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