My RV Building Modifications
by Kevin Faris

(June 21, 2009) -- I made a few changes during the construction of my RV-7 which flew for the first time in October of 2007.  As the manufacturer, we are able to make changes as we see fit.  The important thing is to carefully think the process through and get engineering help if the modification is structural.

If you have ever flown the Piper Cherokee 140’s you learned to appreciate the “tabs” inside the fuel tanks.  If you filled the tanks above the tabs you have a two passenger airplane with four seats.  Over the years several pilots have commented to me on how nice it would be to be able to accurately know how much fuel is on board.  It seemed to me it would be really easy to do this in an RV where you build the tanks and seal them yourself.

My inspiration came in the form of a carpenter’s rafter square.  It is made of heavy aluminum about 3/16” thick so it is quite rigid and not likely to fracture and break off in the fuel tank.  The following pictures show the preparation of that framing square for insertion in the tank.  I apologize for the photo quality as these were taken with an early Sony Mavica with a whopping 0.3 megapixel sensor.

This is the square I selected to use.  These units were purchased at a Menards store in the area.

The base flange was trimmed to a length of three inches.

Here is the “indicator” square all trimmed and deburred.  The length is about 5 ¾ plus the
base flange.

Here is an edge view of the indicator which shows how thick it is.

 
Then I experimented with placement to determine the best location to be easy to see and not be bent or broken by the fuel nozzle.  This is the view through the fuel filler hole.

I decided four AN426AD-3 series rivets and a good coating of tank sealer would be sufficient.   

I positioned the indicator on the floor of the tank and drilled it to the tank skin.  Use cleco’s
to ensure it remains flat and aligned with the holes. The skin is dimpled for the four rivets and
the indicator is countersunk.  As you can see the fuel nozzle will not contact the square if you
are reasonably careful refueling your tanks.

 
Here you can see the indicator is about an inch shorter than the tank height.  The fuel cap is
several inches forward of the indicator.

Not only is the unit sealed to the skin, it is also sealed over as are all of the stiffeners and
rivet shop-heads.  I have had no tank issues of any kind in 130 plus hours of operation.

Once the airplane was ready to start, Sandy and I calibrated the indicators by slowly filling
the tanks with measured quantities of 100LL fuel.  Below is the calibration chart we produced.
As you can see there are some minor variations between left and right tanks.  I assume this is
measurement error and not indicator placement.  In any case they are quite accurate assuming
a level ramp.   Below is the fuel page from my P.O.H. and chart attached to my fuel tester:

Note:
Use 100LL or 91/93 Mogas
Each tank is 21 gallons.  20.4 useable

Calibrated fuel gauges in level attitude,
reads full @ 16 gallons, empty  0.0
      
Tank dipsticks in ground attitude:
STICK                LEFT                RIGHT
1/2"                  5 gals.             5 gals.
1"                       6 gals.             6 gals.
1.5"                   8.5 gals.         8.25 gals.
2"                      9 gals.             9.5 gals.
2 3/8"           10 gals.           10 gals.
2.5+"             11 gals.           10.5 gals.
3"                    12 gals.          12 gals.
3.5"                13.25 gals.    13 gals.
4"                    15 gals.          15 gals.
4.25"              16 gals.          16 gals.
4.5"                 16.5 gals.      16.5 gals.
4.75"               17 gals.         17 gals.
5"                     18 gals.         18 gals.
Top of stick  19 gals.         19 gals.
Below ring      21 gals.         21 gals.

As with all RV projects, it is up to the manufacturer to determine suitability, safety, and sanity
before proceeding with this modification.