WOW, this is a good one!
George Hicks did a whole thing of why a flat plate airfoil is best compromise for this area of flight IE very low RE#'s and wing loads for 3D flight not RC training, after he designed the Tribute, i forget where it was posted though, if you don't know George Hicks he is a aero engineer for Gulf-stream and an excellent pilot part of Team JR.
And as far as designing a better airfoil for these models, sure it could probably be done, but i bet it resembles a flat plate, like thin and blunt. I would love for somebody to compile a list of airfoils that harrier well for 46 size planes

That's just peanuts, I know a better one:
He is called Walter Pix and can throw stones like a king, all they fly like a dream, hands off, do 3D, but best of all they all land on a spot.

I know I shouldn't let it, but this whole flat plate thing grates on me. Each of us needs a neurosis, I guess. I think two issues are being confused: thin and flat. It is definitely true that as the Reynold's number gets smaller, the airfoil section should get thinner. Now, if you take some 'standard' airfoil section and make it thinner, it will start to look more and more like a flat plate, at least from a distance, but there are important differences. With no exceptions that I can think of, performance will be enhanced if the leading edge is rounded (perhaps with a fairly small radius, if one wants to stall on command for some maneuvers), and the trailing edge is tapered. For a particular plane, it may well be that the effort of modifying the leading and trailing edges this way is more trouble than is justified for the performance gain, but that doesn't mean the flat plate is better. The only real superiority that I can think of for the flat plate is that it is much easier to get it truly symmetric than a traditional airfoil section. Strength is NOT an advantage of a flat plate. For the same construction technique, strength depends very strongly on the thickness, and very little on the shape of the section. In fact, the strength to weight ratio would almost certainly be better for a traditional airfoil section of the same thickness.

If you value the simplicity of a flat plate wing, and find the performance to be good, then you should use it happily. If you think that the flat plate performs better, I think you are mistaken. With any luck, this little outburst will get it out of my system, and I can lurk quietly for a while.

banktoturn

Don't know why it should bother you -
I aways felt that the wing shape/foil etc., should be whatever is best for a particular application.
THIS application fits perfectly.
We have sanded the edges round - just to se what effects there are and honestly I cannot find any differences -other than time spent.
Your point of getting things accurate - is a real pet subject for me
Years ago I abandoned the constnt curve in airfoils for models
Why?
One simply can not get left and right panels exactly the same - So I opted for a smooth LE -which blends into the high point of the foil then blends to a straight line to the TE.
These are easy to buildaccurately and tho it may sem clumsy to you - It really should not as they work extremely well .
There are a hell of a lot of international /national / regional trophies awarded on models using that setup .
I know George Hicks and he is in my opinion , very good at his theory.

Hi Dick,
I'm not surprised to see you here ruffling feathers. The funny thing is that Dick provides truth in small portions that are for some reason hard for many to swallow. I only wish I could imitate your brevity of thought so I'll try:

Thin flat plates (no camber) with a sharp disturbance at the leading edge are the best low Reynolds number sections for aerobatics because they possess a linear response in angle of attack over a larger Reynolds number range (speed). Thickness simply causes aerodynamic problems on small models...if you don't believe me build a hand launch glider with a thick section.

Dick would be proud...No theory here...just measured data.


banktoturn..personally I despise rounded leading edges on true low Re sections (60,000 and below) because you don't get full transition which is what makes these things really work. It's not like you're going to support much of a pressure distribution on a section like this and for the foamies that's not the point...make it linear and get your CLmax from a source that capable of providing it...the propeller.

As an aerodynamicist it's sacrilege to make such statements but unfortunately it's true. Dick's theory for years has been pretty much don't ask much out of your wing and you won't be disappointed. When I first met Dick I was against his philosophy because I thought that we could do better...in the end I realized that his airplanes flew good because they were linear, not because CLmax or L/D is high. Aerobatics doesn't benefit from these sort of things...that's when I shifted my focus to Stability and Control.

Airfoil design for aerobatics is what I call "piddling about a point"...fun to do but pretty much worthless. My design criteria is to make the airfoil have as linear a lift curve (especially around +/-2 degrees) as possible over as wide a Re range as possibly. Then you have to start talking to you Structures guy because he's bloke who demands that the airfoil be thick.

It's funny because when you start working with models and using with the square-cube law of scaling you quickly realize that the aerodynamics simply can't keep up.

Once again I've said too much...

George Hicks
Misguided Aerodynamicist and Theory Monger


BTW, Dick are you going to Joe Nall for the Team JR meeting?

Yes you seem to be misguided, Dick starts talking about planes that weights zero - yes ZERO telling us that -of course - CG doesn't matter then…
And if the plane is too heavy it will not fly at all and - of course- CG doesn't matter then either and the problem is resolved… very much impressive.[8D]

Then he says that he likes flying flat plates of styrofoam because they stall and do exciting flick rolls whereas thicker airfoils refuse to stall and are boring for him - so what? Who has denied that?
I think we are just talking on different things, different flying styles and needs.

Adam,
I must admit that I only read page 4 of the discussion. I'll go back and read the rest. Knowing Dick I'm sure it will be thought provoking.

As for zero weight airplanes and CG doesn't matter...I suppose if an airplane had no mass or infinite mass his statements are true. At times I think Dick was the one who invented Calculus because he's always taking everything to the "limit". We call the zero weight scenario the "trivial" solution but a solution nonetheless. Even the mighty Dick Hanson won't fly his foamie with the CG aft of the maneuver point.

Come to think of it discussions of weight and CG have absolutely no place in a "pure" aerodynamics forum. These are stability and control topics. Wouldn't that be terrible if we had to test every possible CG position seperately during wind tunnel testing...you'd never get done.

George

"Aerodynamics are not a function of weight and CG"

George,

You seem to start out by lumping thinness and flatness together again. A section does not need to be flat (i.e. have constant thickness) in order to be thin. Nor does it have to be flat to have a 'disturbance' at or near the leading edge.

I'm not sure what you mean by 'full transition'. If you mean transition to turbulence, that can certainly be arranged, even with a rounded leading edge. Certainly, a thin wing can't generate the kind of CLmax that a thick one can, but if one is building very light planes, a high CLmax shouldn't be needed. If you rely on thrust rather than lift, though, I suppose it makes more sense to call those flat things fins than wings.

Your point about not asking too much of the wing is well taken. I made the same observation, and it certainly makes it more likely that a flat plate will be satisfactory. I'm not exactly sure what you mean by linear, but I doubt that the lift curve for a flat plate is much closer to linear than that of a thin conventional section.

The structures guy would sure prefer a thick section. However, once he concedes that it must be thin, he doesn't much care whether it is flat or has a more conventional section. The strength comes from the thickness, not the section shape.

banktoturn

George - I have always admired the design philosophies of aircraft executions of Steve Wittman.
These seemed a bit "off track" for some --but went like hell .
Reduced to one line it looked like " If it isn't really necessary - don't use it.
After trying basically most of the current aerobatic designs - for IMAC
I am pretty sure the Cassut design really has a good chance of being very good for the new "presentation" criteria.
The "why" goes like this

it can be built strong - and very light -the layout is well suited for this
The low aspect ratio wing should allow a huge speed envelope -even better than any of the bipes.
roll rates even at walking speeds should be rapid.
The fuselage lateral area is almost equal to the wing area and well placed.
The fuselage frontal area is low.
The relative wing span to fuselage overall length needs no fudging - wing is 15' fuselage is 16 '.
As for power to weight - I have that one covered .
Do you see any downside on this setup?
other than the unusual appearance?

Banktoturn,
It was not my intent to make a correlation or distinction between the two... it's obvious to all that as one approaches zero thickness the section must become flat.

FWIW, A thin section even if slightly rounded will typically force transition very near the leading edge which is what makes these "thin" sections Re independent. If the Re gets too low you'll want to force transition which is easy too.

When I say transition I do mean laminar to turbulent. Thin sections do this naturally without any help. Clmax simply doesn't matter for aerobatics...you can get the force you need from area or dynamic pressure while we're talking dimensionally.

Call them fins, wings, foam, wood, flapper-dappers...call it whatever you like...if it supports a pressure distribution that's all any of us are interested in.

If you study handling qualities you'll find that pilots opinion of an airplane are often driven by how linear the airplane's response is to a control input. From and Aero/S&C point of view we've found that when the aerodynamic forces and moments changes linearly with changes in angle of attack, sideslip and control surface deflection the pilot's opinion is typically high especially when tasked with precision flying (not necessarily 3D). This also goes for these forces changing with Re.

I would agree that a flat plate's lift curve is probably just as linear as a thin section of equal thickness. Also the CLmax of both sections will be similar because both sections are almost identical. The flat plate is certainly easier to produce and more readily available which makes it the section of choice if you subscribe to the Ockham's razor approach to aerodynamics and engineering. We're piddling again...


George Hicks

Since we're stating the obvious... the strength actually comes from the combined material properties and the moment of inertia (which can be a function of section shape) of the structure not just thickness as you mention.

Well this thread has gone off-topic about 100 times, and I don't think any point is really being made here.

Just because Dick "feels" his plane is flying good, doesn't mean there isn't room for improvement using hard fast rules. Remember the rules? Thats why Dick started this thread. He tried to sit here and tell us that those rules are messed up or something. So he came up with his own rules, lets review them:
Review of rule#1
He gives us these rules them goes on to say that he does indeed move the CG around to make his "plane" do what he wants to do. So I guess CG does indeed matter hey Dick?

Rule #3 seems to cancel out rule#2, and is a mute rule anyway, and has nothing to do with the almost weightlessness of a foamie. Besides, as soon as you throw Dick out of the plane to make it light enough to fly, then the CG does matter. This also does not prove the rules of aerodynamics wrong.

Rule #3, if you got enough power, you still need a good airfoil design relative to the plane you are flying to maximize your flight time. After all the rules were created in the spirit of making a plane most effiicient for what it was designed to do.

Rule #4 Not enough power means that there isn't enough power, it doesn't mean that the CG doesn't matter, because as soon as you put enough power, you better have the CG right. This also does not prove the rules of aerodynamics wrong.



As far as planes that wieghs zero, that is a misnomer, becasue according to webster, airplanes have weight. If an "airplane" wieghs zero it would not give a hoot about aerodynamics, as it could just "float" without any CG or power. This statement does not prove anything.

When you hover, you are not relying on wings for lift anymore, therefor the airfoil would be required for lift, so it is not important to have one.
As a matter of fact I believe that the thinest airfoil or "flat plate" would be best as it would give the least amount of resistance to prop wash. When a plane is hovering, the prop wash is pulling it back to the ground, so you would want to minimize it as much as possible.
So Dick your fllat plate has room for improvement in areodynamics. If you decreased the resistance of the leading edge of the flat plate, you would indeed improve the performance of the plane, however small that may be.




Followed by:

First he says Cg doesn't matter much, then he has to move it around to make it fly well.
Hmmm, it seems that CG is of utmost importance to achieve the flight characteristics you want.
I mean if CG doesn't matter, then why not mount the battery in the top of the tail fin?

CG always matters when your "flying". Planes that weigh too much can't fly, and planes that weigh zero, aren't planes.
Everything else needs a good CG based on application.


I am sticking to my guns here saying that Dick does not make any valid points whats so ever.
About the only thing that I got from him, is that he knows how to fly a foamie well, and enjoys it very much.

I think you need to rephrase what it is you are trying to say.

OK no more obtuse stuf -
Basically -here is what I am trying to say -and if it doesn't make sense to you -it's likely you don't ever try any hands on experiments.
In our models - futzing with airfoil -is a nice way to pass time -but actually the finely developed airfoils for full scale stuff it of no advantage .
those shapes were done for FINITE purposes.
based in almost every case on power available and weight constraints.
trying to fix an overweight model with some better airfoil is 99% a waste of time .
add more power to a too heavy setup?- -it just changes the crash speed for most.
the real criteria for decent flying models is the same for decent flying full scale aerobatic stuff -
Keep the fu-- weight down.
the rest is secondary as is any "critical" cg.
The CG usable envelope increases with lower wing loading -
doubt it all you want -

so -still doubt it?
try flying more and cogitating less.

Dick,

This is certainly a more defensible statement than "if it's light enough, CG doesn't matter". It is probably true, in the limited sense that one is more likely to be able to fly an unstable or barely stable plane if it is flying slowly, as has been mentioned before. I don't think that it is news to many people that lighter is better for an airplane. Nor do I think the ridiculous 'rules' you've larnt help get that point across.

banktoturn

Honestly - after reading the wind tunnel extrapolations I have seen here (not singling you out!)
I really think my " larnin" is far more applicable to the real world of models and what typically the modeler experiences, than a bunch of formulas which are for many simply too obscure to deal with.
I deal with modelers as a matter of business - and have done so for almost 30 years .
real hands on fixing/ setting up and designs which work - successfully .
Your background in full scale stuff - is based on tried and true for YOUR use in applications of far greater RN than any of these models see.
Now to ask a pointed question.
Do ever experiment with these types -or any design work on model aircraft?
I spent many years with a old friend (now gone) who worked for wind tunnel and prototype design for full scale--also a supurb modelbuilder of extremely light /strong designs.
There are differences in the two vocations -
This is where I larnt whut makes things fly---both from theory and practical application.

One thing I have noticed here is the assumption among some modelers that the "professional" aerodynamicists posting here, those working with aerodynamics in their daily lives, have a knowledge limited to the study of books and theory taught in the classroom. I have spent 11 years in total at university for my degrees in mechanical and aeronautical engineering, and I do admit that what is taught in the classroom is quite theoretical, although it is based on centuries of research in the fields of physics, mathematics and aerodynamics. However, I am also a modeler, and I built my first flying balsa model when I was only six years old. I have never stopped modeling and I have designed, built and flown models that range from very simple free-flight gliders, rubber powered models, control line, etc, all the way through to warbirds. I also fly full-scale sailplanes and I have some full-scale powered flying experience. (just as an aside: when it comes to graduate work, you spend very little time in the classroom.)

I do not post the above to boast about my own experience, because despite my studies and long experience with model airplanes, I admit that I learn new things about aerodynamics and flight mechanics every day. I do not regard myself as an "expert" in aerodynamics, even on the specific topic on which I focused my PhD research, because I don't think it is possible to ever know everything there is to know. What I did want to show is that my own interests go far beyond the technical jargon I have learnt in the classroom at university or the small library of books that I surround myself with. I also know I am not the exception, since almost all fellow aerodynamicists I have met during my work and graduate studies had similar interests - many of them are modelers, or fly full-scale sailplanes, some are military or commercial pilots, and some of them even spend their spare time sailing, which, by the way, have many similarities to aircraft.

In the thread on Bernoulli there is a poster who posted some very technical stuff on control volumes and momentum transfer. From that post you would think that this person is purely a theoretician - however I happen to know that he is one of the most knowledgeable and experienced people there are on handlaunched and discus-launched model gliders, which include designing, building and flying.

Dick, you mention 'wind tunnel extrapolations', referring, I assume, to some of the more technical posts in this and the other threads. I have tested airfoils in wind tunnels down to Reynolds numbers of 20,000 - in case you are suggesting that aerodynamicists don't know what is going on at that low Reynolds numbers. Much work is going on at even lower "insect-size" Reynolds numbers; Micro UAV research is currently very active all around the world. You often mention that the answer to most airplane design problems is low wing loading and more power. That may work for the planes we fly for pleasure, but what about something like a micro UAV where you have a size limitation, so you can't make the wing bigger and yet you also have to carry an array of equipment onboard? Of course you put a motor in that is as powerful as you can find, but even Li-poly batteries are heavy and as mentioned before, you cannot make the wing bigger to compensate. It is here where a "flat plate" airfoil doesn't quite cut it anymore and every trick in the aerodynamicist's book has to be used to get the lift required while still limiting drag so your limited engine power can be used to the maximum. Things such as flexible membrane wings enter the picture, and to get the most out of all of these, thousands of hours of analysis and wind tunnel work is required. It is more than just designing something that "will fly" - duration and range, both of which depend on aerodynamic and power system efficiency are just as important, and the final design has to comply with a long series of specifications.

Finally, I have never encountered an airplane or model on which CG location did not matter. As some have said, under certain circumstances the range of workable CG locations may be wider than on others, and sometimes a small amount of instability may even be acceptable, but it always matters. I guess whatever works for you is fine, but I think your words may deceive beginners that all they need to do is build it light, give it lots of power, and not to worry about CG.

Dick,
Sorry it's taken me so long to get back with you. It's really funny that you bring up this airplane because I was going through some of the older FS pylon designs with Pete Goldsmith and came across this airplane in my research...my first thoughts were that this would be a good aerobatic model. I've also been impressed by Wittman's designs ever since I read one of Raspet's papers on the Tailwind. I think your assessment of the airplane is a fair one and it would probably do quite well in competition...many would have trouble accepting the pylon-style shape of the airplane but as long as it snaps well I'm sure that it will do the rest as well as other designs.

George

Nicely said -
OK, in self defense:
I have spent much time , professionally, unravelling technical gibberish proposed by "experts" in personal injury litigation.
Your thoughts are understandable -and clearly put forth.
Thank you.
I proposed very simple "rules" I have learned for models , simply because I saw mostly answers to questions - which really failed to address the point or, more pointedly - many times, the technical answers failed to address the real problems the modeler was experiencing.
So - I put for some of my own experiences -
and pointedly - the business of CG .
My feeling was and still is that as WINGLOADING diminishes to ZERO- the CG usable range increases and in some cases tho not all, dramatically.
This is why I said "if the plane weighs nothing - then the cg does not matter."
Silly?
All my own tests show that as wingloading increases - CG becomes MORE critical - and the reverse for decreased loadings.
Is this not true?
If true - what are the limits on low loading.- so far I have not found them.

Why because tho they are not stable gliders - they are still, easily controlled craft .
The F16 sure as hell isn't a stable glider.

George - thanks for the reply - I have been flying my Spitballs -basically same layout and the inside and outside snap character is superb
instant and predictable.
So -just as a quick peek -i am slicing up a scale profile of the Cassut as yet anoher look

The aspect ratio is about the same as my Spitball.
My first real model will be 44% - for ZDZ80 single on tuned system
weight based on oher stuff done in same manner-well under 20 lbs - more like -18-19 lbs. wing area 1900+
this one really has my interest piqued!.

Dick,
Nobody here has argued against that.
An unstable (negative stable) plane with a very low wingloading may be controllable thanks to its low inertia, which gives the pilot time enough to react and make corrections.
But the CG location still matters for stability as long as the plane is heavier than the air - and I think all planes are!
Here you have brought up a theory that hardly can be applied into practice (despite you claim you are a practical one).
The fact that you have built and flown models successfully for years, doesn't mean that your assumptions are always right and/or unquestionable.
I think the way you present your standpoints may lead to misinterpretations.
Also referring to someone's professional background or showing disdain for what you call "a bunch of formulas" (despite nobody has presented a formula here yet) hardly helps in clarifying your standpoints.
There's nothing wrong with your practising but there are theoretical explanations for the most including why your foamies fly like they do.

OK-- I am a blowhard but I do try to explain why my stuff works - Stating that " there are theoretical explanations ", clarifies nothing in my book.
But please , enlighten me but leave the wind tunnel at home -

Dick,

Now that I've stopped laughing long enough retrieve my my keyboard, I'd like to put in my .10 cents. (inflation, you know). It's always real muche funn to see a very knowledgeable person "tweak noses". I thoroughly enjoyed this string
re: C.G. "Does Anyone Really Know Where It Is"??
Way to go! Now; let's start talking about "Center of Mass".

Dick,
You say "enlighten me," but your door often seems to be shut tight when you follow up with things like "but leave the wind tunnel at home." As I see it, the "theoretical explanations" he alludes to have already been covered, in layman's terms, in this thread several times.

-David

I haven't had a chance to read the whole thread, but I am wondering: whatever happened with "nose heavy flies poorly, tail heavy flies once" ?
Luis

piffle ---

Dick,

In point of fact, you don't explain why your stuff works. You simply say, over and over again, that you know it works because you've flown it. At no time have you explained how lower wing loading broadens the usable CG range, or why flat plates are better wings at low Reynold's numbers. Telling people to 'leave the wind tunnel at home' is perhaps the most ricidulous thing you've said. The only problem with wind tunnel results is that they are usually not performed at Reynold's numbers that are applicable to models. Wind tunnel results for the right Reynold's numbers would be very valuable to see.

banktoturn