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  #1  
Old 04-16-2011, 11:57 PM
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Kidagain Kidagain is offline
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Default Estes Interceptor -E Wing Cracks

I recently noticed wing cracks on my interceptor-E. The cracks are very close and align with the joint where the sub-wing assembly and leading edge of the main wing meet. I have repaired the other wing already for same issue. My question is could these cracks be the result of a hard landing(s) or might it be wing flexing in-flight? I have only flown this bird on Aerotech F-32T6’s and its been flown more than 6 but less than 12.

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Old 04-19-2011, 12:03 PM
Daddyisabar Daddyisabar is offline
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My guess is that it is repeated stresses from landing. If you have that wing pod tightly glued on an it lands on angle moving sideways the force of impact has to go some where and that joint is the weakest spot. All it takes is a bit of super thin super glue and your flying again in seconds.
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  #3  
Old 04-19-2011, 12:29 PM
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CPMcGraw CPMcGraw is offline
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Could also be a release of tension from a root edge that didn't fully meet at the BT along its length. The root joint looks undamaged ahead of and behind the crack, which also conveniently happens at the joint of the two panels. The two pieces may have induced a slight inaccuracy in the root when they were glued together, and that in turn put extra tension along the root-BT joint. The long root joint, having more contact with the BT, was stronger than the glue joint between the two panels (or more likely, stronger than the grain of the main panel just behind the joint).

Without having a long, stiff sanding block to sand against, it's difficult to get such a long edge perfectly straight.
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Old 04-19-2011, 12:57 PM
Daddyisabar Daddyisabar is offline
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But then again if there were pretty ladies at the launch you would say it was from wing flex caused by near super sonic speeds being achieved from pushing the aerodynamic envelope on this most expensive Estes kit. Be sure to have a pair of Tom Cruise Top Gun aviator sun glasses and a super bike to be really cool.

If there were just crusty old farts at the launch all you can do is complain about the speed of balsa or the need for bulletproofing Estes kits to take mid power motors. In that case all you need is a dirty, undersized T-shirt and a junk 1984 Caprice Classic.
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Old 04-19-2011, 01:10 PM
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GregGleason GregGleason is offline
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Have you simmed it to see what the v-max is? I am wondering what the peak velocity it is seeing.

You could mount a key-cam and point it at one of the wings to see if you see flexing.

Or you could try this experiment:

Put a higher-impulse motor and a big chute as an experiment. If the cracks come back, it is likely due to wing flex.

Launch on a lower-impulse motor with your standard chute. If the cracks come back, it is likely due to rough landings.

Greg
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Old 04-19-2011, 04:05 PM
Daddyisabar Daddyisabar is offline
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A stock built Interceptor E rebuilt with a 29mm motor mount for a higher impulse I motor!!! WOW!!! With like a 40 inch chute. Dude I want to be there! With all that flexing maybe Iceman will show up and a spontaneous game of skins volley ball will break out. And with all that manly action I bet the real Tom Cruise would show up with the key cam and plenty of electrical tape to put it on the nosecone! Hopefully no wind or it is a real boot camp like run for the recovery, but hey, that is the price you will have to pay for flying like a real Top Gun!
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Old 04-19-2011, 04:23 PM
BPRescue BPRescue is offline
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Building an interceptor myself, I noticed this issue right away as it conflicts with basic woodworking practice. The problem stems from gluing the end grain or the trailing edge of the forward section, to the leading edge of the aft section’s edge (cross) grain. The grain patterns of these components are of a 90 degree-ish angle. In reality, the only thing worse is end to end. With wood, edge to edge/matching grain direction is quite strong, as is gluing the face and both will be stronger than the wood itself, however anytime you incorporate end grain gluing, you are up for a challenge at best. End grain is extremely porous and will absorb the glue/provide little surface area for the joint. Wood moves, there is no way around it and this joint is/was destined to fail be it from usage, or time/expansion. I do notice that on landing, the interceptor takes a beating on the wing pods, so that will help this issue along as well. A larger parachute “may” compensate, but that is really not the issue here; and a resolution I am not sure is even valid.

To be honest, I would argue it is a design flaw from Estes. At the same time, understanding, the approach in wing shape, I am not sure Estes could resolve using balsa since using a single piece would cause other issues, including poor adhesion to the tube which now causes a real problem, vs. the cosmetic one you have. Instead, I think they should add some instructions on how to deal with the fragility of this type of joint. If you look at furniture, they will typically get away with end to end by using finger joints; which increases the glues surface dramatically. For 90 degree angles such as kitchen cabinets where grain patterns conflict, you will also note splines, tenons and or matched rail/stile bits which add surface area as well. Unfortunately, balsa is so fragile and thin, these are not options for you.

I dealt with the issue on mine by gluing the joint with wood glue as Estes outlines. In addition, I laminated paper over both faces of the wood; which adds considerable strength to the wing. Not only does it add shear strength which helps on landing, but also limits movement of the wood, helps seal it better, not to mention all the benefits of a plywood type system. Another way around this if you don’t want to “laminate”, would be the careful use of Gorilla Glue. This glue expands, so it can aid in testy end joints, not to mention deals with water/humidity quite well. Epoxy is yet another possibility. Wood glue does allow for minimal expansion, but in this instance, it just will not work well for this task. As I indicated earlier, it is not a matter of if; but when…



The good news, is that the joint of the forward section as it mate’s the tube should be more than strong enough to hold said wing. In other words, I think your issue is more cosmetic than anything else. As for repairing it; now you have a problem of the used wood glue conflicting with other glues. Forcing wood glue in again will not resolve the issue. In this instance, you may get way with superglue, be it regular or gel. If you put tape underneath the joint on the bottom of the wing as to seal the crack, then use a popscicle stick or similar as to tape that to the outside of the tape you just placed, you should be able to turn the rocket over, apply superglue to the top and let gravity work its magic. In the case of gel, force it in the joint with a putty knife, flat piece of wood, whatever. Hopefully that made sense. With the tape, you are trying to stop the glue from passing through the bottom side per gravity when you apply glue to the top of the wing/crack. It will adhere to the glue a little, but you can sand that off. The addition of waxpaper will resolve the stickage issue, however could result in an uneven glue joint, which is why I like tape to butt the edges perfectly. The Popsicle stick will add rigidity and ensure the wing is strait for gluing as well. You could even investigate Gorrila Glue since it expands, it may work here; but also may conflict with wood glue, so I would have to perform more research before trying it.

The end result is once it happens; I fear you may not be able to repair it. As I indicated, wood moves, with these two joints movement conflicting since the end grain will be more severe. That coupled with limited surface contact, and other stresses means this joints failure is inevitable by constructing in the manner described in the instructions. There are more things I would try, but only after I tried the above and said failure occurred again.

Don’t jump from a window though. I do believe the superglue may help, but ultimately this is not going to impact strength of the forward section. The rear section is open for debate per its size and pod weight, but will likely be fine. The forward section actually helped structure in this regard.

Anyway, sorry so long. Hope this helps…

Last edited by BPRescue : 04-19-2011 at 04:41 PM.
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Old 04-19-2011, 04:25 PM
Daddyisabar Daddyisabar is offline
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I better get serious here before they throw me off and I have to go back over to the Care Bear Forum. I have flown my Interceptor Es many, many times on the stock chute using the mighty E-9s that I doubt would cause any wing flex. I too got the same crack after like 6 flights. Then at about a dozen the crack spreads to the root edge on the trough the wall section of the joint. All you can do is watch in horror as you beloved Estes kit with its’ balsa fins, paper tubes and wood glue succumb to age and the stress of many landings. Then on about flight 25 your luck will run out and something weird will happen. On mine the chute got skewered by the rear fin point on to the back of the nosecone after using an E30 – 4 into a stiff wind, idiot! Down she came real fast for about a two foot dead cat bounce on the prairie sand. The cracks were enormous and the wing and broken pod were about 1 foot away. All Daddy could do was to grab my Daughter’s Care Bear and hold on real tight.
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Old 04-19-2011, 04:47 PM
Daddyisabar Daddyisabar is offline
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I laminated my second and third kits with Avery label paper using two different techniques on the joint in question. On one I used more Titebond than usual, ending up with a slight bump that is still slightly visible under the label paper, way uncool. On the other I soaked the joint with thin superglue, then label papered over it. Strong, yet brittle and probably won’t last. All in all without high end bulletproofing techniques or serious G-10/carbon fiber modifications you are just going to have to man up and live with the crack, chalking it up to trying to make an Estes kit with its balsa wings a true mid power wonder.
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Old 04-19-2011, 06:58 PM
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Quote:
Originally Posted by BPRescue
...To be honest, I would argue it is a design flaw from Estes. At the same time, understanding, the approach in wing shape, I am not sure Estes could resolve using balsa since using a single piece would cause other issues, including poor adhesion to the tube which now causes a real problem...


A partial solution is to increase the glue surface area at that cross-grain joint. This will require a re-design by Estes, namely to cut "saw teeth" in both panels so that the glue-contact area is doubled. With a laser cutter this is not an issue; it just needs to be programmed in.

An alternate to this might be to recess the strake into the leading edge of the main panel by about 3/8".

A "home-spun" repair might be to cut narrow slots (about 1/32" wide and maybe 3/8" long) across the joint line, and to insert small strips of basswood into those slots. About four or five should suffice to reinforce the joint.
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