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DFC · 15th June 2009, 22:58

Cats five,

The answer I had in mind was of course drag.

Why is it drag and not gravity that forces a glider to descend and prevents it from sustained horizontal flight?

The First answer is that if there was no drag then there would be no requirement to have a thrust force to oppose it. Imagine getting a tow along at 100Kt in a glider with zero drag. When that pull force was removed i.e. thrust disapears, since you have zero thrust and zero drag you can fly level for ever!!!!! You would have to increase drag to get yourself down!! - think BRS !!!

Secondly but more relevant to reality - if you fly at the speed for minimum drag, then you can stay airbourne for say 30 minutes. Any speed above or below that speed means that you spend less time in the air because you are forced to descend in order to harness the force of gravity and use it to offset the drag.

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To explain the NASA terms - Gravity is the acceleration force caused by the mass of the earth attracting other bodies. This force varies over the surface of the earth but on average is 9.8 m/s/s near the surface.

The mass of an object is simply the amount of matter in an object which would not chnage even if you put it in outer space (almost zero gravity).

Weight is the force caused by the combination of the mass and gravity.

Hence why you weigh x on earth and somewhat less on the moon despite your mass being the same in both places.

Therefore, your aircraft is loaded to a certain mass (airframe, balast, you) and that mass combined with gravity gives a force called weight which requires a force to oppose it for steady flight.

If you had the same atmosphere on the moon, you would use less lift for level flight than you do on earth!!

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Regards,

DFC

15th June 2009, 22:58	#37 (permalink)
DFC Join Date: Mar 2002 Location: Euroland Posts: 2,391	Cats five, The answer I had in mind was of course drag. Why is it drag and not gravity that forces a glider to descend and prevents it from sustained horizontal flight? The First answer is that if there was no drag then there would be no requirement to have a thrust force to oppose it. Imagine getting a tow along at 100Kt in a glider with zero drag. When that pull force was removed i.e. thrust disapears, since you have zero thrust and zero drag you can fly level for ever!!!!! You would have to increase drag to get yourself down!! - think BRS !!! Secondly but more relevant to reality - if you fly at the speed for minimum drag, then you can stay airbourne for say 30 minutes. Any speed above or below that speed means that you spend less time in the air because you are forced to descend in order to harness the force of gravity and use it to offset the drag. -------- To explain the NASA terms - Gravity is the acceleration force caused by the mass of the earth attracting other bodies. This force varies over the surface of the earth but on average is 9.8 m/s/s near the surface. The mass of an object is simply the amount of matter in an object which would not chnage even if you put it in outer space (almost zero gravity). Weight is the force caused by the combination of the mass and gravity. Hence why you weigh x on earth and somewhat less on the moon despite your mass being the same in both places. Therefore, your aircraft is loaded to a certain mass (airframe, balast, you) and that mass combined with gravity gives a force called weight which requires a force to oppose it for steady flight. If you had the same atmosphere on the moon, you would use less lift for level flight than you do on earth!! ------------- Regards, DFC Last edited by DFC : 16th June 2009 at 01:06.