June 9, 2023, ©. Leeham News: This is a summary of article Part 16P. Airframe with lower induced drag. It discusses the Truss Braced Wing type of airframe that increases the practical wing span of an aircraft and thus reduces induced drag.
Last week we laid the foundation for how to reduce induced drag. Here are the fundamentals in repeat:
The induced drag of aircraft is dependent on the parameters in the induced drag formula;
Induced drag = Lift^2 divided by 0.5 * Air density * Speed^2 * Pi * Wingspan^2
In the formula:
For more than 10 years, Boeing and NASA have looked into the Truss Braced Wing to build an airframe that has reduced induced drag through a very wide wing. Figure 1 shows how this technology could be used to produce the next generation of single-aisle airliners.
If you build a very wide wing as a normal cantilever wing, the wing bending moment will force you to make the inner wing beefy to take high tensile and compression loads, making the wing heavy. The truss brace in Figure 1 reduces the wing bending moment to a fraction of the cantilever wing for an equal span.
The TBW is very challenging to realize, especially flutter-wise, as the wing is long and has a narrow chord. Figure 2 shows all the challenges Boeing must overcome to realize a TBW airliner.
NASA and Boeing will build a demonstrator to prove that it has solved these challenges. It’s based on an MD-90 fuselage and will fly in 2028, Figure 3.
What drag gains can be achieved?
Article 16P starts calculating the gains from a TBW for an airliner the size of a 737 MAX. The induced drag is reduced, but at the same time, the wing with its struts has an increased wetted area, which increases the dominant air friction drag.
To see how this all plays out in practice, we put the standard 737 MAX-sized aircraft with a normal wing and then with a TBW in the Leeham Airliner Performance and Cost model and fly a typical route in next week’s Corner.