Is JAA re-writing the theroy of flight?

[CathayBrat]

or am I just having a (not so) senior moment?
A friend of mine is doing his ATPL studies, using Bristol, and he just showed me the following question:-

Lift is generated when:

A: The flow direction of a certain mass of air is changed
B: A symmetrical aerofoil is placed in a high velocity airstream at zero angle of attack
C: A certain mass of air is retarded
D: A certain mass of air is accelerated in its flow direction

I have seen the answer JAA want, but I would not say it’s correct, and neither do the other aircrew I’ve asked. I know its jumping through hoops to do the exams, but surely they should be teaching the correct theory of flight!
All answers gratefully received.....

[Lafyar Cokov]

I believe there is still a big debate over the 'correct theory of flight' at the moment (well - since time memorial).. The change of flow direction/downwash camp seems to be in the ascendancy - although the benouli, longer path reduced pressure is still probably the majority. The only main problem with Benouli and the longer path/higher speed flow theory is that it should work for any shape - the most efficient aerofoil should be a semi-circular cross-section. This is obviously not the case so it points to a basic problem with the theory.

My personal opinion is that Bernouli was obviously pretty much correct, however there is a little more to it and downwash/change of flow must also play a part, as without change of flow direction, lift does not appear to work. The exams all point to this change of flow direction as being a critical part of it and that - I believe - is the answer they are expecting. There again - I just learnt the answer they wanted and passed the exam!! :)

[Lurking123]

Where is Mr Da Vinci when you need him?

For me, the sky sucks up and the ground sucks down. After that, it is all a question of balance. :ok:

[olliew]

Unless you're in a helicopter, in which case the ground repels due to the ugliness of the beast. Da Vinci never thought of that one!?

[Mikehotel152]

Your friend ought to show you the other 500 weird JAA questions and answers. For every multiple choice question, if there is an odd or partly correct answer amongst the four on offer, it's probably the JAA's preferred answer even if there's another, equally correct or even, perish the thought, entirely correct answer available.

Welcome to the world of JAA...;)

[Dick Whittingham]

Try this one:

How Airplanes Fly (http://www.allstar.fiu.edu/AERO/airflylvl3.htm)

Basically, to get lift you have to divert a mass of air downwards. Bernouilli illustrates the effect on pressure distribution caused by imparting downward momentum to the air by using airfoil shaped wings. It is a good, easy way to visualise a very complex reaction. It is not the complete truth.

There used to be an RAF question "what is the air doing after an aircraft has passed through?" The answer is " it is going down, because of lift, going forward because of drag and round and round because of the wing vortices"

But, I repeat, the essential element is that without a downward momentum shift of the airflow there can be no lift.

Dick

[gfunc]

Thanks for the link Dick,

That page nicely describes the basic physics behind lift with reference to the most basic laws. It's nice to know that wannabees are becoming more aware of the science behind flight, rather than relying on a 'magical' theory that only works until you actually start thinking about it.

I still find it funny that throughout my education at the beginning of every new step (i.e. GCSE, A-levels, Degrees) the first thing that you are told is that everything you learnt for the last step was kind of a lie or gross oversimplification!

I wonder if we reach some point in the future where the impact of the Bernoulli effect is played down in favour of real physics in the PPL texts?

Cheers,

Gareth.

[Peto]

Lafyar Cokov, thank you for your eloquent and succinct explanation. As a flying insructor who has delved in to lecturing theory as well, your summary has pretty much said all that needs to be said at pilot level. In years to come, people may well be laughing at us for believing either of these theorys, but one thing will remain constant. Pilots do not design the machines nor will we be able to answer such complex questions. Best to leave that to the poindexters (god bless them) that design and make the machines that we get to have so much fun in. I concur 100% with the premise that there is a little of each theory responsible for that magical force that propels average punters like me airborne. However it works, I like it.

[dream747]

What was the correct answer reckoned by JAA? Wouldn't D come closest to the Bernoulli theory?

[CathayBrat]

D would have been my choice, using Bernoulli, airflow over wing, lift created etc, but the JAA want A. Guess we will soon see aircraft with wings at 45 degrees, and all previous theroy of flight discredited so to fit in the JAA mould!

[Mordacai]

A makes most sense to me - I never did buy into the Bernoulli theory of lift. Nice and easy to explain, but not really correct :=

[dream747]

All the materials by schools seem to focus on the Bernoulli theory as the main contributor of lift so I'd reckon most people would choose D?

Wouldn't the Newton 3rd law theory be pretty practical and acceptable when it comes to flaps? The notes I have with me explain that flaps only serve to increase the mean camber of the wing but how does this effectively increaase the lift by so much at lower speeds? The curvature of the trailing edge does seem to provide a force on the airflow and change its direction downwards which in return provide lift according to Newton's 3rd law?

[gfunc]

Well I give up! Both A and D produce a force (mass x acceleration). D is the closest thing to the Bernoulli option, but even in Bernoulli theory the flow does change direction as it goes around the wing, so its not an exact fit. The key is "in its flow direction". I guess they are trying to trip up people who are think of the the text book diagram of the straight flow away from (above) the wing. Like Roy Walker says on Catchphrase: "It's good but its not right".

Answer A sort of fits the Bernoulli theory if you first discount D as above and obviously B and C. A is the non-magic answer and I would have picked this on the day (I'm not being smug after being told the answer, honest!), but I would have spent the rest of the exam worrying if the JAA partly line is based science or magic!

Cheers,

Gareth.

[BigGrecian]

I'm sorry but your all wrong! :ok:

Check out NASAs website - the page below is a link to why Bernouilli is wrong - explore for their version of lift - if you can understand it! :8
NASA Theory of flight (http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html)

[OneIn60rule]

B wrong since it's zero angle of attack.
C wrong obviously
D Is not clear enough. You need a bit more than just acceleration to get lift.


A Because although it does say anything regarding angle of attack, it does tell you that " The flow direction of a certain mass of air is changed".

Take a regular assymetric wing, get an airflow over it. The airflow after passing has changed direction.

This is a more "complicated" answer and it's not the first one of it's kind.

[Charlie Foxtrot India]

Hmm, I remember wondering how you can have a venturi effect with only half a nozzle and never really believed that theory explained it...Newton's third law does it for me. Pretty easy to demonstrate, otherwise aeroplanes wouldn't be able to move forwards and get airspeed in the first place!

Had a student once who had done a project at uni where they had to make a stop sign creat lift, and they succeeded. No venturi effect around a stop sign!

It does bug me when the theory is dumbed down, but hey as long as you can predict lift I guess it isn't vital to be as nerdy as me.

[dream747]

Coming to think of it, if only Bernoulli's theory hold true planes won't be able to fly inverted!

[David Horn]

Newton's 3rd law only accounts for about 5-10% of the lift. My understanding is that a wing does not "water ski" through the air... a point proved, in a fashion, by the way a non-symmetrical airfoil will produce lift at a negative angle of attack.

[gfunc]

I think you have to remember that the Bernoulli and Newton theories are not mutually exclusive. If my 'history of physics' class I dropped out of in my undergrad was correct (most boring class I've ever done!) Bernoulli was a fan of Newton and the Bernoulli theory was originally derived as an application of Newton's laws for a specific purpose. If you look at it, the Bernoulli theory does not break any of Newton's or any other physical laws (conservation of mass etc).

Bernoulli's theorum nicely describes and accurately predicts the flow velocity and pressure distrubution , because it is couched in Newtonian physics. The problem comes when assuming that a wing acts like a constriction in a pipe. More confusion comes about from someone at some point adding in the equal transit time bit into aviation textbooks. Maybe it was a simplification for understanding, but it is total bollocks!

David Horn: "Newton's 3rd law only accounts for about 5-10% of the lift." I'm not sure what you mean by this: The third law is every action has a equal and opposite reation. Perhaps you are thinking that in the case of a wing, the 'waterskiing effect' is the only contributer to the lift. This really isn't the case: The flow going up the leading edge and then down the 'back' of the wing is the main contributer. Any motion perperdicular the free flow will produce acceleration of air parcels and result in pressure perturbations (since air is a fluid) that can produce enough force for flight. Even at negative angles of attack the fluid going around the wing will produce accelartion of fluid perpendicular to the flow and thus a perturbation to the pressure field. If the pressure gradient on top and below the wind surface is enough, you can still fly.

There's a nice fluid mechanics website that shows this nicely, I'll post a link
when I find it again.

Cheers,

Gareth.


Edit. Here's a nice site: Airfoils and Airflow [Ch. 3 of See How It Flies] (http://www.av8n.com/how/htm/airfoils.html)

[civil aviation]

:ok: How refreshing to read this unusually polite exchange on theory/theories instead of the regular rubbish (e.g. free advertising scams, UKvUSA, anyone going to XXX?, questions which should be addressed to the FTO, do I have a better chance of employment if I go to ABC?, disgruntled dreamers, genuine nutters and tiresome tossers).
BTW, My problem is not how they work (or look) but just cannot comprehend why anyone rational would risk helicopters.:eek:

[poss]

If people are still debating the exact cause(s) of lift, is it an appropriate question to be asking in an exam? I'd say no. The possible answers that were put up are very vague and I'd probably have said D at first but now i'm undecided between A or D.

[.Aero]

Can Bernoulli's theorem describe how an asymmetrical wing produces lift upside down? ie inverted flight?

Can it describe lift generated by symmetrical wings found on acrobatic aircraft?

What about a flying plank of wood? (no joke)!

Other very academically credible areas to explore are Circulatory effects, and the Coanda effect. There's a big difference between creating POSITIVE lift and creating EFFICIENT lift.