March 17, 2023, ©. Leeham News: This is a summary of the article New aircraft technologies. Part 4P. Fuselage trades. In the article, we discuss the trade-offs involved in designing a fuselage of an airliner with 250 seats using different architectures.
We examine what parameters decide the performance of an aircraft and how fuselage changes like single aisle versus dual aisle affect these parameters.
In last week’s article, we discussed that it might be better to design the next generation ”Heart of the Market” airplane as a dual aisle airplane when conventional wisdom says it should be a single-aisle plane.
It’s all about the time period of its operational use and how much the heart of the market has moved by then.
The design parameters we look at to understand if it should be a single or dual aisle are:
We show that the cross-section area of a fuselage has a minor influence on airframe drag. Air is very supple and easily curves around a bulbous shape, only causing small amounts of what’s called pressure drag.
The dominant drag for an airliner is the skin friction drag. It’s the drag force created when the air rubs the aircraft’s skin. This drag can also be labeled drag due to size.
The second major drag component is induced drag or drag due to weight. It’s created by the global circulation of air from under the wing when it circulates to the top of the wing, Figure 1.
Induced drag only depends on the wing’s span and how careful the spanwise lift distribution is controlled (it shall be close to elliptical in the spanwise direction). It does NOT depend on wing aspect ratio, a widespread misunderstanding.
This misnomer comes from when induced drag is divided by wing area to normalize the drag to a lift coefficient,so that drag values from different size aircraft can be compared.
Drag changes a lot with speed. Figure 2 shows that at low speeds, like liftoff, induced drag is dominant. It’s typically 85% of the total drag at rotation.
At cruise, when we fly at high speed, the friction drag is the dominant drag. The air friction decreases with thinner air at altitude. Even though induced drag increases with thinner air, it’s beneficial for an airliner to fly high, to reduce the dominant friction drag.
It’s why airplanes that can climb high, like jet airliners, fly between 30,000ft and 40,000ft, where the air density is one-third of the density at sea level.
Friction drag depends on the skin area that rubs with the air, the so-called Wetted area. If we develop the parameters that drive wetted areas of different fuselage types, we find the drivers for the dominant drag. But we also find the driver for fuselage mass and weight (mass is mass, weight is a force).
It comes from airliners being stressed skin designs. The skin of the aircraft creates the dominant drag and, as it’s the premier carrier of the loads, also the mass of the plane.
So if we can find which fuselage construction has the least wetted area per carried passenger, we understand which fuselage type is best for our next-generation heart-of-the-market aircraft.
This is the task for next week’s Corner.