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RE: basic aerodynamics - 5/10/2004 9:19:02 PM   
Spastic



Posts: 440
Joined: 7/8/2003
From: Buffalo, NY, USA
Status: offline 		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

(in reply to dick Hanson)
       Post #: 101

RE: basic aerodynamics - 5/10/2004 9:47:07 PM   
adam_one


 

Posts: 410
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline 		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.

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(in reply to Spastic)
       Post #: 102

RE: basic aerodynamics - 5/10/2004 10:10:45 PM   
banktoturn



Posts: 713
Joined: 8/22/2002
From: Bloomington, MN,
Status: offline
quote:

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


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

(in reply to Spastic)
       Post #: 103

RE: basic aerodynamics - 5/11/2004 12:32:24 AM   
dick Hanson



Posts: 10192
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		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.

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       Post #: 104

RE: basic aerodynamics - 5/11/2004 2:47:19 AM   
GRH



Posts: 217
Joined: 5/31/2002
From: Savannah , GA, USA
Status: offline 		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?

(in reply to dick Hanson)
       Post #: 105

RE: basic aerodynamics - 5/11/2004 10:02:17 AM   
adam_one


 

Posts: 410
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline 		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.

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.

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       Post #: 106

RE: basic aerodynamics - 5/11/2004 2:38:48 PM   
GRH



Posts: 217
Joined: 5/31/2002
From: Savannah , GA, USA
Status: offline 		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"

(in reply to adam_one)
       Post #: 107

RE: basic aerodynamics - 5/11/2004 4:01:56 PM   
banktoturn



Posts: 713
Joined: 8/22/2002
From: Bloomington, MN,
Status: offline
quote:

ORIGINAL: GRH

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?


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

(in reply to GRH)
       Post #: 108

RE: basic aerodynamics - 5/11/2004 4:07:17 PM   
dick Hanson



Posts: 10192
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		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?

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< Message edited by dick Hanson -- 5/11/2004 11:10:53 AM >

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RE: basic aerodynamics - 5/11/2004 5:23:53 PM   
GRH



Posts: 217
Joined: 5/31/2002
From: Savannah , GA, USA
Status: offline 		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.

(in reply to banktoturn)
       Post #: 110

RE: basic aerodynamics - 5/24/2004 2:52:16 AM   
antslake



Posts: 393
Joined: 8/3/2003
From: Greenwood Lake, NY, USA
Status: offline 		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:
quote:


Wow - lots of great info out there .
Here are some rules I larnt:
1- If the plane is extremely light - the CG does not matter
2-If the plane is too heavy - it still don't matter.
3- If you got enuf power - nuthin else matters .
4-If you ain't got enough power - same thing.


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.



quote:


On my IMAC models - the CG is very important -but on my extremely light electrics - it is of very little importance- these things have wing loadings of only a couple of ounces per sq ft.



Followed by:

quote:


Ah--- pitch stability - I really don't get excied about that -
I just adjust cg for easy control--and extreme pitch--the radio settings of expo make this possible.
these things do fly hands off quite well


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.

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RE: basic aerodynamics - 5/24/2004 4:16:45 AM   
dick Hanson



Posts: 10192
Joined: 12/12/2001
From: slc, UT, USA
Status: offline 		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.

< Message edited by dick Hanson -- 5/24/2004 10:06:33 AM >

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