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In most POHs, the flap configuration for short-field is usually recommended as partial flaps, for instance 10˚ for C172.

I remember the arguments are that full flaps would reduce the acceleration rate, and in no flaps the airplane would have to accelerate to higher speed to off the ground. In either case, the ground roll distance is longer than the optimal partial-flap configurations.

However, this assumes a fixed flap setting during ground roll. How about let the airplane accelerate to a certain speed (in zero flaps) before adding to full flaps? We could achieve best acceleration as well as best lift-off speed?

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    $\begingroup$ Timing this correctly would add more workload on the crew, I guess that's a pretty good reason not to do it. $\endgroup$ – JustSid Mar 13 '16 at 8:32
  • $\begingroup$ @JustSid perhaps this feature could be added as an automation, for example in a FBY system (yes I know certification process, risk reduction, reliability, ...) $\endgroup$ – Manu H Mar 13 '16 at 8:40
  • $\begingroup$ @JustSid I figure there are some tasks of comparable workload, like deploying spoilers/thrust reverser during landing. However, this is even not recommended for multiple crew. $\endgroup$ – skyoasis Mar 13 '16 at 8:50
  • $\begingroup$ @skyoasis It would be interesting to find out what military pilots do when taking off of short runways-or those threatened by ground fire. Anything you can do to make your lift off quicker would be better, IMO. $\endgroup$ – user2479 Mar 13 '16 at 12:03
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    $\begingroup$ @kevin The maneuver you describe sounds like a soft field takeoff, which you have tried if you are an FAA ASEL pilot. $\endgroup$ – J Walters Mar 14 '16 at 0:04
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I found this answer is a good analysis.

13.3 - Obstructed-Field - Skimming versus Wheelbarrowing or Flap-Popping

"Another possible procedure (which is usually not recommended) is to keep the flaps retracted until you are ready to leave the runway. Less flaps means less incidence. A big disadvantage is that “popping” the flaps like this increases your workload at a time when there are lots of other things you should be attending to. Another disadvantage is that you run the risk of extending the flaps past the takeoff position to the landing position, creating lots of drag, which is really not what you want in this situation. If your POH calls for this procedure, go ahead, but be careful. Make sure you have some sort of detent to block inadvertent over-extension."

-- John S. Denker, See How It Flies

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Flaps have the dual function of increasing lift and drag. When they are fully extended, the drag greatly outweighs the extra lift. So whilst you might have a slightly shortened takeoff run (though I doubt that there is a significant reduction in a C172), you could very well struggle to clear any obstacles. And not forgetting that the takeoff distance required is defined as the distance to climb to 50ft, climbing out on full flaps could actually increase this distance.

You might be tempted to say you should extend the flaps just in time to rotate, then retract them immediately once airborne. Retracting flaps at such a low speed and altitude is incredibly risky. Plus this is a lot of extra work for the pilot at an already critical phase of flight.

In most aircraft, using the smaller flap setting provides extra lift with only a bit of the drag. Always follow the POH - it's written by the folks who know the aircraft and its performance better than anyone.

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  • $\begingroup$ Maybe a special setting for takeoff would help. The flaps would be set at 10 degrees, but would automatically retract when the squat switches on the landing gear opened. $\endgroup$ – Howard Miller Mar 16 '16 at 13:05
  • $\begingroup$ @HowardMiller I see where you're coming from, but retracting the flaps right after rotation is not safe. The point of having flaps out for a takeoff is to lower the stalling speed, and thus the takeoff speed required. By retracting the flaps, suddenly the stall speed is raised, when you're already at a very slow speed. Flaps at 10 degrees/1 stage in most GA aircraft still provides enough climb performance and lowers the takeoff distance required, so it is the recommended procedure. $\endgroup$ – Ben Mar 17 '16 at 5:41
  • $\begingroup$ it's just an idea. My personal philosophy in design is to keep it as simple as possible. $\endgroup$ – Howard Miller Mar 18 '16 at 4:36
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Yes. But only partial flaps will help; full flaps are almost always a poor choice for takeoff.

What you describe was the standard operating procedure for the Messerschmitt Me-262s in the JV 44. Due to the low thrust at low speed it had a very long takeoff run, and to operate from short airfields designed for propeller aircraft and hastily repaired after attacks it needed to employ this technique. Note on the picture below the small deflection angle, however: This only worked well when the takeoff run was made with zero flaps and then takeoff flaps (not full flaps!) were added shortly before rotation.

The downside was the risk of an overshoot in case the flaps failed to deploy. If your airfield is long enough, it is certainly safer to set the flaps at the beginning of the takeoff run.

Me-262 two-seater replica taking off

Me-262 two-seater replica taking off (picture source)

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  • $\begingroup$ This is highly dependent on airplane type. Some airplanes have very effective full flaps and some do not. $\endgroup$ – GdD Mar 14 '16 at 9:29
  • $\begingroup$ @GdD: The biggest dependency is flap deflection angle. A split flap is a poor choice for short takeoffs because of its drag, yet for small deflections even this type of flap helps to shorten the takeoff run. The next dependency is flap type, and I assume that is what you meant by airplane type. Sure, other parameters like aspect ratio and sweep also play their part, but when you look at the same aircraft with and without flaps, the deflection angle determines whether those flaps can be helpful or not. For some types of flaps the helpful range of angles is rather small. $\endgroup$ – Peter Kämpf Mar 14 '16 at 10:56
  • $\begingroup$ @GdD Full flaps are designed for landing (high lift and high drag) and are almost never helpful for shortening the takeoff run. I overlooked that in the question and edited the answer. $\endgroup$ – Peter Kämpf Mar 14 '16 at 10:57
  • $\begingroup$ I know, that's why I said it's highly dependent on type. $\endgroup$ – GdD Mar 14 '16 at 10:59

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