"Principles of flight" by Jeremy Pratt
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Pompey till I die
Joined: Nov 2006
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From: Guildford
I guess you missed this ?
Go on then, give us the benefit of your extensive scientific education and tell us why that's wrong.
f = m . a
Force is weight
Mass is the amount of matter there
a is the acceleration
Weight is not equal to mass
Weight is equal to mass * acceleration
So, we've got to 3 pages. Can we make it 5 ? Could this possibly be the longest ever discussed topic on pprune ? Might we even break the 10 page mark ? A few more posts on the same thing and we could even break 20....
Thread Starter
Pompey till I die
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From: Guildford
lol
I hope your GFT examiner doesn't read your threads on PPRuNe or he may wind up being just as pernickerty and bloody-minded as you...
A little less conversation,
a little more aviation...
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From: Bracknell, UK
Paul,
Pragmatism is the name of the game - the books are there simply to help you pass the exams, they don't pretend to be definitive texts on Aviation Physics.
I have a horrible feeling you might be the type of punter who would take his Air Law text as holy writ (because you're a geek, not a lawyer), and shortly thereafter fly directly into the right-hand side of some poor git who simply hadn't seen you, your last words as you tumble to earth in a cloud of broken metal being "I think you'll find *I* was in the right...."
Ed.
Pragmatism is the name of the game - the books are there simply to help you pass the exams, they don't pretend to be definitive texts on Aviation Physics.
I have a horrible feeling you might be the type of punter who would take his Air Law text as holy writ (because you're a geek, not a lawyer), and shortly thereafter fly directly into the right-hand side of some poor git who simply hadn't seen you, your last words as you tumble to earth in a cloud of broken metal being "I think you'll find *I* was in the right...."
Ed.
Joined: Jan 2006
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From: An island somewhere
I guess you have missed the previous 3 pages of posts but the theory is simple.
f = m . a
Force is weight
Mass is the amount of matter there
a is the acceleration
f = m . a
Force is weight
Mass is the amount of matter there
a is the acceleration
The point is, you had originally observed:
Have just found out that apparently an 800kg aircraft parked on the ground weighs 800kg and has a mass of 800kg.
Mr Newton, indeed anybody with GCSE Physics, would strongly disagree.
Mr Newton, indeed anybody with GCSE Physics, would strongly disagree.
But hey, don't let a little reading around the subject cause you to apologise for being mistaken!
Joined: Oct 2005
Posts: 913
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From: somewhere in Oz
Anyway PP, who says you have to measure acceleration as m/s/s? If you measure acceleration as g, which has the units of kgf/kg, or kg/kg if you want to be brief, this still fits Newton's second law.
F (kgf) = m (kg) . a (kgf / kg) - see? Consistency of units and all that...
If an airframe is stressed to +6g/-3g, what is that if it isn't a measure of acceleration?
A
PS: Admiration will not get you a pass on your GFT.
F (kgf) = m (kg) . a (kgf / kg) - see? Consistency of units and all that...
If an airframe is stressed to +6g/-3g, what is that if it isn't a measure of acceleration?
A
PS: Admiration will not get you a pass on your GFT.
The Original Whirly
Joined: Feb 1999
Aviation Qualifications: CPL
Posts: 4,327
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From: Belper, Derbyshire, UK
Jeeeeeezzzzz!!!!!!!!!!!!!!!!!!!! Arrrgggghhhhhhhhhhhhhhhhh!!!!!!!!!! Help!!!!!!!!!!!!!!
I think I speak for the majority of PPLs when I say that I'm now forever grateful to Jeremy Pratt for having simplified this stuff to a point where I can understand it. To me, and I'm sure many others, this discussion is turning into high-powered gobbledygook. I'm not sure how I've managed to fly around for so long not understanding it, but I have, so I think it's probably not necessary for me to know.
But don't let me stop you. I'll go away now and you can enjoy the discussion.
I think I speak for the majority of PPLs when I say that I'm now forever grateful to Jeremy Pratt for having simplified this stuff to a point where I can understand it. To me, and I'm sure many others, this discussion is turning into high-powered gobbledygook. I'm not sure how I've managed to fly around for so long not understanding it, but I have, so I think it's probably not necessary for me to know.
But don't let me stop you. I'll go away now and you can enjoy the discussion.
Thread Starter
Pompey till I die
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From: Guildford
AndyRR
PS: Admiration will not get you a pass on your GFT.
Note!!! For the sense of humour impaired the above is a joke and not to be taken seriously!
Although, the bit about premiership transfer deals being corrupt may actually be true.
Last edited by PompeyPaul; 21st June 2007 at 09:43.
Thread Starter
Pompey till I die
Joined: Nov 2006
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From: Guildford
Units of measurements
Ok guys, I think the pre-GCSE, pre-60's class are getting confused about units of measurement here.
The overiding principle is that mass != weight. An aircraft with mass of 800kg does not have a weight of 800kg. To demonstrate this I will make up a whole new set of measurements so that we are on a clean footing.
Thus I have am using the PP standards of bags of sugar for mass, football pitches per hour for velocity.
Thus
I don't think there's any disagreement in this ?
Everybody agree so far ?
Everybody still in agreement ?
I am presuming we are all happy with the concept of gravity acting downwards, and will define it thus
Gravity = [0.0 -y 0.0]
I.e. a 3d vector that shows gravity acting downards.
Therefore
Thus
Hence it can be demonstrated that weight and mass are not equal, the big stumbling block being that mass is a scalar, acceleration and thus weight is a vector.
Under certain circumstances, i.e. a gravitational field with strength 1, the modules of weight and mass may be numerically the same, but they are still not the same given the fundamental problem of mass being a scalar and accleration or weight being a vector.
QED (Quite easily done)
Ahh, "Principles of flight" by Jeremy Pratt - the thread that keeps on giving
The overiding principle is that mass != weight. An aircraft with mass of 800kg does not have a weight of 800kg. To demonstrate this I will make up a whole new set of measurements so that we are on a clean footing.
Thus I have am using the PP standards of bags of sugar for mass, football pitches per hour for velocity.
Thus
f = m.a
Mass of aircraft = x bs (x bags of sugar)
Weight of aircraft = x . a
I am presuming we are all happy with the concept of gravity acting downwards, and will define it thus
Gravity = [0.0 -y 0.0]
I.e. a 3d vector that shows gravity acting downards.
Therefore
Weight = x . [0.0 -y 0.0]
Weight = [0.0 -xy 0.0]
Under certain circumstances, i.e. a gravitational field with strength 1, the modules of weight and mass may be numerically the same, but they are still not the same given the fundamental problem of mass being a scalar and accleration or weight being a vector.
QED (Quite easily done)
Ahh, "Principles of flight" by Jeremy Pratt - the thread that keeps on giving
Joined: Aug 2006
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From: Somerset, UK
PompeyPaul - technically and hair-splittingly you are correct, however when somebody asks you how heavy you are (what is your WEIGHT? because that is what is being asked as it is found by using a gravity dependent measuring device ) what units do you use?
I'll put a wager that you answer in pounds, kilos or stones and lbs - so let's just get realistic here for a moment. The books are to try and explain the principles of flight not a GCSE physics primer.
If we want to be really exact (as far as modern knowledge allows us) the books should of course throw out the newtonian formulae and use relativistic calculations.
The oft quoted f = m * a is of course only true when the velocity with respect to the observer is zero as mass will change with velocity in the Einsteinian world.
I'll put a wager that you answer in pounds, kilos or stones and lbs - so let's just get realistic here for a moment. The books are to try and explain the principles of flight not a GCSE physics primer.
If we want to be really exact (as far as modern knowledge allows us) the books should of course throw out the newtonian formulae and use relativistic calculations.
The oft quoted f = m * a is of course only true when the velocity with respect to the observer is zero as mass will change with velocity in the Einsteinian world.
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From: He's on the limb to nowhere
G-EMMA,
Is this a correct statement?
"if it's parked on the ground then it's weight would be 800kg and it's mass 800kg as it ain't moving and therefore there is no acceleration due to gravity."
Also.
How would the figures change if it was in freefall i.e moving, but not speeding up?
Is this a correct statement?
"if it's parked on the ground then it's weight would be 800kg and it's mass 800kg as it ain't moving and therefore there is no acceleration due to gravity."
Also.
How would the figures change if it was in freefall i.e moving, but not speeding up?
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From: He's on the limb to nowhere
Just retrieved my 'Jeppesen Private Pilot Manual' from the attic to see what it has to say on the subject. It does a good job of explaning the forces on flight, even discussed Newton's three laws, and doesn't mention mass except the once when it quotes F=ma. One suspects mentioning mass is the mistake Pratt made.
Take the word 'mass' out of the book, as you don't really need it, and there is nothing to argue about. This also makes sense in the every day sense where we substitute weight for mass all the time i.e how much do you weigh?
Take the word 'mass' out of the book, as you don't really need it, and there is nothing to argue about. This also makes sense in the every day sense where we substitute weight for mass all the time i.e how much do you weigh?
Joined: Jan 2005
Posts: 523
Likes: 1
From: Kent
PP,
To get a license (PPL, CPL or ATPL) the powers that be specify certain hoops we have to jump through.
The point of Thom or Pratt is to impart the information necessary to jump through the PPL hoops (plus a little more). The terminology used will be the same as you can expect to find in the PPL exams.
It should also be appreciated that this terminology has been refined, over the past century, to impart sufficient understanding, to pilots, so that they can safely manage an aircraft in flight.
If you want to get a PPL then read the book, accept the explanations, practice with the Confuser and pass.
OC619
P.S. If you want a more in depth understanding and/or disagree with the explanations you get during your PPL course then you can research them later.
To get a license (PPL, CPL or ATPL) the powers that be specify certain hoops we have to jump through.
The point of Thom or Pratt is to impart the information necessary to jump through the PPL hoops (plus a little more). The terminology used will be the same as you can expect to find in the PPL exams.
It should also be appreciated that this terminology has been refined, over the past century, to impart sufficient understanding, to pilots, so that they can safely manage an aircraft in flight.
If you want to get a PPL then read the book, accept the explanations, practice with the Confuser and pass.
OC619
P.S. If you want a more in depth understanding and/or disagree with the explanations you get during your PPL course then you can research them later.
Joined: Feb 2007
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From: Amsterdam
I can just see you now loading up a plane with your photo equipment/tent/junk for a weekend away and working out the balance sheet in Newtons.
Mind you, he'd never get there, because ATC will not allow his transit with a speed of 30 m/s and an altitude of 10.000 meters. And if he specifies a certain amount of dm3 with which to refuel his plane, he'd get very blank stares indeed from the refuellers.
Thread Starter
Pompey till I die
Joined: Nov 2006
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From: Guildford
I'm worried
I'm worried that this thread has become far to sensible and is loosing it's purpose since page 2: to discuss meaningless technicalities in a long drawn out fashion; fufilling the purpose of relieving the tedium of reading aircraft technicalities.
Could new posters please come down on either side of the fence, start posting incredibly strong religious opinions on Newton et al (these need not be backed up by any current, previous or future scientific learning) so that we may all degrade to name calling by page 6.
If you could follow the original thread you will see that the program of events should be followed thus
1. Passionate explanation proposed
2. Statement of years experience in some related field
3. Name calling
Thankyou
Could new posters please come down on either side of the fence, start posting incredibly strong religious opinions on Newton et al (these need not be backed up by any current, previous or future scientific learning) so that we may all degrade to name calling by page 6.
If you could follow the original thread you will see that the program of events should be followed thus
1. Passionate explanation proposed
2. Statement of years experience in some related field
3. Name calling
Thankyou
Last edited by PompeyPaul; 21st June 2007 at 12:00.
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From: Amsterdam
I guess I'm the practical scientist type.
I know that SI requires me to use the SI names and constants like N, Nm, kg, m/s2 and so forth, and I understand the relations between them. I can (or rather: could) do differential and integral calculations and such so that I could tell you the speed of a falling object, and the distance it has travelled, after n seconds of letting go, even considering air resistance, and the other way around.
I also know that in real life gravity is constant, and by using weight and mass interchangeably and measuring it in kg (or lbs if it's a US built airplane) the calculations are greatly reduced in complexity and still very accurate (unless you intend to fly to the moon and back). So for all practical purposes I do the W&B (or M&B) sheet in whatever units the airplane manufacturer specifies, and I interchangeably put USG or liters of fuel in the tanks. I do make a point of writing the units used after the number though. so I don't get confused later on.
And it also allows me to claim that I weighted 360 kilos yesterday, and only 45 kilos a few moments later. Try that with the Atkins diet! (I had my third aerobatics lesson yesterday.)
And on a related note: I'm still looking for an E-6B which has the density of Jet-A/Diesel on it, in addition to 100LL. Anybody knows where to get one?
I know that SI requires me to use the SI names and constants like N, Nm, kg, m/s2 and so forth, and I understand the relations between them. I can (or rather: could) do differential and integral calculations and such so that I could tell you the speed of a falling object, and the distance it has travelled, after n seconds of letting go, even considering air resistance, and the other way around.
I also know that in real life gravity is constant, and by using weight and mass interchangeably and measuring it in kg (or lbs if it's a US built airplane) the calculations are greatly reduced in complexity and still very accurate (unless you intend to fly to the moon and back). So for all practical purposes I do the W&B (or M&B) sheet in whatever units the airplane manufacturer specifies, and I interchangeably put USG or liters of fuel in the tanks. I do make a point of writing the units used after the number though. so I don't get confused later on.
And it also allows me to claim that I weighted 360 kilos yesterday, and only 45 kilos a few moments later. Try that with the Atkins diet! (I had my third aerobatics lesson yesterday.)
And on a related note: I'm still looking for an E-6B which has the density of Jet-A/Diesel on it, in addition to 100LL. Anybody knows where to get one?
Thread Starter
Pompey till I die
Joined: Nov 2006
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From: Guildford
I think there is
No debate required.
For that you should have posted in JetBlast