Lift

I’m coming to the conclusion that the most important law in my aeroplane is Ohm’s Law! 😀

So you’re a virtual pilot ? 😀

Do you also get blue screens? :diablo: But at least, consider yourself lucky… It could be Murphy’s law!

As in “Ohm Igod, look how much I spent last month”

We usually just look at each other and say “ohm my god, what’s it doing now?”

can we add our own captions to that?

My ears are burning… 😀

Rather cool stuff you have gleaned from the guys at Bristol, Dean. I asked about momentum theory and was told to avoid for the purposes of JAA. Still good to know.

I’m coming to the conclusion that the most important law in my aeroplane is Ohm’s Law! 😀

As in “Ohm Igod, look how much I spent last month”

I’m coming to the conclusion that the most important law in my aeroplane is Ohm’s Law! 😀

Still, we have Blue Robin – a man with a [edited out].

can we add our own captions to that?

I had always assumed that there was someone way more clever than me who had this whole shooting match put to bed with the definitive answer but it looks like I’m wrong.

You are actually right. There is a whole army of intelligencia out there who are far more erudite than the congregation on this forum who know exactly everything (almost) there is to know about the aerodynamics of lift.

I wouldn’t hold my breath waiting for them to contribute here. Still, we have Blue Robin – a man with a [edited out].

Hi Ian, so true, this is a good read, from Ken who responded again to my post I made, Ken is a truely wonderful and knowledgable guy, when he talks you tend to listen, that’s the impact he has, he teaches PoF & MET and is the best out there

Hi Dean

I have had a look at the Pprune forum and read some of the posts. There is nothing new there, except that some of the posts appear to think that all the lift stops at Bernoulli

There is nothing wrong with Bernoulli, as long as you don’t think that it is the only reason for lift, as I said in my previous post.

Yes, some books do take the simplistic explanation and do not go into detail, but that does not trash the whole concept, it is just not complete.

I suppose you could compare it to electricity. Do you really know exactly how a current flows through a wire? As long as you have the basics you can apply it to systems (instruments, radio, etc) The exact science is better left to electrical engineers.

A lot of the posts are trashing the questions on lift in various exam systems, (FAA and JAA) but the real blame should be laid at the multiple choice system for testing knowledge.

In the old system you would have written me an essay, and I would be able to see from your explanation if you understood the principle.

With multiple choice it is virtually impossible to set one question, that can test a whole learning objective. Provided there are sufficient questions, in a question bank, that tests different parts of the learning objective, and they collectively ensure that you understand the concept, then that’s the best the multiple choice system can offer.

A question on Pprune from the FAA question bank, refers :

Which statement relates to Bernoulli’s principle

(a) For every action there is an equal and opposite reaction
(b) An additional upward force is generated as the lower surface of the wing deflects air downwards
(c) Air travelling faster over the curved upper surface of an airfoil causes a lower pressure on the top surface

This question is trashed because in the writers opinion there is no correct answer because “lift is not a function of the curvature”

The question did not say anything about lift. It said what relates to Bernoulli’s principle. If it wanted to test more about lift it would have had to specify cambered or symmetrical and angle of attack.

It was just testing a fraction of the learning objective of lift.

I could go on and on. The point as I made in the first post, is that the production of lift is extremely complex. It is not required that you know it all.

Bernoulli definitely applies, as does the momentum theory. At different phases of flight the relationship of which one is producing the biggest part of the lift changes.

If you look as NASA’s “Incorrect theory #1” the third bullet point after the Foilsim says Bernoulli in that context is correct.

The same applies in NASA’s “Incorrect theory #2” the fourth bullet point and the last paragraph says that part of the momentum theory is also correct.

If you look at a flat plate. At zero degrees angle of attack it will produce no lift, but if you give it a positive angle of attack then the deflection of the airflow (momentum theory) will provide some lift. We can’t say that Bernoulli will play no part, because there is no curvature, but it will be a very small part in this case. (There will be some acceleration of the airflow over the top surface because the airflow has changed direction) However you would need a lot of thrust to fly the flat plate, because the airflow separation would be well forward on the plate, hence high drag.

If we look at a symmetrical aerofoil, now there is some curvature, but Bernoulli will do nothing at zero degrees angle of attack as the top and bottom surface pressure changes will cancel each other out. At a positive angle of attack you will have some momentum theory lift and a lot more Bernoulli than the flat plate.

With the cambered aerofoil the extra curvature means Bernoulli gets even more prominence.

So you see each aerofoil shape will make different use of the different factors that produce lift, and at different angles of attack that relationship will also change.

Hope that helps a bit more.

This has been a really fascinating thread from the perspective that it highlights the fact that after more than 100 years since the Wright brothers events at Kittyhawk we still cannot define lift generation! I had always assumed that there was someone way more clever than me who had this whole shooting match put to bed with the definitive answer but it looks like I’m wrong.

Ian

Thanks for the info, a nice insight into the workings of the 340, appreciated.

I posted in my ATPL provider’s forum regarding this subject, and this is what 2 instructors had to say, don’t know what one does but the other has thousands of hrs in the military and flying 727s in SA.

A mathematically and physically rigorous description of the way lift is generated would be extremely complex. This is why it is only now that airfoil behaviour can be accurately predicted from computer calculations. Until recently the best you could do was to make a wing and then test it.
Nevertheless, there are several ways of looking at the problem; all are incomplete, most offer some guidance. The most complete is the momentum theory, called the Physical Theory in your reference. This says that lift is generated by imparting a downward velocity to the air, and for more lift you either have to push a lot of air down or push a little air down a lot faster. Nothing wrong with that.
A flat plate will do this if given an angle of attack, but it is not very efficient. It relies almost entirely on air being forced down by the bottom surface of the plate. Cambered or symmetrical airfoils work better. Why?
This is because the airflow, up to a critical point, will follow the downward curve of the rear section of the airfoil, giving extra downward velocity to the air. It does so because the boundary layer drag slows the layer closest to the airfoil and the higher speed free stream air turns downward over the boundary layer.
So the on the bottom surface air is pushing the wing up and on the top surface air is pulling the wing up. Bernoulli reminds you that the effective reduction of cross section of the streamlines will increase the speed of the air going over the top and therefore drop the pressure, further increasing lift.
If the boundary layer separates you lose some of these lifting effects, but not all. Remember, at the stall lift does not disappear; it only stops increasing with increasing angle of attack.
In an attempt to give a simple explanation of what actually results from these complex interacting effects most gurus chose to use a Bernoulli based theory – the one you are familiar with. The actual behaviour we describe, the stall, induced drag and so on are actual empirical observations of what happens. Bernoulli is just there to make you feel happy. To my mind the standard teaching does not clearly explain why wing tip vortices exist, and why they increase in strength at low speed in level flight and with increasing lift in manoeuvre. But that is for another time. Just remember that if there is no downwash there is no lift, and that after an aircraft has flown through a block of air it will be going down because of lift, going forward because of drag and revolving because of the vortices

Dick W

And the other instructor

Hi Deano

Sorry I have been on leave for a week as we had a break between courses.
I have not had a chance to read Pprune yet, as I have a back log of 140+ e-mails to answer first, the joys of some leave, so I will add to the post later when I have read it.
For the moment I will try to simplify Dick’s explanation a bit more.
There are a number of reasons why an aerofoil produces lift. As Dick said Bernoulli is the easiest, and then secondly the momentum theory. The others get very complex and thankfully are not required by the learning objectives for ATPL.
In normal flight (small angles of attack) 80% to 85% (depending on the design – symmetrical or cambered) of the lift is produced by Bernoulli, so a lot of the learning objectives, and exam questions, are based on that.
The second greatest part of lift is caused by the momentum theory (changing the direction of the airflow) and a few exam questions touch on the basics of this.
As you increase angle of attack the ratio changes, and by critical alpha (the stall) Bernoulli has a marked decrease in the amount of lift produced because of the separation of the airflow.
The increase in drag, because of the flow separation, means most aircraft can’t fly much past critical alpha.
However if you have sufficient brute force (thrust), like the Mig’s and SU’s that do the Cobra manouvre, then the momentum theory will give sufficient lift to maintain level flight for a while.

Ken

…Ian you fly the 340 for Virgin so maybe you can clarify, generally aircraft have their CP aft of the CofG, which creates stability, and this stability causes a nose down pitching moment, this is why the tailplane has a downward force & negative angle of incidence which forces the nose up and level again, but the Airbus has it the opposite way? i.e. the CP is forward of the CofG, which means that the tailplane must cause an upward force to cause a nose down pitching moment to counteract the nose up tendancy (our ATPL instructor told us Airbus manufacture in this way), but one thing I have noticed with the Bus is that the tailplane appears to have the same angle of incidence as Boeings etc, if this is the case what causes the pitch up moment? surely the downwash theory must hold true for the tailplane as well? because if the first link I gave is to be believed, Bernoulli’s theory would suggest that there would not be enough lift provided by the tailplane airfoil?
(of course if my instructor was talking out of his proverbial disregard the above)

If the cg to cp layout is different in the bus compared to normal aircraft well that’s news to me! Below approx FL250 our tailplane works conventionally but above that a whole new set of ball games kick in. At higher levels a pair of FCMCs (fuel control and monitoring computers) pump 6.5 tonnes of fuel from the center and 4 inner fuel tanks into a tank in the tailplane called the trim tank. What it is trying to do is to stick the c of p right over the c of g. This will unload the tailplane so that it creates no upforce or downforce. If the tailplane is not producing any lift then it also producung minimum drag. The stability issue is resolved for us by the fly by wire. The FCMCs then shift fuel forwards and backwards throughout the flight to maintain the c of g to within 0.5% MAC of its optimum position. Within certain parameters before landing the fuel comes forwards again so the trim tank is empty on arrival. To give you an idea of how important this is I recently did a return flight from Hong Kong to Heathrow with an unserviceable transfer pump so we could not use the system. The recalculated plan upped the fuel burn by 4 tonnes to 114 tonnes compared to a serviceable system (approx 3.5% increase)! The A330, 340 and 380 are the only airliners utilising this capability today.
When you multiply those savings by the number of sectors our aircraft fly every year the savings run into millions (and fwiw we spend roughly a million sterling a day on fuel!).

That’s all lovely, but some of my RC planes just have a straight plank wing with no shape, they rely on grunt and angle of attack to make lift.

Moggy, I assume by the colour of the mud on the foot that the crabbing was taking place in Walberswick?

Melvcock your theory is the most viable theory by far, I’ll go with that! 😀

Nice therories everybody, but THIS is how list is really produced:

http://leocat.free.fr/shadok/generalites/images/pomp.gif

Lift is the ultimate result of a large amount of money thrown at an aeroplane.

I hope this helps.

These days, eager young ATPL students are far too busy to notice such things. No good will come of it.

Bloody ‘ell I wish I was young 😀 I guess in relative terms I may be 😉

Ahhh…. there was a time on the internet when people looked at a smiley like this 😉 and understood when a comment was tongue in cheek.

These days, eager young ATPL students are far too busy to notice such things. No good will come of it.

Damn, you mean we are supposed to have a sense of humour as well? sorry no brain capacity left for that 😉

Nobody is trying to impress anyone Trinny, this is a misconception on your part

Ahhh…. there was a time on the internet when people looked at a smiley like this 😉 and understood when a comment was tongue in cheek.

These days, eager young ATPL students are far too busy to notice such things. No good will come of it.

I just thought of another point Ian, (the spin thread prompted me) when approaching the stall at high alpha the airflow separates further & further up the wing as alpha increases, moving the CP forward, if you imagine a wing at high alpha with the separation point, let’s say for argument’s sake, ½ way up the wing chord, this will, in relation to the wing be separating and moving “upwards” away from the wing even though to the relative airflow it will be near horizontal, so in effect you have no downwash from the top of the wing, all you have is the relative airflow striking the underside of the wing, whilst all this is happening the aircraft stays airbourne, and I would have thought going on what I read in the first link if you have no downwash, you have no lift, but maximum lift is achieved at or just before critical alpha.
I start my CPL tomorrow with the CFI of my training establishment, I’ll put this argument to him to see what he “prefers”

Well I think Bernoulli’s theory must hold some water, even if it is only a small percentage, it does provide some lift, however small, the first link is indeed excellent and uses Newton’s theories very well, I think the thing with Bernoulli is the theory is relatively simple and easy to explain, probably why it is inherant within the training structure especially at PPL level, Ian you fly the 340 for Virgin so maybe you can clarify, generally aircraft have their CP aft of the CofG, which creates stability, and this stability causes a nose down pitching moment, this is why the tailplane has a downward force & negative angle of incidence which forces the nose up and level again, but the Airbus has it the opposite way? i.e. the CP is forward of the CofG, which means that the tailplane must cause an upward force to cause a nose down pitching moment to counteract the nose up tendancy (our ATPL instructor told us Airbus manufacture in this way), but one thing I have noticed with the Bus is that the tailplane appears to have the same angle of incidence as Boeings etc, if this is the case what causes the pitch up moment? surely the downwash theory must hold true for the tailplane as well? because if the first link I gave is to be believed, Bernoulli’s theory would suggest that there would not be enough lift provided by the tailplane airfoil?
(of course if my instructor was talking out of his proverbial disregard the above 🙂 )
If the downwash theory was conclusive, I wonder why member states (particularly JAR & the FAA) have not embraced it and use it in ATPL theory, or do you think it is a case of saying “ahh, the lift theory we have been using all these years is wrong, we need to use this now” ?, does this point to the fact that the downwash theory is not conclusive enough? or does it point to the fact that member states have been teaching the wrong theory all these years?