pplm exam principles of flight confusion

[Drav]

Hello, I have been doing some mock exams on the Microisland website

Aircraft Techinal (http://www.islandmicrolightclub.com/education/exams/aircraft_technical.htm)

And here, the answer to how much lift is generated from the upper surface of a wing, is opposite to what I have read in my textbook. As I understand it, two thirds of lift is generated from the lower relative air pressure on the upper surface of the wing, and one third from the airflow being deflected downwards by the lower surface of the wing.

The screengrab below however, has the answer different:

http://i.imgur.com/u3nJlwJ.jpg


So, am i missing something, or is the exam wrong?


Cheers

[Mach Jump]

There is a never ending argument about how a wing produces lift.

The conventional wisdom though, is that 2/3 of the lift is produced by the smooth airflow over the upper surface, and 1/3 by the downward deflection of the air by the lower surface.

In reality, the two effects cannot be treated entirely seperately, as they each affect the other.


MJ:ok:

[Genghis the Engineer]

If you have any reasonable understanding of the principles of flight, then you come to realise that all of the exam syllabi are gross and often innacurate simplifications.

However, the objective in the short term is to pass the exams - and it is true that if you replaced the wing section with just the upper surface you'd lose about 1/3 of the lift. So, the answer works - but is still grossly simplistic and pointless.

Learn to pass the exam, then get a decent book on PofF like Kermode or Anderson, and learn it properly!

Daft, but the way to do it.

G

[soay]

<i>"principles of flight confusion"</i>

That should be covered in the Human Performance and Limitations syllabus.

Sorry, I'll get my coat on the way out ....

[xrayalpha]

The "trial" exam is wrong.

For this sort of exam question, the expected answer is:

The upper surface of the wing generates around 2/3rds of the total lift

Usually put that way, rather than "the lower surface of the wing produces about 1/3rd of the lift", since the idea is that in a stall the wing does not lose all its lift-generating ability.

You can always tell the smart student, they read this and ask: but what if the aircraft is flying inverted!

[Shaggy Sheep Driver]

You can always tell the smart student, they read this and ask: but what if the aircraft is flying inverted!

Or at zero 'G'. Or at negative 'G'. Shows why these simplistic explanations are such tosh.

Read 'Stick & Rudder'. Everything you need to know about why an aeroplane flys (and sometimes doesn't) is in there.

[Genghis the Engineer]

Read 'Stick & Rudder'

Oh come on, a great book, but there's stuff vastly more up to date and at-least as well written. Teaching PofF from Stick and Rudder, is like teaching aerodynamics from Glauert - you can do it, but there are vastly more up to date texts.

This is my current favourite:-

Introduction to Flight (Int'l Ed): Amazon.co.uk: John D. Anderson: 9780071086059: Books

G

[Shaggy Sheep Driver]

At £40 a copy I don't think I'll be reading that any time soon. Stick & Rudder is certainly dated, but I haven't come across another book that cuts through the crap quite as well as it does and tells a pilot what a pilot needs to know.

Should I start saving?

[Genghis the Engineer]

You spend how much a year on flying?

Or for something only 26 years old instead of 70 years old, you can have an earlier edition of Anderson for just over a fiver...

http://www.amazon.co.uk/Introduction-Flight-Engineering-Mcgraw-Hill-Aeronautical/dp/0070016410/ref=sr_1_3?ie=UTF8&qid=1423825676&sr=8-3&keywords=anderson+introduction+flight

G

[ChickenHouse]

As usual in all these exam questions - go have a look which software is used in final real exam and then learn the pushbutton answers. Afterwards switch on again your brain ...

[Shaggy Sheep Driver]

Genghis, there's a copy of 'Stick' on my bookshelf. It's far and away better than anything I've read before on 'pilot's aerodynamics'. Why do you think I'd need to supplant it?

I've no idea if those you've recommended are as good (the only thing 'Stick' falls down on is it's dated presentation). Since 'Stick', and not those other books, continues to be recommended, and it 'does it for me', it's the one I will stand behind as a recommendation. I'm not in the book review business so won't be buying those others to see if they're as good; I don't need to as I know 'Stick' does it just fine.

If you think the others are as good, by all means recommend them. But please recognise as well just how good 'Stick' is for someone who wants to fly aeroplanes, not design them.

[Crash one]

One thing EASA have not yet been able to screw up are the laws of physics. Stick & rudder is still current.

[Genghis the Engineer]

I really do hope that we've learn a few things about aeronautics since 1944 however.

Newtonian physics and incompressible aerodynamics haven't changed yet, but our understanding of human factors, use of instruments, navigation, meteorology - certainly have.

G

[Drav]

Cheers guys, I have Brian cosgroves book call the microlight pilots handbook. Good to know the test answer is wrong in this case. Ive also come across afew other wrong questions from this site, so it might be an idea to avoid these online tests for now....


And to all those trashing the simplistic explanation of flight, well, we all have to start somewhere, and i think a simplistic explanation works for me now, much like a lot of GCSE chemistry and physics is not actually correct, and then retaught in a more complex (and contradicory) manner at A level.....

[Shaggy Sheep Driver]

Newtonian physics and incompressible aerodynamics haven't changed yet, but our understanding of human factors, use of instruments, navigation, meteorology - certainly have.

Um, 'Stick' doesn't address those other subjects so your comment is irrelevant. It's purely about pilots' aerodynamics. And it does it very well, perhaps because it doesn't dilute itself by addressing too wide a spectrum of subjects.

[Heston]

The exam question that the OP refers to is ridiculous. Definitely one just to learn the answer for the exam and then forget about.

Genghis and Shaggy - you're discussing different things. "Stick" is no good for learning about the science of aerodynamics, but is brilliant at getting across enough info for a pilot to understand what s/he needs. You don't need to understand the detail of how a car works in order to drive one safely - but you do need to know a little bit about how it works: flying is the same.

Neither "Stick" nor a good aerodynamics text book will help answer the OP's question though.

No one has mentioned Bernoulli yet...

[Genghis the Engineer]

Daniel Bernoulli was a Dutch born mathematician (1700–1782), although he spent most of his working life in Switzerland. He studied mathematics and medicine, becoming professor of mathematics at St Petersburg in 1725.

In 1732 he became professor of anatomy at Basel University, continuing to become a professor of botany and finally physics. He worked on many areas including trigonometry, mechanics, vibrations and fluid mechanics—including anticipating the kinetic theory of gasses. His solution of a problem of gas properties became known as Bernoulli’s equation and was published in 1738.

Didn't write a single book on flying - ignore him.

G

[Heston]

Agreed! :)

[Mach Jump]

Halfway through Stick and Rudder now. I thought I should read it, as some people rave so much about it.

I'll let you know what I think when I've finished it, but so far, if you can look past the rather dated style and language, it seems to explain things in terms that 'ordinary' people can understand, without falling into the trap of 'dumbing down' to a point where what it says is actually wrong.

From what I've read so far, I think you're a little harsh, Genghis. I think even Kermode, and Anderson seem a little highbrow to many.

Let's see how the other half goes.


MJ:ok:

[hoodie]

...but there are vastly more up to date texts.

This is my current favourite:-

Introduction to Flight (Int'l Ed): Amazon.co.uk: John D. Anderson: 9780071086059: Books (http://www.amazon.co.uk/gp/product/0071086056?*Version*=1&*entries*=0)


From 1978. Just sayin'. ;)

SSD, an earlier edition is available as a PDF (http://ae.sharif.edu/~iae/Download/Introduction%20to%20flight.pdf).

[n5296s]

I'd be very curious to see a wing without an upper surface, ghat could be used to test this. Maybe it's full of magnetic monopoles?

[Ka6crpe]

So how about symmetrical wings?


That is a very poor question that does not really test a student's understanding of the theory of lift.

[Genghis the Engineer]

I'd be very curious to see a wing without an upper surface, ghat could be used to test this. Maybe it's full of magnetic monopoles?

Many 1970s era ultralights or hang-gliders.

I've flown one or two, and can verify that they do fly - just not usually very well.

Search online for "Chotia Weedhopper" for a good example.

G

[Crash one]

n5296s I'd be very curious to see a wing without an upper surface, ghat could be used to test this. Maybe it's full of magnetic monopoles?


I did that experiment as a kid in ~1950 building models. Covering the top surface only, it flew more or less normally, cover the bottom surface only and it would not lift anything like as good. Level glide compared to 30deg dive.

[Baikonour]

Lots of aerofoils used at the dawn of flight were 'concave' - the Wright flyer, Bleriot, Nieuport and, to a lesser degree, later ones too, such as the Fokker Dr 1 (Triplane) and Sopwith Camel.

However, even if you just had a single 'sheet' curved aerofoil, it would still have a top and a bottom.

Magnetic monopoles made from an Unobtanium/Eludium wishalloy might fit the bill :)

The whole concept of 'which part of the wing makes the lift' is bunkum. You could make a 'lifting surface' out of a rotating cylinder...

;^)

B.

[Crash one]

However, even if you just had a single 'sheet' curved aerofoil, it would still have a top and a bottom.

This is more a question of a curved aerofoil compared to a flat one, as most aerofoils have a flatter underside than top. Even concave ones.
I don't think it's bunkum. Try stripping the fabric off the top of one side and the bottom of the other and see which way the thing will roll, even with a concave underside.
A rotating cylinder yes, non rotating, like most wings, no. Different principle, slightly.

[Baikonour]

It's the dumbing down I object to. It is annoying to see questions which try to make the whole subject 'easy' with pat answers when reality is that it is actually a complicated subject.

The closest thing to a right answer would be 'it depends on AoA and actual aerofoil shape'.

Here's another one which you never see but which would maybe be far more interesting to discuss:
Which part of the aerofoil section creates most drag?

:^)

B.

[paco]

As usual, EASA questions only go halfway, or less. To understand how lift works, we must understand how static pressure works.

The weight of the air inside a column that is 1 foot square at sea level is 2116.16 lbs (on a standard day). This pressure surrounds the aircraft from above and below and all around. That is, the aircraft is being squeezed from all directions at a static pressure of around 2000 lbs per square foot. If you can reduce the pressure above its aerofoils by more than the weight of the aircraft, it will fly, which is what we do mechanically, by moving forward to concentrate the airflow over the top of the wing and bring its streamlines closer together, because we cannot affect static pressure directly.

Instead, we mess with the dynamic pressure over the upper surface of the wings, especially in the first quarter, which makes the changes we need. This is often discussed with reference to Bernoulli’s Theorem, whose involvement is b*llocks anyway for this discussion (except for altering the pressures), partly because it assumes a closed system, and no friction but, mainly, the theorem only applies if no energy is imparted to the system - a propeller is an actuator!

The aircraft is not sucked up into the air, as forces generally do not pull. Atmospheric pressure from underneath pushes the aircraft up. There is enough upwards pressure on a 10 foot square ceiling to support a 737. No air movement would be necessary were it not for the need to reduce the pressure on the upper surface.

Of course, we also start with the flat plate to deflect air downwards, with some shaping to help with the turbulence and to create the lower pressure, and a down force also comes from wingtip vortices (NASA).

An inverted aeroplane will still fly under the above conditions, of course, but with a much higher angle of attack.

Hopefully, that's a less gross simplification, Genghis! :)

[mikehallam]

I started reading the on line book above referred to, page 181 with pressure below wing exceeding pressure on top wing shows agreement with the last poster' text !
[I can't post the JPeg directly]

mike hallam

[n5296s]

Many 1970s era ultralights or hang-gliders.
Sorry, I should have maybe used some appropriate emoticon. My point is that any wing has an upper surface, even if it's just the top of the lower surface (hence the remark about monopoles). So the question is completely meaningless. It seems to be a barely-disguisued variation on whether it's the pressure difference or the downwash that generates lift. Answer: since it's physically impossible to generate one without also generating the other (using an aerofoil), the question doesn't make any sense.

Of course if you take a normal wing and simply strip the top covering, it will make an extremely BAD aerofoil, and will not generate lift very efficiently. But it will still have a (rather bumpy) top surface. But as others have said, a single sheet of material, correctly shaped and braced, makes a reasonable aerofoil.