Tuesday, July 20, 2010
Vne doesn’t necessarily mean what you think it means
KNOW YOUR AIRPLANE. Bill Cox has an in-depth understanding of how his Mooney handles speed.
When that work was complete and Curt LoPresti was happy with the result, Darren Tillman of Power Flow Systems in Daytona Beach had flown the airplane north for installation of a newly tuned exhaust, a fairly simple process that contributes as much as 10% more horsepower, but one that added a definite and easily quantifiable speed advantage, if at a higher fuel burn.
Before I had left California a few months earlier, I had made repeated round-trip runs above a timed, 5.43-statute-mile course between two piers near Los Angeles to establish the “before” speeds. Using a combination of average two-way runs and simply comparing the GPS groundspeeds, I knew exactly what the airplane would do at a high-cruise power setting with new platinum plugs installed, all vents closed, minimal winds aloft, full fuel and 350 pounds of people up front, wings and fuselage waxed smooth and leaning forward as far as possible. Now, the question was, how many knots would I realize from the speed mods?
I had done my share of research on what does and doesn’t work in search of more speed, and I knew some of my preparation would be wasted. I also knew for a fact that the LoPresti mods most definitely DO work and the Power Flow tuned exhaust DOES produce more power. I had tried to replicate the techniques used by the manufacturers to maximize cruise performance. No matter what you may think about production aircraft-cruise claims, most of the modern ones are as honest as a Kansas sunrise, provided you can duplicate the absolutely optimum conditions under which they’re flown. The reality is that sometimes you can’t, and therein lies the rub.
I had departed Daytona Beach for points west, climbed to 10,500 feet and set course for Shreveport, La., 685 nm distant. It turned out the Mooney was running about 12 knots quicker than it had before the mods. I did several 180-degree course reversals, and the numbers kept coming out 12 knots faster. LoPresti had reduced the airplane’s equivalent flat-plate area, and Power Flow had increased power sufficient to improve cruise by 12 knots.
Just over 4.3 hours later when I started downhill toward Shreveport’s Downtown Airport, I was flying more on instinct than instruments and was surprised to see the airspeed climb rapidly right to the redline. The air was clear and calm that morning, but my reaction was the same as anyone’s. I eased back on the power until speed had dropped well below the redline, put in slight back pressure to further reduce speed and thanked the Weather Channel that the sky had been smooth.
Twelve knots didn’t seem like much, but pointed downhill, the Mooney picked up more speed than I had ever experienced in my old friend. Fifteen hundred hours in the same airplane had tricked me into believing a standard descent would not push the ASI into dangerous territory. Indeed, it would not have if I had simply been awake.
Page 1 of 2