Stress Sandbagging

[Peter in Fairfax, VA]

In full scale home built aircraft, there is a custom, if not a requirement, that designs be tested, perhaps to breaking strength, by piling bags of sand or cement on the wings.  They call it prototype static destructive load testing.

Could this be applied to CL stunt?  If so, how, in detail?

The reason I'm curious is that I've been using epoxy fiberglass to fix some wrecks, so far with good result.

thanks,

Peter

[Peter in Fairfax, VA]

A result

[Peter in Fairfax, VA]

Example full scale test

[John Rist]

People lives are not a stake for an U-Control airframe.  The test is - Just go fly the thing!  I did see one ringmaster wing inflight failure.  Fun to watch but nobody got hurt.  We already have pull test requirements at contest and supposedly at local flying sessions.  Don't need any more testing.      

[Colin McRae]

Talk about full scale destructive testing, some might remember a TV series called Wings. The hour-long show used to feature the development of various commercial and military aircraft.

One episode I remember was about the development of the Boeing 747. As part of the engineering and development, Boeing fabricated a real wing and the associated fuselage section, anchored it in the center, and used a hydraulic powered system with framework to deflect the wing up to simulate loading until it broke. Was an amazing video to see.

(I don't think I will try that on my CL models)

Other actual 747 flight tests included:

The 747 on a takeoff roll, the test pilot on purpose over rotated the plane and forced the tail to drag on the runway until the plane went airborne. The plane was required to safety take off.

Another test involved a landing. Once the plane touched down, the test pilot locked all the brakes and kept them locked until the plane came to a stop. The plane was required to come to a stop with only landing gear damage.

The show narrator said they used to pay these test pilots quite a bit.

[Dave Hull]

Peter,

Sure, you could do static testing. However, it does not have to be to limit loads. You can test to the max working load.

On experimental amateur-built aircraft I don't believe there is a load test requirement, but instead they emphasize flightworthiness testing (FAR 21.191g?). Any stress testing would be up to the builder/owner. And they would not very likely test to destruction. If you were going to make and sell kits, then maybe. The manufacturer would want to separate design flaws from construction flaws for liability reasons.

Sandbags are a reasonable way to span-load structure. You have to make sure your distribution is a credible approximation of the distributed air loads. Taking your plane weight, multiplying by your G-factor and applying it to both wingtips while you hold down the fuselage is a severe overtest, and doesn't directly prove much. On full-scale distributed load tests they often use a whiffletree arrangement. It helps distribute loads and does not apply moment constraints when there shouldn't be any. (Again, avoid non-representative testing.) One question then is what lift distribution are you expecting from your fat, flapped, single-spar wing? Torsional loading also occurs. And then the discussion of whether we are really designing for stiffness and aerodynamic stability or just for strength. Both are requirements, and would require different tests.

For a model airplane you could build test articles (wings, stabs, etc.) and test to failure and reinforce where needed and take away structure and weight where you guess it is not needed. (Akin to HALT/HASS testing.) In my experience, buckling is a factor in a high percentage of the failures I have seen. And for real shapes, predicting buckling is a pain. I would note that I have seen very few people "engineering" a model, meaning that almost no one calculates stresses in wings, covering, etc. because it is a composite itself (even if just wood and doped polyspan) and that makes it difficult. Further, most won't have a good number for the actual max G's in the wind (how much wind?) or how much FOS to use due to the non-isotropic nature of wood, or for the construction inconsistencies. So everyone uses rule of thumb, tribal knowledge and PLANS. If the design is weak and a good builder blew out a wing, then upgrade where he tells you he had the problem....

Building models is lots of fun, but it really isn't anything like designing a full-scale aircraft.

Dave

[Jim Svitko]

As Dave said, it could be done, but it would not be a simple task.  I spent years in the structural test labs of General Dynamics, Lockheed, Gulfstream, and Bell Helicopter.

To test a structure, we were given data as to the load distribution.  The amount of load was also given.  Whiffletrees and other arrangements were used to apply the required load.  The loading data was obtained from sources such as wind tunnel data, finite element models, and maybe calculation.

The structure was also instrumented with strain gages and deflection gages at specified points.

I would think that for any such test to be meaningful on a model aircraft, the test would have to be done pretty much like the real thing.

[Peter in Fairfax, VA]

Well,  I guess I'll google "Whiffletree."

[Dave Hull]

I actually went thru a structural load test on a model last night. Somehow, a possum got shut into the garage without me realizing it. In the middle of the night, I heard a lot of banging, and I groggily assumed it was coming from the construction site next door; something left loose on the scaffolding and getting blown around by the wind. I must of woke up the second or third time enough to realize it was my aluminum garage door--which meant I had a cat or a possum stuck in there. Turned out to be a possum. With all the airplane projects strewn around I could just imagine how much stuff was destroyed. Possums like to climb which means they knock a lot of stuff down--and onto things like half-finished models! After "extracting" the possum at some ungodly hour this morning I was afraid to go look at the carnage once the sun came out. Yep, the possum walked all over an airplane, load testing the covering. That particular plane was on the long list of OPPs (Other Peoples Planes) waiting for refurbishing since the silkspan was brittle and had split in 3-4 bays along with a plethora of other age-related issues. The amazing thing was that the little guy walked all over the wing and left footprints in the dusty tissue--and didn't poke a single hole! Never trust a possum to do a legit fabric test on your ragwing--they are waaaay too gentle!

[Jim Svitko]

Well,  I guess I'll google "Whiffletree."

https://en.wikipedia.org/wiki/Whippletree_(mechanism)

[Jim Svitko]

To those who do not know any better, model aircraft are far more than a toy.  They are just like the real thing.  They obey the same laws of physics, the same laws of flight, and they are not very tolerant of poor judgement, pilot error, or shoddy construction.

[Paul Wescott]

https://en.wikipedia.org/wiki/Whippletree_(mechanism)

I think this is a slightly better link:  https://www.precisionballs.com/Whiffletree.php


.

[Howard Rush]

I saw the headline and thought Paul Walker was flying Advanced. Never mind.

[Ken Culbertson]

I saw the headline and thought Paul Walker was flying Advanced. Never mind.

[James Mills]

 

I saw the headline and thought Paul Walker was flying Advanced. Never mind.

 

[Brett Buck]

As Dave said, it could be done, but it would not be a simple task.  I spent years in the structural test labs of General Dynamics, Lockheed, Gulfstream, and Bell Helicopter.

To test a structure, we were given data as to the load distribution.  The amount of load was also given.  Whiffletrees and other arrangements were used to apply the required load.  The loading data was obtained from sources such as wind tunnel data, finite element models, and maybe calculation.

The structure was also instrumented with strain gages and deflection gages at specified points.

I would think that for any such test to be meaningful on a model aircraft, the test would have to be done pretty much like the real thing.

   It is not a simple task and it's not necessary to test to destruction.  I did tests kind of like this a long time ago (to my surprise, 45 years ago...) with the goal of trying to determine how various construction methods warped. Results were as expected.

    Brett

[Ty Marcucci]

People lives are not a stake for an U-Control airframe.  The test is - Just go fly the thing!  I did see one ringmaster wing inflight failure.  Fun to watch but nobody got hurt.  We already have pull test requirements at contest and supposedly at local flying sessions.  Don't need any more testing.      

That was a loud "CRACK" as the wing came of fin an inside loop.


Yep, that was one of my Ringmasters from a kit by Tom Morris,,He was experimenting with lighter wings and the LE was way to light,, Inboard wing stayed intact.

[Paul Smith]

There was great video posted of Boeing stress testing the wing assembly for a new airliner.  The wing was suspended in a contraption that suspended the wing while applying hydraulic load to the center.  Employees were allowed to witness the test from behind a protective transparent shield.  The goal was 150% of maximum load.  To me, 50% safety factor didn't seen like much, but airplanes need to fly.

A large digital indicator showed the load.  The employees loudly cheered when it hit 150%.  Then they continued to increase the load with load cheers at every percentage point 151%, 152%,,,

At something over 159% the wing exploded into tiny pieces in massive failure.

For a model, the test setup would be more expensive than just flying the plane.

[Paul Walker]

The idea of testing a wing to failure is something a number of us have thought about. I spent a fair part of my career doing that. I saw various F-18 tests, static, fatigue, and drop while on that program. While on the 757 program I witnessed the static failure test. It had already passed the static ultimate  and Boeing simply wanted to know how much extra strength was there. On that test I was standing directly behind it. As the wings passed ultimate load ( 1.5 times the max load it could see in flight) the wing tips had deflected 17 feet vertically and they looked like they could touch each other ( which was far from possible) but the sight of them deflecting so much was amazing. To this day I still snicked when on a commercial flight in turbulence and some passengers freak out by how much the tips deflect, maybe 4 or 5 feet.

My next job was running a test for an AWACS plane that was based on the 707 airframe. The Navy determined that the loads on the outer section of the wings as they flew them were higher than the 707 was designed for. They had Boeing run another static test to failure to show that it was good for those loads. It failed at 143% of limit load, thus a failure. Boeing's CEO's son had run that test. He was removed and I was assigned the task of re-running the test and making it pass, but I couldn't change anything....hmmm..

It turned out to be a very interesting job. I had not appreciated the complications and magnitude of tolerances that went into predicting the exact failure point.  It's a long story but the prediction I gave was 162% of LL, and it failed at 162.5%. Everyone was relieved!  The bottom line is it gave me a real understanding of the complexity of those type of predictions.

In terms of testing a model wing there are several things that must be considered to make the test worthwhile. First you must determine the flight loads on your model. Brett suggested 15g's as a baseline. If you can estimate what radius corner your plane will execute, it can be calculated. Then it can be compared to your max CL and wing size to see if that is possible. You need to balance these two. Next you must determine the span wise distribution of that lift. Constant is too conservative. For setting up the load fixture to apply the loads you will also need the chord wise distribution. It is not accurate to distribute it evenly chordwise, as that is not how it works. Then to design the fixture to restrain it you need to know the mass distribution of the model. Is it a single motor design in the fuse, or a twin or greater motor'd  plane. If more than one motor,  loads will have to be applied to represent the mass of those mass concentrations. In the simple case, part of the fuse will have to be replicated to get an accurate reaction to the applied loads.

This can give you an idea of the load system. Next  the test model. It should be made of representative materials. The wood should be the exact same density as the real model, and the finish should also be exactly like the flying model. Prior to designing the first B-17 I did tests on balsa to determine its strength. I found the strength to be very linear to the density of the wood. Building with 4 pound wood builds light planes, but they are not as strong as their heavier brother made the same way but with 6 pound wood. Good wood selection is critical the the failing strength of a model, as well as a good design. Your test article and flight article must be identical.

These are a few of the issues that you will have to address. The item of loading will require a waffle tree to make it accurate. Most people don't want to mess with that, and I understand. Short of that, your test will not be accurate, and could easily be unconservatine. Sandbags are quite inaccurate as most of the vertical load is not far from the LE, and holding them there is difficult, and the force they create is not "exactly" in the right direction. Yes, I have seen planes placed upside down and sandbags placed on the bottom side of the wing. I question how accurate that test really is. Is the shear, moment and torsion correct at the root??  How did they represent the drag forces?

This is an simple example of some of the issues you will run into to do a representative and accurate test. Yes,  you could make numerous conservative assumptions to make things easier. But how will you know that every decision really is conservative, or the magnitude of each. You could guess right on 9 which were not significant, and guess wrong one one that is and end up with an unconservative test.

I have pondered that for a long time. Then I got the idea of back calculating what I really wanted, which was the strength of the center section of my wing. I started looking at various models and their center section cross sections that carried the wing root bending loads. From a strength basis  there really isn't much that carries the root moment. I am talking about built up balsa wings  not foam wings....this is another discussion for another time. To my surprise, well not really, was there was not much carrying that load. I then took these sections and used the strength of 6 pound wood and back calculated the moment they carried. I wanted this as the B-17 was take apart and I wanted to know the load on the spar caps at that takecapart joint. I could then compare that to a load based estimate from the aerodynamic loads. The B-17 had the advantage of the reduction in root moment because of the distributed mass of the motors!  The result was a load of about 400 pounds at the pin. I have designed all my take apart planes with that magnitude of load on the take apart hardware. So  think about this next time you build your wing at the center section.

BTW, the tensile strength of balsa is around 1000psi *lb density.

Hope this highlights some of the issues involved in testing a model.
 

[Dan McEntee]

That was a loud "CRACK" as the wing came of fin an inside loop.


Yep, that was one of my Ringmasters from a kit by Tom Morris,,He was experimenting with lighter wings and the LE was way to light,, Inboard wing stayed intact.

   Most of the strength of a stock Ringmaster comes from the leading edge, unless you discard the biscuit and the ply doubler brace. If I'm building a scratch built S-1 Ringmaster, I always make the splice for leading edge on the outboard wing tip. In the search for that elusive 20 ounce Ringmaster, I think they lost sight of this fact. Sean built an RSM S-1 Ringmaster kit and it eventually got weak in the leading edge center section and needed beefing up, and he did question it before building it but did it as it was on the plans anyway. The 20 ounce Ringmaster is a perfect example of the argument that you can build a model too light!! I have been very happy with my 29 to 31 ounce examples and that includes a Brodak ARF.
  Type at you later,
  Dan McEntee

[Jim Svitko]

I had a great time working in structural test labs.  Those of us in the labs had some interesting experiences.  Everything from testing a rather simple component, like a control horn, to full scale static and fatigue testing.  A little bit of everything.  Some of the tests, and the resulting failures, were spectacular to watch.

Talking to the structural designers and the stress guys was very interesting.  They would witness tests and be sweating bullets.

I saw an F-16 wing taken to failure.  This poor test article had been thru the wringer already.  Fatigue tests, static tests, then a test to failure.  I remember it made it to 138%.  Not too bad for a worn out test article.

Not much boredom in a test lab.  At least, not for me. 

[Brett Buck]

In terms of testing a model wing there are several things that must be considered to make the test worthwhile. First you must determine the flight loads on your model. Brett suggested 15g's as a baseline. If you can estimate what radius corner your plane will execute, it can be calculated. Then it can be compared to your max CL and wing size to see if that is possible. You need to balance these two. Next you must determine the span wise distribution of that lift. Constant is too conservative. For setting up the load fixture to apply the loads you will also need the chord wise distribution. It is not accurate to distribute it evenly chordwise, as that is not how it works.

   For my testing, I presumed the lift distribution was elliptical. I didn't try it, but if you assume each square inch shares the lift equally, or spread it equally, 15gs will almost certainly break any normal construction. I was not interested looking at the ultimate strength, but the way it distorted/warped when loaded. For this, you also need to deal with the chordwise lift distribution. I never figured out how to estimate that, so I just shifted it fore/aft

  So, you have a (nearly) point load of the fuselage (15g x mass of fuselage)+ the total lift distrbuted spanwise, and somehow arranged chordwise.

    This was a Imitation wing built 3 different ways - foam, D-tube (with construction nearly identical to a Nobler), and an I-beam. The Foam and D-tube were monokote, the I-beam was covered with GM silkspan to give it a chance. Qualitative results were:

Foam - least total deflection, least dihedral, and very small warps no matter how you arranged the weight chordwise
D-Tube - small total deflection, small dihedral. Warped to "wash out" no matter how you arranged the weight chordwise, but almost didn't care about which way you put the mass on in chord.
I-beam - very large deflection to the point I thought it might break, usually warped towards wash-out, but varied greatly depending on where you put the weight chordwise.

    At the time, I could build these size airplanes at about upper-40s/low 50s, and I was supporting one half of the wing in a cradle while cantalievering the other one. So the total weight was about 22.5 lbs distributed as indicated. I was pretty impressed how much it deflected, even the foam deflected maybe 3/4" and the I beam actually bottomed out on the bench, at about 1.5", so I had to go back and shim the cradle higher. 

    An arguable defect of the experiment is that I (consciously) disregarded the weight of each wing, although they weren't greatly different. I figured it was built how I could built it, and that the wing was maybe 20% of the total weight of the airplane, and I was looking to confirm my guess that foam warped less, more than trying to save weight or shave it as close as I could. I was looking for construction that varied the least under load*, and was at least a viable weight.

    A 625 square inch airplane at 48 ounces with an ST46 was about the low end of what people were doing at the time, and while some people might do a little lighter, I reasoned that the consistency would make mine easier to fly and stay that way from day-to-day better than others - some of which were gangbusters one day, and a disaster the next weekend.   I still think that, and I still think it is worth a few ounces here and there to make sure I am not continually chasing whatever structural variabilty there is (day to day or airplane to airplane).

   Brett

*a concept of robustness not from model airplanes, full-scale aviation, spacecraft design (although it applies there, too...) but from a completely tangential area - stereo amplifier design!  It's why you can get a Pioneer or Yamaha amplifer with .0005% total distortion, but it doesn't sound too good compared to something very much more modest design (say a Dynaco ST-150) with 1% distortion. It's because it is very easy to design an amplifier with low distortion driving a (very specially constructed) 8 ohm resistor and not nearly as easy to drive a real loudspeaker with all sorts of reactive loads varying all over the place with time and frequency. A really good transistor or hybrid amplifier might also make a good arc welder, need 10 amps, no problem.

[Ty Marcucci]

   Most of the strength of a stock Ringmaster comes from the leading edge, unless you discard the biscuit and the ply doubler brace. If I'm building a scratch built S-1 Ringmaster, I always make the splice for leading edge on the outboard wing tip. In the search for that elusive 20 ounce Ringmaster, I think they lost sight of this fact. Sean built an RSM S-1 Ringmaster kit and it eventually got weak in the leading edge center section and needed beefing up, and he did question it before building it but did it as it was on the plans anyway. The 20 ounce Ringmaster is a perfect example of the argument that you can build a model too light!! I have been very happy with my 29 to 31 ounce examples and that includes a Brodak ARF.
  Type at you later,
  Dan McEntee

HI Dan.. This LE was a formed 1/16" balsa "C" with no internal bracing.. Later a horizontal brace the length of the LE was glued in place making it an "E" . This was/is stronger.

[Ken Culbertson]

I am trying to draw some conclusions from this thread.  There appears to be a tradeoff between failure from massive stress on the center section and fatigue failure from repeated flexing.  Building lighter fuselages would help but it is nearly impossible to distribute the weight of a piped IC or an electric.  The remaining balsa is trivial compared to these.  Putting a heavy fuselage on a light wing seems to be the trend and this also may be the problem.  Personally, I have only had one wing fold, and I have always had tighter than average corners.  An autopsy showed that it was defective wood that allowed for the fold when the plane hit a rather violent vortex in a triangle.  The reason I bring up wood is two fold.  When you get down into the 4-5lb range you are getting very close to pulp which has very limited flex before snapping.  The other is that we don't double sheet center sections anymore - why is that?

Next is lift distribution.  The more that lift is concentrated at the center where the weight concentration is appears to be better.  A twin will do that by increasing the airflow from the props but so would a variable airfoil that produced less lift at the tips.  I was the eventual recipient of a wing build by Tom Neiburh before his passing that had such an airfoil.  I did not have the plane long enough to fully evaluate it but the few flights I had before losing it in a fire were eye opening in how effortlessly it turned and locked.

So what is the conclusion?  Mine is that the amount of junk we have to put into our fuselages means we need stronger wings which adds up to heaver planes or less/lighter junk in the fuselage.  Then there is always the non-starter perfect solution - stressing softer corners. 

Ken
   

[Jim Svitko]

Ken, now you understand what airframe designers and stress engineers go thru.  I have to give them credit.  They do a remarkable job.

I think if we stay with known techniques, and not try to get too cute and overwhelmed by saving weight, everything will hold up just fine.

I once tried to build a version of the Coyote.  Thicker wing, I think covered with polyspan.  The plane came out very light, high 30 ounce range.  That wing lasted about 20 flights before I started seeing cracks right at the edge of the center sheeting.  I should have known better than to do that.

Another experience:  I built a partially sheeted foam wing, with capstrips, made to look like an all-wood wing.  Covered with polyspan.  I was told that on a plane of that size (600 squares or so), I needed the internal spar reinforcement in the foam wing since it was only partially sheeted.  So, I did as recommended.  Again, I should have known better.  Where did that wing fail?  The inboard wing folded right at the outboard edge of that spar reinforcement.

The lesson here is to avoid abrupt changes of cross section.  I built another one, fully sheeted, no spar reinforcement.  That plane is still flying and no indication of cracks or other structural issues.  It might be a bit heavier but so what.  It performs very well.

Just build, using what has been working for you, and move on.  I tried to make something lighter and better but it did not turn out to be better.

[Paul Smith]

I have gone to this setup on planes I build.  No spars. No spar notches.  There is 1/16" x 1/4" spruce piece hidden among the 1/16" balsa sheeting.
The center section is also covered with extra light fiberglass.

The taper wing of the Ringmaster series helps by bringing the center of lift inboard compared to the rectangular planform of simpler designs.  Centerline spars are useless.

[Jim Svitko]

Maybe a diagonal cut of that center sheeting, from the forward edge of the trailing edge sheeting, to the aft edge the leading edge sheeting.  Eliminate the stepped pattern.

[EricV]

As someone who has broken over a dozen planes over his knees to throw them away, I can tell you with great confidence that we on average overbuild our planes to a great extent. There was one particular plane that required a swing of my axe or two to finish off the fuselage which had left a bruise on my knee while trying to dispatch with the previous method sans axe... (mostly for fun and payback)

Silliness aside, while the above is all true story, I really do think we overbuild in some areas and under build in others. the most fragile and easy parts to destroy seem to be the stab joints, and the wing at the edge of the fillets on foamies where I didn't use a stub spar yet in the old days. Unlike some ARF's that have a poorly placed joint at the flap hingeline, I think most of our fuselages could serve successfully as baseball bats. Wing flex doesn't seem to be an issue for fully sheeted foamies, but they can snap at the edge of the fillet if you are careless and sand the sheeting there... or in the center if not joined properly. But overall, I don't see a lot of weak structural issues in modern times as much as I see over built structures.

EricV

[Brett Buck]

But overall, I don't see a lot of weak structural issues in modern times as much as I see over built structures.

   Depends on if you believe that "stays together" is the sole criterion.

     Brett

[Peter in Fairfax, VA]

In fact the yellow sheeted foamie at the start of this thread failed originally at the fillets, due to a crash.  Joined again with epoxy and light glass cloth, flies fine.

[Brett Buck]

In fact the yellow sheeted foamie at the start of this thread failed originally at the fillets.  Joined again with epoxy and light glass cloth, flies fine.

   How was it covered? Just up to the fuse sides, or all the way across?

     Brett

[Peter in Fairfax, VA]

The sheeting went all the way across with no breaks.  That is, until it failed when I crashed it.  Butt joined it, then glassed it.  Flew it this week, as well as last season.

[Brett Buck]

The sheeting went all the way across with no breaks.  That is, until it failed when I crashed it.  Butt joined it, then glassed it.  Flew it this week, as well as last season.

   I meant the silkspan/graphite matt. It needs to go across the center section joint, if you don't, it is pretty likely to come apart right at the root. That's what got the New Jersey guys, and was diagnosed by Ted - which they ignored because they were mad at him!

  If you are covering with silkspan, polyspan, graphite, etc - anything that stiffens up the surface, you need to cover the wing from tip-to-tip before you install it, or at least beef it up structurally to tolerate the stress concentration you caused.

     Brett

[Doug Moon]

>>> Personally, I have only had one wing fold, and I have always had tighter than average corners.  An autopsy showed that it was defective wood that allowed for the fold when the plane hit a rather violent vortex in a triangle.  <<<<
\
Ken
 

Well your construction method on that wing bears out with this thread. IIRC you removed a large section in the center to install a "component style" control system. Like a box installed with all the "stuff" in it. Cool idea but in the end that removal of the section did that wing in. Even with carbon rod reinforcement the box installed in the middle did not allow for the transfer of stress and force across the entire span of the wing. The forces found the weak point and broke it.

[Ken Culbertson]

Well your construction method on that wing bears out with this thread. IIRC you removed a large section in the center to install a "component style" control system. Like a box installed with all the "stuff" in it. Cool idea but in the end that removal of the section did that wing in. Even with carbon rod reinforcement the box installed in the middle did not allow for the transfer of stress and force across the entire span of the wing. The forces found the weak point and broke it.

It broke at the edge outboard wheel.  If I had been a foot higher it would have continued flying like yours did.  Strangely, that is where the pieces landed.  The wing flipped up in the air about 6' and fluttered down right where it is in the picture.  You are right about it finding the weak spot, it just wasn't the center section.

Ken

[Brett Buck]

It broke at the edge outboard wheel.  If I had been a foot higher it would have continued flying like yours did.  Strangely, that is where the pieces landed.  The wing flipped up in the air about 6' and fluttered down right where it is in the picture.  You are right about it finding the weak spot, it just wasn't the center section.

   Thats a very common place for a failure, two of my airplane developed large cracks in the sheeting just outboard of the gear blocks. I now extend the gear spar out beyond the end of the gear block, on the bottom only to add some toughness/crack stop ability.

   One of these cracks I found on the way to the field, a week after a contest. It was maybe 9-10" long, chordwise. It was incredibly lucky that it managed to stay together through the previous flight - which was 1st place at Golden State!

     Brett

[M Spencer]

7.781 Mosquito's cant be wrong . https://billabbott.wordpress.com/de-havilland-mosquitos-what-woods-was-the-wooden-wonder-made-from-footnoted/

[Doug Moon]

It broke at the edge outboard wheel.  If I had been a foot higher it would have continued flying like yours did.  Strangely, that is where the pieces landed.  The wing flipped up in the air about 6' and fluttered down right where it is in the picture.  You are right about it finding the weak spot, it just wasn't the center section.

Ken

Aha! I thought it was the center my mistake. You had referenced double sheeting so I thought it was the middle.

Mine folded because I messed up the spar. I even thought it looked wrong but proceeded... oh well.

[Matt Colan]

   Thats a very common place for a failure, two of my airplane developed large cracks in the sheeting just outboard of the gear blocks. I now extend the gear spar out beyond the end of the gear block, on the bottom only to add some toughness/crack stop ability.

   One of these cracks I found on the way to the field, a week after a contest. It was maybe 9-10" long, chordwise. It was incredibly lucky that it managed to stay together through the previous flight - which was 1st place at Golden State!

     Brett

Funny you mention that Brett. I’ve been wondering what the cracking was on the Ferrari plane at the landing gear mounts. Next plane will have to have the spar modified it seems.

[Jim Svitko]

Funny you mention that Brett. I’ve been wondering what the cracking was on the Ferrari plane at the landing gear mounts. Next plane will have to have the spar modified it seems.

Can you make that radius larger, where the crack is coming from?  That is a fairly sharp corner and a stress riser.

[Brett Buck]

Funny you mention that Brett. I’ve been wondering what the cracking was on the Ferrari plane at the landing gear mounts. Next plane will have to have the spar modified it seems.

   That looks pretty familiar! You can inset balsa stringers spanwise without hurting anything, just avoid cutting anything chordwise.

     Brett

     

   

[Dave_Trible]

Funny you mention that Brett. I’ve been wondering what the cracking was on the Ferrari plane at the landing gear mounts. Next plane will have to have the spar modified it seems.

Matt BEFORE that wing parts company I'd sand the finish back a little and put a layer or two of carbon veil around that whole area out two or three inches all around and dope it down,  then repair the finish.   It should stay together.   I have one (and only one) foam winged Desperado and it did the same thing but worse.   Joe was the last to fly that airplane a couple years ago and I was shocked to find the crack when I picked the airplane up, and amazed the wing stayed on.  It was repaired as I mentioned.   

Dave

[Doug Moon]

Funny you mention that Brett. I’ve been wondering what the cracking was on the Ferrari plane at the landing gear mounts. Next plane will have to have the spar modified it seems.

What are you guys doing? I dont get the cracking right there. I have seen it on others.

[Brett Buck]

What are you guys doing? I dont get the cracking right there. I have seen it on others.

    You have a sharp corner that concentrates the stress right where the landing gear loads are also applied. That's also were the spar reinforcement ends.

    If it was me (and it was), I would inset a maybe 1/8 stringer, spanwise, across the front of the gear block and extending about an inch inboard and 2 inches outboard. This will move the load away from the gap in the sheeting, and be pretty easy to refinish (as opposed to large patches on the surface.

    My gear spar has an extension past that corner, on the bottom only, about 2".

      Brett