KNOW YOUR AIRPLANE. Bill Cox has an in-depth understanding of how his Mooney handles speed.
I was flying home to California from Florida in my Mooney Executive a few years ago following what amounted to a medium makeover of the airplane’s aerodynamic drag signature. The Mooney had spent several weeks at LoPresti Speed Merchants in Vero Beach getting a new cowling, installation of a ram air system, a new spinner, a one-piece windshield, gap seals, flap-hinge fairings and a variety of other drag reductions.
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.
Fast-forward to this year’s Sun ’n Fun Air Show in Lakeland, Fla. I recounted this experience to Tom Bowen, COO of Lancair International in Redmond, Ore. Lancair knows a thing or three about high-performance flying, since their Lancair IV has become, pretty consistently, the fastest, piston-powered homebuilt above the planet for 20 years. Tom Bowen joined Lancair last year after stints as VP of engineering for Columbia, Swearingen and Mooney.
“Of all the V speeds, Vne is perhaps the most intimidating,” says Bowen, “and it should be. ‘Velocity—never exceed.’ What could be more telegraphic and inviolate? Don’t exceed this speed—period. No exceptions.”
Question is, what does Vne really mean and how is it derived? Bowen explained that engineers and test pilots determine Vne as a function of Vd, maximum dive speed. When an aircraft designer comes up with a new design, he develops a mathematical model of the airplane’s maximum speed capability to the point of flutter. He advises the test pilot of that probable number, after which it’s up to the pilot to determine the exact speed.
Test pilots deserve every penny they’re paid. Their job is to push the airplane right up to the very limits of flutter, five knots at a time, but not actually into it. This is a job for the VERY astute aviator, a pilot with years of experience in a variety of airplanes who can sense when the airplane is on the verge of flutter, control reversal or dynamic divergence, any of which can tear the airframe apart.
For this reason, Vd tests are conducted only in extremely smooth air, typically in CAVU weather when there’s a low risk of encountering a mongrel up- or downdraft. The pilot controls the onset of flutter with more or less G.
Like many of you, I’ve seen video of high-speed testing on a V-tail Bonanza, where flutter was deliberately induced in a steep turn. The results are beyond scary. The Bonanza’s ruddervators appear to blur in real time and shudder violently in slow motion.
Control surfaces obviously are the most critical at high speed. If they’re not perfectly balanced and rigged, they’re more liable to become catastrophically divergent at very high speeds, at which point their time to failure may be measured in milliseconds.
“Once a test pilot determines the maximum dive speed without flutter,” says Bowen, “Vne is set at 90% of Vd (or Vdf, the maximum demonstrated dive speed). Other factors may dictate a slower Vne. The important point to remember is that flight at Vne is only assumed to be safe in presumably smooth air with a perfectly rigged, well-maintained airplane and an airspeed indicator that’s dead-on accurate. That means for the average general aviation pilot flying a typical aircraft, perhaps slightly out of rig in a questionably calm sky, there are no guarantees that redline will be as posted.”
In other words, a wise pilot will avoid operating anywhere near redline. Accordingly, since I sometimes do make the same mistakes two or three times, I had a set of Precise Flight SpeedBrakes installed shortly after the experience outside Shreveport.
Operationally, the aerodynamic spoilers can be deployed at any airspeed right up to and even past redline. If you encounter flutter with speed brakes installed and do nothing other than extend the boards, there’s a good chance you’ll defeat the problem and land the airplane without further damage.
My Mooney still isn’t the fastest of its type, and I doubt it ever will be. At least its pilot now has a better understanding of the need for—and the possible consequences of—speed.