"Principles of flight" by Jeremy Pratt
The Original Whirly
Joined: Feb 1999
Aviation Qualifications: CPL
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From: Belper, Derbyshire, UK
The PPL books are there to teach you to pass the exams, the purpose of which is to enable you to know enough to fly safely. They are designed to be read by people who don't necessarily have the benefit of all the education you fortunate people have! I taught ground school to a chap in his 50s who'd left school at 13! He struggled, but passed. And why not? But do you think people like that would manage if they had to have all the finer points of Newtonian physics - which are irrelevant to flying - thrown in?
For the record, as a psychology graduate I can tell you that the PPL Human Performance books are over-simplified rubbish too. Can't remember exactly what, but certainly practically all the personality related stuff...and I learned that way back in the 1970s!!! But they serve their purpose. And I remember my tutor for my CPL ground school saying that if you were a radio ham, you'd realise that the Radio Aids exam was over-simplified too...but I struggled to understand and pass the exam as it was, so if it had been more complicated I'd probably have had a nervous breakdown!!!
Jeremy Pratt has written a readable, easy-to-understand series of books which has given generations of pilots most of what they need to get a PPL and fly safely. And now some clever-clogs has to point out that strictly speaking it's not all accurate!!! So what else is new (yawn). And excuse me, but I'm decidedly underwhelmed and unimpressed.
For the record, as a psychology graduate I can tell you that the PPL Human Performance books are over-simplified rubbish too. Can't remember exactly what, but certainly practically all the personality related stuff...and I learned that way back in the 1970s!!! But they serve their purpose. And I remember my tutor for my CPL ground school saying that if you were a radio ham, you'd realise that the Radio Aids exam was over-simplified too...but I struggled to understand and pass the exam as it was, so if it had been more complicated I'd probably have had a nervous breakdown!!!
Jeremy Pratt has written a readable, easy-to-understand series of books which has given generations of pilots most of what they need to get a PPL and fly safely. And now some clever-clogs has to point out that strictly speaking it's not all accurate!!! So what else is new (yawn). And excuse me, but I'm decidedly underwhelmed and unimpressed.
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Pompey till I die
Joined: Nov 2006
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From: Guildford
Whirls
One thing I will say...I think the Jeremy Pratt books have been great. He's managed to make some very dry subjects interesting enough to read through, and I highly recommend them to anyone. He seems to know when to put in a little joke just to keep you reading when things get very dry.
I would just say though, he's obviously more an artist than a scientist, and I disagree that what he has done is dumbed down the subject matter, Although he basically gets the principles he's trying to talk about, he really didn't understand the physics behind it and is using some very confused terms. I.e. "Centre of Gravity" is another one. If you were to remove gravity then you would still have a point in the body of mass where upon all mass appears to act. Therefore it is a COM or "Center of Mass".
Also I strongly disagree teaching a subject incorrectly makes it more accessable. If you are teaching the principles of flight then it's important that you truly differentiate between a force and mass.
It's been the weakest part in the books so far, but overall they have been an excellent set to learn the PPL from.
I would just say though, he's obviously more an artist than a scientist, and I disagree that what he has done is dumbed down the subject matter, Although he basically gets the principles he's trying to talk about, he really didn't understand the physics behind it and is using some very confused terms. I.e. "Centre of Gravity" is another one. If you were to remove gravity then you would still have a point in the body of mass where upon all mass appears to act. Therefore it is a COM or "Center of Mass".
Also I strongly disagree teaching a subject incorrectly makes it more accessable. If you are teaching the principles of flight then it's important that you truly differentiate between a force and mass.
It's been the weakest part in the books so far, but overall they have been an excellent set to learn the PPL from.
Joined: Jan 2005
Posts: 523
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From: Kent
Whirly 
Anyway, if we are being pedantic, I have always understood "weight" to be:
You will notice that there is no reference to "the force exerted on a body when in an aeroplane that is accelerating rapidly".
So:
OC619
P.S. Whirlybird is not Whirls - that's someone else
Anyway, if we are being pedantic, I have always understood "weight" to be:
The force that gravitation exerts upon a body, equal to the mass of the body times the local acceleration of gravity.
So:
- Mass is constant.
- Weight is constant (near as makes no difference) - unless you have a facility for inter-planetary travel.
- 'G' is a term used to refer to weight on earth.
- It is common, in aviation, to measure the forces accelerating a body in multplies of 'G'.
OC619
P.S. Whirlybird is not Whirls - that's someone else
Joined: Jan 2006
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From: Oxford
yeah, I'd mostly echo what Whirlybird says. I'm studying using the Pratt books at the moment. The content of his books is sufficiently correct to pass the exams, generally promotes a safe attitude, and is accessible to sub-genius intellect.
On the matter of psychology, I note that he actually put a disclaimer in (in the 2003 edition) to say that the approach he describes is old-fashioned and not entirely correct, but serves the purpose!
He sometimes says some stupid and anachronistic things, like his comparison of the relative merits of alcohol and illegal drugs: apparently, boozing it up is harmless if you allow it to clear from the body before flying, but if you have a few puffs on a spliff or do a little bit of speed one day, it goes without saying that you'll never be safe to fly an aeroplane again, because unlike alcohol, they apparently have long-term and irreversible effects. (I'm certainly not advocating their use, though!)
And his little foray into statistics at the beginning of the Flight Safety section is a brilliantly ironic illustration of his title of for the chapter, which is "Lies, Damned Lies and Statistics". He presents an obviously daft mis-interpretation of a statistic, and says "to get the best out of any statistical study, it is necessary to approach the subject with a fair dose of common sense." WRONG. It is necessary to approach the subject with informed logical thinking. His "common sense" analysis then presents an laughably fallacious conclusion that "statistically, a pen top or a sock is far more dangerous than an aircraft in flight", because more people were injured by them in one year than by aircraft! Er... that's because only a miniscule proportion of the population encounter light aircraft, and then only for a small period of time on average, whereas everyone encounters pen-tops and socks almost every single day...
He occasionally gives a sense of "common sense says this, so it must be worth heeding" - which is wrong (and inconsistent with much of the good advice he gives elsewhere), and is a fallacy that may lead to the death of aviators. There is no valid substitute for evidence-based truth and rational thinking.
On the matter of psychology, I note that he actually put a disclaimer in (in the 2003 edition) to say that the approach he describes is old-fashioned and not entirely correct, but serves the purpose!
He sometimes says some stupid and anachronistic things, like his comparison of the relative merits of alcohol and illegal drugs: apparently, boozing it up is harmless if you allow it to clear from the body before flying, but if you have a few puffs on a spliff or do a little bit of speed one day, it goes without saying that you'll never be safe to fly an aeroplane again, because unlike alcohol, they apparently have long-term and irreversible effects. (I'm certainly not advocating their use, though!)
And his little foray into statistics at the beginning of the Flight Safety section is a brilliantly ironic illustration of his title of for the chapter, which is "Lies, Damned Lies and Statistics". He presents an obviously daft mis-interpretation of a statistic, and says "to get the best out of any statistical study, it is necessary to approach the subject with a fair dose of common sense." WRONG. It is necessary to approach the subject with informed logical thinking. His "common sense" analysis then presents an laughably fallacious conclusion that "statistically, a pen top or a sock is far more dangerous than an aircraft in flight", because more people were injured by them in one year than by aircraft! Er... that's because only a miniscule proportion of the population encounter light aircraft, and then only for a small period of time on average, whereas everyone encounters pen-tops and socks almost every single day...
He occasionally gives a sense of "common sense says this, so it must be worth heeding" - which is wrong (and inconsistent with much of the good advice he gives elsewhere), and is a fallacy that may lead to the death of aviators. There is no valid substitute for evidence-based truth and rational thinking.
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Pompey till I die
Joined: Nov 2006
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From: Guildford
Hmm, so to get back to the original point
What is the answer to this question ?
The 4 principle forces active in an aircraft in flight are lift, thrust, drag and
A) Weight
B) Mass
C) Gravity
D) Bipolar
Do you give the correct answer or do you second guess what sort of bizzare view of the physical world the CAA may have and try and go for what they want ? It's tricky....
The 4 principle forces active in an aircraft in flight are lift, thrust, drag and
A) Weight
B) Mass
C) Gravity
D) Bipolar
Do you give the correct answer or do you second guess what sort of bizzare view of the physical world the CAA may have and try and go for what they want ? It's tricky....
Joined: Feb 2006
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From: london
Just my opinion but i think the Pratt books are some of the worst text books i've ever read , and i've read lots.
Looking at the ppl confuser and trying to find the answer to your question in the Pratt books is a nightmare . I've just bought the Trevor Thom ones and stuck the Pratt things on ebay.
I must point out i come from the school of pass the exam rather than understanding why you passed in the first place.
Looking at the ppl confuser and trying to find the answer to your question in the Pratt books is a nightmare . I've just bought the Trevor Thom ones and stuck the Pratt things on ebay.
I must point out i come from the school of pass the exam rather than understanding why you passed in the first place.
Blah Blah Blah
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From: Malmesbury VRP
I found the Pratt (AFE) books good enough to get you through the exams. He does simplify things and tries not to throw in tons of tagents like you get in the Thom books.
Either way the Pratt or Thom books will get you through the PPL exams (if you half a brain that is).
Either way the Pratt or Thom books will get you through the PPL exams (if you half a brain that is).
Joined: Aug 2000
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From: UK
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.
I haven't read the text of Pratt's book, but if the most serious crime he commits is saying that an aircraft "weighs 800 kg" then it doesn't really dent the physics.
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Therefore it is a COM or "Center of Mass".
Strangely enough, aircraft performance data specifies "centre of gravity", not centre of mass and that CG datum becomes very important to safe and legal flying.
Time to put this erudite stuff to bed, pass the exams, get a license and then learn to fly, where I can assure you that you won't be too worried about the difference mass or weight, when you are flying around on a glorious VMC day with bandits in all sectors.
By the way, just so it does not come as a terrible shock, when you set the flaps to 25 degrees, it isn't really 25 degrees, its just an approximation, like the stuff you need to know to pass your exams.
Strangely enough, aircraft performance data specifies "centre of gravity", not centre of mass and that CG datum becomes very important to safe and legal flying.
Time to put this erudite stuff to bed, pass the exams, get a license and then learn to fly, where I can assure you that you won't be too worried about the difference mass or weight, when you are flying around on a glorious VMC day with bandits in all sectors.
By the way, just so it does not come as a terrible shock, when you set the flaps to 25 degrees, it isn't really 25 degrees, its just an approximation, like the stuff you need to know to pass your exams.
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Pompey till I die
Joined: Nov 2006
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From: Guildford
On a more serious note...
...I guess it just highlights how flying is often presented and taught as a science, but my limited experience is showing it to be anything but. It really feels like an art with some guiding principles.
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From: Small dot in the Caribbean
Ehh, does your average person trying to get a PPL REALLY care about all this stuff you're discussing anyway? Do they really care about CoM, CoG, Weight versus Mass and their respective units? Not implying that they don't just asking 
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From: somewhere in Oz
PompousPaul, when you've figured out circulation theory in your head and derived the optimal span-wise lift distribution for minimum induced drag, you can re-write Pratt's books for him.
...or when you manage to fly an aircraft outside the effects of gravity, you can re-name centre of gravity to centre of mass.
Until then, best you understand that flying is applied science, but only just enough science to make it understandable and meaningful. Lets face it, how many rules-of-thumb and approximations are used for your navigation? Does it matter that the 1-in-60 rule is a few percent out? Is the clock face rule worthless for being as crude as it is? Have you ever measured track miles with your thumb?
And as Bookworm says, kgf is a widely understood unit of force in the engineering world.
A
...or when you manage to fly an aircraft outside the effects of gravity, you can re-name centre of gravity to centre of mass.
Until then, best you understand that flying is applied science, but only just enough science to make it understandable and meaningful. Lets face it, how many rules-of-thumb and approximations are used for your navigation? Does it matter that the 1-in-60 rule is a few percent out? Is the clock face rule worthless for being as crude as it is? Have you ever measured track miles with your thumb?
And as Bookworm says, kgf is a widely understood unit of force in the engineering world.
A
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Pompey till I die
Joined: Nov 2006
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From: Guildford
Hmm...
And as Bookworm says, kgf is a widely understood unit of force in the engineering world.
f = m * a
Where m is the mass in Kg. So how you guys are getting a force out of that without a value for a...is erm....magic.
Anyway I'm giving up the debate, we've entered the twilight zone of the internet forum where people insist something complete wrong is right and then have a nob measuring contest to proove it. It goes thus:
o) I think this
o) If you think that then you are an idiot, I am right
o) No you're not
o) Yes you are I've got 5 years xxxx experience
o) Well I've got 10 years xxxx experience
o) Well I've got 20 years xxxx experience and my mate knows the king of xxxxx
Last edited by PompeyPaul; 19th June 2007 at 21:54.
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Pompey till I die
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From: Guildford
Ok, I'll bite
Ok, I'll bite!
The original post was NOT about kgf, that was somebody else
Was the original post.
No kgf mentioned there.
This is from a modern day text book, NOT from a text book that was written before 1960.
Hence it is still incorrect. A mass of 800kg != weight of 800kg
Today 800kg!= weight of 800kgf as it is a deprecated term.
Even if it were from the 1960s then the statement must stick with like terms.
I.e. an aircraft that has a mass of 800kg does not have a weight of 800kg because mass is not measured in kg (short hand for kgf).
Even if you ignore the whole measurement of things then it is still incorrect due to mass != weight. Pure and simple.
Hence it is still incorrect no matter which way that you look at it.
And it's still incorrect for a large number of O level physics pupils, and it would've been incorrect for anyone from Newton's Principea Mathematica onwards
The original post was NOT about kgf, that was somebody else
Have just found out that apparently an 800kg aircraft parked on the ground weighs 800kg and has a mass of 800kg.
No kgf mentioned there.
This is from a modern day text book, NOT from a text book that was written before 1960.
Hence it is still incorrect. A mass of 800kg != weight of 800kg
Today 800kg!= weight of 800kgf as it is a deprecated term.
Even if it were from the 1960s then the statement must stick with like terms.
I.e. an aircraft that has a mass of 800kg does not have a weight of 800kg because mass is not measured in kg (short hand for kgf).
Even if you ignore the whole measurement of things then it is still incorrect due to mass != weight. Pure and simple.
Hence it is still incorrect no matter which way that you look at it.
And it's still incorrect for a large number of O level physics pupils, and it would've been incorrect for anyone from Newton's Principea Mathematica onwards
Last edited by PompeyPaul; 20th June 2007 at 09:40.
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From: An island somewhere
PompeyPaul wrote:
Go on then, give us the benefit of your extensive scientific education and tell us why that's wrong.
If you believe the world has settled exclusively on the SI system of units, you perhaps need to get out more! The CGS system of units is still widely used today (have a look at some domestic scales and tell us whether they are marked in kg or Newtons), and in common usage the 'f' is invariably omitted from 'kgf'. Pleasingly, my home scales are still marked in 'lb' from the grand old days of FPS, again with the requisite 'f' omitted!
Hence it is still incorrect. A mass of 800kg != weight of 800kg
If you believe the world has settled exclusively on the SI system of units, you perhaps need to get out more! The CGS system of units is still widely used today (have a look at some domestic scales and tell us whether they are marked in kg or Newtons), and in common usage the 'f' is invariably omitted from 'kgf'. Pleasingly, my home scales are still marked in 'lb' from the grand old days of FPS, again with the requisite 'f' omitted!