747 descent rates: heavy vs. light

[zerozero]

I've heard that a light 747 will descend more quickly than a heavily loaded 747.

This seems counterintuitive to me.

Can anyone provide a good explanation from an aerodynamic/performance standpoint?

Thanks.

[411A]

Not just 747's, all jet transports, flying at the same IAS... the lighter aircraft will descend more rapidly than the heavier.

The hint is the IAS.

[Capt Fathom]

These figures are rough, but should help with your query.

At higher weights, the best Lift/Drag Ratio speed is 300 kts.
At lower weights, the best Lift/Drag Ratio speed is 260 kts.

Descent is generally made at around 300 kts, which is great when you are heavy, but not when you're light.

So at light weights and 300 kts (which is the speed for heavier weights), there is a large increase in drag over a 260 kt descent.

The increase in drag brings you down faster.

Now if you understand all that, you doing better than me . :)

[Keith.Williams.]

If both aircraft are gliding at the same IAS then they cannot both be at their best lift to drag ratio speed (Vmd). If we assume that the heavier aircraft is at its Vmd, then the lighter aircraft is flying faster than its Vmd. As stated by Captain Fathom, this higher speed will create higher drag.

The aircraft are gliding so this increased drag must be matched by the component of weight acting down the glidepath. In order to increase this component the nose must be pushed further down to make the glidepath steeper.

So if two aircraft glide at the same IAS but at different weights, the lighter aircraft must use a greater nose down attitude and a steeper glidepath. With both aircraft at the same IAS, the lighter one, with the steeper glidepath will have the greatest ROD and the smallest glide range.

This appears to be contrary to the conventional wisdom that "weight does not affect glide range", but that statement is based on the assumption that the aircraft in question is gliding at the correct Vmd for its weight. In the situation described above only one of the two aircraft can be at its Vmd.

[zerozero]

Good responses guys, thanks.