Friday, January 13, 2012

Tiger Moth Gets Her Wings Back

By Heather Fleck

Over the last few months, our Tiger Moth has undergone quite a bit of restoration. Last summer, during the installation of a fuel transfer pump, a keen-eyed Aircraft Maintenance Engineer (AME) spotted some corrosion on the fuselage tubing. Further hole-punch tests revealed a few more soft spots, and the decision was made to ground the aircraft until appropriate repairs could be made. This was easier said than done; the repairs
required that the fuselage fabric be completely removed in order to provide welding access to the corroded parts.
Over the winter, the aircraft was disassembled and the fuselage fabric was removed, new steel tubing was spliced into place, and the frame was then recovered and repainted.
Last Saturday hard-working volunteers passed a major hurdle in the reassembly process: they attached the wings to the fuselage. Now, after months of being spread morosely around the hangar in parts, the Tiger Moth is beginning to look like a sprightly airplane again.
Simply bolting the wings to the fuselage by no means completes this stage of the reassembly process. The wings must now be rigged. Rigging
consists of adjusting various attachments points and wires to ensure that the aircraft flies straight and level and performs aerodynamically to published standards.
The first step when rigging an aircraft is to make sure that the aircraft is longitudinally and laterally level. To help accomplish this, tricks of the trade may be employed. A deflated tire can compensate for an uneven floor (level laterally), and the tail may be propped up (level longitudinally).
The centre section is rigged first. The objective is to have the centre section strut square to the longitudinal axis. This is done by adjusting the adjustment fork, which moves the top wing forward or aft.

Adjustment Fork on the Tiger Moth - it sits at the forward edge of the front cockpit.

Next is the wing stagger. The same stagger is desired on both the left and ride side. However, the catch is that the stagger changes subtly from wing root out to the wing strut.
Once the stagger has been adjusted you have to set up the dihedral, or the angle the wing makes with the aircraft fuselage (waterline). Now we get into the wonderful world of landing wires and flying wires, a topic that has admittedly always slightly confused the author of this article. Here is the straight and narrow of it: the landing wires support the lower wing tips upon landing, while the flying wires prevent the top wing tips from
flapping during flight. The landing wires pull the lower wing tips up, and the flying wires hold the upper wing tips down. On the ground,
therefore, the landing wires should be under more tension than the flying wires.
AME Paul Tremblay works on rigging the Tiger Moth. The flying wires run from bottom left to top right, and the landing wires run from top left to bottom right in the picture.

Once all the wires are adjusted, final adjustments are made to the angle of incidence (the angle the chord of the wings makes with the longitudinal axis of the aircraft). In the case of the Tiger Moth, the angle of incidence should be 4°.
The next step is a dual inspection of all flight control surfaces to make sure that every bolt is safetied and every cotter pin in place. Once the engine is remounted, a flight test is in order. This usually results in some more minor adjustments. The aircraft is then ready to start the 2012 flying season!

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