At 11:34 AM 1/26/2014, you wrote: >Not sure if there is some adverse CG consideration in using the >Diehl wing skins on a KR-2 such as on my airplane. My KR-2 seemed >to have that aft CG. I don't have any definitive data, but I have >had to move my 2180 VW 2-inches forward from the plans location to >get a more stable CG. Do the Diehl Wing Skins tend to move the design CG aft? >Sid Wood ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
To my knowledge, the Diehl skins are simply a less labor intensive version of the original stock, plans built, RAF48 airfoil and in no way change the CG over the stock built wing. The reason the original KR design tends to come out tail heavy is an error in design, using the original VW engine. The 2S plans simply adds 2 inches forward of the spar in the boat. If Sid were to replace his VW with an 0-200, I'm guessing he would not want to move it forward at all. Sid, try adding 80 pounds to your engine weight on the computer program and see what happens to the CG. I have no idea how the CL on the RAF48 compares to the CL on the new airfoil shape and what effect the difference, if any, would have on stability. On the horizontal tail, my KR is stretched 24 inches over stock ( all behind the cockpit ), 0-200 on a VW mount with 1 inch spacers added, wing at 3 1/2 degrees with 3 degrees washout, and horizontal at 0 degrees. I wish I had more horizontal stabilizer, set to maybe 1/2 degree negative AOA (wild guess here) and maybe a bit more rudder. If I set my elevator trim to zero and don't touch until cruise flight, I'll have to hold a fair amount of back pressure on the stick. Letting go of the stick at that point (150 mph ) will produce a near violent nose down pitch. On my KR, with me at 200 pounds and 150 pounds of fuel, all centered within 10 inches behind the forward spar, (1100 pounds gross) my calculated CG is centered on the plans CG range. Given that, it is the aerodynamic effects, not weight location that creates this handling characteristic. Starting from scratch, I'd go with the KR2S plans boat, new wing airfoil with all fuel in the wing like my setup, Langford's or Jeff Scott's sized horizontal and rudder, and wait until finished to build motor mount placing the CG , with a reasonably sized pilot and full fuel, in the forward 2 or 3 inches of the design CG range. I'd probably stick with the 3 1/2 degrees AOA on the wing inboard with a 3 degree washout. I can fly through a stall with my feet on the floor, using only aileron to keep wings level. I like that. As to always flying "full throttle", I'm guessing the Wright Brothers always flew full throttle with their whopping 25 hp engine. I, on the other hand with 100 hp, don't need full throttle to obtain a reasonable , fuel conserving , easy on the engine speed. If I had a variable pitch prop, I'd probably fly "wide open throttle" above 5000 feet MSL and use propeller pitch to control RPM. With a fixed pitch prop we have to compromise given available horsepower. Flat enough pitch to give us the power for comfortable take off and climb may let the engine over speed at altitude. Pitched for a good RPM at cruise with wide open throttle, it may not give us the power for takeoff and restrict the climb rate. No single design can do all things well as the Navy and Air Force learn each time they strive for a "one size fits all" airplane. Each of our KR's are a compromise to one extent or another. We just need to build our own KR to meet the characteristics we want it to have. On a lighter note, for those that bothered to read this far, I was startled awake at 5:00 AM this morning with the police shining a flashlight in my face. (no Jone's , I didn't fall asleep at a brothel) It seems the front door was closed but not latched and the wind blew it open during the night. The man delivering the morning paper called the police. That is NOT the way to start your day. Sallie has been laughing her A%* off all day, every time she thinks about it. Larry Flesner
