Sunday, 13 April 2008

The Harer STC

As part of the restoration project I will be implementing the Harer STC (Supplemental Type Certificate) which will have the benefit of allowing an increase in the aircrafts all up weight from 1200 lbs to 1280 lbs. Not a great increase but in addition to a target reduction of 50 lbs to the aircrafts current weight will bring a very useful increase of 130 lbs to the usable load.

In essence, the Harer STC calls for:
  • An 85 hp engine to be fitted.
  • The wing attachment fittings to be strengthened.
  • The lift strut attachments (at the wing) to be strengthened.
  • Additional cross bracing on the fuselage.
  • Provision for a larger baggage compartment.
I already have the 85 hp engine and don't intend to add the larger baggage compartment as this goes against the idea of keeping the aircraft as light as possible, but the other modifications will be implemented.

So while I await the delivery of all the hardware, paints and fabrics from the States, I decided to do as much of the airframe work, that will ultimately require welding, as soon as possible.

The first job requires welding a diagonal bracing tube across the bottom of one of the fuselage bays. In fact two tubes are required so that they will form an X with an existing bracing tube. The first job was to work out how I could accurately cut the tube ends so that they would fit into the tube clusters and the existing bracing tube. Luckily I found a very useful web site some time ago and remembered to bookmark it for later use. provides a tube coping calculator that will give you a cutting template for joining two tubes. For the tubes I have there are various intersecting angles from 40 to 90 degrees. I've had a look at a couple of other programs for doing the same job but this one is very simple and straight forward to use.

Here's a sample template, downloaded in PDF format then printed and cut out ready to be wrapped around a tube.

The Template is wrapped around the tube, which can then be cut and ground to shape. I used a dremmel for grinding.

A finished 90 degree intersection.

The final join in the center of the X. This was straight forward as the intersecting angle is 90 degrees.

The rear tube cluster showing the new bracing tube in place. Cutting this pattern is slightly more complicated as there are two horizontal tubes which intersect at different angles. I used a mashup of two templates to get the basic cutting template and then had to finish the grinding by eye plus a lot of trial fitting until I was satisfied with the result.

The forward tube cluster was even trickier because there are two tube sizes and different angles. Again I used two templates to get the general shape.

The tubes will be welded in place once I have prepared all the other fittings such as door hinges, harness attachments and repairs to the rudder and horizontal stabilizers.

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