Fuel lines and Cabanes, part 2

Builders,

Here is part two of the Pietenpol Fuel line – Cabane story. The pictures and text below are taken directly from our 2010 Brodhead/Oshkosh story on Flycorvair.com. I mention this to show that there is a wealth of information there, and that my comments on these Pietenpol components are not new positions.

I recognize that some people have difficulty finding everything on the old site and on this one, even with the built in search capability. To fix this we have the reference pages, both here and right on the front page of Flycorvair.com. This story will be first published as news on Flycorvair.net, but it is also now immediately cataloged with part #1 on the Pietenpol reference page on Flycorvair.com: Corvair – Pietenpol Reference page

 I am putting a lot of effort into making the information more accessible to builders working on specific airframes. The other half of the equation that builders can really help with is sharing the specific links to our stories on discussion groups they work with.  For example, there are several Pietenpol discussion groups, and these two stories really apply to any Pietenpol builder, not just people working with Corvairs. Builders can help by mentioning this stuff and pointing out that it is all organized at the link above.

Below, I put the original 2010 text in blue. Newer, additional comments are in black.

Although this photo is taken from a flying plane, this is not the best way for the front cabane strut to be done on a Pietenpol. The Piet is a very strong aircraft  with a very strong wing. It would be very difficult to break a well made one in flight. This said, in an off airport landing or accident, the weak link in the airplane  is the connection between the wing and the fuselage. In a sudden stop, the forward diagonal cabanes get a massive compression load, and if they’re set up like this, they  will bend like cooked spaghetti, allowing the wing to parallelogram forward, potentially trapping the passenger. The primary reason why people make the cabanes this way  is that they believe the old wives’ tale that the wing of the Piet can be moved forward and aft to resolve any CG issue.

Get a look at two things above: This type of tubing end is what I refer to as “1960s swing set technology.” You can do better than this. Also look at the hard line with the metal clamp fixing it to the rear cabane. This is exactly what I was speaking of in part #1. Don’t think I am picking on a particular builder, I don’t even know who’s plane this is, and about 75% of flying Piets have this kind of issue. It costs very little to correct.

Here is the much preferred methodology of cabane attachment on Pietenpols. While it won’t make the plane as crashworthy as a Grumman AG-Cat or AD-1 Skyraider, it will vastly  improve the strength of the wing/fuselage connection in a survivable accident. This means that the wing could very likely stay in place in a small event. Keeping the  center section in the correct location is also an important factor in not rupturing the fuel lines from the wing tank. The primary reason why people do not make  their cabane struts this way is that they lack the weight and balance data to be sure of the wing’s location before the plane is finished. Now that we have the data,  making a cabane attachment like this can be done with confidence in the final wing location.

Above, Note that this plane has very good cabane arrangement, but it has a rigid metal line. I pointed this out to the builder who corrected this. A small number of well known Piet pilots have made upgrades to their planes on these two issues. Kevin Purtee changed his fuel line before is accident in 2012 and later told me that he thought it was one of the factors that prevented a post crash fire. Think that over, and decide if it is worth a few hours and $80 to change. -ww.