Bjorn’s Corner: New aircraft technologies. Part 21. Changed flight profiles

By Bjorn Fehrm

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.

Figure 1. A Truss Braced Wing airliner shall fly higher. Source: Boeing.

Why are airliners flown at high altitudes?

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.

Figure 2. The speed and altitude sensitivities of parasitic and induced drag. Source: Leeham Co.

  • Parasitic drag, which can be labeled drag due to size and which is the dominant drag, increases with speed but decreases with altitude.
  • Induced drag, which is drag due to weight, is the other way around. It decreases with speed but increases with altitude.
  • The minimum of total drag is where these drag components are equal in size.

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.

Consequences of technology changes

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.

Figure 3. JetZero Z5 BWB passenger jet concept lowers parasitic drag through wetted area reduction. Source: JetZero.

The influence of engine technology

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.

7 Comments on “Bjorn’s Corner: New aircraft technologies. Part 21. Changed flight profiles

  1. This is great, really getting into the depths of the design trade off and why they are chosen as the options.

    The MD-11 was interesting in what they chose to do with it though in the end it has had a less than stellar crash record though I think the more hot rod oriented freighter pilots have contributed that.

  2. Based on your analysis, one might think a BWB with a truss braced, very high aspect ratio wing would be the best solution for efficiency.

  3. In the Breguet range equation, the performance parts of the equation are V/(sfc) x L/D, where V is aircraft true speed, sfc is engine specific fuel consumption and L/D is airplane lift/drag ratio. We want to maximize these combined terms for maximum range. Assuming sfc is independent of speed, and speed of sound is constant, which are reasonable assumptions for most cruise conditions, we want to maximize M x L/D. For the DC-9 a graph of M x L/D as a function of Mach number and lift coefficient can be found in Roger Schaufele’s “The Elements of Aircraft Preliminary Design”. For the DC-10, a graph of the same set of variables can be found in Dick Shevell’s “Fundamentals of Flight”. Comparing these graphs with those of L/D with the same independent variables (also shown in both of these books), indicates that for maximum range the aircraft does not fly at maximum L/D, but at about 90 – 95% of maximum. These graphs can also be found on the website in the class presentation “Ch. 12 Aerodynamics”

  4. You also have speed lapse as the engine exhaust speed is fixed at just below M=1 and as your flight speed thru air increases and approaches the exhaust speed the thrust drops. I think a 767 engine producing 60 k thrust at T-O is down to 7 k at cruising altitude. The Boeing-NASA wing need some active anti-flutter help to reach normal cruising speeds but with good engineering should be able to do it. The RISE engine as well as careful aero design of the fan should not drastically drop efficiency up close to normal cruising speeds. Not easy but doable.

    • That is another area I look forward to as engines have changed hugely from the old tube type (thrust) to the fan jets with far more bypass.

      My curiosity is does the larger and larger bypass lower the speed efficient flight is done at?

      And as noted, the RISE engine though I continue to disbelieve its viable. GTF will keep increasing efficiency as they learn on that type and props are perceived by the public to be inferior (archaic, a step back, not trusted)

      The other issue a prop job (no matter what you call it) is a one off install, you can’t hang an alternative jet engine in its place.

      LEAP and the GTF have proved to be both viable mounted on the same wing (A320 series).

      As Bjorn has noted, Airlines will factor in speed and compare to how much daily utilization you get.

      • If you have exhaust speed at M=1 the massflow gives the thrust as long as it is axial flow. Getting props and stators to do it is hard. You quickly loose efficiency in supersonic shockwaves. The turbofan slows down speed in the inlet and accelerate in the nozzle, the RISE lacks both. Besides doing anti-ice and noise reduction. Lots of analysis to get the DC-7 size prop working efficiently at airliner speeds and slove anti-ice on the movable fan and stators

  5. Parasitic drag?
    Isn’t that when executive management sucks all the cash out of an enterprise for the benefit of themselves and major shareholders at the expense of the long-term health of the company?

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