Maintaining speed in a descending turn

[fireflybob]

Mainly considering the turn from base leg to final, I was asked today why it is necessary to lower the nose a tad as you roll into the turn to maintain airspeed.

This assumes fully trimmed etc before starting the turn.

It's been a long day so go easy on me but I was struggling to find an answer.

Ok on a medium turn where we want to maintain height we need to increase the angle of attack to increase lift etc which (if we allow it) will lead to a speed decrease.

Thanks for any help.

[Edgington]

During a medium level turn you loose speed due to an increase in drag, which is fine if your doing 100 its. You will only loose 5 kts or so.

During turn from base to final you are already flying at slower speed, but the turn will decrease your speed due to the increase in induced drag. That with the increased stall speed, makes speed loss unacceptable. Lower the nose to maintain speed.

[fireflybob]

During turn from base to final you are already flying at slower speed, but the turn will decrease your speed due to the increase in induced drag. That with the increased stall speed, makes speed loss unacceptable. Lower the nose to maintain speed.

Edgington, thanks - I follow that but why as you roll into the turn does the a/c not naturally pitch down to maintain speed without pilot input?

[Edgington]

ah good question. Trying to think through, I guess there is a change in the thrust/drag couple. Increasing induced drag in a turn causes drag to increase thus increasing nose up couple? Thus you have actively lower the nose? Does that make sense?

[Pull what]

I follow that but why as you roll into the turn does the a/c not naturally pitch down to maintain speed without pilot input?

The a/c (when loaded correctly) does naturally pitch down when you roll into a turn (Further effect of aileron or rudder) If your student is MAINTAINING the same pitch attitude from level flight and is attempting to maintain level flight the speed will reduce because of the need to increase the angle of attack and hence the drag, which in turn decreases the speed. Ideally in the circuit the turn onto final is a continuous descending turn anyway

During turn from base to final you are already flying at slower speed, but the turn will decrease your speed due to the increase in induced drag. That with the increased stall speed, makes speed loss unacceptable. Lower the nose to maintain speed.
Not necessarily we/I teach 5 knots additional speed on base for this very reason. A descending turn when stabilized(ailerons returned to neutral point) doesnt increase induced drag or stalling speed. A completed LEVEL turn increases induced drag and stalling speed due to an increase in wing loading.

Many instructors fail to realise and teach that its the wing loadng that increases the stall speed not the bank angle

[fireflybob]

But surely in any sustained turn (be it level, descending or climbing) there is a need for extra lift as the lift vector is now tilted and weight has to be balanced by something?

Some more here:-

Lift and Weight in Turns (http://selair.selkirk.bc.ca/training/aerodynamics/turns.htm)

[Edgington]

Not necessarily lift would have to be increased if you want to maintain a certain rate of descent, thus increasing the load factor and induced drag.

Can also be acceptable to allow a increased rate of descent during turn to final. Caused by the reduction in lift, but load factor and induced drag won't change.

Still haven't answered the op's question then. Could it be adverse yaw?

[fireflybob]

Hey let's just leave the rate of descent out of the equation!

[fireflybob]

This is the way I see it; for turning flight you have to increase the Lift (irrespective of whether you are level, climbing or descending).

Other than increasing speed (or power but let's assume we are in a glide to keep things simple) the only way of increasing the Lift is by increasing the angle of attack. This will cause an increase in the induced drag. However in a descending turn we want to maintain speed. Therefore we need to lower the nose.

Are we conflating a change in angle of attack with a change in attitude and/or trim?

[Capt Pit Bull]

Pull What.

I hate to be a bore but you need to review basic POF.

[Pull what]

This is the way I see it; for turning flight you have to increase the Lift (irrespective of whether you are level, climbing or descending).

You only have to increase the lift if you want to MAINTAIN the altitude or the airspeed, if you just turn and let go the nose will fall below the horizon the speed will increase, if you want to maintain the speed you have to apply back pressure and in doing so you increase the loading and the angle of attack and the total drag(its not just induced drag)

[Mach Jump]

You only have to increase the lift if you want to MAINTAIN the altitude or the airspeed

You have to increase lift as you turn if you want to maintain a constant rate of descent too, so even in a descending turn, if you want to maintain the rate of descent, you will have to increase the AoA, and drag. This will reduce the speed, and bring the stall closer because of the increase in AoA.

Most pilots choose to temporarily accept a small increase in rate of descent, and speed during the critical turn onto final, maintaining the AoA constant, thus not bringing the stall any closer.


MJ:ok:

[Pull what]

Correct or basically- Power plus Attiude = Performance