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Monday, June 1, 2015

Leading Edge

Building a Bird, With Forethought

Frank Lombardi
My 82-year-old Italian dad likes to cut out news articles about helicopter accidents to show me, so he can then tell me what went wrong. Apparently, according to him, most helicopter crashes occur because “those damn things just get shaky and fall out of the sky.”

I suppose I am taking it too personally, but as an engineer this tweaks me.

While I concede that even the best designs can be flawed and experience failures, I find it difficult to get him to believe that a bit more forethought goes into building a successful rotorcraft. Of course, we all know otherwise, but for anyone whose dad thinks we mess around with machines that are just haphazardly thrown together, a general outline of the design process of a helicopter might help to clear that up.

While pilots love a helicopter that looks and handles like a dream, performance really drives sales. So the primary considerations that have the most influence on design are customer requirements of payload, range, endurance, max speed and the ability to hover, climb and maneuver. There are then usually constraints on these design requirements due to safety regulations, military specification, engine choices, max physical size, noise level, single-engine performance, disk loading and autorotative capability.

With a defined set of requirements and constraints, it is then the job of the aerodynamicists, weight and structural engineers and other engineering specialists to come up with the smallest, lightest, least expensive helicopter that can satisfy all of them. Each area of expertise must consult with the other and make compromises while still satisfying their own requirements.

The first step in this iterative process usually starts with a market survey of existing aircraft, studying the specs of each. Over the years, all the parameters of each helicopter design have been tabulated and plotted, and key performance trends have settled out. This gives engineers initial design values to aim for.

An initial estimation of the gross weight and installed power is made based on existing helicopters of similar performance. Ultimately, weight drives nearly every aspect of design, as “heavier” directly translates into “costlier.” Years of gathering data has allowed weight estimation equations to be derived for each aircraft component, as a function of rotor disk size, horsepower and gross weight. These are continually updated as more modern aircraft are added to the database. Starting points for the estimated weight of the blades, fuselage, drive system, flight controls, hydraulics, fuel system, avionics—every system and subsystem—are calculated, then recalculated as each evolves and begins to affect the weight of others.

Aerodynamicists develop the size of the main rotor based on gross weight estimates. They aim for a particular disk loading (gross weight/disk area), keeping in mind that whatever helps in hover tends to hurt forward flight. They bias their choices accordingly. Further consideration is given to the rotor tip speed, airfoil shape, blade configuration (swept, straight, tapered, etc.) and hub design. Then drag estimates are made.

Choosing the number of blades is actually not so much an aerodynamic consideration as much as a weight and cost savings one. Generally, having more blades means reduced rotor vibration and noise but increased rotor cost and weight. Tail rotor sizing and configuration tend to follow after the main rotor, since main rotor diameter will decide how far the tailboom must extend. This, of course, will affect tail rotor control power, as well as weight and balance.

While aerodynamic choices are being made, structural and mechanical needs are also being met. Material properties for each part are considered and selected. Stress and strain analyses of every component must be done; fatigue-life and wear tolerances must be calculated. Flight control, electrical, environmental control and fuel systems must be designed, and their plumbing routed with the approval of the weight and balance and structural teams.

Prototype models go through rigorous flight testing to gather actual data for comparison to projected stress, strain, performance and handling qualities predictions. Problems arise and are solved. Developmental flight testing gives way to operational flight testing, which gives way to production flight testing.

Thanks to advances in structures and aerodynamics, the useful loads of helicopters have been pushed from less than 40 percent of total gross weight in the 1950s to beyond 60 percent in today’s designs.

If a design is good, it gains a following. It continues to evolve and be improved throughout its lifetime. Through the efforts of many people, we end up at the controls of an amazing piece of technology, which is anything but a fortuitous accident.

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