From the March 2012 issue of Electric Flight Magazine. By Dan Savage
The F-4 Phantom II needs no introduction here. Like the Piper Cub, P-51 Mustang and F-86 Sabre Jet, it is one of those airplanes with a long and storied career. My desire to build an F-4 came from watching my brother Daren fly his scratch-built EDF F-4. What I admired most was the simplicity of his design and how it always flew like it was on rails. It was a box with wings that looked like an F-4 and flew like a sport model.
~ Photos by Dan Savage and Ken Adams
The basic construction of mine retains the simplicity I admired in Daren’s F-4. Although it’s still a box with wings that looks like an F-4, now it’s a more refined box. Once the fuselage skeleton is complete the rest of the construction, including the inlets, is done off the building board. After the fuselage is sheeted, the square corners are rounded. The inlets are built up using sheet balsa and 100# Bristol paper and are area-ruled to maximize efficiency. The wings are jig-built and have 2 degrees of wash-out.
Like Daren, I chose to use a single fan with a single tailpipe hidden inside the twin engine nozzles. Mine is designed for a 90mm fan and mini retractable landing gear. The nose cone and engine nozzles are molded fiberglass. I sell these as well as a short kit of laser cut parts and a clear canopy on my web site, listed at the end of the magazine article. You can also download a full-color, photo-illustrated construction guide here.
The cockpit/battery compartment hatch, fan hatch and tail cone hatch use magnetic latches. The tail feathers are sheet balsa. The horizontal tail uses an inverted Clark-Y airfoil and the rudder linkage is hidden under the elevator hatch. I like to build the tail feathers and wings before constructing the fuselage and I recommend building yours in this order. You’ll need the wings while adding the bulkheads to the fuselage and you’ll need the tail feathers when building the tail boom.
I made a few corrections to the design after the prototype was built. The horizontal tail volume was a little too large and the horizontal tail mount was too low in the tail boom. The tail volume has been reduced and the mount was raised. The nose gear mount has been raised to eliminate interference between the steering yoke and the bottom sheeting.
I prefer to cut all the pieces at one time to make a “kit”. The parts on the plans are organized by the type and thickness of wood. I used spray glue to stick the templates to the wood for cutting. For duplicate pieces, I used spray glue to stick two pieces of wood together before sticking the templates down.
Vertical Tail Construction
The rudder is actuated via a torque rod made from a Du-Bro 4-40 strip aileron set (No. 557). The vertical fin slips into a pocket in the top of the fuselage. The torque rod extends down through a hole the horizontal stab center section and into the elevator hatch area. If you do decide to streamline the fin, wait to do so until after the fuselage is built so that you can trace the top of the tail boom on the fin.
Horizontal Tail Construction
Because of the pronounced anhedral, the horizontal stab is built in three pieces –a center section with right and left stabs glued to it. The plans include a stab jig pieces that are used to sand the bevel in the center section and when gluing the two stabs to the center section. These glue joints must be reinforced with 4-ounce fiberglass cloth and epoxy resin.
The wings are built using the tried-and-true D-tube construction. They have 2 degrees of wash-out between the root and tip rib of the inner panel that is built in during wing construction. The outer panels have no twist. Two 1/8” birch ply stub spars interlock into two 1/8” birch ply rib doublers. These interlock with the landing gear and aileron servo mounts so the landing loads are transferred directly to the spars. This makes the wings extremely sturdy without adding excess weight. Once the wings are pulled up from the board, the tabs are removed and the remainder of the wing construction is done in the jigs. The inner and outer wing panels glue joints are reinforced with 4 ounce fiberglass cloth and laminating resin. The ailerons are cut from the wings after they are built.
If you’ve built a sport model before, you’ll be very comfortable building the F-4’s fuselage. Because you have access through all four sides of the fuselage, building the inlet ducts on the F-4 are a piece of cake. There are two sets of bulkhead pieces used to mount the fan unit on the plans. These are B7, B7-1 and B8. Each set has a one-letter suffix -A and -B. The -A parts are intended for 90mm fan units and the -B parts are meant for 3-1/2” fan units. Before beginning construction, check the fit of these parts on the fan unit you’re going to use and then use those parts when you build your model.
Although an F-4 is a pretty boxy design in its own right, it does have some curves. The top and bottom corners of the bulkheads are already beveled to accept 1/2” balsa triangle stock. Balsa sheets are glued to these and then the corners are rounded. Other, curvy areas, such as the top of the nose section, the fuselage spine and tail boom, all use semi-monocoque construction.
The magnetically latched hatches are simple to build and are secure in flight. The cockpit hatch gives you access to the radio equipment, retract hardware and flight batteries. The fan hatch gives you easy access to the fan as well as the elevator and rudder servos. The elevator hatch is underneath the horizontal stabs. The tail hook/splitter plate is attached to the elevator hatch and is also held on with magnets. If you are going to use a receiver battery pack, I suggest mounting it above the fan on the front of the elevator and rudder servo mount. I placed the receiver pack in the nose section on mine and it was nose heavy and needed 3 ounces of lead under the elevator hatch to balance it.
The inlets are built up using sheet balsa for the inner walls and 100# Smooth Bristol paper for the outer walls. Bristol paper doublers and triplers are added after the initial inlet construction is done. The aft inlet duct is built first, then the front. The first step in building the inlets is to assemble the aft inner inlet walls. The back of these are beveled so the air flows smoothly into the fan. Glue these together over the plans. If you don’t do this, the glue joint or the inlet walls will probably crack when they’re inserted into the fuselage. Finish the outside faces of the inner inlet walls before gluing them into the fuselage. The top and bottoms of the inner walls should be left unfinished so the outer inlet walls can be glued to them.
The recess formed by the joint between B7 and B7-1 doubler creates a shallow pocket. This pocket is what seals the fan and inlet together. Make sure that the back of inner inlet wall does not go past the back face of B7-1 otherwise the front of the fan will not get a good seal. Curl the rear part of the outer inlet wall so it fits smoothly into the opening between B7-1 and I2 and wrap the front of the outer wall around a 3/16” dowel to form a rounded corner. If the outer wall is a little too large, cut a little bit off the top and bottom corners of the outer wall. The front inlet is built the same way as you did the rear inlet duct.
When you’re ready to add the doublers and triplers, slip the rear doubler into place in the opening in B6, below the rear duct before you apply the glue. If you wait to slip it into place until afterward, it’ll turn into a mess. Thin the carpenter’s glue to the consistency of latex house paint. Make sure it’s not too thin, otherwise it will warp the paper. Apply a very thin layer of glue to the rear duct. Slip the doubler into place and gently press it against the outer duct wall. Be careful not to dent the outer duct wall as the glue will slightly soften the paper. Do the same for the tripler. Next, apply the forward inlet duct wall doublers and triplers.
The bottom of the fuselage is sheeted with the grain running across the fuselage and the rest of the fuselage is sheeted with the grain running lengthwise. Route the wires and air lines before sheeting the top of the fuselage and the engine nacelles. Once the fuselage is buttoned up, it will be much more difficult to do so. Once the fuselage has been sheeted, round off the corners on the fuselage bottom, and then do the same to the tops and bottoms of the engine nacelles. The inlet lips are built and shaped on the model. Use masking tape to protect the sides of the nose section while you shape the inlet lips.
The incidence and dihedral of the wings are pretty much set when the stub spars are inserted into the spar pockets. I used a 1/8″ balsa stick to make sure the leading edges of the right and left wing panels were even with each other. One side’s leading edge was higher than the other, so I removed the wing and used a hobby knife and sanding block to take a little bit off the bottom of the front stub spar. I also had to take off a little bit off of one of the rear spars to level the trailing edges. I used 30-minute epoxy to glue the wings and tail feathers to the fuselage. Once the stab is mounted, shape the hatch sides to match the stab center section. Make a “Y” pushrod for the elevator and a straight pushrod for the rudder. Notch the sides of the hatch for the elevator pushrods. Make sure the notches are large enough that the pushrods do not touch the hatch.
I used .010” PETG clear plastic sheet to make the tailpipe. Make sure that the tailpipe fits snugly over the back of the fan and inside the oval opening in the fiberglass engine nozzle. Sighting from behind the fuselage, verify the alignment of the tailpipe to make sure it’s parallel to the centerline of the fuselage. Slip the engine nozzles over the tailpipe and into place onto B9/B9-2. Make sure it doesn’t distort the tailpipe or alter its alignment. If it does, then carefully sand the front of the engine nozzle to change its angle until the opening is centered and doesn’t distort the tailpipe. The nose cone is fitted to the fuselage in the same way. Sighting from the front and rear, verify that the nose cone is aligned with the centerline of the fuselage. Sand the entire model smooth. Since the entire model is fully sheeted, it can be covered with plastic film or it can be fiberglassed and painted. I’m lazy so I used plastic film on mine.
Flying the Model
The model should be balanced upside-down on a balancing stand at the range shown on the plans. Set the control throws to low rates for the initial test flights. The model will require a take-off run of about 150-200 feet on level asphalt. Don’t horse it off the runway. Instead, as the model accelerates, apply gentle back pressure to the elevator and the nose should come up. As it continues to accelerate in this attitude, it will fly off the runway on its own. The F-4 is stable and responsive at all speeds and is capable of very slow flight. As you descend for landing, hold the model into a level flight attitude with the elevator and control the rate of descent with throttle. At about feet off the runway, reduce power to idle. Raise the nose to flare and touch down on the mains. Perform a thorough post-flight check to make sure that no problems have cropped up during the first flight. Take your time to get to know the model’s handling characteristics. It is a blast to fly and should present you with no surprises.
Congratulations! You’re now a Phantom driver!
Captions for Gallery Photos
Photo 01 This F-4 Phantom II is designed for a single 90mm or 3-1/2” fan and mini-retracts. It uses sport model construction techniques. It has area-ruled inlet ducts and a single tailpipe for maximum efficiency.
Photo 02 The fuselage is built up from lite ply bulkheads that are balsa-sheeted. The square corners are rounded after sheeting so it looks like an F-4 should. Tail feathers are sheet balsa. The elevator uses an inverted Clark-Y airfoil.
Photo 03 The single tailpipe is hidden at the top of the inside the fiberglass engine nozzles. The tail hook is made from 1/8” lite ply and is held on using round magnets. To help preserve the distinctive lines of the F-4, the control linkages are hidden as much as possible.
Photo 04 The horizontal stab is sanded to an inverted Clark-Y airfoil. The included stab jig is used to accurately sand the bevel in the stab center section. The hole in the center section allows the rudder torque rod to be hidden in the elevator hatch between the stabs.
Photo 05 Once the stabs are joined to the center section, the glue joints are reinforced top and bottom with fiberglass cloth and laminating epoxy.
Photo 06 Like the stab, the vertical fin and rudder are sheet balsa. The rudder has been hinged and slotted for the rudder torque rod. The torque rod is made from a Du-Bro strip aileron set. The sub-fin below the rudder is slotted for the torque rod and is ready to be glued to the vertical fin.
Photo 07 The wings are jig-built and use D-tube construction. The tabs on the ribs assure accurate rib alignment. The stub spars interlock with the doublers on the wing ribs and the landing gear and aileron servo mounts to make the wings very sturdy and grass-field capable. Leading edge is shaped after construction is complete.
Photo 08 The rib tabs have been removed and the wings have been tack-glued in the wing jigs over the plans. The 1/8” balsa wing spars and 1/16” shear webs have been glued to the upper and lower balsa spars. The wings are ready for top sheeting.
Photo 09 Once the wings are sheeted and the trailing edge stock is glued to the wing sheeting, the wings are given their final sanding and then they are glued together in the wing jigs. The clear sandwich wrap acts as a glue barrier so the ailerons won’t get glued to the outer wing panels.
Photo 10 Once the glue has dried, the ailerons are cut from the wings in preparation for adding the balsa aileron spars.
Photo 11 The aileron spars have been glued into place and sanded to match the wing. The leading of the aileron is beveled and the hinge slots cut.
Photo 12 A jeweler’s file chucked into a hand drill makes an excellent tool for boring through the soft balsa aileron spar for the hidden aileron linkage.
Photo 13 The hidden aileron linkage has been built from 2-56 hardware. The aileron servo is removable.
Photo 14 The glue joints between the inner and outer wing panels are reinforced with 4-ounce fiberglass cloth and laminating epoxy. It’s a little difficult to see in this photo, but the masking tape is holding a small piece of clear sandwich wrap tightly stretched over the epoxy so it will have a glassy-smooth surface and feathered edge that requires very little sanding.
Photo 15 The fuselage skeleton is framed up over the plans. Once the skeleton is built, the rest of the construction is done off the building board. A jig holds the nose-cone bulkhead at the proper angle so the model has the right profile. The bottom of the fuselage has triangle balsa that gets rounded after the fuselage is sheeted.
Photo 16 The fan unit is being fitted to the fuselage and the fan mounting rails have been drilled. 2-56 blind nuts retain the fan. The doubler glued to the front of the fan bulkhead forms a shallow pocket to seal the fan to the inlet ducting.
Photo 17 The wing stub spars fit into spar pockets built into the fuselage. The placement of the bulkheads is verified early in the fuselage construction. Once they’re flush with the mounting bulkheads, the top corner blocks are glued to the upper corners of the nacelle bulkheads.
Photo 18 A building square is used to make sure that this bulkhead is perpendicular to the building board. The tail boom is glued to this bulkhead, so its alignment is essential to a straight model. Once verified, the top corner blocks are glued to lock it into position.
Photo 19 The tail boom is framed up from 1/8” lit ply. At this point, the stab mount portion is measured to make sure that it is flat and level to the building board and aligned to the fuselage center line.
Photo 20 The fuselage skeleton is complete and off the building board and ready for the rest of construction. Even though these photos show the 1/8” balsa stringers glued into place, you’ll want to wait until the last possible minute to glue these on your model. I regularly broke mine during routine handling.
Photo 21 The inlet ducts are built from balsa sheet and 100# Smooth Bristol paper. Inlet duct construction begins by building and finishing the balsa inner duct wall. The back inside edges are beveled so the inner walls fair smoothly to the front of the fan. The top, bottom and front edges are left unfinished to form strong glue joints between the inner and outer walls. The half-moon cut-out gives clearance for the fan spinner.
Photo 22 The inner duct wall is slipped through the fan opening and into place. The shallow pocket seals the fan to the inlet ducts. The inner wall should not extend past the back of the doubler otherwise it could cause interference with the front of the fan preventing a good seal.
Photo 23 The inlet ducts are built in two stages. The back gets built first, followed by the front. The Bristol paper outer wall has been glued to the rear inner wall and to the bulkheads. Notice the broken 1/8” stringer above the inlet duct. These broke with maddening frequency.
Photo 24 If the outer wall is a little bit too big, snip the top and bottom corners to give a little bit of working room. These are sealed later with a scrap of Bristol paper. The inner inlet wall was cut out to clear the fan spinner. The inner face of the cut-out was further beveled to allow the fan to pull a little air in from the battery compartment to help cool the flight batteries.
Photo 25 The front inlet duct inner wall has been finished, then glued into place. The Bristol paper front outer wall has been glued into place and the excess trimmed from the front of the bulkhead. To keep from creasing the Bristol paper, wrap the corners around a 3/16” dowel before inserting them into the fuselage.
Photo 26 Because 1 layer of Bristol paper is not strong enough for a 90mm fan, Bristol paper doublers and triplers are added after the initial inlet duct construction is complete. I’ve used spray glue and thinned carpenter’s glue. Of the two, I prefer carpenter’s glue because it gives you more working time to make sure it’s in exactly the right place. Insert the doubler into the fuselage and slip it into the opening between the bottom of the duct and the bulkheads before applying the glue. If you wait until afterward, it will make a big mess.
Photo 27 The Bristol paper doublers and triplers are done. The open corners at the top and bottom of the outer walls have been sealed with small pieces of Bristol paper.
Photo 28 A 1/8” lite ply bottom keel is glued to the bottom of the fuselage to make a strong base for the wings. The spar pockets are assembled from 1/8” birch ply and are glued to the bottom keel and bulkheads. A small piece of scrap ply is used as a spacer for the spars.
Photo 29 The location of the nose wheel steering servo allows short, direct connections to the nose-wheel. The nose gear plate fits into slots in the bulkheads. 1/2” balsa triangle stiffens the plate to minimize flexing. Overall, this is a very sturdy mount that is rough-field capable.
Photo 30 The bottom of the fuselage and the sides of the engine nacelles have been sheeted and the wiring and air lines have been routed. After the fuselage is sheeted, it would be much more difficult to do this job.
Photo 31 Fiberglass engine nozzle is being fitted to the fuselage. The oval recess gets cut out for the single tailpipe. The square corners of the fuselage will be rounded to match engine nozzles.
Photo 32 The square corners of the fuselage has been rounded. The cockpit hatch base plate is sanded to match the contour of the fuselage. All hatches use magnetic latches.
Photo 33 The coaming formers are added to the base plate. To keep from gluing the hatch pieces to the fuselage, I laid a piece of clear sandwich wrap down, then put the hatch on the fuselage.
Photo 34 The inlet lips are built on the fuselage. Wrap the sides of the nose with masking tape to keep from gouging the balsa sheeting. 1/4” balsa triangles fill the inside corners and will be rounded to match the inlet duct. The outside of the inlet lips have been rounded to match fuselage contour. Next step is the shape the inside to an airfoil cross-section.
Photo 35 The rounding of the inlet lip is done and the F-4’s distinctive splitter plate is being fitted to the inlet lips. The splitter plate is tapered to 1/8” in the front. This is glued to the fuselage after finishing.
Photo 36 Use clear sandwich wrap to keep the elevator hatch parts from sticking to the fuselage during construction. I used masking tape to keep the hatch from warping while it dries. I wet the outside of the balsa sheeting so it would form to a compound curve.
Photo 37 The vertical fin fits into a pocket in the top of the fuselage for a strong joint. I wanted to streamline the fin, so I traced on the fuselage profile on the sides of the vertical fin, then shaped the fin before gluing it in the rudder pocket. The rudder torque rod extends down into elevator hatch area so the linkage is completely hidden.
Photo 38 The tail hook/splitter plate is being built. This is glued to the elevator hatch and is held on with magnets, which makes removing the tail pipe a snap. It’s important to make sure the splitter plate is aligned with the fuselage otherwise it will act like a vane and alter the thrustline.
Photo 39 After the tail hook and elevator hatch is built, the stab is mounted and the elevator hatch is sanded to fit the stab. The inside of the elevators are sanded to match the tail boom contour.
Photo 40 Simple, direct linkages to the elevators and rudder. Even though this photo shows that the horizontal stab is bolted to the mount, it is glued on the final version of the model. I wasn’t sure how well the inverted Clark-Y airfoil would work, so I wanted to be able to remove the stab without hacking up the fuselage.
Photo 41 It’s easier to finish the inside of the inlet lips when they’re off the model. Here, the inlet lip has been removed and is ready for finishing. It will be permanently glued into place after the model finish is applied to the inside of the lips.
Photo 42 The fuselage is weighted over the plans for wing joining. Setting the incidence and dihedral is automatic when the stub spars are inserted in the spar pockets in the fuselage. A hole has been cut into the side of the fuselage to thread the aileron servo extensions and retract air lines after the wings are joined to the fuselage. Matching holes are cut into the wing root ribs.
Photo 43 I used an 1/8” balsa stick to measure the leading edges of both wings. Adjustments are made by shaving a little off the bottom of the front stub spar until the height of the leading edges of both wing panels are equal.
Photo 44 An 1/8” balsa stick is also used to make sure the trailing edges of both wings are even with each other and are also level with the building board. Any adjustments are made by shaving a little off the bottom of the rear stub spar of the high wing.
Photo 45 Access to the nose gear and steering servo is through the nose gear well opening. The large hole in the front was necessary on the prototype due to the mounting plate being placed too low in the fuselage which caused the steering arm to interfere with the bottom fuselage sheeting. This has been fixed in the final version.
Photo 46 The main gear mount is placed deep enough in the wing to allow enough clearance for gear doors. At this point, all construction is complete and the model is ready for finishing, then my favorite part –flying!