Bjorn’s Corner: Aircraft drag reduction, Part 20

By Bjorn Fehrm

March 9, 2018, ©. Leeham Co: In the last Corner, we started to go through a typical mission for an airliner and study which drag types are important when and why.

We went through the take-off and climb phases, now we continue with the cruise phase.

Figure 1. An aircraft’s drag profile as airspeed varies. Source: Leeham Co.

The drag of an Airbus A320 during Cruise

We are assuming our aircraft is the A320neo type with a cabin with 180 seats, all filled with passengers.

When we climb, the Flight Management System (FMS) is programmed with a climb profile, where we will fly at the Climb Mach limit of 0.76 from around 30,000ft.

As we climb, the air gets thinner and we lose thrust because of thinner air and forward speed (thrust lapse). At 37,000ft (FL370) we will be at Top of Climb (ToC) for this rather light aircraft. Our climb speed is then reduced to 600ft/minute from 3,500ft/minute when we started our climb.

In level flight at FL370, our cruise drag, at our cruise speed of M0.78 and average mission weight, is 7,900lbf. This means our engines need to produce 3,950lbf each to keep a constant Mach of 0.78.

The 7,900lbf of drag is composed of 4,700lbf of Parasitic drag or drag independent of lift and 3,200lbf of Induced drag or drag caused by lift.

The Parasitic drag has Air friction drag as the dominant part but also contains drags we’ve discussed like Form drag, Transonic or Compressibility drag and Interference drag.

Form drag would be around 7% of Parasitic drag, mainly coming from airflow which is disturbed by air-conditioning inlets and outlets and airflow separations caused by the upsweep and contraction of the tail of the aircraft. Separations are also caused by gaps around ailerons/rudders/flaps and the end of flap fairings and engine pylons/nacelles.

The Transonic drag, stemming from the supersonic areas of the wing, would be around 5%.

Finally, Interference drag, mainly formed around the engines, would be around 3%.

This means 75% of our Parasitic drag is made up of air friction drag against the aircraft’s wetted surface.

There are other drag factors, but these are the main ones and the ones we have discussed. The important ones are Air friction drag and Induced drag. These represent 85% of total drag of an aircraft.

This is why aircraft designers try to minimize the total surface of the aircraft at the same time as they try to make the wingspan as wide as possible.

It’s also why a figure of merit for the wing is aspect ratio, that is, the wing’s span squared divided by the surface of the wing. The wingspan reduces the induced drag and keeping the wing surface low keeps the air friction drag low.

Descent and landing

In the next Corner, we’ll finish the mission by discussing drag during descent and landing.


10 Comments on “Bjorn’s Corner: Aircraft drag reduction, Part 20

  1. It could be interesting to see the effect of a new carbon wing with 2m high blended winglets on an A321neo and slighty bigger fans of +1″, +2″ diameter on performance for the same and increased fuel loads.
    The new slender carbon wing should have better L/D and allow a lower rotation speed for the same mass or higher MTOW for the same V1.

  2. What’s the upward force on the aircraft in cruise, 120K? It’s amazing the efficiency of flight that 4K forward thrust through induced drag produces 30 times the lifting force.
    In cruise, how much induced drag on the Gossamer Albatross for it’s weight , and how much induced drag on the Valkyrie for it’s weight? Which one has better lifting efficiency. What is airspeed on the graph where the drag lines cross?

    • Hi Ted,

      in cruise, the upward force is the same as the aircraft weight in US units lb. The aircraft is neither gaining nor losing height so the force balance in the vertical direction is zero. In SI units, you take the mass in kg and multiply with g to get the force in N.

      In this case, the average cruise weight was 146,000lbm or 66t.

      The lift to drag ratio for modern long range airliners is around 20 at best, during take-off around 13. For short range aircraft somewhat lower.

      For the Albatross you can guess a Lift over Drag ratio of 25 and calculate the forces.

      • Thanks. Since the parasitic drag on the A320 amounted to about 60% of total drag, what do you think the percentages were on the Albatross?

        • I wouldn’t want to guess. The Albatross is flying real slow but on the other hand, the wetted area is large. Look at the graph in figure 1. You can be sure the project has aimed for the bucket minimum between the parasitic drag and induced drag when designing the aircraft.

          You can calculate the total drag as equal to what power a cycling person can generate times the propeller efficiency, which shall be about 80%.

          • It is cycling power multiplied by propeller efficiency then divided by the flight speed to get thrust force (in this case equal to the drag force).

          • F=P/v So, given say 180w power delivered to the aircraft’s forward motion, at 13km/hr, that would be about 50N.

            Or for a downward force on the aircraft of about 1,000N, a 50N horizontal force.

            How that 50N splits up between parasitic drag and induced drag is unknown. By the graph, induced drag at slow speeds is high by some relative measure, so that seems to indicate induced drag could be a larger percentage in this case.

  3. Good evening Bjorn,
    Thank you for the education. While much of this is over my head,
    the part where you describe the engine power requirements at take off is fascinating. You always hear about max engine thrust, but to see it broken down through the flight profile was a treat.

  4. A very interesting corner.

    I have been wondering what effect the wing sweep has on drag. It appears that most airliners seems to have a similar degree of sweep to their wings. Is this an optimal figure ?

    For two wings with the same aspect ratio but different sweeps how does this affect the L/D ratio, is it directly proportional ?

  5. I had always been under the impression that surface area was more important than wingspan in creating lift. However, now that I understand the effects of drag, it makes sense that you would want to keep the wing surface area low and increase its length, which is actually what is producing the lift. I have a new appreciation for the effects of drag.

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