The Douglas A-4 Skyhawk was created in 1952 as a lightweight shipboard attack airplane. It has been produced from 1954 for 25 years. During that period some 2960 units have been built, serving with the U.S. Navy, U.S. Marine Corps and in the air forces of Australia, New Zealand, Israel, Malaysia Argentina, Singapore, Brazil, Indonesia and Kuwait.
The Chief Designer of the A4D (the original US Navy designation) was Ed Heinemann. He decided to reverse the existing trend of growing size, complexity, weight and cost of shipboard aircraft and came with the simplest, lightest and thus the cheapest attack airplane conceivable – a complete opposite to the general development of the period. He chose a delta wing, structurally compact, yet of high internal volume by its very nature, offering the necessary area for a limited span, avoiding thus also the heavy wing folding mechanism. The main undercarriage legs were hidden in the streamwise canoe fairings beneath the wing lower surface, leaving the wing spars lighter because uninterrupted for only a small drag penalty. All undercarriage legs retracted forward, therefore their emergency opening was assisted by the ram effect of the airstream, avoiding weight and complexity of any back-up power system for emergency lowering. To provide electricity in case of failure or damage a generator with a propeller driven by the ram air was automatically flipped from the fuselage well into the airstream, this method being akin to the way the electric power was generated in the period of the W.W.I.
Ed Heinemann was pretty thorough in is effort to cut down weight, and this zeal was not limited to his subordinates. Among other things they’ve succeeded in reducing the weight of the air conditioning unit by one third, the standard ejection seat, weighing normally some 45 kg, “shed” some 18 kg after the redesign, the weight of avionics dropped by 22 kg. There was no loss of reliability or capability and the production costs were also lowered substantially. All these weight savings told on the increase in payload, and on the growth potential of the machine. Altogether 17 different versions were produced of the Skyhawk, the lightweight pugilist was also known as the Heineman’s Hot Rod, Scooter, Ford and Bantam Bomber. The Skyhawks took part in many conflicts – beginning with the Vietnam war and ending with war in the Gulf in 1991.
The set of decals enables you to build one of three A-4E/F Skyhawks.
The model is not intended for complete beginners. Thanks to the ratio of the model’s weight and the ducted fan’s thrust any catapult is unnecessary – the model is hand-launched. The model equipped with an undercarriage could be flown from hard-surfaced strips. Provided a sufficiently powerful propulsion unit is fitted (e.g. the EDF 60/25), , the model could be flown off a short turf.
The A-4 Skyhawk model you have bought is noteworthy by several features:
Assembling the model
It is quite an easy affair, easily done by an average modeller. We nevertheless ask you to read the following text carefully. Keep in mind all the time that even minor increase in weight shall affect the flight performance of the fan-powered models, much more so than with the conventional propeller-powered ones. Therefore – save every gram of weight!!!
The model is finished in a combination of colours, providing the most common colour scheme, as used by the majority of the A-4s. The tactical markings and the national insignia are of waterslide decals. Their primary advantage is the negligible weight and a minimum risk of damaging the model during application. They require, however, an attention and care. Therefore we recommend that you follow the following instructions:
A) Assembling the power unit
The model is propelled by the Alfa Model EDF 60/15 Mk.2 ducted fan. This ducted fan is designed for the Speed 300/6V motor and/or for AC motors with similar performance characteristics ( MP Jet 25-25/26 Mk.2/Mk.3, MM 1215/12, Mega 16/7/4(3), Hacker B20 22S). The batteries and controllers are listed in the survey of the recommended powerplants. The model flies well even with the Speed 300/6V motor, but count with a limited life of this motor, as it has to run at currents around 9 Amperes. The AC motors are better suited for the propulsion of the fan – their shaft is more solidly supported in the bearings and some are even dynamically balanced. Both these facts markedly influence the total performance of the fan; also their operational life is much longer than with the “Speed” type of the DC motor.
The performance of the model could be markedly enhanced by using the EDF 60/25 fan, differing from the EDF 60/15 Mk.2 only by the rotor. For powering the EDF 60/25 fan it is necessary to use an AC motor of a minimum output of 200 W (e.g. a Mega 16/7/4 or a 16/7/5) and properly sized batteries and controller. In any case the maximum recommended weight has to be strictly adhered to. Also the proper cooling of the controller is mandatory – a suggested was making an air intake slit of 2 x 20 mm size in the intake ducting at the location of the controller – the slit would serve to extract the hot air from the vicinity of controller, or, to use a more powerful controller (around 35A), that would stand the higher operating temperature.
The motor is centred in the fan core by its (motor’s) circumference. Inside the core tube there are wide and narrow ribs. The narrow ones are there to support the Speed 300 motor, the wider ones are for the MPJet 25-25/26 motor. To install the MPJ 25-25/26 motor, it is necessary to cut short the narrow ribs, using a knife, a chisel or a sandpaper (sanding block), without damaging the wide ones. If need be to use a motor of even smaller diameter (such as the MM1215/12), it would be necessary to install taller longerons, gluing them to the inside of the motor casing walls. The diameter of the Mega motor 16/17 requires a marked enlargement of the opening in the motor casing. Therefore it is preferable to use the version made from the outset for the installation of the Mega motor, including the plug. As the outer diameter of the motor casing varies slightly due to manufacturing tolerances, it may be necessary to sand (ream) inside of the fan core tube carefully to ensure the motor a good fit.
The duraluminium plug of the fan is either pressed (Speed 300 with the splined shaft end) or glued onto the motor shaft. The correct relative position of the plug and the motor’s drive shaft is one of the deciding factors affecting the correct operation of the fan. If the plug would wobble, it would vibrate the fan rotor, too, leading to a pronounced decrease of rpm and therefore to a big loss of thrust. Also the front bearing of the motor would suffer a failure soon, indeed. Therefore ensure that the plug and the shaft would be exactly coaxial when pressing them together, also providing that all the forces would be supported by the shaft itself and the plug, not by the motor body. It is prudent to oil lightly the shaft and the plug before pressing, too. Conversely, when gluing the plug to the shaft, remove any trace of grease from the shaft and the plug; some roughening of the shaft with very fine sandpaper would not do any harm, either. If the diameter of the shaft of the motor you are going to use is greater than the diameter of the hole in the plug, it is necessary to drill (ream) the plug hole. Using a series of drill bits with diameters growing in 0,1 mm increments, and a fair measure of patience and care, it is possible to do it by hand; without a lathe. Once reamed, the plug could be also glued to the splined shaft of the Speed 300/6V motor. For the eventual disassembly this method is better than pressing the plug on.
The gap between the motor outer shell and the rotor should be about 0,5 to 1 millimetre. If screws are used to attach the motor to the core, it is mandatory to check that the rotor does not bind on their heads. Even if the recessed-head screws would be used, the distance between the plug and the motor front plate must not be less than 3,5 mm. To set the distance (gap) properly, it is advisable to use a scrap of balsa plank or of a polystyrene sheet of suitable thickness (about 1,8 mm, depending on the size of the bearing box, it has to be checked); slide on the motor shaft once the “tight” hole in the back plate is drilled. This “gap gauge” ensures a proper position of the plug on the shaft during the gluing, also preventing glue from eventual getting into the front bearing. Apply a small amount of the glue included in the kit both into the plug hole and to the end of the shaft. Insert the shaft into the plug. Turning it, spread the glue on the whole inside surface of the hole and leave the whole assembly aside in a vertical position to dry for a minimum of 30 minutes. Once the glue dries, remove (gouge out) the “gap gauge” shim between the motor and the plug. At the low motor rpm, you can check both mechanically and (or) optically, whether the plug wobbles or not. During the optical check, observe how much the reflection of the light oscillates on the rotating plug. During the mechanical check, touch the rotating plug very lightly by the ball of finger or a fingernail - do not push! Eventual vibration could be felt. If the vibration seems to be substantial, it is advisable to remove the plug (heat it with a soldering iron and pull it off the shaft), clean thoroughly the shaft and hole in the plug and repeat the procedure again. Little vibration could be either tolerated or removed by careful sanding of the rotating plug with a sanding block; ensure that the motor is well fixed. It is necessary to assess one’s own manual abilities squarely, as an indelicate action could worsen the state considerably.
Before inserting the motor into the motor housing, remove all stickers from the outer surface of the motor case, cleaning the motor as necessary (e.g. with petrol). Tighten the motor screws lightly, as they serve only the purpose of preventing the motor from turning or sliding backwards. Secure the screws from loosening by vibrations, using a drop of glue (PU, Epoxy, CA). The motors that could not be secured by screws have to be tack-glued to the ribs inside the motor housing, using the PU or Epoxy glue. If using a Mega motor, it is necessary to carefully gouge or rather scrape with a fine file axial (streamwise) grooves beneath the stator vanes. Do not scrape them to the motor casing as it is very thin and some damage might ensue. Once the motor is inserted in the casing, let run a few drops of a thin CA glue into the grooves, cementing the stator vanes directly to the motor outer surface. Solder the power cables and the noise-suppressing capacitors in such position that it would not prevent the motor from sliding inside the fan core.
The gap between the rotor and the plug must be minimal – if the gap between the rotor and the plug is too wide, it is necessary to attach to the plug a strip of a thin self-adhesive tape. If the opening in the rotor is too small, sand the plug carefully. The fan rotor is manufactured with a minimum clearance (gap) between the blades and the inner surface of the fan casing. Once the rotor is fitted to the plug and the securing screw is screwed in place (the hole in the plug has to be clean, with no glue inside!), check and ensure that the rotor spins freely. If the rotor binds (this could be caused by offset (out of axis) position of the engine, by adverse matching of the manufacturing tolerances during the assembly of the fan, etc.), look down the air duct of the fan and find where the fan blades touch the inside of the fan casing. Mark that spot, and, once you disassemble the fan again, carefully sand away the rib(s) on the opposite side of the core, to move the motor to a position where the blade would not touch the inner wall of the casing. Another cure is to shorten the blades by sanding - place a piece of 200-grit sandpaper inside the fan casing and slowly turn the fan to sand the circumference of the blades. This method is quicker than the previous, but, increasing the gap between blades and casing, it lowers the efficiency of the propulsion unit. Sometimes a removal of the motor from the casing and a reinserting it slightly turned might help.
The spinner is snap-fitted to the fan rotor. Especially when using the more powerful motors, it is prudent to secure the spinner in place by tack-gluing it with CA glue or a thin layer of the PU glue (the tack or spot gluing facilitates the eventual disassembly of the joint later). Do that, however, only after balancing the fan. Also the rear (efflux) cone is tack glued to the fan motor core casing. When using a Mega motor, it is necessary to sand away the shoulder (recess) at the rear cone and glue the cone directly to the engine rear face. The streamlined sheath for the power cables is directly downstream of the stator vane, located in the middle between two holes of the attachment flange.
When checking the operation of the fan, hold it so that its casing would not be deformed – such deformation could cause a potentially damaging contact between the casing and the blades of the running fan. By the “trial and error” method adjust the relative position of rotor and the plug and the position of the spinner and the rotor, so that the fan would run with minimal vibration. It is essential to mark the initial relative position of both the rotor and the plug, as well as of the spinner and the fan – otherwise you would soon “get lost” in the balancing process. This procedure requires a considerable dose of patience, but the thorough balancing pays off substantially by increasing the thrust and life of the power unit. As every rotary system, also the fan has its so called “critical rpm” range, that makes itself felt by a sudden increase of vibrations. Therefore pass these rpm ranges swiftly, both when spooling the fan up or down.
The motor in the ducted fan installation is almost without cooling, which does not make any problems when flying at normal regimes up to about 10 minutes. It is necessary to let the motor cool down between flights, this applies especially to the Speed 300/6V. If the motor used has the internal cooling -flow or its small diameter permits airflow to pass between the motor outer surface and the motor housing, it is advisable to enlarge the opening at the front of the motor housing. Air enters the motor casing by the opening in the motor housing’s rear cone and is extracted by the slit between the engine housing and the fan rotor.
B) RC equipment
The general wiring diagram is shown on Fig. B. We recommend that you test assemble and connect the RC equipment outside the model and check its function. When soldering the controller, check that the sense of rotation of the ducted fan is correct and insulate all the cable connections. The power leads from the battery to the controller shall probably need to be split and made longer. For motors of about 100 W power input the cables of about 0.5 to 1mm2 cross-section should suffice, for the more powerful motors the cross-section of cables of 1 to 1.5mm2 should do. Observe the recommendations of manufacturers as listed in the directions of use for the respective components. Check the compatibility of the receiver with the crystal used – the over-the-land range test of the transmitter, albeit it may seem unnecessary nowadays, may later save you much more than it would cost…
C) Aileron control circuit, gluing the wing and fuselage
The aileron control is provided by a single servo , located in the middle of the wing 1. Provided the installation is executed carefully enough, this arrangement of control suits perfectly the requirements on the normal use of the model. The aileron servo should be powerful enough to overcome the friction in the control circuit – we recommend to use a servo of not less than 15 Ncm torque. It is possible to reduce the friction in the control circuit by removing the push-pull rods and lubricating them with a thin oil such as the WD-40 and also by bending them co follow the curvature of the guide (outer) tube.
When removing the cockpit canopy 8, lift its rear edge held in place by magnets (Fig.D1). Slide the canopy backwards so as to disengage the front lock. When putting the canopy back, use the same sequence in reversed order.
The access hatch is, already from the manufacturer, attached to the rest of the fuselage by a self-adhesive tape. On the left (port) side, in the narrow gap between the fuselage proper and the hatch, cut through the adhesive tape. – the hatch could be swung open that way. To close the hatch again, you can tack-glue its free edge to the fuselage using a PU glue – if need be, this joint could be opened again after a cut with a razor blade. Another possibility is to, once the decals are applied, and thoroughly dry, wrap a thin clear (transparent) self-adhesive tape longitudinally so that it would wrap the edge and adhere to both outer and inner surface of the hatch. The hatch then could be held in closed position by other piece(s) of the self-adhesive tape without danger of damaging the paint or plastic when removing the tape.
The fan air duct consists of three parts (Fig.D2). The front one is firmly joined with the lateral air intakes and with the fuselage. The middle section 9 consists of two parts and it has to be removed every time, with the exception of the access to the elevator servo. The exhaust duct is connected to the fan casing 10.
E) Attachment of the horizontal tail, elevator control
For the rudder control a servo having a torque of 7 Ncm or more is suitable.
F) Completing the model
Place the controller and receiver as needed for balancing the model. The recommended position of the receiver is at the left (port) wing side in front of the motor bulkhead, with the antenna led to the belly of the fuselage. The proper place for the controller is on the right (stbd.) fuselage side, the leads to the batteries are threaded through an opening above the intake duct into the cockpit. In any case it is important that the power wires are kept as far as possible form the receiver. If sliding of the accumulator battery to the vicinity of the intake ducting is necessary, it is advisable to prevent a damage to the ducting by gluing a balsa partition to limit the space available for the movement for the accumulator battery. The controller, receiver and the accumulator battery as well are all attached by a self-adhesive- backed Velcro strip.
Glue the completed and painted pilot figurine into the cockpit, or stiffen the canopy by a crosswise brace.
The required position of the centre of gravity is marked by lines moulded on the wing underside (Fig.F). The model, supported in this place on the fingers should remain either in a horizontal position or should slightly dip the nose.
G) Test flying the model
Balance the complete assembled model by shifting the battery in the tray. For the first flights the position of the CG could be moved as much as 5 mm forward, but in no case backwards! Mark the correct position of battery on the tray (better done at leisure at home), as well as the check of the RC equipment – i.e. the correct sense of the movement and the deflection of ailerons and elevator and the operation of the controller. If your RC equipment allows for that, set the exponential deflections of ailerons and elevator to 60%.
When launching the model, hold it behind the wing at the main bulkhead. It is important that the model flies straight, not pitching or yawing (rotating around the horizontal or vertical axis). If you can have a help from a more experienced modeller at least for the first flights, do not hesitate to ask him!
Due to the wing loading it is useless to test-glide such a model. Give the model full throttle, and hand-launch it at some 10° up into a shallow climb. When you fit a weaker power unit, the model would in all probability “sag” slightly but it will still remain fully controllable both by the ailerons and elevator. The more powerful power plants will ensure that the model will continue straight in the direction of the launch. The reaction of the A-4 Skyhawk model to changes in thrust are neutral and the controlling it is easy, provided the recommended values for weight, control deflections and centre of gravity position were observed. When flying, always keep in mind some specific characteristics of the delta wing. One of them is the comparatively steep increase of drag at higher angles of attack, showing itself especially in tight turns. Yet even in these turns the machine shows quite docile and benign stall behaviour. The delta wing shows a quite low damping along the longitudinal axis – both mass and aerodynamic. This may show especially during stronger gusts and crosswind as a certain twitchiness of the model along its longitudinal axis. .
The A-4 manages easily all aerobatic figures with the exception of those that need the rudder control. There is no need to be afraid of flying inverted, of stalls and spins – for the recovery from them it suffices to push the elevator down. The model offers a surprisingly large range of speeds. There is no problem flying to the wind speeds of up to 5 m/s. When going for a landing, always bear in mind that the landing sped is much slower than the maximum one.
It is necessary to bear in mind that the fan characteristics are completely different from the propeller’s ones. Especially at slow speeds the propeller-driven model are able to accelerate much more rapidly that the fan-driven ones. The fan-propelled models also lack the effects of the energetic propeller slipstream over the empennage, which may help in some critical situations.
When flying the Skyhawk model with the powerful fan, a special danger may show – which calls for a constant attention. The model is quite small and not so well “readable” at greater distances. Yet, at the maximum speed it may “disappear” into the visibility-critical distance quite fast. In such case please immediately reduce throttle, start to climb and try to establish the actual position of the model.
Due to the low-wing layout it is advisable to fly at the prepared surfaces. The low–set wing is easily damaged when landing out. Equipping the model with undercarriage, which is almost invisible, will allow you to take-off and land on prepared surfaces. Installing the undercarriage is simple – the shapes of all components are described in the instructions, the places where to install the legs are marked on the appropriate locations on the model airframe.
Do not expose the model and its power unit to the sunlight for long periods of time, especially behind a window of car.
We wish you many a happy landing.
ALFA MODEL s.r.o.
A list of parts and tools necessary for finishing the model that are not supplied in the kit: