Wing Stress Cracks

How To Inspect and Repair
by Bob Violett

  Full size aircraft must undergo airframe and structural inspections based on time, flight hours, exposure to extreme air turbulence or hard landings.  As our model jets get larger, more complex, and hopefully endure many flight hours, we must pay attention to the wear and tear that even normal use will inflict.
  Man carrying aircraft have designed-in removable inspection panels purposely placed to ease the inspection process.  Because most of our model jets have the main landing gear mounted in the removable wings and have multiple servo access pockets for flaps, ailerons, leading edge devices, etc., flight loads and landing loads are absorbed primarily by the wings and their fuselage mounting structures.  Structural inspections are therefore in order.
 
  Frequently, the first sign of an impending problem will be stress cracks that appear on the bottom of the wings, usually starting from a corner of an access pocket. 

 

  You will want to remove the cover panel and with the aid of lights, small mirrors, scope, etc. look for cracked ribs, spars or separated glue joints. Hand flexing the wing may expose a serious problem beyond just a cracked skin. It may be necessary to cut and remove a piece of wing skin to repair the affected internal parts.
 
  After the internal repairs are accomplished or you determine that the problem was just skin deep, make an appropriate size skin patch from .010" Poly Ply (BVM # 5710).  Apply Thin C.A. to the skin tear, then thoroughly scuff the mating surfaces with #80 grit sandpaper and apply the skin patch with Slo-Zap C.A.

For clarity, we painted these skin patches black.
 
 
You can add some rivet details to the patch, tape around it and paint it.  It will look like it belongs there.  See BVMJets.com/Tips & How To's for more on these techniques.
 
  To address the causes of this wear and tear on our jets, we must first recognize that excessive speed and high "G" loads are the problems.  BVM publishes Vne(s) (Velocity to Never Exceed) for all of our jets.  It is posted on the front page of the model Assembly and Operating Manual after we have thoroughly flight tested our products at that speed. 

  If your model jet does not have a published Vne, I would suggest that large scale jets be limited to 140 mph.  If you are flying them in high wind and gusty conditions, reduce the speed to 120 mph maximum.
 
There are several speed detecting / limiting devices recently available. Take advantage of this new technology.
 
Part of holding an A.M.A. "Turbine Waiver" is assuming the extra responsibility for safe operations of a high performance turbine powered model aircraft.

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