by Dick Petit
Designed by Geoffrey de Havilland in 1916, the D.H.4 was the only British design manufactured by the Americans. It was easily identified by its rectangular fuselage and deep frontal radiator. Versatile, heavily armed and equipped with a powerful twelve cylinder engine, this biplane daylight bomber was fast. Sometimes called the “Flaming Coffin,” its huge fuel tank was dangerously positioned between the pilot and observer, hindering communication. Produced in vast numbers, many D.H.4s were modified for civilian air service after the war.
The Airco DH-4 was easy to fly, could travel at over 100 mph and had a high ceiling of 23,500 feet (7,163 m) and was considered the best single-engineered bomber of the First World War.
Thousands of DH-4’s were left in America on Armistice Day and even more Liberty Engines. The DH-4 hadn’t been worth much in combat, but it was a big solid stable biplane with a tubular steel frame and what turned out to be a fine engine. When you crash-landed a DH-4, you walked away alive.”
As I was walking around the show floor at the 2009 Toledo Expo, I came upon an extremely large model of a WW1 biplane that was still “in the bones”, i.e., not covered yet. It was sitting at the Balsa USA booth which attracted me over to take a closer look. It was a quarter scale model of a de Havilland (AIRCO) DH-4 biplane that was in the process of being developed for release later in the year. I asked to be placed on the list to have a kit sent to me for a review project and I have been waiting for several months for those three boxes to come through the shop door.
Although the Balsa USA kit was designed around the reconnaissance version of the DH-4, I began looking on the Internet for photos and drawings of the plane to be used as documentation on my model. During this time, I found a website that showed the only flying DH-4 left in the United States, but it had been rebuilt into a post war mail plane. Just the color scheme made me take a closer look at the plane and it was then I decided to convert the kit into a DH-4M2 mail plane.
This is from the Historic Aircraft Restoration Museum web site:
“Many DH-4s were used by the Postal Service during the 1920’s, and many of the later ones were M models. This unit, being built up for the Historic Aircraft Restoration Museum, is an M-2,and will be converted to a Mail Plane as used by Robertson flying Service on the C. A. M. 2 route between Chicago and St. Louis. Charles Lindbergh was a mail pilot for Robertson and one of his log book entries refers to a metal DH-4. That particular aircraft was actually an M-1, but the sketchy records still available indicate that Robertson did have a few M-2 versions sometime after Lindbergh left the company.”
The seemingly small single kit box arrived at the shop and it fit neatly into my car, after wrestling the 30 pounds of wood into the front seat. I opened it and found it was packed to the brim with balsa sticks (lots of them!) sheets, die cut plywood and balsa ribs and other parts, plus hardware and several formed plastic cowl pieces. The 108 page manual shows all the assembly steps in photographs and includes an inventory of the die cut sheets. However several typographical errors at first caused me to wonder if all the parts were in the box. I had to count up how many ribs of each type were needed when compared to the plan sheets and found that I had all the ribs needed. What I did not have were the firewall, the fuselage floor and several other die cut sheets, plus a few sticks of hardwood. I contacted Balsa USA and they shipped out the replacements that day.
The manual begins with the fabrication of the two wing joiner tubes, made from plywood that is glued together around the aluminum wing tubes, after wrapping them in wax paper. Care must be taken to avoid gluing the joiners to the inside of the boxes and the joiners have to slip easily through them to make assembly possible.
The wing panels are made from die cut plywood and balsa ribs, hardwood spars, balsa leading and trailing edges and several plywood hard points. The ailerons are built along with the wing panel to match size and shape, and are hinged using standard plastic pinned hinges. After sanding and shaping the laminated leading edge and wing tips, that panel was almost ready to final sand. Only three more panels to go.
The upper wing center section has a few more plywood and balsa laminated ribs, hardwood spars, a solid leading edge sheeting piece and a space for the wing box to slide into. Once the center is complete, the joiner box is slipped into the holes, and center section is secured to the flat work surface (I used several worn-out starter batteries to do this) and the outer panels are pushed over the joiner box ends. Each panel is raised from the work surface by 1 inch using a small block of wood and the joiner boxes are secured to the outer panels and center section using epoxy and some die cut balsa reinforcement parts called “scabs”.
Now it’s time for “Weird Instruction #1”. The manual says to get a saw and cut the center section at the very center. No, not your puny little hobby saw, they say to use a big carpenter’s hand saw. It seems that the center section must be cut in half so that the wing panels can be separated easily. I took my ancient hand saw and began cutting up all the hard work I had just completed. Once cut, the remaining balsa and spar material was cut and sanded away, leaving a pair of balsa ribs that will mate together when assembled.
The tail surfaces are built using the die cut balsa parts that have to be laminated and then reinforced with more balsa cap strips. Once the glue dried, the structure was pulled from the board, pinned down and more perimeter parts glued in place and sanded flat. I used Robart point style hinges instead of the flat pinned hinges in the kit. A bevel was cut into all the leading edges and final sanding was completed.
It was time to start on the fuselage with more laminating to be done. Oh, goodie! I used thick ZAP adhesive to laminate the balsa parts because wood glue might have warped them. The front parts were pinned into place, several hardwood parts glued to them and the longerons began life as balsa strips which were doubled and laminated for strength. Balsa vertical and diagonal pieces were glued in place and allowed to dry. This made the first fuselage side.
After it was pulled from the board, the first side was sanded flat and re-pinned to the board. Wax paper was placed on top of it and a second side was built directly over the first one. This technique made sure that both the sides were identical. Once the second side was dry, it was removed from the board and sanded flat and smooth. It was then pinned to the first side and the perimeters checked to make sure there were really 2 identical sides. Minor sanding was needed but they were as close to identical as I could hope to make them.
The front five formers have balsa reinforcements on their upper and lower ends which were added using the diagrams on the plan sheets. The flat front crutch is pinned and weighted to the board over the top view of the fuselage and using the formers, one side is tack glued into place. The opposite side is then glued to the formers making sure everything was straight. I made some minor changes to the positions and sizes of several top fuselage formers to make the shape of the upper deck resemble the mail plane version of the DH-4 I am modeling. Once the formers were completed, balsa sticks were added and a new piece of 1/32” plywood was used to create the upper fuselage decking.
I decided to include the scale elevator and rudder pull-pull linkages that run outside the fuselage. A piece of aluminum strip was used to make the rudder bar which is driven by a servo. It rides in a nylon bushing and sticks outside the fuselage about an inch. The elevator linkage was a bit trickier but once I began browsing through the McMaster-Carr online catalog, I found everything I needed to build the linkage. Half inch brass tubing, riding in a pair of nylon bushings stick out each side of the fuselage and are kept in place by aluminum bushings that look a lot like huge wheel collars. The control horns are more collars on which I mounted some old servo arms that connect to the servo inside the fuselage and the elevator wires outside. 
The dummy engine radiator is built from die cut plywood and balsa frames and a 1/32” ply wrapping. It has to be built flat on the board which ensures it is perfectly straight when complete. Strip wood shutters and the crankshaft exit are glued into place and the screen wire mesh is then added. It bolts up to the front of the fuselage and has a lot of air inlet for engine cooling.
It’s time to build the cabane struts using an assembly jig and some plywood parts. Metal tabs are soldered to music wire struts and bolted to the jig for alignment. Once the struts are complete they are plugged into the fuselage and aligned fore and aft using another jig which is basically two pieces of balsa to set where the front of the top wing will be. The cross brace wires are then soldered into place and the cabanes are then faired over using some pre-grooved hardwood fairings.
The cabane bolt layout is drawn on the underside of the top wing assembly, and holes are drilled for the blind nuts and reinforcement blocks. The top wing is bolted into place and checked for squareness to the fuselage using a tape measure from wingtips to the tail post. Mine came out to be within 1/16” right off the bat and I left it there.
The lower wing is then fitted to the fuselage making sure it aligns both vertically and horizontally with the top wing. The spar box is glued to the fuselage framework and is then cut to separate the wings from the fuselage. With the metal spar tube reinserted, the wing panels are ready for the interplane struts, made from thin plywood inner parts and thicker hardwood outer fairings that must be custom made for each strut location. Metal tabs are bolted to the plywood strut ribs and the building process starts. It took me the better part of 10 hours to assemble all of the 8 struts, their cross brace wires and to sand each strut to the correct airfoil shape. With the struts bolted in place, both wings on each side can be removed as a unit for storage or transportation, but it isn’t all that difficult to remove each strut assembly and separate both the upper and lower wing panels.
I glued in the landing gear blocks and braces and then soldered up the main gear legs using heavy buss wire to keep things together. I also used brass tube over the axles to be able to secure the wheels with a cotter pin. Balsa fairings were laminated and glued to the gear legs and they will be covered and painted later. The kit’s main gear uses a spreader bar with a bungee cord mounted sideways to keep the wheels from spreading. The full scale plane uses bungee cords to keep the axle from moving up and down too far. I guess the kit version will work OK. I used a pair of Williams Bros. Vintage wheels that looked small but they are pretty close to being scale size. The fuselage sides are sheeted balsa at the front and rear and are cap stripped in the middle. This adds to the strength of the fuselage and makes it easier to apply covering. 
I decided on a YD-A 56cc gas engine from R/C Aero Products. This is the same distributor that used to sell RCGF and Aerovate engines, and still have several of them available. With a rear mounted carburetor and with the use of a tight fitting MT flexible exhaust header, the YD-A 56 fits inside the skinny engine bay snugly. I also chose a canister muffler from JTEC to keep the exhaust noise down.
After measuring the engine length and doing the math as detailed in the manual, the firewall was glued into place without any down or right thrust built in. That is handled by adding spacers behind the engine. Now for the fun part. You have to stand the fuselage on its tail, put the spacers on the firewall, put the engine on the spacers and then bolt the radiator assembly in place. Then you move the engine and spacers on the firewall until the prop shaft is in the middle of the opening of the radiator. Then you get to try to keep everything in place while you remove the radiator and mark the location of the mounting bolt holes, all while trying to keep the engine and spacers from moving. Oh, I forgot to mention that the fuselage is about 7 feet tall and you have to do all this standing on a ladder, while juggling 5 rubber balls, blindfolded. (I was joking about that last part) It can be done if you work carefully and my engine installation was completed perfectly.
The formed plastic engine top and bottom covers have a number of louvers that have to be opened up to provide proper ventilation in the engine bay. I used a thin cutoff wheel on my high speed rotary tool and cut the 32 openings in the top part and the 4 openings in the bottom part. The molded plastic exhaust headers come in top and bottom parts and have to be glued together, filled with some sort of filler material (I used good old Bondo) and then sanded and painted. Small balsa blocks are carved to fit inside each of the 12 pipes and are trimmed flat. The blocks are glued to the fuselage while still inside the pipes. The pipes are removed, the blocks trimmed square and you can add the pipes back on after the fuselage is complete.
Speaking of covering, I started to do the finish sanding on the wings in preparation for covering. I used SIG Koverall applied with Stix-It and will use water base polyurethane to keep everything in place. I used safety nuts on all the bracket bolts just to make sure they stay in place. The narrow rib spacing made it hard to tighten the bolts and nuts but after poking a few accurately located holes in a few ribs, the hex wrench fit just fine. The ailerons and tail surfaces were also covered at this time.
Old school modelers will want to apply the first coats of filler using some brand of nitrate dope, which works fine and does a good job. I chose to use water base polyurethane which is brushed onto the Koverall making sure it does not drip through to the other side. After the first 2 coats are dry, they can be sanded and painted if you want to have a nice fabric finish. My scale subject has a smoother surface so I filled the weave with more WBPU that was mixed with baby powder which filled the weave really well.
The fuselage is covered using two pieces of Koverall, one on the bottom and the second wrapped over the top and down the sides. Since there is a lot of sheeting on the fuselage sides, I brushed on two coats of lacquer base sanding sealer to the sheet wood to keep any of the WBPU from soaking through and warping the wood.
While I had the time, I sprayed the wing panels, aileron and tail surfaces with silver paint. When they were dried, I assembled the wing panels to the fuselage to start cutting and fitting the rigging wires. The Balsa USA kit has no provision for flying and landing wires and the prototype didn’t use them. I wanted to install them just for a little more “insurance” and a lot more visual effect. The braided wire, crimps and thimbles came from McMaster-Carr and the rod ends came from DuBro Products.
The 4-40 wire links that connect the upper and lower ailerons were measured, cut to size and screwed into place. The aileron servos and 4-40 rods and clevises were then assembled with homemade servo leads and pulled through the wing panel holes. I will connect them with “more up than down” programmed by the transmitter.
Lots of details needed to be completed such as making a seat for my Premier Pilot figure, adding an instrument panel and painting the fuselage. Once the fuselage had a few coats of “deep burgundy” applied it was allowed to cure for several days in preparation for the application of the graphics from Kirby’s Kustom Graphix.
Finally the fuselage brackets, mounting tabs and other rigging hardware could be installed permanently. The rudder and elevator control linkage was installed along with the servos and I decided to mount my receiver up inside the bottom hatch. The batteries, a set of 2300 maH A123 2 cell packs, will be placed wherever necessary to help balance the completed plane.
Using the lower wing panels as a reference, the stab was epoxied into place followed by the fin. Tail braces were originally made from 2-56 rods and clevises and hold the tail parts in place quite well, but I changed them to braided wire to reduce tail weight. The rudder and elevators were hinged into place and the pull-pull wires were cut and run to their proper locations. I also remounted the engine and set up the fuel tank plumbing, ignition box and battery and the throttle servo with its linkage. The exhaust header and canister muffler was installed and plenty of air circulation was provided. Once the main landing gear and tail skid were installed, the DH-4 was really looking great and just about ready to be assembled and test run outside.
It took me the better part of an hour to assemble all the parts because I was trying to formulate the most efficient way to put it all together. I decided it would be best if the lower wings were bolted in place, followed by the upper panels and then all the pre-adjusted flying wires. Last, the interplane struts would be installed along with the aileron connector rods. If all goes well, I can put it together in about 15 minutes.
It was getting darker as the day went on and I wanted to at least see where the DH-4 balanced and where any weight had to be added. I was really shocked when I found that almost 3 pounds of lead needed to be added
It was getting darker as the day went on and I wanted to at least see where the DH-4 balanced and where any weight had to be added. I was really shocked when I found that almost 3 pounds of lead needed to be added as far forward as possible to get the plane to balance at the 28% mark. I put a pound in the lower part of the cowl and the rest in a wood box just behind the firewall. The A123 batteries were placed above the fuel tank on a shelf and the plane finally balanced quite well.
I then hung the plane on my digital fish scale and was equally surprised to find that the total finished weight was just below 35 pounds, even with all that lead in the nose. I can understand why Balsa USA used such a large engine on their prototype model, not for power, but rather for nose weight.
Engine testing was done on the driveway and I determined that both the Xoar 26-6 and the VESS 25A props would be used on the first flights. There isn’t a lot of prop clearance so I’ll have to keep the tail down while taxiing.
The weatherman was the determining factor in the scheduling of the test flights of the DH-4. We had been having a lot of rain and the weekend selected for flying was forecasted to be chilly and wet on Saturday, but clear and calm on Sunday. I packed the DH-4 into the trailer, picked up my flying buddy and assistant test pilot Rick Cawley and headed to Piedmont Aeromodelers field which is a lot more open than my normal test flying site. It took the better part of 45 minutes to assemble the plane but everything fit just as planned and I took a number of ground photos prior to flying.
The YD-A 56cc engine fires up easily despite the chilly weather and I asked Rick to take video of the first flight rather than still photos. I was planning on making a few taxi runs first, but since the winds were calm and the plane looked as if it wanted to fly, I asked Rick to start the camera and I lined the plane up with the centerline of the grass runway.
Power was added, never going over half throttle and the tail lifted gently as the plane increased speed. Once flying speed was achieved, it just left the ground and was airborne after less than 100 feet on the ground. A bit of down elevator was needed for hands-off level flight but no other trims were needed. I flew around the field a few times and made some low and slow passes for the camera. I was not planning to do any “aerobatics” but a stall turn was made and the DH-4 did it easily.
I asked Rick to see if there was any flexing in the wings due to excess movement. The additional brace wires tightened up things quite nicely and they kept the wings perfectly aligned throughout the entire flight.
After flying for about 5 minutes I set up for a landing and the plane just glided easily to the ground. Full up elevator at the last 2 feet of altitude and it settled to the ground for a perfect three point landing. That was a really great first flight! I checked the brace wires and they had loosened just a bit, which is expected after a flight or two. I left them as they were since they seemed to be doing an admirable job as they were.
The second flight was made by Rick while I took some still photos. Rick flies his 1/3 scale Balsa USA Sopwith Pup with his 2 buddies flying theirs and he said the DH-4 was a bit slower but every bit as maneuverable. He commented that the engine had plenty of power and responded well to throttle inputs. He made some low passes and then set up for a landing that resulted in another perfect 3 pointer.
I made the third flight of the day and that was just to see how little room the DH-4 needed to take off and land. It got off the ground under full throttle in less than 20 feet and stopped moving in less than 10 feet. During that flight I lowered the throttle to just above idle and the plane just putted around under complete control at little more than walking speed.
It took about 20 minutes to take the plane back apart and on the way home, Rick and I discussed the results of the test flying session. He really liked the way the huge wings responded to just a bit of aileron input, and the way the YD-A 56cc engine pulled the plane with authority. The JTEK canister muffler maintained the realism by lowering the sound level to the point where you could mainly hear the prop spinning. In all, everything worked perfectly and I have to say that this had been one of the best flying test models I have ever flown.
In conclusion, the Balsa USA de Havilland DH4 kit was a real treat to assemble, despite a few very minor problems and all those cap strips! Everything fits as designed and once the major structures were put together it was easy to rig the struts and my added brace wires kept the wings perfectly aligned and allowed zero flexing. The YD-A 56cc engine is a pretty good match for the plane, the only problem being the lack of engine weight for which I had to compensate with almost 3 pounds of lead in the nose. The plane still only weighted 35 pounds which resulted in a trainer-like 18 ounces per square foot wing loading.
Add to all this the fact that it looks so darned good both on the ground and in the air, and you have all the incentive to build one for yourself. I like my DH-4 and I hope to see many more at the field next year.