Help with wind on a descent

[rossrobertson]

I have been reading through the "Aeroplane TEchnical manual" and i have a couple of questions that i would like to ask.

In the boook it says that wind does not affect descent through the air and another bit of text from the book says that a taiolwind will increase the glide distance and a headwind will reduce the glide distance.

With regards to the text that says that wind does not affect descent through the air is this to do with the angle of attack? if someone could help with some further information.

Thanks

Ross

[NZScion]

I am not familiar with the text that you cite, however I think the first part is referring to a descent through an airmass, while the second section is referring to a descent relative to the ground.

If you were flying in nil wind for 6 minutes at 100 KIAS (100 knots groundspeed), you would cover ca. 17 NM. However, if you were doing the same in a 25 knot tailwind (125 knots groundspeed) you would cover ca. 21 NM. Think of it as the tailwind pushing the aircraft from behind, and the headwind pushing against the aircraft from the front.

[Agaricus bisporus]

How disappointing. Thought this was a medical thread...:confused:

[Rainboe]

Ross, I always found it helped using an exaggerated example to try and work out what happens. Think of a wind matching your average forward speed, headwind or tailwing. Does it affect your descent time? No- to the aeroplane, it is just descending whilst flying forward. During that time, what is the aeroplane doing? Two things affect it- it is flying through the air forward normally. But the air is moving in a mass either forwards or backwards. If backwards, the aeroplane will appear to an observer on the ground to descend vertically. If forwards, it will appear to that observer to fly at double speed. Make other windspeed assumptions and work out what happens.

[bfisk]

In the boook it says that wind does not affect descent through the air
This is entirely correct. Think of it as the ground moving below the air, as opposed to the air moving across the ground. Your aircraft, in a mass of air, is not concerned with wind.


and another bit of text from the book says that a taiolwind will increase the glide distance and a headwind will reduce the glide distance.


Also entirely correct, as the glide distance is usually (and surely for all practical purposes) measured in ground distance. It is as such affected by how much the ground moves below the airmass, or in other terms, the wind.


A tailwind will increase your gliding distance, and a headwind will decrese your gliding distance. It does also affect your best-distance-glide speed: With an infinite tailwind the speed of minimum rate of descent (usually lower than you best glide speed), would allow you to benefit from the infinite tailwind for the longest period of time. On the other side, a very strong headwind would require you to maintain a higher airspeed, where the increased descent rate would be compensated for by the higher forward speed. This very concept -- fly fast in a headwind and slow in a tailwind, works out good in a lot of flight regimes :-)