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I've read that some aircraft (examples include the Saab 340 and the Fokker 100) can perform flapless take offs, provided there is enough runway available. It's also my understanding that, in terms of lifting off the ground, any aircraft would be theoretically capable of that, as long as there were a long enough runway (which isn't the case in practice).

However, I was wondering why aircraft such as the ATR-42/72 or the Dash-8 still lower their flaps for take off, even from runways that are over 3000m/10000ft long. At least those I've flown with always did that.

Is there some other reason for non-flapless take offs, even with very long runways, for some models? For instance, anything related to pitch/roll stability?

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For an aircraft designed for cruising efficiency it is important to have as little trim drag as possible.

See ATR 72 link:

The wing may appear unusually small, but it is made to produce the needed lift at cruising speed at its most efficient Angle of Attack, where lift to drag ratio is highest. If the wing produced too much lift for level flight at its cruising speed, it would have to be "trimmed" to a lower AOA, at the expense of greater drag, there for a greater rate of fuel consumption.

The contribution of velocity to lift is described in the lift equation:

$$ L = \frac{1}{2} \cdot C_L \cdot \rho \cdot A \cdot V^2$$

with $L$ being the lift, $C_L$ the lift coefficient, $\rho$ the density of air, A the wing area, and $V$ the aircraft velocity.

The ATR 72 cruises at 280 knots. In order to generate adequate lift at lower velocities it either has to dangerously increase its AOA, or increase its coefficient of lift by adding camber.

Lowering flaps add camber, which is why this plane does it, as part of its take-off procedure. Higher flap settings are generally avoided because they produce too much drag.

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  • $\begingroup$ There have been many great answers – thank you all – but I accept this one because it specifically addressed model variation, which was a central part of my query. $\endgroup$ – Digital Dracula Dec 31 '19 at 8:45
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    $\begingroup$ Your link just goes to Google results. Was that intentional? $\endgroup$ – Lightness Races in Orbit Jan 1 '20 at 16:53
  • $\begingroup$ Yes, good pictures and info about the ATR72. $\endgroup$ – Robert DiGiovanni Jan 1 '20 at 17:39
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One function of common inner span flaps that's not often discussed is stall management.

Not only does some flap lower the stall speed, but it ensures the wing root stalls while the tips are still flying; this prevents an incipient stall from turning into a low altitude stall-spin if a wind gust or unplanned maneuver pushes the aircraft from "controlled climb" into "accelerated stall". Given that low altitude stalls have a very high fatality rate, avoiding stalls is important, but a "barely a stall" may still be recoverable as long as the aircraft stalls straight ahead.

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    $\begingroup$ I challenge you to design a wing with slotted flaps inboard and no flaps or slats outboard. Go and test the stall characteristics: They will be awful. You absolutey need to add slats outboard for good stall and highest lift. $\endgroup$ – Peter Kämpf Dec 31 '19 at 8:18
  • $\begingroup$ Look at any 1970s Cessna or Piper light plane. Not a slat in sight, flaps only inboard of the taper break (on Cessna) or about 1/2 of half-span (on Piper). Neither brand was known for bad stall characteristics in the day. $\endgroup$ – Zeiss Ikon Dec 31 '19 at 12:06
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    $\begingroup$ Straight wing aircraft can be designed to have benign stall characteristics with large stall bucket in the clean configuration. Swept wing aircraft is a totally different matter; deflecting flaps without slats won't help with stall characteristics. $\endgroup$ – JZYL Dec 31 '19 at 13:13
  • $\begingroup$ @JZYL That's because the sweep puts the flapped portion of the wing ahead of the unflapped portion, relative to center of mass. When I answered this, the question didn't mention specific models or straight vs. swept wing. $\endgroup$ – Zeiss Ikon Dec 31 '19 at 13:22
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Aside from the decreased takeoff speeds, there are a couple reasons why typical Part 25 aircraft do not allow flapless takeoff:

  1. There is usually a sweet spot at lower flap settings that generate the best climb gradient at V2, and it's usually not with flap retracted. It is typical to see several takeoff flap settings that cater to best climb and best field.

  2. Unless there is a performance reason, for example the point above, manufacturers would prefer not to add extraneous flap settings for takeoff. In order to certificate a takeoff flap setting, one would need to do a bunch of expensive/high-risk flight testing, coupled with tons of analyses and paperwork.

  3. Allowing flapless takeoff does not seem to make sense from an operational dispatch reliability perspective. Therefore, if there is no performance improvement, it would not add any value to operators either.

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In addition to the previous answers, you could argue that yes: provided you had a long enough runway you could theoretically perform a takeoff run that accelerates you to a speed which is sufficient for taking off and remain airborne with no flaps (and for this exercise let's assume an A320 has a flaps-up speed of 210 knots, give or take), but you'd be failing to include very important aspects in your analysis:

First, as you move faster, the amount of runway you are "consuming" increases at an ever growing rate. A 50 knot increase in groundspeed when you are rolling at 80 knots consumes far less runway (as in several thousand feet less) than a theoretical increase of 50 knots when you're travelling at 160 knots (even if acceleration was linear, which it's not, and it took the same amount of time. So, the longest runways that you generally find in commercial airports (around 14-15 thousand feet) would probably need to be in the vicinity of 40-50 thousand feet to takeoff with no flaps in a loaded A320.

But, second: landing gear are not designed and built to withstand the stresses of such speeds. They'd probably wear down or even be damaged/destroyed in a single takeoff run and likely EverbodyDies™

Also, third (and probably most important), aircraft are built specifically to minimize the amount of energy required to put them in the air and physics, being the crazy rascal it tends to be, will find a way to make this even more complicated by means of the ground effect. As you travel faster through the ground on a vehicle which is designed to fly and generate lift (or, seen in the opposite way, to generate as little downforce as possible) would tend to generate enough lift to get separated from the ground before actually achieving the desired flaps-up speed and this would (and has been) fatal: the aircraft would initially climb a bit, maybe 50ft and then lose the lift provided by the ground effect ... You'd be left flying insanely fast (let's say, for the sake of argument, at 180 knots), 50ft above the ground and suddenly without the ability to generate enough lift to remain airborne (because you are now too far from the ground for ground effect to exercise any influence over your aircraft). You now plummet into the ground like a piano, destroy your landing gear and severely damage your aircraft's structure, break up and fire up in a massive ball of flames at 180 knots (over 200 miles an hour). And yes, this has happened: 20 years ago, an Argentine Boeing 737 started its takeoff run with no flaps and killed 65 people when it fell back onto the runway after losing the small amount of lift provided by the ground effect... You can read more about that accident HERE.

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    $\begingroup$ Reading that accident report, the aircraft never lifted off at all. Which makes more sense than your description—ground effect does not make significant difference to the stall speed or critical angle of attack, it mainly reduces induced drag. When a plane can fly in ground effect, but not out of it, trying to climb out of it will not create abrupt stall, but will make the plane start losing speed and settle back to the ground effect (unless the control column is pulled too hard, but than it's that pull that's causing stall, not leaving ground effect). $\endgroup$ – Jan Hudec Dec 31 '19 at 0:54
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    $\begingroup$ Ground effect does in fact increase lift at a given speed and angle of attack. So - for a given lift, you could reduce speed. $\endgroup$ – Carl Berger Dec 31 '19 at 9:58
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    $\begingroup$ Regardless of the cited report, the assumption that the airplane - when leaving ground effect, will gently return to it if it stalling outside the ground effect, is quite optimistic.. too many nonlinearities here.. control surface authority, pilot training … plus you may be in unstable flight regimes or where the velocity vector is pointing such that ground effect won't be enough for recovery. $\endgroup$ – Carl Berger Dec 31 '19 at 9:59
  • $\begingroup$ I never thought of that point - of course you don't want the landing gear going excessively fast! Good one $\endgroup$ – Fattie Dec 31 '19 at 17:32
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    $\begingroup$ It's more the tires than the landing gear which has a limiting speed on most airplanes. $\endgroup$ – Lnafziger Dec 31 '19 at 19:56
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Several other good answers, but one reason I haven't seen yet is pilot proficiency. You don't want pilots only practicing short-field takeoff and landing technique on actual short fields, which might be rare depending on 5he routes any given pilot happens to fly. If they use that technique on every takeoff and landing, though, then you know they'll always be proficient for when it's actually needed.

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It is theoretically possible to do a flaps up take off if you have a long enough runway, but why would you want to? An airplane is designed to be most efficient in the air, so the sooner you get there, the better.

The lift available to an aircraft is proportional to the area of the wings (and flaps increase that area), however, the drag those wings produce is proportional to the square of the airspeed, so at low speeds (ie at typical take-off speeds) extending the flaps a little gives a lot of lift for relatively little extra drag. That extra lift means a) you can climb harder, faster and sooner, and b) you are can go slower before you stall and c) (as Zeiss Ikon explained) if you do stall, it will be more controllable/recoverable. While landing, extending the flaps at the higher landing speeds means a) increasing the drag (so slowing you down) b) increasing the lift (so slowing your descent) c) reducing your stall speeds (so slowing your landing speeds) and d) making stalls more manageable (making landing safer), which, you have to admit, is win, win, win and win.

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Tires. Tires are under very high stress normally due to the compromise between extreme performance requirements and low weight. Tires aren't cheap. By doing a higher-speed-than-necessary takeoff, you're increasing tire wear and inviting tire problems that you just don't need to invite.

It would be a very false economy to save a little in what exactly, actuator motor cycles? and pay for it in increased tire wear and safety incidents.

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