Flaps and effect on take off roll

[desertduster]

An interview question the other day was: on 747-400 if we use more flaps for take will be be using a shorter runway or a longer runway, and what will the effects be on stage 2 climb.. would the normal flap setting not already be the optimum lift vs drag, so an increase would increase ground run?

any insight would be great?

[org]

In general, the less flaps used the longer the takeoff roll and the better the climb angle will be for 2nd segment.

[Dream Land]

I am clueless about this stuff but I remember using maximum flap for maximum lift and using the least amount of flap possible when climb gradient is important.

[Keith.Williams.]

Increasing flap angle reduces lift-off speed but also reduces acceleration rate.

The reduced lift-off speed tends to reduce take-off run.

But the reduced acceleration rate tends to increase take-off run.

If we increase flap angle up to the optimum angle, the dominant factor is the reduced lift-off speed, so the take-off run is reduced.

But if we increase flap angle beyond the optimum angle, the reduced acceleration becomes the dominant factor so the take-off run is increased.


Climb angle is proportional to excess thrust, which is thrust minus drag.

Increasing flap angle increases drag.

So any increase in flap angle decreases excess thrust and decreases climb angle.

So climb angle is maximum when climbing with zero flap.


Putting it all together.

Increasing flap angle up to the optimum value will decrease take-off run and decrease climb angle.

Increasing flap angle beyond the optimum value will increase take-off run and decrease climb angle.

[desertduster]

Thanks for the reply s. That is along the lines of what I told him. some flap will decrease the roll, and at the stage where flap turns to drag TO roll will start to increase. I guess we will see in a few weeks if I conveyed my message right.

[tbaylx]

Not quite..
The more flap you use the shorter the takeoff roll. At low speeds (such as T/O runs) the drag is much less of a factor than the increased lift. So the more flap you put out the shorter the t/o roll will be in distance over the ground. At full flap you have alot of drag so i suppose there may be a slight increase in ground roll, but it would be relatively minor i would think. Anything in the t/o flap setting range is always more flap=less ground roll regardless of the drag increase.

The shorter your t/o run you get airborne at a lower speed. That kills your climb cause you have to accelerate much further towards best rate of climb speeds than if you got airborne with zero flaps for example. At this point the drag of the extra flap becomes an issue in accelerating the aircraft towards best rate of climb. That and the fact that the speed you got airborne at is much lower means you sepnd more time accelerating forward than climbing upward and your 2nd segment climb suffers.

You're basically trading energy you would have gained on the ground for getting airborne earlier the more flap you put out.

[p7lot]

If you increase flaps for your t/o run then the runway is going to be shorter
More flaps will increase your lift and require less runway.
However, there is always a trade off when not using optimum settings for the weight of the aircraft, pax, fuel etc.
The Boeing 747/400 is equipped with a very good FMC and flight director/auto throttle system which should always be utilised to assist with manual calculations.
There should never be a situation where you are unsure of the optimum flap settings for a t/o run as the increased costs of such an error will be reflected in your wage packet

[Keith.Williams.]

Anyone who believes that increasing flap angle will always decrease take-off run might like to try taking-off with full landing flap. But please ensure that you have a very looooooooooooooooooooong runway before doing so.

For some aircraft full landing flap will be 30 or 40 degrees and this will give a very long take-off run indeed.

Increasing flap angle towards the optimum will decrease take-off run. But increasing flap angle beyond take-off run will cause it to start increasing again.

[Junkflyer]

The 747 classic uses flaps 10 or 20 for take-off. Normal is 10, for shorter runways we use 20. Landing for our company is normally 25, 30 if deemed necessary.

[Old Smokey]

Within the bounds of normal reason, and providing that the Flap settings to be used are approved and certified for Takeoff -

(1) A higher Flap setting will provide for a shorter Takeoff roll, but a decreased 1st and 2nd Segment Climb gradient. Even though acceleration will be slightly less, TODR will still be lower because the V1/Vr/V2 to be achieved are lower. An additional advantage is that because the aircraft is airborne earlier, clearance of "close in" obstacles will be improved, even at the lower climb gradients available.

(2) A lower Flap setting will provide for a longer Takeoff roll, but an increased 1st and 2nd Segment Climb gradient. Even though acceleration will be slightly higher, TODR will still be higher because the V1/Vr/V2 to be achieved are also higher. Because the aircraft is airborne later, clearance of "close in" obstacles will be degraded, but clearance of more distant obstacles improved because of the higher climb gradients available.

Somewhere along the way, in the 1st or 2nd segment, the 2 possible flight paths intersect, where both flap settings are equal.

As a general rule, if Accelerate-Stop or "close in" obstacles are the more limiting, use the higher Flap setting. If more distant obstacles are most limiting, and Accelerate-Stop or "close in" obstacles are not limiting, use the lower Flap setting.

There are cases for the sometimes quoted "infinitely long runway with no obstacles" where the higher Flap setting will actually lift more weight than the lower Flap setting. This arises, particularly in down-wind Takeoff cases, where operation with the lower Flap setting hits the Brake Energy limit first, whereas (for one aircraft that I do the number crunching for), Vmbe doesn't come into play at all at the higher Flap setting, due to the lower Takeoff speeds (V1 in particular) at all.

[PantLoad]

Interesting Thread

As usual, Old Smokey provides and excellent explanation of the issue at hand. Thank you.

This thread is related to another that is currently running about the use of Flaps 2 on the 737. It is important to note that airplane manufacturers have takeoff performance figures for several different flap settings. For example, Flaps 2 on the 737 is approved for some models. It is the option of specific operators regarding what data are to be provided to the line pilots.

I remember years ago, my company provided Flaps 2 performance data for Flaps 2 (on our 737-200) for certain airports, in addition to Flaps 1 and Flaps 5 data. Generally (and the key word is 'generally'), Flaps 2 provided a nice balance between takeoff run and second segment climb performance (Flaps 1 vis'-a-vis' Flaps 5).

My old company (I've been retired for several years, now.) published in the Pilots Operating Handbook (POH) the statement that "Flaps 5 [was] the preferred flap setting for takeoff". Unfortunately, many of my fellow pilots took this to mean that Flaps 5 was the ONLY flap setting to be used, unless conditions (weight, runway length, density altitude, etc.) mandated a lesser takeoff flap setting.

In one specific situation, we were taking off from a sea level airport on a hot July day (OAT maybe 35c)...relatively heavy weight...but we had well over 10,000 feet of runway. We were legal and safe for either a Flaps 1 or a Flaps 5 takeoff. I can't remember the specific numbers, but here are the approximate figures:

Actual takeoff weight: 127,500 lbs

Max weights for Flaps 1 and Flaps 5: (In this particular case, we had no Flaps 2 data.)

Runway Limit - Climb Limit
-----------------------------------------------------------------------
Flaps 1 - 128,500 - 128,700

Flaps 5 - 129,200 - 127,800



As you can see, Flaps 5 provided a much better safety factor in the event of an RTO, but an engine failure at V1 (continuing the takeoff on one engine) would have been 'tight' as we were only 300 lbs below the max for second-segment climb.

Conversely, Flaps 1, while generating higher speeds...and making an RTO at V1 minus 1 'interesting' (thus soiling underpants)...provided a better overall safety factor (in the event of either an RTO of continued climbout OEI).

We had published speeds along with the above data (We received all this over ACARS around pushback time.), and the V1 speed for Flaps 1 was about four knots higher than the V1 speed for Flaps 5.

Tire and brake energy speeds were respected in either case...as either flap setting was legal.

I discussed this issue of takeoff flap selection with the first officer, and he insisted that we HAD to use Flaps 5...to be in compliance with the POH doctrine. My point of view was that we had a better safety cushion if we used Flaps 1.

I believe the F/O's confusion existed because some of the captains with whom he had previously flown didn't, themselves, understand basic performance...and the fact that he didn't properly interpret the POH statement.

We took off with Flaps 1...being legal and safe and in compliance with our company's SOP...but not without heated objection from the F/O.

The moral of the story is that we should consider all factors when choosing takeoff flap settings....and look to the manufacturer's published performance figures (as supplied by your company) as a guide in this decision process.

Another factor to consider, by the way, is runway surface condition. If it's rough...to where you really need four-wheel drive for your takeoff run..you might lean towards a higher flap setting...all other factors being approximately equal.

Fly safe,

PantLoad

[bubbers44]

Old Smokey stated it best concerning take off flap settings. Short runways usually require more flaps and climb restrictions on hot and high altitude airports especially require minimum flaps. These are calculated on losing an engine at V1 and either stopping or continuing when using normal balanced field length V speeds at the take off weight. Sometimes using more flaps makes sense if a high speed abort would put you at more risk such as wet runway or a cliff at the end of a fairly short runway and climb is not critical.

[john_tullamarine]

I suspect that the question was intended to lead into a general discussion on takeoff performance so that you could demonstrate whatever level of understanding you had at the time .. the question, per se, is a bit silly. The best initial answer is "it depends" .. and then get into some meaty discussion on the sorts of things OS has suggested.

A bit like a question I had to field at an interview for an engineering job years ago ... "tell me about supercharged turbine engines" .. .which led into a discussion for the next 10 minutes or so on engine design... and, yes, after the interviewer sated his interest in that one he commented that one of the other applicants had agreed, without comment, that supercharged jets were all the go ..... one wonders what people pick up in the hallowed halls of learning sometimes ...

Keep in mind that most sensible interviewers will be working on the presumption that you know the organ grinder questions and answers .. they were addressed in the various exams you have done through your early career ...

[Keith.Williams.]

"Keep in mind that most sensible interviewers will be working on the presumption that you know the organ grinder questions and answers .. they were addressed in the various exams you have done through your early career ..."

I would agree that it would be safe and sensible for inreviewers to do that in an ideal world. But the world of ATPL training is becoming increasingly unideal (is that a real word?).

Far too many students are passing their JAR exams simply by memorising the answers to the questions in various databases. A brief examination of the Wanabees forum will illustrate this attitude all too clearly.

Because of this, many of them know absolutely nothing about the basics of subjects such as aerodynamics or aircraft performance.

If the interviewing process is effective, then such people will be weeded out.

But it is also quite clear from some of the posts that appear regularly in this forum, that a good many practicing airline pilots know very little about the basics.

[Red Top Comanche]

Try it in a 152

If you want to try it when life is marginal, and lets face it this is when it usually matters, then try setting full flap on a C152 with max TO weight.

But remember to pick a runway that you cant see the end of, preferably as a result of the curvature of the earth.

Good luck, and hope the ground effect is higher than the fence at the end.

[Lantirn]

I am mindblowing my head to find out what is the effect of more flap setting in the ASDR.

I have some performance numbers, 737.

Constant weight/constant field length

FLAPS 5
V1 149 (Balanced V1)
VR 155
V2 163

FLAPS 15
V1 145-147
VR 149
V2 157

This new V1 range shows that not only TODR decreases (Vgo 145) but also ASDR increases a bit (Vstop 147).

Initially I thought that the Vstop reduction was due to lower VR but, this doesnt make sense obviously.

Taking into account ever possibility separately,

1) TODR with more "approved t/o" flaps is less due to earlier rotation (AEO and OEI)

2) ASDR should be also less, as far as I can understand, because of lower rotation speeds. The slightly less acceleration possibility is rulled out because the TODR is reduced with more flaps.

How the heck ASDR is slightly increasing?

I am misintepreting something.

[john_tullamarine]

Again, it all depends ... on the basis for the sums.

Not speaking to any particular Type/Model here but, generally, the AFM provides a range of bases for takeoff calculations. There are three main areas of calculation -

(a) general takeoff charts for cockpit use will be constrained significantly to reduce the number of flexibility options. This has nothing to do with whether pilots are clever or not .. simply a case of keeping the exercise simple to permit a quick and dirty (but acceptable and safe) calculation to permit the operation to depart without too much delay.

(b) manual calculations by an ops engineer may be quite complicated (if the runway is commercially important) or fairly constrained (for standardised operations or a sensible balance of back room time and cost). Not many folk have to bore themselves with this these days, given the ubiquitous extent of capable PCs and other computers.

(c) a full computer analysis will exhaust all the nuances of the AFM to extract the very last kilo for the operator .. and all done in a few blinks of one's eye. (Note that setting up the program will be a long-winded exercise unless the OEM provides it to the operator)

As to the ASDR, again it depends on the basis for calculation. One of the main concerns of effect will be the selection of V1/VR, presuming that is provided in the AFM data.

If the ratio is kept sensibly similar througout, and in the absence of overspeed schedules, then one would expect the selection of a higher flap setting to result in a lower V1 and a less critical ASDR.

The specific data you are looking at is inadequate to draw much in the way of conclusions as the basis for calculation is not disclosed.

I would suspect a case of comparing apples and oranges resulting in considerable head scratching and confusion.


On another point, having scanned through the thread, is the following comment -

The shorter your t/o run you get airborne at a lower speed. That kills your climb cause you have to accelerate much further towards best rate of climb speeds than if you got airborne with zero flaps for example.

The first observation (presuming min speed schedule) is correct as the selection of flap and speed schedule initially is driven, inter alia, by runway length.

However, the basis of takeoff certification calculations is that V2 is the target (OEI) and there is no airborne acceleration permitted other than that associated with the rotation phase which provides the VR to V2 increase. For AEO takeoff, the technique is such that the flight path is constrained to remain above the OEI flightpath. While that generally sees some acceleration, there is no specific intent to accelerate to achieve a better rate of climb.

I suspect that the poster's comment was directed at some aspect of light aircraft operation rather than heavy iron.

[Lantirn]

Thank you,

These are supposed to be calculations from cockpit performance laptop

My point is that its strange to lower V1 when ASDR is less limiting

[Natstrackalpha]

RTC

If you want to try it when life is marginal, and lets face it this is when it usually matters, then try setting full flap on a C152 with max TO weight.

But remember to pick a runway that you cant see the end of, preferably as a result of the curvature of the earth.

Good luck, and hope the ground effect is higher than the fence at the end.

That`s silly!!

You should not go around or take off with flaps full.

Once upon a time - a B17 pilot from the second world war - took off from a grass strip in a C172. Two fatties and me a skinny and a large heavy suitcase.
He said he would do the take-off (as he had flown B17s full of heavy payloads and stuff and he owned a C172 in the States and was current it sounded like a good cross section of experience to me)
So he took my original flap setting 10. lowered some more, by inspection, and proceeded with the takeoff. The aircraft lifted off with best perf.
I was left to clean up and accelerate - no problem.
Point? Use the flaps for the perf required.
(Best takeoff I`ve seen at any weight)

[john_tullamarine]

These are supposed to be calculations from cockpit performance laptop

.. in which case you should be able to quote from the user manual what the basis for the two calculations might be .. and that might give us a chance to offer pertinent comment ?

My point is that its strange to lower V1 when ASDR is less limiting

maybe yes, maybe no .. what are the circumstances and bases for calculation ? Is there an operator philosophy (within the bounds of the range of calculations) at play here ?