• 30 Nov 2024 07:47 AM
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Coming to Terms: Range

By Dan Newman
Vertiflite, Nov/Dec 2024

This series addresses the uses of terminology to seek alignment across all users and consumers, so as not to be misleading, unfavorable, erroneous or prejudicial. This installment addresses one of the most significant performance metrics of a vehicle, and so it is imperative that its use and context are commonly understood by all parties.

Mobility is about the movement of people and things, and a decisive metric for any vehicle is its operating range for a single trip on its organic, stored energy. The manner and time to refuel or recharge are also important in its use in varying mission applications, but this mission range capability is central to selecting among existing candidate vehicles and in designing new systems.

The range of a system — the one-way distance it can travel directly from origin to destination — is distinguished from radius, which represents a round trip that finishes at the origin. The radius is not simply half the range. It is used when there are important tasks to accomplish at some distance from the base that involves loitering for observation/communications relay or changes in weight delivering/retrieving payloads or passengers. While range is an attribute of the vehicle, a radius must be specified in terms of a specific mission at the midpoint or waypoints.

The conditions at which a vehicle’s range is calculated, by prediction or test, are important. This is especially true for aircraft, as ambient conditions and altitude can have a significant effect on performance. Often the predictions are made at ideal conditions, as are the mile-per-gallon measurements for automobiles, as specified in the US on the Monroney Label (named after the senator who championed them in the 1950s), each with the disclaimer that begins, “Actual results will vary.” Achieving that many miles on a gallon of fuel is unlikely in real applications, as driving habits and cycles will inevitably vary from the mandated US Environmental Protection Agency (EPA) test procedure, but the quoted performance is a useful metric to compare the fuel efficiency of different candidate vehicles.

The velocity at which the range of a vehicle is calculated or measured is important. Claims are made for operation at the best range speed (V_BR), which is mildly different than its best endurance speed (V_BE), but far lower than its maximum speed (V_MAX). Referring to a typical plot of rotorcraft power required (see figure 1), the speed for best endurance is at the lowest power condition (barring any unique powerplant features), and the best range is the tangent of a line from the origin to the power polar. As an aircraft must overcome gravity, a major factor in quoting range is the operating weight at which the prediction or test of range is done. The range achievable varies with overall weight, as in a typical payload-range diagram for a fueled aircraft (see figure 2). This curve is made from a series of operating points, each at the aircraft’s maximum weight capability.

At the far right of the curve, where no payload is carried and the fuel/energy is topped off, is the maximum achievable destination. The aircraft is at its operating weight empty (OWE) with no payload, able to carry itself to this “ferry range.” It is important to note that the OWE is higher than the empty weight of the aircraft as it must include everything required to fly it (e.g., a flight crew), all fixed equipment and mission equipment required to perform its assigned tasks, and any/all trapped unusable fluids (e.g., hydraulics and fuel). An autonomous, all-electric aircraft would not have flight crew or trapped fluids, but still must account for all equipment.

Moving along this curve to the left represents configurations loaded with a continual reduction in fuel/energy weight, and an equivalent increase in payload weight. The slope is primarily the energy density of the fuel. A battery-powered aircraft would have a figure that is generally similar, but as there’s no weight reduction from burning fuel, the smooth curve would be replaced with a stair-step line representing different quantities of battery packs installed. It is also steeper as the lower energy density doesn’t achieve the same range.

At the far left, at the “zero range payload,” the aircraft has no useful fuel/energy and is carrying its maximum payload weight. Again, this must include all flight crew, fixed and mission equipment, and trapped fluids to accurately represent the maximum payload.

Often, performance claims for existing and emerging aircraft quote maximum speed, maximum range and maximum payload together, but these rarely represent the same configuration or the same mission. Similarly, if empty weight is used instead of OWE, the payload and range data would be overstated.

So, when assessing an existing aircraft, or developing a specification for a new one, it is important to fully understand the conditions and context for each of the claimed performance metrics, individually and together.