Building an Airport Tetrahedron

By John Chirtea, EAA 741127

Whether you call it a tetrahedron or an airport wind indicator, the objective is the same: It tells incoming pilots the direction of the wind. At Eagles Crest Aerodrome (DE25), we decided that having one of these would be a great addition to our private, 26-lot, 3,500-foot grass strip airport community in Milton, Delaware. We started with the tail fuselage of a Bell 47 helicopter that was abandoned and hanging from the rafters of a hangar in our community.

This appeared to be about the right size to act as a wind
indicator, so it was trucked to my hangar. A number of fellow pilot experts put
their heads together and then the fun began. We never had a shortage of
opinions as to how the construction should proceed. At about 20 feet in length
and cylindrical in shape, it did not seem to have enough bulk to be seen from
the 1,000-foot TPA.

So in order to increase visibility from the air, a 3-foot triangular winglet was added to each side. These were secured with a 3/4-inch threaded rod that extended through each winglet, and bolted where it passed through the fuselage and secured with two supports located within the winglets. The winglets were attached to a 1-1/2-inch aluminum bar that was bolted into the fuselage, and covered on each side with 0.060-gauge sheet aluminum. The aluminum sheets were riveted to the bar. A finished rudder unit was donated by a fellow pilot, and it fit perfectly to the large end of the fuselage. This entire assembly was then balanced on a round bar to determine the precise balance point of the unit. With the balance point, or center of gravity established, pillow block bearings were added to the bottom and top, inside of the fuselage, to act as the swing point.

The unit was then taken outside the hangar and installed on a temporary pivot bar to test how and if it would catch the wind. The location was close to the hangar, and as a result, it was catching quite a bit of wind bouncing off the hangar. In fact, on a very windy day, shortly after setting it in place, it started spinning around the pivot like a drunken sailor at 30 revolutions per minute. We thought for sure it was coming through our sunroom windows.

It occurred to our panel of experts that perhaps it needed more rudder authority. After a number of additional opinions, experimental pieces were added to the rudder using some 1/4-inch sheets of Styrofoam. Various sizes were used, but none seemed to make a difference. This led us to the conclusion that the upper pillow block bearing needed to be moved forward, which would change the center of gravity. This resulted in an upward tilt of the nose, and it seemed to do a much better job of tracking the wind.

Having done this, it was decided to relocate the unit to a
temporary location where it would not be impacted by wind deflecting off the
hangar. Modifications at this point included adding tabs to the top of the
rudder, adding tabs to the back of the winglets, and extending the fuselage by
about 18 inches using another piece of sheet aluminum. All of these additional
changes led to the unit catching more wind, which was the original intent.

Additionally, to give the unit more rigidity, two bars were bolted to the bottom of the fuselage. Further support and strength was provided by adding iron bars inside the unit, running parallel to the bars along the bottom.

At this point, we were satisfied with the performance of the unit, and decided it was time to move it to its permanent location at the north end of our field. Four threaded rods were assembled to be dropped into the concrete base. A talented blacksmith friend built a solid pivot to hold the unit. A 24-inch Sonotube was used as a form for pouring the concrete into a 3-foot deep hole.

Using a backhoe, the unit was picked up with straps and moved across the field to its final location at the north end of the field. A solid piece of 1-1/2- inch stainless steel bar was used as the swing rod. To give the unit a little character, four small metal pails were bolted to the end of the unit to simulate a rocket, and a rubber traffic cone was riveted to the nose.

After about five months of labor, 1,000 rivets, and many nuts and
bolts, the unit is working exactly as it should. It took a lot of trial and
error, but we finally figured it out to our satisfaction, and incoming
airplanes now have no difficulty in determining the direction of the wind. It
was an ambitious project, but fun was had by all the participants.

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