Question on forces acting on an aircraft in climb

[Vibes]

Hi Guys,

I was reading through Gary Bristow's book,Ace the Pilot Technical Interview and in the first chapter,there is a question asking about the forces acting on the airplane in flight and the answer is as follows:

"Drag,thrust,lift and weight.When thrust and drag are in equilibrium, an aircraft will maintain a steady speed.For an aircraft to accelerate,thrust must exceed the value of drag.When lift and weight are in equilibrium,an aircraft will maintain a steady,level attitude.For an aircraft to climb,lift must exceed weight of the aricraft. In a banked turn,weight is a constant,but lift is lost due to the effective reduction in wingspan.Therefore,to maintain altitude in a banked turn,the lift value needs to be restored by increasing speed and/or the angle of attack."

What I find peculiar is the part when he mentioned about the aircraft climbing.He states that lift must exceed the weight of the aircraft.But I supposed he failed to mentioned that thrust also plays a part when an aircraft climbs cause without thrust,your airspeed will decay and lift won't be enough to sustain the climb.That's what I feel.

However,I would like the opinion of fellow PPRUNERS on this topic as I feel that you guys are in a better position to answer.Thank you.

[JulieFlyGal]

If I recall correctly from PPL lesson 2, in a climb, lift is less than weight and thrust is greater than drag. I remember this because it was somewhat bizarre as you'd expect lift to be greater than weight because you're going up. But not so. What gets you up is excess thrust.

[18-Wheeler]

If I recall correctly from PPL lesson 2, in a climb, lift is less than weight and thrust is greater than drag. I remember this because it was somewhat bizarre as you'd expect lift to be greater than weight because you're going up. But not so. What gets you up is excess thrust.

Correct - The only time left is (much) more than weight is during a turn or descending. It's a little more in stable level flight, but not a great deal - The wing has to generate a little more lift than weight to counteract the downforce from the tail.

[ChristiaanJ]

Vibes,
The text you quote is so full of mistaeks and ambiguity that it's not worth commenting on.

There is a long thread on this subject elsewhere in 'Tech Log'... not sure I can still find it.

What JulieFlyGal says is correct. It was also the conclusion of the other thread.

CJ

[Vibes]

Hi All,

First and foremost,thank you very much for answering to my queries especially JulieFlyGal,I really appreciate the comment given and I tend to agree with you.

As for CJ,I don't see why I am not able to make any clarifications on this great forum when the fundamental purpose of this forum is for us to come and share our ideas and thoughts.If you feel that my queries are not worth your time to answer,then I feel that you shouldn't be here in the first place.

And CJ,why don't you look up the book itself before accusing what I've quoted as "so full of mistaeks and ambiguity that it's not worth commenting on".And if making a mistake is wrong,why don't you start correcting yourself first by learning how to spell?:rolleyes:

[Capt Pit Bull]

Vibes

CJ said the text you quoted was full of ambiguities.

I have to say, I agree. There is almost nothing in that paragraph that is correct.

"Drag,thrust,lift and weight.When thrust and drag are in equilibrium, an aircraft will maintain a steady speed.For an aircraft to accelerate,thrust must exceed the value of drag.When lift and weight are in equilibrium,an aircraft will maintain a steady,level attitude.For an aircraft to climb,lift must exceed weight of the aricraft. In a banked turn,weight is a constant,but lift is lost due to the effective reduction in wingspan.Therefore,to maintain altitude in a banked turn,the lift value needs to be restored by increasing speed and/or the angle of attack."

The stuff in red is wrong, or only true in particular circumstances (that are not specified). The stuff in green is correct.

IF its quoted correctly, then the book seems (to me) to be of very dubious quality.

Now, you can take that opinion or leave it; neither I (nor CJ) are obligated to go out and buy it to conduct a full review for your benefit. Likewise, I am not required to provide you with a complete lesson covering these topics; (although I am available for consultancy for a very reasonable rate!).

pb

[ChristiaanJ]

As for CJ, I don't see why I am not able to make any clarifications on this great forum when the fundamental purpose of this forum is for us to come and share our ideas and thoughts.If you feel that my queries are not worth your time to answer,then I feel that you shouldn't be here in the first place.Sorry if you misunderstood me.
It was simply the text you quoted that got my hackles up, because it contained so much rubbish (as Capt Pit Bull also points out), that I really saw no point in refuting every single ambiguity.
But I agree I didn't reply correctly to your final question, and as you say, that's what PPRuNe is about.
See below.

And CJ,why don't you look up the book itself before accusing what I've quoted as "so full of mistaeks and ambiguity that it's not worth commenting on". And if making a mistake is wrong,why don't you start correcting yourself first by learning how to spell?If your quote is anything to go by, I certainly see no reason to look up the book itself.... as an aeronautical engineer I do have better literature on my bookshelves....
And apologies if you didn't get the deliberate "speling mistaek".

What I find peculiar is the part when he mentioned about the aircraft climbing.He states that lift must exceed the weight of the aircraft.But I supposed he failed to mentioned that thrust also plays a part when an aircraft climbs cause without thrust,your airspeed will decay and lift won't be enough to sustain the climb.That's what I feel.What I find interesting that your "feel" was right from the start - in the other thread I mentioned it took a fair amount of kicking the idea around, and drawing diagrams, before everybody agreed.

Much of the confusion seems to come from the differences between what I would call 'flight dynamics' and steady-state flight.
You're exactly right. If you start climbing by pulling back on the stick, momentarily the lift is greater than the weight, but if then you don't increase the thrust, the airspeed will decay.

Another thing that seemed to cause confusion is the coordinate system....
Lift, thrust and drag are all defined relative to the flightpath, while weight/gravity always acts along the vertical.

In a steady-state climb, lift, thrust and drag on the one hand, and gravity on the otherhand, are no longer at right angles. Hence, lift is less than in horizontal flight. Thrust is no longer equal to drag, but more, because it now has to compensate for the component of the weight no longer 'supported' by the lift.

The extreme case, quoted in the other thread, is of course a steady-state vertical climb, where the lift (force at right angles to the flightpath) is now zero, and thrust minus drag is equal to the weight.

When the question was first asked in the other thread, most of us had some trouble fully getting our head around it.
So my compliments to you for getting it right first time!

Cheers,
Christian

[captainsmiffy]

Guys, having read the bristow book at length some 2 years ago, not having interviewed in over 12 years prior, I was very disappointed to say the least! It is simply full of holes! There is some good stuff but you would have to really know your stuff to sort the wheat from the chaff here....

[PantLoad]

I find the information contained in this thread to be quite enlightening. In my almost 40 years of flying, I never knew some of these things...and
I take Bristow as 'gospel'.


NOT!!!!!!!!!

(Only joking with you, captainsmiffy!!!!)

Fly safe,

PantLoad

By the way, the U.S. Navy has great pubs explaining how an airplane flies.

[SNS3Guppy]

Bristow's book is a known quantity, and is well understood to be full of errors.

[ChristiaanJ]

The U.S. Navy book is: "Aerodynamics for Naval Aviators".

I haven't read it yet, but everybody here has been unequivocally recommending it, so I'll be ordering it too.

Just Google the title. It can be had for about $10, and even less second-hand.

CJ

[Rainboe]

Nobody please ever spend any money on ACE the Pilot Technical Interview! IMO, I think it is complete and utter confusing erroneous garbage. If you walk past a bookshop table with it remaindered out very cheap (or free), stillwalk on by! This rubbish does not merit discussion here.

[Vibes]

Hi Guys,

First and foremost,thank you for all the responses you've made to this thread.I really do appreciate it especially the clarification made by CJ.My apologies to CJ for the misunderstanding in the earlier posts.With regards to the book by Bristow,as far as I understand,I have heard about it containing misleading info.Hence,I decided,on my own accord,that the best way to learn is by taking his questions and clarifying whatever answers that were given by him.Though its a long method of learning,I feel that it is good as I get the opinions of many professionals in this field.Hence,its been great to be in the company of you guys as IMO the experience you guys have in this forum beats any books that's been written.And aviation,with a huge range of subjects from meteorlogy to aerodynamics,could be quite misleading in a sense that you have so many books and so many different authors,each expressing their own opinion.Thus,that's why I've asked time and again,many silly questions with regards to aviation cause books can only tell you so much.And I supposed in aviation,experience is the best lesson.And this forum has lotsa of them ;)

[Rainboe]

I was so concerned at the inconsistencies I found in this book, I started folding over the corner of the pages I wanted to check out and study further. Let´s say it quickly became apparent that something was amiss with the book! It looks to me like an author overreached themselves, and proof readers were used who didn´t know a whole lot about aviation, and if the spelling seemed OK, that was just dandy by them!

[ChristiaanJ]

... proof readers were used who didn´t know a whole lot about aviation, and if the spelling seemed OK, that was just dandy by them!Well, isn't that what proof readers are supposed to do?

In this case it sounds as if the book could have done with a couple of expert copy editors,before being handed to the proof readers. :ugh:

...I've asked time and again,many silly questions with regards to aviation...Questions are rarely silly.
The problem is usually with the answers: how much does the person asking the question already know, and how much needs explaining?

CJ

[Wizofoz]

In a banked turn,weight is a constant,

Even this bit is arguably wrong. Weight is a force. F=M*A. In a turn the aircraft is subject to the sum of acceleration due to gravity AND the acceleration of the turn, so it's effective weight is increased. The wing needs to produce additional lift to support this, which is why it requires either a higher AofA or extra speed to maintain altitude.

[pineridge]

It would probably be of great asistance to the instigator of this post if someone could find and post a series of simple explanatory vector diagrams. I seem to remember that such diagrams were used to great effect during "theory of flight " briefings when I was learning to fly.
Such a diagram would show, for example, how the resultant of the lift vector
(lift and thrust) would be balanced by the resultant of the weight vector (weight and drag) in a constant climb, etc., etc.
I am unable to recall where I have seen the diagrams published, but it could have been "Flight Briefing for Pilots" by Allan Bramson.
Does anyone know what I am talking about?

[ChristiaanJ]

pineridge, Vibes,


http://img.photobucket.com/albums/v324/ChristiaanJ/Total_drag.gif


Here's my earlier scribble (call it a vector diagram if you like) I did for the thread I mentioned earlier.
This one is strictly for the force balance in a steady-state climb, drawn for the horizontal and the vertical balance.
Re the sin(ac) and cos(ac) ... the "ac" is "angle of climb", happened to be the term used in the other thread for the flight path angle or climb angle.

Turns is another story.
Wizofoz, I've seen so many badly worded explanations I won't start on that one yet...

CJ

[Wizofoz]

Wizofoz, I've seen so many badly worded explanations I won't start on that one yet...


Mine or his?:eek:

[shanx]

In a banked turn,weight is a constant,

I think that the statement above and other similar ones are often the result of a lot of people (strangely) assuming MASS to be synonymous with WEIGHT.

[ChristiaanJ]

Wizofoz,
No, I was talking about the book versions, of course.

CJ

[Capt Pit Bull]

Quote:
In a banked turn,weight is a constant,

Even this bit is arguably wrong. Weight is a force. F=M*A. In a turn the aircraft is subject to the sum of acceleration due to gravity AND the acceleration of the turn, so it's effective weight is increased. The wing needs to produce additional lift to support this, which is why it requires either a higher AofA or extra speed to maintain altitude.

I don't really have the inclination to get into this in detail, however (wiz) your explanation suggests a misconception. The fact that the aircraft has (after vector addition) an unbalanced force means that it will accelerate, in this case towards the centre of the turn. Its weight is unchanged.

Weight is not equal to the F in F = MA. (unless the object is in frictionless free fall of course).

In straight and level flight the aircraft is NOT subject to an acceleration due to gravity. It IS subject to a force due to gravity, but since equal and opposite lift is being applied the resultant force is zero and the aircraft is not accelerating.

One common problem with F=MA is that people get the idea it implies a cause; i.e. that a force exists BECAUSE a mass is accelerated. Then they try and apply that concept to weight, mass, and acceleration due to gravity and get themselves in a muddle. Objects have weight whether they are being supported or not and whether they are being accelerated or not.

Its often handy to re-express F=MA as A=F/M which gives us a better pointer as to what's going on; accelerations are caused by unbalanced forces, not vice versa.

[ Additionally, regarding the OP, a purist might argue any of the following:

- Fuel is being burnt, therefore mass is decreasing so weight is decreasing. Techically correct, but perhaps a bit pedantic and missing the point of the question.

- Actually, even in straight and level flight, lift is fractionally less than weight, (due to Earth Curvature, the aircraft must be following a curved flight path and is not therefore in a 'uniform state of motion' as Newton would have said). This effect is pretty negligable for most aircraft, although it becomes detectable for fast moving things (e.g. SR71) or spacecraft. However this is a depth of analysis beyond most interviewers so I wouldn't go there, nevertheless, strictly speaking there is NO steady flight phase where lift>weight. ]

pb

[Wizofoz]

CPB,

Yes, my explanation may have been a bit "Clunky". My point was an aircraft in a turn IS being accelerated, and this produces a force which is indistinguishable from weight. The wings must therefore produce additional lift to balance that force, and can only do so by increasing either speed or AofA.

It was mearly intended to show how barking the concept of "Reduced apparent wingspan" as laid out by ATPTQ is.

[shanx]

Capt Pitt Bull,

Im still not very clear about that "weight remains constant during a banked turn" statement.

During a steep turn, when an aircraft pulls say 1.5 Gs, the pilot would definitely feel "heavier" than he usually does.

Is it not therefore correct to say that during a banked turn, the "apparent" weight of the aircraft Increases?? (By virtue of the centrifugal force that it counteracts ?? )


Further, is it correct to say ...

1. Weight of an aircraft remains constant during a banked turn, WITH RESPECT TO the Earth ? (for an outside observer)

2. Weight of an aircraft increases during a banked turn, with respect to the ROTATING FRAME OF REFERENCE ? (ie. if the observer is inside the turn)

[airbus757]

In order for a plane to climb lift must be increased. One does this by increasing thrust which increases the drag due to a higher angle of attack required to maintain the speed, which in turn increases the lift. Then as if by magic the airplane climbs...very simple.

7

[Wizofoz]

airbus,

simple-yes

correct-no!

In a steady state climb lift is actually reduced compared to S&L at the same speed. The vertical component of thrust puts the forces in equilibrium.

[Checkboard]

The mass remains constant (disregarding fuel use, bits falling off, bug and bird strikes etc :) ) The weight is the force exerted by a mass as a result of gravity - and changes with altitude and geographic location.

Most people have problems in defining their frame of reference when looking at forces, especially in cases of acceleration. There are two techniques - using an accelerated frame of reference (and thus producing a 'balanced' equation by including the "imaginary" centrifugal force) or a static frame - in which the forces are out of balance, and the resultant force is providing the observed acceleration. In either case the weight is unaffected.

[Wizofoz]

Check,

We might simply be getting into semantics here, but an observer on board a turning aircraft feels an increase in force on the normal axis which is exactley the same as an increase in weight. From his frame of reference, his and the aircrafts weight has increased.

Astronoughts in space are still subject to gravity, but as their orbital acceleration exactley cancels it out are said to be weightless. I would think it therefore not incorrect to say someone experiencing an increased force due to acceleration is more "weightful" i.e has experienced an increase in weight.

[Checkboard]

We might simply be getting into semantics here,
Of course! - This is a technical forum, not a layman's forum. :)
but an observer on board a turning aircraft feels an increase in force on the normal axis which is exactley the same as an increase in weight. From his frame of reference, his and the aircrafts weight has increased.
No it hasn't. The weight has remained the same, the observer is feeling the force which is accelerating them around the corner, in the same way that if you hammer the accelerator in a car you feel the back of the car seat pushing you forward.

[Wizofoz]

I agree with the reason for the force, I disagree that this cannot be characterised as weight.

As I said, the observer in the aircraft would have no way of distinguishing which part of the forces he was feeling were due to gravity, and which to acceleration, just as an astronaught in free-fall has no way of sensing whether he is in orbit or in deep space.

Do you agree that an Astronaght in orbit is "weightless" even though he is still subjected to a force very close to 9.8N due to gravity, but neutralised by his orbital acceleration?

[Capt Pit Bull]

Do you agree that an Astronaght in orbit is "weightless" even though he is still subjected to a force very close to 9.8N due to gravity, but neutralised by his orbital acceleration?

This is a misnomer...

"weightless" astronauts in orbit are no such thing. They are in orbit because of their weight, not inspite of it. Rather, this situation should be referred to as 'free fall'.

What you are talking about is a conditon of 'apparent weight'.

We sense our weight by virtue of being held up against it

Gravity, being a force caused by a field, is applied universally across all the mass in our body. If you're floating around inside a space shuttle - in free fall - you have no way of feeling if you are subject to gravity. You could be falling under 1 G somewhere close to the Earth, or 2 ish G close to Jupiter or 1,000 G about to dissapear down a black hole (*note 1).

Every part of your body is subject to the same acceleration, so you feel nothing.

Whereas when you are being held up against a gravitational force (e.g. me sitting in my chair) this a contact force. i.e. its applied at distinct places on my body... right now, my butt cheeks and left foot (I'm sitting cross legged). This force is distributed to the other parts of me by squashing parts of me (e.g. cartlidge in my back and my blubbery... I mean perfectly toned gluteus maximus!).

Because parts of me are being squashed, I can perceive the force. If I'm accelerated, e.g. upwards, then the extra (unbalanced) force that accelerates me has to be transmitted from the points of contact to the rest of me. I feel that extra force as my connective tissues get squashed. So I may feel like my weight has increased, but it hasn't.

So we have 2 concepts, 'Weight' and 'Apparent Weight'. And although they sound similar they really aren't, and using them interchangably is a barrier to understanding.

Hope thats of interest / use.

pb



note 1 - discounting gravity gradient when R is small

[Wizofoz]

Again agree with the mechanics of what you are saying, but not entirely with the conclution.

In order that an aircraft maintain altitude it must produce lift equal to its' apparent weight, not just the force due to gravity.

In terms of predicting the result of any physical interaction a bodies apparent weight is actually more important than it's actual weight. Objects in free-fall act exactley as if they were not in a gravity field. Objects subject to both gravity and a normal acceleration act exactley as if they weighed more. Weight is measured with a scale, and in both cases, that measurment would be different to 1g, and there would be no way for an observer to tell the difference between force due to gravity and force (or lack of it) due to acceleration.

[wing span]

:ok:I think u all guys need to look at oxford ATPL books which are availaible in every subject..........& take a look at oxford ATPL principles of flight.....& technical....then you may not need any other book for references..........tc:ok:

[Checkboard]

Yes, bodies react according to the resultant force which acts upon them.

Weight, however, is an individual vector with a technical definition.

If you use "common use" terms you end up with an inexact definition of exactly what you are attempting to describe, as the definition of "common use" terms is affected by the vagaries of a living language.

[Capt Pit Bull]

Wizo

I think you are in real danger of tying yourself in knots over this.

In order that an aircraft maintain altitude it must produce lift equal to its' apparent weight, not just the force due to gravity.

In order to maintain its existing vertical velocity, the vertical components of all forces acting on the aircraft must (by vector addition) sum zero.

Taking a few assumptions: slowish aircraft (i.e. not a spacecraft), lowish angle of attack (i.e. vertical thrust component negligable), in level flight to start off with.

In this situation we can say that the vertical component of lift must be equal and opposite to the weight.


In terms of predicting the result of any physical interaction a bodies apparent weight is actually more important than it's actual weight.

Well, for calculating interactions between objects in the system, contact forces are obviously important. But for external 'wheres-the-damn-thing-going' kind of calculations there is a lot of virtue in recognising that the aircraft weight is constant (in the short term) and has a fixed orientation.

Objects in free-fall act exactley as if they were not in a gravity field.

Agreed, its just newton 1,2 and 3

Objects subject to both gravity and a normal acceleration act exactley as if they weighed more.

Their apparent weight has increased, their actual weight is unchanges (assuming no change in gravitational field strength.)

Weight is measured with a scale,

Apparent weight is measured with a scale

and in both cases, that measurment would be different to 1g, and there would be no way for an observer to tell the difference

It is true (and crucially important) that an observer inside can not tell the difference. However, an external observer may well wish to quantify between them (weight and an acceleration caused by an unbalanced contact force) in order to relate the situation to his/her own frame of reference.

between force due to gravity and force (or lack of it) due to acceleration.

The danger in this statement is that people can get in a muddle about Newton 2. Forces cause accelerations, not vice versa. I get very concerned when someone starts talking about a force due to an acceleration.

F=MA tells us what force must have been present in order to cause the mass M to undergo acceleration A.

It does not imply that force F exists because object M accelerated.


In summary, I'm with Checkboard. Weight is a defined term. Its constant (in this context). Apparent weight is also a defined term, its variable (in this context). Use them interchangably at your peril.

pb

[Wizofoz]

Cool,

Certainly see what you mean.

Regards

Wiz

[airbus757]

airbus,

simple-yes

correct-no!

In a steady state climb lift is actually reduced compared to S&L at the same speed. The vertical component of thrust puts the forces in equilibrium.


I find this very difficult to swallow.

There are airplanes out there that can actually have a nose down pitch in level flight and for that matter can climb with a nose down pitch. How do you explain that.

Also if you are cruising a jetliner the attitude is somewhere around 2.5 degrees up. When you add additional thrust the attitude increases in order to prevent the airspeed from increasing which increases lift and the altitude increases.

As for the vertical component of thrust, it doesn`t matter where the lift comes from it is still lift. By your argument one could say that a hovering helicopter is not producing lift.

7

[Exaviator]

Vibes, if you really want to understand aerodynamics I would suggest reading Kermode's Mechanics of Flight. If you want it more simple try Kermode's Flight Without Formula. They have both been around for more years than I care to remember but are still valid.

Happy reading!:ok:

[seejonfly]

Ok dokey to put this in simple straight forward language let's ask what determines an aircraft's ability to climb (for a sustained time period not for a few seconds by just pulling up the nose/dumping flaps)? In a nutshell climb is dependent upon having more power available than is required to maintain level flight in a given configuration. This is why multi-engine aircraft can loose most or even all of their ability to climb after an engine failure.

Also, let's not forget the difference between weight and mass. Mass is an object's resistance to acceleration. Weight is a the product of mass and the acceleration of gravity. The pound is a unit of force that equals a mass of one slug acted on by the local constant of gravity expressed in feet/second/second...32ft/sec^2 is a common value.

Hope this helps!

Jon :)

[Wizofoz]

Airbus,

Good explanation here-

Pilot's Web The Aviators' Journal - Forces Acting on an Airplane (http://pilotsweb.com/principle/forces.htm)

But first, you must get some basic concepts right.

Attitude is not directley related to angle of attack, and thus not directly related to lift, as attitude is angle reference the horizon whilst AofA is reference the relative airflow.

Vertical component of thrust is NOT lift. Lift is force perpendicular to airflow. Thrust is (more or less, depending on the thrust line of the engine) parralel to airflow.Weight always acts directly donwards, so a compponent of lift plus thrust must equal weight for equilibrium. All of this is true of a hovering helicopter.

If lift (or indeed, the sum of all forces acting vertically upwards) was greater than weight, the aircraft wouldn't just climb, it would continuously Accelerate vertically upwards, that is it would be under a constant g force of greater than one, with a constantly increasing climb rate. When you climb your 'bus, are you constantly pushed down in your seat by a force greater than your normal weight? Does it maintain a constant rate of climb?If not, your aircraft is in equilibrium, which couldn't happen if the vertical component of lift was greater than weight.

I think (and it's not uncommon) that you are confusing Lift with Co-efficient of lift, which is only one part of the lift equation.

[airbus757]

Ahh, so what you are saying is the upward pull of the downward thrust has increased so in order for the aircraft to remain in equilibrium the lift must be decreased. But what about the downward pull of the increased drag?

7

[italia458]

Ahh, so what you are saying is the upward pull of the downward thrust has increased so in order for the aircraft to remain in equilibrium the lift must be decreased. But what about the downward pull of the increased drag?


If I understand correctly, the drag would have decreased. As you climb, like you said lift would decrease and so would the drag associated with the lift. If you climbed at the same airspeed or at a slower airspeed the form/skin friction drag would remain the same or decrease respectively.

I just searched on google and came to this thread. I just wanted to lite a spark to this! :)

It makes things more clear if we say weight is equal to the F in F=MG. It won't change one bit while in a turn or while being accelerated at 2G. F=MG is your mass, which is constant, and the force of gravity exerted by the earth, which again is constant! The only time it will change is when you change your distance from the center of the body which is exerting the acceleration on you (earth). As you get higher in altitude your "weight" will change, gravity decreases. It is very slight however for civilian aircraft. We can't think of weight and apparent weight while analyzing the same set of forces because you are switching back and forth between frames of reference to try and explain the actual force you are feeling. So for all this I'm going to talk in the frame of reference of weight, which is an inertial frame of reference.

I think the definition of weight should be clear. Now, looking at the forces in the turn we should agree that the weight doesn't change. The aircraft has an acceleration force applied to it. We have to think in 3 dimensions here. Let's take this to a 0G environment so that we can isolate the force in the turn, space. If we fix a vertical pole in space, and space meaning deep space away from ANYTHING that could affect the results of this. Then take a string and attach it to small box with a small rocket engine on the back. As we fire the rocket it will speed up the box on a circular track around the pole, assuming it doesn't wind around the pole. At a constant speed around the pole, the box will have "1" force on it. Since we are in the inertial frame of reference, the box has a centripetal force acting on it toward the center. Centrifugal force does not exist in the inertial frame of reference. (If I were to sit in the box I would be in a non-inertial reference frame and I would feel the centrifugal force (a pseudo-force), which would translate to me feeling an "apparent weight" that is greater than my actual weight"). So that 1 force, since it is obviously unbalanced, will be 'accelerating' the box inwards. The box's weight HAS NOT changed!!

In a turn, the aircraft's weight DOES NOT change!! Once you understand that, you'll be able to clearly understand the forces in a turn.

The concept that in a turn you have a reduced effective wingspan is still correct when talking about lift according to the y axis. If you enter a level gentle turn and do not increase power, you will have to trade off airspeed for some extra lift to counteract the reduced effective wingspan by adding up elevator.

Now adding up elevator can't be looked at as adding "apparent weight" to the aircraft. We still need to remain in an inertial reference frame to analyze this. The horizontal stabilizer and elevator are a separate airfoil, just like the wing, that happens to be attached to an extremity of an aircraft. It will create it's own lift and it's own drag. The most correct way of looking at it is that drag will be the "apparent weight" in the non-inertial reference frame. It won't be the lift of the horizontal stabilizer!! The lift, or lack of lift, will only "point" the aircraft in an attitude. There is a range of lift it will produce from positive to zero to negative lift. It will have induced drag if it's producing lift at all and that's what will be the "apparent weight". If it's at zero lift, it will only be experiencing form/skin friction drag.

[PBL]

So let's stir this up a little.

There you are, standing on the surface of the earth. You have your mass, and you have your weight. You can tell your weight because you are standing on a scale.

Suppose the earth were to be spinning twice as fast. Your mass is unchanged. Are you heavier? (That is, does the scale show more?)

PBL

[Wizofoz]

No, your apparent weight would actually be less, as the angular velocity of the Earth would impart a force in the opposite direction to gravity. It does that now, but if you increased it's rate of rotation, that force would also increase.

[bookworm]

Suppose the earth were to be spinning twice as fast. Your mass is unchanged. Are you heavier? (That is, does the scale show more?)

After the groundspeed thread, I thought for a moment you were going to come up with another definitional question: Is the "acceleration of free fall" changed? ;)

[Wizofoz]

I think the definition of weight should be clear.

Not as clear as you'd think.

From Wiki:-

The ISO standard ISO 31-3 (1992) defines weight as follows:

The weight of a body in a specified reference system is that force which, when applied to the body, would give it an acceleration equal to the local acceleration of free fall in that reference system.[9]

Note that it does not specify an inertial frame of reference.

If you are using a NON-inertial frame of reference- such as an aircraft undergoing an acceleration- your weight will be greater than your mass.

I actually want to renew my dis-agreement with Checkboard and Captain Pit Bull on this point- by the ISO definition weight is not defined soley as force due to gravitational pull from a mass- or rather, the force felt due to acceleration IS force due to gravity, and force due to gravity IS force due to acceleration- they are actually the same thing. As I said previouly, there is no experiment that would allow you to differentiate between the two.

[PBL]

I asked Suppose the earth were to be spinning twice as fast. Your mass is unchanged. Are you heavier? (That is, does the scale show more?)

Wizofoz replied No, your apparent weight would actually be less

Thanks for biting! I'm not -yet- saying what I think the answer is, because I don't want to spoil the UTC-Monday-morning fun :) However, I don't think your reasoning works: as the angular velocity of the Earth would impart a force in the opposite direction to gravity

Capt Pit Bull has already pointed out that the appropriate way to think of things is as forces causing accelerations, and not motions causing (or, as you say, imparting) forces.

After the groundspeed thread, I thought for a moment you were going to come up with another definitional question: Is the "acceleration of free fall" changed?

I like that! So, what's the answer?

PBL

[Wizofoz]

Capt Pit Bull has already pointed out that the appropriate way to think of things is as forces causing accelerations, and not motions causing (or, as you say, imparting) forces.


And I think he's wrong.

An EQUATION means the two sides are EQUAL, meaning they are the same thing. One side doesn't CAUSE the other, both sides ARE each other.

If it is correct to say F=M*A it is equally correct to say A=F/M.

[PBL]

[PBL: Capt Pit Bull has already pointed out that the appropriate way to think of things is as forces causing accelerations, and not motions causing ... forces.]
And I think he's wrong.

An EQUATION means the two sides are EQUAL, meaning they are the same thing. One side doesn't CAUSE the other, both sides ARE each other.

You are correct that equations do not express causality. Indeed, they cannot express causality, because equations, as you point out, are symmetric, whereas causality is not. But just because equations are used, it doesn't mean that you can infer that there is no causality! Indeed, that would go against 350 years of experience with this particular piece of science (which is still valid at low relative velocities).

For example, suppose I drop a ball and it hits the floor. This entire interaction is well described by Newtonian mechanical principles, all expressed in equations as you note. But the action of gravity along with my releasing the ball are causes of its impact with the ground. Similarly, its impact with the ground is not a cause either of gravity or of my dropping it.

Back to the original problem. Say I weigh 80 kgs on the scale. You think I will weigh less (that is, the scale will read less) if the earth spins faster. So you probably think I will weigh more if the earth spins slower. How much would you think I would weigh if the earth just stops rotating? What would be the cause of my weighing that much? Indeed, what would be the cause of my weighing anything at all in any of these scenarios?

PBL

[Wizofoz]

Don't know what you mean about causation. I'm saying it is wrong to say "Force causes a mass to accelerate". Force IS the acceleration of a mass. Thus, the acceleration of a mass IS force. If you have one, you can calculate the other. If my aircraft is accelerating, there is force. If I know my Mass, I can derive my Force.

As to the problem:-



Trick question perhaps?

Are you at the pole or the equator?

Assuming you are at the Equator:-

F=MV^2/R

= 80*465^2/6 378 000
= 2.7N

Normally, you weigh 80 kg ( at the equator) = 784N

Twice the rotational speed, you weigh 781.3N
If the Earth stopped, 786.7

See Eötvös effect.

Indeed, what would be the cause of my weighing anything at all in any of these scenarios?


Are we sticking to Newton, or bringing Einstein in off the bench?

Newton- because Masses attract each other.

Einstein- Because Mass causes Space/Tme to curve.

Interesting discusstion, but please don't keep me in suspence if I'm missing something.

[PBL]

I'm saying it is wrong to say "Force causes a mass to accelerate". Force IS the acceleration of a mass. Thus, the acceleration of a mass IS force.

I don't think you would get much agreement with this amongst physicists. Consider: force is measured in Newtons, acceleration in meters per second per second. Physicists consider mass and energy to be the same, so they use the same units (electron-Volts). If force and acceleration are the same, why are the units different? If they are the same, there must be a conversion factor: X Newtons = Y meters per second per second. What would be this factor?

You asked not to be kept in suspense. OK, how does the following strike you?

[PBL: ....what would be the cause of my weighing anything at all in any of these scenarios?] ....
Newton- because Masses attract each other. Einstein- .....

Let me run with this suggestion. At lower relative velocities, Newtonian and Einsteinian mechanics say the same thing to most levels of approximation, so let's go with Newton.

Earth is big and close, so as you say it attracts me and gives me my weight. Sun is big but far away; we can ignore its contribution to my weight. Other planets are smaller and far away; we can ignore their contribution also. To a good approximation, we can throw them all away! And the rest of the universe beyond our planetary system as well. To a good approximation, there is just me and the earth giving me my weight, according to you.

But wait a minute: you said rotation of the earth had something to do with my weight. Rotation? What rotation? Rotation relative to what? There isn't anything any more except the earth and me........

Conclusion?

PBL

[Wizofoz]

Physicists consider mass and energy to be the same,

No, they don't.

If force and acceleration are the same, why are the units different?

A Newton= 1kgm/s^2 Note that the unit for Mass AND time is there. I never suggested anything different.


I didn't say they were the same. I said force was the same as mass TIME acceleration. What you are saying is that 2=2*2.

But wait a minute: you said rotation of the earth had something to do with my weight. Rotation? What rotation? Rotation relative to what? There isn't anything any more except the earth and me........

Angular velocity is an acceleration. Acceleration is absolute, not relative. The Earth is rotating relative to ANY inertial frame of reference.

[rudderrudderrat]

Hi PBL

Sun is big but far away; we can ignore its contribution to my weight. Other planets are smaller and far away; we can ignore their contribution also.

Then please explain the tides.

[Wizofoz]

rudder,

The sun exerts about 0.0006N at the radius of the Earths orbit. The Moon somewhat more. It certainly is enough to cause the tides, but isn't that significant when it comes to weight.

Interesting question came up elsewhere, though- why is there also a high tide on the OPPOSITE side of the Earth to the moon? (Hint, you can ignore the suns influence for this example).

[PBL]

[PBL: Sun is big but far away; we can ignore its contribution to my weight. Other planets are smaller and far away; we can ignore their contribution also.] Then please explain the tides.

Sure, the moon is there. I didn't think it relevant to what I was suggesting.

PBL

[Wizofoz]

PBL,

Sorry, offending comment removed. I thought YOU were being condescending, whilst also being mistaken.

Anyway, as I pointed out, the rotation of the Earth DOES change your apparent weight as, as I pointed out, acceleration is absolute, not relative.

Do you dis-agree with that?

[rudderrudderrat]

Hi Wizofoz,

why is there also a high tide on the OPPOSITE side of the Earth to the moon?

Would you agree it's because the earth and moon orbit one another about their common c of g? Since that point is not at the centre of the earth (must be somewhere say about 1/10? distance to the moon), then there is a high tide on earth's moon side (due closer to moon) and another on the far side because it's "centrifuged" out.

[Wizofoz]

Hi Rudder,

No, that's not it. Consider that the orbital period is 28 days, the centre of the rotation is very close to the earth, and you'll see any centripetal force due to the Earths orbit of the moon (YES, it orbits the Moon just as surley as the Moon orbits the earth- they are a non-symetriacl binary system) is very small.

No, it's because the far side of the Earth (from the Moons POV) is sufficiently further away from the Moon than the near side for the Moons gravity to be have appreciably less effect there. This Gravity Gradient means on the near side it pulls the WATER away from the EARTH, on the far side, it pulls the EARTH away from the WATER!

PBL,

Hope bridges are mended.

Now, do you see what I mean about the Earths rotation being an absolute, as it is an acceleration?

[italia458]

what i find interesting is why PBL implied that you might feel heavier if the earth spun faster, opposite to what i would think a normal mind would think. even more surprising is that many high school students think that if the world stopped spinning, gravity would be 0G.

I actually want to renew my dis-agreement with Checkboard and Captain Pit Bull on this point

You are quite confused!.. regarding that quote and what you said after. Does this clear it up: gravity is not a force, it is an acceleration, 9.8m/s2.

Thanks for biting! I'm not -yet- saying what I think the answer is, because I don't want to spoil...blah blah blah

Just come out clean to what you think it is! :)

I'm saying it is wrong to say "Force causes a mass to accelerate".

wiz... suppose you are standing still and i run up and crash right into you. im applying a force to you, to make you accelerate backwards. to accelerate backwards, a force has to be applied, correct? so without that force, you won't accelerate. you can't have acceleration without a force, but you can have a force without acceleration!!!

yes F=MA balances out, but an equation doesn't explain everything.

This discussion is filled with ridiculous banter... going back to PBLs dilemma, no one has mentioned one thing about the gravity equation while trying to explain gravity.

I'm waiting for the Lord PBL to explain his views because I think you and I will have some interesting points! :)

[Wizofoz]

You are quite confused!.. regarding that quote and what you said after. Does this clear it up: gravity is not a force, it is an acceleration, 9.8m/s2.


Agreed, Italia. My point is, it is an acceleration just like any other acceleration. If an aircraft is under a load of 2g, it is undegoing an acceleration of 19.6m/s2. F=M*A so so the airccraft is under a force of M*19.6. This force can be described as it's weght, as there is no way of descerning which part of the force is "Real" wieght and which is "Apparent".

This discussion is filled with ridiculous banter... going back to PBLs dilemma, no one has mentioned one thing about the gravity equation while trying to explain gravity.


The Gravity Equation quantifies Gravity. It doesn't explain it.

As to this-

but you can have a force without acceleration!!!


No, you can't. You are confused on that one.Force is measured in kg*m/s2.If you dont HAVE any m/s2, you don't have a Force! Please describe a net force that does not produce an acceleration.

[rudderrudderrat]

Please describe a net force that does not produce an acceleration.

Say the differential pressure inside the hull of my aircraft is 8 p.s.i. The net force on a passenger door with an area of 1,000 square inches is 8,000 lbs.

Are you sure you can't have a force without acceleration?

[italia458]

regarding your first point, yes that would be apparent weight which would be in the non-inertial reference frame!

No, you can't. You are confused on that one.Force is measured in kg*m/s2.If you dont HAVE any m/s2, you don't have a Force! Please describe a net force that does not produce an acceleration.

no, im not confused about anything! it was a general concept about the equation F=MA and how i stated that the equation doesn't explain everything! it was more for conceptualization purposes. if you had a box on a carpet floor, you have friction resisting it's movement so if you apply a small force, there is no acceleration. yes, if you have a NET force there will be acceleration.

[italia458]

The Gravity Equation quantifies Gravity. It doesn't explain it.

wiz... so because one equation doesn't explain the whole concept of gravity, it shouldn't be used IN the explanation?! wow you really come up with some good things. for starters, it's very valuable as it shows WHAT FACTORS AFFECT GRAVITY!!!

Say the differential pressure inside the hull of my aircraft is 8 p.s.i. The net force on a passenger door with an area of 1,000 square inches is 8,000 lbs.

Are you sure you can't have a force without acceleration?

rudder... yes conceptually i was going along the same lines and i agree! however, wiz is correct that if there is a net force there will be acceleration. the 8,000lbs applied on the passenger door is counteracted by the passenger door applying 8,000lbs to the air inside.

[Wizofoz]

Say the differential pressure inside the hull of my aircraft is 8 p.s.i. The net force on a passenger door with an area of 1,000 square inches is 8,000 lbs.


Assuming you mean a force equivelent to the weight of 8 000lbs, if that was a net force on the door, the door would be accelereting away from the aircraft.

It is NOT a net force, however, as the structure of the aircraft exerts an EQUAL and OPpOSITE force, leaving a NET force of zero.

[Wizofoz]

if you had a box on a carpet floor, you have friction resisting it's movement so if you apply a small force, there is no acceleration.

Because the friction supplies an opossing force. The net force in the system is therefore zero.

wiz... so because one equation doesn't explain the whole concept of gravity, it shouldn't be used IN the explanation?! wow you really come up with some good things. for starters, it's very valuable as it shows WHAT FACTORS AFFECT GRAVITY!!!


OK, it varies with the mass of the two objects and as the square of their distance, but I don't think that is really relevent here. For our purposes gravity on Earth imparts an acceleration of 9.8 m/s2.

[Wizofoz]

To try and get this back on track and summarise my points-

By the ISO definition of weight, force due to acceleration of a mass is weight from the frame of reference of the object being accelerated, so an aircraft in a turn can be said to increase in weight.

..And PBL is wrong about what his scales will read as he does not appear to understand the concept that angular velocity has an absolute value, and therefore is present in any inertial frame of reference.

Oh, and I though the reason for the high tide on the far side of the Earth was pretty cool!!

[ChristiaanJ]

...so an aircraft in a turn can be said to increase in weight.I would think that in most discussions where one first defines carefully what each word, like "force" and "weight" really mean.... you will find, that what you "feel" in a turn is referred to as "apparent weight".

So yes, it can be "said to" increase in weight... but only after you've defined carefully what the word really says...

"In the beginning was the Word". Remember? But then we went and used the same word for so many related and similar but not identical concepts.... and then thought the word meant the same thing every time.

You've just noticed it yourself... by finding you have to substitute "net force" for "force"... :ok:

CJ

[Wizofoz]

Christian,

All fair comment. I would point at that, in most technical discussions, ISO values are usually the ones used. By THAT definition, what you refer to as "Apparent weight" would simply be "Weight".

As I'v said, there is no experiment or observation which could distiguish what was "Real" and what was "Apparent" weight, without invoking the Earth as an absolute frame of reference.


Yes I used the term "Net force". But a net force of zero has the same physical results as a force of zero, net or otherwise.

[italia458]

All fair comment. I would point at that, in most technical discussions, ISO values are usually the ones used. By THAT definition, what you refer to as "Apparent weight" would simply be "Weight".

No it would not be. Read the ISO definition below.

The weight of a body in a specified reference system is that force which, when applied to the body, would give it an acceleration equal to the local acceleration of free fall in that reference system.

The reference system for "apparent weight" and "weight" are different. Apparent weight is measured from a non-inertial reference system. Weight is measured from an inertial reference system. Note the definition says "specified" reference system.

[Lonewolf_50]

Capt Pit Bull:

Weight constant in a banked turn?

Deleted. Wizofoz had already addressed that. Sorry. :O

[Dimitris]

Not pilot but:

1. The Moon-Earth's two-body system center is below the surface of the Earth (I don't argue the tides explanation, but the barycenter is below the surface of the earth)
2. A force can do various things to objects, not only accelerate them. It can deform them. Your passenger door in differential pressure is deformed. You are probably thinking of kinematics and dynamics of rigid bodies which is excellent for the initial question.
Your wing flexes up during level flight even though there is no force unbalance, lift deforms your wing.
When I was in high school, 'force' was defined as 'the reason to alter the motion or shape of a body' (dodgy translation, but you get what I mean!).

When a train passes over a coin, the coins turns to foil. No (rigid body) acceleration involved.

Cheers!!:)

[Wizofoz]

The reference system for "apparent weight" and "weight" are different. Apparent weight is measured from a non-inertial reference system. Weight is measured from an inertial reference system. Note the definition says "specified" reference system.

italia,

Where in that description does it use the term "Apparent weight" or specify that the frame of reference must be inertial? Once again let me ask- what would you use to measure weight that would distinguish between "Real" and "Apparent" weight?

[italia458]

Wiz.. you're hard to reason with!

Where in that description does it use the term "Apparent weight" or specify that the frame of reference must be inertial? Once again let me ask- what would you use to measure weight that would distinguish between "Real" and "Apparent" weight?

It doesn't say apparent weight in the definition and it never says you have to be in an inertial reference frame... and I didn't say that either!

Apparent weight is in the non-inertial reference system and weight is in the inertial reference system. I'm just saying what it is! Go look it up.

[PBL]

I cut out of this discussion because it was no longer fun.

It is fun to use some physics, indeed to hint to people that they weigh a quarter of a kilo less at the poles than they do at the equator, for example, but it is not fun to deal with silliness such as
...PBL is wrong about what his scales will read as he does not appear to understand the concept that angular velocity has an absolute value, and therefore is present in any inertial frame of reference.
by someone who doesn't appear to understand the difference between a question and an assertion.

Wizofoz is a phenomenalist about physical science. That is, he correctly observes that equations are symmetric and that no one side is preferred to the other, but he concludes that this is all that can be said. Not so. I mention a couple of features of causality, and he claims not to know what I mean.

Causality is an important concept, determination of which is probably the main activity in explaining accidents, which people in commercial aviation generally consider a very important activity. And indeed which demonstrably confuses lots of people (including, unfortunately, some accident investigators). So one cannot dismiss it. Wizofoz is right to point out that it plays no role in, say, calculations in Newtonian dynamics, but that might prompt him, and others, as it prompted Kant, to wonder why not.

PBL

[cwatters]

Incorrect reply deleted.

[Capt Pit Bull]

Oh my.

Look away for a couple of days and a year old thread gets necroed and gains a couple of pages.

I really don't want to get into it again, so many things being talked about and so many red herrings. There's about 8 different discussions underway. Stop reading if you're of a nervous disposition.

Here are a few random thoughts:

1. Forget about the ISO and start reading Physics textbooks. Since when did international non government organisations know bugger all about anything?

2. An equation is equally correct however you express it, and causality should not be inferred. However, as an aide-memoire, there are a great many basic physical relationships that lend themselves to being expressed as Effect = Cause / Opposition, and therefore purely as a convenience it can be a useful route to memorisation to express a relationship in a particular way.

3. cwatters. Sorry mate, that's bollox. How you think a climb can be analysed by ignoring thrust is so far out there I'm gobsmacked. Do yourself a favor and analyse the forces parallel to and perpendicular to the flight path instead of vertically and hrizontally before my eyes start bleeding and my head implodes.

<incoherently grumpy>

[cwatters]

> Do yourself a favor and analyse the forces parallel to and perpendicular to
> the flight path instead of vertically and hrizontally

You're right of course...

Lift + weight*cos(climb angle) = 0

so

Lift = - weight*cos(climb angle)

Since cos(any angle) < 1

Lift < weight.

I'll delete my earlier post.

[PBL]

cos(any angle) < 1

What is cos(0)?

Doesn't apply in the case you're considering of course; a simple mod should suffice.

PBL

[Wizofoz]

Hi CBP,

My point on weight remains this- What instrument would you use to determine whether weight is "Real" or "Apparent"? If you can't differentiate between the two, are they not the same thing?

You earlier brought up the idea that an observer on the Earths surface could determine which part of the aircrafts weight was due to Gravity, and which to acceleration. True, but this gives the Earths surface a privileged frame of reference.

Take an observer in freefall- from HIS POV, ALL the force on the aircraft would be due to it's acceleration, NONE due to Gravity (as from his frame of reference, there isn't any). The Aircrafts acceleration is absolute. Which part is "Real" and which is "Apparent" is relative.

I understand what you are saying about Causality, but still think it doesn't apply- Take a meteor striking the Earth- It undergoes an acceleration. Has a force caused the mass to accelerate, or has it's acceleration caused a force? My answer is "It's the same thing, as they are EQUIVALENT

PBL,

Several posts by you since I explained you were in error regarding rotation. It exists (as does it's effect) in any frame of reference. Do you acknowledge this?

ETA,

Actually PBL, reading your last reply to me- Are you saying you DO agree about rotation, and that your statement that implied rotation was relative was a red herring?

I'm sorry, but you write in riddles. I'd appreciate a clear explanation of your understanding of this.

[PBL]

My point on weight remains this- What instrument would you use to determine whether weight is "Real" or "Apparent"? If you can't differentiate between the two, are they not the same thing?


The view of science that quantities which you cannot discriminate with instruments are the same thing is about the only philosophy of physics which has been definitively discredited in the last hundred years. So it is rather inappropriate to try it on here, don't you think?


I understand what you are saying about Causality, but still think it doesn't apply- Take a meteor striking the Earth- It undergoes an acceleration. Has a force caused the mass to accelerate, or has it's acceleration caused a force? My answer is "It's the same thing, as they are EQUIVALENT

Repeating an assertion doesn't make it any more persuasive to me. It rather suggests that you might benefit from deeper understanding of causality. Let me recommend the counterfactual notion, proposed first by David Hume but not really pursued very deeply until David Lewis did so in the early 1970's. You might start with his papers Causality and Causal Explanation, both in his Philosophical Papers vol II. Peter Menzies has a pretty good account in the on-line Stanford Encyclopedia, Counterfactual Theories of Causation (http://plato.stanford.edu/entries/causation-counterfactual/)


Several posts by you since I explained you were in error regarding rotation. It exists (as does it's effect) in any frame of reference. Do you acknowledge this? ..........
I'm sorry, but you write in riddles. I'd appreciate a clear explanation of your understanding of this.

I note, again, that you are confused about my understanding of mechanics. I am here to have fun talking about topics of mutual interest. If you can't interpret what I say, fine. There is no law saying it has to be sorted out.

PBL

[Wizofoz]

PBL,

Interesting that you would say this:-

The view of science that quantities which you cannot discriminate with instruments are the same thing is about the only philosophy of physics which has been definitively discredited in the last hundred years.

Followed by this:-

Repeating an assertion doesn't make it any more persuasive

Kindly site your source for this "Philosophy"

Meanwhile, I am discussing Physics with people willing to put clear, easily understood principles in writing for discussion. You seem to think philosophy somehow contravenes the universes physical laws.

Good luck with that.

In any case, angular velocity is absolute. If you understand that, great. But if that is the case, why did you ridicule me for correcting a statement you made which apparently YOU knew to be incorrect?

[italia458]

Wiz...

My point on weight remains this- What instrument would you use to determine whether weight is "Real" or "Apparent"? If you can't differentiate between the two, are they not the same thing?

Instrument:
I believe the result you get from "measuring" or "observing" something is actually based on the instrument you use. This is particularly important when analyzing quantum mechanics and particles at the atomic level. I still believe it is relavent to the "normal" world tho. Take light for example, when observing it to see how it travels in waves, the instrument will indicate that it does indeed travel in waves, but when observing how it travels in a straight line, the instrument will indicated without a doubt that it does travel in a straight line. So if our instrument is not giving us an accurate "view" of what we are observing, how can having no difference in observation from a different instrument, prove that two different objects are actually the same.

Another example, it's like looking at a real apple and a fake apple sitting on the moon through a telescope and since you are detecting no change, saying that both apples are real based on only visual observation, through one instrument, and then through our eyes, which inherently aren't perfect. However, if we were actually on the moon right beside the apples, we could probably tell which one was fake just by observation. So it depends on how "accurate" the instrument is and also if it's giving us all the information that we need, feel of the apple, weight of the apple would definitely help determine if one was fake.

Frame of reference:
"Real" and "apparent" weight are based on your frame of reference and can be measured using the same instrument. "Real" shouldn't even be used cause it infers that "apparent" isn't a real weight, yet it is, in a different frame of reference.

Put a scale under your butt when you're sitting in your airplane on the ground. Say it says 200lbs, that's your weight while observing from a non accelerating frame of reference (inertial FoR). If you were flying level at 3000' with no acceleration, the frame of reference would still be the same (inertial FoR) and your weight would be the same. Also, in any other inertial FoR (for example standing on earth), observing you would reveal that you appear to weight 200lbs also. Now in a turn (observing from earth), you would still appear to weigh 200lbs, the observer hasn't seen you eat any Big Mac or grab onto a 20lb weight or something. To him, you're still 200lbs. But to you, in the aircraft in the turn, you are accelerating and you FEEL that extra force and sure enough when you look down at the scale in a 60 degree bank at 2G you indicate your weight at 400lbs. You're in a non-inertial FoR because you are accelerating. This does a pretty good explanation of non-inertial FoR (Non-inertial reference frame - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Non-inertial_reference_frame))

This is inertial FoR (Inertial frame of reference - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Inertial_frame_of_reference))

You earlier brought up the idea that an observer on the Earths surface could determine which part of the aircrafts weight was due to Gravity, and which to acceleration. True, but this gives the Earths surface a privileged frame of reference.

I'm not sure what he meant but maybe that observer on the earth has a weight scale inside the aircraft and is getting data sent down to him? The privileged frame of reference the earth is in is called a rotating reference frame.

Take an observer in freefall- from HIS POV, ALL the force on the aircraft would be due to it's acceleration, NONE due to Gravity (as from his frame of reference, there isn't any). The Aircrafts acceleration is absolute. Which part is "Real" and which is "Apparent" is relative.

Exactly! I think you might be meaning the same thing I am but just saying it differently. "Real" and "apparent" are relative to your FoR. Does any of this make sense?

[Pugilistic Animus]

well at the least, we can say definitively that:

the product of uncertainty in position and momentum, must be greater than or equal to Plank's constant divided by 2pi:8

for airplanes Stick and Rudder and the FAA Airplane Flying Handbook answers best the original question....
So, far everytime I wrote anything about climb performanceThe British said "wot are you on about":}:}:}
...so:\

also, for those who really can't get enough You have Hurt's text

and beyond that, you'd really better know engineering math and physics:eek::\

:)

[Wizofoz]

Hi Italis,

We are on the same page if not the same verse.

Another example, it's like looking at a real apple and a fake apple sitting on the moon through a telescope and since you are detecting no change, saying that both apples are real based on only visual observation, through one instrument, and then through our eyes, which inherently aren't perfect. However, if we were actually on the moon right beside the apples, we could probably tell which one was fake just by observation. So it depends on how "accurate" the instrument is and also if it's giving us all the information that we need, feel of the apple, weight of the apple would definitely help determine if one was fake.


Yes, but there is NO experiment, even theorecically, which can differentiate between "Real" and "Apparent" weight. They will both make a mass accelerate in the normal direction in the relevent FofR. They will both make a mechanism distort and give the same reading on a scale. Without perfect instrumentation, things appear to be the same. Without any WAY of differentiating between two things, they ARE the same.

I think you might be meaning the same thing I am but just saying it differently. "Real" and "apparent" are relative to your FoR. Does any of this make sense?

Exactly.

An observer on the ground would observe the aircraft in a 2g turn, whilst ALSO feeling the force of his own mass times gravtiy, evidenced by the scales HE is standing on. He would assume the Pilot he is observing has 200lbs "Real" weight, and another 200lbs "Apparent" weight.

Now take an observer in a uniform acceleration due to gravtiy (yes, yes, gravity gradient, but close enough). The scales HE is standing on read zero. He observes the aircraft not only turning, but also in a uniform vertical acceleration.

He ALSO concludes the pilots scales reads 400lbs, but all of it "Apparent".

From the Plots POV, it's ALL "Real".

As such, it seems the whole concept of "Apparent' weight is relative to the observer, and the absolute term is, well, just "Weight"!

ETA do YOU have any idea what PBL is trying to say?

[Pugilistic Animus]

A scale measures weight and a balance measures mass:)

a scale for a 200 lb man would indicate 400 at a 60 bank angle in steady coordinated flight...a balance would remain at 200 and not move;)

[Wizofoz]

Correct, Pug. No-one is suggesting Mass changes.

[SomeGuyOnTheDeck]

No-one is suggesting Mass changes.
Except possibly Albert Einstein.;)

Invariant mass - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Rest_mass)

[PBL]

Kindly site your source for this "Philosophy"
For example, Operationalism in the Stanford Encyclopedia (http://plato.stanford.edu/entries/operationalism/). Notice, in the first paragraph, that: operationalism is nowadays commonly regarded as an extreme and outmoded position

BTW, I am here to discuss things which I find fun to discuss with people who like to discuss them. So if you want to continue schimpfing at me, Wizofoz, please do so privately and not burden others.

PBL

[Wizofoz]

PBL,

To complete the statement you quote-mined out of context.


Both in philosophy and in psychology operationalism is nowadays commonly regarded as an extreme and outmoded position, but that is not to say that the potential of Bridgman's original ideas has been exhausted.


We are not talking about either Philosophy ao Psychology, but Physics.

Physical sciences do indeed depend on what can be demonstrated and measured. How else do you propose we can differentiate between things if we cannot measure the difference?

You appear to have the hump with me because you made a plainly wrong statement about a physical reality and I corrected it. You will now not even show the common courtesy to state whether you actually agree with what you wrote, or intentionally made an incorrect statement.

Do so and the "schimpfing" (???) will end.

[italia458]

Wiz...

regarding this...

Yes, but there is NO experiment, even theorecically, which can differentiate between "Real" and "Apparent" weight. They will both make a mass accelerate in the normal direction in the relevent FofR. They will both make a mechanism distort and give the same reading on a scale. Without perfect instrumentation, things appear to be the same. Without any WAY of differentiating between two things, they ARE the same.

I think there is a way to measure real vs apparent weight. If the aircraft is in a level turn you'll find that it has to produce enough "energy" to counteract the force of gravity. You could then calculate the energy that the aircraft system (majority from the wings) would produce downwards, which should equal the calculated energy required to keep the airplane in level flight.

Now enter into a turn and you require a lot more lift to stay level. In the 60 degree turn you're at 2G and therefore your apparent weight would be doubled from your real weight. So the whole aircraft system, (body, fuel, wings, people inside, etc) would now weight twice as much (apparent weight) which would require twice as much energy to counteract. This is a way to calculate real vs apparent weight.

[Wizofoz]

Italia,

But how do you know whether your "Real" weight was caused by gravity, or by acceleration?

[Capt Pit Bull]

Hi CBP,

My point on weight remains this- What instrument would you use to determine whether weight is "Real" or "Apparent"?

In general terms, you never perceive your own weight directly, only when something prevents you from accelerating in a gravitational field.

You can't measure your weight inside the aircraft UNLESS you specify that you are unaccelerated in relation to whatever is causing the weight, (i.e. the earths mass). In that condition only apparent weight happens to be equal and opposite to weight. Your other flight instruments will tell you whether the aircraft is unaccelerated.


If you can't differentiate between the two, are they not the same thing?

ummm.... sounds like more of a philosophy discussion to me, but as far as this goes....

They definitely aren't the same thing. Draw a freebody diagram of a pitbull sitting on a chair in an aircraft flying a constant speed turn at 60 degrees of bank. There are 2 forces applied to the pitbull; first is its weight, lets call that W Newtons, acting straight towards the Earths centre of mass. Second is a force of 2W acting from the chair to the backside of the pitbull, acting in the direction of the aircraft's Normal axis.

So we can see that (with the sole exception of the special case), in general weight and apparent weight have differnt magnituds and different directions.

How much more differnt would you like them to be ?


You earlier brought up the idea that an observer on the Earths surface could determine which part of the aircrafts weight was due to Gravity, and which to acceleration. True, but this gives the Earths surface a privileged frame of reference.

You say that like its a bad thing. The fact of that matter are that the OP was asking about forces in a climb. And in a climb, or any other analysis to do with the balance of forces on an aircraft to determine what its future flight path will be, the Earth is a pretty darn sensible frame of reference.


Take an observer in freefall- from HIS POV, ALL the force on the aircraft would be due to it's acceleration, NONE due to Gravity (as from his frame of reference, there isn't any). The Aircrafts acceleration is absolute. Which part is "Real" and which is "Apparent" is relative.

(Assumption: you are talking about actual free fall rather than the innacurate colloquial parachuting term describing terminal velocity)

I think I can see what you are trying to say. You'd have to have some dude up in space above the atmosphere looking down and saying "Hey! That aircraft is accelerating towards me....". Yes, on that basis, he could ascertain the apparent weight of the aircraft (assuming the mass of it was known)

But he would also look down and see a large planet accelerating towards him as well.

In fact this guy, by virtue of being external to the aircaft, would be in a position to see if the aircraft was accelerating towards him at the same rate as the planet (in which case he could deduce the weight of the aircraft in the planets frame of reference). But if he looked at the aircraft and saw it was accelerating at a different rate then he would known that the aircraft was accelerating relative to the planet. Thus the occupants would be experiencing an apparent weight not equal to their weight in the planets frame of reference.

In other words, nothing is different, the point of view of a free falling observer is basically a red herring. Its a useful exercise but doesn't really change anything. At least if I understood you correctly.

I understand what you are saying about Causality, but still think it doesn't apply- Take a meteor striking the Earth- It undergoes an acceleration. Has a force caused the mass to accelerate, or has it's acceleration caused a force? My answer is "It's the same thing, as they are EQUIVALENT

My answer...... Newton 1

It says words to the effect of "A body will continue in a state of uniform motion unless acted upon by an unbalanced force"

It does not say something like "from time to time a body will spontaneously accelerate and when it does so forces will be created"

You've expressed this viewpoint several times over the last year, and you are just plain wrong I'm afraid. As others have said in the thread, you can have forces without acceleration (if they are cancelled out by another force) but the presence of acceleration has to mean the existance of an unbalanced force causing that acceleration.

In the case of your meteor example, attempting to place a rock and air molecules in the same space causes electrostatic replusion (very strong compared to gravity which is a pathetically weak force), which slows the merteor down and speeds the air up. Large amounts of work are done leading to the release of a lot of heat... boom etc....

anyway, got to go.

(RL has intruded, the above is not proof read and may include bollox)

pb

[Wizofoz]

CPB,

Yes, I'll give you the force/acceleration argument. thanks for patiently getting it through my skull.

I STILL don't agree with the "Apparent weight" bit.

For instance, when you say this:-

But he would also look down and see a large planet accelerating towards him as well.


Why would the relative acceleration of a third body effect the observation of our soon-to-be-strawberry-jam Free-Faller? From HIS POV, the aircraft is accelerating. he might DEDUCE the reason for the aircrafts acceleration from the rapidly oncoming spheroid, but he would have no other way of MEASURIBG it other than it's chang of relative position to him.

As to the aircraft, lets assume it is maintaining the same AofA at the same TAS, only THIS time it's in a terraformed Martian atmoshere which has the same ambient pressure as the Earths atmoshere. Will what the pilot feels and his G meter measures be any different to what it would have been on Earth?

[Capt Pit Bull]

Nice thought exercise... I like it! Kind of pushed for time but here goes.

(Mars surface gravity about 0.4 of Earth)
(Lets say 100kg man, approx 1000 Newton weight on Earth, 400 N on Mars)



In all cases, weight constant, but less than on Earth.

S+L apparent weight = weight.

In a level turn, apparent weight increases as 1/cos bank angle. So in a 60 degre banked turn, apparent weight on Earth = 2000N, on Mars 800N.

So the mans apparant weight in that turn is 0.4 x 2, not 0.4 + 1.

Of course this means that the lateral component of the lift vector (same speed and bank angle) is smaller than on Earth. Which means the radius of turn is much larger.

However, that's not the end of the story. Since gravity is less, to get the same ambient pressure our martian atmosphere would need to be much denser; 1/0.4 in fact, so density would be 2.5 ISA sea level.

Thus, production of a given amount of lift would require lower lift coefficient. If was assume the same aerofoil our martian aircraft would be flying around with much lower Alpha. Looking at it another way for the same TAS our martian aircraft has a much higher IAS.

Lets imagine our Earth aircraft was flying relatvely slow, and was at CLmax at 60 degrees AOB, sitting in the light buffet. At the same TAS, our martian aircraft would still have plenty of alpha to spare.

I've scibled down a few sums here, and I reckon if it increased it AOB to 77 degrees it would fly the same radius of turn as on the Earth. At that AOB the pilot's apparent weight would be 1778 Newtons; that's 4.44 higher than S+L (Mars) and 1.78 higher than Earth.

Note that aircraft could still 'pull' 2 G, so the pilot still has some Alpha in reserve. (AOB could be increased by a couple of degree).

In summary to follow the same radius of turn at the same TAS, compared to our 'nibbling the buffet' earth aircraft, our Martian pilot:
- experiences less G
- has higher IAS
- Uses higher bank angles but lower alpha
- Sill has some Alpha in reserve

How does that sound?

[Wizofoz]

Sorry, sounds like bollox!

(Well, what you say is correct, but you missed the point)

Note the question- Same AofA, same TAS in same ambient pressure= same Lift vector.

You are correct about the aircrafts weight from a FofR of the Martian surface, thus it will be producing more lift than weight and thus accelerating vertically.

Lets assume the aircrafts mass is 1000kg.

Lets assume it is maintaining the AofA to fly level on Earth, so it is producing 1000kg*9.8kg= 9800N of force.

On Mars, its weght is 400kg, so the force needed to fly level would be 3920N.

Thus it is producing an excess of 5880N.

This will produce an "Apparent weight" of 5880/9.8=...600kg.

Thus, the aircrafts "Total weight" is STILL 1000kg, and it's (Earth calibrated) G meter will still read 1.

This is what I'm saying- fly an aircraft at a particular AofA and you will measure a force. You can try and deduce which part of this force is "Real" and which is "Apparent", but it makes no difference to the aircraft.

[Capt Pit Bull]

Ah, I thought you meant a constant AoA not the same AoA as the earth aircraft, and presumed we were talking about turning, but now I'm not sure what our original Earth flight path was; I'm assuming straight and level.

You are missing the crucially important point that IF pressure is the same THEN density would have to be radically different.

But it doesn't hugely matter. If I may be so bold, I think you want standard earth density rather than pressure. I'll assume that to be the case:

Lets assume the aircrafts mass is 1000kg.

Lets assume it is maintaining the AofA to fly level on Earth, so it is producing 1000kg*9.8kg= 9800N of force.

On Mars, its weght is 400kg, so the force needed to fly level would be 3920N.

Thus it is producing an excess of 5880N.

Correct end result, but the second and third line are dodgy.

Second line: lets get the dimensions correct. You say "1000kg*9.8kg= 9800N of force." I say "1000kg*9.8 N/kg= 9800N of force."

Third line: You are muddling mass and weight concepts. Its mass remains 1000 kg, its weight is 400 N. Neither its weight nor it's mass is 400 Kg.

This will produce an "Apparent weight" of 5880/9.8=...600kg.

Again, weight and mass confused. Weight is a force, leave it as Newtons. But MUCH more crucially, the 5880 is NOT the Apparent weight, its the increase in apparent weight (above the 400N needed to maintain S+L). In otherwords it is the unbalanced force, and could be used to calculate the resulting flightpath change.

This is what I'm saying- fly an aircraft at a particular AofA and you will measure a force. You can try and deduce which part of this force is "Real" and which is "Apparent", but it makes no difference to the aircraft..

You mean appart from the fact that the flight path will be different? Call me picky, but I would consider that to be a difference.

Lets try and cut to the nub of the issue:

Do you understand that:
- in order to move you around during manouevres, a portion of the unbalanced force needs to act on you.
- the proportion is set by the relative mass. e.g. if you mass 100 kg, and the aircraft mass is 1900 kg, then your mass proportion is 0.05
- The sensation of that force being applied to you is how you perceive your weight.
(ignoring vertical thrust components, and assuming not doing extreme manouevres involving high angular rates or high alpha the unbalanced force is mostly about changes in lift. Drag is a nil factor assuming the manoeuvre is sustained as it is implicit that the thrust is increased to sustain the manouevre and maintain speed)
- In otherwords your apparent weight = LIFT x Your mass proportion.
- Therefore whenever lift changes your apparent weight changes.

Meawhile your attraction to the nearby planet due to gravity is constant (ignoring gravity gradient and curvature of the planet), so your actual weight is constant.


((You seem to be hung up on the use of a set of scales. Yes, it is a very important point that you can't tell whether you are accelerating or being held up against a gravity field (or a bit of both) IF you are sitting inside a sealed box with just a set of scales. BUT as soon as you have external reference it is entirely possible to figure out what's happening.))

pb

[Wizofoz]

CPB,

We are in oerfect accord with regard th physics of the situation, except I disagree with this bit:-

You mean appart from the fact that the flight path will be different? Call me picky, but I would consider that to be a difference.


The flight path is either different or the same depending on the frame of reference. Different relative to what?

But that is relatively minor.

The nub of my argument lies here:-


You seem to be hung up on the use of a set of scales. Yes, it is a very important point that you can't tell whether you are accelerating or being held up against a gravity field (or a bit of both) IF you are sitting inside a sealed box with just a set of scales. BUT as soon as you have external reference it is entirely possible to figure out what's happening.))


Basically I'm saying this:-

You define weight as being due to gravity and "Apparent" weight as being due to acceleration.

But gravity IS an acceleration. ALL the force being felt by the aircraft is due to acceleration, and there is no need to divide the too into different "Types" of force.

[Capt Pit Bull]

You mean appart from the fact that the flight path will be different? Call me picky, but I would consider that to be a difference.

The flight path is either different or the same depending on the frame of reference. Different relative to what?

But that is relatively minor.

Look, I hate to keep banging on at the same point, but the whole point of the thread is how do you analyse the forces in different phases of flight.

Therefore the fact that the aircraft doesn't move relative to itself, although true, is totally useless!!!!

Since (for the OPs question) things like climbing and descending are by definition relative to the Earth its pretty obvious that the aircraft as a frame of reference is singularly unilluminating.


Basically I'm saying this:-

You define weight as being due to gravity and "Apparent" weight as being due to acceleration.

No. You're the guy that thinks acceleration causes forces, I'm the guy that thinks forces cause accelerations.

I say weight is caused by gravity, whether you are accelerating or not. You can not perceive your own weight directly because gravity it is a field effect.

You can only perceive contact forces acting upon you (because they are non uniform). This is your apparent weight.

They are fundamentally different types of force, one is gravitational, the other is electrostatic.

But gravity IS an acceleration. ALL the force being felt by the aircraft is due to acceleration,

To say gravity causes acceleration does not mean "gravity IS acceleration". You still seem to be struggling with cause and effect. We've already been over newton 1 and although you said you accepted my point its clear you haven't taken it on board. An acceleration tells us an unbalanced force exists, but its the force that causes the acceleration, not vice versa.

You also don't seem to want to address the point that objects that are in a gravitational field but not accelerating (becuase they are supported) still have weight. This clearly demonstrates that gravity is NOT synonomous with acceleration.

Fundamentally I have shown you have Weight and Apparent weight usually do not have the same orientation. We've seen that they do not have to be the same magnitude. I've just explained how one is a field effect and the other is a contact force. You yourself needed to distinguish between them in order to analyse your martian flight path And yet you say

"and there is no need to divide the too into different "Types" of force."

The bottom line is this:

I say the aircraft's weight is constant.
The fact that the aircraft weight is constant is the starting point for drawing a free body diagram of climb, descent or turning.
Yet many pilots have perceive changes in their apparent weight due to manouevre; however this because LIFT has changed, not because WEIGHT has changed.
But because the effect of accleration is so compelling (especially if you've been squashed under 8 G) pilots tend to really *believe* their weight has changed, and when they come to draw the forces acting on the aircraft a fair few of them are totally confused and really struggle as a result.


pb

[italia458]

Italia,

But how do you know whether your "Real" weight was caused by gravity, or by acceleration?

First off, gravity is "acceleration".

Your apparent weight is like an "addition" to your weight caused by gravity (real weight). So your apparent weight will include your real weight and the remaining weight will be caused by the other acceleration, ie. in a turn.

Think of it this way. If you're straight and level, you have a certain lift vector, say it's 17640N (1800kg * 9.8m/s2) (1.0G). When you enter a 60 degree level turn you will be feeling a load factor of 2.0G. So now my airplane has to increase it's lift to 35280N (2.0G) to stay level. The lift always acts perpendicular to the wing. So now this has a vertical and horizontal component, the latter of which will pull the aircraft into the turn. 35280N * cos 60 = 17640N which is our vertical component of lift which counters the weight of the aircraft caused by gravity.

So using the load factor in the turn, the cos of the angle of bank and the weight (in newtons) of aircraft on the ground, you can find out the apparent weight at any angle of bank. With that you can subtract the weight of the aircraft and find out specifically how much the acceleration, due to the turn, is adding on to your real weight to end up with your apparent weight.

Does this help?

[Pugilistic Animus]

Fly at Vmo and do a High g pull up I think the wings would feel the weight because apparently they wont be there anymore:}

W =mg:)

y component = mg*sin theta

x component mg*cos theta

[Wizofoz]

CPB,

You make sense, but there are still loose ends.

If you won't define Gravity as an acceleration, what is your understanding of what causes it?

What little understanding I have of general relativity talks of gravity a being a curveture of space-time, which governs the motion of inertial objects.

In other words,it talks exclusivley about acceleration, not force. Force is a consequence of the mass of the object and the acceleration due to the space-time curveture.

Thus, the response to "Why do you have weight when not accelerating" is that you ARE accelerating in curved spacetime. You consider us inertial while on the ground. GR considers us inertial while in free-fall, but NON inertial when on the ground.

Newtons laws can be used to calculate the effects of forces very accuratley, and in Newtonian physics, there can be no acceleration without force. What Newton does NOT do is explain WHY gravity does what it does. GR does, and it does indeed tret gravity AS an acceleration, rather than just a CAUSE of acceleration.

[Capt Pit Bull]

As far as I'm concerned, being an individual that experiences life in a relatively weak gravity field whilst never moving more than a tiny fraction of C relative to anything that is of interest to me , I do NOT see the need to invoke GR when dealing with every day mechanics.

Accordingly, Newtonian physics is perfectly adequate for teaching PofF. Introducing more advanced physics simply adds confusion to a student cohort who often have only the most tenuous grasp of maths and physics.

So, yes, there are loose ends. But GR has loose ends as well. Where do we go next? Einsteins fields equations before first solo? Unified field theory before ATPL?

Physics is like a tool box. You use the right tool for the right job. In this case, newtonian physics is the right tool for analysing forces in climb, descent and turning.

Why do you want to invoke GR when by your own admission you have little understanding of it? (That's a rhetorical question by the way... it means don't).

Now, fascinating as its been, I am done with this thread. Its become very circular and tending to drift well off topic.

regards,

pb

[PETTIFOGGER]

Capt PB how does that sound
Er. Interesting but not very convincing, since the atmosphere of Mars is 1 % of that of earth and is mostly carbon dioxide.
Wizofoz Basically I'm saying this:-
You define weight as being due to gravity and "Apparent" weight as being due to acceleration.
But gravity IS an acceleration. ALL the force being felt by the aircraft is due to acceleration, and there is no need to divide the too into different "Types" of force.

Not quite. Gravity or the gravitational constant (with a big G) is a weak force which exerts an attraction on all objects near and far. Gravity (little g) is the acceleration imparted to objects on or near the surface. Gravitational acceleration is the force or acceleration on an object caused by Gravity.

Classical Acceleration is the change in velocity over time, and is caused by the result of the total of the forces acting on a body of constant mass.[Newton a=F/m]. But there are several types of acceleration – proper acceleration, coordinate acceleration, uniform acceleration, Euler acceleration etc. And just to make life more interesting, we are in a rotating reference frame and subject to 2 or 3 pseudo or fictitious forces.

Wizofoz Newtons laws can be used to calculate the effects of forces very accuratley, and in Newtonian physics, there can be no acceleration without force. What Newton does NOT do is explain WHY gravity does what it does. GR does, and it does indeed tret gravity AS an acceleration, rather than just a CAUSE of acceleration.
The relevant bit in GR is the Einstein Field Equations where the use of Euclidian geometry is eschewed in favour of Gaussian coordinates to allow accurate curvature calculations. It has been my understanding, possibly imperfect, that it is the need to preserve the equivalence principal that gives rise to discussion of acceleration. Other than that, Einstein treats gravity as a force, as can be seen from the following quotes:
“We must note carefully that the possibility of this mode of interpretation rests on the fundamental property of the gravitational field of giving all bodies the same acceleration, or, what comes to the same thing, on the law of the equality of inertial and gravitational mass.”
And
“Bodies which are moving under the sole influence of a gravitational field receive an acceleration, which does not in the least depend either on the material...”
(source: A Einstein , 1920, Relativity: the Special and General Theory, Chapter 19)

Fortunately, one does not need to be au fait with all of this to fly. The OP’s question can be satisfactorily answered by referring to page 3-19 of the FAA’s ‘Pilot’s Encyclopaedia of Aeronautical Knowledge’ has a section on ‘Forces in Climbs’.
Pilot's Encyclopedia of Aeronautical ... - Google Books (http://books.google.com/books?id=m5V04SXE4zQC&pg=PT33&lpg=PT33&dq=+%22angle+of+bank%22&hl=en#v=onepage&q=%22angle%20of%20bank%22&f=false)

[PBL]

One can argue all one likes about what causes what, but if one uses as an example a primarily phenomenological theory, such as GR, then one is going to find it hard to answer questions about causes. Phenomenological theories describe what happens, and like Newtonian dynamics or GR tie disparate phenomena together elegantly. But they have little or nothing to say about causes.

I pointed this out before, but it didn't seem to resonate. I take it no one (else) discussing here is familiar with the literature on causation.

I looked for deeper grounds behind Wizofoz's argument about forces and mass-accelerations being the same thing. So far I have understood little more than a simple assertion of the phenomenology (with which no one at this point 350 years later can really disagree), coupled with a strong dose of operationalism to say there is nothing more than that. It beats me how one can, after embracing an argument which says they don't exist, still try to argue about causes. But then, this is PPRuNe.

PBL

[PETTIFOGGER]

PBL
I pointed this out before, but it didn't seem to resonate. I take it no one (else) discussing here is familiar with the literature on causation.Yes, I noticed, and also noticed that the philosophy of physics appears to have been banned by Wizofoz. I am aware of some of the literature on determinism/causation.

[Wizofoz]

Well at least I kept the conversation lively!

Thanks to all, I have learned a lot from the discussion.

I didn't want to cede my position simply as a surrender to authority. Those who have posted here obviously have a greater education in physics than I, but unless you could explain it to me, I didn't want to just say "I guess you were right".

Yes I understand (and always have) that part of the forces an aircraft experiences are due to gravity and (lets see if I can phrase this to CPBs satisfaction), when acted upon by an additional force, the aircraft will undergo an acceleration. the pilot will then feel a force in the opposite direction, and the sum of the two are the "Apparent weight".

But unless you can explain what gravity IS, not just what it DOES, and if you conceed that you can't objectivley MEASURE (as opposed to OBSERVE) which force is which, I didn't necessarily agree that they can be seperated into different catagories. If this is operationalism, then I'm not prepared to abandon operationalism as being valid at least in the study of purely physical phenomena, and I don't think the literature PBL cited says I have to.

[PETTIFOGGER]

Wizofoz
Fair enough. Concerning what gravity IS, not just what it DOES, there is much to choose from, e.g. http://www.imsc.res.in/~iagrg/IagrgSite/Activities/IagrgMeetings/25th_Iagrg/VRtalk.pdf (http://http//www.imsc.res.in/%7Eiagrg/IagrgSite/Activities/IagrgMeetings/25th_Iagrg/VRtalk.pdf)
or http://arxiv.org/abs/1001.0785 (http://http//arxiv.org/abs/1001.0785)
and many others besides.

[Pugilistic Animus]

Classical mechanics is for 'everyday stuff'
Relativistic mechanics is for things going very very fast
Quantum mechanics is for very small things---it's not too tough... just the same seven postulates repeated endlessly:zzz:

For spectroscopic work the relativistic and quantum mechanics are both important...and that's a very nasty topic::E

Einstein's time dilation theory was proven using an two atomic clocks and a DC-8...the moving one slowed a bit---:}

according to the correspondence principle QM will converge to suit the classical case:hmm:

All measuring devices introduce a perturbation in the system... a thermometer or electric metering transformer,..flowmeter.... for example...thereby altering the system and creates a definite inaccuracy...albeit infinitesimal is most cases...an NOTHING CAN BE DONE ABOUT IT:ooh:

Lastly wrt to momentum and position there is no limit, to accuracy of measurement only if one of those variables is zero [i.e not moving],..if not, then it is impossible to measure either quantity with an accuracy greater than h/2pi...now ..
and lastly
they have some guys already working on the final unknowns in theoretical and experimental physics...I know one...:suspect:


:cool:

[Wizofoz]

SPECIAL relativity deals with speeds approaching C

GENERAL relativity is applicable at any speed.

BTW PBL, how does your view on Operationalism fit with the concept of Equivalence, which is one of the cornerstones of GR?

[rudderrudderrat]

Hi wizofoz,

I've enjoyed following your discussions over the last few days.
GENERAL relativity is applicable at any speed. True - but Newtonian physics is a good approximation to GR and is much simpler to handle mathematically. GR can explain the advance of the perihelion of Mercury's orbit (1 degree per Century? something of that tiny magnitude)

Do we really need that sort of precision when discussing forces on an aircraft in the climb, or turning when pulling "delta g"? We know what earth's gravity feels like, so if we measure anything different - then it's due to some other acceleration.

[Wizofoz]

Hi rudder,

Have you considered that if you were in a fast elevator, accelerating towards the earth with an accelaration of 19.6m/s2, you'd be standing on the roof with no way of knowing that you weren't stationary, upright wrt the surface of the earth?

Or this- what AofA would an aircraft, flying inverted, have to achieve in order to show +1g on its g meter?

[rudderrudderrat]

Hi Wiz,

Yes I have considered these.

Unless the elevator lift shaft was infinitely long - I'd realise I was standing on the roof shortly after we hit planet earth and stopped. I've been standing in the present lift shaft for a lifetime and it doesn't hurt yet - so I guess I'm standing on the floor and it's stationary- but in earth's gravitational field.

The aircraft would have to pull 1G towards earth - so it will fly the same AofA as it would in level flight the normal way up (wheels towards earth).

[Wizofoz]

Rudder,

Exactly so.

Point being, in neither case can you tell what is gravity and what is acceleration without referencing the planet. In both cases any physical interaction will be identical to being in 1g on (or not accelerating wrt) the Earth.

PBL seems to insist that because of causation, they are not the same thing.

I'm not saying he's wrong, but it does seem that that defies Equivalence, which is quite defiantly still a current concept in physics, being one of the principles of GR.

[rudderrudderrat]

Hi Wiz,

Because we've all spent a lifetime experiencing what 1g feels like, measuring weights using spring balances, predicting the flight of tennis, squash, foot balls etc. - we reckon we can tell which acceleration is due to gravity. If we measure or notice a difference with respect to the reference frame of planet earth (e.g. looking out of the window or Artificial Horizon in IMC (same as lift shaft with no windows))- then it must be due to some other acceleration.

[Wizofoz]

Rudder

Again completely true, but the Earth is not a priviledged frame of reference, and measurments made in any OTHER FofR are just as "Real".

[PBL]

PBL, how does your view on Operationalism fit with the concept of Equivalence, which is one of the cornerstones of GR?

I am not sure what's being asked.

My view on Operationalism, which I take to be the view that concepts are to be identified with algorithms to measure them, is that it has been refuted in this form. The Stanford Encyclopedia author appears to think it can be rescued in some form.

My view on pure GR is that it is phenomenological, it describes. It doesn't assign causes. There are many equivalence principles you could be meaning; (a) that there is no difference between dynamics on the surface of the earth and dynamics inside a craft in free space which is accelerated at 1g by motors (b) that free fall in a gravitational field is independent of inertial mass; (c) ...... As far as I know, and that is not necessarily very far, (a) is refuted by tidal effects. GR in general appears still to be refuted by quantum theory (that is, it appears to be mutually incompatible and no one is prepared to give up quantum theory).

So what am I supposed to be "fit"ting? And does such a discussion fit at all on an aviation forum?

PBL

[Wizofoz]

Insofar as Aviators are generally not dumb, and are usually interested in scientific discussions, sure, why not.

I in fact raised the issue on a more "Sciency" forum (JREF) and got replies which were interesting.

I'm not saying they refuted you, as it was me putting my interpretation on things you said, and I may well have mis-represented you and your arguments, but one poster (who I know to have a PhD in Physical Sciences) participateded in this exchange:-

me-

Hi guys,

I need some help from a physics Guru, even if it is to tell me I'm wrong so I can drop it.

On an Aviation forum, a discussion came up regarding the forces on an aircraft in a turn.

As you'd be aware, and aircraft in a 60deg Angle of Bank turn experiences 2g. People started refering to this as the Aircrafts "Apparent Weight".

I chimed in and said it was meaningless calling it "Apparent Weight" as it was indestiguishable from "Real" weght. Basically the the opposite force felt as a reault of the aircrafts acceleration was the same thing as that due to Gravity, so it was incorrect to label them as different things.

I got pretty soundly shouted down, and one posted invoked that I was using "Operationalsim" which was out-moded.

What are the physics heavy-weights opinions?

Thanks

Sol Invictus (PhD dude):-

There's a deep principle of physics - the equivalence principle - which says you're right. Inside a sufficiently small laboratory, you cannot tell whether you're in a gravitational field ("sufficiently small" means too small to measure tidal effects). If you feel an acceleration it could be because you're in a room or plane that's being supported against the force of gravity by the ground or air, or because the room or plane is accelerating in zero gravity due to something else (like an elevator cable or a banked turn), or some combination of the two as in your example.

Of course you could also just look out the window and observe you're flying over earth, and use your knowledge of its mass to determine which component of your weight is "real" and which is "apparent". But I suppose you consider that cheating.

ps - I'm not sure what "operationalism" is, but I think I believe in it.

Me (Responding to a query)

Well, I copped a speil and a link to a philosophical document, and was told

"The view of science that quantities which you cannot discriminate with instruments are the same thing is about the only philosophy of physics which has been definitively discredited in the last hundred years. So it is rather inappropriate to try it on here, don't you think?"

I didn't understand what he meant...



Sol:-

Insofar as I can parse that, I disagree with it completely. The idea that the only real things are those we can (in principle) measure not only has not been discredited, it's one of the most powerful ideas around. It has repeatedly proven its worth over the last century (the equivalence principle being an excellent example).

So, firstly you can see that EVERYONE (including me) knows that at a practical level, invoking the Earth as a Frame of Reference makes sense in the real world, but also that from a purerly theoretical standpoint, operationalism appears to be alive and well.

The thread is here:-http://forums.randi.org/showthread.php?t=179962
If you'd like to read or participate in it.

[Pugilistic Animus]

SPECIAL relativity deals with speeds approaching C

GENERAL relativity is applicable at any speed.

well for earthbound relativistic endeavors i.e electrickery or lasers; SR is important

For other activities since the system is so comparatively small against a massive and just too obvious reference [Earth] that use of GR reference frames would not increase the number of signifigant figures to be worth it, as stated...not an expert on it at all, but...you could try

:p

[PBL]

Wizofoz,

Craving indulgence from those (I am sure most discussants) who don't really see what this has to do with aviation, here is my rational reconstruction of part of the debate.

You suggested that (paraphrasing) there is no causality between force and acceleration because of the symmetry, namely they are related by an equation
(at this point, we were discussing Newtonian dynamics).

I said that causality is an asymmetric notion and equations obviously not, so that one cannot argue about what causes what just using equations.

You said that indeed one can: since you couldn't extract a notion of cause from the equations, it meant there was no such notion at work.

I said that was an operationalist argument (it is) and said operationalism has been discredited (it has). I didn't say how, and I don't think this is the place to discuss such philosophies of science.

I haven't been involved in any of the discussion about "Real Weight" and "Apparent Weight".

Your correspondent doesn't apparently know what operationalism is, but believes that the EEP is well established (let me provide a link to Clifford Will's 2001 survey (http://relativity.livingreviews.org/Articles/lrr-2001-4/index.html) ) and thinks this establishes operationalism (whatever he thinks it might be) as equally well-confirmed.

The logic behind these views escapes me. You take a phenomenological theory (such as Newtonian dynamics or GR), say it has been well-confirmed (which it has) and use this to say that nothing not mentioned by the theory has any reality.

Quite apart from the operationalist twist you gave your justification, this kind of argument cannot be right. GR is well-confirmed. Brans-Dicke is also well-confirmed (mostly by the same tests) but differs from GR in certain conceptual respects. It is not actually known at this point which is preferable. You could use the argument style to say nothing not mentioned by Brans-Dicke has any reality (if you believed in that) as well as use it to say nothing not mentioned by GR has any reality (if you believe GR). Now you would have turned a discussion about which phenomenology agrees better with measurement (GR or B-D) into an argument about what things exist!

Similarly, the Standard Model is well-confirmed. It is also, by almost unanimous opinion, wrong. If you followed your argument scheme and said that nothing not mentioned in the Standard Model has any reality, I imagine you would be roundly contradicted by almost all particle physicists (certainly by almost all I know, including the one I helped hire), who know that there is more stuff that the Standard Model doesn't cope with but they don't know what it is.

I don't see that you have established that an assertion, that forces cause acceleration rather than acceleration causing forces, has no meaning. (Your physicist correspondent didn't offer a comment about that, I guess because you didn't ask.) Second, I don't think you can reasonably make ontological decisions on the basis of a phenomenological theory.

I am happy to stop here.

I am not sure anyone can win the "Apparent Weight" versus "Real Weight" debate. You would deny any distinction, say by appeal to an equivalence principle. I imagine you think that those who think they are distinct are appealing to a privileged frame, and I think they would agree they are. You would say, I imagine, "no frame can be privileged". They would say "here, I just privileged this one". It is not obvious to me that one view is to be preferred over the other.

PBL

[Pugilistic Animus]

I am not sure anyone can win the "Apparent Weight" versus "Real Weight" debate.

he did not win the debate for sure
YouTube - Wings fall off at an airshow (http://www.youtube.com/watch?v=LPbhQS6IljU)

[Wizofoz]

Thanks, PBL. It's been enlightening if not definitive.

Just to clarify, though-

I don't see that you have established that an assertion, that forces cause acceleration rather than acceleration causing forces, has no meaning.

I thought I'd made it clear that I conceded I was wrong on that point. If not, I do so now!