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Monday, August 1, 2011

Matched to the Mission?

By Frank Lombardi

Performance, stability and control. These are the main ingredients in the recipe for the most suitable aircraft for your mission (cost notwithstanding). Blending the proper amounts of each gets your mission accomplished safely, efficiently, reliably and happily. Messing up that recipe leaves you as frustrated as a chef with a deflated soufflé.

The military usually has the luxury to design an aircraft specifically to meet their mission goals. In the civilian world, this is not so. Aircraft are certified to meet the FAA’s required level of safety, not mission requirements. Of course manufacturers do their best to design aircraft that will appeal to a broad customer base, while perhaps highlighting a particular niche. In doing so, they design in various amounts of the above ingredients with purpose. It is beneficial for you, the buyer, to understand what each ingredient brings to the design, so you can best choose what fits your needs.

An aircraft’s performance characteristics are the result of pairing a powerplant to an airframe. The airframe’s size, shape and weight will dictate the power required to propel the aircraft through the air and the engine will dictate the power available to do so. Parameters such as range, endurance, speed and hover capability are dependent upon this combination. An aircraft’s degree of stability and controllability will dictate its flying qualities—how well it handles while performing a task. Some mistakenly believe that if an aircraft is very stable, then it is very controllable. While quite related, the opposite is actually true. A stable aircraft is essentially one that resists being disturbed from the condition it is trimmed at. Obviously this is beneficial in many cases, but it can be problematic in others, if it is the pilot who is disturbing it with a desired command. An aircraft that resists its pilot’s commands is not a very controllable one.

Deciding exactly what elements of performance, stability and control you are going to require begins with deciding exactly what your mission is. If forethought and discipline are not used here, the lack of it will most certainly show up in various forms such as large maintenance bills, perplexed owners and most often, cursing pilots. An aircraft that is a “jack of all trades” is well, you know.

Next comes a deeper understanding of your mission’s “task elements,” or what is intricately required to accomplish each aspect of your flying. Do you perform electronic newsgathering (ENG)? Then plan on doing lots of OGE hovering for extended periods of time. Do you provide corporate transport? You’ll probably need a fast, smooth ride, good range and IFR capability. Do you perform scene medevac? Then you’ll want exceptional vertical lift and something that can be handled easily in confined spaces.

If you’re getting the idea by now, good. But don’t stop there, as this is too general. Get more specific and dissect what’s involved. Don’t worry, no test pilot schooling required. Let’s revisit the task elements of the OGE hover in the ENG role, for example. Just ask a few ENG pilots to describe a helicopter that hovers “nicely” and you’ll hear such technical answers as, “One where I’m not churning butter while doing the pedal dance as I pray I don’t over-torque it.” What they are describing are elements of performance, stability and control as they relate to pilot workload, or, how much attention they have to put towards holding a “nice” hover. So scrutinizing one important task element of ENG operations results in realizing that not just a helicopter with power to hover OGE, but low workload in that hover is certainly a requirement. This approach can be taken with all the aforementioned examples, breaking down each mission into its most rudimentary elements.

Now here’s a bit of a rub. It’s usually basic performance that drives aircraft sales. Most shortcomings in an aircraft’s handling qualities end up compensated for by pilot skill, and sometimes we even like the ego boost! We often pride ourselves on being able to master a machine that’s been called “a handful,” or we make fun of stability augmentation systems (SAS) saying, “real pilots don’t need SAS.” Point being is its easy to overlook the importance of low pilot workload when making choices in favor of an aircraft that boasts excellent performance—one with seemingly endless power or massive useful load. But shouldn’t low workload always be our goal?

Helicopter expos showcase rows of beautiful helicopters along with large picture cards that display their pertinent performance parameters on the back. All those specifications may have purpose, but rarely can you make any decisions by doing a side-by-side comparison of those cards alone? In upcoming articles, we will continue to broaden our understanding of the aspects of the performance listed on those cards, as well as examine flying qualities more thoroughly, and see how designers strive to bring us pleasurable machines to fly.


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