Control Line Cutaway

[Doug Moon]

Here's a cool cutaway of my model.

Pretty tough way to get that pic though. Spar failed. This is the ONE and ONLY time I will build a wing this way... I built it a long time ago and just finished it out this year. Around 40 flights. Was a promising model.

It is NOT repairable. The main structure of the wing broke. To repair it would require re-engineering the plane and that's not something that can be done in a way that would be worth it. This was a good plane, not my best, but a good one that was fun to fly. But I can risk motors and props on repaired models.

[Steve Berry]

What happened? It looks like a millennium-style wing with the bellcrank mounted in the spar instead of next to it.

Steve Berry

Sent from my SM-N986U using Tapatalk

[Mike Griffin]

Here's a cool cutaway of my model.

Pretty tough way to get that pic though. Spar failed. This is the ONE and ONLY time I will build a wing this way... I built it a long time ago and just finished it out this year. Around 40 flights. Was a promising model.

It is NOT repairable. The main structure of the wing broke. To repair it would require re-engineering the plane and that's not something that can be done in a way that would be worth it. This was a good plane, not my best, but a good one that was fun to fly. But I can risk motors and props on repaired models.

Doug was that a Millenium wing or one of your own design?  Hard to tell from the cutaway.

Mike

[Doug Moon]

I believe this was the millennium wing. I did mount the BC through the spar and I sure that was the end of it. I also fully sheeted the wing as well.  I am sure the failure is due to my building error and not the system.

I was exploring other ways of building back when I did this one, 2015, but I will keep on my current path.

[M Spencer]

well , I hope you havnt jumped on it .
Bryce gibso gave me a wrecked lite weight foam wotsit with the wing pulped at the center .
Being irrited I gave it to somwone who said ' I can fix that ' . ( He had brain cancer but well leave that out . Dont drink riverwater downstream ofd a farm dump for 20 years . They use toxic sh*t )

So the little monkey cut the middle six inches out , and rather'n 54 inches for a LS had 48 inches and someoneslse gave him one of those old max 35 ballrace rc things . The contyrols were a bit messy .
After going to a doctor for 18 months complaining of severe headaches , and told to pull the other one . He changed doctors . This one sent him to hospital for emergency surgery with a 60 % survival chance .*

The MORAL f this story is Dont Give Up . Whack the   ! #* middle out , Throw it in a profile . And SOMEONE will have a GOOD SHIP , that they can take out when they dot want to risk something flasher .
Effectively It'd cost diddly squat . ovelaid joiners in that side - a few peg holes in the other with both surfaces pre coated after the r h is cured . blah blah etc .

Sorry ya broke it , but its not the end of the world .

* Which he did . But that came later .

x # * etc . A N.S.W. Govt. Initative is POT HOLES . We even have them at the flying field . So doublers oif neccesity extend to the landing gear .
But if thats 1/32 ply , A few more each side aint gunna weigh 2/10ths of 5/8ths of S F A , as was the saying . ( like use enough glue for the joint , not enough to hold the world , whatever that meant .

I figure these guys usually knew a thing or two .



R.J.Mitchell . " If  anyone ever tells you something bout aerodynamics which is so complicated you cant understand it , you can take it from me , it's b*llsh*t " . But a few ,say tapered ( down to 1/8 x 1/16th at the ends -- 3/16 x 3/8 spruce across the next one , will cost maybe a 1/2 ounce .



The general idea is progressive , so as load bearing capeability is near uniform - for the actual load . There ALL going through the middle there , whereas out at the end theres not as many , so tapering things makes sense .

Cor . The pictures make sense , some of them , anyway . TRY FIGURE SIX . http://pdobson.weebly.com/uploads/1/0/2/7/10278742/wing_spar_cw_-_pdf.pdf never mind the gobbledegook . That might explain it . Lotta LOADS there . 
( theres a pile of stuff on spar design in the confuser ) .

[Matt Colan]

Here’s an action shot shortly after the wing departed the airplane. Heard a crack in the horizontal 8, and the square 8 finished it off

[M Spencer]

Not as annoying as it would be if it were MINE , but none the less irritated .
Funny Fing ,
The 10 ton ( Er , 2 Kilo ) MB-3 , when it was ' blowing ' gave big ' thumps ' in the lines ( jerks / short sharp tugs ) in the BOTTOM corners of the Squares , In & Out of the bottom leg .
Which''d indicate thats where its under the most strain . ( It started opening the bottom join on the centerline - I wiped in some epoxy )

Built with old tough unfinished fully sheeted 1/6th plus ( had to sand it ) wood a bloke had had in his tin shed rafters for ten years . Which in part explains the WEIGHT .

That ' diagram 6 ' explains the massive spike in load at the root , with the combined lateral ( lift ) and TORQUE / Twisting Moments . Something Id always discarded as neglgable . if not others .

Double Sheeting was a old standard through the center in the ' old days ' ( 1960s ) on many designs . Sanding bevels across so its not a sudden transition at the edge ,
Either pre sanded - block on glass sheet - edges marked - sometimes ends in scrap - creativity in clamping while laminating . Or
Pre Laminated Then ( Double thickness ) masked at edges ( Sides ! ) and knocked back to the taper with say 80 Wt. fresh paper - ' polishing ' smooth with 120 / 180 ?
a intresting things ' undressed ' timber's said to be more durable ( time wise ) than ' finished ' , for weatherboards at least .  So even if someones looking , its not neccesary for a super finish on the tapers . But STRAIGHT accurate tapers are best . 

Annoying to see that much work on anything go to waste . Dont discard the debri in a fit of pique . Some young snot could make good use of it , if you dont get the inclination to do a cut n shut job . Be good for a ball race 35 or fp 40 / LA 46 ship ,
reconfigured .

[john e. holliday]

Did you forget the ply reenforce on each sidem of the spaR/  I've the bell crank post throught the spar with ply on each side .

[Doug Moon]

Did you forget the ply reenforce on each sidem of the spaR/  I've the bell crank post throught the spar with ply on each side .

Yes I had ply on each side across the middle and out most of the way on each side. Still not enough...

[Gaylord D Elling]

Sorry for your loss, Doug. Like losing a friend. Parallels life, from a stack of sheets, to a beautiful piece of art, back to ... dust to dust.

[Jim Svitko]

Are you able to determine if the initial failure was right at the bell crank location, or somewhere farther outboard?

The last time I had a wing failure, it was a partially sheeted foamie, with the internal lite ply reinforcement glued into a slot of the forward internal foam spar.  I was told to do this on partially sheeted foam wings.  Well, the inboard wing failed right at the outboard end of that lite ply insert.

I should have known better.  The abrupt change of cross section at the outboard end of the lite ply spar caused the failure.  From now on, I fully sheet foam wings and leave that lite ply spar reinforcement out.  I do not think it is needed with a fully sheeted wing, anyway.

I am wondering if your wing failed for similar reasons.  You added ply reinforcement to the spar but how far out did the ply reinforcement extend?  And, was the outboard end of the ply tapered in depth to minimize the abrupt change in cross section that a squared off end would produce?

[Brett Buck]

Yes I had ply on each side across the middle and out most of the way on each side. Still not enough...

      Sorry about your airplane. Unfortunately, while I am sure you do, a lot of people still do not appreciate how much load you get on these airplanes with modern powerplants.

      Brett

[Jim Svitko]

When I got back into the hobby in the late 90s, I was at a contest where I was able to see, for the first time, an I-beam wing in flight.  I was positioned so that I saw the plane head on, thus getting a great view of the wing flex in maneuvers.  This was not a large plane, most likely something from the classic era.  Yet, the wing flex was quite noticeable.

Seeing this makes you wonder about the loads that these "toys" have to withstand.

[Ken Culbertson]

A fully sheeted D tube wing is extremely stiff.  There is very little flexing.  What this means is that the full force of turns is being transmitted to the wing fuselage joint.  With our obsession with weight we leave out everything we don't view as essential, like formers over the wing, double sheeting center sections, glass cloth on the inside of wing joints.  Then we reduce the crush strength by using molded 1/16" sheet on the top and bottom with soft fuselage sides and few formers.  Then epoxy it all together around the wing.  This works moderately well until you add heat.  The Epoxy's that we use start to soften at 140 degrees and what is directly under the center of the wing - a pipe.  Last Friday the temperature on our circle was 121 degrees at 10:00am.  With these behemoth motors vibrating away, it doesn't take much softening of a joint to start it moving culminating in a break.  From the looks of the break, I would guess that it first broke the top half of the spar on an outside corner then snapped the rest on a inside.  The forces must have been tremendous to snap plywood vertically.  That spar almost looks like it was hit by a laser. 

Ken

[Brett Buck]

When I got back into the hobby in the late 90s, I was at a contest where I was able to see, for the first time, an I-beam wing in flight.  I was positioned so that I saw the plane head on, thus getting a great view of the wing flex in maneuvers.  This was not a large plane, most likely something from the classic era.  Yet, the wing flex was quite noticeable.

Seeing this makes you wonder about the loads that these "toys" have to withstand.

   The wing sees about 45-50 lbs in a hard corner, maybe a little more.  They *all* flex, far more than you might think. What happens to the alignment and twist in the wing determines what it does, performance-wise.

     I have done some guesses at the lift distribution, and for a once-piece wing it's not all concentrated at the root or wing/fuse joint. One thing that is known to be a problem is failing to cover the entire wing with silkspan/graphite before assembly. If you build the whole airplane with bare wood, then cover up to the fuse sides, then it really does concentrate the stress right at the weakest point. That used to be entirely acceptable normal practice (although the Nobler plans show otherwise...) but not any more. This was a problem even with ST60s, and it is a vastly bigger problem with piped or electric.

      Brett

[Dave Hull]

Here's an approximation of the total load on the wing, assuming my brain didn't fall asleep....   zzzzz

[Matt Colan]

Here's an approximation of the total load on the wing, assuming my brain didn't fall asleep....   zzzzz

Wow!! Those numbers are fascinating!! Thanks for posting that

[Steve Helmick]

That's a shame, Doug. I guess the good thing about it was shedding the outboard wing, so the engine wasn't damaged.

Dave's calculations are interesting. Being a long retired F1A flier, I'd be interested in how those numbers would compare to Lauri's World Champion F1A. Wing typically around .30" thick at the spars, model weight around 16 oz, and pull on the towline at maximum zoom launch is extreme...but I don't have a number for you. Years ago, I did some calculations it looked to me like the loads were about the same as a typical F2B. But I'm not an engineer or math whiz.  Steve

[Lauri Malila]

  .

[Dave Hull]

Here's a single parameter variation for turn radius....

As is made clear by the v^2/r term, going from a 10 foot turn radius to the magical 5 foot radius DOUBLES the load on the wing. "Slamming a corner" means exactly that--the wing is getting slammed with bigtime loads....

Note that this study assumes a track made good, ie. the center of gravity of the plane traced thru the radius of turn specified. If your plane "skidded thru the turn" and effectively flew a larger radius, then the loads would correspond to that larger radius.

Just more food for thought.

[Doug Moon]

Here's an approximation of the total load on the wing, assuming my brain didn't fall asleep....   zzzzz

Dave, that is very cool!!  The plane in question was at 70oz and the wing weight approx 20 of that. The wing with flaps was around 15 so add in 5 for paint and clear and 20 is a good number. I wonder if that changes it much?

[Dave Hull]

Data for Moon case.....

Note the other setup parameters of 65' flight radius, 5.2 laptime, and 10' turn radius. If you think you are flying tighter than that, we need new numbers. The turn radius is the plane killer.

Dave

[Dave Hull]

To maybe better answer Doug's question, we can look at the fairly flat (boring?) curves for plane weight versus wing load, with everything else held constant. The fixed parameters are noted in the table.

Note that oversizing the engine/motor/battery/tank etc. is all suspended weight in the fuselage. It is great to have a ton of power margin and for the engine to "loaf" thru the pattern. But as is true in all aircraft design, the tradeoff will show up somewhere. We don't usually hear of the nose breaking off in a hard maneuver, so I'll suggest that the wingroot is where we see it first, most of the time.

There are consequences to weight, no doubt. But banging the corner as hard as you can go is the killer.

[Dave Hull]

For the academically inclined, here's an interesting article on the strength of birch plywood, and characteristic failures under tension/compression/shear using lab test coupons.

https://www.sciencedirect.com/science/article/pii/S0950061822015252

Note that main wing spar during a hard turn is experiencing all of these types of loads at once, and is not a simple case for a number of reasons. One that may not be commonly understood is the difference in stiffness (modulus) between the plywood and the balsa spars. Essentially, the plywood has to carry all of the load to failure (structural engineers call this "participation") since the balsa spar material is still stretching and supplying little loadsharing--up to the point that the plywood fails. I'd have to do the numbers for a composite beam to see how much the balsa was actually participating. In the joint shown in Doug's picture, I would start off with the assumption that all the balsa was doing was keeping the plywood from buckling. And that is a perfectly good combination as long as the plywood can carry it all.

[Ken Culbertson]

Dave, that is very cool!!  The plane in question was at 70oz and the wing weight approx 20 of that. The wing with flaps was around 15 so add in 5 for paint and clear and 20 is a good number. I wonder if that changes it much?

You think, last time I had an 8oz finished wing was on a Flight Streak...in 1959.  If that analysis is true, then we need to test our planes by propping up the wingtips and putting two 20lb dogfood bags on the center section.  Given the way the stress hits we might want to drop them on.

ken

In response to Dave's post while I was typing this.  You are correct that "Banging" corners is the issue.  The problem is what we call banging.  A 10' corner is really tight.  Most I am guessing are around 13' on 65' lines.  In a corner all of the forces are additive at the wing center section.  The weight of the fuselage is pushing down from above through momentum and the lift of the wing is pushing up from below.  If you have a solid wing center section, they tend to offset.  If however, the wing center section can flex you have problems because the weight of the fuselage far exceeds that of the wing.  Cracks will develop.  Doug does not "bang" corners.  They are tight and smooth.  One thing I have taken from this is that all future planes of mine will have a double planked center section.

[Dave Hull]

I'll assume that Ken's "dogfood bag" static load test is hyperbole.

The way aerospace does static load test is with a "whiffletree." It distributes the lift load on the wing per the analyzed lift distribution while restraining the fuselage. Propping up the wingtips and then loading the fuselage to the limit load is certainly going to be an overtest condition, meaning all you proved is that you can break things.

But when the wing failure being evaluated is at the root and you can calculate in the other direction (ie. coming from the defined maneuver load, not the lift distribution, AOA, etc.) then the simplified analysis is justified. The reason the model is set up parametrically is so that someone can put in numbers that are relevant to them and their model.

I will assume that the materials and workmanship in Doug's wing were excellent--which leaves us with the conclusion that the center section was underdesigned.

I will defer to the real stuntheads what banging means. What I meant was that a tighter and tighter turn radius is the wing killer. And, having seen the paths travelled in that discussion before, I'll let the ProStunt-types discuss actual turn radius for their style and equipment, again, staying out of it....

Dave

[Tom Luciano]

I broke a lotta stuff over the years not just in modeling , I'm looking at it slightly different.

   It appears to me that the fuselage failed here. It's hard to determine where it gave way first but I'm thinking for a break to happen that far inside the wing it had to be flexing that far in the fuselage. It doesn't appear that there are any formers(no criticism) in the fuselage between fuselage sides, and top and bottom blocks.
  My last crash with my Crossfire ended with a break similar to what Doug has here. When I was rebuilding it prior to the '21 Nats I butt glued the two wing halves together as there was no way to get the joining spar out of the foam wing without making an absolute mess. In turn I ended up reinforcing it with 1/2oz. cloth. After, rebuilding the fuselage around it, I was unsure it would hold. So, I ended up glassing an oval shaped pattern over the wing and fuselage out over the wing maybe 6" out on the spar/highpoint.

  The plane has flown in some pretty nasty conditions since the rebuild and is still going. I'm thinking a made a truly monocoque joint.

My $.02 $2.00

[Brett Buck]

You think, last time I had an 8oz finished wing was on a Flight Streak...in 1959.  If that analysis is true, then we need to test our planes by propping up the wingtips and putting two 20lb dogfood bags on the center section.  Given the way the stress hits we might want to drop them on.

    That is not what it says. The load is distributed over the entire span to one degree or another, the weight is also distributed, hence my comment about lift distribution.

   I think the 68 lbs in the example spreadsheet is also not realistic, note the corner radius and then try to work backwards to the required Cl. Another thing (that makes the load higher, not lower) is that the load described is for the entire airplane, not just the wing, the tail is also lifting, but in the opposite direction. This can be substantial and if you thought that the maximum load on the tail occurred at the same time as the maximum load on the wing, that would add another 5+ lbs. It probably *doesn't* occur at the same time, and there are substantial dynamics that are only partly-understood.

   I also note that the level-flight velocity is *also* not considering the effects of maneuvering slowing the airplane. This is the thing that made such a big difference in the structural failures. Construction techniques and materials that were fine from 1940 to about 1985 suddenly weren't any more, hence Doug's problem an everyone else's. I have had two outright failures and a couple or near misses, one where I found about a 10" long chordwise crack right at the end of the landing gear spar on the way to the field one day. Most of my airplanes since 1989 have had repairs from structural failures or weaknesses, and my airplanes are built like tanks compared to what we used to do.

  Use the same calculation (which is pretty easy, it's mv²/r) for 13 feet then it gets down to the 45-50ish lbs.

    Brett

p.s. anticipating a bunch of issues - the conventional unit for mass is *slugs", a mass of 1 slug has a weight/force of 32.174 lbs at 1G, v is in feet/second, r is in feet.  A 64 ounce/4 lb airplane has a mass of 0.124 slugs.

    Note for those cursed with French units, 1 slug = 14.59 Kg, and also note that kg is a unit of *mass*, not force, and that the SI unit of force is the newton. 4.45 newtons = 1 lb. Lbm (pound mass) and Kgf (kilogram force) are not fundamental units.

[Mark wood]

Generally, for loads calculation of a wing we use an elliptical lift distribution. It's a close enough approximation to design a spar for a wing. Basically take the calculated flight load apply it elliptically. That then is used to generate the bending and shear along the spar from tip to the root. There are other ways of distributing the loads for analysis but they generally lead to less efficient structures. Empirically works too such as in this case. 

The failure on the plane appears to be overload of the center section. I can't tell if there were spar caps or not but the appearance is that there was basically only a plane beam, web, with reinforcement plates at the center. Not the best beam (spar) design for the application. Next model add some doublers in the center. They don't have to be full size webs. Otherwise I think the design is good enough as it didn't buckle out board.