Thursday, January 19, 2012

A smaller restoration project

By B. Reid

There seems to be no end to the interesting and diverse maintenance related tasks being performed by the staff at Vintage Wings of Canada. In addition to their dedicated work, there is a small team of volunteers that have made a significant contribution and continue to do so. For example, a large portion of the restoration of the Lysander was done by a team of volunteers.
Recently, a small out of the way project has been slowly but steadily progressing. It has no deadline but has kept several of the volunteers busy under the watchful eye of Oscar Verdugo. This is the restoration of much of the woodwork of our original Tiger Moth, CF-DHQ(c/n DHC1671). It has brought to attention the interesting structure of this 1920’s technology biplane.

While no firm decision has been made on how much restoration will be done on this aircraft, one can only hope that as work progresses, the day that DHQ returns to the skies gets closer to reality.

The Tiger Moth was really an update of the DH.60 Moth. The major changes were the inverting of the Gipsy engine allowing better visibility as the four inline cylinders now pointed downward and the staggering and sweepback of the wings. In order to sell the aircraft to the RAF, the Air Ministry insisted that the parachute wearing pilot in the front seat be able to more easily abandon the aircraft. By moving the top wing forward 22 inches, there was no longer an overhead obstruction to escape while the upper mainplane support struts on the fuselage no longer surrounded the front pilot. The rear flying wires were moved as well. For centre of gravity purposes, the both wings were then swept back. When it was decided that a further small amount of sweepback was required, only the top wing was further swept resulting in a 9° and 11° sweep for the two wings. With the wing tips now further aft and lower, deflected ailerons were felt to be too close to the ground. This was corrected with increased dihedral on the lower wing(which also increased stability). Although the ribs are no longer aligned with the relative wind, there is no detrimental effect, however, unlike its similar predecessors, the wings cannot be folded.

Like the Avro Tutor and the Hawker Tomtit, a name that started with a T was needed for an RAF elementary trainer. In order to still have a Moth named after an actual moth, a previously used name was chosen, Tiger Moth. Named after the orange-striped insect, the original de Havilland Tiger Moth, the DH.71, was a testbed for the Gipsy engine and known for its high speed, participating in the King's Cup air race. However, it’s much slower follow-on was the one that would would make the name legendary.

It is not unusual for proud Tiger Moth operators in the New World to list off the differences between the Canadian-built DH.82C model compared to the DH.82A models built in the U.K., Australia, New Zealand, Sweden, Norway, and Portugal(don’t forget that there was also the original DH.82 model and a few DH.60T’s). Some of the differences seen in the wing and empennage area include the lack of an elevator trim tab on the A model, an internal balance weight on the leading edge of the A model rudder, two external balance weights on the elevator of the C model and a plywood reinforcement on the leading edge of the lower wing on the C model(as well as Australian A models). Metal interplane struts on the C model versus larger wood ones on the A model. If you know of some other differences for the flying surfaces, please leave a comment.

There are several ways to identify this upside down wing as a lower wing(lower mainplane). At the far end is a cutout for an aileron. The leading edge has a full length plywood reinforcement. There are extra ribs(platform ribs) at the near end which are covered by the plywood tread-board(walkway). Note how all the ribs are designed so that they can slide onto the two spars from the far end and then be secured in place.

While mostly made of wood, the Tiger Moth wings do have some metal. Here you can see the curved light metal tubing for the wingtip. These tubes are located in several wing edge areas where there are curves including the inner trailing edge of the upper wing eliminating the need to build time-consuming wood pieces. The silver tube is a drag strut. There are three of these fore and aft compression members in the wing that are part of the internal bracing. Partially visible are drag and anti-drag wires that attach to the struts and provide bracing against forward and aft forces. At the tip you can also see that the handle connects to two ribs. While mostly used for ground manouvering when the engine is shut down, they can be vary useful in strong wind conditions with a wing walker at each wing tip.

This is the top side of the aileron control box where the cables from the control stick arrive(via turnbuckles on the left) onto a chain that operates the sprocket wheel which is attached to a concentric bellcrank above it(seen to the right of the sprocket wheel) which actuates the rod that moves the aileron. The green circlular piece is a cover plate. There is a nice view of the drag strut and drag wire combination.

The lower side of the aileron control unit. Placing the attachment of the control rod in the proper position on the circle allows the rod to move farther in one direction than the other from the ailerons neutral position allowing differential deflection of the aileron to a great degree. The Tiger Moth’s predecessor, the DH.60 was the first aircraft with differential ailerons which de Havilland patented. In fact, as the control stick is moved toward full deflection, the down going aileron reaches full deflection at about two-thirds stick travel and then returns to neutral when the control stick is at full deflection. The desire was to reduce adverse yaw. The enduring result has been sloppy aileron response. Note the strengthened spar area on all sides where the drag strut is attached.

The horizontal stabilizer in a vertical position. The curved portion is the leading edge and is a laminated spar, the straight portion is the rear spar. Notice the thicker area of the leading edge by the two ribs farthest to the right(also at the other end). This is a strengthened area where two small bolt holes can be seen for attachment of a metal strut on the lower side of the stabilizer to the fuselage.

The horn balanced rudder of DHQ nears completion. Shaping that metal pipe is a heck of a lot easier than making the trailing edge out of wood. The two black arms are where the rudder pedal cables attach. The plastic tube houses a wire for the rear nav light.

The vertical stab is quite small. The lower portion is the trailing edge solid spar with its lowest part, the tailpost that the tailwheel attaches to, hidden behind the jacket. That lower portion bolts onto the aft fuselage along with a single bolt at the lower leading edge. A good place to check carefully on the walk around. The part number, serial number, date of manufacture and inspectors stamps can be seen at the bottom.

With original factory green paint still visible, both ailerons await restoration work. On the left at the leading edge of what is the spar is a long thin wooden strip. These wood strips are also on the elevators and rudder and control the airflow between the airframe and the associated flight control. They are covered with fabric like the rest of the flight control unit. None of them touch the mainframe at any time when the flight control is deflected. I am told that without these strips to reduce the gaps, there is a noticeable difference in flight control effectiveness.

The spruce spars have been routed into an I-beam type with the one on the left showing the root attachment end. There are a total of eight wing spars on a Tiger Moth. From left to right, the spars you are looking at are rear inner end, rear outer end and forward outer end. While the Tiger Moth has a tubular steel airframe as its basic structure, most of the rest of the structure is made of wood, using spruce and plywood along with a fabric covering which minimized the use of strategic materials.

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