Horizontal Stabilizer/Elevator

It is no secret that the KR2 is an extremely responsive airplane. Roll sensitivity is snappy, but manageable. It is a bit more of a stretch to say the same of the pitch sensitivity. Pitch sensitivity, especially near the aft 2 inches of the CG range can be difficult to manage. To understand this, we first have to examine the original KR2 plans. There is a common understanding among the KR community that while the KR plans published by RR Engineering contained a well-designed aircraft, they were extremely preliminary without much refinement. In retrospect, we find that the sizing of the horizontal stabilizer and elevator suit a plane that is much longer than the stock KR2. This is what pushed some builders to stretch their KR plans. Roy Marsh built N211RM which became the prototype for the KR2S (Here’s the link to the KRnet post: http://www.krnet.org/krs/rmarsh/ ). Since we’re ready to start the fiberglass and carbon fiber work on our stock KR2, we have to find another way to tame the pitch sensitivity on the plane — we can do this by adjusting the horizontal stabilizer and elevator to better suit the length of the stock KR (we’re also going to build it in the front of the cg range, but more on that later). We’re certainly not the first to do this, so I take no credit here. Here’s the link to Troy Petteway’s plane, who (to my knowledge) first tried this ( http://www.n56ml.com/troy/ ). Our build guide/log (call it what you would like) will show how we modified our horizontal stabilizer and elevator size as well as some other modifications we made to the assembly.

Elevator Hinges

One of the changes that we decided to make with the plans was the hinges for the elevator. The plans call for heavy, draggy brackets (three of them to be exact). Some countries have actually mandated that builders include 5 brackets instead of the three that the plans call for in fears of the three wearing out too fast. I don’t know if thats true, but we figured we might as well explore other options. Rather than go with the brackets, we decided to try a method that was proposed (and I believe he actually used them) by Dr. Dean Collette back in 1999. What Dr. Collette did was exchange the three brackets for seven rod end bearings. These are not only much lighter, but also stronger, less prone to wear (since the load is spread out over all seven), and more forgiving when it comes to keeping alignment during the installation. Here’s the link the Dr. Collette’s write-up of the design (http://www.n56ml.com/dean_hinge/) which actually includes a list of all the hardware necessary. I should note that a lot of this content is hosted on Mark Langford’s website. He’s the resident KR guru, so if you’re building a KR, definitely check his build guide out as well. Anyway, we ordered everything for the hinges on Wicks. We had to search for the right part numbers, since not everything was the same on Dr. Collette’s list, so I’ll also include our list here with quantities:

• MW-3 , Female Rod End Bearings (7x)
• AN42B-10 , Eyebolt AN42B:
  3/16″ Shank, 3/16″ Hole, 1032 Threads (7x)
• AN3-7 , Bolt AN3 -3/16″ (1032) (7x)
• AN3-11A , Bolt AN3 – 3/16″ (1032) (7x)
• AN310-3 , Castellated Nut (AN310) (7x)
• MS21044C3, Fiberlock Nut Stainless (MS21044) (7x)
• AN970-3, AN970 Flat Washer (28x)
• AN960-10, AN960 Washer (1 pack (25 washers))

Here’s what the rod end bearing will look like. It doesn’t look like much compared to the brackets that the plans call for, but when you put seven of these together there’s no comparison.

We used a caliper to measure the center of the rod end bearing and eye bolt, then divided them in half. We added these values together to find the offset necessary for the holes on the opposite spar (whether you use the horizontal stabilizer or elevator spar as the one where you drill the offset holes doesn’t really matter). You can’t drill the hole at the same point on both sides of the assembly for obvious reasons. Simple, but worth noting if it could save someone the dismay of realizing they drilled the wrong holes.

We marked the center of the spars at each end, as well as in the center where there isn’t any taper. Then, we took a long straight-edge and drew a pencil line down the center of the spar. We were really happy that all three center points were perfectly on the line, so I guess the old table saw that my dad used back in the day didn’t do too bad a job after all. This line will give us a plane on which to space out the hinge points.

These aren’t the actual spars. We ripped a couple of pieces of old rough cut that we had to test out the spacing of the holes. We figured that if we could get three hinge assemblies spaced correctly, then we could do the same with all seven. we found the center points of the mock spars, just like the real ones, and drilled the holes on one of them. Then, we marked the other board with holes offset to the first by the value we measured with the caliper and drilled those holes. What we found is that the drill bit we were using must have been slightly bent because it wanted to wander. Not much. Then again, any error that can be corrected is too much. We’ll get a new bit before we drill the holes in the spar.