Tail spar structural issue

[markallison]

I need some information regarding a structural issue.

I recently imported RV-7A 71975 to South Africa from USA. The rear empennage cover was secured with rivets instead of screws so when the aircraft was re-assembled here in SA, the re-assembler put rivnuts in the rivet holes so that the cover could be screwed in place. The original builder put quite a lot of rivets in an the guy this side put rivnuts in every hole. When fitting one of the rivnuts they drilled through the skin and also through a small piece of the spar.

approximate position of the hole

approximately how much has been drilled out of the edge of the spar i.e a half moon piece the size of half an 8/32 rivnut.

Has the hole compromised the integrity of the spar and if so what would need to be done to rectify the problem?

 

[scard]

Others should chime in here but I think that since it's the flange you're in good shape. Dinging into the web is another story and should be avoided as much as possible. BTW it's 1330 here and Vans tech support could be called for the real answer.

A call to Van's is the only generally accepted action on something like this.

 

[penguin]

As well as Van's, AC43.13-1B may also have something useful to say.

For your own piece of mind I would remove the offending rivnut and rivet in a doubler, the same thickness as the flange. If you want to re-install a rivnut then make the doubler twice a wide as the flange. I would use probably 4 rivets either side of the cut out.

Pete

 

[terrye]

Tail Spar Structural Issue

I completely disagree with Rivethead. The vertical stabilizer rear spar reacts the loads from right and left rudder deflection. Thus, it is loaded in reversed bending depending on whether the rudder is deflected left or right. A beam (in this case a channel section) reacts bending loads by a tensile stress at the outboard fibres of one flange, decreasing to zero tensile stress in the center of the web, and increasing to a compressive stress at the outboard fibres of the opposite flange. So it is the flanges that have the highest stress, not the web.

So, yes, the extra hole has compromised the structural integrity of the spar. Now, the spar flanges already have rivet holes in them, but these are already accounted for in the design. To add another hole(s), especially at the location above the support bolts which is the point of maximum bending moment is not good and is a big stress riser.

Your choices are: repair or replace. For repair, you should consult AC43.13 Acceptable Methods Techniques and Practices Aircraft Inspection, Repair and Alteration. See Section 4-58 Repair Methods and Precautions for Aluminum Structure and especially Figure 4-24 Typical Repairs to Rudder and to Fuselage at Tail Post. At a minumum you will need an inserted channel doubler inside the existing spar for perhaps 6" above and 6" below the extra hole. However, check the clearance to the rudder before doing this.

The best repair would be to replace the vertical stabilizer rear spar. This is not as big a job as you might at first think, and maybe less than the repair scheme above. The cost of the part from Van's is probably cheaper than you think as well. I do agree that you should consult Van's before you decide.

Hope this helps.

 

[L.Adamson]

I completely disagree with Rivethead. The vertical stabilizer rear spar reacts the loads from right and left rudder deflection. Thus, it is loaded in reversed bending depending on whether the rudder is deflected left or right. A beam (in this case a channel section) reacts bending loads by a tensile stress at the outboard fibres of one flange, decreasing to zero tensile stress in the center of the web, and increasing to a compressive stress at the outboard fibres of the opposite flange. So it is the flanges that have the highest stress, not the web.

On the other hand, the "skin" in this case is taking much of the load, as the rudder is deflected. So........it will be interesting to see what Van's says.

L.Adamson --- RV6A

 

[terrye]

Tail Spar Structural Issue

On the other hand, the "skin" in this case is taking much of the load...

Sorry to disagree Larry, the skin isn't taking any of the load at this point. The skin ends at the lower vertical stabilizer rib and I don't consider the fiberglass fairing that attaches to it a structural member. If the vertical stabilizer rear spar was completely cracked through from one flange to the other at the location of the added rivnut or platnut, then the only thing holding it onto the aircraft are a couple of -4 bolts on the vertical stabilizer front spar and they wouldn't react the rudder loads for long.

 

[Andrew M]

Fix it

After 20 plus years of doing aircraft structures, sometime the hardest thing is deciding to fix it, or why. This damage is not in a good spot. Repair or replace. It really isn't to tough for someone familiar with the work to get it done
Andrew
AP IA
-9 preview

 

[markallison]

Thanks for the replies so far everyone. I will try an take a photo today as that will be clearer than my diagrams. I will alos try Vans later but a 9 hour time zone difference is a pain.

 

[Andrew M]

Blended repair

It's worth a shot, run it by Vans.
Good luck,
Andrew

 

[RudiGreyling]

The close proximity to existing rivet hole (Not 2 Diameters Away) and the fact that it is almost 1/2 the flange width has me worried.

You might get away with scalloping and making a nice round 1/2 moon cut-out including the close rivet hole out the spar, but then I will always worry about it.

You might do as above and then rivet a angular doubler to the spar flange and web too as an acceptable repair, but I don't know if you will have the rudder clearance.

Better talk to Vans cause this is not an easy cut and dry answer you can solve on a forum...

 

[Bill Wightman]

I wouldn't like to see this blended out. And a repair doesn't look good to me (even though one could be engineered to work) because adding a doubler involves drilling even more holes in the spar web flange, which is what we're trying to get rid of in the first place. Problem with holes in bending components is that stress cannot cross the void that a hole presents. So, any hole(s) you add will effectively reduce the cross-sectional moment of inertia and only drive stresses higher elsewhere.

Suggest replacing the VS spar. Drill it out, re-use the spar flange doubler and other pieces. You could have that thing apart in a couple hours. Lots of peace of mind considering the overall time/cost on the project. Failure isn't an option here.

 

[Bob Kuykendall]

I agree with the repair or replace crowd. Notching an edge is how you tell fatigue, "Hey, come over here and make a crack!"

Thanks, Bob K.

 

[terrye]

It's a flange blend the rivet hole into the scallop and move on. The web is the structural part of the spar and shouldn't be encroached on.

The vertical stabilizer rear spar is actually a composite beam composed of a channel shaped section of .032" aluminum, with an insert of 1/8" aluminim riveted to it. So you might wonder, which part is really carrying the load. Well, they both are, but only in proportion to the distance from the neutral axis which is half way between the flanges. In any beam loaded in bending the outermost fibres (that is the flange in an I or C section beam) have the highest stress (see my post #5). So the failure mode of this spar if it is loaded high enough or repeatedly enough (fatigue) will be a crack initiation from the location of the additional (rivnut or platenut) hole since it is near the edge of the flange and near the point of maximum bending. The crack initiation site will gradually propagate across the flange and into the web of the .032 portion. It will not affect the 1/8" aluminum which will then carry more of the load. Provided this 1/8" insert is not overloaded it will probably continue to carry the rudder loads.

If it were my airplane, I would replace the .032" spar with a new one. They are available from Van's (or can be made locally) and it should only take afew hours of drilling out rivets and then re-riveting.

 

[johnfurey]

I made the big mistake of cutting the lightning holes through BOTH the
808PP AND the 803PP rear spar. I did not catch my mistake until I went to install on the plane many months later. I called Vans and was told it was probably OK which is what I wanted to hear but didn't have much confidence in the answer. I called again and was told NO WAY, it must be replaced. I called and spoke to Ken who said it didn't sound good to him but he wanted to talk to an engineer who determined that it must be replaced. Long story short, Ken told me to just devote a morning to carefully drilling out each rivet and reuse the 808PP and it would all be behind me. No worries, No regrets, No questions, it will be fixed for good. I took his advice and never harmed a rivet hole. By mid afternoon it was ready to install on the plane, Never been happier with a decision.

 

[markallison]

Got a reply from Vans

Yup you're right it is a composite spar I'll give you that but I disagree that this "scallop" it's now an imminent point of failure. Properly treated this doesn't have to be an area of much concern. If it does become a problem it's in an easily inspect-able area and the repair of a problem can be dealt with at the time there may actually be a problem, but to start ripping the HS apart because you dinged into the flange just doesn't make sense. My opinion would be somewhat different if the damage were in the bend between the flange and the web but it doesn't look as if it even gets close.

Thanks for the replies guys, special thanks to Rivethead for your in-depth replies! Rivethead, your thoughts are exactly the same as Vans. They say it doesn't look like too big a deal and dont think any action is needed. They did say if I want i could add a doubler on the inside of the flange 2-3 inches abouve and below, 1/2" wide. They said that obviously the spar could be replaced but said that that would be a big job especailly if the VS is painted.