Tuesday, July 20, 2010
Vne doesn’t necessarily mean what you think it means
“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.
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