To G or Not To G - or How To Avoid Breaking Your Airplane

Vance W. Jaqua
Jaqua Systems, Aerospace Engineering Consultants
EAA Chap 1000 Det 8, Camarillo CA

Originally published February 1998

When people ask how strong a particular airplane is, the answer is usually given in "G's". When they ask me this question I am a bit hesitant in response because I know that this is not a totally true, and a somewhat misleading, answer. The "G" answer is only proper for the specific load assumption that went into the original calculations. Also, in spite of the fancy diagrams, you do not fly around with your eye on the "G" meter saying "oops, that's enough - back off". This may be OK for the lads and lasses bounding about the aerobatic "box", but that is not very applicable to the way the average pilot treats the airplane.

Those of you that may know me, are aware that I am never satisfied with the "pat" established view of engineering problems. "Just because that is the way it has always been done" is like waving the red cape at the proverbial bull. My approach is to step back and ask something like "What is it that is likely to break an airplane"? Unless you hit something solid (sorry that is not a design point) the culprit has to be aerodynamic air loads, generally on one of the lifting surfaces. The maximum force that these surfaces can react on the structure are limited by the maximum lift coefficient and the local dynamic pressure (the technical term for this pressure is "Q" just to confuse you ordinary people). Fortunately, almost every airplane has a handy little meter that is measuring this "Q" - it's called an airspeed indicator.

If your airplane designer did his job right, the critical level of this dynamic pressure is labeled "Max. Maneuvering Speed." Strange as it may seem, this is not really a speed at all, but a pressure level that corresponds to an INDICATED air speed. No corrections for temperature or altitude - the critical thing is indicated air speed which is an established, directly measured pressure level. The maximum maneuvering speed is generally defined as that value where you can make any input with the controls without fear of damaging the structural integrity of the airplane. It also is described as the operating point where you can run into any PROBABLE level of gust without structural distress (bonafide funnel clouds are also not a design point - any force that can drive a straw through a tree trunk is not some place where you can survive in your airplane).

Now the "G" number that would be indicated (if you had the meter) can be over a wide range of values depending on the loading conditions of your plane when this gust is encountered. For most systems the stated design value corresponds to the expected "G" levels if you encounter these conditions at maximum gross weight. If you are lightly loaded the indicated "G" level will be higher (and the seat of your pants would be given a bigger whack), but the structural stress is the same. conversely, if you are loaded over gross weight - do you break? NO, the stresses are the same, but the perceived "G" forces are less. The limiting indicated air speed remains the same.

There is a rough correlation between "G" levels and maximum maneuvering speed, and it points out why those slippery, high efficiency designs have to be made extra stout. Everyone wants one of those airplanes that will land at a brisk walk, and cruise at over 200 mph. Well that is a real tough call because a basic 4G type structure can be over stressed if it flies any speed over twice stalling speed. For example - 50 is a nice landing speed, but twice that for potential 4G forces is only 100 mph. The fabled "unbreakable" 9G design only takes you to 150, and if you intend to hit big gusts while cruising at 200 you better be ready for a 16G hit. The light wing loading for those slow stall speeds will really loosen your teeth in rough air, which is one reason that effective flap systems are so popular for high performance planes.

That is one problem with aerodynamically "clean" airplanes. Another alligator waiting to snap up the unwary pilot is the rapid speed buildup if you fall out of a botched aerobatic maneuver. The speed builds up and the fast approaching hard ground pumps the adrenalin into the efforts to keep the blue side up and the green side down. This has often folded the wing halves up in a low lift "prayer like" position as you plummet to earth.

The more optimistic side to this is that the key word in max. maneuvering speed is INDICATED. This is another good reason to fly fairly high. At altitude the actual true airspeed is quite a bit higher than indicated airspeed, and you can cover quite a bit of ground without venturing too far above the magic indicated number in regions of questionable roughness. The vertical speed of the gust that might break your airplane has to be high enough that the resultant angle of attack on your wing approaches the 15 or so degrees of stall angle. The likelihood of the first gust you encounter at the edge of "turbulence" being this high, is very small, and you should have sufficient time after your teeth have been slammed together, to drop the speed to a safe (but not necessarily comfortable) level.

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Revised -- 20 September 1998