By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by CML

What makes models zoom

Is it to do with wing section, or not

All Topics | Latest Posts

Search for:  in Thread Title in  
John Cole08/02/2009 10:59:05
615 forum posts
24 photos
JW: you do the Wright Brothers an injustice.  They built a small wind-tunnel to test the efficiency of their wing sections, and much of their work with gliders was to test those results at nearer full-scale before using them on their manned Flyers.  They initially used the results of Lilienthal, taken from (essentially) tethered gliders, but limitations of his work caused them to build the wind tunnel.
Gemma Jane08/02/2009 12:14:14
avatar
1349 forum posts
53 photos
Lilenthal copied the sections of birds John, so the injustice is slight as the Wright Brothers simply took this onboard and carried out crude experiments.
 
This is simply getting overly complicated, I'm not going to resort to Wiki for an explanation after eight years of formal study of a subject.
 
 
Quite simply put:
 
Can one visualise why a flat bottom aerofoil would display 'speed sensitivity', 'pitch sensitivity' and observed ballooning, well yes one can.
 
Can one back up this visualisation with empirical data from a reliable source such as NASA/Langley, well yes one can.
 
In the context of models what conclusions might be drawn?
 
Would I personally choose a flat bottom aerofoil in a model design? No I would not.
 
Would I build a model from a kit/plan with a flat bottom aerofoil? - perhaps yet I know it will display some undersirable traits and I may look to change the section or look to another model.
 
As this thread is now getting near the top of a google search on flat bottom aerofoils and models, I think a little less wool is required, it is quite obvious that there are raging debates that run in model circles based on little more than a very basic knowledge. Aerodynamics is not a subject that takes kindly to such behaviour.
 
For the modellers out there wondering is there anything in this, this makes an interesting read:
 
 
As for the debate regarding naming such as "semi-symmetrical sections", it is quite clear that it is a body of terms used in modelling that has no technical basis but is clearly understood by many. I think we can live with it!
 
 
Myron Beaumont08/02/2009 12:35:59
avatar
5797 forum posts
51 photos
Don't know why I got out my soap box ?No answer to me " came the stern reply "  Having spent some time at RR Aero-engine Div. as an engineer .I'm not particularly impressed by formulae  etc -I'm used to them would you believe !.Especially where compressor & turbine blade profiles ( sections) are concerned .All I know is the practical aspects of model aerofoil sections & if your model " zooms" or "balloons " then it''s being flown beyond its design parameters In other words -Too much power = too high a climb rate on anything but a symetrical or "semi-symetrical"  wing section 
John
I see from your brief profile your are quote "retired" . So am I
Have you ever flown anything I ask myself?
A bit like Gemma ,I've flown full size including twin engined Islanders ,Have been a life -time modeller-built & flown my own buildhang-gliders( Rogallo & Sunspot )
Not impressed is all I can say
Steps back off "soapbox"  Temporarily I fear !
John Cole08/02/2009 12:45:24
615 forum posts
24 photos
Gemma: you say the Moment of a cambered aerofoil produces a nose-up pitch.
 
I say the Moment Coefficient about the AC is nose down.  I gave a Wikipedia link for other readers to see, and an academic  book-reference which supports it. 
 
If you mean that I am wrong an you are right then only one of us can be correct.
 
The link you just pointed to correctly states that the CofP moves forward as alpha increases.  That fits exactly what I said when saying that CofP is a somewhat-strained term: that as alpha DECREASES  the CofP moves backwards, and in fact goes behind the wing TE at low alpha. That's why I prefer the use of CM, but my preference has nothing to do with whether the moment is nose-down or nose-up.
 
The AC is defined as the point where the Cm is invariant with alpha.  So if the Cof P moves forward so it is 50% closer to the AC then by definition the lift has doubled (and the moment stays the same).  But it (almost) always remains BEHIND the AC, giving a nose-down couple.
 
If you think I am wrong and you are right, then explain why.  Cite a reference for positive Cm.
Tim Mackey08/02/2009 15:06:51
avatar
20920 forum posts
304 photos
15 articles
I am watching this thread by the way.....
Gemma Jane08/02/2009 21:12:28
avatar
1349 forum posts
53 photos

Timbo, don't be concerned it isn't worth my time.

Terry Whiting09/02/2009 08:54:29
154 forum posts
And I second that Gemma.
Tim Mackey09/02/2009 09:06:54
avatar
20920 forum posts
304 photos
15 articles
Being concerned and vigilant is what I am paid all these millions for  
Evan Pimm11/02/2009 09:42:19
146 forum posts
Terry Whiting, Yes, I know the term 'semi-symmetric' is locked in modelling yore, but it is a nonsense term, something is either symmetric, or it ain't, and if accurate exchange of information is the goal, then we shouldn't use nonsense terms, specially with single interest subjects. As for the camber line, you will note that I said it separates equal areas above and below the line itself, to do that then, it must be equal distance between the top and bottom of the section at any point on the section. Depending on the shape of the section then, the camber line will either be straight or curved, ie the section will be symmetric (non-cambered) or cambered. Even that old term 'undercambered' shouldn't really be used, it is simply a cambered section where the bottom curve of the section has gone past the 'flat bottom' bit and, because of the amount of camber and the thickness of the original streamline, is now curved to follow the top. But I am only an old grouch and no matter what I may say, it won't make any diff at all, but it is fun to natter away about it all...
Evan.
Gemma Jane11/02/2009 12:22:55
avatar
1349 forum posts
53 photos
With respect Evan you best sort out your own definitions before you correct others as your definiton of 'camber line' (I pressume it is "mean camber line" you are referring to but you are not specific) is technically incorrect and a nonsense in itself. Nice try though.
 
Perhaps before you reply you will understand that an infinite number of lines could divide the upper and lower parts into equal areas and best not call them the top and bottom of the section as that is defined by the chord line not the mean camber line....you see Evan one must be terribly specific before one is able to correct other people.
00112/02/2009 08:40:58
2212 forum posts
1 photos
After yesterdays political episode I would be very wary of quoting Wikipedia in connection with anything technical or numerical!
 
Without wishing to add gasoline to the conflagration. IF there can be semi symmetrical lenses and semi symmetrical graphs, can there not be semi symmetrical airfoil sections?
Lima Hotel Foxtrot12/02/2009 11:11:54
avatar
391 forum posts
This is all fascinating stuff and it's nice to see so many passionate views from modellers.
 
However, at the risk of derailing this thread even further from it's original intent, can people please post pictures (or maybe a link or two) of what they are describing in words? Frankly, unless it's in a diagram I have extreme trouble deciphering what you mean! I'm sure I'm not alone!
Gemma Jane13/02/2009 13:34:42
avatar
1349 forum posts
53 photos
Hi Lima Hotel Foxtrot,
 
There is a good explanation including pictures of the principles that are being aired here:
 
 
4.1 Illustrates some of the terms that have been used such as mean camber, camber etc
 
4.9 Covers the principle of using the aerodynamic centre as the basis of stability analysis
 
Wait for John to say, 'Oh but it says there Gemma....'
 
Martin Harris13/02/2009 14:55:55
avatar
9337 forum posts
249 photos
 Quote: "I know the term 'semi-symmetric' is locked in modelling yore, but it is a nonsense term, something is either symmetric, or it ain't, and if accurate exchange of information is the goal, then we shouldn't use nonsense terms, specially with single interest subjects."
Perhaps we should rename semi-symmetric as nearly-symmetric?
Gemma Jane13/02/2009 15:13:32
avatar
1349 forum posts
53 photos
I don't see the problem with what it is called Martin.
 
In context with the above 'discussion' nothing in aerodynamics is fact or explanation.
 
It really is nothing more than a collection of design tools based on what understandings we have of the subject. It allows engineers to design aircraft.
 
In the same way the term semi-symmetrical allows modellers to choose an aerofoil with particular qualities they desire within their limits of understanding. There are so few sections in common use in modelling you could call them whatever you wanted just as long as other people know what is meant.
 
This is John's sticking point with his argument. He looks for explanation in something that is merely a design contrivance, as if it were some sort of fact when it is only really intended to allow comparisons and data to be collected. Hence why I couldn't be bothered to argue it any further when he continued to argue the same hot air whilst ignoring everything I had acutally said. Of course he will obtain that result, because aerodynamicists came up with the analysis to make it so!
 
It is perfectly normal for such 'theory' to not provide explanation of observed events, sort of aerodynamics business as usual. Hence why my own analysis looked to the movement of the centre of pressure. I can take my moments from the LE, the TE the AC or the moon if I want to, I'm not constrained to only one view of the situation when looking for a possible physical explanation of  what appears to be a readily observable phenomenon.
Martin Harris13/02/2009 16:22:31
avatar
9337 forum posts
249 photos
"I don't see the problem with what it is called Martin."
 
Neither do I but it seems to be a major problem to some of the thread contributors!
 
I don't know how accurate it was, but it was once an often quoted  "well known fact" that according to aerodynamic principles, a bumble bee was incapable of flight.  A perfect example of your last point!
Stephen Grigg13/02/2009 16:36:51
avatar
8691 forum posts
1128 photos
When I first read the opening statement I thought Zoom was the noise some models make when they zoom past,but then you start talking about ballooning and im lost .Then along comes Gemma , Phooee mate you lost me and I think you know what your talking about,so gemma why does a piece of paper fly ,and how can a bumble bee,when theoretically it cant,but can
Gemma Jane13/02/2009 17:13:54
avatar
1349 forum posts
53 photos
Stephen I would be rich if I could explain either in simple terms.
 
Though most people will argue it for a month of Sundays, aerodynamics doesn't even explain why aeroplanes fly. To understand this one needs to know the historical origin and the motivation of the farther of aerodynamics namely Ludwig Prandtl. Aerodynamics as a subject is little more than a mathematical tool box that allows aeroplanes to be designed, it has so many contradictions and 'untruths' the actual explanation is never found, Prandtl never sort to provide an explanation, he simply wanted to understand the criteria that would allow better aircraft to be designed. Most people simply will not comprehend the fact and argue that in fact we know it all, though any student or lecturer in aerodynamics will put them straight on the subject.
 
Regardless though you deserve some answers, the bumble bee issue is due to very low Reynolds Numbers, we don't know so much in that area so trying to apply the theories that work for say full size aircraft or an average size model simply don't apply to a bumble bee, hence there is truth in the fact that the usual principles do not apply. It can't be explained with theory, but obviously it does actually work.
 
As for the paper plane, 'flat plate lifting theory' gives us an insight into why a flat plate with an angle of attack generates lift, it is also one of the theories that NASA employ to discount the Bernoulli principle as an explanation of lift generation, even though it is the most commonly cited explanation given in basic books on flight, the Bernoulli principle simply does not offer any real explanation of lift generation.
 
 
Stephen Grigg13/02/2009 17:18:56
avatar
8691 forum posts
1128 photos
Thankyou Gemma clear as mud to me ,wouldnt want to be in an argument with you to clever,the wings on the TSR2 look quite small,
Myron Beaumont14/02/2009 07:39:42
avatar
5797 forum posts
51 photos
Just to start up more controversy !   There's no such thing as a true flat-bottomed airofoil either ! It's just what we have always called Clark Y and the like .
Well explained Gemma -I agree with you on everything you've explained so clearly
Myron

All Topics | Latest Posts

Please login to post a reply.

Latest Forum Posts
Support Our Partners
CML
Sussex Model Centre
Slec
electricwingman 2017
Sarik
Advertise With Us
Has home isolation prompted you to start trad' building?
Q: The effects of Coronavirus

 Yes - for the first time
 Yes - but Ive bashed balsa before
 No - Ive existing projects on the bench
 No - Im strictly an ARTF person

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us

Magazine Locator

Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!

Find RCM&E!