Wednesday, February 8, 2012

Tiger Takes to the Air

By B. Reid

After a serious refurbishment involving a complete teardown of the Vintage Wings Tiger Moth due to corrosion issues, our swept wing biplane has returned to the skies of eastern Ontario and western Quebec. And it is about time after being grounded for almost 7 months.

Our most experienced Tiger Moth pilot, Dave Hadfield with flying time in both the A and C models was on hand for her first flight several days before her scheduled participation in the 22nd Annual Montebello Challenger Ultralight Rendezvous 30 miles to the east not to mention the very local upcoming 23rd annual Mo
s fly-in, both of which are on the frozen Ottawa River.

With a nice winter high pressure system hovering overhead and moderate temperatures, the conditions were perfect for a test flight. The Tiger Moth has minimal crosswind capability especially on pavement and unfortunately, the Gatineau Airport has frequent crosswinds but the winds were light for several days in a row.

Overall, things went quite well. When a biplane is rigged, it is not unusual for some minor re-rigging to be subsequently required and this is a work in progress. With hands off the controls, excessive yaw was noted necessitating an adjustment on the fixed rudder tab. Yet a certain amount of aileron was still required to maintain wings lever requiring a flying wire adjustment to change the wing incidence.

A large number of Vintage Wings maintenance people participated in this project. A special thanks has to go out to all of them for their dedication and hard work in getting this aircraft airborne again. It is planned to visit many former BCATP sites this summer on the Eastern Yellow Wings tour.

While most Tiger Moths built by de Havilland Canada at the Downsview Airport in Toronto were the C model designed for winter flying, over two hundred of the A model aircraft were built here as well, many of them with canopies. They were also manufactured in New Zealand, Australia, Norway, Portugal, Sweden and of course in England. A large number of the British built Tigers were built in mass production by Morris Motors.

Rigging of the wings is done while in the level position. Note the two plumb bobs, one hanging from each leading edge which, when measured on the ground should equal 22 inches of stagger. This is the amount that the upper wing was moved forward from the earlier DH.60 version to comply with Air Ministry requirements in 1931 to allow better egress capability from the aircraft for parachute-equipped crew. This led to the sweepback that you can see to keep the C of G within limits(9° for the lower wings and 11° for the upper wings, which were the only ones adjusted a second time). This led to the increased dihedral on the lower wings for better wingtip clearance as the sweepback had moved the wingtips farther aft. If the stagger is not correct, then loosen the struts surrounding the cockpit and adjust the whole upper unit forward or aft. The really sharp-eyed may have noticed that the walkways are made of cork, which is the way Canadian Tiger Moths left the factory.

A not particularly clear diagram of some of the Tiger Moth rigging. Some points noted are a 4° incidence for the upper wing. Lower wing dihedral is greater than the upper wing. The elevator deflects upward more than downward but interestingly it is at slightly different amounts than for the A model. The original DH.60 predecessor to the Tiger Moth had a large amount of aileron differential, which was patented by de Havilland, designed to reduce adverse yaw. As you can see, max upward aileron deflection is 10° while max downward deflection is only 1.5°. Uniquely, the downgoing aileron deflects downward and then returns toward neutral after about 3/4 stick deflection. However, your leg is always in the way to get much more than 3/4 stick input anyways. A 1/4 inch droop on both ailerons with stick neutral is by design, so don't call maintenance if you notice this on the walkaround.

On a nice -5°C day with engine running and prop bent by the camera shot, Tiger Moth 4947 prepares for flight. The method of priming prior to hand propping is a little unusual. In coordination with the pilot for proper setting of brakes, mag switches, throttle, and fuel selector, the ground crewman pulls a small cable at the engine compartment for several seconds which activates a lever on the carburetor to flood it, which then overflows into the intake manifold.  You can hear the fuel trickling down. The excess fuel drains out through an overflow in the intake manifold onto the ground. The prop is the pulled through 5 times to suck the fuel into all the cylinders. Then the prop is swung for start(which no doubt starts successfully on the first try).

Looking at the Tiger Moth from the Cornell chase plane during a formation flight to the Montebello Ultralight Fly-in. This picture gives a good view of the horizontal stabilizer strut along with the aileron rods. The Moth series of aircraft were the first to use differential ailerons to reduce adverse yaw. It was patented by Geoffrey de Havilland. We don’t have the DH painted a different colour on the wheel covers but then again, I haven’t found any WWII pictures that has it painted that way. Please let us know if it was done this way.

Nice bottom view of the Tiger Moth from its WWII replacement, the Cornell. A trim tab can be seen on each elevator. Only the Canadian versions had this, replacing an adjustable spring loaded elevator trim system on other versions. The Canadian version does not have a type certificate in the U.S. but the A model does.

Descending into Montebello. Our small registration, CF-ANN(formerly C-FIME) requires special approval from Transport Canada which has to be advised if we change the paint scheme in any significant way. The pitot tube is on the forward interplane strut for the right wings. It looks like two pitot tubes but one of the tubes is for the static with several holes on the side of the tube.

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