March 16, 2018, ©. Leeham News: In the last Corner, we looked at the drag of an airliner during cruise. We could see the thrust required to counter the drag in the thin air of 37,000 feet was low, about 4,000lbf per engine.
Now we continue with the drag created by the aircraft during descent and landing.
We are assuming our aircraft is the A320neo type with a cabin with 180 seats, all filled with passengers. In the last Corner, we saw the cruise drag was 7,900lbf at our cruise speed of M0.78 and cruise altitude of 37,000 feet (FL370). This meant our engines needed to produce 3,950lbf each to keep a constant Mach of 0.78.
When we want to descend to our destination, we start the descent at a point where we can keep a steady descent with our engines at flight idle (a higher RPM than ground idle as the engines must produce electrical and hydraulic power to our systems and in addition, bleed air from the engine’s compressor to our cabin pressurization and air-conditioning).
We use our potential energy of Mach 0.78 at FL370 (37,000ft) to combat the aircraft’s drag during descent. We put our engines at flight idle so we can get a good descend speed of 2,000 to 3,000 feet per minute without speeding over M0.78 (our aircraft is not certified to fly faster than M0.82).
The drag of our aircraft is the same as our cruise drag at FL370 (7,900lbf) as we start the descent. To this, we shall add the thrust the engines produce at flight idle to get our force balance in the forward direction.
The engines still produce thrust at flight idle RPM, but their intakes now generate additional drag. The intakes are sized for climb and cruise thrust air flows. At the low engine airflow at flight idle, the intakes are spilling air over the sides, air which no longer can be consumed by the engines.
This spilling of air over the intake sides causes Form drag, as the curving of the air around the outside intake lips causes separations. This means the engines’ flight idle thrust will be compensated by added Form drag from the engine nacelles. The result will be a low net thrust from the engines during descent.
As we pass FL100 (10,000 feet), we must reduce our speed to 250kts indicated airspeed to coordinate all airport approaching traffic speed-wise. We are now flying at a real airspeed of 290kts or Mach 0.45.
Our drag is now 7,500lbf, composed of 5,000lbf parasitic drag and 2,500lbf induced drag. Our engines are contributing a minimal thrust of 200lbf each. The drag of the aircraft minus the engine thrust gives us a descend speed of 1,500 feet/minute.
As we pass 2,000 feet on the way to landing at the airport we extend slats, flaps and landing gear. The aircraft is now in landing configuration. This configuration is deliberately draggy.
When flying on the final stretch before touch down, we don’t want the engines to remain in flight idle. If for some reason we have to abandon the approach, we want the engines to accelerate fast to full thrust. The spool up time from flight idle is too long; we want a fast reaction as we raise the nose to make an aborted approach. Therefore, we welcome the drag generated from the extended landing gear and the slats/flaps, which forces us to have about 70% RPM on the engines.
A pilot feels how the landing flap causes the air to curve strongly around the wing, by it producing high lift and lots of drag at our low speed (we fly around 140kts on approach). This curving of the air causes the nose to dip down and the pilot must compensate with horizontal elevator trim. He also feels how he lurches forward from the aircraft reducing speed due to the drag when the gear and flaps extend.
As we are on final approach with gear and slats/flaps extended, our aircraft generates 18,000lbf of drag. Induced drag dominates at 10,500lbf as we are at low speed (Figure 1). Parasitic drag is 7,500lbf.
With the landing, we finished our trip through the drag types experienced by an airliner on a typical mission.