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pj997
14th Aug 2001, 10:42
Hi

I always wonder why a small amount of flaps (15%?) is applied before the plane takes off, and not during the take off run..

I always understood that the flaps increase the amount of lift, but at the same time the amount of drag increases.

So surely the take off run would be shorter, with faster acceleration and less fuel used if the flaps were applied when the aircraft was say at 50% of the rotation speed?

Any thoughts

PJ

Kermit 180
14th Aug 2001, 11:07
From day one pilots are taught to apply flaps necessary for take off as part of pre-takeoff checks.

Flaps do increase lft, but, as you rightly point out, also increase drag. The flap selected for takeoff is an amount of flap (determined by the Aeroplane's flight manual or checklist) that gives a good increase in lift over the penalty of an increase in drag. The idea is that the aeroplane gets airborne quicker (using less runway) and the initial climb angle (over obstacles or for noise restrictions)is increased. The flaps are then retracted at a safe altitude to provide better climb performance. Some light aeroplanes do not use flap at all for takeoff. The necessity of using or not using flap for takeoff is determined by test pilots.

Hope this helps.

Kermie http://www.stopstart.fsnet.co.uk/aircraft/stuka2.gif

Arm out the window
14th Aug 2001, 11:17
Also, I'd say that the difference in accelleration between not having the flaps down for the first half of your takeoff run and having them down would be relatively insignificant, as drag increases with the square of the speed and is therefore going to be on the low side at low speeds.

I have heard of people doing what you said, i.e. putting the flaps down partway through the roll, timed so that they get to the desired takeoff setting as rotate speed is reached, but I couldn't see it making too much difference, especially considering that the real drag penalty of flaps doesn't really manifest itself until you get to the high angle settings used for landing.

Additionally, the chance of forgetting to put them down at all is increased if you don't do it and check it during the pre takeoff checks.

[ 14 August 2001: Message edited by: Arm out the window ]

max_cont
14th Aug 2001, 12:53
Some problems with this idea.

What happens if the flaps fail to deploy on the t/off run? You may not have enough runway to stop!! and no flap to get airborne.

What happens if you get assymetric flaps? You most certainly don't want to get airborne in that config.

How long do the flaps take to run to F5 or F15, exactly?

Is it the same for each A/C in the fleet? This will determine how much runway we use before we get to the rotation point.

What happens if there is a delay in running the flaps on a short runway due say the PNF not hearing the command to run flaps?

How much performance is going to be lost in the above scenario?

How do you determine the amount of safety margin when writing the performance manuals with so many variables?

That was just a few very simple examples of why we do not run flap on the t/off roll. I didn't want to get into perf "A", which is a nightmare for most of us "drivers airframes" let alone anyone else. I am not saying you wouldn't understand it, merely I don't want to keep typing all day. :)

If you did use the suggested method for departure, you would need runways at least 20,000'long to account for all the unknowns and variables. Since I have
never seen a runway this long, nobody would be going anywhere. I like to keep things simple, that way even I tend to get it right :D :D

Kermit 180
14th Aug 2001, 13:29
MaxCont, those are good examples why flap is selected prior to takeoff.

Kermie http://www.stopstart.fsnet.co.uk/aircraft/stuka2.gif

PAXboy
18th Aug 2001, 22:01
Time is critical here. If you consider the 'standard' 737 and 320 family of machines, with 140/190 seats, the time from start of t/o roll to rotation is between 30 and 40 seconds, depdent on all the the usual factors. A heavy 747 and 340 family machine will take up to 50/55 seconds. These are typical at sea level and with normal temperature conditions.

If, say, Flap 15 is required the time for them to travel from 0 to 15 can easily be 12 seconds. Thus, they would have to start to deploy just as the roll starts, with no real gain in the time that they would be closed.

The reason for the slow deploy time is that they, typically, use a threaded screw-drive. This is able to stand against the enormous air-pressure when the flaps are set on descent. If straight hydraulics were used, the rams would have to be very large. The screw-drives used for flaps (and some other surfaces) are small and can be slotted into the wings (and fairings), they are powerful but slow to operate.