Descent Aircraft performances

Descent Aircraft performances


The Descend post is similar to the climb aircraft performances. As far as we are talking about the concepts and how they interact among them. That´s why I do recommend to read the post: Aircraft climb performances.


Why do we descend?


Because all that goes up, sooner or later falls down. That´s what Newton´s realized and could demonstrate, discovering a force called gravity.

The most important is how. The power required is bigger than the power available.





What is Rate of Descend and Angle of descend?


In the picture above shows how the different forces interact during a descend.

Angle of descend, is the angle which the aircraft is descending. Its formula is angle of descent (a) is equal to (Drag – Thrust)/Weight  (of course  we consider sin a = a).

a= (D-T)/W

Rate of descend (u in the picture) is a vertical speed measured in feet/min and its formula is:



What is a Gliding?


Gliding is an specific part of the descend, where the engine is set to idle. So the Power available is almost cero, we consider it cero.

During a gliding we have to decide if we are interested in avoiding an obstacle or being the most time in the air.

If we want to avoid obstacles we have to choose a speed where we descend the less in the shortest distance. It is the minimum angle of descend speed.

The angle of descend is minimum when the Drag is the lowest possible (a= (D-T)/W), which is the same concept as Vx.

As I wrote in the cruise aircraft performance post, we get the minimum drag speed when angle of attack corresponds to the maximum fineze.

If we want to be the most time in the air while descending the Rate of descend must be minimum (R/D=(Pr-Pa)/W), and this is reached when the required power is minimum.







Speed:  If we maintain another angle of attack besides the maximum fineze , the speed will  change, so we will not fly the most distance.



Weight: The weight has two contradictory effects on una hand the rate of descend increases, and the speed of minimum descend increases. So if we have 2 identical aircrafts, one heavier than the other gliding. The result is that both planes will land at the same point, but the heavier one arrives earlier.

Configuration: For travelling the highest horizontal distance, the drag must be the lowest. So it will be reached without flaps ( flap 0). 

  • Note: This consideration it is for gliding. When we land we use flaps to reduce the landing distance among other advantages.



Wind: The wind does not affect to the time that the airplane is in the air as it does not affect the power required, however it does affect the distance travelled with respect to the ground. With tail wind decreases the angle of descent by increasing the distance of glide, reciprocally happen with the head wind.




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