Spar Damage

After an in-flight aileron flutter episode all 4 lower wing spars cracked at the fuselage attach points.  This is nearly impossible to see without de-mating the wings.

 
 
Loose internal wing drag wires and crushed compression ribs lead to this total loss of wing geometry in flight.  You cannot adequately tighten the internal drag wires if the compression ribs are hollow.  Cascading failures like this are difficult to catch, because once the limits are reached, the break up occurs rapidly, perhaps all in 10 seconds like this one.

These spar attach straps are poorly designed with regard to cycles in aerobatics and age.  The distribution of load causes the spar to crack along straight lines with the wood grain.

If you examine the damage, you'll note the aft wing spar straps had been "working" for some time, with nicely rounded out holes showing perhaps years of wear.  Clearly, the right rear bottom spar cracked first, which lead in seconds to a violent flutter breaking all the remaining spars.  The left rear, while unbroken up to the incident itself was also loose and shows rounded out attach holes.

Steel plates on the top and bottom of the "I" struts prevent the spar box crushing that is also evident.  The "foot" of the "I" struts crush the spar box over time, leading to loose flight wires.  The is particularly evident at the top and bottom forward bolts on the standard "I" struts.  

Higher flight wire tensions can be evenly maintained over longer periods of time, and many cycles with 4 steel reinforcement pads on top of the wing spar boxes.  The loads are evenly distributed in these sensitive areas and damage to the spar caused by the "I" strut is eliminated as a result.

Check your Pitts, Eagle, or biplane for these problems!!!  Look for the following chronic problems, especially during recovering:

Hollow compression ribs.  Between internal drag wire attachments.

Loose interal drag and anti-drag wires.

Loose metal leading edges.  (Best to replace with wood)

Crushing around the "I" struts.  

Signs of loose or "working" spar butts.

Dry and brittle wood showing any signs of water damage or rot.
 


 
 
 

 
 

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