Hello,

I'm a little unsure of this question someone asked me today:
State the effect on rate, angle of descent and attitude resulting from changes in the following:

1. power – constant IAS
   
2. flap – constant IAS

From my understanding, when flap is extended in a decent, rate of decent does not change (I'm a little unsure of the technical details about this), but angle of decent will increase (once again, I'm a little unsure about the technical detail about this). As for attitude, when flap is deployed, we get an initial balloon affect, that if not controlled for will cause us gain altitude, so we want to pitch down and keep a nose down attitude. Am I correct? Could someone explain the technical details?

As for power, I am totally lost on this and have no idea. Could someone explain this? I'm assuming that, if you decrease power, to keep a constant IAS you must lower the nose to compensate (decrease AoA), and thus would have a low nose attitude and your angle of decent would also increase, but I am unsure about rate of decent.

Help is much appreciated!

Gidday Deeroy

For situation 1. It's probably helpful to consider the two extremes. If you descend at a constant IAS with no power at all, you will be in a full glide and have the highest rate of descent. If you added power you would have to raise the nose attitude to maintain a constant IAS. If you increased power sufficiently at constant IAS, you would end up flying level at that IAS and eventually, with further increase in power, you would commence to climb at that same IAS. So adding power in a descent at constant IAS would result in a decreased rate of descent, a higher nose attitude and a shallower angle of descent.

For situation 2. The application of flap will always increase drag by a greater proportion than lift so the lift/drag ratio will be spoilt and the angle of descent will become steeper. If you fly down a steeper path at the same speed it follows that you will lose more height in a given time - rate of descent will increase. To maintain the same IAS with the extra drag created by flap extension, you will need to adopt a lower nose attitude. Flap extension at constant IAS will result in an increase in both rate and angle of descent and require a lower nose attitude.

It is true that the initial increase in lift just after flap is deployed would cause a momentary ballooning of the flight path, but that effect would quickly vanish.

Hello Bob,

Thanks for the reply! With respect to situation 1, if we were to decrease power (as in for a landing), to maintain a constant IAS we would have to lower the nose attitude. So with that being said, we would also have a steeper angle of descent. So we should also have a decrease in rate of decent. Is this correct?
Why is that? Is this only because we are keeping a constant IAS?

With respect to situation 2, could you explain why the rate of decent increases with the application of flap? I'm a little unsure about this

If you maintain the same speed you will travel the same distance in one minute. With flap deployed, the angle of descent will be steeper. If you travel down a steeper path at the same speed you will also lose more height in one minute. With flap deployed, both angle and rate of descent will increase if speed is kept constant.

Ah, makes much more sense now! Thank you Bob!