"Principles of flight" by Jeremy Pratt

[PompeyPaul]

Is it possible to take a book seriously that thinks mass is a force, has direction associated with it, and is countered by lift ?

Tut, tut. He should've listened to his physics teacher a bit more.

It's also incredibly frustrating learning something that is being discussed in incorrect terms.

[maxdrypower]

Is this a new book ? Not seen it , is it a book for exam purposes? atpl/ppl?

[PompeyPaul]

It's part of the series, PPL 4, for err PPL. It's on amazon:
http://www.amazon.co.uk/Principles-Flight-Aircraft-Knowledge-Performance/dp/1874783233/ref=pd_bbs_sr_1/203-7102446-4806335?ie=UTF8&s=books&qid=1182112902&sr=8-1

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.

I wonder if the CAA exams are based on this warped view of physics too ?

[maxdrypower]

ahhh good already passed PPL so I should be okay

[Vee One...Rotate]

Jump through the hoops. Learn the Mickey Mouse material. Pass everything. Get the licence. Smirk and tut at the powers that be as you pootle around the sky.

Just like countless generations before you.

It's naff but a means to an end :bored:

V1R

[maxdrypower]

Pisser of it is I keep having to ask the powers that be stuff that i think I should know , :O:O
But spose rather that than do summat silly

[Blinkz]

to be fair he's just trying to explain things in a simple way. How else are you going to explain the fact that the aircraft will way more when the G increases etc?

[davidatter708]

I think he is keeping it to the earth only here and not changing planet :}

[PompeyPaul]

to be fair he's just trying to explain things in a simple way. How else are you going to explain the fact that the aircraft will way more when the G increases etc?

Weight is the force that is pulling on a body to make it accelerate to the planet's surface. Just as you weigh less if you walk on the moon, due to there being a smaller gravitational force, you also weigh more as you pull Gs. In fact, G is the short term for gravity, thus the more G the more weight, the less G the less weight.

However THE MASS ALWAYS STAYS THE SAME. The mass is the measurement of the amount of matter present. It always stays the same (unless you start throwing bits out of the aircraft) but the WEIGHT can vary depending on what is happening.

[maxdrypower]

Does that make my diet pointless?

[Blinkz]

Yea I know, but its just easier to explain that on the ground they are both the same and when you pull G the weight goes up but mass stays the same. Why confuse people who may not know exactly what the difference is between them by saying that mass is actually less then what they think. He's trying to teach people what they need to know to be safe! Remember the PPL is studied by a wide variety of people!

[PompeyPaul]

Does that make my diet pointless?
Nope, it should still make you less MASSive.

[davidatter708]

Int MASSive chav language man like safe init

[maxdrypower]

yeh man , street !!

[davidatter708]

The girl has it as Force=mass*acceleration therefore no acceration =no force. I should have known I do A level physics and lots of mechanics in maths.
Dave

[PompeyPaul]

New to the forum haven't worked out how to do a quote yet... 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.Nope, f=ma acceleration due to gravity is approx 10m/s/s so in this case it's mass may be 800kg but it's weight (f) would be 8KN (KiloNewtons). The reason why it is not moving is due to the equal and opposite force applied by what ever the aircraft is on, in this case the ground, of 8KN. If it were on a glass plate and the plate was unable to apply that force the aircraft would fall through it to the ground. Just because the plane is not moving does not mean the force has gone away, you would still find it hard to lift the plane.

The main point is that weight and mass are NOT the same.

Weight is a force and therefore can have direction, mass can't

Weight can change (by varying a), mass can't (ok it can if you start sawing the wings off but I wouldn't recommend doing that if you are flying at the time :=)

[JP1]

The reason why we aren't physically accelerating throught the floor is beacuse of the normal reaction. The net force on us ZERO.

But the gravitational force acting upon us would cause us to accelerate at g (10m/sec^2) if the floor wasn't stopping us.

Just beacuse we aren't moving does not mean that there is no force due to our mass in a gravitational field.

Weight is a vector and has magitude and direction. The units of it are Newtons

Mass is a scalar and has only magnitude. The units of it are kilograms

Mass is NOT equal to weight.

The term weight is used for the general public when they are talking about mass. But in scientific fields the two are different.

[Jenni Morton]

I am amazed that flying schools recommend Pratt
so many errors imho
exit stage left

[BHenderson]

Remember that the aircraft is constantly accelerating around the centre of the earth. To stay in orbit the aircraft must have an acceleration towards the centre and therefore a force.

[nano404]

Is it possible to take a book seriously that thinks mass is a force, has direction associated with it, and is countered by lift ?

Ehh, can someone make a list of safe and unsafe books to read. That is ridiculous.

[Whirlybird]

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.

[PompeyPaul]

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.

[OpenCirrus619]

Whirly :D:D:D

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.

You will notice that there is no reference to "the force exerted on a body when in an aeroplane that is accelerating rapidly".

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 :ok:

[michaelthewannabe]

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.

[IO540]

Why does the CAA go for these word plays in their exams?

You can do the FAA PPL, CPL, ATPL and not have to learn this double talk.

[PompeyPaul]

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....

[maxdrypower]

Unfortunately Im thick as two short pig shxts and dont understand any of this

[neilcharlton]

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.

[gcolyer]

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).

[bookworm]

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 wouldn't disagree. Historically, grammes, kilogrammes and pounds have invariably been used for units of force, on the basis that the conversion rate between weight and mass is, for all practical purposes, standard. The newton as a unit of force is a relatively recent invention. Principia also refers to "centre of gravity" (gravitatis centrum), by the way, so it's hardly surprising that the term stuck.

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.

[Final 3 Greens]

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.

[PompeyPaul]

...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.

[sternone]

what is a good place to buy the 'latest' version of the jeremy pratt books ? amazon.com does not have them all, i guess on the amazon.uk ?

[Final 3 Greens]

On a more serious note...

Science is a set of temporal and provisional hypotheses, no more or less. Some of it applies to flying.

[nano404]

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 ;):}

[Andy_RR]

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

[PompeyPaul]

And as Bookworm says, kgf is a widely understood unit of force in the engineering world.You guys work in some strange engineering worlds. The equation is quite simple

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

:ugh::ugh::ugh::ugh::ugh::ugh::ugh:

[PompeyPaul]

Ok, I'll bite!

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.

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 ;)

[Andy_RR]

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...

[Islander2]

PompeyPaul wrote:
Hence it is still incorrect. A mass of 800kg != weight of 800kg :=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!

[PompeyPaul]

Go on then, give us the benefit of your extensive scientific education and tell us why that's wrong.

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

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....

[PompeyPaul]

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...I admire anyone that's waded through 3 pages of the same arguments back and forth.

[eharding]

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.

[Islander2]

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 accelerationWell, thanks for setting me straight on that, I can tell you it's a real Eureka moment for me!! :ooh:

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.Yet there is nothing whatsoever incorrect with the statement other than the letter 'f' omitted from the kg weight, and as has been said already, omitting that letter has been the norm throughout the 40 years of my engineering career.

But hey, don't let a little reading around the subject cause you to apologise for being mistaken! :ok:

[Andy_RR]

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.

[Whirlybird]

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. ;)

[slim_slag]

....who says you have to measure acceleration as m/s/s? Now we are really getting our knickers in a twist :)

[PompeyPaul]

PS: Admiration will not get you a pass on your GFT.
What about brown envelopes stuffed full of cash ? Oops, not you are right, that only works in premiership transfer deals.

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.

[PompeyPaul]

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

f = m.a

I don't think there's any disagreement in this ?

Mass of aircraft = x bs (x bags of sugar)

Everybody agree so far ?

Weight of aircraft = x . a

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

Weight = x . [0.0 -y 0.0]

Thus

Weight = [0.0 -xy 0.0]

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

[Choxolate]

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.

[slim_slag]

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?

[PompeyPaul]

Paul you are a prize plonker

Do you think we'll get another 3 pages if we start debating that ?

[Choxolate]

No debate required.

[slim_slag]

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?

[OpenCirrus619]

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.

[BackPacker]

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.

Except for, he would not call it a "weight and balance" sheet, but a "mass and moment" sheet. And of course it would be corrected for the different gravitational "constants" across the globe.

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.

[PompeyPaul]

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

[Choxolate]

For that you should have posted in JetBlast

[BackPacker]

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?

[PompeyPaul]

No debate required.
I think there is. There's got to be a few pages there...
For that you should have posted in JetBlast
Doesn't the Jeremy Pratt angle confine it to "Private flying" ? I guess that's how it started out before the non-SI brigade arrived on mass (get it! boom boom!).

[Choxolate]

No debate required.
I think there is.
Unless you want a massdebate - now THAT is a pun :rolleyes:

[BackPacker]

Okay, we want to extend this thread to at least five pages so here's a serious question, concerning Va (manoeuvring speed).

Va is defined as the speed where the wings stall at the point where they reach the design limit (normal category that would be -1.8/+3.8g I think) when pulling up hard. As long as you stay below Va you can't break the wings by pulling the stick or moving the ailerons to maximum deflection. (This does NOT apply to the rudder, but that's another story.)

Common wisdom is that Va reduces as the aircraft gets lighter. The reasoning behind this is that with a lower weight (mass), the angle of incidence in level and unaccellerated flight is also lower (due to less lift needed). If you pull up hard, the angle you can pull up until the wings stall is greater and therefore you can pull more g's, assuming the same speed. So lower weight/mass requires a lower Va, in order not to exceed the design limit (measured in g) of the wings.

BUT... My gut feeling tells me that wings don't break due to accelleration. They break because a force is applied to them, and that force is mass x accelleration. As you reduce the mass, the wings are able to withstand a greater accelleration. That cancels out the mass from the equation, leaving Va to be constant.

In other words, if you have an aircraft of, let's say, 1000 kg MAUW with wings designed to withstand 4g (at that MAUW) and you fly it at 500 kg MAUW, at Va for 1000 kg, pull up hard, then the wings will encounter 8g before stalling, but this 8g will only need to accellerate 500 kg. In both cases the force on the wings is 4000 kgf. (About 40.000 N for the purists.)

Where does this reasoning go wrong? Am I assuming a linear relationship somewhere where it isn't?

[PompeyPaul]

Unless you want a massdebate - now THAT is a pun :rolleyes:
dum dum dum dish!

It is indeed - and one I'm very keen on (boom boom). Ok, stand up comedian I am not.

Wait, nobody has said anything scientific for 3 posts and we've only got 6 to go until we get to page 5.

Given we've done Jeremy Pratt to death shall we move onto Trevor Thorn ?

His books suck and they are full of spelling mistakes. He can't spell colour correctly, he spells it color instead. What an idiot etc etc

Oh god, will I EVER finish this aircraft technical book ?

[Mariner9]

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?

Just scribble an arrow on the "8" and label it "Jet". (assuming you're using the metric system):8

Though given the scientific musings on this thread, it should technically be the range between two arrows pointing at .7739 and .8389 (and the reasoning behind those particular numbers could extent this thread by another few pages :E)

[PompeyPaul]

Okay, we want to extend this thread to at least five pages so here's a serious question, concerning Va (manoeuvring speed).

Va is defined as the speed where the wings stall at the point where they reach the design limit (normal category that would be -1.8/+3.8g I think) when pulling up hard. As long as you stay below Va you can't break the wings by pulling the stick or moving the ailerons to maximum deflection. (This does NOT apply to the rudder, but that's another story.)

Common wisdom is that Va reduces as the aircraft gets lighter. The reasoning behind this is that with a lower weight (mass), the angle of incidence in level and unaccellerated flight is also lower (due to less lift needed). If you pull up hard, the angle you can pull up until the wings stall is greater and therefore you can pull more g's, assuming the same speed. So lower weight/mass requires a lower Va, in order not to exceed the design limit (measured in g) of the wings.

BUT... My gut feeling tells me that wings don't break due to accelleration. They break because a force is applied to them, and that force is mass x accelleration. As you reduce the mass, the wings are able to withstand a greater accelleration. That cancels out the mass from the equation, leaving Va to be constant.

In other words, if you have an aircraft of, let's say, 1000 kg MAUW with wings designed to withstand 4g (at that MAUW) and you fly it at 500 kg MAUW, at Va for 1000 kg, pull up hard, then the wings will encounter 8g before stalling, but this 8g will only need to accellerate 500 kg. In both cases the force on the wings is 4000 kgf. (About 40.000 N for the purists.)

Where does this reasoning go wrong? Am I assuming a linear relationship somewhere where it isn't?

I vote backpacker best scientist on here. He has outscienced all of us. :D

[BackPacker]

Just scribble an arrow on the "8" and label it "Jet".

That's about what I did, although I put the arrow on the "9". But that's because the USG arrow is not at "1" but at 1.28 and the 100LL arrow is at 7.7 on my E-6B.

The DA-40 POH, by the way, defines the specific density of whatever you put in there as 0.84, regardless of whether its diesel or Jet-A or a blend of both. Safe side of caution I suppose. I've also seen the number 0.81 being used and WikiPedia claims that the density of Jet-A can get as low as .775.

[BackPacker]

Backpacker - I can't answer the question due to your use of the word speed instead of velocity.. speed being the absolute value (scalar just for Paul) of velocity which is a vector blah blah....

Sorry. I meant to say air pressure in the pitot tube minus air pressure in the static port, projected on a dial which indicates "IAS" but is presumably calibrated as to be very close to "CAS".:{

[slim_slag]

Va is defined as the speed where the wings stall at the point where they reach the design limit I thought Va was calculated from the stall speed, and then the airframe was required to be strong enough to survive any abrupt maneuvers at that speed. So it's a number derived from a regulation, not an engineer? (Note question :) )

[BackPacker]

Well, the design rules for a "normal" category airplane require, as far as I know, +3.8 g limits (at MAUW). Based on that requirement the actual strength of spars etc. is designed and Va is calculated. I think.

In any case, the DA-40, the DR200-120/160 and the PA-28 all have +3.8g as the load limit in the normal category and +4.4g in the utility category. The Robin R2160 (aerobatic) does not do a normal category but has +4.4g in the utility category and +6g in the aerobatic category. All taken from the POHs I have lying around here. Coincidence? I think not.

[slim_slag]

Heh heh, but I think I will do some more research before I put my foot well and truly in my mouth.

[OpenCirrus619]

Ripped off from: http://www.auf.asn.au/groundschool/umodule2.html. Note the last sentence (in brackets) which should please PP immensely.

OC619
Va – design manoeuvring speed. Sometimes referred to as the 'speed for maximum control deflection' or perhaps the 'rough air airspeed' though the latter is usually designated as Vra. It is unwise to make full and abrupt applications of any one primary flight control if you are flying at a speed greater than Va because the force applied could exceed the aircraft's structural limitations and particularly so if you apply more than one control e.g. apply lots of elevator and aileron together. Also when flying above this speed gust-induced loads can exceed the structural design limit, and gust loads in Australian arid region high temperature conditions can be very high. Va is the recommended indicated cruising speed (CAS) when flying in moderate turbulence – strong intermittent jolts. At this compromise speed the aircraft will produce an accelerated stall, and thus reduce the aerodynamic force on the wings, if it encounters a vertical current imparting enough energy to exceed the design wing or tailplane loading.

Va is a fixed theoretical calculation relative to Vs1 for all aircraft within the same category; for a normal category light aircraft (whose certificated vertical load limit factor is +3.8g) Va = Ö3.8 Vs1 or nearly twice Vs1. For a utility category light aircraft (whose certificated vertical load limit factor is +4.4g) Va = Ö4.4 Vs1 or just over twice Vs1. Va is not marked on the ASI but for non-aerobatic aircraft you can assume it's twice Vs1.

If you look at the V-n diagram for a particular aircraft type below you will note that Va is 90 knots whereas the stall speed at a 4.4g load is 94 knots.You can also see from the accelerated stall curve in the diagram that flying at speeds much below Va in turbulent conditions also enhances the possibility of stalls induced by vertical gusts – and also may reduce aileron and rudder effectiveness.

Va decreases as the aircraft's weight decreases from MTOW because it is directly related to Vs1, which decreases as weight decreases – refer rule of thumb 3 above. The aircraft's flight manual may specify lower design manoeuvring speeds for weights below MTOW. (Actually Va decreases with mass rather than weight, but that is splitting hairs a bit.)

[Mariner9]

Back to the thread within the thread...

The DA-40 POH, by the way, defines the specific density of whatever you put in there as 0.84, regardless of whether its diesel or Jet-A or a blend of both. Safe side of caution I suppose. I've also seen the number 0.81 being used and WikiPedia claims that the density of Jet-A can get as low as .775.

Does the POH actually use the words "Specific Density", "Density", or "Specific Gravity", coz technically, they're all different ;)

Assuming they meant density, then the DEFSTAN 91-91 specs for Jet A-1 specify a density @ 15C (in vacuum) in the range 0.775 to 0.840

For diesel, the EN590 specs are for a density @15C in vac 0.820 to 0.845

If you wanted to do the job properly you'd then need to convert the density for both temperature and the buoyancy effect of air. :8:8

Cutting through the BS, if fuel supply is in litres, multiply by .8 for jet and .83 for diesel to get the weight in Kilos

If your fuel supply is in US galls, complain! :E (or to get lbs from US galls multiply by 6.7 for jet and 6.9 for Diesel)

[gcolyer]

Oh god...someone please make it end:ugh:

[BRL]

Your wish is my command..............