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Home arrow Engineering arrow Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
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Airframe Topology

Unmanned versus Manned - Rethinking Topology

Commercial transport aircraft - in particular large jet airliners - are phenomenal demonstrations of technological progress. Indeed, over the last half century they have seen steady improvements in performance (most importantly in terms of fuel burn), environmental impact

(they are getting quieter and their emissions have fallen significantly), safety, dispatch reliability, and so on. This progress is the result of the advent of digital avionics, leaps in material science, careful refinement of aerodynamic surfaces (both external and internal), and so on, and advances in the design tools that enable the effective and efficient integration of all of these aspects of aeronautical engineering.

There is one aspect of transport aircraft design, however, that has not changed over half a century: airframe topology. In other words, the airframe is made up of the same major components and these components connect to each other in the same way as they did in the 1950s. Modern commercial transport aircraft trace their lineage back to the Boeing 367-80, sometimes referred to simply as the “Dash-80,” which rolled out of The Boeing Company’s Seattle assembly plant in 1954, see Figure 10.1. If the Dash-80 were sprayed in a contemporary livery and pulled up at a gate at Heathrow, few passengers would notice anything out of the ordinary.

Of course, aeronautical engineers and camera-wielding plane spotters would immediately notice the relatively low aspect ratio wings and the slender nacelles housing now obsolete turbojet gas turbines but, ultimately, the Dash-80 is a “tube-and-wing” design,

OnMay 14, 1954, Boeing officially rolled out the Dash-80, the prototype of the company’s 707 jet transport. Source

Figure 10.1 OnMay 14, 1954, Boeing officially rolled out the Dash-80, the prototype of the company’s 707 jet transport. Source: This photo, by John M. ‘Hack’ Miller, was taken during the rollout (Image courtesy of the Museum of History & Industry, Seattle https://creativecommons.org/licenses/by-sa/2.0/ - no copyright is asserted by the inclusion of this image).

with pylon-mounted nacelles slung under the leading edges of the swept wings and fuselage-mounted tailplanes and fin - much like a twenty-first century transport jet.[1]

The multibillion dollar question is: why has airframe topology not changed in over 60 years when, for example, the avionics of a modern jet has about as much in common with that of the Dash-80 as a magnetic levitation train has with the Flying Scotsman?

There are three possible answers:

  • • The Dash-80 type configuration is the engineering equivalent of, to use an example from biological evolution, a shark. In other words, it is as good as a transport jet topology can be and will ever be.
  • • The complexity of a modern, clean-sheet design is such that the likes of Boeing and Airbus effectively bet the company on each new aircraft[2] and, in the face of such terminal commercial risks, deviating from the tried and tested template is seen as unacceptable by the respective boards of directors and shareholders.
  • • Public acceptability is often quoted as the reason for conservatism, though the only significant deviation - Concorde - was not noted for being shunned by prospective passengers due to its slender delta wings, rectangular nacelles, or elegant area-ruled body.

The truth is likely to contain elements of all of these. The “tube-and-wing” design may not be as good as a topology can be, but it is very good. It allows for easy stretching and shortening (i.e., it naturally forms the basis of a family of designs, whose members can be instantiated at relatively low costs - something a tapered fuselage would not easily permit, for example), it can accommodate a structurally efficient pressurized cabin, it has some manufacturing and maintenance advantages, it is easy to evacuate in an emergency, and so on.

At the same time, for example, there are few technical arguments against and many in favor of over-wing engines (a relatively minor deviation from the standard set by the Dash-80), and yet, with the exception of Honda’s innovative HA-420, we are unlikely to see such aircraft in the skies before 2025 at least.

Wherever the exact truth might lie, how should we readjust our airframe topology design thinking when we remove the pilot (and any passengers) from the aircraft? Here are some of the relevant reasons why a design paradigm shift may be required:

  • • The development costs of unmanned aircraft tend to be orders of magnitude lower and so are the commercial risks of making the wrong topological decision.
  • • The fuselage rarely needs pressurization - this removes numerous structural engineering constraints.
  • • The ranking of design objectives and constraints is different, with cost often dominating ahead of, say, performance, in a way it rarely does in the case of manned transport aircraft.
  • • The range of propulsion system types worth considering is generally much broader than in the case of manned aircraft (e.g., distributed electrical propulsion, etc.).

• The removal of the uncontained engine failure constraint adds significant topological freedom on unmanned aircraft (passengers on transport aircraft generally have to be shielded by the wing from detached turbine blades liberated by an uncontained failure of the turbomachinery).

  • [1] The reader wishing to get up close with this intellectual ancestor of the modern airliner can only do so at the Smithsonian Institute’s Steven F. Udvar-Hazy Center at Dulles Airport, near Washington DC - only one Dash-80 was everbuilt.
  • [2] The order of magnitude of the costs of developing a clean sheet twin-aisle transport aircraft are currently estimatedat tens of billions of dollars.
 
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