July 14, 2023, ©. Leeham News: Developments in engines and airframe technologies require that the aircraft are flown differently to maximize the benefits.
We start by locking what changes in parasitic and induced drag mean for how airliners fly.
Our typical airliners climb to between 30.000 to 40,000 feet after takeoff, then cruise there before descent and landing. Why?
An aircraft that climbs consumes more energy and thus fuel than if it flies at a level altitude. And the extra energy consumed for the climb can’t be regained in full in the descent before landing.
As it costs energy to fly at high altitudes, why is it done? We need to look at how drag changes with airspeed and altitude (Figure 2) to understand why.
At takeoff, over 90% of the drag is induced drag. At landing, we have 80% induced drag. In both cases, induced drag is high because the speed is low. It’s why a wider wing or improved winglets that reduce induced drag help with field performance for an airliner. It means you can operate with a lower maximum thrust engine.
As we increase speed to climb speed, which is typically around Mach 0.7 to Mach 0.8 at altitude, parasitic drag shoots up, and induced drag reduces. An airliner’s climb speeds are adapted to be lower at denser air and faster at altitudes where the air is thinner.
The aerodynamically optimal cruise altitude and speed are where parasitic and induced are equal in size, giving the lowest drag. As lift is set by aircraft weight, it’s also the highest Lift over Drag point.
The engines play a role at what altitude an airliner can cruise. As engines are air pumps, their thrust declines when the air gets thinner (thrust lapse). The cruise altitude at a certain weight is, therefore, often set at where the climb speed is dipping below 300 feet per minute.
If a new airliner generation is designed with reduced induced drag, like the Boeing Truss Braced Wing (Figure 1), the way to use this benefit is to fly the plane higher. The balance between parasitic and induced drag shifts to a higher altitude where total drag is lower.
The balance could also shift to a lower speed, but this reduces the productivity of the plane (fewer passenger miles covered per day). It’s a challenge to get a wide and narrow Truss Braced Wing to cruise at the typical narrowbody cruise speed of M0.78. But this is the target, not to have a discussion about productivity with the airlines (we will discuss why a slower cruise speed has a negative operating cost consequence as well).
A design for lower parasitic drag, like the JetZero Blended Wing Body (Figure 3), could find low drag at a lower cruise altitude. Lower cruise altitudes benefit short-haul airliners as typical climb and descent profiles then allow longer cruise segments.
The developments on the engine side also influence the optimal altitude and speeds for an airliner. We will look at this in the next Corner.