Aerobatic Airfoils?

[Luffberry]

I’m trying to find out what makes a good aerobatic airfoil. That probably means I need a semi-symmetrical airfoil. Any suggestions on what makes a good aerobatic airfoil?

I am also operating on the assumption that, if the airfoil is mounted with 0 degrees AoA, that would make inverted flight look less unbalanced, and would also decrease drag during normal flight. But does that mean I will have unrealisticly little lift? I was hoping to be able to fly level, with 0 degrees AoA, and have my plane be supported at about midrange speeds. Is this possible with an airfoil that needs to be aerobatic?

How much should I factor in Reynolds numbers? And can I get some info on drag characteristics?

About how much wing do I need per gram at 0 AoA, and how much at a more reasonable AoA?

[slipstick]

Symmetrical (more correctly zero mean camber) airfoils are normal for aerobatics planes and are often mounted at 0 degrees Incidence. I suspect you're confusing incidence and angle of attack (AoA). They're not the same thing.

Of course zero mean camber foils don't fly at zero AoA because they have no lift at at that AoA. But they do fly at more or less the same (small) AoA either upright or inverted.

Once you've understood that rather crucial difference you might be able to rephrase your other questions so they are a bit more meaningful.

Steve

[Luffberry]

Yes, I suppose you’re right. Fine, I want to know if I can get enough lift with my airfoil at 0 degrees of incidence in level flight, which means 0 degrees of AoA. But if you are in level flight in equilibrium, and your wings are at 0 AoA, they are probably at 0 degrees of incidence anyway. So your correction is rather moot. :P

And since I want to generate lift at 0 degrees of AoA, it has to be semi-symmetrical. So, since I’m still looking for a good aerobatics airfoil that is semi-symmetrical so it generates lift at 0 AoA (which is still my original question), can the non-jokers out there kindly respond to my question?

[Wingin' Wayne]

Ther are a lot of smart guys on this forum who will give lots of complex answers! So you need to determine how complex you want to be. I'm a novice designer/builder working in foam. It's cheap and easy. A fully symmetrical is easier to build and gives good lift even at slow speed. I'm a machinist not an engineer, so I just go with what I've read and seen in these forums. Go to the foamie forum and look at how simlpe some of the wings are. A folded Blucore wing with a spar is simple and it works at zero incidence. The size of wing has variables. Wingspan should be about 3/4 the length of fuselage. You need a ballpark estimate of weight to determine how many square inches of wing you need to give you the desired wing loading ratio. That will determine the width or chord. Wing thickness is a function of desired characteristics. Do you want a thin wing that flies fast and lands fast or do you want a fat draggy wing that flies and lands slow? Wing thickness is a percentage of wing chord. I like 16%, slows down nice and has fair pitch speed. Good luck and HAVE FUN !!!

[Ollie]

"Fine, I want to know if I can get enough lift with my airfoil at 0 degrees of incidence in level flight, which means 0 degrees of AoA."

Your myth is not true. The plane flies at a range of wing AoA angles that change with air speed and with constant lift and in level flight.

Read and understand:
http://www.rcgroups.com/forums/showthread.php?t=392962

[Ollie]

More:
A normal configuration of a best aerobatic plane has a symmetrical airfoil, wing zero incidence, tail zero incidence, zero thrust line and almost neutral stability. The lift is equal to the wieight in a level flight. To enough lift, the pitch attitude angle of the plane is so the wing enough AoA (pitch up attitude) to generate the lift. The elevator up control puts the tail at a new incidience angle to put the nose up and keep the direction of the plane level. The symmetrical perforance is mirrored, upright or inverted. The plane must be piloted at every moment. If you want the have the cake (trainer set up), you can't eat (aerobatic set up) it too.

[slipstick]

You could certainly design a plane with an airfoil which is mounted at 0 incidence and flies level at a particular speed at 0 AoA. It will only fly like that upright. Inverted it will require a high AoA to get enough lift. Very different flight and control characteristics upright and inverted is not what most people call "aerobatic".

BTW just for interest why this obsession with 0 degress AoA. What is it that you are actually trying to achieve ?

Steve

[kartoffel]

A symmetric airfoil at 0 degrees AoA generates no lift. It's no crime to fly straight and level at a positive AoA; airplanes do it all the time. If you want an airplane that's equally adept at upsidedown as rightside up flight, go symmetrical with a zero incidence surfaces and a zero thrust line. Just remember that it's not mandatory for aerobatic planes to be 100% symmetrical.

Now, setting all that aside let's discuss what makes a good aerobatic airfoil.

First, consider what type of aerobatic flying you want to do: (1) fast & crisp pattern aerobatics or (2) slow and floaty 3D style? A thinner airfoil will let you go faster, but it will induce more drag when you pull G's. Flat plates are fine for low Reynolds slowfliers flying at high AoA, but they suffer from laminar separation bubbles and don't track very straight in "normal" flight.

Second, if you're not dealing with flat foamies, determine LE radius. Decide how snappy you want the aircraft to behave in stalls and rolls. A deliberately small LE radius will encourage flow separation, making the aircraft snap more easily.

[Luffberry]

"Fine, I want to know if I can get enough lift with my airfoil at 0 degrees of incidence in level flight, which means 0 degrees of AoA."

Your myth is not true. The plane flies at a range of wing AoA angles that change with air speed and with constant lift and in level flight.

Level flight is not the same as level airspeed. What you are trying to debunk my supposed myth with is level airspeed. Level flight requires both the fuselage and the airspeed to be level. If you aren’t going to help me with my airfoils, find some other thread to troll.

[Luffberry]

You could certainly design a plane with an airfoil which is mounted at 0 incidence and flies level at a particular speed at 0 AoA. It will only fly like that upright. Inverted it will require a high AoA to get enough lift. Very different flight and control characteristics upright and inverted is not what most people call "aerobatic".

Inverted will always be considerably different from upright. Increasing the incidence on the wing just makes it WORSE.

BTW just for interest why this obsession with 0 degress AoA. What is it that you are actually trying to achieve ?

Angle of attack effects always trump airfoil shape contributions. If I can find a semi-symmetrical airfoil that will give me enough lift at 0 degrees incidence, then when I go inverted, because angle of attack trumps the airfoil contributions, I can bring the angle of the fuselage (and the control surfaces) closer to the upright angle; and thus have considerably closer control to upright.

I believe this also means I will have less drag in either orientation, but especially in upright flight (you have to get the plane high in the air so you have room to play, too).

But, in addition, I also don’t like the look of wings mounted at incidences far from 0 degrees. Also makes it harder to mount anyway.

In short, I want to overcome the increased lift requirement of upright flight by making/using an airfoil that is slightly less symmetrical, as opposed to increasing the angle of incidence on a fully symmetrical airfoil. Not only does this cause all sorts of other problems I’d like to avoid, but it just plain seems to be the “bludgeoning barbarian” approach.

[slipstick]

Level flight is not the same as level airspeed. What you are trying to debunk my supposed myth with is level airspeed. Level flight requires both the fuselage and the airspeed to be level. If you aren’t going to help me with my airfoils, find some other thread to troll.
That's complete rubbish. "Level flight" means flight at a constant altitude i.e. neither climbing or diving. It has nothing at all to do with angle of attack.

Ollie is about the most helpful person round here and you're very rude. OTOH if you aren't intelligent enough to take good advice when you get it I'm afraid he's wasting his time. I've already given up. Have fun with your physically impossible semi-symmetrical fully-aerobatic zero AoA wing :(.

Steve

[Luffberry]

First, consider what type of aerobatic flying you want to do: (1) fast & crisp pattern aerobatics or (2) slow and floaty 3D style? A thinner airfoil will let you go faster, but it will induce more drag when you pull G's. Flat plates are fine for low Reynolds slowfliers flying at high AoA, but they suffer from laminar separation bubbles and don't track very straight in "normal" flight.

Ugh, flat plates? I’ll try and pretend you didn’t say that... ^_^;

I’m not interested in 3D; while I suspect that my plane will have enough power to, I won’t be building the excessive wings and control surfaces that 3D requires. I’m looking for classical aerobatics, with classical aerobatic shapes to match. I’m interested in going fast enough to make the classical stuff fun, and I’m hoping to pull alot of “g”s (although I worry about the increased structural complexity). I’m also trying to pull off less than 15 grams, so it can be quite micro indeed.

Second, if you're not dealing with flat foamies, determine LE radius. Decide how snappy you want the aircraft to behave in stalls and rolls. A deliberately small LE radius will encourage flow separation, making the aircraft snap more easily.

Now you’ve lost me, which is probably a good thing. But this is probably less about airfoil shape than size, right?

[Luffberry]

That's complete rubbish. "Level flight" means flight at a constant altitude i.e. neither climbing or diving.

Hardly. That’s just level airspeed.

Ollie is about the most helpful person round here and you're very rude. OTOH if you aren't intelligent enough to take good advice when you get it I'm afraid he's wasting his time. I've already given up. Have fun with your physically impossible semi-symmetrical fully-aerobatic zero AoA wing :(.

If he was helpful, he would talk about asymmetrical airfoils like I asked. Instead, he went off on a misinformed rant about the nature of angle of attack vs. incidence, when I was specificly asking about AoA all along. This discussion is about airfoils, not really planes themselves. Changing the subject is what is really being rude, when someone goes off-topic from a specific question.

As far as intelligent advice, neither one of you has yet to talk about semi-symmetrical airfoils as opposed to fully symmetrical airfoils. How can what you have to say be intelligent when it is specificly NOT about the question posed? If this is where your intelligence maxes out, I’m quite happy to not receive your advice.

Such a wing is also not physically impossible, although it may be insufficient for my particular need. But this discussion is about the physics of airfoils, so possible and impossible are irrelevant to the discussion of the physics involved. Why do you think I posted this in “Modeling Science” rather than some other forum, if not to discuss the science behind aerobatic airfoils, which neither of you have produced?

[Ollie]

http://142.26.194.131/aerodynamics1/Lift/Page6.html
http://142.26.194.131/aerodynamics1/Lift/Page7.html

[kartoffel]

I’m looking for classical aerobatics, with classical aerobatic shapes to match. I’m interested in going fast enough to make the classical stuff fun, and I’m hoping to pull alot of “g”s (although I worry about the increased structural complexity). I’m also trying to pull off less than 15 grams, so it can be quite micro indeed.

Interesting! :) So, a very small 15 g micro RC plane that can perform classical aerobatic type maneuvers? Hmmm..... If it's going to be that tiny you really don't need to worry much about the airfoil at all. At that scale the reynolds number will be extremely low, and a flat plate is fine. Really.

[Ollie]

Air is normaly not seen directly. The flow of air is not seen directly. That is why air is complex to see directly. You have to measure airspeed and angle direction with out seeing. In air flow around an airfoil is not seen and the behavior with out speed and direction are subject to myths.

http://www.amasci.com/wing/airgif2.html
http://142.26.194.131/aerodynamics1/Basics/Page4.html
http://www.desktopaero.com/appliedaero/airfoils1/airfoilhistory.html

You have to put a a flat airfoil into a smoke windtunnel too see the airflow. At a positive AoA the flow splits just under the leading edge on the bottom. The top flow goes back and around the leading edge and over the top. The air sticks to the airfoil. The boundary layer has some thickness before getting to full speed. The boundary layer thickness depends laminar some spots, turbulent other and turbulent type eddy or bubble. Just at the leading edge on the top, the flow can't bend the flow around the too sharp LE. The flow makes a short turbulent bubble in the thick boundary layer. It is the streamline flow for the solid airfoil plus the boundary layer and bubble. The streamline flow applies with all Bernoulli principle, Newton, etal. You can't see the flow behavior without smoke or fog.

The boundary layer is more important at very, very small scale flow.

[BMatthews]

Lufberry, if anyone around here is confused it's you. And you may just want to check the hostility at the door before you enter the site.

You've got a lot of concepts mixed up and based some core beliefs on bad info. Most, if not all, the stuff others said is spot on.

A zero camber airfoil MUST fly at an angle of attack to generate lift. That angle will change as the airspeed changes. Slower means it's more nose high. Faster means it's closer to, but never, level. When inverted the model must fly with the airfoil at the same angle of attack as when upright. That means that if the fuselage is pointed up 2 degrees when upright it'll be pointed "down" 2 degrees when inverted. If keeping the fuselage level is that important you need to realize that you need to build in an angle of attack, sometimes known as incidence. But with the wing at a positive angle compared to the fuselage when upright it means the fuselage will be angled when inverted.

The only airfoil that can generate lift at zero angle of attack is one with camber. But if you use such an airfoil the angle of attack when inverted will be be higher than when upright since the camber is then lifting down rather than up and you need extra angle of attack to counter that camber lift.

If you want to obtain a better understanding of lift, camber, angle of attack and other airfoil related knowledge then go to Foilsim and play with some airfoils. In particular add some camber to one and then set it at 0 degrees angle of attack. Note the lift amount and then force the angle of attack negative until you see the same lift value generated in the negative direction. Note how much the new angle of attack is for your "inverted" flying. Change the airspeed and note how the lift decreases. Then increase the angle of attack until the lift value is restored. That simulates how far your angle needs to change due to flying speed variations.

[Ollie]

Steve and BMatthews,

I agree with you both. Thanks!

kartoffel,

"At that scale the reynolds number will be extremely low, and a flat plate is fine. Really."

Right on!

Wayne,

Your good advise is right.

Luffberry,

All are trying to help you. Please, open your mind a little.

[Luffberry]

Interesting! :) So, a very small 15 g micro RC plane that can perform classical aerobatic type maneuvers? Hmmm..... If it's going to be that tiny you really don't need to worry much about the airfoil at all. At that scale the reynolds number will be extremely low, and a flat plate is fine. Really.

I’d rather engineer a proper airfoil for this need. I know enough about flat plates to know that it won’t be good enough for what I want. If you have anything more to say about semi-symmetric airfoils, I look forward to your contribution.

[Luffberry]

A zero camber airfoil MUST fly at an angle of attack to generate lift.

I stated from the very beginning that I do not want an airfoil with zero camber. In order to be SEMI-symmetrical, as opposed to FULLY symmetrical, it MUST have some camber in it! Do pay attention to the subject of discussion.

I am quite aware of Foilsim. It doesn’t simulate small enough wings. I’ve had to approximate with it by shrinking my wing length and increasing my wing width by 4, to keep the same area. Bad for results.

Foilsim only simulates one category of airfoils; I think the NACA 4 digit airfoils. Could be wrong on which NACA series it is, but I hardly care. The point is, there are more airfoils than that, and certainly ones that are properly tuned to model-sized needs; particularly with our atypical Reynolds numbers. That’s why I’m here. But you guys have yet to talk about anything meaningful to that end. It’s very disappointing that people keep opening their mouth without paying attention to the subject being discussed.

[Luffberry]

Ollie, I have no interest in proving or debunking the supposed “myths” of flight. While I am all for re-evaluating scientific viewpoints from time to time (popular perspectives on quantum mechanics, and all of string theory, for instance; what a pathetic joke), that particular discussion seems rather silly and pointless. While conceptually they are relevant to this discussion, ultimately they have not had meaningful impact on the equations used, nor their application.

I’m trying to discuss the shape of airfoils, particularly semi-symmetrical airfoils, for the purpose of aerobatics. Preferably, I would like this discussion to lead into airfoils that can be mounted at 0 degrees of incidence, still generate enough lift to support the weight of the plane at a specific speed, and still have very good aerobatic performance. I’ve yet to hear anything about specific precision shapes made for our Reynolds numbers, how to make others, or pretty much anything beyond “Og say symmetrical good, asymmetrical bad! Og say 0 degrees incidence bad!”. New contributors will continue to go off-topic if this keeps up. Note Matthews and how he talks about fully symmetrical airfoils instead of semi-symmetrical ones.

[slipstick]

I think the problem is we're not good at reading minds. We only read what you actually write:

I’m trying to find out what makes a good aerobatic airfoil. That probably means I need a semi-symmetrical airfoil. Any suggestions on what makes a good aerobatic airfoil?
If you didn't want suggestions on what makes a good aerobatic airfoil, i.e. zero camber, you should have asked a different question.

Semi-symmetrical airfoils don't have any particular characteristics. They're just one variant of a cambered airfoil. Any cambered airfoil will always be inferior for aerobatics. So presumably there are some other flight characteristics you want that you think makes using a cambered airfoil a sensible choice. If you say what these other flight characteristics are perhaps someone can help. But at the moment you just keep saying that you want a good airfoil for aerobatics but you won't accept any of those airfoils that are actually good for aerobatics. It's very confusing.

Steve

[Ollie]

For your specification:
http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1058
S8025 airfoil
semi-symmetrical
good for aerobatic
fits the size of the plane
it a little better than a flat airfoil
http://www.nasg.com/afdb/contour/s8025.dat

Other than that, I will keep my mouth shut.

[BMatthews]

Well since you apparently seem to know everything about everything I'll just leave you to your own efforts.

You also apparently glossed over my comments about cambered airfoils and their operation at 0 AoA.

Bye.....

[flyingwings]

Since the question at the beginning was related to semi symetrical airfoils to answer your question requires more information.

First off semi symetrical will work for a fine aerobatic machine. Now the question is more what type of semi symetrical do you want.

Personally I like an airfoil with minimal pitching moment change at the various speeds that a model will experience through it's manouvers.

In general any semi symetrical airfoil will generate lift at the zero incidence angle. However, until you know the wing loading and the speed that you require for level flight the amount of lift generated by the wing can not be determined.

Since you are talking about a small airplane how are you proposing to build the wing? As indicated in another post in this thread you could use fan fold doubled over to achieve a good semi symetrical airfoil. Or you can cut ribs or you can cut foam. What will the wing be covered with? That will definitely change the lift characteristics.

My suggestion is take a pencil to paper. Draw an airfoil that looks right to you and then go ahead and make your wing based on your design. Small wings are easy to build. Experiment with different airfoil shapes but do your own. Learn what it takes to achieve an acceptable airfoil for your purposes.

If you take a look at the various airfoils that are available to try you will see that there is no perfect answer. Every designer strokes the design his way. Furthermore, even if you find a super duper airfoil it does not mean that you will be able to get the same results as the designer. Simply you will probably build your wing and ribs entirely different than the designer of the particular airfoil.

Paul

[biber]

I’m trying to find out what makes a good aerobatic airfoil. That probably means I need a semi-symmetrical airfoil. Any suggestions on what makes a good aerobatic airfoil?Generally for aerobatic airfoils drag is not a big issue anyway since the installed power should be able to overcome that easily. Some other characteristics are of much more interest. E.g. the clmax, clmin, cm0. Very important for precise maneuverability is not having any deadband effect wich is especialy a low Re-number issue.
So, that makes a good aerobatic airfoil.
I am also operating on the assumption that, if the airfoil is mounted with 0 degrees AoA,Do you mean incidence (regarding the fuselage)? Or do you really intend to mount an airfoil(or maybe wing?) fixed in the airstream? Please try to formulate more precise to avoid getting answers not precisely matching your expectations. that would make inverted flight look less unbalanced, Yes, maybe, but you could get that balanced look wether you use a symmetrical airfoil or any other.and would also decrease drag during normal flight.What makes you assume that? But does that mean I will have unrealisticly little lift? I was hoping to be able to fly level, with 0 degrees AoA, and have my plane be supported at about midrange speeds. Is this possible with an airfoil that needs to be aerobatic?Do you really know what you want? That sentences would make an aerodynamicist wonder if you know what you are talking about. About how much wing do I need per gram at 0 AoA, and how much at a more reasonable AoA?No answer possible. First you have to declare the projected airspeed, the chosen profile's cl0, and explain what you think a reasonable AoA is.I’m also trying to pull off less than 15 grams, so it can be quite micro indeed.If you are dealing with such small Re-numbers as with a 15 gram micro flyer there won't be any available computerprogram capable of giving serious info on the questioned wingsections. Nor will there be measured windchannel data for suiting candidates.

biber

[Ollie]

biber,
Have you seen:
http://www.news.cornell.edu/releases/March00/APS_Wang.hrs.html
or
http://www.ipfrontline.com/depts/article.asp?id=4775&deptid=5
as applies to micro flyers?
Comments?

[biber]

No, I didn't yet.
I thought of the eppler code or xfoil as free available software that won't be able to work properly for such REs.
And I have no information on any airfoil being reasonably better for micro aerobatics than a simple flat plate. At least it won't be worth the effort to search for another highly sophisticated airfoil for that application. There will be minor benefit in that. At that REs nearly all we can do is trying to avoid having a big laminar separation issue all the time at any cl. So the airfoil will have to be very thin. A plate matches that quite perfect. For the capability of inverted flight the plate will have to be little cambered. So a flat plate again will match that very well.
But Luffberry didn't yet give us any idea of the airspeed that the little thing will fly at, so we don't know the RE. Maybe he wants it to go 200 mph wich could raise the RE upto a decent level :rolleyes: .

biber

[Ollie]

biber,
About the reported news as applies to micro flyers:
"The old bumblebee myth simply reflected our poor understanding of unsteady viscous fluid dynamics," explained Wang, an assistant professor of theoretical and applied mechanics in Cornell's College of Engineering, in an interview before the APS meeting. "Unlike fixed-wing aircraft with their steady, almost inviscid (without viscosity) flow dynamics, insects fly in a sea of vortices, surrounded by tiny eddies and whirlwinds that are created when they move their wings."
Comments?

[Ollie]

More:
"Biologists with high-speed video cameras have been able to document the oscillating motions of insect wings as they move up and down and change pitch by tilting the edges, and the complex motions are beginning to be incorporated in the design of robotic "insect" wings. However, the aerodynamics of hovering insects, such as dragonflies, is still not fully understood. Wang chose the dragonfly as "the worst case for quasi-steady state theory" as the first test of her computer simulation."

[biber]

That's the main prob with micro aerodynamics, they are still a white patch on the map to quite some extend and are just about to be explored. Things are going a different way there.

biber

[Ollie]

biber,
Thanks! I will keep my mouth shut on micro aerodynamics.

[vintage1]

"Here there be Tygers" or is it draginflies? :D

[Ollie]

I want to keep my mouth shut on micro aerodynamics ( flying insects and hummingbirds) but open my mind.

Why don't motorcyclers smile?
Answer: They get insects stuck between their teeth.

[biber]

Please don't shut up at all, Ollie!
I personally think (if that may count) of your posts as being very helpful, on topic, and mostly hitting the point.

biber

[flyingwings]

biber,
Thanks! I will keep my mouth shut on micro aerodynamics.

Ollie

Unfortunately, micro aerodynamics does not fit with the question of this thread.

Since we don't want to be "thread robbers" start a new thread. I would like to discuss a theory about why bees can fly. I was writing the theory when it dawned on me I would be changing the theme of the thread.

Paul

[Ollie]

Paul,
This thread can be yours and anybody else who wants it, even Luffberry.
Be the leader if you want it.

[biber]

Come on Paul, now you can't leave us without your theory!

Luffy doesn't seem to bother anyway ;) .

Please go on.

biber

[kartoffel]

Very important for precise maneuverability is not having any deadband effect wich is especialy a low Re-number issue.

If you are dealing with such small Re-numbers as with a 15 gram micro flyer there won't be any available computerprogram capable of giving serious info on the questioned wingsections. Nor will there be measured windchannel data for suiting candidates.

Agreed! :)

Although, you could still use a general purpose Navier-Stokes solver in this case. I'm not an airfoil guy, but I believe OVERFLOW-D permits slip flow boundary conditions. Perhaps somebody could confirm whether FLUENT or some other tool can do it too.

Alternatively, if you have lots of time to kill, do your CFD with direct simulation monte carlo (DSMC). It's numerically intensive to simulate flows at standard sealevel conditions in the Earth's atmosphere, but you won't need to worry about fudging N-S. DSMC is good stuff :cool: It's an ideal simulation method for both micro-scale flows and rarefied flows. SMILE(1) tool (Statistical Modeling In a Low density Environment) will do the job nicely.

(1) Ivanov M.S., Markelov G.N., Gimelshein S.F. "Statistical simulation of reactive rarefied flows: numerical approach and applications", AIAA Paper 98-2669, 1998.

[flyingwings]

Come on Paul, now you can't leave us without your theory!

Luffy doesn't seem to bother anyway ;) .

Please go on.

biber

I would like to talk further about the "birds and bees". :D

Concerning the bees first. Years ago I worked up a theory as to why the bees could fly. If you don't mind I will pose it here since the information provided fits in with it.

First the vortex theory is very important. I had been reading about Schauberger and his vortex theories. Since I have worked in the aircraft industry for 40 years I have always wondered about the Aerodynamacists comments that "the bee, aerodynamically can not fly!" and looked for an explanation that would explain why they were wrong.

Schauberger used his vortex theories to float logs through a multi curving flume without having the logs touch the walls of the flume. My thought was can this be applied to why a bee can fly.

Bees flap their wings creating vortices but apparently not creating much lift. Bees are small and compared to an air molecule the air molecule is still much smaller. However the flapping of the wings creates compressed vortices and large air bubbles.

Now the bee is covered with hair that stands up like a man with a brush cut.

It is an established fact that air bubbles can be quite large. Sailplane pilots put these bubbles to good use.

http://www.expandingknowledge.com/Jerome/PG/Article/Technique/Cross_Country/DennisPagen_ThermalLore/Main.htm

The flapping of the bees wings heats and compresses air bubbles as the vortex is formed. The heated bubbles will agglomerate to form larger bubbles.

The heated bubbles flow along the bees hairy body creating a cushion of warm air that the bee rides on. More important the bees hair will capture the air bubbles sustaining the bubbles bouyant effect over the body of the bee.

I kind of suspect that the bees hair is also sticky from the nectar and honey process. Air molecules will stick to the hair.

That's the theory.

Luffberry

Go to the Profili site - http://www.profili2.com/eng/default.htm - and buy Stefano's program. I have used it for 3 years now. It is super and does seem to give some reasonably good results down to Re= 30,000.

It is really more fun trying to develop your own airfoil than trying to copy someone else's.

I guarantee you can spend many hours studying the many airfoils that are provided with the program while developing your own.



Paul

[Ollie]

Paul,
Please read the profill2 from Menu News,Nov and Oct 2002. Stefano included XFoil in his program. With out XFoil, profill2 (except plotting) can't function in aerodynamics.

[biber]

So, Paul, I guess you are suspecting the bees to act like biological hot air balloons? Now, that sounds weird :rolleyes.
Did you once figured out about what size of air bubble and what differential temperature would be neccessary to lift a bee's weight? Maybee (sorry couldn't resist ;) ) the theory could then easily be checked with a thermometer.

For the Profili thing, as Ollie noted, its aerodynamic features are completely driven by Xfoil running in the background. To my knowledge Profili just sets a nice and comfortable visual interface upon it plus the plotting options. I don't use Profili though but think it's nice and worthy software.
That being said, I just doupt that one can expect getting serious info for REs below 100000 from Profili/Xfoil. The only guy I think who may be able to handle that is Mark Drela.

Kartoffel, what has driven you to choose such a funny user name? To avoid any misunderstanding, I do like (eating) Kartoffeln but OTOH wouldn't want you to be fried...

biber

[Ollie]

More:
Assume we try to find or design an airfoil with profill2 (including XFoil).
Specifiction(Sp) applied to a plane:
Re>30,000
Gross wt. 15 to 30 grams, including structure, motor, prop, rec., batt., and 4 servos.
Launch at airspeed (___), working airspeed (___) and landing airspeed (___).
Wing span and chord.
Max G's with maneuvering and working airspeed within sight.

Who can propose a R/C model design to the Spec? Who can defend the design with the airfoil? Is this spec. reasonable? If you don't like the spec, change it but defend it.

[biber]

That's why I'd choose the flat plate and be happy with it like so many others are ;) .

biber

[kartoffel]

Biber: My online handle has been Kartoffel (or variations thereof) for longer than I can remember. It doesn't have anything special to do with potatoes ;)

[kartoffel]

For regular airplanes and R/C models, the molecular mean free path is much much less than the characterstic dimension, and we can apply standard Navier-Stokes equations. At smaller scales (or in rarefied conditions such as extremely high altitude and low Earth orbit), the flow is different: velocity at the wall is nonzero.

In these cases, the molecular mean free path is non-neglibile compared to the characteristic length of the object being studied. The dimensionless Knudsen number (Kn) represents the ratio of the mean free path to the characterstic dimension (airfoil chord, duct diameter, etc.)

Image borrowed from wikipedia:
http://en.wikipedia.org/math/3e9a48117255e661fb1a908d08da8661.png

We can apply CFD tools used for one regime to solve problems in the other. In both cases, the Knudsen number determines the type of flow regime we are in.

Regular airplanes, birds and standard R/C aircraft all operate with where Kn is essentially zero. The mean free path is essentially negligible relative to the scale of the flying object.

Insects and microscale devices operate with nonzero Knudsen numbers, yet Kn < 0.1. This is the flow regime we're interested in. Classical Navier-Stokes can still be applied, as long as one consideres slip-flow boundary conditions at the wall. Some of the classical rarefied "free molecular" flow characteristics can be applied (ref. 1) but it's not 100% valid. We are operating in a gray area.

At even smaller scales, or in low earth orbit (extremely rarefied flow), Kn > 1.0. Navier-Stokes falls apart completely. Now flow can be modelled as collisions between individual particles (ref 2). In this regime we've gone beyond slip-flow to what's known as free molecular flow.

There are two ways to simuate conditions for microscale aircraft: Navier-Stokes with slip-flow wall conditions; or direct simulation monte carlo (DSMC) (ref. 3). Navier-Stokes is faster, but it struggles to handle unsteady flow and weird microscale effects. N-S is only as good as the boundary conditions we give it. DSMC, on the other hand is dog slow because we attempt to statistically model collisions between individual molecules. DSMC is fast enough for satellites, but when you increase the molecular number density to standard sealevel conditions, you need a heck of a computer.

The attached file DSMC.gif shows a simulation I did of a flat plate at 40 degrees angle of attack and Kn = 0.1, corresponding to hypersonic flow in the upper atmosphere. The very simple flat plate was divided into 100 segments. It took about 4 hours to solve on a 350 MHz computer. If you wanted to solve actual airfoils, and at standard sealevel density, you'd need to be rather patient. I still like DSMC though :cool:

(1) S. A. Schaaf, P. L. Chambre, Flow of rarefied gases, Princeton University Press, Princeton, 1961.
(2) Bird, B. A., Molecular Dynamics and the Direct Simulation of Gas Flow. Oxford Science Publications, Oxford, (1994).
(3) Müller, M. & Herrmann, H. J., Simulations of Granular Media, Institute for Computer Applications, Stuttgart, (1999).

[flyingwings]

Paul,
Please read the profill2 from Menu News,Nov and Oct 2002. Stefano included XFoil in his program. With out XFoil, profill2 (except plotting) can't function in aerodynamics.

Ollie

I am very familiar with the Xfoil portion of the program. It is Stefano's interface that makes the program worth learning.

Dr. Mark Drela is the most knowledgable of the Aerodynamacists that I have reviewed. His development of Xfoil and Stefano's interface make Profili a simple and great program to work with.

Give it a try Ollie. You will be impressed.

Paul

[flyingwings]

More:
Assume we try to find or design an airfoil with profill2 (including XFoil).
Specifiction(Sp) applied to a plane:
Re>30,000
Gross wt. 15 to 30 grams, including structure, motor, prop, rec., batt., and 4 servos.
Launch at airspeed (___), working airspeed (___) and landing airspeed (___).
Wing span and chord.
Max G's with maneuvering and working airspeed within sight.

Who can propose a R/C model design to the Spec? Who can defend the design with the airfoil? Is this spec. reasonable? If you don't like the spec, change it but defend it.

If the radio control system weighs 15 gm. That leaves 15 gm for the airplane. Attempting to use any airfoil from Profili would be irrelevant.

Using a well documented airfoil requires upper and lower surfaces that are for all intents and purposes rigid. A rigid surface is a heavy surface well beyond the weight restrictions applied here.

If you go to a lighter flexible surface the material will not retain it's shape during flight. Thus a Profili airfoil would be irrelevant as the program cannot account for shape change.

Furthermore at the specified Re it would be wise to consider installing a turbulator at 40 to 50 percent.

To fit the Profili criteria the aircraft would be:

Re= 51,000
12" wing span
constant chord 4"
Airfoil PS SN32
Lift factor at best cruise .5
Drag factor at best cruise .022
Lift factor at landing .8
Drag factor at landing .1
Cruise speed 16.9 mph
Weight of aircraft 47 gm. including RC
Landing speed 12.2 mph

If you would like the airfoil co-ords and polar diagrams I can email tomorrow night.

Note it is a semisymetrical airfoil with a camber of 1.32% and a max thicknes of .25 inch. Basically a flat plate.
I can show you how to sand Depron to approximate the shape. An exact shape is utterly impossible.

Note a thicker airfoil could be designed but at the low Re required the drag at cruise sky rockets increasing the power requirements.

Tomorrow night I can give you the size and location of a turbulator that will improve the flight characteristis.


Note the lift factor at zero incidence angle is .05.

Paul

[biber]

For the small chucky gliders have made of sanded depron years ago I've found it to be sufficient to improve the used airfoils just by trial and error (with some general ideas and basic rules in mind). That chuckies had wingloadings in the 2.5g/dm² upto 5g/dm² region. Chords from 15mm upto 100mm tested. Wingspans from 200mm upto 1240mm (limited by the depron sheet size). Used 3mm and 6mm depron. At first I had (just like most modellers do) some sort of clark-y clone in mind when sanding an airfoil shape on it. LE sanded round (:D) TE tapered to almost zero. The wing sections where never undercambered due to my limited skills to sand that properly, so they remained flat except for a little reflex on some tailless ones. Soon I found it to be much better to taper it down to practically zero towards the LE aswell. After some more testing, tossing, sanding and all that I came to something like a personal standard recipe. Sharp TE and LE, flat bottom, simple arced top (i.e. camber is half the thickness), thickness never exceding 8%, better 6% or even less. Worked quite well for me so I stuck to that. The RE-numbers applying to that would be roughly between 5000 and 50000 I guess.

That e.g. is one of the chuckies I've once done:
http://www.rcgroups.com/forums/showpost.php?p=3266143&postcount=21
I have to admit that this particular one wasn't built for maximum endurance or L/D but for the plane beauty of AR and to explore how fare one could drive the AR.

biber

[Ollie]

Paul,

I like your spec!

I think the SN32 airfoil is much like the S8025. I used the S8025, 60,000Re polars from Selig measurements. I checked your design results and my calculations using S8025. Right on but with a little more drag. I vacuum bagged S8025 2-meter V-tails using foam with 1.5 oz. glass. I think the process will be fine.

I would like your itemised weight budget for structure, battery, ESC, motor, prop, rec and 4 servos, please. I would like your itemised drag budget, too.

I am cooking crow now. (Yuk!) If your budget is ok, I will eat crow.

[Ollie]

Paul,

Are you OK?

Please don't drop this conversation. I have cooked the crow and frozen it. What next? Reheat it?

[flyingwings]

Paul,

Are you OK?

Please don't drop this conversation. I have cooked the crow and frozen it. What next? Reheat it?

Ollie

I won't drop it. I will reply tonight.

The difference in the drag and the lift polar are because I adjusted the airfoil for the lower Reynolds number. All airfoils are designed for a specific Re range. They are not suitable for all Reynolds numbers.

Low Re in the 30 to 100 k range require the max thickness and Camber moved forward. Many hours of playing with Profili emphasized that fact.

Changes in the leading edge are required as well however the average modellor cannot achieve the desired radii and curvatures. Mark Drela can. He is a real master at it.

Paul

[kartoffel]

Paul, as I understand things, profili is a user-friendly frontend to xfoil? Sounds like a great tool (both xfoil and profili). Gonna have to check those out.

Biber said:
I just doupt that one can expect getting serious info for REs below 100000 from Profili/Xfoil. The only guy I think who may be able to handle that is Mark Drela.

As the author of xfoil I'd expect Mark Drela to know better than anyone what xfoil is capable of. If it can generate good results for low Reynolds numbers, all the better!

Ted (the flat-plate potato)

[biber]

As the author of xfoil I'd expect Mark Drela to know better than anyone what xfoil is capable of. If it can generate good results for low Reynolds numbers, all the better!

Ted (the flat-plate potato)That's the point! He may know as how thrustworthy the data can be considered or how it has to be interpreted to get at least an idea of what's really happening.
To my knowledge the reliability of xfoil is quite good above RE 500000 and still acceptable down to about 100000. Below that I wouldn't thrust it too much. I'm sure though, Drela e.g. is capable of operating xfoil quite far into little RE areas decently.

biber

[Ollie]

biber,
"I'm sure though, Drela e.g. is capable of operating xfoil quite far into little RE areas decently."
Dr. Mark Drela's XFoil is the best tool that we have. The windtunnel tool we have compared to the XFoil program. Any tool is better than no tool.

[biber]

Hm, I think I'd better written "low Re areas" instead of "little", so that it couldn't be as easily missunderstood. I myself use xfoil quite often and think it is great software, but my skills in operating it are narrow limited. I don't think that I could really tell how trustworthy the results are that I get in the very low Reynolds regions. Drela certainly knows exactly what he is doing, what his software is doing, what it is capable of and how far he can trust it to match reality. So I think the AG series can be considered as being some of the most decently designed low RE airfoils available in the moment.

biber

[kartoffel]

Any tool is better than no tool.

True.

Admittedly I'm not an airfoils person, but I have mucked around with SMILE/DSMC and found that its particle-based approach works fantastically in the nonzero Kn and low Re regime. It's not the sort of thing where you could generate a set of polars on the fly as in Profili (great software, by the way), but I think it might be valid for low Re.

As far as optimizing low Re airfoils, I can think of several approaches, but ultimately it would take a bit of high performance computing combined with some real world tests.

[Ollie]

biber,
Thanks.
I know how you think, now. The XFoil "tool" has the Dr's skill depending on his results. I have no skill using XFoil.

I know that Dr. Selig and his crew got runs with large differing results, at 60,000RE, for same airfoil. However, one run only for S8025 at 60,000 Re. The reason Dr. Selig designed the S8025 airfoil by minimizing effects of turbulent bubbles changing around zero AoA. My faith but not my knowing hard facts about it.

[flyingwings]

Paul,

Are you OK?

Please don't drop this conversation. I have cooked the crow and frozen it. What next? Reheat it?

Good Evening Ollie

I went to an air show tonight. It's midnight now but I wanted to compare the S8025 to the PS SN32 with reflex at similar RE. The two compare almost identical with the exception that the PS SN32 is 2% thinner and as a consequence has slightly less drag.

The S8025 is basically a symetrical wing and does not fit withip Luffbery's criteria for a semi symetrical airfoil.

The S8025 will be heavier if built as a solid wing. However, foam can be cut away aft of max thickness to reduce the weight.

Paul

Paul

[biber]

As far as optimizing low Re airfoils, I can think of several approaches, but ultimately it would take a bit of high performance computing combined with some real world tests.At least that is what e.g. Prof. Loek Boermanns does at the Technical University of Delft in the Netherlands. On this side of the pond he is well known as a great expert in subsonic aerodynamics and he made the airfoils for many gliders and powered planes (ASW 24, ASW 26, ASW 27, ASW28, soon aswell the ASG 29 I guess, Antares, Extra 400...). He is comparing windtunnel test results and reality to the results of the developing software constantly all the time and may adjust the software from time to time. That's the only way to improve the softwares exactness and they seem to have come quite far on it. I guess that's basicaly the way Drela is going.
Our students gliding club (Akaflieg) once visited his institute and windtunnel in Delft where they were researching on BL suction at that time. Impressing! They know how to make real small holes into surfaces and lots of it within little time.

biber

[Ollie]

I'm a big fan of Dr. Drela. I built one of his first R/C designs (Apogee).
http://www.charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm
"Notes:
1. Polars not provided for tip airfoils as flow not really two-dimensional (2D) at the tip, so the analysis wouldn't be particularly meaningful... "

With a four aspect ratio for a wing, much of the wing, have no airfoil polars applied. Also, the 3D flow into the prop and the wash behind the prop applied to the wing. We have 3D prop flow and with no polars applied for part the wing airfoil. We must be careful how we apply airfoil polars.

You guys, kartoffel, biber and Paul,
You stir up my thoughts! Thanks, again.

Paul,

Please, with enough time for budgets.

[Ollie]

Paul,

Are you still OK?

Please don't drop this conversation.

[flyingwings]

Paul,

Are you still OK?

Please don't drop this conversation.

Good Evening Everyone.

Profili was developed to simplify the use of Xfoil. Stefano has accomplished that extremely well.

No matter what program is used the results will always be suspect. Dr. Drela and others have made comparisons of the various programs and found that there is always some variance between program results.

From a modellers or aircraft designers perspective Profili is ideal for comparing airfoils at various Re and alpha while being very simple to use.

Geez, I sound like a commercial. However, I have spent many hours over the past 3 years working with Profili and therefore have aquired an appreciation for a good program.

Attached are three JPG files showing a comparison between airfoils at Re=30,000

Spend a few minutes and study them. You will see how simple they are to read and understand.

Luffberry

Take a look at the polar diagrams. You will see why Profili would provide the answers you are looking for.

Attached is a PDF printout from Profili.

Ignore the ripples as that is the resolution of the system.

Profili will provide the DXF file as well.

Paul

[biber]

If I got it right from the diagrams, I'd say the S8025 has a quite sharp bordered narrow laminar dragbucket around zero lift, not much benefit in this case. And it has a dead band wich neither is any good thing at all. You may try once where the laminar separation bubbles seem to have their front ends, trip the BL just 5% in front of that and redo the calculations. That will add a little drag, but might also remove the dead band. In the pressure distribution diagram you can locate the bubble, the alfa function/command in xfoil offers in addition to that also a visualisation of the BL thickness where bubbles can literally be seen.
But the S8025 is reasonably thicker than the compared airfoils and its thickness of 8% does contribute to the dead band ond other low RE issues. I wouldn't expect too much of an airfoil much thicker than 6% at RE 30000.

biber

[Ollie]

I use a Mac. I want a Window capability so I can use Profili. I am behind times!

Coming back to the question, "Which airfoil for the job?" I can accept the SN32 as a better posibility for the job. I think a wing is better with a higher aspect ratio than four, taper in plan view and taper percent thickness. As the wing tapers, the Re decreases to the tip. The tapered wing airfoil increases the applicability for the polars, too. Maybe, at least two airfoils are better for the job.

I still have a question about the gross weight budget of the plane.

From my point of view, the airfoil(s) changes with many assumptions included in design.

We could not help Luffberry because of his narrow point of view.

I hope my point of view is not too narrow.

[Ollie]

This afternoon I got Quiet Flyer, August, 2005 and read about Park Side. This brought me up to minute about micro motors, Rec, ESC, micro servos, batteries. I agree that Paul's 48 gram weight, for the whole plane, is ready to fly.

Paul,
I EAT CROW!

[flyingwings]

This afternoon I got Quiet Flyer, August, 2005 and read about Park Side. This brought me up to minute about micro motors, Rec, ESC, micro servos, batteries. I agree that Paul's 48 gram weight, for the whole plane, is ready to fly.

Paul,
I EAT CROW!

I have never enjoyed the taste of crow. However, I have had to eat it on occassion. My wife is usually the one serving it.

Paul