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Exploring the Envelope

by George Anderson on June 1, 2010  •   Print This Post Print This Post   •   

Anyone hanging around aircraft test pilots or watching movies about them doing daring things probably remembers hearing the statement: “We are exploring the envelope”. Alternately, if you recently read a Boeing plush piece for the new B-787 aircraft prototype, you may notice the statement: “we are extending the envelope.”

This pilot talk has an interesting instantiation in the world of the systems engineer and you may hear it used by persons who may or may not totally appreciate its original meaning. Before we consider the system engineering implications or the wisdom of adopting this phrase, we should briefly explain just exactly what it is the pilots are referring to. After all, they share a common compulsion with systems engineers to take complex situations and reduce them to two-dimensional charts and use these charts to make important decisions.

All air breathing aircraft have a range of speed capability that varies with altitude, fuel load, temperature, atmospheric pressure and power plant characteristics.

Simplified down to a two dimensional chart, the aircraft envelope can be drawn as a combination of two variables. On a Cartesian chart, the x-axis depicts the range of speeds and the y- axis covers the range of altitudes that the airplane can attain. The result of plotting all the combinations of speed and altitude results in a closed polygon shape that that bounds all the possible points that the aircraft can pass through in controlled flight. Figure 1. Presents a hypothetical aircraft performance envelope.

Figure 1

The vertically oriented minimum speed line on the left indicates where the aircraft wing stalls. The line to the right documents the maximum speed that can be safely reached without danger of structural failure. Note that the two lines converge as the operating altitude increases. The narrowing of the envelope can have serious implications for operating the aircraft at or near the highest altitude in the envelope. This is because the range of speed between maximum and minimum is becoming so small that it is difficult to maintain the aircraft speed between the two limits. This is especially true when turbulence causes un-commanded airspeed variations. All pilots call this area on the chart, coffin corner. Smart pilots imprint these charts in their memory.

What then, does it mean to explore an aircraft’s envelope? A new aircraft is not born with an envelope ready for the pilot to use. Instead, the test pilot flies until he verifies the limits of safe flight. This is not a job that can be done safely without a lot of previous flight experience and a solid theoretical understanding of many subjects coupled with courage to face the unexpected.

Expanding the envelope on an aircraft can only suggest that fundamental changes are being made to the design that allow it to fly higher and/or expand the range of permissible airspeeds. Thus when Boeing says it is expanding the envelope of the B787 they are making yet another change to the design and lengthening the delivery dates to anxious customers. It could also mean that the recently completed test envelope is below par.

Other Systems have performance envelopes just like aircraft. Every system operates within boundaries defined by physical laws and a plot of this data can aid the system engineer in performing development activities. For instance, information systems represent a significant subset of all man-made systems and are defined by gross parameters such as bandwidth, delay, data integrity and so on. If two variables are selected carefully, the resulting test envelope will provide a valuable record of performance. For instance, plotting operating temperatures vs. input voltage variations can establish what environmental conditions are acceptable for routine operation.

The system engineer should define the initial equipment performance envelope and fully explore and document this envelope during testing. Following the deployment of the system, the envelope should be continuously monitored by instrumentation designed or adapted specifically for the purpose.

There are other types of data charts, but the concept of the performance envelope is very useful in almost every system and the systems engineer can make good use of this concept in presenting clear and actionable data to the end user’s maintenance and operations functions.

Exploring and extending the envelope is, indeed, a suitable exercise for the systems engineer and, in conclusion, I leave one question as an exercise for the reader. Is there a man-made system that does not have continuously bounded performance limits.

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