Every engineer has their own opinion about aircraft design, but there are three important characteristics of an airplane that the end user cares about: 1) Performance, 2) Flying Qualities, and 3) Systems. What makes airplane design difficult is that you cannot change one without changing the others. I’ll illustrate with pictures of the F-35 JSF.

Performance
Performance answers questions like: How high, and how fast? For example, how fast can the F-35 accelerate from 0.8M to 1.2M, the critical transonic region? A glance at it’s underside reveals a large cross-section, a weakness that mother nature exploits with wave drag. Initial design called for longer and more slender, but it was too tail heavy.

Systems
Systems allow the pilot to use the airplane to get it’s job done. In the case of the F-35, that’s to fly and fight. You can see part of the DAS (distributed aperture system), a collection of sensors, that appears as a bump under the nose of the aircraft below. Systems are needed to get the job done but add drag and take up valuable real estate inside the airplane.

Flying Qualities
Understanding how the airplane responds to mother nature’s laws will reveal how it responds when pilots ask it to turn tighter while looking over the shoulder in a dogfight. That’s the objective of flying qualities testing like high alpha and spin testing, as shown here. Pilots want the airplane to be responsive and maneuverable, but that can make it unexpectedly hard to fly in places like high angles of attack.

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