ATPL Performance Question

[m4x]

Hi!

Im going through the "General Principles: Climb and descent" chapter in the oxford book "Performance" and would be pleased if some of you could explain the answer on this question:

With the flaps in the take-off position, compared to the aircraft clean, the climb gradient (i) and the speed for best climb angle (ii) will:

------(i)------(ii)------
A: Decrease Increase
B: Increase Decrease
C: Decrease Decrease
D: Increase Increase

The answer sheet says that C is correct. Why is that?

With flaps settings compared to clean config, Vx will decrease...but shouldn't the climb angle increase relative to the ground?

/m

[usedtofly]

Flaps will ALWAYS induce extra drag, therefore will also reduce climb gradient. However the use of flap will allow the aircraft to get airborne in a shorter distance (useful for shorter runways). Once airborne and at a safe height cleaning up will enable aircraft to fly faster and climb steeper.

Sounds odd I know,

UTF

[BHenderson]

What's the difference between a climb angle and a climb gradient?

[Keith.Williams.]

Strictly speaking % climb gradient = ROC/Ground Speed.

But for small climb angles in still air it is close enough to say that it is ROC/TAS

For this type of question it is also close enough to say that
% climb = 100 x Sin Climb angle.

Sin Climb angle = (Thrust - Drag) /weight

So assuming that thrust is constant ( which it is for a jet), for best climb angle (and best % climb) we need the speed that gives minimum drag. This is Vmd.

Deploying flaps increases drag, so it decreases best climb angle and best % climb.

For a jet best climb angle occurs at Vmd and for a prop it occurs at about Vmp. Both of these speeds decrease with deployment of flaps.

So deploying flaps decreases both the best % climb and the speed for best % climb.

[m4x]

thanks for the answers. But IS climb gradient THE SAME as climb angle? Im getting confused here.

Let's say that there is a high obstacle close the end of the runway, near the threshold. You would like to start with flaps rather than clean config. This is because you want to get a better climb angle/climb gradient/angle of climb to clear the obstacle. You will climb steeper than with clean config. Is it right?

That's why I answered B.

[Keith.Williams.]

Climb angle and climb gradient are not the same thing in that one is measured in degrees and the other is measured in %. But they are simply two different ways of measuring the same thing.

As an example of this, a 3 degree climb angle equates to approximately 5% climb gradient. This is because the sine of 3 degrees is approximately 0.05 and % gradient is approximatley 100% x the sine of the climb angle.

If you increase climb angle you will also increase % climb gradient.

For JAR examination purposes both the climb angle and % climb will be greatest with zero flap.

Generally speaking increasing climb angle and % climb gradient will increase obstacle clearance. But we need to be careful when considering obstacles that are very close in.

Using zero flap will increase the take-off run, so we will lift-off closer to the obstacle. But our climb angle will be greater than when using flap.

Using flap angle up to the optimum value will reduce take-off run, but decrease climb angle. So the aircraft will lift-off sooner but climb at a lower angle.

If you draw these two situations on the same sheet of paper you will see that there is a point where the two climb paths cross. Before this point the aircraft with zero flap is lower than the one with optimum flap.

So for obstacles that are located before this cross-over point, optimum flap gives best clearance. But for all obstacle that are beyond the cross-over point clearance is best with zero flap.

[m4x]

Oh thanks for the great answer. I get it now!!