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Why can we use flaps in short field take off but not in high density altitude airports?

Why can't they be used in high density altitude to shorten the take off distance as in short field take off?

Why short field take off with flaps doesn't apply to high density altitude airports?

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With more flaps, you will lift off at lower speed, but will have worse climb performance due to the increased drag.

On short field you are limited by the field length, so you need to lift off early and therefore need to use more flaps.

But in the mountains you are limited by the climb performance. Therefore you need to use less flaps.

Of course, this is just general rule. You should look up performance for given density altitude and compare it with available runway length and required climb gradient.

Update: Extending flaps never improves rate of climb. Up to some points it does improve angle of climb though, because it reduces Vx and the engine can produce more thrust at lower speed. The setting for best angle of climb is higher than for normal take-off and is used on obstructed field.

However as excess power reduces (with density altitude), Vx increases towards Vy (at absolute ceiling, Vx = Vy and rate of climb is 0) and the flap setting for best angle of climb decreases. So even for best angle of climb you shouldn't use as much flaps at high density altitude as at low.

And most mountain airports are not obstructed. In most cases the valley ahead is large enough, but you want to get above the tricky winds fast and you want high rate of climb in case you encounter a downdraft. And that means no or little flaps.

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  • $\begingroup$ I was taught to use same indicated air speed to rotate in short field take off and normal take off. Are you talking about true speed here? $\endgroup$
    – pmoubed
    Commented Nov 14, 2015 at 5:11
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    $\begingroup$ @PMoubed, no, indicated. And at low altitudes they are the same anyway. What you say is strange, because you achieve the same speed after the same distance—and if the field is short, you don't have that distance. On the other hand if the field is obstructed (runway is long, but then you need to climb steeply), this actually advocates rotating at higher speed (if paved, accelerating in ground effect if not). $\endgroup$
    – Jan Hudec
    Commented Nov 14, 2015 at 9:34
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    $\begingroup$ Some wing designs achieve best lift:drag with some flap deflection rather than flaps0. Flaps0 is still used for cruise because of reduced total drag at the lower lift coefficients needed for cruise. Thus best climb rate and angle (and ceiling) can be had with some use of flaps. However this is highly dependent on the design and it is possible the gains may be too small to balance the extra operational complication. Especially when including the variables of zero AoA ground roll, ground effect, and angle of climb vs point of rotation, for a 50ft obstacle. So POH settings are based on testing. $\endgroup$
    – Max Power
    Commented Jan 24, 2022 at 9:13
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A cursory glance of this site reveals that the slight increase in lift is not worth the larger increase in drag when deploying flaps at high-altitude/low air-density conditions. http://www.newconthenet.com/mountainflying.html

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    $\begingroup$ Since the list gives, as a second point, a rule about X% of take-off speed at Y% of runway length, while as explained here no such rule can be given with any generality, I would not trust it too much. Many of the rules are right, but the performance ones tend to be over-generalized—your performance calculation might yield different result. $\endgroup$
    – Jan Hudec
    Commented Nov 12, 2015 at 10:31
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It depends on how much excess horsepower you are packing.

Large passenger jets can and do deploy high-lift devices before takeoff because they are more likely to have enough power to overcome the drag penalty in the interests of increasing lift.

Small planes can't do exactly the same because they are more likely to be power-limited, and generally they have to take off in a low-drag configuration- meaning 10 degrees or less flaps on takeoff.

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    $\begingroup$ I'm sure I recall seeing at least 10% flaps recommended in some cases for takeoff in flight manual for Ces. 152. Just saying something is "power-limited" is too absolutist-- everything in aviation is affected by the fact that power is not infinite. It's just a matter of where on the curve the data point happens to fall, as to whether or not it is worth deploying flaps etc. $\endgroup$
    – quiet flyer
    Commented Mar 17, 2023 at 19:44
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    $\begingroup$ Here we go: "Using 10° wing flaps reduces the total distance over an obstacle by approximately 10%." and "On a short field, 10° wing flaps and an obstacle clearance speed of 54 KIAS should be used. This speed provides the best overall climb speed to clear obstacles when taking into account turbulence often found near ground level." $\endgroup$
    – quiet flyer
    Commented Mar 17, 2023 at 19:47
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This is not a question that can be answered with a general rule.

What must be examined, when taking off in a condition of lower atmospheric density, is the altitude limit for that weight.

The issue is: can the engine/prop generate enough thrust to fly?

The best hope is at the speed where prop efficiency is the highest.

is this Vmin power?

Well, no, because at full RPM the prop spins faster, so it needs a faster forward speed to keep relative wind at the best angle of attack.

what is that angle of attack?

Well, why not put a Clark Y on your wing and your propeller?

The magnificent Clark Y maintains optimum Lift/Drag efficiency from about 3 to 7 degrees, making it ideal for fixed pitch prop applications and wings for GA aircraft.

One can then evaluate the effects of lowering flaps even 10 degrees on lift and drag: it shortens gliding distance. In other words, it's L/D ratio is less.

At high altitudes, where thrust is limited, it is best to fly in the "sweet spot" where Lift/Drag ratios are optimum for prop and wing. This is where climbing performance, so important at higher altitudes is best.

Flaps may get one off the ground faster, only to provide a nasty surprise, once trying to climb out of ground effect, if power is insufficient.

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