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Losing Airspeed In Turn

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Losing Airspeed In Turn

Old 6th Jul 2014, 17:12
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Losing Airspeed In Turn

So I was asked by an instructor about losing airspeed in a turn. I understand why we lose airspeed, because by increasing AoA we increase induced drag.

Normally this would create an adverse situation as reducing airspeed would require increased AoA to maintain height. Further increasing AoA causing further decrease in speed until flight cannot be maintained.

In medium turns the speed reduces and settles at speed only a few knots lower. Why does the speed settle? Why do we not have to keep increasing AoA?

Can't find the answer anywhere in books or internet, all they say is increase is so small speed only slightly reduces. No explanation why it stops going down.

Any help would be appreciated!
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Old 6th Jul 2014, 18:07
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Hope this helps..

OK..remember these basic principles....

1 To maintain height/altitude, lift must equal gravity... any difference between the two will result in the aircraft climbing or descending

2 Remember that lift is perpendicular to the lifting surface(s)

3 In a turn, some of the vertical component is lost..we therefore increase AoA to compensate for that loss.

4 The necessary increase in AoA is determined by the angle of bank... thus if the AoA is increased too much, the aircraft will climb.

Cheers, TP
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Old 6th Jul 2014, 20:54
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That part I understand, but why does the speed settle?
If you increase AoA then you produce more lift, thus more induced drag, causing the speed to decay. Lower speed means you need to increase AoA to maintain the same amount of lift, but then you will increase induced drag, so speed decays even more. This will go on until you can't fly anymore

Why does this stop during a 30 bank turn?
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Old 6th Jul 2014, 21:21
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Why are you only concerned with LI drag?

Also increasing alpha doesn't necessarily mean you produce more lift.
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Old 6th Jul 2014, 21:53
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Very quick reply as I am busy with other stuff.
Though you increase induced drag, this is not the only drag involved and drag overall reduces as you slow down, thus allowing you to stabilise at the lower speed
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Old 7th Jul 2014, 02:29
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As the forces equal out (lift required is met which means no more AoA is needed which in turn, stops drag increasing)
It's the same reason why airspeed settles as you slow down for approach / decent / climb.

For any given speed, there is a corresponding AoA (and power to a certain extent).
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Old 7th Jul 2014, 02:37
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In addition to the preceding helpful comments, think of this as vectors:

If you're flying directly north, and want to fly directly east, in an aircraft which is flying at 100MPH, at the instant you begin your turn, you are flying north at 100 MPH, and east at zero MPH. By the time you have finished your turn, you'll be going 100 MPH east, and zero MPH north, so, in the radius of the turn, you have to accelerate from zero to 100 MPH. The amount of acceleration depends upon the radius (smaller equals more).

Accelerating means adding power or reducing speed (or a combination). Once this adjustment has been made, the acceleration can now be constant as long as you like, and the difference in power to fly and accelerate is now there. If you also want to climb, add more power, or slow down more. If you're happy to descend, not so much....

And no, you don't get credit for the energy you give up from flying at 100 MPH north to flying at zero north.
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Old 7th Jul 2014, 06:51
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Pilot DAR:

You most certainly do gain credit for the energy flying north. Remember energy cannot be created or destroyed. You can use that energy to turn, in the same way that a glider or wheeled vehicle can turn using only their momentum to provide acceleration in a new direction.

Cheers
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Old 7th Jul 2014, 07:36
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Another explanation:

The total drag curve is more-or-less U-shaped, with Vy/Vbg at the bottom of the curve. On the left hand of the curve, most of the drag is caused by induced drag, at the right hand of the curve, most of the drag is caused by parasite drag.

Anytime you fly at a speed above Vy/Vbg, and you either increase drag (induced, by pulling g, or parasite, by for instance sticking your hand out the window), your speed reduces and will get closer to Vy/Vbg. Anytime you increase thrust the reverse happens. In this part of the curve the aircraft will automatically find speed stability due to the way the curve lies.

What the OP describes is actually the situation that you are flying below Vy/Vbg. Due to the increased induced drag, caused by the g-loading in the turn, the total drag increases with such an amount that the aircraft slows down even more, which requires even more alpha to compensate, which increases the drag even further. Up to the point where the aircraft stalls.

Try entering a steep turn with a speed that's less than Vy/Vbg, and then hold the angle of bank, power and altitude constant. I give it about 15 seconds before you stall. Make sure you have plenty height under the wings because you may end up in a spin or spiral dive.
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Old 7th Jul 2014, 10:31
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You can use that energy to turn, in the same way that a glider or wheeled vehicle can turn using only their momentum to provide acceleration in a new direction.
Definately.

However, any energy you have to add to make the turn is not recoverable, as it opposes drag, so it cannot be stored. Its like using the brakes on a engine powered car to come to a stop, you turned your energy of momentum into heat, and it's gone - you did not store it, so now you have to replace it.

If you dive the plane, all other things remaining unchanged, it speeds up, and stores that energy. You can recover most of that energy from the stored speed climbing back up again shortly after. A turn does not work the same way.

For every aircraft, there will come an angle of bank which id great enough that the aircraft does not have enough power to sustain it, then you stall, or exceed a G limit, if you try to maintain it co ordinated.
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Old 7th Jul 2014, 20:28
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Why does this stop during a 30 bank turn?
... because you stop at 30 degrees of bank. The lift you require is now a constant, so the drag you generate is now constant, so the thrust you have counteracts the drag by a constant amount. Therefore the the speed you end up with remains constant until you change either the lift, the drag or the thrust.
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