Trim Tab system on N1852Z I started with the plans design, but I changed that out pretty quickly. I did use the trim tab size per plans.
Bottom line - I have a fixed but ground adjustable trim tab on the elevator. It is set up of 1 person cg and flight. I use a bungee cord in the cockpit for when I need extra help with forward push on the stick when I have two people on board. Pretty simple. Thanks, Rob Schmitt N1852Z -----Original Message----- From: KRnet [mailto:krnet-boun...@list.krnet.org] On Behalf Of Sid Wood via KRnet Sent: Wednesday, January 24, 2018 12:18 PM To: krnet@list.krnet.org Cc: Sid Wood <smw...@md.metrocast.net> Subject: Re: KR> Trim I used the Nyrod trim tab system per the plans. On the take-off run on my first flight the system failed and put in full nose up trim just as I nudged the stick to lift the nose. Estimate the speed was about 65 knots at that point. The aircraft immediately pitched vertical. I over corrected with forward stick and the aircraft pitched straight down. I recovered from that dive pulling 2.6 g. Ground observers said I cleared the ground by about six feet. I had to hold about ten pounds forward stick to maintain steady climb and level flight. Trim tab control in the cockpit had zero effect on the trim. After landing found the Nyrod sheath had broken at the forward part of the horizontal stabilizer. The slightest aft stick movement would move the trim tab to full nose up and would stay there. Fixed that with two Adel clamps at the aft end of the sheath. Moral of the story: Bowden cable sheath must be firmly attached at both ends of the sheath. Sid Wood Tri-gear KR-2 N6242 California, MD, USA ---------------------------------------------------------------------------- I would avoid trying to control a trim tab with a single bowden cable. I did that on my SuperCub Clone per the Wag Aero plans. When I trimmed nose down, the cable was pushing the trim tab up. The bowden cable started flexing, then broke, leaving the trim tab flapping free in the breeze which brought on quite a case of tale flutter. Slowing to minimum controllable air speed did not moderate the flutter, which was quite violent. The Cub has a braced tail structure, but even with the bracing, the tips of the elevator were moving roughly 6" vertically while the trim tab was in flutter. You could both feel and see the shock from each flutter cycle propagate through the air frame and out the wings. You could quite literally see the shock waves moving out the length of the wing from the violent shaking in the tail. Pulling in full flaps changed the air flow over the tail enough to moderate the flutter enough that I was able to make my way into the nearest airport where I mad! e temporary repairs. After I returned home, I stripped the tail and installed a Ray Allen trim servo. I also found a few cracks in the elevator, likely a result of having flown for several minutes with the trim tab in full blown flutter. I know of numerous other failures of a single bowden cable on the trim tab with similar results. I recommend avoiding that type of configuration. -Jeff Scott Cherokee Village, AR ------------------------------ >I?m adding an autopilot and will need trim availability. I?m thinking >Bowden cable and trim lever. A Bowden cable (manual push-pull arrangement) is exactly what's called out in the plans. This was sold by Rand Robinson in the form of a "ny-rod" plastic pushrod in a plastic housing, straight from your local RC model store. This is a somewhat low-friction design, but it quite springy due to the low stiffness of the housing and flexible pushrod. This leads to a very springy trim tab, which as Jeff pointed out, is not a good thing. And after 20-30 years, the housing cracks and you have nothing at all! I second the Ray Allen trim system. It's quite stiff, by comparison. There are some details on my trim tab installation at http://www.n56ml.com/trimtab/index.html. This setup easily handles all of the CG changes that I ever need on N56ML, with probably half the range unused. I'm happy to keep that in my back pocket though, as you never know when landing without an elevator or something similar will make me thankful that I have the capability. One point to make on the Ray Allen system...use shielded cable to run the wires from the switch and display units back to the servo, and ground one end of the shield, and avoid close proximity to the radio and transponder antenna cables. When keying the mic, the trim position display LEDs goes bright and dim, and eventually it fries them! My middle three LEDs are cooked, and it's because those three LEDs have been lit while the transmitter was broadcasting. The same thing happened on N891JF, and I've heard several other folks have had the same problem. Keep those wires shielded and isolated from RFI (radio frequency interference). It's not a big deal to not know exactly where the trim tab is anyway, as it's so easy to overcome by stick pressure anyway. It is kind of nice to set it neutral on takeoff, as the plane will takeoff by itself with no stick pressure from the pilot. That's good advice that Troy Petteway gave me for my first flight...."just keep it centered on the runway and when it's ready to fly it will lift off and climb. You shouldn't have to pull back on the stick to make it climb...it will do that all by itself". See http://www.n56ml.com/trimtab/index.html .... Mark Langford, Harvest, AL ML "at" N56ML.com www.N56ML.com _______________________________________________ Search the KRnet Archives at https://www.mail-archive.com/krnet@list.krnet.org/. Please see LIST RULES and KRnet info at http://www.krnet.org/info.html. see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change options. To UNsubscribe from KRnet, send a message to krnet-le...@list.krnet.org _______________________________________________ Search the KRnet Archives at https://www.mail-archive.com/krnet@list.krnet.org/. 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