The SAL Mustang originally came to be as a modification of the Jurca MJ7 scale Mustang, and was originally called the Falconar 2/3 Mustang. Two prototypes were built, the first looking very similar to the MJ7, and the second having more changes, the most obvious being the firewall angle and location. The structure was greatly changed also, with the wing being redesigned to use built up spruce ribs instead of the sawn out plywood ribs of the Jurca.
The second prototype was pretty much identical to the first issue of plans, which I have. Since then, numerous detail changes have been issued, but nothing large, at least for the single seater.
The SAL is basically a wood airplane with a fabric or fiberglass cover to protect the wood and make a better base for the paint. The structure is of spruce wood members faces with aircraft grade plywood of Birch or Mahogany.
As for glues, Iíll confine myself to what I actually have experience with. When I started construction, Resorcinol was the widely used glue for wooden aircraft, due to itís availability, proven record, and not least itís FAA approval for use in certified aircraft.
It works very well and makes a good bond that lasts for at least decades. Thatís the good part. It requires VERY close fitment of parts and is very temperature sensitive. Itís not for use below 70 degrees F. It makes a nasty stain and is difficult to clean off (it looks like dried blood). Youíve probably figured out by now, I donít like the stuff.
When it became available, I tried T88 and used it for some time. T88 was better in all respects (in my opinion) and was more tolerant of low temperatures and gaps. T88 is an epoxy and cures with some flexibility. In common with most other epoxies itís very sticky and hard to clean up, but is more or less clear so it doesnít have the dirty appearance of the resorcinol.
Next was Hexcel Structural Adhesive. Everything I said about T88 is true of this glue, plus it comes with the choice of two hardeners for different temperatures or to shorten/extend working time. Most of my aircraft is held together with it.
The last epoxy adhesive I tried is West System. The West System is based on a single epoxy and several hardeners. It can be used as a bonding adhesive, laminating adhesive, and to lay up fiberglass, carbon fiber, or Kevlar. Itís less viscous than the others but by adding a small amount of colloidal silica it can be used as a wood glue. Since itís originally meant for the wood boat building trade, itís very durable.
All these glues are available from Aircraft Spruce or Wicks, as well as other mail order suppliers. West System is available locally in larger cities.
The fuselage is constructed as a box, with the sides built flat on a table at least 15í long (more like 17í for the two seater) and about 40Ē wide. The spruce longerons and vertical members are placed (with blocks screwed to the table) and glued. Then the whole thing is planed flat and either the outside ply skin or inside gussets are glued on. The side is turned over and the ply side or gussets are placed. The process is repeated for the other side. This is a good time to make the engine mount fitting plates and drill the boltholes in the fuselage sides.
The sides are joined by built up bulkheads of spruce members covered with ply. The rear fin spar doubles as the rear bulkhead. The bulkheads are carefully fitted and squared. The fittings for the various bellcrank brackets and the tailwheel attach should be built and the holes drilled before assembling the bulkheads and sides. This process is extremely important, since if the fuselage box isnít true, it canít be taken apart without destroying it. It takes lots of measuring and fitting. After the box is constructed, the braces for the firwall are fitted and the wood floor is fitted and attached.
In order to get a curved (instead of flat) contour on the fuselage sides, I used urethane foam attached with contact cement, sanded to shape, and covered with two layers of fiberglass. I used two layers to protect the foam from damage during normal handling.
My windshield frame is different from the plans in that itís laminated plywood instead of steel tube. Itís also set about 1.5 inches to the rear of the plans position to place the pilot a bit farther forward in the canopy visually. By making it from ply, Iím able to use a three piece windshield and glue the panels in place. The center panel is Ĺ inch thick and the sides are 3/8Ē thick.
By glueing the panels in place and using fiberglass and foam to construct the fuselage area in front of the windshield, I have a more weatherproof assembly that provides a little more structural strength to the forward fuselage. The engine controller computers will reside in this area.
My canopy bubble came from Airplane Plastics Company, of Tipp City, OH. Jeff Rodgers owns the company and was really helpful in forming a canopy bubble that fit perfectly and literally fell into position once trimmed. I sent him a full size outline of the canopy rim and front bow, the angle between the two, and a side drawing of the shape I wanted. He sent me the bubble. Doesnít get much easier than that.
I glued the bubble into the frame, the only holes in the plexiglass being those in the front bow to hold the metal trim strip. The adhesive I used is made to bond plastic auto body panels to each other and is specifically made for various plastics including plexiglass. The brand name is FUZOR127EZ and itís made by Lord Company and available at auto body paint supply stores. It uses an applicator gun (looks like a caulking gun) that has mixer tips that mix the two parts, eliminating handling and stirring. This stuff doesnít sag or run, and itís not stringy like lots of epoxies.
Fitting the canopy and windshield was quite time consuming and involved lots of fitting and trying and fitting (and on and on) , but since itís such a large part of the appearance of the airplane, it was worth it.
The jettison system is pretty similar to the plans in theory, but a little different in construction due to the wood WS and canopy frames.
The channel is 1X1 aluminum square tube and the sliders are nylon.
The SAL wing starts out as a robust box spar, one piece tip to tip, laminated from 10mm thick by approximately 5Ē wide spruce planks. As can be seen in the pictures, thereís lots of structure. The spar is one of the more difficult parts to build in the whole project, mainly because of the sheer size and need for rigid jigging to laminate the booms while incorporating the 4.75 degree dihedral. The dihedral is set by curving the booms over a 26Ē length in the center while the glue sets up. An overbend of about 1 degree is needed to allow for springback. Internal blocks are placed to bear the bolts used to attach various parts as well as the passthrough for the control torque tube and wing attach bolts. Itís then boxed with ply of various thicknesses front and rear.
The 15 ribs require 15 jigsÖlots of work just getting ready. The ribs from #6 to #15 are built up truss type, made from 10mm x 10mm spruce, with 1.5mm birch ply gussets. The inboard ribs form the tank bay and have open tops.
As with the fuselage, itís a good idea to make the metal fittings that attach to the various parts of the wing and drill the holes for them BEFORE assembling the wing where possible. Itís lots easier and more precise to do it with a drill press while standing up than with a hand (or snake) drill while bending and twisting to reach the part.
The fuel tank sits in the open tank bay between R1 and R5.
The aileron control cables run through the ribs over pullies to the bellcranks.
The wing is covered with birch ply, varying from 4mm on the bottom of the tank area, 3mm from R5 to R11, and 2.5mm to the tip. This is fairly straightforward except for the need to bend the leading edge a little more than it wants to go. Hot water applied with towels and allowed to soak the ply makes it a little easier.
The ailerons and flaps are built up from light weight ribs and spars. The balance weight brackets can be seen here.
Inboard leading edge and main gear
I chose the wood option for the inboard leading edge. This set of supplemental plans replaces the original sheet metal construction with spruce and plywood. It's actually the only option when using the scaler landing gear. I suggest building this structure AFTER doing a trial installation of the main gear, since the fit is very tight and some adjustments might have to be made.
There are two options for the main gear. The first is simply a pivot tube extending through the main spar all the way to the rear spar and anchored there. The gear leg assembly rotates from the extended to the retracted position on this fixed tube. This setup has been used on a number of SALs and works well. Two disadvantages are that the tube extending all the way to the rear spar takes up quite a bit of space in the fuel tank bay, and it weighs quite a bit.
The second, pictured here, is the "Scaler Gear" that more closely mimics the real P-51 gear geometry. Overall, it uses many of the same parts as the original, but the gear leg and pivot tube are altered. The wing structure is changed to add a wood plate (false spar?) in front of the main spar. This allows the gear position to be moved forward, closer to the scale position. The angle between the pivot tube and the gear leg is changed to allow the leg to fit into the center section while leaving the position of the tire when extended at the same place as the original gear.
The scaler is supposed to use Delrin bushings for the bearings to allow the pivot tube to rotate. The leg is meant to be fixed by bolts to the tube. We found a potential problem with this due to the high expansion of the Delrin when exposed to temperature changes. The bushings went from being an easy slip fit at 75 degrees F to being impossible to move at 40 degrees. A couple of minutes with a hair dryer and they were easy to turn again. This didn't appear to be a good thing, so we decided to fix the pivot tube and rotate the hub around it, like the original gear. We still use the Delrin bushings to fit the pivot tube through the wood, but it no longer needs to turn.
A disclaimer: I haven't actually flown this mod, and I don't advocate anyone else doing it. I bring up the Delrin expansion problem mainly to allow the next guy to be aware of it ahead of spending time and money then having to figure out a fix. There may well be a better way than mine, but I did it this way because it involved the least amount of "redoing" and expense, and it appears it will work.
The finished leg and springs. My gear was built by David Spencer, and I can't say enough good things about his work. David was very helpful in suggesting work arounds and meticulous in his workmanship.