Effect of Larger Flap Setting.

[Medwin]

Can somebody help me with this question?


What is going to happen to T.O. run and screen height with a larger flap setting?

I'm just trying to study for an oral exam.

Thanks ya'll

[oldebloke]

gets off the ground sooner ,suffers in the 2nd segment climb..But some times one has to get off the short runways and accept the climb penalty!!:ok:

[Medwin]

So take off run will be shorter.

Thanks oldbloke

[swh]

Medwin,

Depending on the aircraft, the airborne segments maybe better or worse with a higher flap setting.

It will normally get airborne sooner with a higher flap settng as the stall speed is reduced, allowing a lower rotation speed.

Airborne gradients are effected by the IAS and rate or climb.

The rate of climb is generally less with flap out, due to the additional drag at a given airspaeed, and and hence less excess power to enable the aircraft to climb.

The lower IAS for a given flap setting will generally result in a lower gradiet, if you were to draw a triangle with IAS along the horizontal scale, and ROC on the vertical scale you would get the gradient as being the resultant slope.

The overall obstacle clearance is generally better as the aircraft lifts of the runway sooner, further away from the obstacle, which overall will give a better obstacle clearance

Most aircraft types have a lower gradients airborne, however airbus aircraft have a higher gradient with a higher flap setting due to the advanced geometry/lift capabilty of the wing with flap/slats out.

:ok:

[john_tullamarine]

.. one caveat ... higher flap may be better for close-in obstacles, but generally is dreadful for, say, 4th segment problems... you get airborne sooner but the overall gradient is worse. All a case of horses for courses and optimising the flap for the particular runway and ambients.

[Alex Whittingham]

e.g.

http://www.bristol.gs/download_files/flaps.jpg

P.S. Swh, how does improved lift help the climb gradient?

[mutt]

Medwin.

Just to help you study... :):) What happens to the VMCG??


Mutt.

[Keith.Williams.]

It all depends on how tricky the question is intended to be. If they really want to be devious then the effect on the take-off run depends upon the flap setting chosen.

With zero flap the lift-off speed will be high so it will take a long run to accelerate up to it.

With maximum landing flap set the lift-off speed will be lower but the drag will be much higher. So the acceleration rate will be dreadful. This will also require a long take-off run.

But at some flap setting in between zero and maximum there is an optimum angle which gives the shortest run. It is the angle at which the trade-off between increasing drag and decreasing lift-off speed is best. The value of this optimum angle depends on aircraft type (and probably a few other things).

[john_tullamarine]

.. don't think I would be too concerned about high flap drag on TOR acceleration .. measurable, but probably not of concern other than for the case of critical runway length with no obstacles and for the one flap setting .. far more important consideration is the speed schedule differences between flap settings which will swamp any minor acceleration delta.

The other consideration is WAT limit .. higher flap normally has a lower WAT limit.

A case of balancing the runway limits against WAT and obstacle limits ... circus juggler is a good CV point to look for when employing ops engineers.

[Old Smokey]

Medwin,

Too many variables for a general question, here are some of them -

(1) Higher Flap means lower Vs, which yields Lower V1/Vr/V2, and thus lower Takeoff Run, EXCEPT at lower weights with lower Vs, Vmcg/Vmca will govern V1/Vr/V2, and may be the same speeds for a Low or a High Flap setting. For the same speeds, lower flap would be advantageous due to improved acceleration.

(2) Screen height should be the same, assuming a performance limiting situation. Takeoff data looks at TODA, not TORA, which requires the aircraft to reach 35 feet at the end of the TODA. The airborne distance to, and gradient to achieve 35 feet will be worse with a higher flap setting, and this is added to the TORA, BUT it is unlikely that the manufacturer would have certified the aircraft for such a higher flap setting if distance to TODA was greater, and it would not be advantageous to use the higher flap setting.

(3) 1st segment gradient will be lower with higher flap. Not a problem if there are no close-in obstacles. The only advantage in the 1st segment will be a shorter 1st segment distance due to the lower speed for a fixed gear retraction time, thus the aircraft can enter the 2nd segment climb at a shorter distance from the end of TODA.

(4) 2nd segment gradient will be lower with higher flap. Again, not a problem if not obstacle limited, but if obstacles are limiting, it's time to cross compare the overall takeoff scenario to find the best flap setting.

(5) 3rd segment at higher flap can be a 'monster', particularly if a high 3rd segment acceleration altitude is required. The time taken to accelerate from the lower 'High Flap' V2 to Final Takeoff configuration (Clean) can be MUCH longer. Add the time from Brakes release to commencement of 3rd segment level flight to the LONGER 3rd segment time, and you may 'bust' the 5 or 10 minute limit at Takeoff thrust. In several airports where I've done the Airport Analysis, higher flap was necessary due to a relatively short runway, but then time to a higher than normal acceleration height plus the longer acceleration necessitated further REDUCTION in RTOW to contain the time at Takeoff thrust within the 5 or 10 minute limit. In 7 runways where I encountered this problem, solution was gained in 4 cases by reverting to the lower flap setting, with it's attendant faster climb and acceleration times.

(6) There are variables within these variables - many!

swh, a good response, but may I suggest that climb gradient depends upon Ground Speed and Rate of Climb, not IAS and Rate of Climb. Whilst we are discussing Climb Gradients, isn't it more appropriate to discuss excess THRUST, not excess Power.

Regards,

Old Smokey