Author: Robert Zimmerman

Picture this scenario – an internal or external customer contacts a team, possibly through a Request for Proposal (RFP) or as an exercise. The customer knows that your team makes a certain product, and the customer desires to have a similar product. However, the original product is either too big or too small for the desired application. What generally happens?

A small team of engineers, along with a support team of management, financial, and business experts, convenes to define a solution that meets the customer’s desires. Since a similar product already exists, that product becomes a starting point for engineering, cost, and schedule studies. Usually, the preferred outcome is a product like the existing one but with more – more range, more seats, or more payload capacity. Occasionally the preference is for less – less range, less payload, or more likely, smaller external dimensions. The first course of action the team comes to is to take the current product and scale it to fit the new product’s specifications.

The problem then becomes how to do the scaling in a credible manner. I first realized this phenomenon when I was 6 years old, sitting in the rear seat of a four-passenger Piper Comanche. We had stopped overnight in Little Rock, Arkansas, and were sitting on the ramp with the engine idling when a North American P51D Mustang went by on a taxiway. I stared at the Mustang, my mind boggled by its sheer size. Up until that time, I had seen pictures of the legendary World War II fighter, and even had a model of one. But in my six-year old mind, I had made a basic scaling error – I took what I knew, the size of a four place single engine aircraft, and had scaled down that size for my internal picture of how big a Mustang ought to be, for it only had one seat, not four. And so, looking out the window from my vantage of a four place aircraft, and watching this behemoth roll by, caused a definite case of cognitive dissonance.

I had let one parameter guide my understanding of how big a Mustang is. A more thorough investigation would have uncovered some more pertinent parameters, such as engine horsepower (Mustang: 1490, Comanche: 250, or about 6 times the power), cruise speed (435 mph vs 180), and max gross weight (12,100 pounds vs 2900). Then I might have realized my mistake and understood that the Mustang was 50% longer and twice as tall as the Comanche, even with a similar wingspan.

Therein lies the fallacy of scaling – the scaling parameters must be categorized and prioritized, and for that you need knowledgeable experts. These are people who understand the interactions of different parameters, which ones to deprecate, and the ones that should dominate. And most importantly, what parameters really matter. If you are taking a four place aircraft and turning it into a six-place aircraft, there are lots of changes besides adding two seats. Most likely the fuselage length grows, and this upsets the balance of where the center of lift is for multiple reasons. Not only are you adding aluminum (or additional composites), but cable lengths (both electrical and mechanical) change. The outer mold line of the fuselage necessarily changes, the question becomes whether a “plug” is installed or the whole fuselage changes. These decisions have aerodynamic consequences. Think of the difference between the Beechcraft Debonair straight-tail Bonanza (four seats) and Bonanza 36 series (six seats) vs the Grumman American AA1 (two seats) and AA5 series aircraft (four seats). Both initial aircraft added two seats. The Bonanza took the route of blending the fuselage across the new length whereas the AA1 vs AA5 has a constant-width plug. Consequently, the AA5 has control problems in certain flight regimes, where the slab sides result in aerodynamic blanking of one or more tail surfaces. The Beechcraft solution was undoubtedly more expensive, but the 36 series of Bonanzas has endured in the marketplace whereas the AA series is a footnote.

So, what can you as a mass properties engineer do when faced with this very familiar scenario? We are in an irreplaceable position on any program because mass properties engineers have visibility across the breadth of a program. Use that to your advantage. Put on your systems engineering hat and look at how the various parts of your total system come together and how they interact. This is where mass properties engineers shine. We are among the few who have insight into every aspect of the product from where components are placed to how these components operate. And, because mass properties engineers have this insight, we are able to influence design and design changes, including functional and aesthetic aspects of a proposed design. Unlike most other engineering disciplines, we are not “pigeon-holed” into affecting one of a design’s parameters. We can interplay multiple factors and guide the program’s management towards arriving at a better solution than one by multiple engineers, each looking only at a subset of the available options. This is the true Unique Selling Point of Mass Properties Engineering, one that has immense value to a company employing mass properties engineers and it is one of the major reasons engineers choose to stay in mass properties.

Finally, what is the SAWE’s role in enabling an individual mass properties engineer to perform this powerful position? Look no further than the SAWE’s Mission Statement: The Society of Allied Weight Engineers is an international, professional, nonprofit organization dedicated to the promotion, practice, and innovation of the field of mass properties engineering. The SAWE executes its mission via a variety of initiatives encompassing peer reviewed papers, conferences, industry leading training, mentoring by experienced mass properties engineers, and establishing industry specific Standards and Practices. The SAWE has enabled countless mass properties engineers to better serve their companies through all the above methods of knowledge transfer, in the process creating the next generation of mass properties experts. Tying this all together, the engineers who become tomorrow’s experts will lead the innovations that power tomorrow’s products.