MEASURING GRAIN SLOPE

by Victor A Boyce, CSN

Here's a handy tip that all woodworkers should know. This knowledge is most important when choosing woods to be used in aircraft building that will allow you to get the best pieces available

The best wood to use when building an aircraft is wood that has a perfectly straight grain. i.e. wood where the fibers of the grain are parallel to the edges; it is easier to work with, less likely to split, can be laminated and bent into many shapes. Stronger structures can be made from this class of wood.

But is not always possible to obtain this quality with a straight grain that runs the entire length of the piece without some kind of grain runoff. So an allowance must be made for some deviated or sloping grain however, if the grain slope is too steep, the piece must be rejected. So the answer is to measure the slope to determine if the piece can be used.

The grain slope must be determined and carefully measured on both the flat and edge grain surfaces. These two measurements being at right angles to each other must then be combined to obtain the total grain slope. This will be steeper than either of the individual slopes.

In cases where one of these faces has straight grain with no run-out, the true slope of the grain can be seen on the other face, just measure this slope, no calculations needed.

For general aircraft work the grain slope must not be steeper than 1 in 15 and for wing spars the slope should not be steeper than 1 in 20. As previously stated, wood with no slope at all is best.

If the annual rings can be seen on the edge-grained face, simply measure the slope of the annual rings. If the annual rings cannot be distinguished, consider this face to be a flat grain surface.
On a flat grain surface a little more effort is required to determine grain direction. This can be accomplished in several ways. Look closely at the surface and note the following items that will indicate grain direction.

1. Small resin ducts which will be in the same direction as the fibers, small checks (separation of the fibers), streaks of different colored wood in the grain. Then draw a line parallel with the grain and measure for any grain deviation or slope as it is called.

2. Small drops of ink placed on the surface will spread along the fibers and indicate grain direction. A line can be drawn through these small indicators, from which grain direction can be determined and measured.

3. If a small surface scratch can be tolerated, use a scribe with a long handle about 2' long fitted with a stout pin or needle to make a light scratch indicating grain direction. The operator holds the scribe by the end of the long handle and pressing the point of the pin into the grain, pulls the scribe in the direction of the longitudinal axis or what appears to be the grain direction. The long handle prevents the operator from steering the scribe and allows the pin to follow the grain. If this is done carefully the pin scratch will follow the direction of the fibers and clearly indicate grain direction; several parallel scratches may be made if desired.

The total combined slope of the grain is determined by taking the square root of the sum of the squares of the two slopes.

 

About the author, Victor A. Boyce

Australian born, at 17 became apprentice as Carpenter & Joiner; At 20 took his first flying lesson in a Ryan STM with a world War II bomber pilot/instructor who would later send him solo after just 6 hours.

Entered a full time career in Aviation as a Licensed Aircraft Maintenance Engineer and DCA authorized Inspector with Airframe, Engines, Electrical, Instruments and Radio endorsements and many accomplishments' one of which was Top Engineer of the Year in 1962 winning the Coveted Ryan Award;

Instrumental in making a new improved reinforced wing walk on the DH82 Tiger Moth. The removal of Urea Formaldehyde glue from the Dept. of Civil Aviation and FAA Approved List of Glues for use in aircraft building and repair.

A noted Aerobatic Glider pilot often doing demonstrations at airshows in his Hutter 17, a small Austrian design that was extremely maneuverable that he had so painstakingly restored.

Did "Flood Rescue Flying" in the northern outback, with an old Gypsy Moth requiring good flying skills as landing was often on ground covered with water and obstacles producing hazards not normally encountered by pilots.

Has been called upon as an expert in wood and glues by lawyers in court cases in Australia and Canada and from time to time by the D.O.T when investigating Sabreliner problems or accidents.

While in Georgia participated with the FAA Safety Seminars as an expert panel member. Designated Airworthiness Representative for the FAA in the amateur aircraft building program. Assisted FAA in accident investigations as an expert in wood aircraft construction.
Active as:
Chairman hands-on Workshops at Sun 'n Fun EAA Fly In with some 14 shops and over 150 volunteers - the largest convention in Florida and second largest Aviation Convention in the United States and the world.

Technical Editor, bi-monthly newsletter with construction methods, building tips and technical support for building the all wood design Corby Starlet CJ-1.

Conducts Forums on Aircraft Woodwork and Glue Joints at EAA AirVenture Convention in Oshkosh, Wisconsin
Sun 'n Fun EAA Fly-In at Lakeland, Florida.
Popular Flying Association Rally at Cranfield, England

Published articles in The EAA Experimenter, Technical Counselor Newsletter and other publications

EAA Technical Counselor

Spokesperson for sport aviation at news media interviews - particularly after a plane crash or other such news-making event

He is more than willing to share his knowledge with anyone who seeks his help. He is a true grass root's aviator and likes to share his love of flying and building planes with all who will listen. There are many people from many walks of life in aviation but none as dedicated as this man. He is genuinely an unpretentious, down to earth individual who has devoted his whole life to aviation.

Over 40 Licenses, ratings, endorsements and awards

Received EAA Major Achievement Award in 1997 presented at EAA Oshkosh Convention