Flight Envelope

- James Albright (a former G450 driver)

Updated: 2013-10-31

"Thar I wuz, pushin' the outside of that darn ol' envelope. I wuz outta speed and outta idears."

So that's "flight envelope" in the glory days of flight test, but what about now, in the genteel world of flying Gulfstream G450's?

We clearly know how fast we can go, the AFM gives us a limit, the FAA gives us a limit, and the engines can only push the airplane so fast before the airspeed indicator stops moving.

We clearly know how slow we can go during takeoff and approach, the book tells us and the stall barrier system keeps an eye on us. But what about at altitude? If we wait for the stall barrier system at altitude it may be too late, the aircraft will likely mush into a descent and that violates our ATC clearance and costs us valuable style points. How slow can you go in the climb to FL 450? We need to know our flight envelope.

Do you really need a V-G Diagram if you aren't in the practice of yankin' and bankin' to keep bogey off your six? No. But we'll derive one anyway and then show you a chart that may be of more use to you.

Everything here is from the references shown below, with a few comments in an alternate color.

 Definition

[14 CFR 25 §25.333(a)] The strength requirements must be met at each combination of airspeed and load factor on and within the boundaries of the representative maneuvering envelope (V-n diagram) of paragraph (b) of this section [shown above].

An aeronautical engineer calls this a "V-G Diagram" because it plots aircraft speed (the "V") versus load factor (the "G"). The chart does not contain reference to altitude, but that is implied since it does include aircraft stall speed. What is often missed, however, is that the chart also does not mention aircraft weight and configuration; there should be a separate chart for each of these variations.

 G450 V-G Diagram

The G450 manual suite does not contain any such charts, but one can be derived knowing G-Limits for every configuration, stall speeds for given weights, and maximum speeds. Using the process outlined in Operating Flight Strength, a V-G Diagram that produces a cornering speed (VA) of 206 knots can be found to require a 1-G stall speed of 130 knots. This happens at several points, among them 70,000 lbs gross weight, flaps up, gear up, speed brakes retracted, and sea level:

This process is useful only because it reveals that our published VA is not valid for most of our flight conditions and that we are better off using lower speeds, for example:

Gulfstream G450 Maneuvering Speed (Sea Level)

Gear Flaps Weight VA

Down 39 50,000 lbs 139 knots

Up 20 74,600 lbs 174 knots

Up Up 70,000 lbs 206 knots

 Buffet Boundary

[G450 Airplane Flight Manual §5.8] Buffet boundary data are presented as a function of speed, weight, and load factor. For speeds below approximately 0.7 Mach, the buffet boundary is based on shaker onset (low speed buffet) while natural buffet (high speed buffet) is limiting above this speed.

This chart can be used to determine a flight envelope for someone who doesn't intend to roll into more than 30° of bank and promises to avoid all G-Limits. I've done that here for three sample altitudes and maximum gross weight.

You could take this chart and tape it to your yoke and vow to keep an eye on it every time your speed gets away from you, or you can run the chart for a worst case scenario. Say you are climbing one day after a maximum weight takeoff and find yourself with the autopilot engaged keeping a watchful eye on the autopilot's performance. Using the 30° of bank line gives you a pad for maneuvering and a bit of turbulence.

At 20,000 feet you will get the stick shaker at Mach 0.40, the green line. At 30,000 feet you will get the stick shaker at Mach 0.50, the blue line. At 40,000 feet you will get the stick shaker at Mach 0.60. See a pattern here?

Rule of Thumb: During a maximum weight climb to altitude you will get the stick shaker at a Mach twenty higher than your altitude in thousands. At 20,000 feet it occurs at Mach .40, at 30,000 feet at Mach 0.50, and at 40,000 feet at Mach 0.60.

 References:

14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013