Radial Orientation of Bolt Holes wrt wood grain

[drrivah]

On old props the orientation of the bolt center-to-center diameter of bolt holes may vary with the blade form. Are there rules about the angle between the wood grain of the prop and the line across the hub between bolt holes center-to-center?

-pete

[Dave]

Don't know, although from an mechanical standpoint it would seem to me to make sense to stagger holes so that they didn't occupy the same logitudinal grain as another hole.

[drrivah]

Bolt holes vs wood grain

Sensible: that would imply 30 degrees off-grain for the 6-bolt hub and 22.5 degrees off-grain for the eight bolt hubs.

[Dave]

Sensible: that would imply 30 degrees off-grain for the 6-bolt hub and 22.5 degrees off-grain for the eight bolt hubs.

Doesn't look like it made much sense to the manufacturers, though. Go down the hubs on this page and you'll notice most of them are oriented right through the grain. On some of the U.S. built aircraft props it might be somewhat different.

[drrivah]

Grain Spec and laminations

The Air Ministry spec for acceptable walnut lumber includes wood grain at no more than 1:12 slope wrt long axis.

In addition, the radial axis of board laminations (4-7) changes with depth in the prop. Meaning, the grain of board No. 1 is offset compared to board 2 and the rest.

If the 1:12 slope and the differential radial orientation through the plan section are considered, the optimal placement of the bolt holes is complicated in terms of stress. Importance of alignment is probably greater for 4-board vs 7-board construction.

Needs more thought and study.

[pmdec]

Hi,

Doesn't look like it made much sense to the manufacturers, though. .../...

+1!

On the picture, the upper prop was directly fitted on rotary engine crankcase. Central hole larger than blade foot width. About 100 of that kind made between 1910 and 1914... And no prop halves scattered everywhere for what I know. Too old to make sense? The lower prop is a Chauvière made in 1947: central hole smaller... but 12 bolt holes...



Regards,
PM