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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?

27th Nov 2012, 07:45

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Lyman

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?
Any aircraft at the AoA for maximum lift is about to stall no matter what the weight. But who is talking about such conditions? Not I for sure! We have been discussing the point at which L/D is the same for two different weights - 226 kts in the example I gave. For those aircraft stall would have been at about 170 kts or 194 kts - close to the LH ends of the lines.

How can one equation serve to express two variable results, when it cannot contain a non common variable?
Allow me to give a Lymanesque answer - How can a bus with #29 on the front go to a different destination from the one with #41?

[Its early]

Last edited by Owain Glyndwr; 27th Nov 2012 at 07:47.
27th Nov 2012, 08:17
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Then again, Owain, is it not true that a #29, full of passengers, would take longer to slow down than an identical but empty bus #41?
27th Nov 2012, 11:06

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Then again, Owain, is it not true that a #29, full of passengers, would take longer to slow down than an identical but empty bus #41?
Sure its true, which is a good safety feature because the empty #29 just behind will be able to stop in time if the first #29 does an accelerate/stop
27th Nov 2012, 11:34

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... if the brakes are too weak to lock the wheels or to activate the ABS.
27th Nov 2012, 11:49

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I hope I manage to paste here this graph...

On the slope of the CD curve is the answer of the effect of weight o a flying airplane slowing down rate.

Absolutely no momentum or Energy analysis is valid without taking this into account

edit, I dont know how to paste the graph alone without the text...

Last edited by Microburst2002; 27th Nov 2012 at 11:50.
27th Nov 2012, 11:58

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Lyman

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.
Too many variables, too many questions to understand what you are getting at.

Thrust provided by drag? Conventionally we talk about thrust as provided by engines.

I was briefly visiting gliding because it got a brief mention in the thread, although the OP was talking about deceleration in level flight.

how is thrust not acceleration due gravity, when descending?
BZZZTTT! Dimensions clash!!!!

How is dissipation of momentum not due to the thrust provided by drag?
BZZZTTT! In level flight unbalanced drag will dissipate momentum, yes, but drag does not 'provide' thrust.

Notice I said

gliders equivalent to thrust is a component of its weight
It's equivalent in being (1) a force that is (2) equal and (3) opposite to the drag, thereby (4) keeping the aircraft at a constant speed. But is not actually thrust in terms of being a powerplant squirting air around.

(Personally, as an aside, I find gliding a lot easier to explain by forgetting about drag as a stand alone force. It's only a method for conveniently resolving the total aerodynamic reaction for times when we want to show 4 forces. For a steady glide it's actually a lot easier to say total reaction is equal and opposite weight. Therefore no unbalanced force therefore steady glide.

Energy burnt of as force x distance, power as force x TAS, therefore KE dissipated as drag x TAS. Easy, no vectors because Energy is a scalar.)

pb
27th Nov 2012, 12:36

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Captain Pit Bull

It's equivalent in being (1) a force that is (2) equal and (3) opposite to the drag, thereby (4) keeping the aircraft at a constant speed. But is not actually thrust in terms of being a powerplant squirting air around.

Agreed. However, relative to the airframe, there is no difference in drag v. Thrust, simply because no energy is being consumed in the form of fuel. With the standard four forces model, cruise flight is balanced among four assumed forces. When one of the required forces is diminished (or enhanced),
a change in E occurs. To slow, Thrust is removed, and the other three are left to establish some form of attitude that will result in arrival.

Simplistically, which of the two aircraft under discussion can remain aloft the longest, the heavier? The lighter? Because that introduces Galileo, no?

Thanks for your response, lyman
27th Nov 2012, 13:43

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What an interesting thread and discussion.

When I go back to the original post, I see a problem statement that asks us to consider deceleration between two aircraft of different mass. (I assume the frame of reference is Earth, where a difference in weight can be equated to a difference in mass).

One is asked to explain the time it takes to change velocity from one value to another, with the only difference in initial condition being difference in mass. (Note that I had to make a number of assumptions to parse that problem statement).

I have enjoyed the seven pages of discussion into the esoterica of deceleration, rates, inertia, momentum, and enery in an aerodynamic context. Good stuff!

From the discussion, I find myself aligned with those people who approach the problem from the change in energy state. Were I asked that question with no help from 20 pound brains, that is the direction from which I would answer it.

Pardon me but..

1)Why do heavier aircrafts {sic} take longer to slow down in the air compared to lighter aircrafts? {sic}
This appears to me to be an example of begging the question, and indeed, assuming the answer to a question in the asking.

A more correctly phrased question would be:
Do heavier aircraft take longer to slow down in the air compared to lighter aircrafts? {sic} If so, why is that?

[QUOTE] 2)Why on high profile, it's recommended to increase speed? [QUOTE]
Is it? By whom?

Depending on how you are flying, you may wish to maintain approach speed and reduce power to increase rate of descent, or you may wish to increase drag by doing things like adjusting flaps, or dropping the landing gear to increase drag. (Or, if in a glider, descending in a slip to also increase drag ... )

Once again, I'd say we have an example of begging the question.

1) Given all other things being equal (which some of the above conversation suggests they might not be) and
assuming the "I need to slow down" action taken will be identical
in terms of what change to the aircraft is executed to change energy state (example, change a flap setting or reduce engine power an identical amount)
then the aircraft with the greater energy at the beginning would more likely take longer to slow down ... On The Assumption that they are both attempting to get to the same "V" when slowing down in the first place. (Can we assume that? )

I have been cheered to learn that "it's more complicated than that" during the discussion, because of aerodynamic unique considerations. Again, my thanks to the 20 pound brains.

I am not certain the person framing the question was clear on that or not.

2) The permutations of what to do with your energy state to change profile if you are high were not, in my training, generally to speed up to change profile errors, unless one were ALSO slow and while being high on profile.

The problem statement omits what actual initial conditions are, which leaves us guessing. I'd answer question number two differently:

If you are high on profile and slow, then speeding up is a part of returning to profile.
If not, then speeding up may not be the best way to resolve your being off of profile to begin with.

For example, if you have time to change configuration early, perhaps you dive, speed up, and then dirty up early so as not to get to close to the plane ahead of you ... there's a lot to this answer left open in the question.

To answer number two, on basic principles, I'd allude to "Power plus attitude (for a given cofiguration) equals performance" in powered flight.

This answer is not provided by an aero engineer, it was provided by a pilot. Sure, I have a degree in mechanical engineering, but I can't hang with the 20 pound brains in this thread. (You all know who you are! Goodonya, all of you!)

Talking about gliders seems a red herring in this discussion.

Last edited by Lonewolf_50; 27th Nov 2012 at 22:09.
27th Nov 2012, 13:54

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Is this thread serious?

27th Nov 2012, 14:10

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The energy or, more practically, the power extracted from the fuel to keep the airplane flying level at a constant speed would give us some measure of the rate of kinetic energy decrease if the airplane maintained altitude after reducing thrust to nil.

Comparing the power delivered by the engine to the airplane at the reference speed with the power the airplane would have at another reference lower speed would give us the average kinetic energy loss between that speed range, for that weight. From that we could obtain deceleration rate. But i don't now how yet...

edit the rest of the post I have quarantined for analyisisssss

Last edited by Microburst2002; 27th Nov 2012 at 14:12.
27th Nov 2012, 14:42
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Originally Posted by LM
Is this thread serious?
- it would be if you were waiiting for the #41...............................
27th Nov 2012, 14:54

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Solo Lupus 50

Talking about gliders seems a red herring in this discussion.

From the original, the question has to do with accelerating a reduction in velocity. It is a Rate question, and demands a little latitude in parameters.

So my assumption was from the beginning that the engines on both slowing a/c were 'mass only', not thrust. We could get into "windmilling" v. Flight idle, but as we see with seven pages, most here are not satisfied unless the discussion can be made more complex, not less.

To some extent it is a "Straw Thread", a vehicle for trotting out some algebra in the interest of appearing cerebrally massive.
27th Nov 2012, 15:25

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it would be if you were waiting for the #41.....
In the rain ...
27th Nov 2012, 15:44

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- it would be if you were waiiting for the #41...............................
Time to go where people know what they are talking about then.

Why is this "professional pilots forum" full of amateur idiots please?
27th Nov 2012, 16:00
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Why is this "professional pilots forum" full of amateur idiots please
- I trust you are not referring to me, Carruthers - I have always prided myself on my professionalism
27th Nov 2012, 16:31

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Time to go where people know what they are talking about then.

Why is this "professional pilots forum" full of amateur idiots please?
Occasionally one needs a little light relief from all the idiocy, both amateur and professional, that has plagued this thread.

There are non-piloting professionals writing here as guests, and believe it or not, some of them know what they are talking about.
27th Nov 2012, 22:06

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Lightning Mate: maybe lightening up would be in order, eh mate?

Here, have a pint.

Lyman:
From the original, the question has to do with accelerating a reduction in velocity.
From the original, the question has to do with deceleration. What you typed there is ...
It is a Rate question, and demands a little latitude in parameters.
Depending upon if you are solving it in the classroom, or in the cockpit.
So my assumption was from the beginning that the engines on both slowing a/c were 'mass only', not thrust.
Engines are generally used on an aircraft to provide sufficient thrust to fly.
Hanging an engine on an airframe for the purpose of mass would be counter productive, would it not? Hanging engines on a glider seems to defeat the purpose of building a glider, does it not?
We could get into "windmilling" v. Flight idle, but as we see with seven pages, most here are not satisfied unless the discussion can be made more complex, not less.
Various means of increasing drag are yours to employ if you know your aircraft well. They become tools for deceleration, which as I noted is the core of the question in post number 1.
To some extent it is a "Straw Thread", a vehicle for trotting out some algebra in the interest of appearing cerebrally massive.
The Original Post, as I noted, begged the questions.

From there to now, it's been fun and games.
I, for one, have been most entertained.

Would you care for a pint as well?

Last edited by Lonewolf_50; 27th Nov 2012 at 22:12.
27th Nov 2012, 23:06

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Wuff...

Slowing is acceleration with a minus, no? Nomenclature and concept.

I'll have a pint, thank you Sir! Sarseparilla....

Now slowing in a helicopter.....requires thrust. No rest for the wicket.
28th Nov 2012, 03:59

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Maybe what seems like weight affecting descent angle has more to do with the fact we descend at about 320 knots and we are closer to our best L/D at that higher speed. If we are light our L/D speed is much lower. Once level I think the same aerodynamics apply. Being heavy always requires more miles because the wing is more efficient at the higher speed. It always worked for me.
28th Nov 2012, 08:50
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Why is this "professional pilots forum" full of amateur idiots please?

One of the thread management problems we are faced with regularly relates to the basic question of what the forum is about.

(a) if commentary is restricted to a specialist group, fine, but the whole thing becomes elitist and, very possibly, we miss out on a bunch of interesting things

(b) if commentary is restricted to folks of competence above a given level, (a) probably still applies and we lose the potential of whatever inherent training/educational value may vest in a given thread.

Senior management edicts indicate that we should transfer appropriate threads to the Questions forum. I interpret that to mean threads which rapidly lose their way/provide little interest for the readership or for which requests are made from the floor for such to occur.

So far I have not had any PMs etc so requesting.