Bill Crawford’s Flightlab Blog

(Note: My monthly column in the Atlantic Flyer is printed across from the comic strip Smilin’ Jack)

I like being across from Smilin’ Jack every month. Although I admire Jack, I don’t envy him—one way or another, he’s always getting clobbered. He has the look of a perpetually annoyed Clark Gable. He’s a magnet for sabotage schemes and for women who tear up the scenery. He wears absurd hats, but he’s infinitely resilient.

Most everyone in Jack’s world has a sideshow name—and the ensemble makes no pretense to political correctness: Women are objects; foreigners are degenerates. Last month, the villainous Toemain the Great arranged to have acid poured over Jack’s parachute. Toemain also managed to weaken the cargo moorings on Jack’s freight plane, so that when Jack flew through Hurricane Pass the cement mixer he was carrying would break loose in the turbulence and “pound the ship to pieces.” I can’t guarantee, but if you look across the fold, that’s probably what’s going on.

Jack’s creator, Zack Mosley, was a pilot with some serious flight time, and friendships ranging from Jimmy Doolittle to Buzz Aldrin (the strip originally ran from 1933 until 1973). Mosley was great at rendering both airplanes and women’s legs. While he idealized the latter, he took obvious care to get the facts about aviation right. Last month, when the cement mixer broke free, he had Jack lecture Cindy, the Incendiary Blonde—who had stowed away and was holding a revolver to Jack’s head—that the rampaging mixer was “shifting the center of gravity,” thus “unbalancing the ship.” Think about it. Jack lives a life of staggering adventure, flies in and out of remote jungle oilfields like Dick Cheney on a field op for energy independence, but Mosley has him retain the abstract soul of a high school physics teacher. “We’re having a little problem with the center of gravity, folks. It might get rough ahead.” You have to respect a pilot who, before whacking you into the side of a mountain, at least has the courtesy to explain why, in technically informed prose.

So what would you do if you were Jack, other than ditch the Panama hat? Ignoring the center of gravity problem until next month, how do you keep the famous turbulence in Hurricane Pass from breaking the wings? Probably your first thought is to make sure you don’t let the aircraft exceed maneuvering speed. That’s the maximum speed at which a full, abrupt elevator movement, a gust, or a combination of the two will cause the wing to stall rather than bend. (The concept of maneuvering speed properly refers to symmetrical flight conditions, meaning no aileron or rudder involved. Somehow, the definition got extended to include those surfaces.) Operation below maneuvering speed guarantees the structure: at least if the structure has kept its integrity through the repeated stress of flight—perhaps doubtful if Jack has previously laid hands on it!

But would Jack have known about maneuvering speed? The strip in question is from 1941. I looked through my ragged collection of aerodynamics and flight training texts from the 1930s and 40s, but found nothing about maneuvering speed as we think of it today. The structural engineers certainly had an equivalent idea, but apparently they weren’t telling the pilots. My old books and manuals generally do warn against high maneuvering loads, but don’t describe a maximum speed for safe operations. Maneuvering speed is also known as “corner speed” in fighter operations. Pulling maximum structural g while flying at corner speed gives the maximum rate of turn and the minimum turn radius. That’s important when you’re trying to get on someone’s tail. But corner speed isn’t mentioned in Pilot’s Handbooks for the fighters of World War II, either, at least not in those I’ve seen.

If you know the basic, one-g stall speed, maneuvering speed is easy to compute. You just multiply the stall speed by the square root of the structural limit load. Say a 1929 Ford Tri-Motor 4-AT-E is good for four g. (I’m guessing, but the thing is built like a railroad bridge.) Stall speed is 58 mph. Square root of four is two, which multiplied by the stall speed makes the maneuvering speed 116 mph. Cruise speed is around 110 mph. So it would be easy to exceed maneuvering speed in a dive. But Jack isn’t thinking in these terms, because the concept apparently wasn’t being taught.

Nevertheless, Jack would have understood the need to slow down in turbulence. In an aircraft subjected to a sharp vertical gust, the increase in structural load—and thus the acceleration the pilot feels—varies directly with airspeed. A high-time rough-air hot-stick like Jack would have known that, because he would have felt it in his butt many times before. Or maybe the old timers would have told him.

It happens that for a gust of a given intensity, the other principal variable is wing loading (weight/wing area). The higher the wing loading, the easier the bumps become. Normally, one surefire way to increase wing loading is to strap in a cement mixer.

But what happens when that cement mixer goes rampant? What occurs when the center of gravity shifts too far forward or back? How does the aircraft then respond to turbulence and to Jack’s efforts at control? In serial style, that’s for next month.

(Stop the press! A couple of months after writing the above, I came across an account of maneuvering speed on page 317 in the Civil Pilot Training Manual–aka Civil Aeronautics Bulletin No. 23, Second Edition, September 1941.)