View Full Version : Mass and Inertia

7th Nov 2003, 00:57
Why does a "heavy" have to start a descent earlier than the same model but "lighter" aircraft?
I have a very basic understanding of Newton laws...
Anyone for an heads up???

Max Angle
7th Nov 2003, 02:57
Check out this old thread. Everything you need to know is here I think.


7th Nov 2003, 10:49
I'm guessing it has to do with momentum. The heavier the airplane, the more momentum, so you must start down early to counteract the momentum. Just a guess, maybe someone else has a better explanation.

7th Nov 2003, 20:31
As a general guide transport a/c are flown so that the wing is at or within reasonable proximity to the most efficient angle of attack.

For argument, consider that to be fixed. For all intents & purposes as weight increases then the speed needed to generate a sufficient amount of lift must also increase.

Starting at a particular height then the faster (ie heavier) a/c will require more distance to descend at the same rate of descent as the slower (lighter) a/c.

Mad (Flt) Scientist
9th Nov 2003, 15:15
Actually, it would apply even at the same speed.

The reason is that a heavier aircraft actually has a smaller descent angle than a lighter one, even at the same speed. Consider the following.

Same aircraft, one weighs 50klbs, one 100klbs. Both wish to descend at 300kts, both have engines at idle and same config (this is emergency descent type numbers, but it works the same everywhere). In both cases the nose is pushed down until the aircraft stabilises at the desired speed. Since the plane needs gravity to provide thrust, the lighter plane needs to dive more to make the weight pull more along the flight path. So lighter aircraft actually will descend faster (in terms of ROD) than heavier ones, even at the same weight. It's all a result of having a limiting speed to obey.

Now, with a higher RoD and the same airspeed, you can start the descent later. So lighter aircraft start to descend later, and follow a steeper profile.

As an alternative: which is easier to keep to a speed limit driving down a hill - an empty truck, or a full one? Obviously, the empty one, because the heavier one wants to go faster. If you had bad (or no) brakes, the heavy truck couldn't go straight down the hill, but the light truck might be able to. Same ideas at work, more or less.

9th Nov 2003, 22:01
I think that's the same as my argument, only this time you're keeping speed fixed & using a variable RoD.

Mad (Flt) Scientist
10th Nov 2003, 06:29
Should be pretty much the same, it's the same underlying physics at work. ;)