Bjorn’s Corner: Aircraft drag reduction, Part 21

By Bjorn Fehrm

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.

Figure 1. Drag of an aircraft at different airspeeds. Source: Leeham Co.

Drag of an Airbus A320 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.

Descent

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.

Landing

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.

8 Comments on “Bjorn’s Corner: Aircraft drag reduction, Part 21”

1. I have really enjoyed this series Bjorn and learned much!

• Bjorn, I echo Mike’s comment and look forward to the next topic at Fehrm Aero University.

2. Great series Bjorn. Is this the last? I hope not…

What’s the fuel flow for this A320 at flight idle? Does it vary with altitude at all or is constant? What’s N1 as a percentage of max or cruise N1?

• Hi Eric,

sorry for the late answer, was traveling and needed my GasTurb tool to answer your questions. The flight idle case is one of the tougher parts in controlling a gas turbine over the FADEC. The aircraft OEM wants the least possible thrust, whereas the engine manufacturer has optimized the engine for cruise conditions.

Full and climb thrust is a matter of going to the limits of the engine while sacrificing SFC as these phases are short in most cases (a regional engine needs more focus on climb SFC).

When you throttle back an engine to flight idle the compressors approach stall, just like a wing does at low airspeed. You have variable guide vanes (= the wing’s slats) but you often also need bleeding compressor air to the bypass duct to keep the airspeed up in the compressors. This increases the fuel consumption (wasted compression work).

The flight idle lower limit is most often not set by compressor stability but the aircraft needed electric, hydraulic and bleed power. To produce these over the engine gearbox the core needs to work at a certain level.

I get an engine thrust level at around 1500lbf as probable but the aircraft trust will be lower due to engine intake miss-match. TSFC is now close to double the cruise value but you haven’t got a lot of produced lbf so actual fuel consumption is insignificant. N1 would be around 55% of full RPM. And all these values vary with altitude and aircraft speed.

3. How many parts will Aircraft Drag Reduction have?

• No 21 was the last one. New theme this week.

4. Hi Bjorn, Again, you do a GREAT job describing the subjects helping ‘new’ and ‘old’ people interested, or active in the Aviation World. Thank you, THANK YOU!

5. Thank you for posting these articles Bjorn, they are fantastic! Can’t wait for the next one.