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Mad Dog 11
4th Sep 2001, 20:35
I still can't figure it out. Why does the B737 sink faster when its light versus when it is heavy, using a constant speed. I would imagine that to maintain the same speed during descent, a heavy aircraft would need less angle of attack thus decreasing its ROD but it is not very clear to me. Is this a correct assumption? Anyone who could explain this one to me??
Thanks,

MD :confused:

mutt
4th Sep 2001, 20:53
At any constant high speed, the rate of descent is higher for light weights than for heavy weights. This is a function of the lift/drag ratio.

Mutt http://www.stopstart.fsnet.co.uk/mica/PNTSAce.gif

[ 04 September 2001: Message edited by: mutt ]

Bellerophon
4th Sep 2001, 22:21
Mad Dog 11

An aircraft’s best glide angle is found at max L/D, which does not vary with weight but is purely a function of the design and aerodynamic efficiency of the aircraft.

A variation in weight however does affect the speed at which the max L/D occurs, the best glide speed increasing as the aircraft weight increases.

If we consider two aircraft, identical apart from their weights, both trying to achieve the best glide angle possible in their descents, we will find that the heavier aircraft, flying at it’s best glide speed (max L/D) will be flying faster than the lighter aircraft, which can still achieve the same glide angle, but only at a slower speed.

In practice, the best glide speed for both aircraft is likely to occur at too slow a speed for schedule regularity or ATC purposes etc., and so many operators have a fixed speed in descent for their aircraft, regardless of weight, and this fixed speed is generally well above the theoretical best glide speeds of either aircraft.

Let us assume that our airline requires us to descend at 300 kts IAS, and that the best glide speed for our heavier aircraft is 270 kts and our lighter aircraft is 250 kts.

Both could achieve exactly the same glide performance if allowed to fly at their varying best glide speeds, but they have both been made to fly at the same speed.

We can see that at 300 kts the heavier aircraft is much nearer to it’s best glide speed (only 30 kts too fast) than the lighter aircraft (50 kts too fast). Being closer to it’s best glide speed will mean a better glide performance, and so our heavier aircraft will lose less height over a given ground distance than the lighter aircraft. As both are flying at the same speed, this means that the heavier aircraft has a lower rate of descent than the lighter one!

This ability to increase the speed at which max L/D occurs by increasing the aircraft weight is used to good effect by competition glider pilots, who, on good thermalling days, will often load up to 500 lbs of water into their gliders, but that’s another story!

Hope this helps!

Bellerophon

Ace MCcoy
4th Sep 2001, 23:19
Guys, you make it a joy to browse these pages!
Belle...you MUST be teaching, when not in the sim, preparing for the big day?? Assuming you are spending a lot of time in the sim now of course :D

m&v
5th Sep 2001, 02:16
The heavier weight develops more enery going downhill than the lighter Aircraft.More energy/speed more lift-lower rate of descent.
Lighter aircraft doesn't develope the same speed(glides down),but better range for time.
Both aircraft would reach the ground 'same place'but different times. :rolleyes:

jtr
5th Sep 2001, 02:59
Bellerophon, great explanation!

chris barlow
5th Sep 2001, 05:03
In actuality an aircraft will glide at constant dynamic pressure. If we assume that all glides are performed at the best L/D and that the aerodynamice are linear, then all glides will have the same best L/D angle of attack and occur at the same best L/D lift coeffieient. This is independent of the weight. The flightpath is also invariant with weight and is given by the L/D ratio. An increased weight at the same lift coefficient and flight path angle will require a proportional increase in dynamic pressure to support it. Therefore an increase in weight should produce a faster speed down the same glideslope.

Capt Claret
5th Sep 2001, 07:05
Mad Dog

Another way to look at it is:

Max glide range occurs at the best L/D ratio AoA. This AoA doesn't change with weight, thus the weight of the aircraft doesn't change the glide range.

However, the speed required to achieve this AoA does vary with weight. In the 146 this speed is defined as Vfto (min speed with zero flap) + 30 kts. Vfto also varies with weight, the higher the weight, the higher the value of Vfto.

So if on descent you are say at MLW, you have the highest value of Vfto. If descent is flown at 250 kts, the difference between 250 kts and Vfto is smaller than say if you are several tonnes lighter, with a comensurately lower Vfto.

As a jet's descent is essentially a glide with minimal/partial power, the IAS flown is controlled by (the secondary effect of) pitch attitude.

As weight decreases, Vfto decreases, the split between descent IAS and Vfto increases and, the attitude required to convert the potential energy (of altitude) into the kinetic energy required to maintain decsent IAS must decrease (lower pitch attitude), thus increasing the RoD.

twistedenginestarter
5th Sep 2001, 11:28
Capt Claret

This AoA doesn't change with weight


Oh but it does.

twistedenginestarter
5th Sep 2001, 11:43
Here's my position.

To maintain flight the aeroplane must generate lift. To generate lift it incurs drag and thus must provide thrust.

A heavier plane must increase its angle of attack to provide extra lift. It thus incurs proportionately more drag and thus requires proportionately more thrust.

The thrust comes from the vertical component of the glide ie the rate of change of height times the weight of the plane.

So the heavier plane needs more thrust because it is heavier. But it is heavier so it provides more thrust for a given angle of descent.

What I am saying here is you don't have to 'pay' more to glide a heavy plane. You have already paid for the weight when you climbed.

This leaves you at the mercy of the wing designer as to whether you will glide better or worse at higher wing loads. Your 737 may do better heavy. Another plane might do better light.

;) ;) ;) :cool:

chris barlow
5th Sep 2001, 14:28
If we're talking pure glide then then there is no thrust. For a given aircraft configuration, the best glide range will occur at a fixed angle of attack at which the L/D ratio is a maximum. At this fixed aoa, the lift coefficient is constant. The extra lift force required for the heavier weight is supplied by the increased dynamic pressure and not by an increase in AOA. I'm not sure this really answers the original post , which concerned itself with constant speed descents. At constant aoa, the glide dynamic pressure is constant, however the glide velocity will decrease as the air density increases at lower altitudes during the glide. To maintain constant velocity during the descent, the aoa must be varied such that the required dynamic pressure increases in proportion to the increase in density during the glide. This will produce a constant speed glide. Note that the glide range will no longer be the best glide range for the aircraft and that the flight path angle will be steepening during the glide.

1.3VStall
5th Sep 2001, 14:30
Chaps,

Belle is absolutely spot on. An aircraft's L/D ratio (i.e. glide angle) is not affected by weight. However, for a given aerodynamic configuration, as the aircraft weight increases the speed at which the best L/D is achieved also increases.

This is precisely why we glider pilots carry hundreds of pounds of water ballast in our high-performance sailplanes on good cross-country soaring days. We are able to fly at higher inter-thermal speeds more efficiently.

Mad Dog 11
5th Sep 2001, 17:07
This is the reason why I ask my questions in this forum. Thank you all for the replies.

MD

go with the flow
10th Sep 2001, 11:57
A marginal point but doesn't skin drag increase with speed at a different order to the directly proportional increase in work "gained" in falling/gliding at increased speed? Therefore won't a heavier but otherwise identical aeroplane although flying at the same AoA and theoretical glide path actually fall slightly short as a result of greater drag?

twistedenginestarter
11th Sep 2001, 01:12
Look chaps. Lot's of people are watching us. So let's not make elementary mistakes.

The lift on a wing is a function of

wing shape angle of attack air density freestream velocity wing surface area

Lift = Cl 1/2 p v **2 S

Now you can see the only variables are angle of attack and speed. Unless you increase speed you must increase angle of attack to maintain the same glide angle if weight increases.

[ 10 September 2001: Message edited by: twistedenginestarter ]

Ignition Override
11th Sep 2001, 08:11
How does this apply to the real world?

In an old airplane at FL 310 with almost no tailwind or headwind, BUT without VNAV, you are at normal cruise speed about 100 miles from a VOR and you are given a clearance to cross it at 12,000' and 250 knots. The descent should have no more than light turbulence (negating the need to fly at "rough air speed", i.e. .79 or 285 knots). Are you going to descend earlier with a jet weighing about 98,000 lbs compared to one at about 82,000 lbs, and if so, about how many miles earlier? Roughly a normal two/one ratio (i.e. double your altitude to lose and add on miles for speed reduction and several extra miles)?

Does most such study material in various countries apply to line flying? Just curious.

[ 11 September 2001: Message edited by: Ignition Override ]

Allotta Fagina
11th Sep 2001, 14:15
Dearest El homoez,
Mutt, Maddog11, Ace Mc Coy & Missle man(parlez vous l'anglais "ÄCTUALITY")

Are you really that stupid or are you just being a f%c&ing tosser you no life sh*t....

wake up to yourself you idjit...