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Are full flaps ever used on takeoff?

One flying book I read strongly discouraged anything more than quarter flaps on most planes due to the amount of drag produced. I was just wondering if there are any scenarios where full flaps might be necessary.

Farhan
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flyingfisch
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    Even on a go-around taking the last notch out is generally recommended, the amount of drag goes up a ton for only a little bit of extra lift on the last setting usually. – p1l0t Jan 30 '14 at 19:10
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    @p1l0t Tell me about it. Going around with full flaps is not a pretty sight. Tower even asked me if I had engine trouble. – Philippe Leybaert Jan 30 '14 at 20:40
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    The POH is authoritative source for the valid flap takeoff configurations. – Steve Kuo Oct 11 '14 at 19:21
  • I did that once (full flap touch and go). It wasn't good. The aircraft felt super sluggish, like I was in slow flight while taking off... Definitely not something you want when you are that close to the ground. – Keegan Oct 12 '14 at 21:55

5 Answers5

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First, you have to remember on some configurations, the lower stages of flap will mostly add drag and very little lift. Sometimes that extra notch of full flaps is only there to change the camber of the wing to add a bit of a nose down attitude to help with visibility on landing. Both are items you do not require on takeoff.

Now, when selecting flaps for take-off there are two things to consider: runway length and obstacles to clear after the runway:

Runway Length

Generally when lowering flaps your $V_{\text{mu}}$ (the speed at which you become airborne, “minimum unstick speed”) will decrease. This means a shorter runway for normal stages of flaps. As you continue to add more flaps your $V_{\text{mu}}$ will be lower, but because of the added drag, which decreases acceleration, it will actually take a longer distance to reach that lower $V_{\text{mu}}$. There is a sweet spot in-between, though. “Take-off Flaps” is not always designed to give you the shortest takeoff roll.

Obstacle Clearance

And that is because once airborne, the more flaps you have deployed, the higher the drag, the less excess thrust available, thus the less you can climb. Simply put, you are now flying at a low speed and barely climbing. Not a good idea if there are tall trees waiting for you at the end of the runway.

It would be illegal to take-off on full flaps if:

  • POH says it is prohibited,or
  • For commercial flying, if the flap setting does not provide the minimum required gross obstacle clearance climb gradient (depending on the type of operation and number of engines, between 2.4%-3.0% ).
Radu094
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    Then there are some crazy bush flying techniques like accelerating w/out flaps to Vunstick, lowering flaps to bubble off the runway, accelerate in ground effect while retracting the flaps, and climbing at Vx. Guaranteed to scare your instructor and/or fail you at your checkride!! – Radu094 Jan 30 '14 at 22:25
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    wow, that's pretty crazy. also, is there a 'correct' term for Vunstick?? – flyingfisch Jan 30 '14 at 22:36
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    Actually Vmu is the correct term, with mu meaning 'minimum unstick'. Can't find a decent page on the internet about it though. – Radu094 Jan 30 '14 at 22:38
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    Ah, ok, I thought 'unstick' was an informal term ;-) – flyingfisch Jan 30 '14 at 22:43
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    I really like this answer because it provides a lot of useful information, but I think the first paragraph has two wrong statements. (1) remember that in a wing, the induced drag is induced by lift, so lift must be increased in order for drag to increase (see http://www.vatnz.net/_media/images/raw/image3.JPG) (2) That last notch of flaps is there to increase the angle of descent, not to provide visibility. I wasn't sure how to edit the first paragraph without changing the whole answer. – rbp Oct 12 '14 at 18:55
  • edited the text for (1), which is true. For (2) I'd still argue with a list of (small) a/c that have the landing flaps there to compensate poor fwd visibility from the cockpit. – Radu094 Oct 13 '14 at 13:16
  • @rbp, I've vetoed your suggested change: As a 1.70m pilot I really struggle with fwd visibility for flaps-up landings on some aircraft. Full flaps really helps in all Diamonds a/c and on some Cessnas. – Radu094 Oct 13 '14 at 13:42
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    @Radu094 just because you personally get better forward visibility, that doesn't mean that full flaps were designed to improve visibility. If you can find any actual evidence that any aircraft manufacturer added landing flaps to improve visibility rather than improve the angle of approach, then I will retract my objection. – rbp Oct 13 '14 at 14:32
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Typically more flaps will get you in the air sooner, but due to the added drag your climb will be shallower. So when the runway is really short and there are no obstacles, full flaps might be the solution.

There are aircraft that are not certified for full flap take-offs and in some cases that additional drag might in fact reduce the performance so badly that the take-off run will be extended. The operating manual will provide guidance.

DeltaLima
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    the standard procedure for a short-field takeoff is to lift off into ground effect, accelerate the aircraft in ground effect until reaching Vx, then pitching the nose for Vx. Full flaps will increase the induced drag as the plane accelerates, thereby increasing the runway length required. – rbp Oct 12 '14 at 18:47
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Not all flaps are the same and you should read the poh for the suggested and prescribed way to operate the aircraft you are flying.

For example the Grumman AA5 series flaps provide a lot of drag and no real extra lift nor slow speed ability. They do allow sharp descents.

Whereas a Cessna 172 flaps will increase lift and lower stall speed.

Read the manuals.

Tim
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Full flaps can be used on take-off whenever it is called for in the POH, but I have never seen it in any of the 40 different aircraft I have flown.

In some aircraft that I've flown (eg Cherokees and the Pilatus PC-12), there is an intermediate flap settings (about 25 degrees) which is used for short/soft field takeoffs because the extra lift from the downwash from the flaps helps the plane liftoff.

rbp
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As a certification requirement, all planes must be able to climb from sea level at max gross weight with full flaps deployed, in standard day conditions. As has been noted by others, the rate of ciimb will be anemic.

In the PA-28 that I fly, the third notch of flaps adds a lot of drag and very little lift. POH procedure is to use two notches, not three, of flaps for a short field takeoff.

Skip Miller
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    Are you sure about the certification part? I would be very surprised if a fully loaded 160HP C172 with full flaps would climb at all. – Philippe Leybaert Jan 30 '14 at 20:38
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    Straight out of the factory it had to, or it wouldn't get certified. But engines do get a bit tired with years of use and abuse. Certification requirements are a one time pass or fail. Then, as long as the planes are made under the same type certificate, they do not need to pass the certification requirements again. – Skip Miller Jan 30 '14 at 21:24
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    @SkipMiller: not necessarily -- if the aircraft can retract the flaps (partially or fully) in 2 seconds or less, it can use that flap setting for the go-around (14 CFR §23.77). – Qantas 94 Heavy Jan 31 '14 at 04:02
  • What year was this in force? – Tim Feb 22 '14 at 22:28
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    Please cite a reference for this certification requirement. My airplane (M20M) will barely climb at sea level with full flaps and 36", let alone to 25,000ft (its certified ceiling). – rbp Oct 12 '14 at 18:32
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    The takeoff performance requirements in FAR Part 23 are predicated on the use of the takeoff flaps setting. – Porcupine911 Mar 12 '15 at 19:36