Ace the Technical interview by G.Bristow
It isn't necessary to include wing area or any other factor into the formula as CL is just that-a co-efficient.
If one wing produces twice the lift of another wing at the same speed and air pressure, it has twice the CL. Could be because of wing area, aerofoil section, planform, angle of attack etc. etc.
In any case you multiply, not add, and "Wing span area" is still a nonsense term.
If one wing produces twice the lift of another wing at the same speed and air pressure, it has twice the CL. Could be because of wing area, aerofoil section, planform, angle of attack etc. etc.
In any case you multiply, not add, and "Wing span area" is still a nonsense term.
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Agreed it needs a a revision but i think its a good book and gives simple straight to the point answers for most stuff. Good for refreshing the memory without having to dust off the old textbooks IMHO
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The book is published through Mcgraw-Hill any errors should be brought to the attention of their editorial contacts, which for aviation related materials is Steve Chapman, e-mail: [email protected]
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If one wing produces twice the lift of another wing at the same speed and air pressure, it has twice the CL
.. now, that statement should generate some discussion ....
.. now, that statement should generate some discussion ....
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Originally Posted by Wizofoz
It isn't necessary to include wing area or any other factor into the formula as CL is just that-a co-efficient.
If one wing produces twice the lift of another wing at the same speed and air pressure, it has twice the CL. Could be because of wing area, aerofoil section, planform, angle of attack etc. etc.
.
If one wing produces twice the lift of another wing at the same speed and air pressure, it has twice the CL. Could be because of wing area, aerofoil section, planform, angle of attack etc. etc.
.
Do the analysis to prove it to yourself.
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Wizofoz
S is required as CL is only applicable to 1 section of airofoil. ie the wing is made up of many sections of airofoil so therefore S is required to find how much lift a wing with 'X' meters wingspan is producing.
I put it to u that CL is the same for a peice of airofoil with the same characteristics that is multiplied by S to determine how many peices of that particular airofoil is present.
S is required as CL is only applicable to 1 section of airofoil. ie the wing is made up of many sections of airofoil so therefore S is required to find how much lift a wing with 'X' meters wingspan is producing.
I put it to u that CL is the same for a peice of airofoil with the same characteristics that is multiplied by S to determine how many peices of that particular airofoil is present.
Well, now you mention it....
Yep. Quite right. To get a force (KG*M/S^2) you need an extra M^2 on the right of the equation. I stand corrected.
barit- Absolutely not, that's why I resisted the idea that area should be part of the equation as twice the area does not mean twice the lift.
That being said, I think we hold the record for getting off topic the fastest as
1) He still should have known the difference between addition and multiplication.
2) "Wing span area" still doesn't mean anything, and
3) That was only one of the errors I highlighted, and my list was not exhaustive.
barit- Absolutely not, that's why I resisted the idea that area should be part of the equation as twice the area does not mean twice the lift.
That being said, I think we hold the record for getting off topic the fastest as
1) He still should have known the difference between addition and multiplication.
2) "Wing span area" still doesn't mean anything, and
3) That was only one of the errors I highlighted, and my list was not exhaustive.
Warning Toxic!
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Wizofoz- you are right. Some of the information is outrageous. I found myself getting increasingly annoyed with it and dumped it 1/3 of the way through. There are some very dodgy areas and i concluded better to not trust anything in the book than try and sort the wheat from the chaff. There has been a previous thread about it.
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I almost bought the book but after speaking to a few friends who did I decided against it. I'd buy the revision if they act on the feedback they're receiving.
At least I didn't have an interview otherwise it would have been a case of "Ar$e the Pilot Interview".
At least I didn't have an interview otherwise it would have been a case of "Ar$e the Pilot Interview".
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I reckon there were some answers in that book that I just didn't understand 
If you have an interview, I would recommend reading "Aerodynamics, Engines and Systems for the Professional Pilot" at Airlife. Very thorough - sometimes more in-depth than needed for JAR (it's written by a group of Oz pilots). Well worth the 30 quid or so.
Cheers
If you have an interview, I would recommend reading "Aerodynamics, Engines and Systems for the Professional Pilot" at Airlife. Very thorough - sometimes more in-depth than needed for JAR (it's written by a group of Oz pilots). Well worth the 30 quid or so.
Cheers
Last edited by FougaMagister; 28th May 2006 at 00:33.
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A few comments on CL (now, if I could write in sub- and super-scripts, wouldn't that be wonderful ..) ...
CL, and a bunch of other terms (eg Re, M, etc.) are derived from a math technique called "dimensional analysis" - a typical story is given here but, unless you fancy brain strain, I wouldn't worry too much about the detail.
This technique arose as a result of the empiricism of the Industrial Age when some bright folk figured that there had to be an easier, more compact way of presenting data for engineering use.
Looking at forces generated in fluid flow, one considers all the parameters which might reasonably be involved ..
(a) mass (density is more appropriate for fluids)
(b) some sort of size measure (a big sheet of ply is harder to hold in a breeze than a little sheet)
(c) airflow speed
(d) viscosity (sticky-ness of the fluid eg compare honey with water)
(e) compressibility
.. viscosity and compressibility reflect the fact that we want to deal with real fluids in the real world.
The technique uses an exponential form of expression and, for force, gives a result that force is proportional to
dynamic pressure * Re * M * a representative area
If we hide Re (relevant to low speed) and M (relevant to high speed) within a constant of proportionality (this doesn't mean they go away .. have a look at any of the text books which will show the effect on CL slope at higher M values .. just that it would be a routine and not very useful complication to have them in the expression for normal use) we might write
force = constant * dynamic pressure * area
or, in the more usually seen form
L = CL * (1/2*rho*V2) * S
and, similarly, we can run up an expression for drag forces.
The area, S, need not be wing area, but that is probably the most important shape (area) consideration so it is conventional so to choose.
The advantage of DA to we boring old engineering folk is that we can put a huge amount of data into a very simple form rather than having reams of empirical data looking at the relationship between one variable and another ..
It is held, generally, that all the meaningful dimensionless values have been discovered .. find yourself a new one and you, too, could get your name in text book lights ...
As an aside, be aware that this use of the term "dimensional analysis" tends to be the engineers' view of life .. other meanings may be seen, including the simple dimensional checks of equations to make sure that the dimensions all balance out ...
I've never seen the book referred to in this thread so I can have no view on its usefulness etc.
CL, and a bunch of other terms (eg Re, M, etc.) are derived from a math technique called "dimensional analysis" - a typical story is given here but, unless you fancy brain strain, I wouldn't worry too much about the detail.
This technique arose as a result of the empiricism of the Industrial Age when some bright folk figured that there had to be an easier, more compact way of presenting data for engineering use.
Looking at forces generated in fluid flow, one considers all the parameters which might reasonably be involved ..
(a) mass (density is more appropriate for fluids)
(b) some sort of size measure (a big sheet of ply is harder to hold in a breeze than a little sheet)
(c) airflow speed
(d) viscosity (sticky-ness of the fluid eg compare honey with water)
(e) compressibility
.. viscosity and compressibility reflect the fact that we want to deal with real fluids in the real world.
The technique uses an exponential form of expression and, for force, gives a result that force is proportional to
dynamic pressure * Re * M * a representative area
If we hide Re (relevant to low speed) and M (relevant to high speed) within a constant of proportionality (this doesn't mean they go away .. have a look at any of the text books which will show the effect on CL slope at higher M values .. just that it would be a routine and not very useful complication to have them in the expression for normal use) we might write
force = constant * dynamic pressure * area
or, in the more usually seen form
L = CL * (1/2*rho*V2) * S
and, similarly, we can run up an expression for drag forces.
The area, S, need not be wing area, but that is probably the most important shape (area) consideration so it is conventional so to choose.
The advantage of DA to we boring old engineering folk is that we can put a huge amount of data into a very simple form rather than having reams of empirical data looking at the relationship between one variable and another ..
It is held, generally, that all the meaningful dimensionless values have been discovered .. find yourself a new one and you, too, could get your name in text book lights ...
As an aside, be aware that this use of the term "dimensional analysis" tends to be the engineers' view of life .. other meanings may be seen, including the simple dimensional checks of equations to make sure that the dimensions all balance out ...
I've never seen the book referred to in this thread so I can have no view on its usefulness etc.
The area, S, need not be wing area,
When you consider that in flight Lift=Weight, then lift is the total reaction of all surfaces (E.G Tail and Fuselage) producing a vertical force in either direction.
In fact the wind area in isolation is probably going to have to produce MORE lift than the aircrafts weight to counter the down force of the tail.
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The point is that the S thing is just a reference area ... doesn't matter what area ... and depending on the actual size chosen, the CL values will fall out accordingly. Not really related to tails and wings as such ...
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In fact, it's not the real area of the wing anyway - most analysis uses some form of idealised wing area.
And often, when producing a derivative aircraft, it's simpler to keep the same reference area, even if the wing is physically increased in size.
And often, when producing a derivative aircraft, it's simpler to keep the same reference area, even if the wing is physically increased in size.
In light of the above two responses, is there any reason why S could not be assumed to be 1m^2 for all aircraft, thus making my original contention that the different lifting ability of different wings could be compared exclusivley by different values of CL?
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You can define it to be anything you want, so it's theoretically possible, but pretty pointless; the usefulness of the coefficients is in comparing the efficiency of different wing DESIGN FEATURES - by a more representative non-dimensionalisation, you can eliminate the influence of purely area changes and compare, say, the effect of sweep, or t/c, on the lift.
Knowing that a wing that is the same design but twice the size generates twice the lift isn't really insightful. Knowing that a change in t/c by 1% changed the CL by 3% for two DIFFERENT SIZED wings allows you to understand the influence of t/c on CL.
Knowing that a wing that is the same design but twice the size generates twice the lift isn't really insightful. Knowing that a change in t/c by 1% changed the CL by 3% for two DIFFERENT SIZED wings allows you to understand the influence of t/c on CL.
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A few comments on CL (now, if I could write in sub- and super-scripts, wouldn't that be wonderful ..) ...
Was it you I emailed about a year ago (or whenever) about getting TeX support for the Forum to make this alot easier?
I used to write my more mathematically inclined posts in TeX script, and various posters were/are conversant with it...
It should be possible for the administrators of the BB to get vBulletin to recognise [TEX] tags and voila.
My $0.02.
Wizo
barit- Absolutely not, that's why I resisted the idea that area should be part of the equation as twice the area does not mean twice the lift.
It does not mean twice the lift coefficient.
The point of C_L is exactly as you suggest - to compare different objects ability to produce lift.