Dan wrote:
How do you drill through a tube the wrong way?

Many ultralightes also have holes drilled through the tubes the wrong
way, seriously weakening the tube.



He doesn't like to see holes drilled vertically in a spar tube.

Actually, he's right in that removing material from the top and bottom
do reduce the amount of bending load the tube can ultimately take before
bending permanently.

The reason is that the top and bottom are the highest stressed areas.
The top skin is normally in compression and the bottom in tension.

Drilling here reduces the amount of material to carry the load.

But, like the sleeved bolt holes he mentioned, it is unnecessary -
if the stresses at that point are below what the structure will stand.

Now, the two seaters that Clare mentioned (there is no such thing as a
two seat ultralight in the US) are considerably heavier airplanes.

At that point they probably do need the sleeves (bushings?) in the tube
to take the compression load that the wing imposes on the root connection.

At the other end of the spectrum, the mast on my sailboat (and on the
bigger boats too) have only a simile bolt pinned through an unsleeved
aluminum tube to secure the base of the mast. AND - the compression
loads on it are right near the same as the wing root compression loads
on the airplane's wing root! Interesting.

To bring it back to the thread topic...

I used grade 8's on my airplane - for the landing gear axles.

5/8" diameter by 5 to 6 inches - to fit the wheels you want ti use.
The heads are cross-drilled for a 3/16 (AN-3!) bolt that attaches to the
shock absorber tubes (telescoping tubes with bungees).

After drilling a 3/16 hole through the heads, there is not a lot of
metal left - but we've never had a failure there.

So we can conclude that the stresses imposed here (gear loads are the
highest on the whole airplane) are lower than what the bolt head can
safely stand.

And - that - is all that matters.

Richard