Defiant Report
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.