NetHeads, Y'all probably remember me saying that I accidently found myself doing 230 mph (249 TAS) a couple of weeks ago, and it really sneaked up on me. I just didn't think I was going that fast. Well, I started wondering about that, and started comparing the GPS to the airspeed indicator's speed, and found a pretty big difference on the top end. I had formerly proven that my ASI was within about 1.5 mph of actual GPS speed under standard conditions, so it was time to do some checking. Since the EIS and ASI agreed within 1 mph, it wasn't an ASI problem, but just for kicks I checked it out, and it's still within 1.5 mph of correct from one end of the range to the other (see http://home.hiwaay.net/~langford/airspeed_calibration/ for how I do that). It reads 1.5 mph slow.
So it was apparent that I had a static port problem. My static system is a small diameter hole on each side of the fuselage about a foot from the firewall, using off-the-shelf ports with barb fittings from Wicks (http://www.wicksaircraft.com/catalog/product_detail.php/pid=4294~subid=10166/index.html). The two barb fittings connect via 1/4" tubing to a tee that goes on to all the instruments that need it. Last night I discovered that one of the two ports had become disconnected from the tee, so I reconnected it and assumed it was fixed. That explained the higher airspeed readings on the ASI, because I've always known there was a bit of a vacuum on the cabin due to the openings in the tail which allow the elevator to travel vertically. The fact that my cabin air and cabin heat ducts work so well tells me it's usually a good thing to have. OK, so I knew it sucked, but I wondered how much? Having just calibrated the ASI, I could make a pretty good guess based on the discrepancy of the readings before and after I reconnected the outside static ports. But I remembered that I still had Oscar Zuniga's differential pressure guage, and it'd be interesting to see what the difference was in terms of hard numbers. So I plumbed into the static system with one end of the pressure gauge and put a static probe on the other, and located it back in the tail right next to the aft openings in the turtledeck. So I went for a flight, closed both air vents and the cabin heat, and recorded the numbers. The numbers were higher than I would have thought. As you'd expect, the curve is fairly linear, ranging from 3" of water at 100 mph to 6" at 160 mph. At 140 mph cruise, opening either of the outside air vents yields a decrease of about .3" of water (cancelling some of the low pressure), and the cabin heat is good for a .4" decrease. Chopping the throttle to idle drops it by about .5", even with the same airspeed, so propwash is a real factor. I disconnected the static source and used cabin air and the gauge dropped to zero, proving that the pressure in the cabin is the same as close proximity to the exit in the tail. After reconnecting to the static, I checked the area between the canopy and longerons with the static probe, and got a 1.2" increase (lower pressure) on the right side, and 2.2" increase on the left side. The difference is easily explained by the propwash, which is "shadowed" by the left side of the canopy, creating an even lower pressure area. It also reminds me that I need to seal that joint better. I originally had an EPDM seal in there, but over the years it has left, and I've neglected to replace it. And when flying the rear of the canopy lifts about a half inch, so that is a real source of drag on the airplane, especially with that big plume of air coming out of the that joint. What I really need it better canopy latches, but that would require that I build instead of fly, so that may not happen. So now I have some numbers to hang on the negative cabin pressure. Using the spreadsheet at http://home.hiwaay.net/~langford/airspeed_calibration/asi_cal.xls , I plugged in the 6" difference at 160 mph (using "Option 1" and "Option 2") to find an error of 33.5 mph caused by using cabin pressure as the static source! No wonder I didn't think I was going 230 mph. After this testing I did a north/south wide open run at 7500' and the TAS was 178 with an average GPS speed of 172, and since the winds were high and roughly at 340, this makes sense until I can find a calm day to test. And 178 is very close to what I'd expect from previous testing at 7500'. The moral of this story is to make sure your static system is isolated from inside the cabin, and don't think for a second that you can get away with porting your static source to the cabin. I suspect most KRs are like mine, and are somewhat open back there around the elevator horn. Another point to be made is to test your airspeed indicator and static system before first flight to make sure it's accurate, or at least to know what the correction factors are. My first ASI was off by 52%, right out of the box. And another takeaway point is to make sure you do some stalls on your first flight so you'll know for a fact what the INDICATED stall speed is so you'll have some idea of the landing speed (1.2x stall speed, many say). The reality is that the KR wing is so close to the ground that ground effect gives you an even lower stall speed for a cushion, but I didn't tell you that. I learned on my first KR flight that landing at the right speed is critical, unless you have a long runway and you can afford to just glide along until it quits flying, but that's not as easy as it sounds on gusty days either... Mark Langford N56ML "at" hiwaay.net website at http://www.N56ML.com --------------------------------------------------------
