What Goes Up

[David Burton 2]

Hi Everyone,

 

I am the author of the article "What Goes Up - Part 1" in the December issue of the mag.This mini-series is intended to offer a non-mathematical introduction to aerodynmics for model flyers. As part of the "package" we are offering;

 

1. some accompanying videos with notes each month - this month''s videos can be found here

 

2. an email address to which you can send any comments, questions or suggestions (legal and decent suggestions only please!). This address is [email protected]

 

3. and finally this forum thread on which I will try to comment on and answer the those emails that seem likely to be of general interest.

 

Hope you find this useful.

 

David

[David Burton 2]

OK let's start with an issue that has been raised by a couple of folks in emails to me.

 

Some people have suggested that the "longer path leads to faster flow" explaination of where lift comes from is not correct.

 

Well in one sense they are right - but its not really a problem! Let me explain.

 

The article set out to make two points very clearly;

 

1. That lift is really a pressure difference either side of a wing

 

2. That the pressure difference comes about because of the Bernoulli Effect and is due to the air travelling faster over the top of the wing than on the bottom.

 

Now these two are indisputable facts - borne out by literally thousands of experiments carried out in wind tunnels every year. But what is less clear is "why does the air flow faster over the top of the wing?" This is where the debate lays.

 

To be honest, there isn't an easy answer to this. One answer is to say:

 

"The air separates at the leading edge then exactly meets up again at the trailing edge. Because the path over the top is longer the air has to speed up"

 

The trouble with this is, while its simple to understand, it isn't strictly speaking true! Take another look at the fourth video in the forum support article linked to at the top of this thread. As I point out in the notes to this video, it can be clearly seen that the air along the top actually goes so fast its beats its opposite number going along the bottom to the trailing edge - and this is generally true. So, I used this agrument because its easy to understand - but it is limited in applicability.

 

So I added a second explaination: I talked about "what would happen if the air didn't speed up?" And then I showed that would lead to a vacuum, which would pull the air along quicker. Now this is nearer to the true situation - and is basically right. But its still not perfect.

 

So how could I have explained it better? I am afraid that the simple answer is that, without recourse to some very heavy mathematics indeed, I can't! Its impossible to describe purely in words - especially in less than 2000 words of plain English - exactly why the air speeds up. I can give you a feeling for it - a concept that will give you a picture in your mind - but I can't give you a definitively correct explaination without the mathematics.

 

Does this matter? Well I would say - no. To have a practical understanding of wings you don't need to understand why the air speeds up - you just need to know that it does and that this speeding up causes a pressure drop which in turn makes the lift.

 

Edited By David Burton 2 on 20/11/2010 18:08:59

[Tim Hooper]

Hello David!

 

I just wanted to report that I read your article over breakfast this morning, and found it to be both enjoyable and informative, so thank you for your efforts!

Given that I'm strictly a meat-and-two-veg modeller, with virtually no understanding of what makes an aeroplane fly, combined with an arithmetically-challenged mind, I think your article was pitched exactly at the right level.

tim

[David Burton 2]

Thank you Tim, I'm really pleased you enjoyed the article and found it interesting. Praise indeed from a modeller of your standard, I have long admired your designs!

 

Moving on, I've had more emails - one theme emrging is "very good - but you didn't mention 'x' or 'y' which is very important".

 

First of all, thanks for these and you're all absolutely right! But as I am sure you appreciate we have to start somewhere! Also as each article has to be reasonably "self contained" its not always going to be possible to include all the "if's and but's" and finer points within a single article. But, rest assured the Ed has given the green light to follow up articles and many of your comments are already in hand and planned for future inclusion - whilst some others have indeed given me new ideas for stuff that could be covered in the future and would be likely to be of interest to modellers.

 

One topic that does seem to be of particular interest is the stall. One correspondant observes, and I agree with him, that second only to "pilot error" stalling is the biggest model killer - much more than radio failure or inteferance. It maybe a surprising, but it seems to be the case that many modellers don't always recoginise a stall when it happens to them - often assigning to the event some other explanation. So definately food for thought there - but before we can really discuss the stall in all its 'beauty' we do need to know a bit more aerodynamics - so if we can cover it we will but it will have to come a little later in the scheme of things.

 

Talking of things coming later, going by the emails there is also a lot of interest in the symmetrical wing profile and how it produces lift. I'm afraid the bad news is it wont be in part two - sadly there just wasn't the space. Part two does cover inverted flight and factors that effect lift - as promised, but within the limits of one article is was impossible to cover these and also do justice to symmetrical wing sections. That's the bad news. The good news is it will definately be in part 3 which I am working on at present. So, sorry about the delay, we will get to it, but I will have to ask you to be a little patient!

 

David

Edited By David Burton 2 on 22/11/2010 11:31:49

[Steve Hargreaves - Moderator]

David, 'tis a brave man indeed who sticks his head above the parapet!!!

 

So well done for being that man....!!!

[Martin Harris - Moderator]

I've yet to read the article so this may have been covered and if so, please accept my apologies but modern thinking seems to be inclined towards lift being generated by the opposite reaction to air being deflected downwards - which goes a long way to explaining why a flat plate wing can fly - and is usually known as the Newtonian explanation or similar.

 

Opinions seem to range from Bernoulli explains all to Newton rules...

 

Personally, and for no deep scientific reason than common sense and empirical judgement I'm quite convinced that they both have a part to play.

 

Of course, if you accept the Newtonian theory the need to explain symetrical wing theory you refer to doesn't exist...although Bernoulli also has a part to play as I'm sure you'll agree.

 

As Steve says, good luck - and if you start explaining stalling off a downwind turn you'll need a bunker!

[Tim Hooper]

Posted by Martin Harris on 22/11/2010 12:38:01: 

As Steve says, good luck - and if you start explaining stalling off a downwind turn you'll need a bunker!

 ......not to mention a conveyor belt........

 

tim

[David Burton 2]

Martin,

 

you are absolutely right - there are indeed several approaches to the lift problem and they are not necessarily mutually exclusive.

 

For students studying aerodynamics the approach used is basically the Newtonian one. We use Newton's equations of motion with conservation mass, and conservation of momentum/energy to derive the flow around a simple geometry such as an infinitely long circular cylinder. We then use the something like the Kutta-Joukowski transform to obtain the flow around an aerofoil.

 

Another way is to see that the flow around an aerfoil is really a straight flow plus a circulation. This is more abstract - but still arrives at the same point.

 

The third approach is the classical Bernoulli explanation which I use in the article.

 

Some people see these as mutually exclusive, competing, explainations - but I don't. In the final analysis the Bernoulli equation is derived from a similar stand point as the Newtonian approach, just with a different "getting off" point. So the same physics underpins both. So the two are not incompatable at all - they are just two ways of describing the same thing.

 

The advantage of the Bernoulli method, as I mention above, is that its relatively easy to understand - and most importantly in this case it can be understood conceptually, without any mathematics. Its disadvantage is that to a small extent it "begs the question" because it fails to account for why the air speeds up. But as I say above I think for our use - where we want a picture in our mind about how lift works rather than a rigourous scientific understanding - its perfectly adequete.

 

But what does excite some aerodynamists is when the "Equal transit time" argument is applied as the explaination of the air going faster in the Bernoulli formulation. Now they are right, while equal transit time is a convient explanation its not strickly speaking scientificall (or observationally) sound. The error the objectors to the Bernoulli method sometimes make is to say that the whole Bernoulli formulation is wrong because it depends on the Equal Transit time falacy. This is not true. We can use Bernoulii without equal transit times and its a fine - if slightly incomplete - explanation. Bernoulli is sound - it just doesn't explain everything!

 

So your "gut feeling" Martin that both Newton and Bernoulli have a role to play is one I would completely agree with.

 

David

[Phil Russo]

Brilliant article David!

 

Pitched just right (for me anyway) and clarified many fuzzy thoughts i had about aerodynamics. Skillful piece of writing. I felt like i was in conversation with you rather than having a lecture. Great links to videos which made my interest soar!  I even got into other fasinating bits of physics. Can't wait to devour next months article.

 

Definately gonna watch my angle of attack more critically and play around with different aerofoils.

 

I'm sure you will cover this, but any clues as to why a flat shockie type, depron wing  (or balsa for that matter) gets it's lift?  Is it just light weight and air resistance that keeps simple flat wing models afloat?

 

Inspirational editing RCM&E (Graham) - I love your mag and always look forward to the 'How to' & 'Why?' features. Keep it comin'.

 

Phil

[Phil Russo]

Wow! I didn't read the previous post by Martin.

 

I suppose this can become a very complex discussion, which is gonna leave me floundering in it's wake!

 

Simple explanations which point one in the right direction is enough for a simple hobby RC pilot like myself.

 

Phil

[David Burton 2]

Thanks for that Phil, very glad you enjoyed it and found it useful.

 

And don't worry if the conversation on here sometimes get a bit "heavy", we are totally commited to making all the articles themselves fully readable and understandable by all rc pilots! And if you have any questions don't hesistate to ask - there is no such thing as a daft question - only daft answers!

 

David

[John Bunting]

David, I think these ideas are sometimes a bit counter-intuitive. I remember -  when I was much younger -  being surprised by Bernoulli's idea, because I felt instinctively that if air flowed through a constriction in a pipe, it would be compressed, so the pressure would increase. Also, if you swung a piece of cardboard round at arms length, with its front edge tilted up a bit, you felt it lifting - obviously, I thought, due to the pressure on the underside!  But the pressure distribution in airfoil test results usually shows that the decrease of pressure over the top surface is considerably larger than the increase on the bottom; (is this also true for a flat plate airfoil?). And when you hold a convex surface against water coming from a tap: it gets drawn towards the stream, although there is no flow at all on its other side.

Could you please comment further on your bit in the article about symmetrical airfoils? You say, "when aligned with the oncoming airstream the air travels the same distance above and below yet we know they generate lift in this condition, but how?"  If by "aligned with the oncoming airstream" you mean that the datum or centre line of the wing is at 0 degrees angle of attack to the airflow - although it might be rigged at some positive angle of incidence relative to the datum of the whole aircraft -  then I don't see how it can give any lift. Surely for a truly symmetrical section, zero lift occurs at zero angle of attack.

[David Burton 2]

Hello John,

 

Thanks for your post. Several interesting points there. Let's talk about the compressibility of the air first.

 

Your point is a good one, why doesn't the air just compress? Well the fact is it does! But at the sort of speeds we are talking about compressibility is a very small effect indeed, so small we can ignore it. The forces imposed on the air by wings such as ours at the speeds typical of our models are too small to cause any significant amount of compression and so other factors are dominant. In fact it is not until a wing starts to approach the speed of sound (say at Mach numbers in excess of 0.6-0.7) that compressibility effects become large enough to need be taken into consideration. In classical aerodynamics (at small Mach numbers) we consider the air to be incompressable. So, in some ways, its like the wing is actually flying through a liquid rather than a gas. This approach was fine until later on in WWII when some aircraft started to approach the sort of Mach numbers at which compressibility becomes significant - then we needed a serious extention to the theory! But of course for models we are lucky - we don't need to worry about compressibility too much. Even the fastest models we fly are way below the point at which compressibility needs to be taken into account.

 

Your point raises an important issue for me. Its very easy for me to forget to point out these assumptions and just think everyone knows this! So thanks for reminding me.

 

I think you are right about some of these ideas being "counter intuitive". But hopefully we can present them in such a way that people can understand and form a conceptual model in their minds which is much closer to what is actually happening to a wing in flight.

 

The symmetrical wing section issue has certianly got folks thinking - that is of course a good thing. But please forgive me if I don't go into detail on it here - as that would rather "steal my thunder" for Part 3 when we will be dicussing that matter in detail. One thing I will say by way of a "hint" - it is related to the difference in the definitions of angle of incidence and angle of attack - but not in the way I suspect you are thinking! Hopefully that's tantilised you and whetted your appetite for part 3!

 

Thanks again for your interest.

 

David

[Stephen Grigg]

Thoroughly enjoyed the article,like the way you wrote it ,and  learnt from it.Did also begin to understand how a Formula One car works,..Im looking forward to Part 2 because I understood your point in Part One,Thankyou.

[David Burton 2]

Hi Stephen,

 

thanks for the positive feedback - always nice to know that we're pitching it right.

 

If you are interested in Formula One cars I hope you will enjoy Part 2 - when we take a brief look at inverted flight and downforce vs lift, among other things!

 

David

[Stephen Grigg]

Yes that will be interesting Dave,looking forward to that.Over the year Ive been starting to look at the shape of wings  knowing that fat ones are better than thin ones and now I know why

[Simon Chaddock]

David

Nice videos and interesting text. 

 

However just a point about the Airbus 380 video in part 2. Surely that was the prototype test of the high angle of attack take off, hence the temporary red tail skid structure under the fuselage. It was all intentional.

 

The Berkut has a remarkable performance but if you have the power and control a plane can continue to fly well beyond the conventional stall. Note the computers working over time with the big canards in slow flight.

 

Of course the early delta wing aircraft wowed air show audiences with unheard of angles of attack and they did it without the benefit of computer aided stability control.

[Ross Clarkson]

Posted by David Burton 2 on 20/11/2010 17:19:21:

Hi Everyone,

 

I am the author of the article "What Goes Up - Part 1" in the December issue of the mag.This mini-series is intended to offer a non-mathematical introduction to aerodynmics for model flyers. As part of the "package" we are offering;

 

1. some accompanying videos with notes each month - this month''s videos can be found here

 

2. an email address to which you can send any comments, questions or suggestions (legal and decent suggestions only please!). This address is [email protected]

 

3. and finally this forum thread on which I will try to comment on and answer the those emails that seem likely to be of general interest.

 

Hope you find this useful.

 

David

 

Great stuff David, i really enjoyed it and think it is an amazing idea as i know some people flying models are not aware of the basic principles, and to be honest they don't need to be, but it does help so thumbs up to you.

 

I just hope most people realise this is as you describe and a non-mathematical introduction series to aide people who may not be aware, and don't pick holes in it!! That would be a shame.

 

Well done.

 

Ross.

Edited By Ross Clarkson on 20/12/2010 13:03:53

[Myron Beaumont]

David

First of all congratulations on your Thread . Just found it and a lot comes to mind about various theories & hypotheses ' As someone who (years ago RR Derby) was involved with turbine blades some of which had to be thick enough to incorporate cooling air flow due to the high temps involved & compressor blade technology and their various "aerofoils"I am not averse to you putting out explanations of what is what & including mathematics for explaining what is happening . There are a few of us on here that I'm sure can understand a few formulae etc .

Keep it up   Regards Myron

[oliver gibbs]

David,

 

 A very good read on a subject that is difficult to understand (with my little brain anyway) and even more difficult observe in practice. One mental hang-up I have is tendency to think of moving air as only having the ability to blow onto things; positive pressure. I find it difficult to create a mental image of negative pressures, sucking at things.

  I was a mere lad during the “great storm” in the mid 1980’s. The wind came in off the North Sea and hit the roof of our house at 90deg head on, the positive pressure as the wind hit the tiles was huge. Then my dad pointed out the real problem was not the windward side being blow away; but the leeward side being sucked away. Although there was no wind flow under the roof (thank God) our house now possessed a rudimentary “wing”: the roof.

  Indeed, on the leeward side, as the wind passed over the apex of the roof, the negative pressure was sucking the roof tiles upward. I could not believe my eyes, all of the roof tiles were banging up and down. After about an hour of this the wind direction changed and it hit the roof at an angle. The roof tiles started to calm down as the patch of negative suck moved away to the new leeward part of the house roof : the gable end wall. The wall had never been tied in correctly so with one big blow, the gable end wall was sucked out and dumped onto the garden below. Being a lad, I thought this all great fun.

Cheers Olly.

[David Burton 2]

Hi,

 

thanks everyone for the kind comments - much appreciated. I'm glad you are finding the articles interesting and useful.

 

Simon - I very much suspect you are right that the video of the A380 is a test of some sort - the fact that the aircraft is in airbus livery and was fitted with a tail protector is, as you say. a bit of a give away. I'm not sure what type of test though, it could a high alpha take off test or (the one I suspect) a max take-off load plus test. Either way its an exciting demo of the effects of high angle of attack!

 

Ross, thanks for your comments. Glad you are enjoying it. Different folks are at different stages in their understanding of this stuff. Sometimes those who are a lot further along the track can forget that they too had to start learning somewhere! They can also get a bit impatient to get at the more detailed stuff. But we should all be able to move along together given a bit of understanding!

 

Myron, gas turbine aerofoils are, as you will know, a highly specialised field of aerodynamics in their own right. The various tricks employed - especially in engines designed to operate at supersonic flying speeds are amazing! As we work through we may well steadily introduce the odd design equation. If you think about it all of part two is in fact about one equation - the so-called lift equation - but I quite cunningly avoid ever writting it out in symbols! Its there in words, and all the terms are there - just not the symbols!

 

Oliver, your comments on the "big blow" are very interesting. People often don't realise the method by which the air can damage their property. Your Dad was quite right - its like the video I showed in the part one set of how to blow a coin into a glass - it by blowing over the top of it not at it that the trick is done. Same with your roof tiles but on a bigger scale!

 

Anyway - thanks again for the feedback everyone. I'm just putting the finishing touches to part three at the moment - meaty stuff all about vortex flow and the wing!

 

If there is any topic within aerodynamics you would particularly like to see covered just mention it here - or PM me, or drop an email to the address given at the start of this thread

[Mark Pritchard]

David,

Nice videos, but please read up on "V" speeds. The A380 was actually undergoing Vmu (Velocity Minimum Unstick) tests which is to confirm the minimum speed at which the aircraft can take off, hence the temporary tail skid (painted red) to avoid damaging the tail.

Rgds,

Mark 

[David Burton 2]

Hi Mark,

 

thanks for the comment. I am familar with the Vu test. That the 380 was undergoing a test I agree is fairly obvious - from both the Airbus paint scheme and the presence of the tailskid. But what makes you so sure it was minimum unstick speed test? I mean it might have been  - I'm not saying it isn't - but there are any number of tests it could have been! Including the one I speculated on - i.e. max takeoff load test. Who knows, unless they were actually there? If you were, and know this specific incident, I freely accept that obviously.

 

The point of the video really, as I'm sure you appreciate, is simply to demonstrate the effect of increasing alpha - but equally the need to rapidly reduce it on climb out in this particular situation!

 

Glad you enjoyed the videos.

 

David

[Phil W]

Hi, yes nice videos indeed. I can confirm that the 380 video is indeed one of the Vmu tests which took place at Istres sometime in July 2005. It is a certification requirement that the calculated Vmu must be demonstrated to be within +1/- 0 KIAS of the verified Vmu over a range of weights before the aircraft is allowed into service.

 

Anyhow, keep up the good work, looking forward to the 3rd instalment.....

 

Cheers

 

Phil

[David Burton 2]

Hi Phil,

 

thanks for that clarification much appeciated. As I say I was just interested in a demonstration of the effect of high alpha - and its certainly that! With little or no information on the You Tube thread one is left guessing at the nature of the test. So thanks again!

 

David

 

PS I don't think Graham will be too cross if I let out of the bag that in part 3 we are going to be looking at the "bound vortex"!