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HOW TO CHECK IF YOUR AIRPLANE IS STRAIGHT
So you have a few hours on your new EZ/Long/Defiant/etc., and
you are buzzing around within your limited 25 mile radius of home base - why not spend the
required hours you have left to take a close look at your airplane. Specifically, checking
the rigging, the 'straightness', if you will, of your brand new creation.
Assume you have built a "perfect" airplane, both wings are mounted to the fuselage at the correct incidence with z e r o relative difference, the canard is straight and at the correct incidence, and the two winglets are correct and exactly symmetrical relative to each other. This airplane should fly at cruise power, level flight, with the ball centered and both ailerons even and faired with the wing trailing edges. Depending on the CG and the speed, the elevator may also be perfectly faired with the canard tips. Since elevator position is a function of speed and, to a lesser degree, to CG position, I will limit this discussion primarily to rudder and ailerons.
How many of you have reached this goal? Not many I would bet. I know my own Long-EZ certainly is short of this state of perfection. How important is it to have a perfectly straight airplane? Difficult to say. Obviously, the straighter it is, the less control surface deflection there will be in high speed flight and the lower the drag and the greater the efficiency will be.
How do you check for a straight airplane? First of all, you will have to have a slip indicator, accurately installed. This can be a short length of yam stuck to the canopy on the aircraft centerline with a small piece of masking tape (this will only work on gliders and pushers!). Place it about 12" up from the leading edge of the plexiglass canopy. If you have a needle and ball, a turn coordinator and ball, or just a ball, it must be mounted in the panel, ball centered with the wings exactly level. Be sure this is correct before attempting to evaluate the airplane.
Now, before you conduct the following flight test, check to see that the two elevators are rigged perfectly, relative to each other. You will have to remove the canard to check this out. Simply eyeball along the elevator trailing edges. They should be in a straight line. If they are not, you must correct this before doing the flight testing. Elevators rigged incorrectly will roll the airplane. Also, stand behind your airplane looking at the center of the spinner. Raise or lower your head until your eyes can see along the top skin forward of the trailing edges of the wings. You don't want to be looking down on top of the wings or up at the bottom skins. You must be able to see the trailing edges and the top skins as a line. Now, without tilting your head, look from the right wing to the left. Any differences? Shouldn't be. If you can see more of the top of one wing, you have a relative incidence problem. Make a note as to which way it should roll and verify this in flight.
Take off and establish a high cruise in level flight, feet off the rudder pedals and ailerons perfectly centered (if you can't see your ailerons, take a passenger along to help you get them centered. Remember, your limitations allow you to carry a passenger if they are essential to the mission)! Now, look at the ball. Is it centered? Are the wings level? Probably not! Bummer, oh well, take comfort in knowing that almost everyone else is in the same boat! Keep the ailerons centered (visually verify this), and "step on the ball", that is, step on the rudder to center the ball. Step on the rudder opposite the direction of the yarn slip indicator. Lock your feet, ball centered (yarn centered), keep ailerons centered, and carefully observe the horizon and your DO (if you have one) to see if the airplane is flying a straight course over the ground or if it is slowly turning. If you have no turning rate and your wings are level with the horizon, you have one or both winglets attached to the wings slightly crooked. Even though you have a small error in your airplane, at least you know what is wrong and it can be corrected.
What if you are turning? Carefully null out the turn. Use just enough aileron in the proper direction to zero the turn. Verify this by watching for zero heading change on your DG or by observing a distant peak or other prominent object on the ground at the horizon. This takes a little time and patience but you can get it perfect if you try. With zero turn rate n@ the ball centered, check how much aileron and rudder deflection you have and in which direction. An assistant can be a great help here. Have them write down, for example, "right aileron up 3/16", left aileron down 3/16" and left rudder outboard 1/4", right rudder at zero." These dimensions can be quite accurately "eyeballed" with a little practice. If you doubt your passenger's ability to judge this, before you fly, have him or her sit in the passenger seat and you move the ailerons and rudders, using a scale and have them call out what they see. Now you know you have a relative wing incidence problem, as well as a relative winglet incidence problem.
Block the rudder out to whatever the eyeball estimate was by taping a small wood block to the inboard trailing edge of the winglet. When the rudder is released, it should close on this block and remain deflected outboard the estimated amount. Repeat the flight test and verify that the ball is centered with zero turn rate.
Now, in the case of a Long-EZ or Defiant, you will have to install shim washers on one of the outboard wing attach bolts such that the wing incidence is altered in the proper direction, i.e., in the example above of the right aileron trailing edge up, this wing would need to be shimmed by perhaps one thin washer (AN960816L) on the bottom outboard bolt. The left wing probably should be left alone until you look at the results of this change in flight.
Fly it and see if this was enough and if it was in the correct direction. Remember, do this kind of adjusting only in small increments. Use thin washers or thin shim stock, one piece at a time, starting with the wing that appeared to be off when you eyeballed the airplane from behind, whichever wing needs to be shimmed to raise the trailing edge. If one washer on one wing does not do it, add one on the other bolt on the opposite wing. Keep both wings even by eyeballing from behind - do not get one wing much different than the other. Continue using small increments until the airplane flies wings level, ball centered with zero turn rate.
You now have a straight but ugly airplane! Unfortunately, if you have already painted it, you will have some work to do. If it is still in primer, fair the fuel strakes to match the wing roots with dry micro (West System). To fair the rudder with the upper and lower winglet (on a Long-EZ), use a hacksaw blade to cut through the outboard skin along the rudder hinge line to the top and bottom of the winglet. If necessary, widened this saw cut as required and cut through the foam core to the inside of the inboard skins above the rudder and below the rudder. Check that you can now flex the trailing edges of the top and bottom of the winglet til it lines up with the rudder (still in its blocked outboard position). Now, reduce the amount the rudder is blocked out by approximately 10%, fill the saw cuts with micro and force the top and bottom outboard to exactly match to the rudder. Clamp them in this position and allow to cure. Layup a 2-ply BID repair over the saw cuts and fill, sand and finish. Install a permanent block, full span along the inboard trailing edge of the winglet to block the rudder in its proper faired position. You can use wood or a piece of pre-cured glass here.
Your airplane should now fly straight and the winglet repair will not be detectable.
This works great on a Long-EZ, but what about a VariEze? Since it is not possible to adjust the incidence of the wings of a completed VariEze, you will have to do surgery to the TOP of whichever wing it takes to correct the tendency to roll. If it rolls left (ailerons centered), you will have to slit the top skin of the right wing, outboard of the aileron along the aileron hinge line and bend this trailing edge up as described for Long-EZ winglets/rudders. If you have to do this to your VariEze, call me at RAF and let's discuss it before you do it.
Well, I hope this is helpful and not too confusing. I'd be happy to discuss this with any builders or flyers who may find themselves have to make this kind of correction.