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  • Date: Oct 14, 2013
  • Posted by Dave Hodges
  • Tags: Aerospace, Engineering

"Getting Off the Ground"

More than just the forces of gravity

More than any other field of engineering, aerospace exemplifies the importance of compromise amongst the many different disciplines that are necessary for designing and constructing an airworthy aircraft. The primary reason for the difference between aerospace and other disciplines boils down to one single physical problem: gravity. Of course, gravity affects all engineering disciplines, but unlike the others, gravity governs the entire aerospace field: if an aircraft is ever to get off the runway, it has to be able to overcome the forces due to gravity.

In structural engineering, gravity doesn't need to be overcome – at least not in the same way. For instance, gravity is part of what keeps buildings from blowing away in the wind. In this case, gravity is a welcome phenomenon for the building designer. Most structures do have to support gravity loads though, and in this case columns have to be designed to take these loads safely. But structural engineers have several options for increasing the strength of a column for a given load. One option is to increase the size of the column. Another would be to add lateral braces to the column to prevent it from buckling. Where the girders connect to the columns, similar adjustments can be made. A larger weld can be used to take a higher load or more bolts can be added. And when in doubt, there is never any harm in adding just a little bit more material to be on the safe side.

In mechanical engineering, gravity poses a similar problem and is overcome in basically the same way. If the motor that sends a lift up and down the shaft is found to be deficient, it is rarely any issue for the mechanical engineer to replace the motor with one more robust. Another option may be to increase the output of the power supply. If the cables that hold the lift are similarly deficient, one can quickly and easily change the design by specifying a larger, thicker, or stronger cable. Or, just by adding additional cables. More strength and more material are easy solutions to any design issues.

In electrical engineering, gravity does not pose a problem the same way it does for structural and mechanical engineers. However, for electrical systems in buildings, motors, and automobiles, problems are generally handled the same way. In other words, an electrical engineer always has the option of making his system more robust without major penalty applied to the design. Bigger, stronger, and more powerful are always options.

In the aerospace field, engineering is practiced completely differently. If an aerospace engineer adds extra material to the airframe in order to be on the safe side, it may end up with a plane that doesn't fly. And whilst a plane that never leaves the runway may technically be very safe, it kind of defeats the purpose of being in the field of aircraft design. So what do aerospace engineers do? Many times, they resort to strong aluminum alloys, perforated metals, and occasionally titanium to get the strength they need without the additional penalty of weight.

Similarly, if an aerospace engineer designs a power plant for an aircraft that is twice as strong as it needs to be, it is not just merely adding a little extra weight. This excessive design will have all sorts of ramifications down the road. The bigger power plant means more weight on the wings and fuselage, which in turn means more material in the airframe. A bigger power plant also means more fuel (or at the very least a smaller range). Aerospace engineers who design aircraft power plants must carefully assess the benefits and risks in order to suit the aircraft.

Aerospace engineers are responsible for the advanced avionics equipment that has become integral in modern aircraft. However, space is just as important as weight in aircraft. Too much space that is needlessly wasted with oversized equipment ultimately causes the same problems outlined above: more material in the airframe. Electrical engineers face the challenge of fitting their equipment into small spaces without sacrificing quality, accuracy, or performance. To top it all off, the systems should be easily accessible for maintenance as well.

If the engineering disciplines remain unchecked with respect to each other and to the main goal of designing an airworthy aircraft, the result is the following cycle:



The cycle can continue to spiral out of control until the plane never leaves the ground. The disciplines must work together to make sure that each design decision is appropriate for the aircraft in concert with all the others. In aerospace engineering, one single discipline can never drive all the others. It is not a one-man show. It requires teams of engineers, all diligently willing to work together to meet the needs of the aircraft and aviation industry.

Let me know if you have some thoughts, or would like to talk through.

dhodges@phoenixengineering.com

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