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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?

24th Nov 2012, 09:33

Join Date: Jan 2008
Posts: 10,596
No need to discuss momentum or energy.
Whew!

Thanks for that - I was beginning to worry that when I did my aero engineering degree I must have missed a lecture where they explained that momentum was a force.
24th Nov 2012, 10:26

Join Date: Jul 2009
Location: France - mostly
Age: 80
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Dave,

We're debating with a person who claims to be an engineer designing aero engines yet writes such nonsense as (see his first post #15 on this thread):

... 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.

... if the objects have different mass as the drag of each object equals the mass of the object acceleration will become zero
Then states his objective as:
water sufficiently muddy i hope.
Regards,
HN
24th Nov 2012, 10:59

Join Date: Mar 2005
Location: Uh... Where was I?
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Using Newton (forces, inertia, acceleration, momentum) is complicated here, because airplanes move a lot of air. It is not just ann ideal body moving in outer space.

For F=m.a we need to know the force, which is D, and it is a complex dorce to work with. It doesn't behave linearly.

For KE= 1/2 m v2, we don't know how much energy is put in the air, so it is difficult to calculate decceleration rate based on that.

The best approach, I deem, is the empirical formulae of L and D, which are based on the empirically obtained values of CL and CD. And then Newton.

Mass increases, Weight increases proportionally, AoA increases proportionally, CL increases proportionally, CD increases but less than proportionally to CL (at typicall AoAs), so D increases less than mass does. Newton's second and Galileo's equations then take us to the conclusion: The heavier a/c will use more time and distance to slow down.
24th Nov 2012, 11:49

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Location: Derby UK
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hen I did my aero engineering degree I must have missed a lecture where they explained that momentum was a force.
and i missed the bit where i claimed that it was well.

But hey you can't let the facts get in the way of an attack now can you.

what i said and repeated was that a force is derived from the momentum to make the glider go up against gravity.

In this circumstance there are 4 forces acting on the glider not 3.

The momentum is not a force it creates a force on the mass to push it in a direction momentum has a quantity and a vector.
24th Nov 2012, 12:02

Join Date: Jan 2008
Posts: 10,596
i assume you can prove this claim that i am not an engineer designing aero engines
For the avoidance of doubt, I'd like to make it clear that it wasn't me who made that claim about you.

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.
Remind me again, who exactly is being "highly insulting" ?

Pot ? Kettle ? Black ?
24th Nov 2012, 12:07

Join Date: Jul 2009
Location: France - mostly
Age: 80
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Bye,

I'm just quoting the facts as you wrote them in your posts. It is up to the reader to form his/her opinion about your sincerity in this debate.

If you're not playing the game of trolling, then take the sound advice offered to you in good faith in post #75.

Regards,
HN

Last edited by HazelNuts39; 24th Nov 2012 at 12:16.
24th Nov 2012, 12:34

Join Date: Jun 2011
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Bye,

You have complained that nobody will explain the basic aerodynamics to you but you yourself gave a reference to a document that does just that in post #27. Is it unreasonable to expect that you have read this?

You are a design engineer and own a company that designs aero. engines so you ought to be able to put some numbers in the equations given in your reference, or employ someone who could do it for you, so if I give you some realistic values you can check what others are saying for yourself.

For an aircraft of the B737/A320 class:

Cdo (profile drag coefficient) 0.018
K (induced drag factor) 0.0425
Wing area 1300 sq.ft
Weights between 180,000 and 120,000 lb

Over to you

Last edited by Owain Glyndwr; 24th Nov 2012 at 12:36.
24th Nov 2012, 15:44

Join Date: Aug 2011
Location: Grassy Valley
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Adding any energy to the system (thrust) was mooted when it was eliminated in the original question. The question had everything to do with reducing velocity, "slowing down". This involves dissipating energy, not accreting it.

I think calling momentum 'thrust' is not appropriate. Momentum is the result of thrust, not its own precursor. The identified heavy aircraft is unpowered by definition.

all respect.
24th Nov 2012, 15:59

Join Date: Jun 2011
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Age: 83
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Bye
Owain, sorry but someone posted that the link i posted contained references that were wrong so i expected them to be corrected before i could use them
I don't know what that means exactly, but if I knew that references in one of my posts were wrong I would feel duty bound to issue a correction. As it stands the reference you quoted DOES contain the explanation you are looking for and you can access it and put in the numbers I gave, so you can, if you wish, make an independent verification of what others have been saying here.

Once again, over to you.
24th Nov 2012, 16:18

Join Date: Aug 2007
Location: Derby UK
Age: 54
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He didn't say what they were. and i couldn't see any errors but then i know nothing.

so i was hoping he would point out where the material was in error.

i have worked through with your numbers and thanks for them, and i am wiser for it thanks to you.

oh and forgot to state that FORCE = MOMENTUM / TIME

GB

Edit, having gone thru again, i still can't get why the 2 aircraft at different weights can't have the same L/D ratio. the graph shows they do at 226 kts, and my sums show they do as well with the data you gave.

I can get them to have the same L/d ratio at a certain V every time, so i would be grateful if you would explain again why its not possible and where the graphs are wrong.

GB

Last edited by Bye; 24th Nov 2012 at 16:40.
24th Nov 2012, 16:43

Join Date: Jul 2009
Location: France - mostly
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Dear mr Bye,

I'm not aware of anything in my writing that calls for the apology you ask for. I've pointed to an incongruence that I perceive between your claim of being an engineer and the understanding of elementary physics that you display in your posts. That leaves two possibilities:

Either you are an engineer and deliberately pretend to misunderstand the physical realities you write about, or -

You really don't understand physics and refuse to learn more about it, which in my mind is not compatible with having had an engineering education.

I do apologize for a mistaken remark that I made regarding the paper referenced in your post #27. That paper discusses speed stability at two arbitrily chosen speeds V1 and V2. When writing my post just before going to bed I mistakenly associated those with the V1 and V2 speeds that are used in the operation of civil transport airplanes. Thinking about it later I realized my error and deleted that remark first thing the next morning.
24th Nov 2012, 17:09

Join Date: Jan 2008
Posts: 10,596
the rest of us can't hope to match your grasp of aeronautics
You're right, I did indeed say that. I've seen nothing in your subsequent posts to make me change my view.

and i missed the bit where i claimed that it [momentum] was [a force]

in a glider the only thrust you have is in fact the momentum you have
yes its called thrust because the momentum is PUSHING the glider upwards against gravity
I rest my case.
24th Nov 2012, 17:28

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Bye

Because you specified that they were both flying at 400 km/hr (250 kts).

226 kts is the ONLY speed at which they can have the same L/D.
24th Nov 2012, 17:43

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Posts: 10,596
Yet you claim i said that momentum was a force, sorry i don't see it, i see the word thrust, but not the word force
OK, I understand now - you don't consider thrust a force.

So presumably the thrust that's produced by the engines you design isn't capable of making the aircraft move.

Words fail me.
24th Nov 2012, 17:45

Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
hi bye,

I see no libel. I also see an ephemeral chance for truce.

IMO. Thrust is a force that is not contained in the object to which it is applied.

So, to apply the word Thrust to the system under discussion, we look for a separate object, divorced from the airframe, such that it is subject to, and gains from, the force contained in the system.

The Airmass is acted upon, and reacts to, the entry of the aircraft into its separate region. The aircraft imparts energy to the airmass, the airmass reacts, the airplane slows. Accelerating the airmass produces momentum therein, the aircraft accelerates negatively, etc.

As I say, ephemeral. But not wrong, and in the proper spirit, etc.

bye, bye

Last edited by Lyman; 24th Nov 2012 at 17:53.
24th Nov 2012, 23:01

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We're debating with a person who claims to be an engineer designing aero engines yet writes such nonsense
I read this in the context that an engineer is writing something that, to the author of the above quote, didn't make a lot of sense for someone who is suppose to be an engineer...not the way you have seen it which I assume is the author saying that because what you wrote didn't make sense to him you cannot be an engineer.

Much like someone saying to me that they can't believe I claim to be pilot if I did a landing like that - They're not claiming I'm lying about being a pilot, they're just claiming that as a pilot they can't believe I actually did a landing like that!

And off the record I nail every landing - on the record though...
25th Nov 2012, 03:14

Join Date: Aug 2012
Location: Overland Park, KS
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Already have one engineering degree, and working toward computer science master's degree.

all this thread is .....mm....weird! I initially (silly me) thought that there is only momentum envolved. Then, ok, if you need to account for different attitude (=different AoA + different profile, obviously) for different weights you can do it too. But after you calculate initial momentum which doesn't change (I don't consider burning fuel here, since that wasn't the initial question and there is no point in making task over-complicated) it shouldn't be too hard to account for different AoA + profile drag too. induced drag will be almost the same, I assume.

I think the thread needs to be closed since there is no point, I am afraid, to continued!
One side tries to stick to physics, others try to develop their own...ehmm...understanding which is a bit strange to me!

All sloppiness that are presented here can't be a sign of good knowledge in physics in any way.
Everything that was said here about legal claims whatever it might be, simply beyond anything logical...don't want to comment on that.

Last edited by Sunamer; 25th Nov 2012 at 03:19.
25th Nov 2012, 05:58

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I don't have an engineering degree, but have been interested in the points raised.

The OP's request related to two similar aircraft, one heavier than the other, and why the heavier aircraft took longer to slow down than the lighter one. Assuming that the initial speeds are the same, the mass of each aircraft along with the L/D ratio (associated with AoA) will become the variable. Discount the flight idle fuel usage and thrust which are equal for both aircraft, then we are essentially dealing with kinetic energy.

EK = ½mv²

The L/D ratio along with AOA will change in a similar (though different) fashion for both aircraft as they slow, so it can be assumed that the Mass and V² are the only real factors we are dealing with. SLOWING is the operate word, and it follows without too much argument that the aircraft with the higher mv² product will always take longer to slow than the lighter one.

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.
25th Nov 2012, 07:27

Join Date: Aug 2012
Location: Overland Park, KS
Posts: 120
yes. all that is true, but

the thing that needs to be accounted for also is:
unlike in the case with two different-weighted railroad engines, that have different mass but identical railways, identical speed and identical friction (drag is the same here)
two identical planes with different masses will have different AoA due to the fact that once needs more lift to keep AC at the same altitude if AC has more mass, than derivation of AoA = aerodynamic profile will be (probably) less efficient and this will create more profile drag (not induced, though). Since it creates more drag, it probably in some cases will be enough to counteract more momentum of heavier AC and thus the result will be the same = two aircraft will slowdown the same way.

everything depends upon particular coefficients and values. There is no general solution here to say that yes, it will slow down slower (or faster), I am assuming.
25th Nov 2012, 07:46

Join Date: Jan 2008