Constant Speed Descent
Constant Speed Descent
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A heavy A/C will use a longer distance than a lighter one (assuming same speed and same thrust setting). Since the weight and gravity is what keeps the speed up, the heavy will need a shallower decend to keep the same airspeed and hence cover a greater distance.
Tor
Tor
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Maybe I haven't understood your explanation!
Seemes to me that if A/S and PWR are constant, the lighter aircraft will not descend as quickly as a heavy.
Ex.
140 tons will have a gradient of 1.5% gradient and at 220 tons, A/C will have a 3% gradient. (Again assuming the power and A/S remains constant)
In effect, whether we are heavy or light, we always descend at idle pwr settings. In this case gravity has the effect of increasing the gradient and hence, the airspeed. The heavy aircraft will descend faster but the speed will increase faster to Vmo. Therefore the rate of descent must be reduced, increasing the TOTAL distance required. The light aircraft can keep the higher rate of descent and not approach the Vmo as fast.
Seemes to me that if A/S and PWR are constant, the lighter aircraft will not descend as quickly as a heavy.
Ex.
140 tons will have a gradient of 1.5% gradient and at 220 tons, A/C will have a 3% gradient. (Again assuming the power and A/S remains constant)
In effect, whether we are heavy or light, we always descend at idle pwr settings. In this case gravity has the effect of increasing the gradient and hence, the airspeed. The heavy aircraft will descend faster but the speed will increase faster to Vmo. Therefore the rate of descent must be reduced, increasing the TOTAL distance required. The light aircraft can keep the higher rate of descent and not approach the Vmo as fast.
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Thanks Chaps,
That is just what I thought. At a constant I.A.S.(Yes, Thrust at idle) a heavier A/C will have a lower ROD tahn a lighter A/C at the same I.A.S. Therefore the heavy will require extra miles.
Will be totally different if the speed was not kept constant.
Thanks again
FuelFlow
That is just what I thought. At a constant I.A.S.(Yes, Thrust at idle) a heavier A/C will have a lower ROD tahn a lighter A/C at the same I.A.S. Therefore the heavy will require extra miles.
Will be totally different if the speed was not kept constant.
Thanks again
FuelFlow
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FF,
I don't think it's quite that simple. As I understand it, the best glide angle occurs at a fixed angle of attack which is constant whatever the weight.
This is covered in the manuals by a table which gives descent speeds for different weights.
In your case the speed is constant but the weight changes. In order that Lift=Weight, the aoa has to change.
So it all depends on how close to optimum you were when you increased the weight. If you were at the optimum aoa, then any change in weight, up or down, will result in a steeper descent because it will move the aoa away from the optimum angle.
If you're light and the speed for the best descent is lower than your actual speed, increasing the weight would result in a steeper glide angle because you will increase the aoa from the optimum.
If, on the other hand, you are heavy and the speed for the best descent is higher than your current speed, increasing the weight will increase the aoa towards the optimum which will improve the glide angle.
I don't think it's quite that simple. As I understand it, the best glide angle occurs at a fixed angle of attack which is constant whatever the weight.
This is covered in the manuals by a table which gives descent speeds for different weights.
In your case the speed is constant but the weight changes. In order that Lift=Weight, the aoa has to change.
So it all depends on how close to optimum you were when you increased the weight. If you were at the optimum aoa, then any change in weight, up or down, will result in a steeper descent because it will move the aoa away from the optimum angle.
If you're light and the speed for the best descent is lower than your actual speed, increasing the weight would result in a steeper glide angle because you will increase the aoa from the optimum.
If, on the other hand, you are heavy and the speed for the best descent is higher than your current speed, increasing the weight will increase the aoa towards the optimum which will improve the glide angle.
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I think of it this way:
Drag has two components, induced and parasitic. At the same IAS, parasitic drag is (about) equal in each (heavy vs. light) case.
But heavier weights, there is more potential energy to be disposed of by parasitic drag. Induced drag is greater, and thus parasitic drag is a smaller fraction of the total. Ergo it takes longer to burn off the potential energy (i.e. descend).
Not strictly accurate I know, but the major factors are there.
Drag has two components, induced and parasitic. At the same IAS, parasitic drag is (about) equal in each (heavy vs. light) case.
But heavier weights, there is more potential energy to be disposed of by parasitic drag. Induced drag is greater, and thus parasitic drag is a smaller fraction of the total. Ergo it takes longer to burn off the potential energy (i.e. descend).
Not strictly accurate I know, but the major factors are there.
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Here we go again.
wnchaigap has it right. A heavier a/c will typically be closer to speed for best descent, therefore slower ROT than a lighter plane.
If a light and a heavy plane were allowed to glide at their respective L/D speed, they would of course glide the same distance (only the lighter plane would use more time to get there).
wnchaigap has it right. A heavier a/c will typically be closer to speed for best descent, therefore slower ROT than a lighter plane.
If a light and a heavy plane were allowed to glide at their respective L/D speed, they would of course glide the same distance (only the lighter plane would use more time to get there).
I think of it this way . . . .
The heavier the aircraft, the higher its minimum drag speed (ie min clean speed).
Therefore, for a given IAS - say, a standard 300 kt descent - the heavier the aircraft, and the lesser the margin between its min drag speed and descent speed. So the aircraft is operating closer to its best glide IAS, and consequently its RoD will be closer to its minimum, which logically must occur at best glide IAS.
Therefore, for a given IAS - say, a standard 300 kt descent - the heavier the aircraft, and the lesser the margin between its min drag speed and descent speed. So the aircraft is operating closer to its best glide IAS, and consequently its RoD will be closer to its minimum, which logically must occur at best glide IAS.
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well here goes
two identical a/c same type same wt same L/D etc....commence descent say maintaining 300kts IAS....one flown by your friend and you have the other....now some time during this happy descent.
..hypothetically...some one added 5 tons to your a/c
in order to still continue to maintain 300 kts what do you
hope that helps
..hypothetically...some one added 5 tons to your a/c in order to still continue to maintain 300 kts what do you
hope that helps
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You have two skier with same skills, one 70kg and one 80kg, they start a race down the hill, which one is to win?
The heavier because of potential energy!
Now for aircraft we are speed limited, MMO/VMO in our case, the heaviest aircraft will have to reduce his ROD not to overspeed...
Hope it helps!
The heavier because of potential energy!
Now for aircraft we are speed limited, MMO/VMO in our case, the heaviest aircraft will have to reduce his ROD not to overspeed...
Hope it helps!
