This plan was first published in RCM&E October 2006. The plan and moulding set can be purchased at www.myhobbystore.com
Just as David Long might be said to have produced the Midget Mustang by scaling down the air racing success of the North American P-51, so I've scaled down his classic design to 1/5th size to produce a model that's easy to handle on the building board, but which has the potential for sparkling performance in the air, even when powered by a regular O.S. 40 four-stroke Surpass.
While my design remains largely true to the original, I have made a few discreet changes. The wingspan has been stretched by six per cent, for example, which increases the wing area by 12 per cent. The flaps have also been shortened in order to allow for longer ailerons, and the wings are covered in film aft of the main spar, although the aluminium-skinned original really calls for them to be fully sheeted.
The fuselage is no more difficult to build than that of an average sport model. Start by making up the two sides with the doublers and triangular stock, then, working over the plan, join the sides using formers F3 to F6, and some scrap in-fill at the rear.
While this assembly is drying, fabricate up the engine mount box. Note that the front of the box is made from two laminations of ply, one of which is cut to sit within the sides of the box. The corners of the box should be reinforced with triangular stock, and you'll also need to make a hole in the front for the neck of the fuel tank.
Once finished, attach the box to F1, ensuring that it is perpendicular to the former in both elevations, and remembering that its position may need to be different to that shown on the plan, depending on your choice of engine. When the glue has set, fit F1 and F2, and pull the nose into shape.
Brace F1 using triangular stock, then add the wing seat treblers. Do note that clamps will be needed to hold them against the curved sides while they dry. After you've fitted the 1/4” square spine to the rear fuselage, you're ready to add the rolled sheet covering.
Start by gluing the first sheet to one of side of the fuselage and then, once the glue has set, wet the outside of the sheet. If you now warm the balsa sheet with a heat gun, you'll be able to carefully bend it so that it sits comfortably on the formers. Trim the sheet to the centreline of the fuselage and glue it into place; repeat the exercise to cover the other half of the fuselage.
The front turtledeck meanwhile, rather than using rolled balsa, is covered with good old traditional planking that can be easily shaped to give the necessary compound curves. This is quite a simple task, but it does take a little care and patience to get right.
Install the control snakes before covering the bottom of the fuselage with 1/16” sheet, remembering that the grain should run spanwise across the fuselage. Okay so far? Good. Once you've fitted the 1/4” ply wing retaining plate with its blind nuts, all that remains to be done is shape the lower corners of the fuselage. These will stand quite a lot of rounding over as the triangular stock on the inside affords plenty of material with which to work. Don’t hold back here, look at the full-size fuselage section (see photos) and keep cutting and sanding until you have it about right.
A LASTING SOLUTION
The cowl is of built-up construction using 3/8 and 1/2” sheet, but first you'll need to give yourself something around which to shape it. This support is provided by the cowling's rear former and the nose ring, so begin by gluing the 1/32 C3 to F1, followed by the 1/16 C3, which should be tacked in place temporarily with four small spots of glue. Next, glue the backplate of the spinner to the nose ring using pieces of 1/16 balsa scrap as spacers. Incidentally, I much prefer the Irvine spinners with the metal backplate because they ensure a constant spacing between spinner and nose ring when you change props.
If you now mount the engine, and fit the spinner / nose ring to the prop' shaft so that the widest part of the nose ring is at the bottom, you have all the support you need to attach the sides of the cowling. Once these have set, you can then fit the top and bottom pieces, using the cross-sections to guide you.
Carve grooves in the cowling at the screw locations shown on C3 (the sharpened end of a length of tubing is ideal for this), then drill pilot holes through the layers of ply. You can now separate the cowling from the fuselage at the temporary join between the two C3s, and remove the engine. Reattach the cowling using screws, and carve and sand it to shape. When you're happy with the fit, reinstall the engine.
Shape and fit the first layer of the apple cheeks (or air intakes, if you prefer to call them that). This is an operation that calls for some care as the balsa will be fairly thin. However, once they're in place you'll be able to cut away the cowling within the area of the right-hand cheek (see pic). If using the vacuum-formed ‘apples’ this layer will make a very secure foundation on which to attach the plastic moulding. Of course, some of you will prefer to go the balsa route and in this instance continue building up the detail as shown in the cross-sections. Finally, carve and sand the intakes to shape, not forgetting that you need to open up the left-hand cheek to make the cooling inlet, outlet, and baffle.
With a little gentle persuasion, you should now be able to remove the cowling so that you can form a small but essential depression underneath the cylinder that'll give you room to screw the exhaust manifold into place. Needless to say, the exhaust must be detached to remove the cowling.
The tail components are simply made from 1/4 sheet. When you've rounded off their edges, you can cover them and set them aside. The fairing blocks, meanwhile, are made by fitting pieces of scrap 1/4 in place of the fin and tailplane, and temporarily attaching the blocks on either side by again using a few small spots of glue. You can then carve and sand the blocks so that they blend with the fuselage before removing them until you're ready for the final assembly.
The wings are quite straightforward to build, but I made things easier still by asking Replikit to produce a set of printwood ribs. This set includes all the liteply doublers, as well as the flap and aileron ribs, and is available in a strong cardboard box from Replikit's Mike Brannan for just £12.95 including postage.
Though the build sequence described below may seem slightly unusual, it's designed to avoid any warping of the wing, while allowing you to incorporate a precise amount of washout. Start, then, by laying down the sheets for the lower leading edge, the centre section, and the trailing edge, together with the capstrips. Next, glue and pin down the lower spar, and lay the undercarriage plate in its approximate position.
Laminate the liteply nose ribs as shown before gluing down the wing ribs using the extension lines on the plan; use the template provided to get the right angle for the root rib. While fitting the centre ribs, you can also fix the undercarriage mounting plate in position, but don't glue it to the lower sheet - you need to be able to cut away the balsa beneath it when attaching the undercarriage.
Fit the top spar and, once dry, add the 1/16 sheet webs in front of the spar. The lower edge of the 1/8 leading edge sheet needs to be chamfered to match the front of the ribs before being glued in position so that it lies flush with the bottom of the ribs.
Next, fit the rear 1/8 sheet spar, followed by the aileron and flap leading edges, and their associated ribs, noting that the lower t.e. sheet ends at the rear of the ribs.
Apply glue to the bottom of the leading edge and the front of the ribs, and then carefully bring them together with the sheet.
To make the undercarriage mounting plate secure, glue 1/4 triangular stock between the edges of the plate and the leading edge sheet, and between the plate and each rib, as shown on the plan.
Fill in the wing-bolt area, and fit the hinge blocks to the rear of the wing, but don't separate the ailerons and flaps from the wing just yet. Instead, drill through the lower sheet at the junction of the flap and aileron and the flap root to mark their position.
Before you can begin sheeting the rest of the wing, you need to build in the washout. This is done by ensuring that the wing's root and tip are fixed firmly to the board, and then removing the pins at the outer end of the trailing edge, and inserting the washout wedges: one wedge is chamfered at two degrees and goes under the tip; the other, chamfered at just one degree, goes under R6. This should tension the wing in such a way as to give the right amount of washout. For the wedges to do this, however, it's important to check while fitting them that the rib at the wing root remains in contact with the building board all the way from the spar to the trailing edge, and that when the wedges are in place, the wing is in contact with them over the same area.
When you're happy that the wing is correctly tensioned, you can sheet the top of the trailing edge and fit the leading edge sheeting. Whilst doing this, you may find it helps to clamp the sheet to the spar with bulldog clips or clothes pegs, and pin the sheet to the leading edge using map pins, whose heads will hold it firmly in place while the glue dries and makes the washout permanent.
Once the glue has cured, remove the wedges, pin the lower trailing edge sheet flat on the building board, and glue the 1/8 x 3/8” trailing edge to it - a method which I find gives a much neater finish than trying to get an exact chamfer on the trailing edge sheet itself.
When you've built both wings, join them with the dihedral brace by pinning one wing to the board and attaching the other to it, using blocks to support the raised tip so that it's 3/5“ (85mm) above the board.
SERVOS, HINGES & CONTROLS
The servo mount is made from 1/8 liteply, and I used Radio Active mounts to attach the servos themselves. The bellcranks in the wing are fitted in the normal way, but unlike the ailerons, the pushrods for the flaps need to move in the same sense at the same time - the plan shows their arrangement on the output disc.
If you can find any of Great Planes' pivot point hinges, then use these for the flaps and ailerons; if you have difficulty getting hold of them - and they do seem to be getting scarce - then the Robart medium-size hinge points are a good alternative. For the moment you only need to dry-fit the hinges to establish their position, and then remove them ready for covering.
Once you're happy that the controls are working smoothly and correctly, you can fit the upper centre-section sheets and the cap strips. For their part, the wingtips are laminated from two layers of 1/2“ sheet, then hollowed for lightness. The last job is to fit the 1/16 ply plate that stops the heads of the wing hold-down bolts pulling through.
The undercarriage legs are made from two large aluminium sections (available from Nexus Model Supplies), which should be cut as shown and then bent. If you know a friendly light engineering shop with a sheet metal bending brake, ask them to fold the legs for you; pressing them both into shape in one pass will make for greater uniformity. However you bend them, though, don't forget to allow a small radius in the bend itself to avoid the risk of a stress fracture.
Mark the inner and outer locations of the undercarriage on the underside of the wing, and then locate the position of the ply plate by pushing pins through the lower sheeting. Cut away the area of sheeting inside the edges of the plate, establish the final position of the undercarriage by screwing it to the wing, and then remove the gear until the model has been covered.
Once again, the plan shows that the spats are made using built-up construction, although you can purchase the ABS version if you so wish. Either way, they're attached using screws that run through the undercarriage leg and into a ply piece in each spat. Alternatively, you can dispense with the brackets shown on the plan and glue some nuts into a small piece of 1/8 ply and fit this inside the spat; you'll then be able fit the spats using machine screws screwed through the leg.
COVERING & INSTALLATION
I covered the model with Solarfilm Supershrink polyester, which I reckon is about the best material for the job. It can be pulled and stretched to cover compound curves very neatly, though to cover the curves of the apple cheeks it's still easier to use two pieces of film apiece.
When it comes to fitting out, there's plenty of room in the fuselage, so getting everything in isn't a problem. Start with the fuel tank then immediately behind, fit the receiver, which sits just forward of F2; the battery, meanwhile, is secured to the former itself using a Velcro strap. In a change to the printed plan, you should fit the servos an inch further back from F2 than shown.
The engine is installed in the usual way, the only difficulty being getting to the locking nut for the exhaust pipe which, as I've mentioned, has to be fitted after the cowling is in place.
Once fully assembled, my model tipped the scales at 4 lbs 12oz, giving a wing loading of 23oz / sq. ft. It also balanced right on the designed point without needing any ballast, which is always a bonus! All that was left to do, then, was to set the control throws - 5/8” each way for the ailerons, 1/2” each way for the elevators, and as much rudder as possible - and go flying!
IN THE AIR
If this model reflects the behaviour of the full-size Midget it's no wonder that it's so popular with home-builders. Overall, the aeroplane is very docile, and with the specified C of G she simply won't stall: even throttled back with full up-elevator, she just mushes along without even a hint of wing drop. However, the model will tackle any aerobatic manoeuvre you care to try. Four-point rolls are very positive; loops and Cuban Eights are easy, and slow rolls are a delight. Outside loops are trickier - the model wants to screw out of them - and its reluctance to stall makes flick rolls a little slow. Spin recovery, however, is instantaneous.
The flaps can be dropped to a barn door-like 90 degrees, but when I tried flying on full flap I had a real surprise: nothing happened! The model simply slowed without any trim change whatsoever, which means that I could make some very slow approaches. If your C of G differs from the published one, however, you should make sure you've got some height in hand before experimenting with full flap.
Overall, then, the Midget Mustang makes an excellent all-rounder. She's pretty, well-mannered, but fun to fly - and with a .52 four-stroke up front, she'd go like stink!
Name: Long Midget Mustang
Model type: Semi-scale racer
Designed by: Peter Miller
All-up weight: 4 lb 12oz
Wing area: 475 sq. in.
Wing loading: 23oz / sq. ft.
Rec’d engine: .32cu. in. two-stroke
.40 - .52cu. in. four-stroke
Control functions: Aileron, elevator, rudder, throttle, flap