Very few airplanes have been used the way they were designed for.
From the trainer B.E.2 (which was abused for reconnaissance missions early in WW I) to the interceptor F-104 (which was used as a ground attack plane later), there are numerous examples. And for every mission that is still within the capabilities of one design you can think of numerous others that aren't. Therefore, dimensioning for the worst possible mission is an exercise in futility.
If an airplane carries too much fuel, it will be less economical. If it has too much strength built in, its structure will be too heavy. If the engines are more powerful than needed, it will have poorer fuel economy. In the end, every design can only fill its niche. But there are some principles which make some designs more useable:
- Since regulations require a minimum number of flight attendants depending on the number of passengers, small passenger airplanes come with 19 or 50 seats but rarely with 23 or 55.
- To be able to fly all domestic US routes, the more successful designs will have enough range to fly to Hawaii from airports at the West coast. If maximum range with full payload falls shorter than that, the design will be less versatile for US carriers.
- Since air temperature is constant in the stratosphere, the most economical flight altitude is in the tropopause. Fly lower and you will not make full use of the cooler air above, fly higher and wing size and engine thrust will be less than optimal. Exceptions apply for business jets which like to fly above all other traffic.
- Redundancy requires at least two engines, and maintenance cost demand to keep the number of engines as low as possible. Therefore, except for the really large airliners, the most successful ones all have just two engines.
- In order to offer one of the fastest connections on a specific route, the reference cruise speed of long-range airliners is traditionally set at Mach 0.85. Fly faster and Mach effects will decrease fuel efficiency (demanding thinner airfoils and/or more wing sweep), fly slower and your design will drop from the first page of connections.
If you wonder how much a deviation from optimum airspeed will reduce range, I plotted that for a subsonic design (transsonic airliners will bump into steep drag increases with only a small increase in Mach, so this diagram does not apply to jets). I used a quadratic polar which gives a good approximation of reality.
Range reduction over airspeed. Blue line: L/D at the specified speed. Red line: Range relative to optimum. The optimum speed in this case is 84 m/s. I also assumed constant engine efficiency over speed, so range purely depends on L/D.
