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Why heavier aircrafts take longer to slow down in the air?

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Why heavier aircrafts take longer to slow down in the air?

Old 23rd Nov 2012, 20:26
  #61 (permalink)  
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THRUST
Pronunciation (Uk):
Dictionary entry overview: What does thrust mean?
THRUST (noun)
The noun THRUST has 5 senses:
1. the force used in pushing
2. a strong blow with a knife or other sharp pointed instrument
3. the act of applying force to propel something
4. verbal criticism
5. a sharp hand gesture (resembling a blow)
Familiarity information: THRUST used as a noun is common.

THRUST (verb)
The verb THRUST has 8 senses:
1. push forcefully
2. press or force
3. make a thrusting forward movement
4. impose or thrust urgently, importunately, or inexorably
5. penetrate or cut through with a sharp instrument
6. force (molten rock) into pre-existing rock
7. push upward
8. place or put with great energy
Familiarity information: THRUST used as a verb is common.
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Old 23rd Nov 2012, 20:33
  #62 (permalink)  
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They cannot have the same drag coefficient or the same lift/drag ratio so the rest of
Read it properly before jumping in.

I did NOT say they have the same drag coefficient did i ????

in fact i seem to think i said that plane A had 1.5 times the drag of plane B which is NOT saying the drag is the same is it ???

i said they DID have the same lift / drag ratio, purely for the maths, as a theoretical calculation.

Why can't they have the same lift/drag ratio. ?????
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Old 23rd Nov 2012, 20:35
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In the context of this discussion:

3. (Engineering / Aeronautics)a. a propulsive force produced by the fluid pressure or the change of momentum of the fluid in a jet engine, rocket engine, etc.

Source:
thrust - definition of thrust by the Free Online Dictionary, Thesaurus and Encyclopedia.
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Old 23rd Nov 2012, 20:39
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Sorry Dave but you're wrong here.

There are 4 forces acting on a gliding some of the time.

Your nasa illustration shows a gliding falling from the sky, BUT if he builds up some speed ( momentum ) and pulls back on the stick good n hard what happens, well he will go back up again against gravity untill he runs out of velocity at Agogee and then gravity will take over again and he will fall to earth again

During this upwards flight bit he is using the momentum built up as THRUST, and yes its called thrust becuase the momentum is PUSHING the glider upwards against gravity with the kinetic energy being converted to potential energy that is known as height, as is the tension on a kite string that stops it fluttering away in the wind.

so during this upwards flight mode using the momentum as thrust 4 forces are acting on the glider, not 3.
I suggest you Google Newton's Second Law before you dig yourself in an even deeper hole.
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Old 23rd Nov 2012, 20:50
  #65 (permalink)  
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In the context of this discussion:

3. (Engineering / Aeronautics)a. a propulsive force produced by the fluid pressure or the change of momentum of the fluid in a jet engine, rocket engine, etc.
As i used the word thrust in the context of a GLIDER and wrongly assumed that people here would know that a glider doesn't have any rockets or jet engines and therefore was simply trying to show that the thrust in a conventional jet engine is replaced by momentum in a glider in certain circumstances.

so as a GLIDER does not have any other propulsive unit i was using the word thrust in its correct context.

sorry i should of realised this is just a troll fest and not actually based on real science.

GB
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Old 23rd Nov 2012, 20:53
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Dave i am genuinely interested as to your belief that Newtons second law is relevant.

please explain
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Old 23rd Nov 2012, 20:54
  #67 (permalink)  
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thank you hn39 WE AGREE.
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Old 23rd Nov 2012, 21:02
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Dave i am genuinely interested as to your belief that Newtons second law is relevant.
Newton's Second Law can be summarised as (F)orce = (M)ass x (A)cceleration.

So in your glider example, pulling up into a climb (acceleration) is the result of a force with a vertical component (lift).

I can't really put it any more simply.
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Old 23rd Nov 2012, 21:10
  #69 (permalink)  
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Maybe not but you could put it correctly.

immediately you pull up into a climb you start to DECELERATE as you are now being pulled back to earth by GRAVITY. not ACCELERATE.

you only have your momentum to FORCE you up which is being taken away by gravity.

this is a VERTICAL climb and there is no contribution of lift from the wings by the way.

and even if there was, it would still be the force of momentum pushing you along against the drag and gravity 4 forces in play not 3.
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Old 23rd Nov 2012, 21:13
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Well ...

The two aircraft shown in post #49 have the same L/D at about 226 kts. At that speed, and only at that speed, both airplanes slow down at the same rate. As the speed reduces below 226 kts the L/D's of the two aircraft will change differently and hence the rate of slowing down will be different, as shown in the graph.

P.S. I deleted the above immediately after posting it Because I saw that while writing there had been several new posts and I wanted to read those first. Sorry for any confusion.

P.S.2 I suggest you could consider the component of weight that acts in the direction of the flight path as the glider's equivalent of thrust, but momentum is not thrust.
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Old 23rd Nov 2012, 21:15
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Read it properly before jumping in.

I did NOT say they have the same drag coefficient did i ????
No you didn't - I did as part of an explanation.

in fact i seem to think i said that plane A had 1.5 times the drag of plane B which is NOT saying the drag is the same is it ???
That is what you said, but it is just as wrong. Aircraft A will NOT have 1.5 times the drag of Aircraft B if the only difference is weight.

i said they DID have the same lift / drag ratio, purely for the maths, as a theoretical calculation.
And that is where you went wrong because you postulate a non-valid equivalence

Why can't they have the same lift/drag ratio. ?????
If I really have to explain that I'm wasting my time. If you understood the basics of how drag varies with lift coefficient and how lift coefficient varies with speed and weight you wouldn't need to ask that question.

As to force vs momentum

Momentum = mass * velocity
Force = mass * acceleration
Acceleration = rate of change of velocity
Force = rate of change of momentum.

Last edited by Owain Glyndwr; 23rd Nov 2012 at 21:21.
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Old 23rd Nov 2012, 21:19
  #72 (permalink)  
 
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the force of momentum
OK, this is where we came in, I give up.

sorry i should of realised this is just a troll fest and not actually based on real science.
Well we can agree on something at least: the rest of us can't hope to match your grasp of aeronautics.
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Old 23rd Nov 2012, 21:30
  #73 (permalink)  
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thank you HN39 we agree yet again, i was merely trying to explain that the gliders equivalent of thrust was its momentum.

But the context police jumped on me looking for a fight.

If you understood the basics of how drag varies with lift coefficient and how lift coefficient varies with speed and weight you wouldn't need to ask that question
i clearly don't and have in fact asked that question, would you now be so kind as to enlighten me please.

Dave think of a skier going down a ski jump ramp, at the end the ramp curls upwards to the sky, now as the skier gets to the turned up bit what happens to him, oh he goes upwards, is he creating lift, no then what carries upwards as he leaves the ramp, i guess it must be the momentum he built up on the way down.

and i don't really care how much you insult me about my knowledge.

the force of momentum
OK, this is where we came in, I give up.
sorry i keep forgetting i have to explain everything and leave nothing open to deliberate misintrepration

The word "of" from the dictionary.

of1    [uhv, ov; unstressed uhv or, especially before consonants, uh] Show IPA
preposition
1.
(used to indicate distance or direction from, separation, deprivation, etc.): within a mile of the church; south of Omaha; to be robbed of one's money.
2.
(used to indicate derivation, origin, or source): a man of good family; the plays of Shakespeare; a piece of cake.
3.
(used to indicate cause, motive, occasion, or reason): to die of hunger.
4.
(used to indicate material, component parts, substance, or contents): a dress of silk; an apartment of three rooms; a book of poems; a package of cheese.
5.
(used to indicate apposition or identity): Is that idiot of a salesman calling again?

so you see i was using this word to try to explain that the force is derived from the momentum or that its source was the momentum, not that the 2 words are the same.


Last edited by Bye; 23rd Nov 2012 at 21:57.
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Old 24th Nov 2012, 04:43
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We have 2 aircraft
plane A weighs 300 Tonnes and plane B weighs 200 Tonnes.
Both are identical and have the same wing with the same Lift drag ratio of 17 similar to a 747.
Both aircraft are cruising straight and level at 400 Km/h.
Sorry, that is not possible. Identical airplanes with different weights an the same cruising speed can't have the same L/D ratio, because they have different AoA. From then on, any conclusion you come to is arguable.

L=q.S.CL
D=q.S.CD

L/D = CL/CD , that is exclusively dependent on AoA. They can'y fly at the same AoA.

As for the slowing down subject:

The heavy airplane has more Drag than the light one, when speed is constant. When you cut thrust to idle the difference between thrust required and actual thrust (let's say nil for simplification) is greater: greater retarding force than in the light airplane case, which tends to decrease stopping distance. However, the higher weight airplane has more mass, which tends to increase stopping distance. Which effect prevails?

D varies because CD varies, when q and S are constant. CD varies because CL does vary too (they are childrem of the same father, AoA). AoA varies linearly with weight, and so does CL. However, CD is not linear. At lower AoAs, CD varies less than linearly with CL. At higher AoAs the opposite is true.

So, at low AoAs, D will vary less than mass for a given weight change. Inertia prevails at low AoAs or high speeds. Mass increase is greater than the drag force increase and as a result acceleration is reduced.

At high AoAs or low speeds, air viscosity prevails over inertia. Mass increase is less than the drag force increase and as a result acceleration is increased.

At least that is the conclusion I have come to thinking about this. I'm standing by for your comments.
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Old 24th Nov 2012, 08:24
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sorry i keep forgetting i have to explain everything and leave nothing open to deliberate misintrepration
I said I wasn't going to argue with you any more, and I won't.

But that remark can't go unchallenged. I can't see any evidence of anyone deliberately misinterpreting your posts. We may not agree with them, in fact we don't, but that not the same thing at all.

PPRuNe is full of spirited discussions on matters of physics, science, engineering, aeronautics, etc, which we all enjoy and which illustrate why many of us chose aviation as a career.

But no poster is infallible. Most of us, if challenged by 3 or 4 people who clearly have some background knowledge of the subject matter, would at least be prepared to consider the possibility that we might be wrong, and to listen to their honest attempts to explain the theory and to provide understandable examples to illustrate it.

You might want to pause to reflect on that.
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Old 24th Nov 2012, 08:30
  #76 (permalink)  
 
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Gezzs what a blood bath in this thread..

I'm assuming "longer" means dt (more time to) slow down from the same speed

Now stop killing the momentum and inertia, it has nothing to do with our problem!

First let's have Newton in here:
a) Force = Mass * Acc
Now take two airplanes with mass M1 and M2 where M2 is heavier:
b) M2 = 1.3* M1
The lift produced must be just as big aswell:
b) L2 = 1.3 * L1
Now if you want airplane nr 2 to decelerate faster, from (a) and (b), the drag needs to be bigger for the heavier airplaine in respect to it's mass:
c1) F2 = M2/a2 remember that F in our case is Drag
c2) F1 = M1/a1 and because we need
c3) a2> a1 then the drag is automatically:
c) D2 > 1.3 *D1
Now combine (c) and (b) a little bit and you end up with a simple condition
(d) L2/D2 < L1/D1

Lift/Drag ratio needs to be bigger for the second plane. We know that L2>L1 from (b), and we assumed they both start at the same speed, so it's only Alpha that changes to create more lift:
e) Alpha2>Alpha1 (note that we do not know the actual A2/A1 ratio .. it's a matter of wing profile)

So we need a better lift/drag ratio (d) for a higher alpha (e) for airplane nr 2.
Take a look at a polar Cl/Cd graph. This only happens at speeds below max lift/drag ratio, ie below best glide. Above that speed the ligher airplane will slow down faster.
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Old 24th Nov 2012, 08:56
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but Dave your so called honest attempts to explain that a glider only has 3 forces acting on it was wrong, and it seems once challenged you just resort to insults.

You claimed that a glider only has 3 forces acting on it, yet in the very same article you linked it even states that the glider has 4 forces acting on it.

So to say you clearly have a background in the subject when all you rely on is google is a bit rich and highly insulting to those of that rely do have a background in aeronautical engineering.

When you fail to win the argument on substance you result to syntax then finally to insults - classic troll behaviour pattern.

i have repeatedly stated that my aerodynamics is flawed, i am a thermodynamic engineer who designs jet engines for living, yet when i ask to be enlightened by the "experts" there is no reply.

It seems a trait here is that people just want to pull every poster down and show the world they are cleverer than the poster rather than actually discussing a subject, as you have shown without resorting to insults when you can't win.

Maybe you would also like to reflect on your posts, re read them and realise that maybe just for this one in a million possibility that you were not correct in stating a glider has only 3 forces acting on it. Oh and i didn't see 3 or 4 other posters supporting your claim as to only 3 forces acting on it, just yours.

Kind regards.

GB
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Old 24th Nov 2012, 09:11
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Urbandictionary.com:
Troll: One who posts a deliberately provocative message to a newsgroup or message board with the intention of causing maximum disruption and argument
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Old 24th Nov 2012, 09:23
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Oh well, it's a damp Saturday and I have nothing else to do ...

You claimed that a glider only has 3 forces acting on it
I did, and it does.

yet in the very same article you linked it even states that the glider has 4 forces acting on it.
It doesn't. Read it again.

"The thrust is determined by the size and type of propulsion system used on the airplane and on the throttle setting selected by the pilot." (those emphases aren't mine, they appear in the original article).

The US convention is to use the term "airplane" to mean a powered, fixed-wing aircraft - I used to argue, once upon a time, that gliders were also "airplanes", but I was wrong about that, it's their word and they get to define it.

The terms "propulsion system" and "throttle" also give a pretty good clue as to what we're talking about here.

So please, one more time, tell us all what this mysterious 4th force (in addition to L/D/W) is that acts on a glider in flight ?
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Old 24th Nov 2012, 10:21
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There are only two forces acting on a glider: an aerodynamic force and a gravitational force. The gravitational force is directed towards the center of the earth and is equal to the product of mass m and gravitational acceleration g.

The aerodynamic force is usually represented by its two components: the drag force D acting opposite to the direction of movement, and the lift force L perpendicular to it.

For a glider moving at constant altitude the lift force is equal to the gravitational force: L = m * g ... (equation 1)

There is no force opposing the drag force so according to Newton's second law there results an acceleration a resulting from the equation: - D = m * a ... (equation 2)

(The sign convention is positive for forces and accelerations in the direction of motion, negative in the opposite direction)

Combining equations (1) and (2) we get: a / g = - D / L

No need to discuss momentum or energy.

P.S. If you measure accelerations in kts/second, then g is about 19.05 kts/second

Last edited by HazelNuts39; 25th Nov 2012 at 17:09. Reason: P.S.
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