I believe the main two pillars of aircraft designing are
- Range and
- Capacity
Every decision is taken to satisfy these two criteria correlated to financing and practicality of physics.
When you say passenger aircraft, you need to specify if it is a short range small to medium capacity aircraft or a medium, extra range/long range aircraft.
For instance an aircraft that fly short range with medium capacity may not require to gain a faster speed and high service ceiling. Hence, a design with straight wings, turbo prop engines would be feasible. Generally, passenger aircraft such as F-27 is designed as hi-wing for certain reason.
- Since the range and capacity is short and small, no need of jet engines, but couple of turbo prop engines would do the job in a most economical way
- Using prop blades need some ground clearance
- Since, airplanes with short range and low capacity doesn’t require higher speed, their wing construction is less complicated (tapered rectangular wing) than of a high speed jet. So most of turbo props place the simple wing on top of the fuselage as it doesn’t require heavy “reinforced wing-carry box” , which is heavy and complicated.
- Hi-wing is aerodynamically has good characteristics at lower speeds-perfect for small/medium props. (pendulum effect)
On the other hand, fast jetliners have totally opposite characteristics. Most passenger jets such as Boeing 777 are having low wing configuration. Following are some of the reasons for selecting a low wing configuration for such aircraft.
- As their capacity is high and range is longer, they require to gain much more speed than others. Jet engines serves this purpose in the most economical way.
- Low wing with high swivel angles has good aerodynamic characteristics at higher speed.
- Since the wing is complicated* in a long range hi speed jet the construction require a heavy wing carry box, which is most unluckily to be place on top of the fuselage.
(* Flaps, slats, ground breaks, ailerons, outboard flaps, inboard flaps, outboard air breaks , in board air brakes, wash out airfoil construction, thinner airfoil with small LE radius, variable sweep angles, etc. These kind of complexity is not seen in prop/turbo prop aircraft)
To summarize all: every aircraft is designed initially looking at two main criteria which are “range” and “ capacity”. All other decisions are taken once these two are defined. so a short range less capacity aircraft may not require to fly fast and at high altitudes. Hence the hi wing arrangement has the best aerodynamic characteristics for such an aircraft. On the other hand a plane with long range with heavy capacity may require to fly fast, at high altitudes which can be complimented by having a low wing configuration.
(some of the fast military jets have hi-wing configuration.Military aircraft design is totally different to designing civil jets. Certain rules and criteria do not apply to military aircraft design. )