Sin 1: invalid assumptions in specifications
- airplane is a tool
- ConOps defines the job and how it is to be done
- engineers design plane to meet the requirements of the conops
::: examples:
- B-58 only useful for penetration with nuke, not other missions after cold war thawing in late 60s.
- Multi-seat WW-2 fighters for anti-bomber role: Bell SFM-1 Airacuda, Boulton Paul Defiant (no forward guns),
- Failed business model: extrapolate last trend one step too far: SST: speed, Brabazon: comfort, Princess: runways built during WW2 eliminated need for seaplanes, A380: ETOPS change eliminated hub and spoke long haul system.
Sin 2: overconstrained
- too many constraints damage ability to perform mission
- Mission related: restrictive operating conditions, too many missions, unrealistic goals
::: Example: F-111
- non mission related: hangar size, configuration, specific size or other geometric constraints
::: Examples: Short Sterling: hangar size, standard RN hangar led to lack of wingspan which reduced operating altitude to below AA ceiling, US hardened shelters are 44 feet wide.
Sin 3. Dependence on new or immature tech: can give advantage, but: project can fail if tech doesn't work, overoptimism common, need to have a plan B.
::: Examples: Rockwell XFV-12,
JSF: both bet on new tech:
Boeing: low risk on propulsion (Harrier); high risk on thermoplastic wing skins to save weight
Lockheed: high risk on lift fan; low risk on structure
Both bet on new tech, the one that made the new tech work won.
Sin 4. Success of airplane depends on success of engine: engine must perform for aircraft to perform, erosion of engine performance hurts plane's performance (thrust, weight, SFC, reliability)
- If airplane is designed to limit of projected engine performance, airframe can out grow engine: weight increase, drag increase.
::: Example: Eclipse 500 very light jet, Williams EJ-22 turbofan: 500 lbs thrust/$40,000, engine did not produce thrust on time to support Eclipse production schedule, Eclipse bankrupt.
Sin 5. One airplane, many innovations (hobbyshop syndrome): all critical systems must work for vehicle to be successful, working out bugs in many new systems is much more difficult than doing one at a time, one significant failure can doom program.
::: Example: X-33, linear aerospike engine, composite crygenic LH2 tanks, metallic thermal protection system, new lifting body aerodynamic configuration; LH2 tank failed in testing, configuration changed dramatically as aero stability and control issues emerged.
Sin 6. Lack of margin and fallback options: design margins necessary to accommodate shortfalls as design becomes real, weight growth, thrust reduction, higher fuel consumption, higher drag, payload growth.
- Highly integrated design may not have the flexibility to adjust, design margin and fallback options must be designed in from the beginning.
The conventional wing-body-tail configuration works because of the ability to adapt the configuration during design.
::: Example: A-12 GD/MD; weight growth caused by poor load paths and difficulty making large composite parts, approach and launch speeds increased requiring wind over deck, no way to increase wing area of CLmax, span constrained by carrier suitability, sweep constrained by observables, tailless configuration cannot trim flaps.
Sin 7. concept driven design: concept believed to offer some vital advantage, concept becomes more important than suitability of the configuration, advantage may not be real, or may be offset by inherent disadvantages in the concept, design compromises to keep concept pure and fix problems it causes.
- Magic configurations: triplanes, aft tail pusher props, turboprop canards, BWB (maybe for very large aircraft, jury is still out).
::: Example: Beech starship: turboprop canard, all composite, pusher engines; there was no performance advantage over conventional layout to justify price.
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