rates of descent

[Keith.Williams.]

FT,

No I did not forget it, I just did not discuss it.

At Vmd the induced drag is half of the total drag. Only the induced drag is affected by weight changes. So at Vmd the increase in total drag for any given weight increase will be half of the increase in Induced drag. In effect this means that for small % weight changes, there will be a 1 to 1 ratio between weight increase and total drag increase.

So using the figures in my previous post (Induced drag increased to 300% of its previous value). If this happens at Vmd then total drag increases to 150% of its previous value. So the drag of the 30 tonne jet is about 2/3 that of the 52 tonne jet.

But at lower speeds the induced drag is a much greater proportion of the total drag. So the increase in total drag is a much greater proportion of the increase in induced drag. The relationship between weight increases and total drag increases become smuch greater than 1 to 1. This means that at low speeds such as Vmp it is entirely possible that the drag on the 30 tonne jet will be less than 57% of that of the 52 tonne jet.

The overall effect of this drag-weight-speed relationship is that the heavy jet will have the greatest ROD at low speeds and the lighter jet will have the greatest ROD at high speeds.

Readers who prefer the "nose further down to maintain speed at low weights - therefore greater ROD" argument, should consider what happens at low speeds. The heavier aircraft will have an inefficiently high angle of attack, so it will produce a great deal of drag. This high drag will use up its energy more quickly.

The fact that gliding at or close to Vmd causes light aircraft to sink faster, does not mean that this is true at all speeds. It will just appear to be inevitable if you do most of your gliding at or close to Vmd.