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BAGGRESSiVE forum  |  Long Distance  |  Articles  |  Topic: Airfoils
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Tibor
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« on: Nov 17, 2005, 11:01 »

Here is a short guideline to contemporary LD airfoiling, to be used for boomerangs of the MegaQuirl, Voyager, Buzz Whip, Backdraft, etc. type.

If we think about what LD booms are supposed to do, i.e. go out far and come all the way back, we can identify two airfoil characteristics:

1. low lift
2. low drag

The boom must have a little bit more lift than required to keep it in the air. Because of the small forces and moments, inertia dominates the motion and the boom will go out far. The boomerang must be launched almost flat, so that the small lift force may counteract the weight of the boom.

Of utmost importance is low drag to maximize the conservation of energy, which the boom needs to come back to the thrower eventually.

Here is an airfoil, not too aggressive, for you to start with: I use it on the Offspring, which is not a top performer, but a steady boomerang with range 100-120 m. Compared to airfoils of shorter flying boomerangs, e.g. booms for Aussie Round, the leading and trailing edges are more pointed, reducing thereby lift and drag. An important feature is the undercut, meaning material is removed from underneath the trailing edge, which reduces lift dramatically. Be careful with undercuts - too much and your boom will just fly straight out and won't return.

Booms with airfoils like the one shown above (Offspring) fly considerably far, while still being quite forgiving and stable in flight. For throws going out much farther, the airfoils need to be more AGGRESSiVE. The Backdraftis my competition boom with a distance 120-170m. Compared to the previously shown airfoils, the faces are carved much further into the wing, with smooth transitions. Also, the leading edge is rather pointed. These features let the boom travel out much farther, but also render it less stable and prone to crashing on the way back. You have to give the boomerang as much forward velocity and rotation as possible to keep it in stable flight.


Here is the airfoil Manuel Sch�tz uses for his Voyagers. It seems less aggressive than the Backdraft airfoil. But maybe he showed here a bit of a conservative airfoil, not his most aggressive one. Anyway, we can see that the undercut reaches up to half the material thickness, giving very low lift.

A special feature Manuel introduced is the concave trailing edge. He has booms with concave carving only on the top, and such where both sides are shaped concave. Manuel points out that this brings about a further reduction in drag, and judging from flights I have seen of Manuel's boomerangs, it appears that the ones with the concave trailing edge return with higher velocity.

In conclustion, it can be said that in order to attain a maximum distance, you have to go to the boundary of the stable to the unstable flight regime. The last bit of tweaking to approach this limit may be effectively achieved by tuning. Reducing drag is of primary importance. So use the thinnest material available that is still stiff and strong enough to withstand your throw without getting deformed or warped.

« Last Edit: Nov 21, 2005, 11:26 by Tibor » Logged
laurent
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« Reply #1 on: Nov 21, 2005, 19:00 »

Hello everybody

I still believe we can improve our airfoils. I have thought about it for years and I would like to share some ideas with all of you.

I am pesruaded that it is useless to carve our airfoil like the wings of a plane, for at least these reasons:

1- The airflow around the boomerang arms , which are 3-4 cm wide, and which fly at maybe 100km/h (just a rough guess), is not comparable to the airflow of a 1.50m wide wing that flies at 250km/h or a lot more. If you calculate the "Reynolds number" you find a very different result and I believe that curving an airfoil with such low Re is another world that classic airplane theorical basis.

2-When an LD boom is thrown very fast (say 100kmh, but of course it changes during the flight and it depends of the thrower -- how fast throws Manuel ?), the straight speed is a lot higher thatn the spin speed. What is the rotation speed contribution ? At 10 full rotation per second, with a 30cm wingspan, you can easiliy calculate .
That means when an arm is at 6 o'clock, it does not go backwards: it just goes a bit slower than the arm in 12 o'clock position, and it is entering into the air with his sharp trailing edge.

In other words, I don't think it has a sense to build a leading and a trailing edge for a fast boomerang. It is usefull for a slow boom and during hover periods, but not during the straight, fast flight periods of an LD flightpath.

I think that we could just build a trailing edge identical to the leading edge . Both pointed and sharp, or both elliptical and round, or whatever you want to build, but identical.

3- Does a pointed airfoil reduces drag ? Hum... not sure. This is an intuitive and interesting idea, but I have the feeling that it has not been proven yet. And I have the feeling too that very sharp airfoils produce very unstbale booms.
Some years ago Manuel S. experimented a concave trailing edge, finding it very efficient at the beginning. I think that he is not sure of the positive effect now (Manuel, If you read that, your comments are welcome Wink). I have tested it too and I am not convinced.
One thing for sure: as more material is removed, the boom is slightly lighter and then you can throw it faster. More speed, more distance, safe return: this is very logical. Is it the consequence of a better airfoil, or a higher throwing speed ? It might be only the speed, after all. Well, thta's what I believe.

I have noticed that this kind of airfoil are making more noise than more conventional ones. Noise produces a nice, impressive speed feeling, but it is caused by airflow turbulences (vortex) and for me it is the proof that the airflow is not so good. I guess that a perfect airflow should produce the less noise as possible, for a given speed.

That concave airfoil was a very interesting idea, though. maybe some other unusal airfoil could give good results ? I'm very intersted in reading your ideas.

Anyway, my best booms are not the ones with the sharpest airfoil. Not at all: I would even say that very simple (but carefully built) ovoid or elliptical airfoils have given the best result for me . 2.5mm thickness showed impressive performances too, but as everybody else I had some stiffness problems...

What is true for me is not necessarilly for other booms and throwers, though. But as mentionned Tibor there seems to be a boundary between performance and stability, and I have the feeling this boundary is near the elliptical airfoils. In any case, we can't compare an airfoil if the weight of the boom changes, and that's the point: with sharp airfoils we throw faster and have the illusion that the performance comes from the airfoil.

4- Has anybody tried tvery thin urbulators ? Like on real soarers, but thinner ? I don't know if it can work (Georgi Dimanchev tried it on MTA's, but flight speed are not the same). I tried some ones on LD booms, it's hard to see understand the dflight differences. But something happens (good or not)

That all for today, dear LD enthousiasts. I'm dying to read your comments, ideas, experiments, (aerodynamics and physics thesis if you want, he he)...

laurent
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GordonShuttleworth
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« Reply #2 on: Nov 22, 2005, 08:14 »

I was very interested in what was said in your airfoil article regarding 'noise'. To me Noise = drag but it would be impossible to thrust an airfoil through the air at speed with no noise. I cannot think of a single object that that travels at speed through the air that makes no noise. As is already known, most distance boomerangs make a high pitched swishy whistling sound as they fly, possibly this is the best we can hope for. I don't believe that a silent distance boom is possible. If there is a man who can make one I will gladly eat his socks.
  One word on airfoils. It is said that the best aerodynamic shape is a teardrop and this goes along with the thought that maybe sharp leading edges are not the best for performance. Maybe a teardrop shape has to be modified slightly to give the required amount of lift,I shall be experimenting.
                                                                                            Gordon Shuttleworth.
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laurent
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« Reply #3 on: Nov 22, 2005, 18:59 »

I don't believe that a silent distance boom is possible. If there is a man who can make one I will gladly eat his socks.

OK. Can I send you my socks via airmail ?  Smiley

What I wrote about a "silent" airfoil was purely theorical. I meant: an ideal,  perfect airfoil should make no noise, or at least very slight noise. Of course perfect things don't exist. I just meant: The best LD airfoil, with the lowest drag possible, should be more silent (or less noisy)  than other ones. And it seems that my best LD booms are not the more noisy ones, and this noise it not only lower, it sounds different. I just suggest that the vortex noise could be an indicator about the airfoil quality, among others.

About the teardrop shape: yes, this is the classic and proven theory, but it is verified for bigger and/or faster objects moving in the air (or any other fluid): cars, airplanes, soarers, some fish and snarks, even birds. But have we any proof that this the best shape for smaller and/or slower object ? At low Re factor, it is not 100% sure. It works very well, OK. But is it the best for us ? I think that this question as to be discussed.
And with an object that translates and rotates, the best airfoil might be different, as I said when comparing rotation speed and transalation speed. Our airfoils are cutting the air with their trailing edge during about 1/3 of the flight, or something like that.
Well, this is just an idea, a discussion to be continued. I maybe wrong, but I think we usually take some classic theories about boomerang for sure and we shouldn't: we should think again , and ask questions. What happens exactly ? Are the usual airflow theory useful for our booms, or not, or  should they be applied in a different way ? For several reason I mentionned in my first message I have the conviction that our usual understanding is  not correct (At least, a part of it is not correct. Of course there are other things I can't deny).

I remember that during the late LD World Champs in france there were an american guy who could help us: I don't remember his name, a thrower from Atlanta I think. Maybe Jason Smucker ?  If I remember well he works in aeronautic engineering. He could have suggestions for us. Ohe, Jason, do you read our discussions ?

Well, it's time to sleep here. I'm going to dream about streamlining...
laurent
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Tibor
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« Reply #4 on: Nov 24, 2005, 13:21 »

Our airfoils are cutting the air with their trailing edge during about 1/3 of the flight, or something like that.
I certainly agree to that and I think we have this fact mentioned in our ld book.
But for me, the implications of this are not clear (at all).
I think it explains partly why ld boomerangs with airfoils that should not create lift still return (or fly for that matter).

The tear drop shape has not been used on ld boomerangs for a long time now. Not sure if you think of the airfoils above as tear drops. If you do, what would a non-tear drop shaped airfoil look like?
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Lorenz
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« Reply #5 on: Nov 24, 2005, 14:46 »

We know that even perfectly symmetrical airfoils create lift if there is a non-zero angle of attack. Think about arcrobatic airplanes, they can fly normal and upside down equally well. It's the same with LD booms. In addition, with respect to booms there is another aspect: the 'angle of incidence'. The plane of the rotating boomerang and the direction of motion are not necessarily parallel, the angle in between is the 'angle of incidence'. This is an important parameter, having large influence on the lift and lift distribution.

Lorenz
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laurent
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« Reply #6 on: Nov 24, 2005, 15:28 »

I have to say that I am very happy to discuss about airfoils with you. I often "feel alone" when trying to think about aerodynamics and flight physics...  Sad

I really wish I had better areodynamics knowledge.

As Tibor, I often asked why a boomerang with a symetrical airfoil --and sometimes a negative angle of attack-- could fly. I agree with you Lorenz: it must depends on the incidence angle of the full boomerang , not only the angle of incidence on the wings. I have some ideas about that, but it is quite difficult to explain here, even with drawings.
Maybe the fact that the wings are cutting the air with the trailing edge is a part of the answer, as mentionned Tibor. But this works only at high speed, at low speed the boom should really dive. And sometimes it dives, sometimes not... There must exist some induced phenomena that we really don't know.

I answer to Tibor question about the teardrop shape: yes the LD airfoils are quite far to the basic teardrop shape, but we have still kept the main feature: very sharp traing edge and more rounded leading edge. I wonder if this feature still has to exist. Maybe a thin elliptical airfoil could do the job, with absolutely similar leading and trailing edges.
The problem of such airfoil is that less material is removed, then you have a heavier boom and you feel like it doesn't keep its speed as well. I am sure that the speed difference comes only from the throw.
It should be tested with a slighy lighter material. But beetween epoxy/glassfiber and paxolin there is to much difference of density.
Anyway... I am currently testing that kind of airfoils.

I wrote about turbulators too. This is a well know method used on full scall soarers.  Have look here too:

http://www.mh-aerotools.de/airfoils/index.htm >aerodynamics > turbulators.

(Yes, I like aeromodelling too, as you can see. I learnt more about aerodynamics in reading airplane and models books thant at school -hence my limited knowledge)

By the same way, I wonder for long if a glossy surface is better thant a rough paint. One thing for sure: when I apply a very glossy varnish on a boomerang that flies well, it doen't fly well anymore after that.

bye

laurent
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GordonShuttleworth
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« Reply #7 on: Nov 24, 2005, 16:01 »

Hi laurent,
              Your thoughts are very similar to my own, and I would think that if the theory of the trailing edges cutting the air first for part of the time is  correct then surely the answer could be a perfectly lenticular airfoil. Also regarding smooth or rough surfaces, we only have to think of golf balls. Another subject for thought is the 'Airobi' (sorry if the spelling is wrong ) The flying ring with a pronounced ridge of material around the outer upper surface of the ring. No sharp leading edge here, although I'm not sure how such an anular wing works. Don't forget, silence equals socks!!!!
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ld_238
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« Reply #8 on: Jul 02, 2007, 18:18 »

Had the same discussion with some LD throwers.
My hypothesis is: An airfoil that creates lift cannot be "silent" at velocities with Re >> 1
Reason: An airfoil can only produce lift, if there are vortexes behind the wing. And vortexes produce "noise" at a certain Re number. With "noise" I mean sound waves we're able to hear.
About concave trailing edges: It is indeed not yet clear, if they help to increase the distance. My second best throw was done with a Voyager variety without concave trailing edge...
Sience has not ignored the sound of airfoils. Example
http://adsabs.harvard.edu/abs/1985sedv.rept.....T
MHR, Manuel
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rikbeter
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« Reply #9 on: Jul 17, 2009, 12:08 »

Interesting discussion about airfoils you have. I am a newbee in LB boomerangs, but not in aerodynamics and aeroacoustics. So i would like to give my opinion.

In my opinion LD boom-foils should have 2 leading edges. (as Laurent has mentioned before) Especially at the highest speeds the trailing edge works as a leading edge half of the time. A sharp leading edge will result in flow separation from the airfoil. This will reduce the lift (which does not really matter, since the airfoils are designed to have low lift) but also induce some extra drag.
Since most of the distance is made at high speed, I think for this part of the flight, the airfoils should be optimized.

I have made an airfoil design with a web-based elliptic airfoil generator ( http://www.ppart.de/aerodynamics/profiles/Ellipse.html )  with 10 % thickness, and I slightly subtracted a small amount from the lower side and added this to the upper side, to let this airfoil generate some lift at zero incidence. One condition I have put in Xfoil to check if lift will be generated this way. And yes, when the underside was 1/3th of the height of the upper side, at 0 degrees AoA the lift coefficient was 0.4 at mach 0.11 and Reynolds 60000. Unfortunately, Xfoil did not give any clue about drag.
I have discussed this item also with an aerodynamic engineer, and his opinion was the drag induced by not having a sharp trailing edge is only small.
The resulting airfoils are attached to this message.

I like your discussion about noise. At my work we did some research on wind turbine blades and helicopters so I know something about aerodynamic noise sources.
I think boomerangs have 2 likely noise sources:
1) trailing edge noise. This is a broadband noise source.
2) blade vortex interaction. This is the tip vortex of the first blade hitting the second blade. This noise can be heard as small buf- buf- bufs in the boom-rotation frequency.

My first voyager will have an elliptic profile with very little lift.


* ellipse.jpg (114.05 KB, 1099x683 - viewed 691 times.)
« Last Edit: Jul 18, 2009, 05:55 by rikbeter » Logged

rikbeter
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« Reply #10 on: Aug 01, 2009, 16:09 »

Hi Everybody,

The more i think about airfoils of LD booms the less certain i become about my last post in here. I did some calculations about AoA on the wing and ask myself if i am right in my assumptions.
With zero wind the AoA is always zero degrees.

The stronger the wind the higher (more negative) the angle of attack becomes in the first part of the boom flight.
With little wind and these parameters:

Arm length from COG [m]   0.15
AoA (tuning) [deg]   0 
V forward [m/s]   25
Rotation speed [rot/s]   10
Wind speed [m/s]   3
Wind angle [deg]   20
Tilt angle [deg]   70

The resulting angles of attack are very small, varying from -0.55 degrees on the tip of the advancing wing to -1.1 degrees on the same tip when retrieving. The more wind, the more slope of the AoA on the boom. With wind, a small AoA is very good for lift distribution on the boom and will result in a longer straight flight until the turn.
But this is nothing new for you.
My question, is my AoA calculation right?

I think in the second part of the flight, AoA are huge and produce the necessary lift to make the turn.
I have no data about the third part of the flight.

In my last post i suggested 2 leading edges on both the leading and trailing edge. But with these small AoA maybe a sharp leading edge is possible. This will reduce the pressure stagnation on the leading edge and so drag. So maybe two trailing edges are the answer to LD boom profiles? This is what you people call an aggressive airfoil.

I searched the web for wind tunnel test data about sharp leading edges, but can't find anything useful about this topic at all. Can somebody back me up on this?

The good news is that i have access to a small demonstration wind tunnel. Its width is about 15 cm (maybe 20?) and the maximum velocity is 23 m/s. But its equipped to measure drag very accurate! When i find some time i will carve some 15 cm wide profiles to test in this wind tunnel, and measure drag at a variety of AoA and wind speeds. Then the difference between a double trailing edge or double leading edge can be measured. This will certainly take some time, because my first goal is to finish some booms and i shape all my profiles by hand with gentle force.

So if somebody is very keen on the results, and maybe has some clever tools to shape airfoils, you are invited to make some 2D profiles and send them to me. I will do the testing and send the results back or post them here.
Material can be anything. Due to the maximum speed of the wind tunnel, some scaling has to be done with regard to Reynold. My idea is to test a few airfoils (10% thickness> sharp - sharp, round - round, naca 0010 normal and reverse, etc) with or without turbulators and zero lift, since drag is most important for LD booms.
If somebody is interested, contact me to discuss the details.

Paul
« Last Edit: Aug 01, 2009, 16:13 by rikbeter » Logged

Lorenz
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« Reply #11 on: Aug 05, 2009, 16:45 »

Hi Paul

Interesting essay, I have tried to understand boomerang physics a few years ago, with the help of Felix Hess' thesis. One of the things I learned is that we have to distinguish between the 'angle of attack' (alpha) of individual boomerang wings and the 'angle of incidence' (psi) of the boomerang as a whole with respect to its direction of motion. The second important parameter of motion is the 'advance ratio' U, given by U = v / (omega * a), where v is the forward velocity, omega the angular frequency and a is the radius of the boomerang disc. The advance ratio U is a measure for the degree of reverse flow over the boomerang. At launch, with your values, U = 2.7, meaning a lot of reverse flow over the receding wing. 

I am not sure if I understood your calculation of the AOA, in Felix Hess' terminology, that would be the angle of incidence psi, right ?

I agree with you that at the beginning of an LD flight, the advance ratio U is large and the angle of incidence psi small. Upon approaching the maximum distance, the forward speed and thus U decrease and psi increases. On the way back, again the boom picks up speed, so large U and smaller psi again.

It would be a good idea to check the behaviour of an airfoil in normal and reverse flow. To me the question then is, how relevant is that for the boom, because these measurements are done in steady uniform flow, whereas we have unsteady flow, vortices etc in case of the rotating boom. What Felix Hess did in his thesis, he mounted different boomerangs on a support and measured forces and torques in a flowing water setup, while he could rotate the boom. Thus he could determine the forces and torques under relevant fluid dynamic conditions.

Please let us know how the experiments are going once you get started :-)

MHLDR
Lorenz
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