Last week we created an airplane's stress envelope. This week we'll look at how gust loads affect the airplane. We'll also construct a gust envelope (which isn't the package the gust lock came in either!).
In straight and level flight, when the airplane encounters vertical gusts of air, it experiences an increase in lift. This occurs in much the same way you experience an increase in lift by pulling back on the elevator. The angle of attack is increased and the airplane accelerates upward. The question is, "How do these gusts affect the airplane?" Let's examine Figure 1 to find out.
Figure 1 shows the typical gust envelope for an airplane at full gross weight (the envelope changes with a weight change). The blue diagonal lines represent the effect of a mathematically determined sharp edged gust on the airplane. These are called sharp edged gusts because, as far as the engineers are concerned, there is no transition gradient between the smooth and rough air of a gust. Of course, it doesn't work this way in nature, but it does make the equations easier to calculate (it works to your advantage too).
Notice that the gust lines have their roots to the left of the diagram, starting at the 1g position (your normal, unaccelerated flight condition). Two gust lines angle upward (the positive-g gust line) and two angle downward (the negative-g gust line).
The upward angled gust lines represent the effect of a +25 fps (foot per second) gust and a +50 fps gust on the airplane. It should be immediately apparent that the faster the airplane goes, the greater the g-load imposed on the airframe by a gust. This is one reason we want to slow the airplane down in turbulent air. Keep this point in mind.
Airplanes certificated after September 1969, are required to withstand a + /- 50 fps sharp edged gust at Vc or the velocity of cruise and a + /- 25 fps gust at Vd or design dive speed. (As additional information, airplanes certificated prior to 1969 have lower gust load requirements. These are +/- 30 fps and +/- 15 fps, respectively. The Beechcraft Bonanza is an example of an airplane certificated under the older requirements.)
Examine Figure 1 again. At Vc, the airplane can withstand a +/-50 fps gust and not exceed the design limit load factor. While the FAA's equations create several values for Vc, the value that's usually lowest is used to reference the high speed end of the green arc on your airspeed indicator. This is also known as Vno or maximum structural cruising speed.
Now you know why the green arc eventually becomes the yellow arc. It has to do with the gusts the airplane's capable of withstanding. Remember, the faster the airplane flies, the greater the load imposed by a gust. In the yellow or caution range the airplane should only be flown in smooth air. What's the definition of smooth air? Air not having sharp edged gusts greater than +/- 25 fps is an appropriate definition.
Of course, the question is, how do you know what sharp edged gusts to expect? Ahhhh, you'll have to wait till next week to find this out. For now, let me be clear in saying that you should not fly in the yellow range unless the air is smooth. As far as I'm concerned, if the air is bumpy then slow to below the caution range.
But what if the air is extremely bumpy? Can we fly in turbulence at speeds up to Vno? Once again, since you don't know if there are sharp edged gusts exceeding +/- 50 fps, don't fly near the top of the green arc in heavy turbulence. Fly at or slightly below the airplane's maneuvering speed (Va). This is the only speed that offers a reasonable guarantee that you won't exceed your airplane design limit load factor, no matter how hard you manipulate the controls nor how strong (within reason) the gusts are.
I write this last sentence with great care and caution. There are gusts out there that are strong enough to shear the wings off an airplane. While very, very rare, it can happen. In fact, the wingtip vortice strength produced by certain transport aircraft weighed in at nearly 300 fps. It's possible that vortices of this magnitude could damage airplane wings even with the airplane operating below maneuvering speed. This is very rare but it can happen. Avoid those vortices!
Next week we'll take a look at how we might estimate the strength of atmospheric gusts while airborne.
For more information on this subject, see "Maneuvering Speed," "Airspeed: There's More To It Than Going Fast, " and "Learning Experiences: Beyond the Limits."
