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Author Topic: wing thickness?  (Read 16247 times)

Offline Howard Rush

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Re: wing thickness?
« Reply #150 on: December 18, 2009, 01:00:34 PM »
Yes, that's my guess.   I think the plots I sent you show that.  Igor has some tricks to avoid it.

I think a stunt wing should have as much lift coefficient as possible without doing perverted things.  I suspect what John and Ted are seeing is separation near the TE at high flap deflection.  For a given loop radius, I can't see how having a thinner wing than that with the highest Clmax would help. 
« Last Edit: December 18, 2009, 01:18:05 PM by Howard Rush »
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Offline Howard Rush

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Re: wing thickness?
« Reply #151 on: December 18, 2009, 01:35:24 PM »
Or as John said in the thread below, "Hey man....its all in how you make it stall!"
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Offline John Sunderland

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Re: wing thickness?
« Reply #152 on: December 18, 2009, 01:44:53 PM »
You and Ted may well be describing a real phenomenon, but it's difficult to figure out what it is.  Best I can tell, you guys have your own definition(s) of "lift" and are describing a reduction in pitch stability with increasing lift.  I usually read Ted's stuff pretty carefully, because he certainly knows how to design, build, trim, and fly stunt planes, but the "excess lift" stuff has me puzzled. 

Modelers use a number of technical terms differently than do disciplines from which the terms were taken.  Other examples are "cavitate", "moment", "inboard", and "power".   A definition is neither right, nor wrong, but workers in a field generally settle on unique meanings for the technical terms they use.  The standard definition for lift is the component of aerodynamic force on the airplane perpendicular to the direction the airplane is going relative to the air.  Thus the more lift, the more the airplane will accelerate perpendicular to the direction it's going relative to the air.   Although you perceive me to be a dummy because I don't understand the simple concept that "'To much lift' is the opposite of 'not enough'!", your superiority might be because I don't understand your private slang.     

Uh, I am superior to none and whatever expertise I have is trial and error. Wasn't taking a shot at you Howard. I went all out to design my own airplane, noticed differences from one to the next and discussed it with those more knowlegable than me. My fat blunt airfoil under the best available power was not settling in after abrupt change of direction and did accelerate out of corners and round shapes suffered. Going backwards a bit with a semi-blunt, thinner section, cured the problem in combination with airfoiled stab/elevator. It took about a year or so for me to let Teds/Davids works, ideals and long explanations sink in. Tried it and it worked! Voila, my scientific method. The Geobolt foil was used in my best effort to date. It had issues too, but corners were hard and flat and it locked in immediately upon return to neutral handle deflection. It had a higher aspect ratio than my nominal fat 58" span self designed fast foil and was now 63" with my tips on it. My current build will employ the same airfoil with 60" span and 630 to 640 sq" area and electric power. We will see next season. For me, I do not have to completely understand a concept in order to accept it and use it. As stunt power moves forward, the aerodynamics, however minimal, is bound to change some also.

Having stepped back and observing more than competing has given me some perspective on CLPA design and less reliance on text book aerodynamics understanding and terminology. I am no one to argue those points with. Proof is in results and I may be wrong here, but hey, I'm back in the game.  H^^

Offline John Sunderland

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Re: wing thickness?
« Reply #153 on: December 18, 2009, 01:49:07 PM »
YES!.... Serge and Howard! #^

Online Igor Burger

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Re: wing thickness?
« Reply #154 on: December 19, 2009, 12:15:15 PM »
Yes, that's my guess.   I think the plots I sent you show that.  Igor has some tricks to avoid it.

I think a stunt wing should have as much lift coefficient as possible without doing perverted things.  I suspect what John and Ted are seeing is separation near the TE at high flap deflection.  For a given loop radius, I can't see how having a thinner wing than that with the highest Clmax would help. 

It is necessary to look to moment polar of the airfoil. I attached polars of 3 airfoils  with the same amount of flaps (in area and deflection). The first is thin and sharp, second is thicker, and the last is “that my” .

Moment polar of such flapped airfoil has always a place, where the moment starts to change. This change of moment makes unwanted input to pitching rate of model in maneuver, also means change of neutral point, and center of pressure. And since it is abrupt, pilot cannot control it well. Result is difficult to make round loop, or corners with unpredictable angle, or height. The only way how to fly such a model is to trim it that model fly outside of that area (AoA) or goes over them quickly. Such model is sensitive to CG position, or flap/elevator ratio.

Moment changes as the separation point on TE goes more and more forward. The sharp and thin airfoil has very early separation and that separation is abrupt and happens on whole area. Thicker airfoil can abruptly separate from TE to hingeline, so it visibly keeps the lift, but moment polar shows problems. And blunt airfoil with smooth surface at hingeline can have late and smooth change of moment slowly with higher and higher AoA.

It means the best will be thick airfoil like that last one and model designed to fly inside its “safe” AoA. All my models are designed to fly at max 8 deg AoA in corner. It has still some reserve and in reality (circular flow) it has even safer properties. Result is that it allows almost any trim and it is usually also very insensitive to tip weight. That wing is used by me with extremely back CG, or Alex who placed 4th on WC with moderate CG location and also Richie K. current EC champion with extremely nose heavy setup.

But those thin and sharp wings can also fly. You can see that the separation happens already at 0 AoA, so it happen “for sure”. It had limited lift, but if wing load allows it, it can fly also well. It is something what we know from sharp versus blunt stab. Both works well … while moderate radius makes troubles  … the same reason is why I used flat airfoil on my indoors, thicker airfoil does not work well.

And one small note – my flap is flat, it also gives some chance for abrupt separation even with flat hingeline at 30 deg deflection. I think airfoiled flap (I mean the flap making his full airfoil, not integrated to airfoil of the wing) can help to some extent. If I remember well, Juno has such flaps.



Online Igor Burger

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Re: wing thickness?
« Reply #155 on: December 19, 2009, 12:31:24 PM »
One note to drag of thick wings.

I do not think it is reason of airfoil itself. Thicker airfoil has more drag, but the difference is only small and its drag in maneuvers (with flaps and at high AoA) is sometimes lower compared to drag of thinner airfoil. But anyway, it is known fact that thicker wings show signes of higa drag in corners. And not only that, it is usualy also visible on 4-2-4 run of wet engines as a boost after every corner as model accelerats to its previouse speed and thus makes motor leaner.

The reason is capability of such wing to carry more weigh. Loaded wing with thick airfoil will easily make high lift coefficient, but induced drag of such wing increases with square of the lift coefficient. Simply if wing exceeds some lift coefficient, then the induced drag is higher that airfoil drag. So if we push such a thick wing to high lifts, it brakes, while thin airfoil exceeds critical AoA, its airfoil drag can be higher, but since lift is limited, also induced drag does not increase so much.

However thick wing with lower wing load will not brake as much and there no need to make stress from high drag of thick wing.

Offline Howard Rush

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Re: wing thickness?
« Reply #156 on: December 19, 2009, 02:00:54 PM »
Wow.  That is very impressive, Igor.  Thank you.  I am glad that I have not cut the ribs for my new airplane yet.

For a given airplane weight, wingspan, and turn radius, wouldn't induced drag be the same, regardless of Cl or Clmax?

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Offline Howard Rush

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Re: wing thickness?
« Reply #157 on: December 19, 2009, 07:55:22 PM »
I got JavaFoil and tried a couple of airfoils at different flap deflections.  It looks like the flow stays stuck on the flap even at high flap deflections.  Does JavaFoil model separation on the flap accurately?

While I was fiddling with JavaFoil, I found that it can make cool plots of the flow field around the airfoil.  Here are two views of an Impact MAC airfoil with 20 degrees flap deflection at an angle of attack of 10 degrees.  The black lines are streamlines.  The color is pressure.  It doesn't give the whole story, because it doesn't have the finite wing effect, but you can see flow angle and "wake" position behind an infinite-aspect-ratio wing.  

JavaFoilshows that the Impact leading edge literally sucks at high angles of attack.
« Last Edit: December 19, 2009, 09:59:47 PM by Howard Rush »
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Online Igor Burger

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Re: wing thickness?
« Reply #158 on: December 20, 2009, 01:06:46 AM »
For a given airplane weight, wingspan, and turn radius, wouldn't induced drag be the same, regardless of Cl or Clmax?

induced drag = something * cl ^2 / AR

so if you extend the chord only (lower cl and lower AR), the induced drag will be linearly lower, but as the area linearly grows, the final drag will be the same

so lower drag needs either increades span instead of chord (like gliders) or even better lighter model  :-)

Online Igor Burger

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Re: wing thickness?
« Reply #159 on: December 20, 2009, 01:34:29 AM »
Does JavaFoil model separation on the flap accurately?

I do not know how much acurately, but does. However it is not visible on flow field analyze, you must look to boundary layer folder. You can see where is transition and separation points on both sides.

If you want play with AR, you can set it in options folder.

Regarding accuracy - I do not know how accurate could such analyze be if we cannot specify surface, ribs, prop disturbance etc, but at least it gives feeling what is happening there.

Offline Doug Moon

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Re: wing thickness?
« Reply #160 on: December 20, 2009, 09:21:30 AM »

Hi Howard

You will notice I said a tiny amount of weight differance...however if i build a wing say 1 inch thick and the same size wing 3 inches thick I willl need a longer metal rod for my BC mount, this will add weight, I also will need more material to glass and epoxy the center section, this will weigh more etc..

However as I stated the tiny amount of weight is not worth talking about

Regards
Randy

Isn't that just darn near exactly what Brad said?

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Offline Larry Cunningham

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Re: wing thickness?
« Reply #161 on: December 20, 2009, 12:46:49 PM »
Wow, I'm impressed with the serious analysis you guys are doing with
Javafoil. Awesome.

I need to watch these threads more closely, interesting and useful work
is going on.

Like everyone else, I'll be happy to absorb the fruits of your labors!  8)

Best,

L.

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Offline Howard Rush

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Re: wing thickness?
« Reply #162 on: December 20, 2009, 02:28:13 PM »
Isn't that just darn near exactly what Brad said?

Yes, and both disregarded the beam part.  The point was that the increase of weight with thickness due to a bigger surface area, hence more foam, sheet, and paint is trivial, the bellcrank axle weight growth is also pretty small, but the structure weight needed for a given wing bending moment goes down pretty fast as thickness increases. Something else could be sizing the spar, of course. If the thickness of wood on a foam wing suffices as a spar on the thinner wing, and is picked not for strength, but as a barrier to the solvents (or the Monokote iron) getting to the foam, then the thicker wing would be-- what was it?-- .09% + delta bellcrank axle heavier.  Also, if a thicker wing has more lift, it would require some more spar.  If you say that weight increases with wing thickness, which is the opposite of most airplane design cases, you might back up your assertion with calculation.
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Offline RandySmith

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Re: wing thickness?
« Reply #163 on: December 20, 2009, 06:50:51 PM »
Yes, and both disregarded the beam part.  The point was that the increase of weight with thickness due to a bigger surface area, hence more foam, sheet, and paint is trivial, the bellcrank axle weight growth is also pretty small, but the structure weight needed for a given wing bending moment goes down pretty fast as thickness increases. Something else could be sizing the spar, of course. If the thickness of wood on a foam wing suffices as a spar on the thinner wing, and is picked not for strength, but as a barrier to the solvents (or the Monokote iron) getting to the foam, then the thicker wing would be-- what was it?-- .09% + delta bellcrank axle heavier.  Also, if a thicker wing has more lift, it would require some more spar.  If you say that weight increases with wing thickness, which is the opposite of most airplane design cases, you might back up your assertion with calculation.


Howard

The extra amount of weight could very easlily  be calculated, and as you mentioned the ply spar that most use in a foam wing would also add a little more weight, as It would be thicker, thus heavier, and I for one use 1/16th balsa and would never think of going thinner, anyone doing so would be asking for problems.
However as i stated the weight is too small to argue over and would not be the determining factor in me building a thinner or thicker wing, I stated above why I don't use very thick wings, and weight has little to nothing  to do with it.

I  did not disregard "the beam" part. I didn't argue that point because there was no need for me to do so. My statement was for practical purposes in building a foam, ply and balsa wood toy airplane wing, which I can't buy  decimal sized wood, it would serve no purpose for me to sand the sheetining down a few thou of an inch for use in a thicker wing, nor to sand down the ply spars a few thou thinner to save weight in a thicker wing. This does make sense in full size ships since we would not be talking a matter of a grams.

Regards
Randy

« Last Edit: December 20, 2009, 07:09:42 PM by RandySmith »

Offline Bradley Walker

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Re: wing thickness?
« Reply #164 on: December 21, 2009, 07:18:04 AM »
If you say that weight increases with wing thickness, which is the opposite of most airplane design cases, you might back up your assertion with calculation.

"which is the opposite of most airplane cases"...  like give some examples?????  Who are you talking about?

Randy Smith = most prolific designer of the modern era (designed and tested dozens of new designs) + most popular supplier of pre built components stunter in the US for the last two decades (a man who has probably built literally hundreds of wings of all different sizes)

Howard Rush = never designed his own stunter (as far as I know) + has been flying the same plane for 8 years and does not like to build so may never build another at the rate he is going---no offense


Howard has taken what I said and twisted it to be something I never said.  I never said that thicker wings cannot be built lighter, I said they are heavier than thinner winged planes...  all things being equal (which I believe Randy pretty much backed up exactly).  No one has seemed to argue any  different.  In fact, everyone is eager to say I was right, however it was "trivial" to what degree.

Also, Howard seems to believe there is some magic to the ultimate strength that can be gained from fatter airfoils.  Within reason, I do not see it.  I agree a thicker beam will be stronger...  but to what end?  Remi Berringer out flew all of those fat winged stunters in Muncie in the 2004 WC.  His airplane did not seem to know it could not possibly be better.  Compared to an American stunter his plane had more in common with a free flight model than an Impact.

Lastly, what is thick?  What is thin?  No one has really defined what examples were used for the supposed "calculations".  This isn't a very well defined discussion at all.  It is just people throwing crap out that, as far as I can see is baseless.  At least *my baseless crap* was based on my personal building experience....  the wall of my shop is covered in weights of every single component from planes I have built.  I also have numersous spread sheets *calculating* what all the components *SHOULD* weigh *BEFORE* I built them.  I did this with several planes....  and that is number is *trivial* compared to Randy...
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Offline Serge_Krauss

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Re: wing thickness?
« Reply #165 on: December 21, 2009, 09:40:14 AM »
 Remi Berringer out flew all of those fat winged stunters in Muncie in the 2004 WC.

No, he - and it - did not.

To us all:

This is a stunt design board, and AGAIN we find discussions of each other - and semantics. ALL of these ideas can be addressed for their own merit and specific content without ever mentioning or obliquely referencing another personality. Regardless of what one feels about another's presentation, personality, or qualifications - in fact, regardless of how grievous one sees any given post to be - there is absolutely no compelling reason to address anything but its design content and/or specific assertions. So let's all cut the crap.

Edited to add a comma. Oh, yeah, and  - sincerely! - Merry Christmas, happy holidays, and a fulfilling, long, and happy life to all...or else!

SK

Offline Bradley Walker

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Re: wing thickness?
« Reply #166 on: December 21, 2009, 11:52:00 AM »
No, he - and it - did not.

I was there...  he did....  by a lot.

He damn sure did in 2006 in Spain...  even the judges saw that.

He was third in 2004 behind the Chinese guy (not flying a fat airfoil that I could see) and Billy which uses the same airfoil I use in the T-Rex plus the thickness of the sheeting top and bottom (if you want to consider his "fat" and mine "thin").

Didn't David Fitz abandon the Coke bottle airfoil for a much thinner airfoil to gain "penetration"?  Didn't he win the WC's after that?

Serge, it makes perfect sense.  Stunt is more about personalities than designs anyway.  People defend *whatever* because of the guy using *whatever* more than they defend the *whatever* because they can *prove* it works better.

This whole argument is stupid...  the guys who use fat wings will say they are the best for all kinds of reasons.  The guys who don't...  won't.  End of discussion.  Oh, and yes there will be some belittling and down talking along the way, just to be sure.
« Last Edit: December 21, 2009, 12:14:47 PM by Bradley Walker »
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Offline Howard Rush

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Re: wing thickness?
« Reply #167 on: December 21, 2009, 02:05:40 PM »
I just picked on you because you made the blanket statement that thick wings are heavier than thin wings. Being a nobody, I had to do some calculation to support my assertion that it's more complicated than that.  It's above somewhere.  I'm sure you reviewed it for correctness.  Examples? Here's some beam stuff.  I didn't spend much time checking it for relevance: http://www.engineersedge.com/beam_bending/beam_bending8.htm , http://courses.washington.edu/mengr354/jenkins/notes/chap3.pdf .  I don't do wing design myself, but I've sat through meetings on the subject.  Aerodynamics for those airplanes favored thinner-than-optimal wings; structures and fuel capacity favored thicker-than-optimal wings.  Sometimes the optimal wings turn out to be amazingly thick:  the wing root thickness of the B-17 and the DC-8 is 18%, of the B-24 and B-29 is 22%.

As I said above, and in the case Randy cited, something else could be sizing the spar. If the thickness of wood on a foam wing suffices as a spar on the thinner wing, and is picked not for strength, but as a barrier to the solvents (or the Monokote iron) getting to the foam, then the thicker wing would be .09% (as I remember) of the surface stuff + delta bellcrank axle heavier. Given such a wing with 1/16" sheeting, as you make it thinner, eventually the 1/16" sheeting + shear web will no longer suffice, and the weight of the requisite beam will make the wing weight go up.  As Randy also said (I think), weight doesn't matter much in picking the thickness of a stunt wing.

Like you, I have selected my stunt wing design based on experience and lots of experimentation.  In my case, as you note, somebody else did the work. I may try tweezing the airfoil some, so I am hoping to get some analysis (also outsourced to somebody who knows what he's doing) to reduce the risk of going to a lot of effort (and time, as you correctly observe) and coming up with a dog.  So far, the old Impact wing looks pretty good.  I know that any slight deviation I've made to it by hunch has come to naught.   

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Offline Howard Rush

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Re: wing thickness?
« Reply #168 on: December 21, 2009, 02:19:59 PM »
Howard Rush = never designed his own stunter (as far as I know) + has been flying the same plane for 8 years and does not like to build so may never build another at the rate he is going---no offense[/b]

You are pretty much correct.  I last "designed" a stunter in 1961, but it didn't amount to much.  I have been flying my current plane for two years; I flew its predecessor for ten.  You are not alone among Walkers at critiquing about my building rate.  No, I'm not at all offended by that. 

Serge, I don't think Brad is making a personal attack here.  I think he's just parodying what he feels is an unjust attack on his work that somebody made challenging his qualifications.
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Offline Jim Pollock

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Re: wing thickness?
« Reply #169 on: December 21, 2009, 05:40:33 PM »
OK Guys,

Let's keep it nice now... O.K.?  Since this is my board, I hereby declare Bradley Walker as good a designer as anyone else who has ever posted anything to this board!

Done!

Jim Pollock, Moderator    :o

Offline Bradley Walker

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Re: wing thickness?
« Reply #170 on: December 21, 2009, 06:58:31 PM »
Serge, I don't think Brad is making a personal attack here.  I think he's just parodying what he feels is an unjust attack on his work that somebody made challenging his qualifications.

That is correct.  I like Howard a lot.  He cracks me up!  Obviously he is quite brilliant.  Smarter than me.

I do, however, think these discussions get rather silly.  



A lot of dancing on the head of a pin, and posturing to protect ones turf.  Not always (I am not saying that is what I going on here), but it is quite common.  Like disagreeing with someone insults their intelligence, or worse yet, their legacy.
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Offline Serge_Krauss

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Re: wing thickness?
« Reply #171 on: December 21, 2009, 09:14:56 PM »
I was there...  he did....  by a lot.

As before, we will have to continue to disagree on this - "a lot." I too was there in 2004 and watched the tail of that plane wag all over the place in yaw and pitch. I actually like some of that plane's design and have some similar things in my sketch pads, not copied, but started from scratch. So I am not necessarily critcizing the design or plane itself, but certainly its trim. 'might have overloaded the wing (aoa) and tail (size, c.g.)? I also carefully noted sizes, shapes, and intersections. So you saw what you saw, and I saw what I saw. I am as amazed by your conclusions as you apparently are by mine. There doesn't seem to be a lot more to be said.

SK
« Last Edit: December 22, 2009, 08:33:59 AM by Serge_Krauss »

Offline Bradley Walker

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Re: wing thickness?
« Reply #172 on: December 23, 2009, 11:36:00 AM »
As before, we will have to continue to disagree on this - "a lot." I too was there in 2004 and watched the tail of that plane wag all over the place in yaw and pitch. I actually like some of that plane's design and have some similar things in my sketch pads, not copied, but started from scratch. So I am not necessarily critcizing the design or plane itself, but certainly its trim. 'might have overloaded the wing (aoa) and tail (size, c.g.)? I also carefully noted sizes, shapes, and intersections. So you saw what you saw, and I saw what I saw. I am as amazed by your conclusions as you apparently are by mine. There doesn't seem to be a lot more to be said.

SK

Planes have a tendency to waller around some when they are flown at the correct size, at slow speeds, with the correct amount of corner.

Of course, you will not see that when everyone else is wizzing around at blistering fast lap times flying 60 degree maneuvers with swooping corners (as was most of the field---including Werwage).

I believe Remi would have won in 2004 (as he did in 2006) if he had not been forced to fly in the rain for his final flight, and the contest was immediately stopped for Werwage halfway through his flight in the rain (which was not a good one), and then restarted again in stunt heaven air...  

The point is that Remi has whipped a bunch of fat winged, US style stunt planes (with pipes and all), with his skinny winged 4 stroke plane that looks like a free flight...

PS:  Shhhhh... don't tell anyone but that Sportster is not really very "strong" either (at least by foam winged jumbo airfoil US standards).  In fact it flexes a lot.
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Offline Serge_Krauss

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Re: wing thickness?
« Reply #173 on: December 23, 2009, 12:19:19 PM »
A couple observations-

1) Having watched Bill fly locally and heard his comments about F2B results, I am convinced that he deliberately flies differently in F-2B than otherwise. Here, his corners and bottoms are quicker and very clean, perhaps sometimes on the low side. There, he flew higher, did larger maneuvers and rounded out the corners. He speaks diplomatically, never to me disparaging the preferred corners, but leaving no doubt that he prefers the cleaner - more pristine appearing ones that also leave slightly longer sides in square maneuvers. It seems from internet comments over the past few years that F-2B corners might have tightened some, but nothing I saw there was like the (bobble-free) sharp corners I've seen him do around here. I did not think his WC flights were as "good" as what I was used to, but I believe he deserved the win.

2) My comments on Berringer's performance came from watching only two (three?) last-day flights, one of which was not from directly behind the judges, but rather close and downwind.

3) I think Berringer is quite good and flew quite well. His intersections were about the same as Bill's (not perfect that day), but on all the flights I watched, the plane wiggled after corners, and the repeated figures and adjacent figures were not as close to the same size. I felt that Bill's rounds were more circular, but then as a certain multi-time oriental champ showed, that was of little concern to anyone but perhaps me. That particular pilot flew diameters and tops that differed by as much as 4-7 feet on his worst scored flight (hard to judge higher in the hemisphere).

If I were to guess, I'd say the plane's wing with it's small flaps and apparently forward c.g. needed a high aoa, perhaps near its stall, and the small stab wasn't as stabilizing, even on the longish arm. 'could of course be wrong, but that would explain what I saw. It's a neat design, but I'm not convinced that it is what a guy of Berringer's talent needs to do his best. FWIW, the most impressive single flight I saw the last day was done by the other Mr. Walker.

As usual, FWIW...

SK

Offline phil c

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Re: wing thickness?
« Reply #174 on: December 24, 2009, 01:02:23 PM »
Yes, and both disregarded the beam part.  The point was that the increase of weight with thickness due to a bigger surface area, hence more foam, sheet, and paint is trivial, the bellcrank axle weight growth is also pretty small, but the structure weight needed for a given wing bending moment goes down pretty fast as thickness increases.

If I'm not mistook, the beam stiffness varies with the 3rd power of the thickness.  The extra lift from a thicker section is only a small fraction, say from 1.6 to 1.8 coefficient of lift going from 15% thickness to 18%.  So even small increases in the wing thickness make it much stiffer.  With some careful design you could probably drastically reduce the wing skin thickness and other material sizes, but as Randy points out, there are some practical limits.  Unless you go to match molded carbon fiber skins with balsa cores and minimal internal structure.

I hear a CNC mold this size only costs $5K or so.
phil Cartier

Offline Bradley Walker

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Re: wing thickness?
« Reply #175 on: December 26, 2009, 05:24:10 PM »
If I'm not mistook, the beam stiffness varies with the 3rd power of the thickness.  The extra lift from a thicker section is only a small fraction, say from 1.6 to 1.8 coefficient of lift going from 15% thickness to 18%.  So even small increases in the wing thickness make it much stiffer.  With some careful design you could probably drastically reduce the wing skin thickness and other material sizes, but as Randy points out, there are some practical limits.  Unless you go to match molded carbon fiber skins with balsa cores and minimal internal structure.

I hear a CNC mold this size only costs $5K or so.

Do carbon fiber wonder planes fly better?
"The reasonable man adapts himself to his environment. The unreasonable man adapts his environment to himself, therefore all progress is made by unreasonable men."
-George Bernard Shaw

Offline Erik Janssen

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Re: wing thickness?
« Reply #176 on: December 27, 2009, 07:57:37 AM »
Stiffness is only better with the 3rd power of thickness if the top and bottom of the I beam cannot shift their position, if they do the stiffness is the cumulative of the top spar and bottom spar alone without the multilication factor due to wing thickness.

So it is no use making a very strong top and bottom of the spar if you separate it by just foam or a 2mm balsa horizontal webbing. I prefer a 4mm vertical webbing between spars. I prefer spar against carbon as it is easier to glue to balsa.

I once flew a detroit wing and my fellow competitors informed me that the wing was flexing more than 4 inches. I wondered where that came from and found out it was held by the silk covering alone. I decided that it was probably broken the week before at another competition and as I came this far it would survive the final flight. It did although accuracy was gone but good enough to secure 3rd place.

At home I took off the covering and found out that the main spar was broken in pieces on both sides of the fuselage and repaired the model. 
   



 

Offline rustler

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Re: wing thickness?
« Reply #177 on: December 27, 2009, 03:21:08 PM »
Not sure if I'm reading this right. The value of thin webbing between top and bottom spars is that it converts two very small section independant spars into one integral I section spar. Stiffness is proportional to "D" squared?
Ian Russell.
[I can remember the schedule o.k., the problem is remembering what was the last manoeuvre I just flew!].

Offline Erik Janssen

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Re: wing thickness?
« Reply #178 on: December 28, 2009, 09:51:58 AM »
True, but if you use 2mm balsa lengthwise you will see that it is not enough to fix the position of the upper spar to the lower spar. My Flying Dutchman design from 1980 had this type of webbing and a very flexible wing.

Vertical webbing is a lot better, 2mm is already an improvement but I feel 4mm is needed to really fix the movement, once fixed you get the strength.

I believe that stiffer wings fly more accurate although my Detroit wings with an "I" beam from 8x3 spruce top and bottom separated by a 8mm vertical webbing of 30mm height worked well on my .46 airplanes.
The D tube on my .51 airplane with 4mm vertical webbing feels a lot more accurate to fly.

Foam wings without spars feel very rigid but I have not enough experience with this material to build them light enough.

Offline Bill Little

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Re: wing thickness?
« Reply #179 on: January 03, 2010, 01:33:27 AM »
Hi Igor,

I was wondering while re reading all of this about the effects that Al Rabe came up with on his Sea Fury.  He actually went to a slightly asymmetrical airfoil on his Sea Fury to help in certain areas of the schedule.  I would have to get that 1973 issue of American Modeler back out to verify the numbers he used, but he did specifically point out a change to the airfoil, and that it was not fully symmetrical.  I know he was working with very scale numbers, sticking to his Scale Stunt regime, but I have always found it interesting as to his thoughts.  The plane did very well!

Any thoughts?

Big bear
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James Hylton Motorsports/NASCAR/ARCA

AMA 95351 (got one of my old numbers back! ;D )

Trying to get by

Online Igor Burger

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Re: wing thickness?
« Reply #180 on: January 04, 2010, 10:33:35 AM »
Hi Bill,

Yes I know about that asymetry. Al wrote that reason for that is extra lift necessary for corners close to ground in triangles and hourglass.

Well ... I see some reasons for that, but I mean that if the airfoil works well at all used AoA then we not need such a trick. The airfoil makes the same amount of lift does not matter if we co corner 10 degrees, 90 degrees or 120 degrees. Important is radius and radius is the same. The only difference between 90 and 120 degrees is speed of the model which slows down more after 120 degrees than 90 degrees.

It is not big difference, because lift necessary for centrifugal force in that radius is of the same second power of speed as the lift of wing. So the wing needs still the same AoA and the same lift coefficient does not matter what is the speed. However gravity has stronger effect if we do it close to the ground at lower speed, so may be there is really some good reason to do something for lower corners of triangles. But I think it is much better to keep some reserve in calculation and not to allow airfoil go to such extremes. For example little lower wing load, or little stronger power train or bigger flaps or so. I wrote somewhere upper, that good airfoil should keep its linear part of lift polar in all used AoA, so we can do it that it will last little more it is possible. I mean that there are airfoils good enough and those others. I do not think that we can find something even better and even better. It is either good enough or not. :-)

And why NOT do do it? just because symetric airfoil is easy to calculate, easier for bench trimming and predictable in positive / negative G in symetric figures (rounds and squares) ... I hate if my model fly well either positives, but not negatives or negatitives and not positives .. but never both  n~

Offline phil c

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Re: wing thickness?
« Reply #181 on: January 04, 2010, 02:36:00 PM »
Yes I know about that asymmetry. Al wrote that reason for that is extra lift necessary for corners close to ground in triangles and hourglass.

....However gravity has stronger effect if we do it close to the ground at lower speed, so may be there is really some good reason to do something for lower corners of triangles. But I think it is much better to keep some reserve in calculation and not to allow airfoil go to such extremes.

And why NOT do do it? just because symetric airfoil is easy to calculate, easier for bench trimming and predictable in positive / negative G in symetric figures (rounds and squares) ... I hate if my model fly well either positives, but not negatives or negatitives and not positives .. but never both  n~

I suspect the reason for the cambered airfoil that helped most was to get more lift early in the control movement.  Al did not have much room for getting lighter weight, or bigger flaps, given what he was trying to do.  The hourglass especially can get very difficult in a bit of wind.  The plane accelerates towards the ground from gravity, plus the normal speed pickup coming out of a corner, plus the help from the wind.  If the controls generate too much hinge torque you simply can't move them fast enough against the extra control loads in the lower right corner.  An airfoil that generates lift quicker with less control movement could help a lot.

Doing things to reduce the control loads are a good idea too, as long as it doesn't affect controllability more than suits you.
phil Cartier

Online Igor Burger

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Re: wing thickness?
« Reply #182 on: January 04, 2010, 03:14:33 PM »
hmmm ... I do not know, but just as a quick look, I would say that if the airfoil is in its linear segment of polar, and if model is trimmed to be neutral, means the airfoil is either at its negative AoA of flaps are little up, then the same amount of controll defelction will do the same change of lift. The shape of airfoil will take effect only if it reaches its extremes. Means if the lift must be so high, that goes to stall, then it can be. ... However I wrote it in previous post - IF the airfoil is in linear segment .... etc

Regarding the handle load - I would say that if model has some weight and wing has some flaps to area ratio, then the hinge moment will be the same, does not matter what is the airfoil - again - IF the airfoil is not stalled ( at least partially). If yes, then it is another storyy and it can be so (for example separated on flaps) but I think we cannot design model to reach its stall AoA regularly in flight.

I agree with one - as Al wrote, that asymetry of airfoil will give higher maximal lift (critical AoA) even if flaps are little up in neutral. (flaps will limit lift in level, but will not limit maximal lift when it is needed). I mean that if the fixed part has say 1% chamber, and flaps needs say -2 deg deflection for neutral, then maximal lift at -3 deg flaps (flaps down, positive G) can be higher in absolute value than maximal negative lift at +7 deg (flaps up, negative G)

Offline John Sunderland

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Re: wing thickness?
« Reply #183 on: January 05, 2010, 03:58:02 PM »
By the way Igor, mw~ I think you explained all of this well...one decade ago or there about, you did. The graphics over time are much more defined!

I like playing with flyable designs based on my WAG! I accept the best available info and move on...got it the first time...still like Bill's foil.

Offline Erik Janssen

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Re: wing thickness?
« Reply #184 on: January 08, 2010, 01:10:10 AM »
http://www.iroquois.free-online.co.uk/rabefury.htm

The article in case you did not want to go through your archive.

Offline phil c

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Re: wing thickness?
« Reply #185 on: January 13, 2010, 01:40:26 PM »
hmmm ... I do not know, but just as a quick look, I would say that if the airfoil is in its linear segment of polar.......

The problem here is that a stunt plane(wing) does not have a "polar" in the sense used in the airfoil analysis graphs.  In level flight the stunt wing is pretty much at 0 aoa, with a very slight bias from either the flaps or stab to generate enough lift to support the plane.  When the pilot moves the controls the flaps move and the stab pushes the tail down.  The airfoil develops an angle of attack(between the chord line drawn from the LE to the TE of the flaps) due both to the movement of the flaps, and the pitch up of the fuselage.  At the maximum control movement used in a maneuver this might be 6-7 deg. from pitch up and 6-7 deg from the flap movement.  None of the conventional airfoil analysis accounts for the change in shape of the airfoil with angle of attack.  The only way to get close is to select the right numbers from a series of plots where the true polar is generated using the actual(airfoil+ pitch) angle.  So, run the program  with zero flap deflection. Then run it with say 5 deg. of flap deflection and select the number from where the AOA is about 10 deg.  Then run with 10 deg. of flap and select the number from ~20 degrees, etc. and plot the result at lift(and/or drag and/or L/D) vs. the true angle of attack.  You kind of have to guess at the appropriate true angle of attack for each specific degree of flap angle, since nobody has any measurements of airplane pitch vs. loop radius and control movement, except for a few examples of a tight loop.

The other thing to keep in mind that it is very possible to move the controls faster than the air can react.  The plane may fly one or more lengths before the airflow settles down. The racers do this all the time, wagging the tail to slow a plane down to hit a particular pit position.  You can do the same thing with a stunter.  So I can see plenty of instances where hitting the controls too fast or hard might cause all or part of the flaps or elevator to stall briefly.  This could cause all sorts of transient control issues.  You can see this if someone biases the loops to far into the wind.  As the plane comes around and goes up past vertical(the left hand side of a round loop) it will jerk with no imput from the pilot as the plane goes from flying into the wind to suddenly flying down wind and one wing loses a significant amount of lift. 

I suspect this kind of transient behavior is what Al was trying to take advantage of by biasing the airfoil like he did.  When the plane is coming downhill in the third leg of an hourglass the wing has to be producing virtually no lift in either direction and the flaps probably are biased up a little.  When you start to move the controls it takes very little effort to apply 5 deg. or so of flap and generate a lot of lift easily, compared to the amount of control force to actually get the controls to their maximum movement.  You may remember somes post from downunder where he built a plane with fully adjustable flap travel.  He ended up with only 5 deg. of flap needed to keep the plane flying in hard maneuvers, making the difference between a stally turn and a smooth one.
phil Cartier

Online Igor Burger

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Re: wing thickness?
« Reply #186 on: January 13, 2010, 02:19:32 PM »
Phil, I agree in most points. That transient AoA is known effect. I tried to estimate it some years ago. You can see data fro my old “Next” on my old pages. It is still alive. Look here:
http://www.netax.sk/hexoft/stunt/thenext.htm

You can see it in right column almost on end of page, value called “angle of attack wing before corner” what is angle where elevator can push the fuselage at its maximal deflection before wing starts to make lift. It is very pessimistic value. Then you can look at data of the airfoil on:
http://www.netax.sk/hexoft/stunt/notes.htm

The table under the airfoil and you can see that it will still work well (still before critical AoA, means on end of linear segment) at that angle and deflected flap.

I cannot speak for Al’s airfoil, but I expect that it will be also close to that value.

Since it is still not stalled, I expect that the airfoil IS in its linear segment.

You are also right that we have moving flaps, but does not matter what, the single points of polar with defined flap and elevator deflection are still on some place of polar, I think well designed model should keep them on that linear part and they usually ARE on linear part. And yes, the polar changes with moved elevator, so it is another curve, but that point can be still on its linear part. Once again here, I cannot speak for Al, but I do not think that he got it outside. May be I am not right, and his model stalls somewhere during normal flight, but if not, then the airfoil will be sensitive symmetrically.

The only difference I can see is that such asymmetric airfoil will have MAXIMAL lift in positive G’s better. And that is what I have seen several times written in his posts.

Where did you see him to speak about such sensitivity in transient?

Offline Dennis Moritz

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Re: wing thickness?
« Reply #187 on: January 14, 2010, 04:19:58 AM »
What's the matter with trial and error and doin' what the other guys do?

Offline Serge_Krauss

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Re: wing thickness?
« Reply #188 on: January 14, 2010, 09:40:03 AM »
What's the matter with trial and error and doin' what the other guys do?

There's nothing the matter with doing whatever you want to do. 'also nothing the matter with seeking to understand.

SK

Offline Dennis Moritz

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Re: wing thickness?
« Reply #189 on: January 14, 2010, 01:37:50 PM »
i was joking of course. But there is something to be said, I think, for evolutions of traditional designs that turn out to be an effective compromise. Evolutions that might be arrived at via dumb luck. The Gieseke Nobler, for instance. If the story told about it's derivation from George Aldrich's original is accurate.

Offline phil c

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Re: wing thickness?
« Reply #190 on: January 23, 2010, 07:54:10 PM »
.......
Where did you see him to speak about such sensitivity in transient?..........

.

I never talked to Al about this.  The transient response is based on my own experience, where getting into the third corner of the hourglass can be a very exhilerating experience because the plane takes its time starting the turn and tends to run wide.
phil Cartier

Offline Jim Pollock

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Re: wing thickness?
« Reply #191 on: January 23, 2010, 09:18:16 PM »
Phil,

The second turn of the hourglass can be very exciting as well, especially when you have too much weight near the outboard tip!!  Ask anyone that was on circle 4 on Thursday at the '04 Nats....

Jim Pollock   :o H^^


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