Here is a list of all the postings David Burton 2 has made in our forums. Click on a thread name to jump to the thread.
|Thread: What Goes Up|
John - did you sort out the download of Foil Sim? If not and you need some help then PM me and I try to help you get hold of it.
Tony - loved the videos - the second one in particular is very good showing as it does the increased lift being demanded as the turn tightens and of course the ineveitable stall when the critical AoA is reached.
you are of course correct. The classical entry into a spin would indeed be via a tip stall. So I suppose its a case of "one man's meat is another man's poison"! If you deliberately stall the aeroplane and then push in rudder in the direction in which the first wing stalled (because 99 times out of a hundred one wing will stall before the other) then its a spin. If on the other hand you didn't intend to do it - then its just a tip stall.
There is I think one major advantage in your position though - if you recognise that "death by tip stall" is really a spin then provided you know, and have practiced, how to recover from deliberate spin then you know the correct recovery procedure for a tip stall as well!
I'm very sorry about the delay in getting back to you all - its been a busy period! I'll try to deal with questions and comments now in a series of posts.
Tom, your question about the flat plate is a very interesting one and not at all a trivial question. You'd think that flat plate aerodynamics would well boxed off, and to an extent at least it is, but only with rather theoretical solutions using a technique known as "potential flow" theory. They tell us that a flat plate wing will produce lift and interestingly suggest that the rate of increase of CL with respect to alpha would actually be higher than is the case for many well know aerodynamic aerofoil sections!
But to be honest practical information on flat plate lifting surfaces is thin on the ground - as John says above. There is the NASA report he refers to and there are a couple of papers by some German scientists in the 1920's but that's about it! So we are in pretty unknown territory here. What is certain is that John is correct in his view - you could make a cardboard box fly if you give it enough power!
You mention that you believe your wings are operating at low Re values - I think that they may not be as low as you suspect! Your wing is 24" chord - that's 0.6m - if your model is flying slowly - let's say 20mph, or roughly 10m/s, the Re value across your wing is 400,000! Only a little below the critical value for flow across a flat surface of 500,000. But is it below at that speed - so all things being equal your boundary layer flow should be laminar. IF it was that might explain the relative insensitivity to pitch you observe.
But - as you rightly say - the square cut off leading edge is very likely to act a turbulator - especially given that fact that you are quite close to the critical Re value anyway - so on balance I would be surprised if the actual boundary layer flow over your flat plate wing is not turbulent. I say I'd be surprised - but I must be honest I do not know!
I'm planning to do a few experiments in the wind tunnel this summer (quite time of year in Universities!) on flat plates as lifting surfaces - just for bit of fun. It seems to me to be an area few people have really looked at in any depth. I'm hoping I'll get some data I can use in a second series to talk about the aerodynamics of shock fliers. I'll let you know how I get on.
So you see, as I said, not a trivial question at all! Keep on experimenting. Real experimental data is king in this field - its not a question of "are your observations correct, can they be aligned with the theory?" Its the other way round! If practical experience says that happens then its the theory's job to try to explain it and if it can't then the fault is with the theory! Maybe if I do those tests in the summer and get some quantifiable measured data of lift on flat plates I might have some better answers for you!
really glad you are enjoying the articles and finding them useful.
I certainly wouldn't feel too bad about realising that stalling might be at the root of the issue with the Merlin - we've all been there! Every R/C pilot has stalled his/her share of models - but it wouldn't it be nice if we could all recoginise the causes, see the danger signals before they develop too much and hence start to avoid a few of these prangs!
Here's hoping eh!
LOL! Oppps! Now there's one that escaped the proof reading! Well spotted John.
It should of course read "shedding"! Although now you mention it shredding them might be an effective strategy.
I will be sending some videos into David for this in the next day or two, so watch out for them.
re your observations - I agree, that's why I didn't think it was hysteresis.
The "S" shape curve into the instability region was quite repeatable - happening in most cases if not all. I don't find the second part of this "S" so surprising - when the angle is below |0.2| as the lift nears the instability. If it didn't do this there would be a risk of the curve having an actual first order discontinuity - which would be very unlikely indeed. But the second part of the "S" - the "hump" around 0.3 degrees is something we considered at the time but, to be honest, can't really explain - even with a hypothesis. If it only happened on one side of the plot I would be tempted to dismiss it an effect of a manufacturing error on one side of the section. But its there both sides of zero alpha - and I've no measurement evidence from the section of a systematic error that occurs on both sides. Indeed, as I stated above, measurement of the section's form indicated a complete absence of systematic form error - only random errors.
Well, one thing about research I have found out: do an experiment and you get some answers - but you also invariably get more questions! Its a sort of technical job creation scheme for researchers! I think I'll just have to put the "hump" down to that for now!
Sorry John, you're quite right 0.006 not 0.06! Tired eyes!
The balance actually did have a very fast response because of the fibre optic strain gauge. The data logging was at 2MHz sampling rate - which is slower that the frequency response of the fibre optic system. So we could get very good temporal resolution. I did have some graphics of this, Cl vs time - not sure I can find them - I'll take a look.
The basic effect was that the aerofoil would not oscillate a constant speed between the two values - oscillate is not really the right word. I know it is the one I used - but in tetrospect it does not describe accurately what we observed. The lift force would sit at one value of Cl, stable for a short period, and then flip to the other, again for a short period before flipping back again. The period for which it was stable would be anything from approx 0.1sec to 1-2sec. Without any decernable pattern. And this behaviour was the same for all alpha values between about -0.07 and +0.07 degress.
sorry I haven't been back for little while, but to respond to some of these points:
John B: I'll have to go off memory as I am away from the office at the moment, but if I recall correctly the aerofoil section was about 17% thickness ratio and its chord would have have been of the order of 35-40cm. Air speed, again from memory, about 30m/s or thereabouts. So Re approx 7x10**5.
One day I will get round to the flat plate experiment - I need to convince an MSc student it would make a good project! And then dig out the fibre optic lift balance kit again!
John C: no the data is not end corrected.
It could be a hysteresis effect (let's face it, it could be a lot of things!) but I doubt it. The reason for that is the instability the flow exhibits at this point. Anywhere in that range the flow can continually "flip" between plus and minus about 0.06. So it isn't just that we "only get these values and no zero" its the ocsillation between them at a fixed (albeit very small) alpha which is so distinctive. and unusual.
The first lift curve we got is shown below:
Nothing too surprising there. The points even fit a straight line very well, and the line pretty well goes through zero. So far everthing is as expected.
Then I tried at smaller and smaller angles. The result of this is probably best summarised by the lift curve below:
Between 0.4 degrees and 0.1 the section is behaving more or less as we would expect. But once we got below, somewhere around 0.07-0.05 degrees the Cl value started to become constant - at approximately 0.06-0.07. This was the case on both sides of the zero point. The two data points at angle zero are in reality the mean of a series of readings around that point - plotting them all confuses the graph.
So, in this range close to zero the Cl value plateaus - remaining almost constant at a value we might call Clcrit. If you start at say minus 0.05 degrees and steady increase the angle to plus 0.05 what happens is that the Cl value remains around minus 0.06-0.07 and then suddenly switches to plus 0.06-0.07. It prooved impossible - despite many attempts to get any stable reading between these two points. And no amount of careful adjustment would produce a "zero-lift" condition.
So I think this is the answer to your question - at small angles, less than about minus 0.05 degrees, the lift curve runs almost horizontal at minus Clcrit, nominally at zero it switches to plus Clcrit and it runs at this value until about plus 0.05 degrees.
The cost of producing the wing section meant I was not able to repeat this experiment for other symmetrical sections - but I have no reason to believe this result is untypical in any way.
I hope this is helpful.
Good question. About three years ago I developed an interest in what happens to symmetrical aerofoils at very small angles of attack. I had notice that very few published lift curves actually indicated that the zero alpha position had actually been measured. There were measurements at, say plus minus 1 degree, but at the zero point the line was simply drawn through the origin - as you might expect without an actual experimental point. But I wanted to know what happened at very small angles - less than half a degree.
I would need a very accurate symmetrical wing section to do this. So I designed a symmetrical aerofoil using "Solid Works" (a professional 3D design software suite suite) This meant that, at least in the computer, the section was perfectly symmetrical because I simply designed one half and mirrored it.
To make the section - I had it machined from a single block of Araldite using a Bridgeport 5 axes CNC Machining Centre. (An advantage of working in a University - we have lots of specialist kit around!) Using this means that the section is going to be about as accurate as I could possibly get it.
To check it was accurate I measured it using a optical 3D measurement system we had developed - it showed that the section was accurate to the computer model, at all points, to better than 50microns (i.e. 1/20 of a millimetre). It also showed that such errors as there where were random - there was no structured error leading to unintentional camber.
So I now had a "perfect" symmetrical section aerofoil. The next problem was that the force balance and the section rotational stages fitted to our wind tunnel would not be accurate enough for what I wanted to do. So I used a micrometer screw rotational head to control the position of the section and a new fibre optic strain sensor based on a Bragg grating system that had been developed by the research of a close colleague to measure the force. The important thing about this force sensor was that it had a very high sensitivity and responses almost flat to signals from static all the way to 2MHz - very fast.
With this I set about obtaining a lift curve. The micrometer screw limited the angles I could use to only approximately plus minus one and half degress - very small but that didn't matter - as I was only interested in small angles.
Continued in the next post.......
Thanks for your comment.
For the figures you quote I totally agree, but I must admit to taking somthing closer to the max and min, rather than the means!
For a cold winter's day here I took 0 degrees C. Not that untypical at the moment - in fact a week ago I was flying at the patch and it was -4!
For a warm summer's day I took 30 degrees C. OK, a little optimistic maybe - but I have known it to reach that here! Not very often - but it does happen and we can all dream!
At 0 degrees air density is 1.292Kg/m3
While at 30 degrees air density is 1.1644Kg/m3
A difference of 9.88%
In the original draft of the article I did in fact cite the temperatures - but in the inevitable editing for length that detail was one of the casulaities! Anway thanks for your point.
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!
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"!
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.
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
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!
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.
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!
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.
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!
Edited By David Burton 2 on 22/11/2010 11:31:49
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