A prominent flyer who lives in the upper Midwest suffered an inflight failure of the wing on a T-Rex today, and commented on the same phenomenon of seeing wrinkles in the covering (film in this case) indicating the stressed area and deformation. Not sure if it can be repaired. I hope Rich's spectacularly beautiful Spit is strong enough to resist breaking; I don't like the looks of those wrinkles, and that is not a particularly high-stress maneuver.

Brett,
far be it from me to disagree with you on a structural issue, but the way I see it, the tips would need to be flexed up towards the viewer to create those wrinkles. this would be wash-in I beleive ( increased angle of attack at the tips. ) if the tips were down, or trailing edge up, that would make the wrinkles the other direction.
just my observation, correct me if my thinkin is stinkin!

Wish I could get a picture of the Avenger this clear, I wonder about the wing on it some times too.

Howard,, it would have to be carbon filled paint to help,,

Twisting moment from the flaps, maybe???
Dean P.

  And the load in general, I think. There's a pretty limited amount of torque that the flaps themselves can apply - line tension + pretty poor mechanical advantage. But if you distribute the entire maybe 60 lbs of load over the entire wing evenly what's going to happen?  It has a hugely stiff D-tube in the leading edge and a little skinny open box at the trailing edge - it pretty much has to warp the TE towards the lift vector.

    I didn't get a good angle to show the dihedral (and it's tough to get the right angle to remove the vision smear effects of a moving slot focal plane shutter (need a leaf shutter to do it right, really)) but I routinely see it with my eyes an almost all stunt planes - and it's not small. In this case it's essentially pulling more dihedral at the TE than the LE.

   The problem of course is that the chances of it being the same on both wings is not that high, depending on a whole bunch of factors we don't attempt to control. It's not a function of Rich's airplane in particular, it's a problem with ALL construction.

  Note that this is *exactly* what happened with the tests I did on different types of construction - D-tube, I-beam, and sheeted foam. The D-tube washed out exactly as implied by the wrinkles, the I beam too, but with about twice as much total deflection both in dihedral and washout, and the foam the least by far and very small to no  measurable warping. Lots of people told me it was either a bad test (bullsh*t, I think, was the consensus) or that it didn't happen in flight for some magical-thinking reason. Or that washing out made it more stable - which it probably does but also creates rolling moments if it isn't exactly the same, which it won't be. Its easy to pick up 1/32" of differential warp, when the whole TE is moving an inch, the chances are high that one side might be 1 1/64 and the other 63/64, oops, untrimmable rolling moment.

    Brett

>>It needs more paint to help distribute the load.<<

If that's true, I have the stiffest wing with the most distributed load in the world. Of course, it's also heavy creating more load.

Great picture!  Its hard to catch it just right, I know, I have been trying to get some dihedral shots on pullouts for awhile at the field and have not gotten a good one yet.  Its fun to see what the structure is actually doing in pictures like that.

  Part of the problem is that most SLRs have focal plane shutters and unless you get the relative motion stopped pretty well WRT the camera, you get image smear that can distort the image and make it seem like you have distortions you don't actually have. Above a certain shutter speed, the shutter is never open all the way - the front and rear curtain actually move at the same time, so the image is taken by a moving slot that gets narrower as the "shutter speeds": get faster. Its the same as the flash sync speed. For really good SLRs this is about 1/250th of a second. On something old like a Leica, it's 1/60th of a second. That has a lot of implications, but one of them is that the parts of the image are not all exposed at the same instant:

http://en.wikipedia.org/wiki/Focal-plane_shutter#Disadvantages

  For moving subjects this introduced distortion. The sensor in most point-and-shoots also has a similar problem because they are "scanned" during exposure like a TV picture, with similar results. Same with video.

   A leaf shutter actualy does open all the way, then close all the way, from the center to the edge and back. This is why they are generally pretty limited in speed but they don't have the same problem. But 1/500 of a second, which is about as fast as they go,  may not be fast enough to avoid motion blur.

   So it's actually a pretty difficult technical photography problem to catch the these inflight airframe distortion issues.

    Brett

Brett, I need to retract my earlier comment, I looked again, I missed that it was in an OUTSIDE manuever,, so you of course are correct in stating it has a washout condition,, sigh,, lesson to self, READ the whole thing before commenting

    The elevator and flap positions can give a pretty good idea how hard the corner was - that's about as much as you normally see.

    BTW, I only have one picture of the Avenger maneuvering inflight, and I *might* be able to convince myself that there is some dihedral, but the deflection is *tiny* and I think it was in a round loop.

     Brett

Context image, closeup image, Pretty level flight (at about 10feet!) and a fromal portrait to see how gorgeous the airplane is

Brett,
love the pictures of the Avenger, the level flight,, or maybe call it a flyover? lol,, is the best inflight one I have seen to date.
thanks for the comments, I am finally starting to be less tenative with her. At the regionals I probably had two dozen flights at that point.
Still not happy with something about the trim, but I have not figured out what it is yet. Paul and I talked, perhaps I am still of on my CG.
anyway, Love that picture,,

Looking closer at the level flight picture, I am somewhat surprised by the amount of up it appears that I am carrying in the elevator. Interesting,, I also see things in my body position I dont like in the context picture, I think I need a staff photographer to shoot me when I fly

Mark,  do you want Randy to use a 50 caliber or would an old 12 gauge work?   I looked at those pics and have to agree with Jim Lee when I told him it looked like his Sunflyer wing was flexing.  He stated it was not flex, that it was an optical illusion.  But, when I see the wrinkles in the wing of the Spitfire you know that is not an optical illusion.  Way back in the 60's a gentleman in Eldorado KS flew a plane the we thought was going to fold it wings, especially in the squares.  But, stated he had several hundred flights on that plane and it always flexed like that.  

   I don't there is any serious debate that the wings flex, and I think it's clear that it flexes a lot. Block the airplane up by the tips, and put, say, 5 lbs on the fuselage. It's going to flex a pretty good bit. So that in and of itself is inevitable. But it's also clear that its *critical* that even tiny amounts of warp cause huge rolling moments. Therefore, when it flexes, you want to minimize the tendency to warp. That's why I think sheeted foam is superior to other forms of construction - it deflects in dihedral with much less tendency to warp, too. It may or may not be heavier but that's a trivial issue with piped 40s and larger.

     Brett

This is one from awhile ago that made me wonder if there was really that much anhedral in the wing, or if it was some just distortion.  The wrinkles suggested the wing was bending quite a bit, but almost entirely near where the center section sheeting ends.

The other picture is a 120% Ringmaster maneuvering.  The even distribution of ripples in the covering is interesting, presumably the wing is flexing over the entire span instead of "hinging" somewhere near the root.

  Both of those look real to me. The first one is alarming - appears that there is a strong stress concentration right at the root and that it's almost in two pieces internally to the fuselage. Most of the anhedral looks to start in teh fuselage, not the open bay section (although that shows some, and some washout). That's a lot like Larry Fernandez airplane looked like right before the wing came in half in the middle - and was then held only by the fuselage. I bet it stresses the fuselage a lot right at the TE of the wing, too. Is that  a Pat Johnston or a Billy P-47 - I find it hard to imagine they wouldn't have done the center sections correctly. Note that the stab is bent the other way (as you would expect) and that, and that the distortion is in straight lines pretty well convinces me that its real.

    The other one looks pretty normal for Monokote airplane.

   Brett

Isn't that P-47 a modified T-Rex?

I am fairly certain that the silver P-47 is in fact a Trex. Looking at the nose opening,shape of wing, and fuse. I also remember hearing something about an issue with one having problems in the wing area, perhaps this was it?

one of the ones that later had a wing failure (albeit a non-catastrophic one).

 Called that one, I guess!  I wonder if that sort of problem (flexing in the center section) might not have something to do with the fuselage problems. Although it seems impossible that the center section sheeting was properly connected in the middle - there's no way you can pull any significant strain in tension even with 1/16 balsa,  over the space of 2". Almost all the dihedral seems to emanate from the center in this case.

       Brett

Definitely, one of the all time BEST threads that I am aware of.

Steve,
Based upon my personal experience, Polyspan will NOT contribute to wing stiffness. The avenger was covered in polyspan, and brought all the way up with dope to a sanding point, when I discoverd to my horror that the wing was still VERY whimpy torsionally. In other words, I could twist the wingtip easily + and - nearly a half inch with little effort, well much less effort than I deemed correct. At this point I covered it with silkspan ( yes over the polyspan) and it became RIGID as you would desire.
I would measure the rigidity enhancement of Polyspan to be somewhere between Monokote and doped silkspan. I will say this, Polyspan is VERY strong for puncture resistance though. I think if you are really concerned about this issue ( as we should be) the IDEAL wing is a foam wing, it has all the structural material essentially on the outermost surface as it should be to maximize the strength/rigidity of the structure.

I got that part Steve, still not sure that it would really make the difference in an Ibeam situation. with the carbon re-flexed (?) into the concave portions of the covering, its not going to be in the optimal position to add rigidity. It really needs to be in a linear form, as in straight , in line with the stress to maximize its strength potential.
Or so is my understanding, perhaps Howard would have better insight on this.
My option would be to add Carbon tow , Unidirectional, on the outer surfaces of the I beam spar cap thereby putting its strength in line with the anticipate flex direction. Of course to prevent the washout situation, you would also need to BALANCE the additional rigidity in the spar with reinforcement in the trailing edge as well, otherwise the rigid spar would not flex, and the trailing edge would flex the same as it did before, In this scenario, it would actually make the situation worse...
Another possible solution would be to "bury" carbon fiber Tow into the clearcoats on the polyspan diagonally, but again, I dont think this makes optimal use of the properties of CF

I think if you are really concerned about this issue ( as we should be) the IDEAL wing is a foam wing, it has all the structural material essentially on the outermost surface as it should be to maximize the strength/rigidity of the structure.

    As mentioned above. The counter-argument is that it is perceived to weigh more, and if you look strictly at the weight of "minimum" structures, that's correct. However, if you tried to make an I-beam as stiff as a typical foam wing, it would wind up *much* heavier because all the structure would be in sub-optimal places, therefore it would have to be much stouter. You can overcome that with exotic materials, but most people don't.

    One of the stereotypical ways to shoot one's self in the foot in this event is to do something silly to save an ounce or two, and *give up* something far more important.

     Brett

Hi All,

This is a great thread, and thanks to Brett and others for the nice pics. I've tried many wing construction methods myself.......although I hate building. The one wing design that I've seen but would never try to build, that does not bend or flex, is Graham Swallow's Wind Dancer designed by his late dad John. It is the old egg-crate construction used in some of the old free-flight designs. If I'm not mistaken, the Goldberg Sailplane used a similar construction.

The Wind Dancer in the photo was built by John Swallow and flown by Graham in the 2000 world champs in France. I watched Graham build the last Wind Dancer and it took 24 sheets of 1/16th balsa. I tried to twist this wing without any covering and it simply does not twist. Graham covered these wings in translucent Monocote. All that work certainly deserves to be seen! It was built using rectangles for each rib, and then sanded to shape with a special sanding block. I suppose that  these days it could be done on a good CAD system and then the ribs could be laser-cut. Either way, this is certainly the most rigid wing that I have ever witnessed.

Keith R

One tube is all it takes for a wing.  Commercial, pultruded carbon tubes are kinda wasteful as torque tubes.  If you wrap your own carbon spars, you can orient the fibers as required.  Here's an O-beam spar that's pretty much infinitely stiff, but still adequate as a beam: http://clcombat.info/sos.html .  My flap torque tubes have +/- 45-degree fibers and about .014" walls (I forget).  The are wimpy enough in bending to flex with the wing without binding, but really, really torsionally stiff: too stiff, according to some.

The JCT is coming to town.
We're plenty stiff from the flaps on down.

Y'all should take a look at a state of the art FAI Free Flight wing. F1A, F1B, or F1C. They're thin and stiff like an oak plank...without covering. They're very much like a stunt wing, except the D-box is a molded carbon cloth "taco shell" (45 deg. bias). The TE is a chunk of carbon strip, maybe 1/16" x 3/16". The ribs are balsa, with unidirectional carbon cap strips, which overlap the D-box and TE strip. The spars are spruce with unidirectional carbon at the outside surfaces (bigger on top...compression), with solid vertical grain between the spars. They're light, they're stiff, durable, take abuse very well, and they fly very well. Most have no diagonal ribs, but if they do, they're inside the D-box, where they do the most good. I will admit that they're not as slick as a stunt wing (for pretty points), but amazing technology. I have an F1B wing that's nekkid and open to inspection, Howard. PW's seen it, years ago.  Steve

Yes this is a great thread and it did answer one of my questions. I was debating whether to lay the glass cloth on the wing skins parallel to LE or TE or to bias it 45 degrees to account for the torsion stiffness. I will lay it 45 degrees and use a few strands of CF to help the wood in longitudinal load.

A wing that can bend two 1/4 inch spring steel bars is very impressive.  That must be a great launch. 

Yeh, Steve, it kind of takes the fun out of it.

Lauri, you are more athletic than this old guy.  I see what you mean about hard launches. 

Disclaimer. a C or D tube wing DONT pull the tips past 1/4 in.Can do it with fuse on bench and packers to STOP exceeding the 1/4 Ea. side. But youll see it does
As th Gs alledged to be 10 + this isnt irrelevant dimnsions.
While where here, the liptical wings bnding 1/2 way out .  Aerodynamically , it is a Square wing in its load inputs and outputs tangntally . Cuts evenly.

A Square wing has a elliptical Lift PATTERN , as the airflow at the tip is irregular to inner span. In both cases , what your seeing and what your getting
are TWO differant things.

And , a spruce spared detroiter , SILK covered ll bnd near an inch at th tips.Nose at belt , canopy at knee on your right leg.All yr mates'll wanna see you do it with there planes # @* ? !  .

Brett,
Thank you for the only shot of the plane I have in flight!  May she R.I.P.  Onward to the completion of my new plane!

Rich

   It's wrecked!? When did that happen? I think I can do better than that picture and I think you will like the Fun(d) Day Trophy.

     Brett