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fabbe92
8th Nov 2009, 17:02
When you bank you pull back a bit to compensate for the increased Induced drag but doesn't a higher angle of attack increase the induced drag? In order to pass an obstacle with a sep, you should have flap 0 to have a higher rate of climb but wy do the airliners keep their flaps out longer when taking off from Innsbruck etc? /thanks!

Bealzebub
8th Nov 2009, 17:12
Innsbruck is in a high narrow valley. It is important to keep the radius of turns to a minimum. That means keeping the speed back, which in turn requires leaving the appropriate degree of flap deployed until the turn has been completed.

Halfbaked_Boy
8th Nov 2009, 17:17
Hi fabbe92,

When you 'pull back', you're not compensating for the increased induced drag, you're compensating for the lesser vertical component of lift - in a turn this is inclined towards the horizontal, giving less force in the vertical, hence the requirement to produce more lift, which in turn increases the load factor. Ergo in a turn, you're pulling higher Gs than in level flight, or in a constant rate of climb/descent.

As for your other query, and I'm not an airline pilot, but I'm making the assumption that Innsbruck has a rather short runway, and littered with mountains left right and centre, so it's in your best interest to get off the runway and be in a position to maximise your rate of climb as soon as possible. Increasing flaps decreases the take off run required to an extent, after which point the increase in drag becomes greater than the higher lift.

So take-off flap settings will decrease your take off run, but they will decrease your maximum climb gradient/maximum rate of climb.

Take what I say with a pinch of salt, I'm no expert, and I'm sure there will be somebody along to give a clearer and more accurate explanation, but I hope this will suffice for a general understanding! :)

fabbe92
8th Nov 2009, 17:18
Aha so they fly at so low speeds that they need to keep the flaps out?

Bealzebub
8th Nov 2009, 17:33
That's right. Flap retraction usually starts at around 1500ft AGL and is completed by about 3000ft AGL for most non restrictive departures. At Innsbruck for certain departures there is a requirement to keep the speed back to around 155 kts until the aircraft has completed a turn back overhead the airport. For a high performance aircraft such as a 757 this will often result in achieving a height of over 9000ft AGL (11,000ft AMSL) before flap retraction is commenced.

fabbe92
8th Nov 2009, 17:51
Okay thank you very much for the innsbruck answers, now I understand!

As for the induced drag question I donīt understand fully. If I remember right, when you bank left with a small SEP, the induced drag increases on the right wing wich will result in the nost drifting to the right, therefore you have to push a little bit with the left rudder, correct?

And after considering all this information and seeing that the book states otherwise than some guys say, how is it really? Is it to compensate for the induced drag or is it to generate more lift? Because if you donīt pull back the plane will lose altitude so I would go for the second option.

Wizofoz
8th Nov 2009, 18:22
fabbe,

Whilst most aircraft will, steady state, climb at a steeper angle clean than with flaps out, with a large jet you must first accelerate to flap retraction speed. During this acceleration it is necessary to reduce your rate and angle of climb.

For the short term clearence of obstacles, therefore, it is actually better to accept the lesser angle with take-off flaps still out, as it will give a higher average angle than the acceleration phase followed by best claean angle for the clearence of most realistic obstacles. As such, minimun clean-up altitudes are specified.

Additionally it is more ecconomical to remain at a lower speed, and thus acdhieve a steeper climb and lower turn radius, when ever a reversal turn is called for after take-off.

You don't need rudder because of increased induced drag, but because of adverse yaw. The down-going aileron is deflecting into higher pressure air, and thus produces more FORM drag than the up-going one. This can be optimised for one speed by differential ailerons, but will require correction at any other speed.

By banking and turning, you at once reduce the vertical component of lift, whilst increasing load factor by causing an angular acceleration. Both therefore require an increase in lift by either/both an increase in AofA or an increase in airspeed.

Intruder
8th Nov 2009, 18:54
Flap retraction usually starts at around 1500ft AGL and is completed by about 3000ft AGL for most non restrictive departures. At Innsbruck for certain departures there is a requirement to keep the speed back to around 155 kts until the aircraft has completed a turn back overhead the airport. For a high performance aircraft such as a 757 this will often result in achieving a height of over 9000ft AGL (11,000ft AMSL) before flap retraction is commenced.
The 2 standard "noise abatement profiles" call for flap retraction to start at 1000' above airport elevation where there is no close-in noise problem, and 3000' above airport elevation where there is a close-in noise problem. The angle of climb is steeper when the airplane is not accelerating and cleaning up, thereby getting to higher altitude closer to the airport.

Microburst2002
8th Nov 2009, 19:09
Hi FAbbe

In straight and level flight lift balances weight.
However, to balance the airplane weight during a level turn, Lift has to be greater that the weight. The more the bank, the more the extra lift you need. When your lift exceeds your weight, you are flying with more than one "g".

Having with more than 1 g is equivalent to being heavier, simply put. For the same speed, you need more lift coefficient to achieve the lift increase. That is why you have to pitch up a little bit when turning.
An increased CL means an increased induced drag and therefore you need to add some power to avoid losing a few knots.

Try this the next time you fly in a SEP: Make 30š turns flying at the "high speed cruise speed". You will scarcely lose speed withour adding power. Do more turns at low speed. You will see how speed drops significantly, so you need to add power to avoid it. This is because an increase in angle of attack induces more drag at high angles of attack than at low ones.

Also notice that at low speeds, if you have a steep bank angle and try to maintain altitude you will have "gs". This means your stall speed is higher than usual. Don't tighten bank angle when turning final to avoid an overshoot when at low speed. You might reach the ground with a 90š descent angle!

DERG
10th Nov 2009, 19:26