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I've been down in Wichita for the last week, doing testing of the Global Vision Flight Deck (Bombardier site) (images) avionics update to the Bombardier Global Express. We've been working long hours, six days a week, but finally took today off to let everyone catch their breath. I did laundry and relaxed. This afternoon I finally found some time to take another look at the wheel pants OFF cruise performance data.

I've been flying with wheel pants OFF for several weeks, as there is frequently some snow on the taxiways and ramp. I don't want to risk having snow collect in the wheel pants, which may lead to frozen brakes, as the warm brakes can melt some of the snow, which will then freeze in flight. I was interested to see how the cruise performance compared to what it is with wheel pants ON, so I gathered some cruise data on flights 103, 104 and 107. I also had some data from last winter, on flight 51, before the aircraft was painted.

I analyzed all the data last weekend, and since then I have been puzzling over the fact that the data from flights 51 and 107 sits about 3 kt slower than the data from flights 103 and 104. I finally realized a few days ago that the flight 51 data was from before the aircraft was painted, and it was fairly dirty then, as I had never washed it. I have done a pretty good job keeping it clean of grit and bugs since it was painted. So, the three knot speed difference on that flight may represent the effect of dirt and bugs.

One other possibility was that I now had a small static system leak, which would make the airspeed read a bit too high. I did a static system leak check after installing the autopilot, and there were no leaks at that time, but perhaps a new leak had developed. To close off that issue, I did a static system leak check before flight 107, and it was completely leak-free.

Or, perhaps the engine power has increased as it has broken in. This is a possible explanation, but the power would have to increase by 6% to explain the observed speed increase. I really doubt it has increased this much between 50 and 100 hours time on the engine - I would expect the big changes in power to occur in the first ten hours.

I checked back to look at data with wheel pants ON, before and after paint. I don't have a lot of cruise performance data with wheel pants, before paint, and the available data isn't the best quality. But it shows speeds about five knots slower than after paint.

I note that the CAFE Foundation did a before and after test to check the speed increase due to waxing their Mooney. They found a 2.8 mph speed increase from a wax job, so it is quite plausible that the speed of my aircraft increased 3 kt from a paint job.


The fact that three of the four data points on flight 107 sit several knots below the line from flights 103 and 104 is puzzling. The engine was running a bit rough during the lowest power point on flight 107, with the EGT on #4 cylinder gradually climbing during the test point. I think one of the plugs on that cylinder was perhaps not firing properly, and that would have reduced the power, which could explain why that point is off the line. But the two middle points on that flight have no obvious reason why they shouldn't match the others. The engine was running smoothly, and the data shows nothing obviously wrong. I'll clean the spark plugs, and then do some more testing in that range of conditions.


Here is the data for wheel pants OFF, all corrected to sea level, standard temperature, 1800 lb weight. Conventional theory says that a plot of Power * TAS vs TAS^4 should make a straight line. You can see that the four points on flight 51, without paint, and three of the four points from flight 107 sit a bit to the left of the line through the rest of the points.


Here we see Power * TAS vs TAS^4, at sea level, standard day conditions from both wheel pants OFF and wheel pants ON.


Here we see Power vs TAS, at sea level, standard day conditions with both wheel pants OFF and wheel pants ON.

Note that the actual testing was not done at sea level, standard day, 1800 lb weight. The data analysis technique corrects each test point to standard conditions, keeping the same angle of attack. Some of the testing was done at fairly light weight, which leads to a low angle of attack. When the points are corrected to 1800 lb, this implies that the speed would be higher to achieve the same angle of attack. It would take a large amount of power to fly at this high speed at 1800 lb at sea level, which is why you see the one point at 280 hp and 195 kt. This does not imply that my aircraft is anywhere close to being capable of achieving that speed, as it only has 200 hp. This analysis simply says that the aircraft could achieve 195 kt without wheel pants, if it had 280 hp.



And finally, here we see Power vs TAS, at 8000 ft, standard day conditions with both wheel pants OFF and wheel pants ON.

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