What 1140 hours of vibration can do...

[AlexPeterson]

I began the annual conditional inspection yesterday knowing that I would need to replace the hinge on the alternate air door, as it has been getting pretty sloppy the last few years. Here are some pictures of the parts.

Big picture, so to speak:


Note below how thin the edge had worn, over 3/4's the .032" thickness is gone:


The hinge is also quite worn laterally:


There are also cracks on the relatively weak portion of the airbox top (the part riveted to the fiberglass) onto which the hinge was riveted. Will need to think of how to best repair that. I will use a SS hinge, and may try to have the door close against rubber in some way.

In general, the whole airbox design has been problematic with regards to maintenance. I've replaced the top plate which fastens to the induction system twice, and I've strengthened the fiberglass bottom once. The fiberglass was wearing almost through where the filter contacts it (and yes, it clamps firmly on the filter). There is a tremendous amount of acoustic vibration in induction systems, that could be a large portion of the problem. Additionally, the prop pulses are hammering away at it. I'd suspect the latter is the main culprit.

This wear I have on the alternate air door (which is outside the filter) should be carefully contemplated by those with similar moving parts inside the filter. My opinion is that doors, sliders, etc., are best not put in a position where parts could be inhaled by the engine in the event of a cracked part coming loose.

[Steve Barnes]

Looks just like mine. If you use pop rivets, that little bulb of steel can come loose and get sucked into the engine. Had to rebuild my door twice in 1400 Hrs. Steve Barnes " The Builders Coach"

[sprucemoose]

I've had similar results, as I suspect most of us had. A couple of changes that I made that really helped:

1. The .063 plate bolted to the carb bottom was replaced with a new one of .090. I put large area washers under the 4 bolts that mount it to the carb. No cracks since then (around 500 hours.)

2. I made a plate of .032 Al that fits the contours of the bottom of the airbox, where the filter wears into the glass bottom. I can make a new one and replace it every year in about 10 minutes if need be. The plate attaches to the bottom of the airbox with screws, using the mounting holes from the 1st generation of the air box alternate air door (the magnetic one.) This solved two problems, as I decided to do away with the ill-advised alternate air door and return it to stock configuration.

3. I made a form-in-place gasket of RTV around the edge of the door where it rubs when closed. This has proven to be more durable that I would have thought, and (mostly) eliminates the fretting that you see.

4. The piano hinge will likely get replaced this winter. I don't see a way around this; it's just a high wear item, just like yours. Not too much trouble to replace, but there has to be a better way. I like this stainless hinge idea.

[DanH]

Alex, might want to try a sheet of ordinary black rubber baffle material over the hinge installation. Rivet the hinge, then attach the rubber sheet with separate rivets and large area washers. The rubber sheet would bend with the hinge, but oppose the axial vibration in shear.

[AlexPeterson]

Dan, Jeff,

Great replies. I will adopt ideas from both - thanks.

[Randy]

Be very careful that any rubber or other sealing material cannot come loose and get lodged to block the intake. This type of modification has been the suspected cause of some very serious accidents that I know of. On a Thorp T 18 the airbox had been reworked and or modified, it crashed and burned on initial climb out, quite literally, not enough good pieces left to really determine the cause. An RV6 went through a fence due to the use of some kind of material from a dive suit being used as a gasket or seal that came loose.

Tread carefully in this area, consider that Murphy is at work, "if it can happen, it will".

RC

[sprucemoose]

The RV induction/ filter system is a little different from what most people are used to. The carb heat door is upstream from the filter, and that has advantages and disadvantages. One advantage is that, in the event that something comes loose on the door assembly (pieces break off, sealing material comes loose) it will be blocked by the filter, just like any other debris that might come in.

A disadvantage is that the carb heat does not bypass the filter as it does in many other installations. If the filter becomes plugged, particularly with snow, there is likely not enough heat there to melt the snow. The solution to this is to immediately apply full carb heat at the first sign of flying in snow and leave it on. Unfortunately some people didn't do this, with disastrous results, and this is what led to the various alternate air fixes for the air box.

[Geico266]

Alex, have you had your aircraft dynamically balanced?

[AlexPeterson]

Originally Posted by Geico266

Alex, have you had your aircraft dynamically balanced?

Yes


blahblahblah for the thing to be happy with enough text...

[AlexPeterson]

Here is a picture of the airbox top showing a crack, which has been opened up during removal:


This shows the small angle I reinforced the top front with:


This shows the small silicone rubber bead to prevent future chafing:


I'll check back after another 1000 hours to let you know how it held up!