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-   -   Why heavier aircrafts take longer to slow down in the air? (https://www.pprune.org/tech-log/501013-why-heavier-aircrafts-take-longer-slow-down-air.html)

extricate 21st Nov 2012 13:34

Why heavier aircrafts take longer to slow down in the air?
 
Pardon me but..

1)Why do heavier aircrafts take longer to slow down in the air compared to lighter aircrafts?

2)Why on high profile, it's recommended to increase speed?

Trying to learn.

Thanks

SMOC 21st Nov 2012 13:38

Newton's second law.

F=MA

RunSick 21st Nov 2012 13:44

If you select a higher speed descending the aircraft will "obtain" this speed by PITCHING DOWN.
That way you get a faster speed but as a side effect also a higher VERTICAL speed, which will help you come back into profile.
(Oversimplified, yes, but hopefully youŽll get the idea)

hetfield 21st Nov 2012 14:10

1) More energy
2) More drag

extricate 21st Nov 2012 14:20

F=MA.

A=F/M. If M increases, acceleration falls. This is why heavier aircrafts travel slower? Does it have the same meaning as why heavier aircrafts will take longer to slow down?

yotty 21st Nov 2012 14:26

Inertia. Related to mass. The same anywhere in the universe!:cool:

AdamFrisch 21st Nov 2012 15:09

Kinetic energy. It's exactly why a truck/lorry will coast much further if you get off the gas than a car.

DaveReidUK 21st Nov 2012 15:25


Kinetic energy.
Related to mass. The same anywhere in the universe!

Okivan 21st Nov 2012 15:29

İntertia , momentum.

Microburst2002 21st Nov 2012 15:57

Extricate has a point.

Newton's second law (F=m.a) is not only about Inertia. It is also about Force.

Bigger airplanes have more mass, that is obvious. But they have more Drag, too, right?

Same with energy (E=F.d). Mass has an effect in the distance, but so does force.

Therefore, if it takes more distance to slow down a big airplane than a small one, it means that the mass increase is greater than the drag increase when airplane size increases. Somehow, increase size is more efficient aerodynamically...

As for the secons question: increasing speed has two effects. One is that you exchange altitude energy for speed energy. However you will have to get rid of the extra speed energy before landing, which in turn takes distance... But exchanging both energies is not free. The air takes a "price" in the form of friction and heating an other forms enrgy loss, so you dissipare some energy when you dive, and also when you level off. You will need less miles to descend if you dive and increase speed 100 kt and later level off to decrease them again that if you just keep constant speed.

The second effect is the change in lift to drag ratio, which is what determines the descent angle. In the normal speed regime (speed above min drag speed) an increase in speed decreases this ratio, which increases the glide angle. For instance, at low speed you are more efficient and glide at 270 fpnm. Then at high speed you make 300 fpnm. And at vey high speed, 330 fpnm.

HazelNuts39 21st Nov 2012 15:59

I'm assuming you're comparing two aircraft of the same type at different weights. I'm also assuming that the deceleration is in level flight and that engine thrust at idle is negligible.

Then it depends. The retarding force F is the drag. At high speed the drag is mainly parasitic (i.e. the drag due to lift is small in comparison) and the drag for both aircraft at the same speed is about the same. Then from A=F/M follows that the deceleration of the heavier aircraft will be less than that of the lighter aircraft.

However, if both aircraft are flying at their 'green dot' speed (where drag due to lift is of the same order as the parasitic drag), both aircraft have the same lift/drag ratio and decelerate at the same rate.

Lyman 21st Nov 2012 16:13

But is not greendot AoA dependent? It is a stablized value, no?

Should we channel Galileo?

HazelNuts39 21st Nov 2012 16:30


Originally Posted by Lyman
But is not greendot AoA dependent?

The green dot speed varies with weight, the green dot AoA does not.

Stuck_in_an_ATR 21st Nov 2012 17:09

Another explanation is that heavier aircraft operates at better L/D ratio for a given speed, and therefore is more efficient => ie. there's less drag to slow it down. That's why high performance gliders take water ballast - to increase performance at high speeds

Bye 21st Nov 2012 18:37

you really need to use the formula for kinetic energy rather than F=MA

KE = 1/2 x mass x Velocity Squared.

So with 2 identical aircraft with the same drag and at the same velocity the aircraft with the higher mass will have higher kinetic energy.

Or the same aircraft both at the same mass with one travelling faster than the other, the faster one will have higher kinetic energy and thus take longer to slow down as well, But bear in mind that drag is also squared with velocity so the faster aircraft will have higher drag until it slows down to the same speed as the other one was at.

so we can't use the F=MA formula as it doesn't allow us to take account of the velocity of the aircraft at the point of forward thrust being replaced with momentum / inertia.

With 2 falling objects you initially ignore mass as the both accelerate towards the ground at the same rate of 1G, until terminal velocity is reached then if the objects have different mass as the drag of each object equals the mass of the object acceleration will become zero and terminal velocity is achieved.

so for a faster terminal velocity with the same drag, you need a heavier aircraft.

water sufficiently muddy i hope.

GB

Microburst2002 21st Nov 2012 19:09

OK, we are talking about same model at the same descent IAS? Like a heavy "full house" A330 and an empty A330 flying at 300 kt?

Then it is all about L/D ratio.

The distance for altitude is equal to the L/D ratio.

L/D ratio depends on AoA. L/D ratio increases with AoA up to a maximum (min Drag AoA) and then decreases again. Normally we fly at AoAs below max L/D AoA.

Imagine those 330s. The heavy one, flying at max L/D (green dot speed) would make, say 240 kt, whereas the light one would be making only 210 kt. But they are both flying at 300 kt, so the heavy one is 30 kt closer to green dot that the light one. Its L/D ratio is better. Its AoA is closer to min Drag, to green dot. That is why the heavier airplane flies more miles for the same altitude.

Another way to look at it. Imagine two blocks on skis on two icy downslopes, sliding at 120 kmh. The heavy one will have enough forward force to overcome the drag of those 120 kmh, with a shallower slope than the light one, due to its weight.

As for the second one: speed increase is the same as reducing AoA, which means getting farther from min drag AoA, and decreasing L/D.

framer 21st Nov 2012 20:29

[quoteOkivan İntertia , momentum.
][/quote]
It is worth remembering that at a given weight you will always have the same inertia regardless of whether you are doing 340kts or standing still. The same cannot be said for momentum.

Chu Chu 21st Nov 2012 21:20

In general, drag is proportional to area, while mass is proportional to volume. Very roughly, if you scaled an airplane up by a factor of two, you'd increase the drag by a factor of four, and the mass by a factor of eight. Since the larger airplane would lower drag relative to its inertia, it would decelerate at a lower rate. Of course it isn't this simple, especially when you bring induced drag into the equation, but I think the general point holds.

misd-agin 21st Nov 2012 23:44

"2)Why on high profile, it's recommended to increase speed?"



L/D (better gliding performance) is almost always slower than descent speed. Speeding up generates more drag which results in your energy being closer to the profile.

Also increasing speed results in a faster rate of descent getting you closer to your profile.

The reserve applies if you get low on profile. Slow up closer to L/D speed. You should get closer the VNAV descent profile as your gliding performance at L/D is shallower than it is using the VNAV descent speed..

Adding power isn't the most efficient way to correct a small deviation below profile.

DaveReidUK 22nd Nov 2012 08:00


It is worth remembering that at a given weight you will always have the same inertia
Inertia is a function of mass, not weight.

On its way to the Moon, Apollo had a lot of inertia but weighed nothing at one point. :O


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