Fokker D7


Up / terug

Partly assembled, it probably spent decades under a tin roof, perhaps in Arizona.

Most of the wood has become brittle as have the balsa cement joints. The plan shows a glow motor installation starting at ,19 (about 3 cc) and the original builder had a VECO 35 lined up. How long could this airframe have withstood such forces?.

What kind of lifecycle did the manufacturers have in mind?

After one rough landing, straightening the undercarriage would tear out the bulkhead.

Think of the effects of glow fuel on the structure and covering below and behind the engine bearers.

Fencing hardware used to guide the elevator pushrod.in the tail area.

The half inch rudder deflection in the instructions came out at about twice that number. The elevator is firmly hinged with nylon strips secured by brass tacks that protrude.

Fortunately the crusty cement joints yield before the old balsa does and so the early building steps are reversed. More recent Sterling products are known to have instruction sheets that include full size examples of every part, but this plan shows few parts and we must recover as much as possible from the wreck or leave the old build intact and strengthen existing joints.

Pleasant weather is most helpful when you need to sand lots of old balsa wood. This step is both a means of checking the strength of the old structure as well as a morale booster.

Because the engine to be installed is smaller than the one picked by the original builder, the engine bearers are in want of change: they are slightly too far apart and the engine sits too high. Restoring old models is an ongoing agony of decisionmaking around how far to go in surgery. The 1/16” balsa may be very brittle and weak but the maple bearers are rock solid. A redwood scrap is placed under the bearers for strength and oil containment.

The upper wing, partially assembled by the previous owner, has its joints re-glued and broken pieces filled in.

In order to attach landing gear and wing struts to the fuselage, various reinforcements are needed. Below, a piece of redwood is glued in.

The upper wing – not quite complete – still needs reinforcements for the strut attachment points.

The lower wing coming together. The plan wants me to glue everything to the fuselage but I intend to create detachable wings, held in place with elastic bands. Vague instructions and horrible wood in this kit!

Brass tubes pass through the fuselage to accommodate the spring wire struts. Everything will be made detachable for the sake of later adjustments and repairs.

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The main landing gear legs. Detachable of course and with credits to Henk de Jong.

The landing gear fitted to the fuselage.

The rejected lightweight wheels from my Suls Record Trainer are recycled here with heavier hubs of desired shape in the making.

The all-important bellcrank.

Lower wing coming together.

Excess material filed out of the heavy crutch.

The streamlined crossmember between the wheels is in position and the wings temporarily placed. Wing mounting deviates strongly from the plan: everything will be attached by means of elastic bands in order to reduce the risk of crash damage. The upper wing mounting struts now follow the example of the full size aircraft.

False ribs are added in order to improve the scale effect and better support the profile.

Upper wing mounting struts are built up from 2 mm piano wire that will later be streamlined.

Below: the streamlined undercarriage cross member clipped between the main struts.

The cross member is supported by buttons residing in recesses and elastic bands.

Another tricky detail: the interplane struts. These play an important role in keeping the biplane a tight ship but they also have to separate freely in the event of a crash if the wings, secured by elastic bands, are submitted to excessive forces. Below, one set of the piano wire struts is sewed and glued to plywood feet that will rest on the top and bottom wings.

The top wing rests on flat beech bearers that have been “sewed” to the piano wire struts by means of thin wire and epoxy glue. Below you can also see the method of attaching the bottom wing to the fuselage using elastic bands.

Top and bottom end plates of the struts have a whaleback tongue that resides in a slot in the wing. This is meant to give rigidity but equally to permit disengagement under extreme forces.

Below the biplane is assembled. Thin braided wire is deployed for rigging with an elastic element for flexible tension – the rigging must give in the event of great force.

Glass and epoxy line the engine bay. As a extra measure the bearers receive tinplate strengthening.

Engine mounting bolts will be accessible from below the fuselage.

Fuselage rear turtle deck with cockpit coaming and Spandau's. Under the guns is a hatch that gives access to the bellcrank.

Behind engine mount is a compartment for the fuel tank. Both engine bay and tank compartment have draining holes and tubes running through to the fuselage bottom in order to discharge spilled fuel and used oil. The entire engine/tank area is epoxy-coated to prevent oil seepage into the wooden framework.

Cowling for engine bay and fuel tank incorporates the kit's metal radiator surround.

Under the nose the drains emerge for engine bay and fuel tank overflow. The larger holes give access to the engine mounting bolts through paper tubes.

As this is a scale model, some detail is required. Here, soft wire is wound to create dummy valve springs for the exposed engine.

The original D7 engine is a 6-cylinder but here there is only room for 4 dummies as the 3.5 cc diesel that powers the model occupies the first rows.

Compare the image below with the second picture of this web page ..

Oracover will be used to finish the model but here the nose has been treated with colored dope in order to toughen the soft wood and get a preview of the end result.

The final result. Credits for the “Oratex” covering job go to my control line partner Klaas!

The entire building period was accompanied by worry about overweight. The beast weighs a kilogram and the wingload compares to the brick-on-a-string Russian PO-2. Hopefully the D7's better aerodynamics and less unfavorable Reynolds Numbers will help out! (in the table below the biplanes' wing area is reduced by 20% for efficiency loss)

The picture below shows the upper wing struts that are reasonably authentic and 100% functional. Both upper and lower wings are fixed by elastic bands.The Sterling kit plan, on the contrary, shows a (non-scale) cabane-like structure and permanent wing mounting.

Originally, I wanted a more scale-like undercarriage but weight restraints, fear of complexity and a deadline forced a compromise.

Ideally, a second model should now be built, making use of all the lessons learned. It is tough to admit, but I believe that the airframe weight might well be halved.



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The Fokker D7 is powered by a FROG 3.5 cc diesel engine. The choice of motor was limited by engine bearers that were already fixed to the partially built fuselage. This engine has a rear air intake and needle valve, simplifying use of the pre-shaped radiator surround supplied in the Sterling kit.



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