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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 25th Nov 2012, 09:13
  #101 (permalink)  
 
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Originally Posted by mm43
My 3 year old grandson proved it to me when he set up two parallel lines of Thomas wooden railway and placed one engine and a carriage on one line, then one engine and a wagon loaded with sand on the other. He pushed them off together, and you know the answer - he observed it also.
Let's study that in some detail. Presumably the speeds achieved by your 3 year old grandson were low enough for the aerodynamic drag to be insignificant. So the retarding force is the result of the friction between the wheels and the track, and the friction in the wheel bearings. Normally one would expect those frictions to be proportional to the loads on the wheels, i.e. the masses of the two trains. In that case the two trains would slow down at the same rate. If that didn't happen, there must be other differences besides the difference in mass. It would be interesting to extend the experiment to find out what these differences were. If you loaded both trains to have the same mass, would they decelerate at the same rate?
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Old 25th Nov 2012, 09:16
  #102 (permalink)  
 
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is your solicitor also recommending that you maintain your novel "thrust is momentum, it isn't a force" proposition that, frankly, wouldn't get you a pass in GCSE mechanics ?
No
Good - then you should take his advice.

Some might think that displaying a woeful grasp of basic mechanics would be enough, on its own, to discredit anyone's standing as a professional engineer, without needing any external assistance.

But I couldn't possibly comment on that ...
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Old 25th Nov 2012, 10:05
  #103 (permalink)  
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Folks,

Please be aware that the basic RoE in Tech Log include -

(a) posts and statements therein do not have to be correct but they do have to be reasonably polite and civil lest they be censored.

Note that this does not proscribe spirited and vigorous discussion including pointed jousting. However, the essential thing is to play the ball and not the player.

(b) one of Tech Log's underlying values is in education which is why we are happy to let discussions go around in circles for a while .. if they don't sort themselves out, folks tend to lose interest and such threads sink to the bottom of the abyss.

A good way to underpin one's technical knowledge is to be forced to explanation and justification and, in the process, those whose knowledge could be a little shakey may end up with a benefit from the discussion

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Unfortunately I hadn't been monitoring this thread until certain matters were drawn to my attention. However, I will do so from here on in. Hopefully, my infrequent forays into censorship will not be needed henceforth.
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Old 25th Nov 2012, 10:29
  #104 (permalink)  
 
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Hi John,

Thank you for your intervention. I wasn't prepared for the emotional response to my remark. I shouldn't have made it and regret that I did.

Regards,
HN
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Old 25th Nov 2012, 10:43
  #105 (permalink)  
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When I get to be perfect (SWMBO indicates that such an outcome is not likely in my lifetime) I might expect others to be so.

Occasionally we all are at risk of shooting from the hip .. fact of life, I guess.
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Old 25th Nov 2012, 15:55
  #106 (permalink)  
 
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A good way to underpin one's technical knowledge is to be forced to explanation and justification and, in the process, those whose knowledge could be a little shakey may end up with a benefit from the discussion
yes, but not in the case where person doesn't consider facts and vigorously defend his PoV.
Going personal isn't a very good option though, but, honestly, I can see why some of respectable PPRuNers did just that.

Such level of misunderstanding of physics can be insulting by itself, and if a person having that "knowledge" claims to be a person who relies on that knowledge PROFESSIONALLY, than this makes it twice more insulting.

Last edited by Sunamer; 25th Nov 2012 at 15:57.
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Old 25th Nov 2012, 18:03
  #107 (permalink)  
 
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Only ducks are safe. If one is a professional, and not a duck, one will be carrying about a load of water on one's back.....
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Old 25th Nov 2012, 19:19
  #108 (permalink)  
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Sunamer.

Such level of misunderstanding of physics
.

i would be grateful if you would point out where i demonstrated this to you.

i have already explaned that i ( wrongly but innocently ) used the word thrust not to describe the thrust from an engine but to describe the effect of momentum in a glider or ski jumper on a ramp that will allow him to overcome gravity without lift for a period of time dependent on the amount of momentum acquired.

So i admit i used the word thrust ( and did so previously but it was ignored rather conveniently ) when i should of used another more appropriate word in this instance, i would like to see where else my Physics was so far off.

GB

Last edited by Bye; 25th Nov 2012 at 19:20.
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Old 25th Nov 2012, 20:38
  #109 (permalink)  
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A few more comments, if I may ..

yes, but not in the case where person doesn't consider facts and vigorously defend his PoV.

I have no problem with folks defending their position. If they are wrong, fine, then one hopes that the discussion can work through to the point where that is accepted and the learning process progresses.

On some occasions, brick walls are maintained and the thread is subject to general loss of interest and dies a natural death.

It doesn't necessarily follow that one who holds whatever qualification or accreditation is the font of all wisdom. Folks should be wary of falling into the trap of presuming such. I'm a chartered engineer, for instance, and know a little about a few things .. however some will attest to my lack of knowledge in a great many areas ... the secret is knowing one's areas of strength and weakness .. and acting accordingly.

Going personal isn't a very good option though

and, generally, is the main reason for the very occasional censoring of posts in this Forum.

Such level of misunderstanding of physics can be insulting by itself

such a comment is a tad precious, I would opine ? Then again, I suspect that you are speaking with a smile so all probably is OK

if a person having that "knowledge" claims to be a person who relies on that knowledge PROFESSIONALLY

in a nominally anonymous website, unless one actually knows the person behind the post, there is an inherent danger in presuming too much about the quality of a given post.
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Old 25th Nov 2012, 22:05
  #110 (permalink)  
 
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So i admit i used the word thrust ( and did so previously but it was ignored rather conveniently ) when i should of used another more appropriate word in this instance
That's gratifying to hear.

And, given that the offending posts have all been removed, we'll forget about the equally inappropriate term that you directed at those who had the temerity to point out that your usage was incorrect.
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Old 25th Nov 2012, 22:18
  #111 (permalink)  
 
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Originally Posted by HazelNuts39
If you loaded both trains to have the same mass, would they decelerate at the same rate?
As usual, you've raised a very valid point.

Well, I took both locos/engines and tried the experiment again with just them. One was Thomas and the other was Billy, with identical wheel base / bearings etc.., and yet Thomas came to a stop earlier. So, I got out some scales and weighed each loco; Thomas = 61 grams and Billy = 54 grams, and to equalize the mass I taped a NZ 10c coin (3.3g) + 20c coin (4.0g) to Billy.

To ensure that variable track friction wasn't a factor, I placed both locos on the kitchen bench top and pushed both in the same direction together. They both came to a stop with Thomas slightly ahead by 2mm after traveling about 80cm from the point where the propelling force ceased.

Now these bench top tests with kids toys are far from the ideal, yet they are capable of demonstrating the not so obvious.
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Old 26th Nov 2012, 07:35
  #112 (permalink)  
 
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Hi mm43,

Thank you for sharing your exciting experiment. Thomas, the heavier engine in the first test, stopped in the shorter distance?

Bye,

The problem lies not in the use of the word thrust. You are wrong when you equate momentum to a force. Momentum and force are like apples and oranges. An apple can equate to another apple, it cannot equate to an orange.

Force = rate of change of momentum (post #71).

You yourself wrote in post #90: Force = Momentum/Time

If that is correct, then Force = Momentum cannot be correct.

Let's examine the ski jumper. If we ignore the aerodynamic forces acting on him, then after leaving the ramp the only force he is subjected to is gravity. Because there is no force opposing gravity, his vertical motion slows down at the rate of 19.05 kts/second. If there were no force acting on him he would continue to move at constant speed in a constant direction.

Last edited by HazelNuts39; 26th Nov 2012 at 07:52.
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Old 26th Nov 2012, 08:11
  #113 (permalink)  
 
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somebody claims a question and you all start to fight ,throwing wild formulas, in the very urgent try to demostrate the own internet superiority without thinking about the question by itself.

we have to ask if the statement"why heavier aircaft take longer to slow down in the air" is in any case true at all .

as an extreme example lets take the space shuttle which is (was) at approach basicly an aerodynamic glider airplane. and for sure significantly heavier than a small one-seater glider.

did it take longer for the space shuttle to slow down in the air than for the small glider ?

cheers
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Old 26th Nov 2012, 08:24
  #114 (permalink)  
 
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From reading the original question I believe the OP, and all subsequent replies, were referring to the same aircraft.
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Old 26th Nov 2012, 08:56
  #115 (permalink)  
 
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Gennlemen

One thing that amazes me about physics is that we don't even know what a force actually is... For us it jIs ust whatever makes a mass to accelerate or decelerate, whatever the hell it might be. There is something that makes that happen, and they called it Force.

But we don't really know what is the nature of forces, or their origin. Again our fate is to recur to the empirical method.

I think I demonstrated to you what is the effect of weight in drag for normal flight AoAs: less than proportional than the mass increase. It coincides with the intuitive notion of higher momentum or higher KE will leas to longer stopping distance. However, at high AoAs drag force will increase more than proportionally to mass, and the heavy airplane will actually slow down faster than the light one, which is against anyones' instinctive answer.

So Newtons second law is the right choice, after the above analisis.

a=F/m.

Sometimes F increases more than mass, sometimes it does increase less than mass. sometimes acceleration is slower, sometimes it is faster.

It all depends on the AoA.

Owain said above the two airplanes can fly at the same speed, 226 kt, with the same L/D, which is amazing to me, but since I tend to believe him, I thought a bit about it and I deem that the heavy at 226kt is above max L/D AoA while the lighter one, at that speed is below it, and they happen to be the same L/D. Is that right?

Last edited by Microburst2002; 26th Nov 2012 at 08:56.
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Old 26th Nov 2012, 13:39
  #116 (permalink)  
 
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Microburst

I thought a bit about it and I deem that the heavy at 226kt is above max L/D AoA while the lighter one, at that speed is below it, and they happen to be the same L/D. Is that right?
Yes, that's right. If you look at the graphs I posted (#48) you will see that at 226 kts the high weight aircraft is flying below minimum drag speed (AoA above that for max L/D) and the light weight above its minimum drag speed (AoA below that for max L/D). I guess that if one had the time and the inclination one might be able to derive some sort of formula to tell you what that speed might be for any given set of coefficients, but to be honest I can't be bothered

Of course if one is thinking about how long it takes for aircraft with different weights to slow down then the drag/weight will vary with airspeed differently for each aircraft so the fact that at one particular speed the values of D/W i.e.1/(L/D) are the same is unimportant, particularly since that speed is going to be pretty close to the lower limit of authorised airspeeds (1.25~1.3 Vs?)

Last edited by Owain Glyndwr; 26th Nov 2012 at 13:50.
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Old 26th Nov 2012, 14:34
  #117 (permalink)  
 
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one might be able to derive some sort of formula
Equation (9) in the paper referenced in post#27 gives the minimum drag speed Vmd. It corresponds to a lift coefficient equal to √(Cd0/K).

If Vmd1 and Vmd2 are the minimum drag speeds at different weights, then the speed at which the airplane has the same L/D at both weights is given by:

V = √(Vmd1*Vmd2)

P.S.
A pretty useless formula, isn't it? It took me a whole evening to work it out. Just for the fun of doing it. But the result is a beauty, for those open to it.

Last edited by HazelNuts39; 28th Nov 2012 at 07:16. Reason: to correct the misunderstanding OG points out in Post#118
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Old 26th Nov 2012, 17:18
  #118 (permalink)  
 
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Hi HN39

Equation (9) in the paper referenced in post#27. It corresponds to a lift coefficient equal to √(Cd0/K)
I read that as an equation to calculate minimum drag speed. Maybe the way I phrased things, but I meant an equation to tell you at what lift coefficient two aircraft of differing weight could have the same L/D. Or am I missing something?
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Old 26th Nov 2012, 19:19
  #119 (permalink)  
 
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Somewhat as a johny come lately, a few observations:

Quite a lot of dodgy Physics in this thread.

You can go at the OPs question using momentum if you like. You can also go at it using KE. Momentum is handy if you want to know how long a time a velocity change requires. KE is handy if you want to know how long a distance a velocity change requires.

(Generally, energy changes are easier to deal with because they are scalars.)

Bye, and gliding etc. Sorry Bye, but your Physics is full of holes.

In my experience people with a dodgy grasp of mechanics usually fail at Newton one. This is evidenced by trying to find some 'Thrust' to explain why the glider keeps moving. If you resolve forces on axes along/across the flight path then a component of weight is equal and opposite to drag, ergo there is no resultant force ergo the velocity is constant.

So, the gliders equivalent to thrust is a component of its weight, not it's momentum.

Pb
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Old 27th Nov 2012, 05:10
  #120 (permalink)  
 
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Capt bull...

If frame of reference can be exchanged, how is thrust not acceleration due gravity, when descending? How is dissipation of momentum not due to the thrust provided by drag? If the calculus is interchangeable due the acceptance of any increase in acceleration as due to thrust, positive or negative, then any loss of velocity can be expressed in terms of only two forces, gravity and drag, both of which accelerate the airframe.

Owain. If the heavy a/c is at AoA producing maximum lift, is it not About to Stall? There can be only an instant of maximum lift without power?

How can one equation serve to express two variable results, when it cannot contain a non common variable?

Sorry, it's late.
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