How does setting takeoff flaps improve aircraft performance?

Question

A study guide I recently found indicates that Vmc and aircraft performance improve with takeoff flaps set. The Vmc improvement makes sense but the performance improvement does not. At a basic level, drag = bad and flaps add drag...a lot of drag.

The question is, "how does aircraft performance improve with takeoff flaps set?"

The Airplane Flying Handbook (FAA-H-8083-3B) page 12-17 indicates that takeoff flaps will degrade aircraft performance:

If flaps were used for takeoff, the engine failure situation becomes even more critical due to the additional drag incurred.

See also FAR 23.149.

Answer 1

I think the term "takeoff performance" in that context means takeoff distance and departure gradient for obstacle clearance. Obviously, takeoff distance is shorter with takeoff flap than without. And a departure with takeoff flap will generally, depending on the airplane, have a steeper gradient than without, even though the total climb rate is degraded with takeoff flap out. On an IFR departure, or a regular airplane clearing trees at the end of the runway for that matter where you fly at best angle instead of best rate, gradient is the key performance factor, not so much rate of climb.

Transport aircraft generally don't start to accelerate to retract flaps at least until reaching 1000 ft agl, to keep the initial climb segment as steep as possible. Some noise abatement procedures for airliners require keeping takeoff flap deployed beyond the normal retraction schedule, typically to 3000 feet instead of 1000 feet. This is to keep the climb as steep as possible to a higher altitude so as not to upset unusually sensitive neighbours farther away from the airport.

That sort of thing is a luxury that jets enjoy because of the massive power reserves. Not so much in a piston twin, which has very little performance reserve when single engine and loaded. Where an AFM says to do so, you would want to avoid climbing with takeoff flap any more than you have to for obstacle clearance, since if an engine quits, the airplane's climb gradient becomes more of a descent gradient until flaps and gear are retracted, and even then what climb you get is pretty stately - and only if your speed and technique is spot on.

So I don't think the seemingly contradictory conditions you gave are inconsistent in that context.

Answer 2

My answer is (as in most cases): It depends. Mostly on the type of flap used and the position defined as zero.

Camber flaps

Camber flaps do not necessarily increase drag, at least at moderate deflection angles. Maybe it helps to think of an unflapped wing as of one where the flaps have been permanently fixed in one setting. And this setting does not necessarily need to be the one which offers minimum drag. Changing the setting of chamber flaps shifts the optimum angle of attack range up or down without changing drag much - until you reach extreme flap settings where the contour break is too big to be left unnoticed by the airflow.

The plot (made with XFOIL 5.4; own work) shows how the laminar bucket is shifted up and down the c$_L$ range without affecting drag much except for the -20° setting - here, the suction peak on the lower side in combination with the strong flap camber is too much for the boundary layer to remain attached. But this setting is really useable for inverted flight only.

Setting flaps to a slightly positive angle does indeed increase performance:

• The maximum lift coefficient is higher, allowing lower lift-off speed and a shorter ground run.
• The maximum climb angle is increased greatly especially for propeller aircraft as the aircraft can climb at a lower speed where thrust is higher for the same engine performance. Note that the polar point for maximum climb angle sits at a high lift coefficient, especially for a wing with high aspect ratio. Therefore, positive flaps might be required to operate the aircraft at its best climb angle speed safely.
• Maximum sustained turn rate is also helped greatly with moderate flaps. The plot below shows calculated turn radii for a large unmanned reconnaissance aircraft at several rates of climb (or descent). The line for 0 m/s is the stationary turn rate.

Turn rate diagram for two flap settings: 0° (blue lines) and +10° (red lines). Own work.

Several fighter aircraft used flaps effectively to maximize their turn rate in dogfights (google for "combat flaps"). Also, the living wing concept of the F-18 adjusts flaps to maximize wing performance in all situations. This would not help if flaps only reduce take-off runs.

Fowler flaps

These flaps increase wing area, so here any actuation will also increase the friction drag coefficient, since that is referred to the wing area with retracted flaps. However, since the increased wing area allows to fly more slowly, the total drag might be lower, depending where on the polar the aircraft flew before extending flaps. A moderate take-off setting will, therefore, also result in reduced drag if the aircraft is able to fly more slowly and induced drag is less than half of total drag.

At full deflection, fowler flap drag grows out of proportion to lift, but that is one of the design goals. The higher settings are meant for landing only.

Answer 3

It reduces Vmc in the same way that having the landing gear extended does ie by creating a ‘keel effect’ with an increased induced drag behind the CG and moving the CL more aft on the chord line. This will increase longitudinal stability and reduce the minimum control speed. Also flaps set to the AFM’s approach or takeoff setting increase lift with only a modest increase in drag. Check with what the manufacturer recommends here.

Answer 4

Flaps do indeed increase drag.

They also increase lift.

The reason flaps are used for takeoff is that the increased lift allows the aircraft to become airborne at a lower speed (which reduces the amount of runway needed for taking off) and - up to a point - climb more steeply for a given speed (which increases obstacle clearance after taking off). The downside of flaps is that the increased drag requires more power from the engine(s) in order to maintain speed; sans engines, or sans some of a multiengine aircraft's engines, the aircraft's airspeed airbleeds off more quickly with flaps than without, due to said increase in drag (this is aggravated by the increase in lift, which makes the plane want to go up - trading airspeed for altitude - more than it otherwise would), which makes you sink more steeply and come down shorter than you would with flaps retracted.