More on G and the Envelope

Yesterday, in G-Loads and the Envelope, I explained the basics of the Vgn diagram and the limits of the operational categories. Today we’ll discuss more about what it means to operate within the envelope.

First, its all about what the wing can lift. Take your basic Cessna 172 with a gross weight of 2400 pounds. Full of people, fuel, and baggage and tipping the scales right at gross weight, the airframe has to support a total of 9120 pounds–3.8g x 2400 pounds–for the total force.

Here is something else you should know about the 172. The airplane does not have enough power available to set and maintain 3.8g’s. The only way you can possibly maintain 3.8g for any length of time on a 172 is by spiraling down. Pilots refer to this as trading altitude for energy. Even then, there is a limit to how far you can go (the ground).

When it comes to young pilots, there is always the temptation to do things you shouldn’t do. For example, aerobatics in an airplane not certified for acro.

Intelligent and enterprising young pilots have been known to do the math: If the airplane is capable of supporting 9120 pounds, if I fly solo with quarter tanks my weight would be … uhmm … 1800 pounds. Let’s see … 9120 divided by 1800 … that should give me 5.06g’s to play with. Enough for almost any maneuver.

The young, inexperienced pilot who uses the math and logic above is right that the airplane can handle 5g at lower weights. What he or she fails to realize is that all the other components of the airplane, things such as the floor of the baggage compartment, the engine mount holding the engine, and even the seat on which they are sitting, is engineered to handle only a 3.8g load.

Now, as a rule, 3.8g is sufficient. And most pilots will keep the g-meter within that limit. The problem happens when the inexperienced pilot accidently “falls” out of a maneuver. It is amazing how quickly airspeed and aerodynamic forces build up with the nose of an airplane pointed at the ground. It is at this point, the pilot realizes he or she has a choice—pull really hard and chance the wings falling off, or pull more reasonably and risk flying into the ground.

Uhmm … choices, choices …

As a note in the case of Cessna 172s, I have read two accident reports in which inexperienced pilots tried to go out and play “Top Gun.” In exceeding the limits of the airplane, their windshields blew out and departed their respective airframes.

One pilot was close enough to make it back to the airport for an emergency landing. The other ended up in a field. The drag imposed by the missing windscreens was so great the airplanes could not maintain straight and level, even with full power.

Luckily, both survived.


©2011 J. Clark

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