CP58, Page 2, (January, 1989)
There are now at least 15
Defiants flying that we know of. All of the builder/pilots who have reported to us are
pleased with their Defiants, however one problem in particular has occurred to many of
these airplanes. During taxi tests and high speed taxi runs, divergent nose wheel steering
has occurred, sometimes getting so bad that the airplane has left the runway. John
Steichen had just such a problem which caused him to leave the runway at around 70 KIAS.
He ended up in a drainage ditch with a broken prop, broken rhino rudder and a failed nose
gear. A devastating situation for someone who has worked so hard to get to this point. Our
discussion and suggestions to John are printed here in the hope that they may benefit
other Defiant builders approaching this stage.
Burt's Defiant, N78RA, has never had a divergent steering problem
but it certainly did have a few serious shimmy problems early on in the program. These
problems were completely cured by rebuilding the nose gear assembly (done by Mooney
at Kerrville, TX), and by the addition of an efficient hydraulic shimmy damper (no damper
was installed initially). No further nose wheel shimmy or steering problems have occurred
in something over 1100 hours of operation with numerous pilots.
This problem has been aggravated by a very confusing drawing put
out by Mooney showing the axle centerline AHEAD of the steering pivot!! The dynamics of a
trailing link nose wheel (such as the Mooney nose wheel) are such that, if the axle
centerline is on or forward of a line extended down through the center
of the steering pivot, the nose wheel will be neutrally stable to unstable and may diverge
or dart left or right, uncommanded. If the axle centerline is aft of this line,
will be prone to shimmy. Shimmy is similar to flutter and can be instantly
destructive. Uncommanded, left or right divergence can be equally destructive by driving
you off the runway across a ditch. So what to do? Set the axle centerline aft of
the line through the center of the steering pivot by 0.6 to 0.8 inches. Control any
tendency to shimmy by installing an hydraulic shimmy damper shown in the plans. Roger Rupp from Alaska had similar problems to John
Steichen's. Rodie Rodewald, also, had divergent steering problems when his nose wheel was
adjusted per the Mooney drawing. Some others who have also had varying degrees of this
problem:. Dr. Yost of Alabama, Don Foreman in England, Dr. George Best in Phoenix,
Arizona. Some saw heavy steering divergence at low speed. Scary, but not destructive,
probably caused by having the axle centerline too far forward. Some did not see this
divergence until traveling much faster, caused almost certainly by the nose wheel swinging
forward of the steering pivot line as the canard started lifting the weight off the nose
wheel.
The nose wheel geometry should be set with ZERO weight on the
nose wheel, to 0.6/0.9 inches aft of the steering pivot line using the Mooney spacer
washer (SB #20-202-3, 1-1/4 x 1.010 ID x .120 thick, 4130 N steel shed in zinc chromate).
John Steichen suggests that it may even take two of these washers, at least it did on his
4 donut Mooney nose gear. (see sketch, page 15)
Another factor that may influence the likelihood of shimmy,
and/or divergence, is rotation speed. If your Defiant rotates at 60 to 65 KIAS, you may
not see any shimmy or steering divergence simply because you have not reached the speed at
which either of these phenomenon may have occurred. Some Defiants do not rotate until
indicating 80 Kts or more. This may be caused by one or more of the following problems: Forward
CG. or too far forward CG. Two large people in the front, nothing in he rear seat, no
baggage and minimal fuel will put your CO forward. Fill her up with gas, and the CG will
march rearward. Main gear axle centerline too far aft will cause the Defiant to act
as though it has a forward CG condition while rolling on the runway but may be
normal to aft CG once airborne. Having the main gear too far aft will really extend the
take-off roll and will require a higher than normal rotation speed. This is tough to fix.
Take great care while building not to end up with this problem. Do not be tempted to move
the axle centerline forward either, because this will result in a real problem when
loading baggage. The Defiant may tip over on its winglets and could break the prop. Put
the main gear axles where the plans call for them to be. Ground attitude. A nose
down ground attitude puts the canard and the wings at a negative angle of attack during
the take-off roll and can greatly increase the rotation speed causing you to use a lot
more runway and putting your nose wheel at risk due to excessive speed. Even a correctly
set up nose gear may shimmy if it hits ruts or bad bumps at too high a speed. If
your Defiant sits nose low with both engines installed and some fuel, perhaps 1/3 on each
side, you have a problem. Burt's prototype, N78RA, with the above conditions sits with the
flat bottom (from forward firewall aft 3 feet or so) slightly nose up. Check with a two
foot level held bubble level. (see sketch, page 15)
If your plane is not close to this angle, you may have to change
the ground attitude. The easiest way to do this is to cut some amount off the main gear
strut. You will have to determine how much by calculation after doing the above check.
NOTE: You should push the airplane forward on a level ramp at least 100 feet to allow the
main gear to relax to its natural position (this is driven by wheel toe-in/toe-out. Your
wheels should not toeout. They should be zero toe-in to a maximum of a total
of 1/20 or 1140 each wheel). If it sits nose low at this point, jack the nose wheel up or
shim it with scraps of lumber until you have the required nose up attitude. Now measure
the stack of lumber scraps under the nose tire - this is the amount you will have to cut
from each main gear strut. The only negative side to cutting the main gear strut is, of
course, less rear prop tip clearance. This is not too much of a problem with a Defiant
since the rear prop has much more tip clearance than, say, a Long-EZ. The added work of
remounting the axles will be well worth the effort. While you are at it, it is best to
mount the brake caliper on the forward side of each strut. Better ground clearance
with a flat tire and a better brake line run inboard of the strut which gives more
flexibility to the brakeline to allow the caliper to move laterally with brake pad wear.
One more important point - all Defiants should be using low profile tires, Goodyear
15x600x6. If you use standard 600x6 tires, you will set your axles approximately 1-1/2 too
high.
There is one other way you can help lower your rotation speed. It
consists of reflexing both ailerons trailing edge up by adjusting the fore/aft aileron
pushrods in the wing roots equally to raise the trailing edges of each aileron up
to a maximum of 3/16" (measured at the outboard tip of each aileron relative to the
wing trailing edge). This should be done with caution and in small increments with careful
flight testing after each adjustment. The effect of reflexing the ailerons is a nose up
trim change. This will lower rotation speed but, and this is important, it will
also allow you to command a higher angle of attack and this may cut down on the stall
margin of your main wing. If at any time you see uncommanded wing rock, even if you can
null it out with a stab of the aileron, DO NOT reflex the ailerons any more. Our
experience has also shown a small aileron authority increase with reflexed ailerons. This
is a test worth trying but we believe you should have some experience in your Defiant
first. You should approach this kind of testing in a professional manner, make small
changes each time. Flight test each change and document the results. Use good judgement
and quit if you see any roll authority degradation or any uncommanded wing rock
(caused by asymmetric-flow separation on each wing, probably outboard of the ailerons). A
few tufts (3" long pieces of yarn taped on the wing with masking tape) on top of each
wing will give you visual warning of impending wing rock before it actually occurs.