Bjorn’s Corner: New aircraft technologies. Part 23. Efficient flight speeds

By Bjorn Fehrm

July 28, 2023, ©. Leeham News: We touched on efficient flight speeds in Part 21, but it was focused on fuel efficiency and how new technologies require the airliner to fly at new altitude-speed combinations to maximise the fuel efficiency gains.

We now look at how to fly the airliner cost-efficiently, which is slightly different from how to fly it fuel efficiently.

Figure 1. The difference between lowest fuel burn speed (Mmr) and long-range cruise speed Mlrc. Source: Airbus.

Efficient speeds

When discussing efficient flight speeds, it’s convenient to define a term called Specific Range, SR, as the distance the plane flies on one tonne of fuel. The maximum range is achieved when we fly at the altitude and Mach of the maximum Specific Range, Mmr, Figure 1.

A more used speed than speed for maximum endurance and range is Long-Range cruise speed which Airbus defines as the Mach where the Specific Range is within 1% of maximum SR. As the Specific Range maximum is flat in the speed dimension, it allows a higher distance coverage per time unit with a one percent sacrifice in Specific Range, Figure 1.

But the efficient flying of an airliner is more complex than the search for the lowest fuel burn per nautical mile. An airline operation has several cost factors that cost money per unit of time:

  • Flight and Cabin crew are paid to 100% by flight Block hour in the US (Block Hour = door closed to door open at destination). In other parts of the world, the pay is mainly by Block Hour. It’s supplemented by certain fixed or semifixed pay components.
  • Aircraft maintenance has three use limits for when parts and systems shall be scheduled for maintenance: Flight Cycle limits, Flight Hour limits, and Calender Time limits. The costs for Flight Hours and Calender Time maintenance affect the optimum flight speed.
  • The airplane’s flown Flight Hours are a component in the depreciation of an airliner’s value. Thus there is a cost part per flown Flight Hour in the aircraft’s value decline.
  • The Capital costs of an airliner are often calendar-based costs. Thus they contribute to the Calender Time cost factors.

This means airlines fly their fleet to a Cost Index speed that optimizes the Direct Operating Costs (D.O.C or DOC) of an airliner (Figure 2). The DOC optimal speed is a faster speed than Long-Range cruise. How much faster depends on how strongly the above factors are weighted into the Cost Index algorithm that is run in the flight deck’s Flight Management Computer/System (FMC for Boeing, FMS for Airbus).

Figure 2. Efficient Direct Operating Cost (D.O.C) speed. Source: Airbus.

The productivity factor

An airline also looks at the overall productivity of an airliner. Fleet management tries to schedule the planes to complete the maximum number of sectors per day to maximize revenue. As Airports can have night curfews, it can mean that flight speeds are adjusted upwards over what a CostIndex would prescribe in order to complete the maximum sectors for a day and, by it, maximize revenue. Domestic airliners are flown up to seven or more sorties per day, covering the maximum flight hours the airport destinations allow.

The above shows that many factors influence how an airline flies its fleet. The lowest fuel burn is important as it’s a major part of the overall operational costs but not the only part.

Flight hour or calendar time costs weigh into the equation and up the flight speed above what the pure airframe and engine combination has as optimum speed.

A reader commented that the ATC will influence what speed and altitudes can be flown. It’s correct; it’s why the next generation Open Rotor aircraft tries to fly at the same 0.78 Mach as turbofan Domestic airliners. Otherwise, there will be queues formed with the Open Rotor airliner leading the queue. There are several other aspects around ATC management of increasing traffic and how to give each airliner an efficient flight. It’s a subject in itself.

11 Comments on “Bjorn’s Corner: New aircraft technologies. Part 23. Efficient flight speeds

  1. For the very common 1hr jumps, the climb and decent are important parts of the flight. With RISE vs. regular 737Max/Neo engines how will the profiles change also taking into consideration RPN and continuous decent?

    • I noted the “tries to fly at Mach .78” for the open rotor, and am wondering if they’ll actually achieve that speed; ii quite seems problematic if they can’t.

      • The CFM RISE project says it will, and I thrust them on that. It’s about increasing the Specific Thrust a bit (by a higher fan pressure ratio) and careful transonic design of the props, spinners, and nacelle. It’s why the shape of the engine’s nacelle and spinner is as it is (Coke bottle shaped).

      • The problem is keeping the fan efficiency at those speeds. Besides the durability and life on wing problem of variable pitch fanblades and its anti-ice system at this power level and flight speeds. You would like to have the USN being the first RISE customer for a submarine hunting aircraft and hit all the problems first before airliners experience them.

        • Claes:

          Good point, so far its all spin (pun intended) and they don’t and can’t know unless they build a true prototype.

          There is a good reason Turbo Props don’t push anywhere close to that Mach figure.

          The only way I have seen to even get what they got to work is more fan blades and exotic shapes (nothing wrong with that). But, other than computer models a Mach .78 props job has never been done.

          There are some hot rod Turbo Props out there, the P-12 and the (Denali?) TBM. A 737 cruise speed is pushing 200 mph faster than any of those.

          You need a higher speed other than cruise if your need is to meet schedules, ie, able to make up time.

          Even assuming RISE met all its goals, the airlines are seeing the cost of new engines vs mature CFM-56. I can see Boeing or Airbus going to airlines and the airlines telling them they are out of their minds if they think they are going to gamble on RISE.

          I continue to firmly believe that RISE is nothing more than a red hearing to freeze the market until CFM comes up with a GTF and CFM gets an engine core and a gearbox out of the European program.

          • For what it’s worth, I also do not think that
            this RISE thingie is going to cruise at Mach .78, or very close to that number.

            I am not an expert, and we will see.

          • the Tu-95 (russian Bear bomber) does ~.75 mach with 1950’s tech turboprop engines and propellers.

            simply updating the propeller design to a fully modern design would likely be enough to get it to .78

  2. I share your concerns about the ability of an open rotor to reach M.078 at cruise speed. The A400M, which uses a propeller technology that is almost 20 years old, still has a maximum speed of M 0.72 at FL370 with a huge 5.3 meter diameter propeller . In view of the evolution of the propeller profile and sweep on one side and low diameter of the high thermal efficiency core engine ( by comparison with current turboprop core diameter ) , isn’t there room for substantial progress based on RISE shown model ? . We will see but having a first “trial ” through a military transport aircraft should make sense before airliners go ahead.

    • Mike,

      You would need a program and I see no new programs to try it on nor any reason for Airbus or Lockheed to do an NEO on the A400 or C-130-J.

      All programs I see the US sort of looking at are jet powered BWB designs of one type or another.

      If CFM builds a true prototype you can hang on the A380 test bird, then you might get an idea.

      But Mach .78 seems to be like 110% power if it can be done at all (other than a dive) .

      I can see why Airbus went with props as they wanted a short field aircraft and then it morphed into more strategic which a jet engine is more suited for.

      Other aspects is swept wings are not done for short field aircraft (Grippen aside).

      CFM will continue to try to hold the competition in place until they get caught up.

      The TU-95 is also worth looking at being a swept wing Bomber where speed is important and cruise is 440 mph.

      I don’t like Mach because its a tricky number
      as defined by what altitude you are at and plain speed numbers puts things into perspective.

      And a note, by swept wing I mean swept front and rear not just a tapered front (DC-3 had a tapered front)

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