.

Get building! Fuselage construction is kept simple with plenty of straight lines, 1/16 ply sides and 1/8 balsa top and bottom. Some 3/8 and 1/2 balsa block in the nose is easily carved and sanded to the correct profile. Note the different wing seating shape, to accommodate the aerobatic or the flat bottomed wing - cut this accurately to preserve the correct wing incidence.

If you choose to build the wing from foam, note that, to spread the bending loads, the 1/8 spruce or obechi braces are different lengths. They also form the ends of the aileron servo box, so measure the length of servo you intend to use and space them accordingly. First join the wing halves using epoxy, then cut the slots for the braces using a steel rule as a straight edge - push in some pins as a guide for the rule. Install using PVA - nice and slow drying. Don't make the slots too tight a fit, because the veneer pulls in as you push in the brace. Cut the braces slightly over-size in depth, and sand down flush after the glue has dried..."

BMT from Radio Modeller, October 1996.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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