Blade trailing edge thickness

[madlabs]

Hi all,

Finally getting around to finishing a set of blades, they are shaped and I just have to do the final sanding. They are 60cm long and made from cedar. I was wondering how thick y'all leave the trailing edge. My last set I sanded to less than 1/16' but they looked a little thin. I'm thinking a little thicker this time. Any thoughts?

Thanks!

Jonathan

[Ungrounded Lightning Rod]

The trailing edge is sharp so the air streams on the two sides come together smoothly without drag.  Round it off or flatten it and you create drag, like the drag behind a station wagon.  The wider the non-sharp part, the more drag.  You'd like it to be a knife edge for best aerodynamics and only round it at all to keep it from being fragile or dangerous.

[redtick]

Here is a video that shows flow over foil profiles at different pitch and flow rates. 

http://www.youtube.com/watch?v=dUhiDctsyfs

[kevbo]

Here is a video that shows flow over foil profiles at different pitch and flow rates. 

http://www.youtube.com/watch?v=dUhiDctsyfs

That is a sweet video, but it is showing 3D flows including tip vortex, which obscures the effect of a blunt trailing edge at anything other than zero angle of attack.  In general, if you can't have a knife edge for structural or safety reasons, then it is better to have two sharp corners than a radius.  The sharp corners give the flow a clean break, and there is a stagnation area behind the TE that sort of fills in for the missing knife edge.  A rounded edge will do the vortex shedding thing as the video shows for the cylinder at the beginning.

[madlabs]

What I am trying to figger out is the best compromise between performance and blade strength. I'm worried that hone the trailing edge to a razor will allow the baldes to break or warp. Any opinions on that?

Thanks!

Jonathan

[SparWeb]

You're doing blades in wood so it's definitely possible to sand the TE down very thin.  After that, though, you have to treat the blades like they're made of paper so that the TE doesn't peel off...  leaving you with a jagged edge 1/4" thick anyway.  You can try doing it a little thinner than 1/16" but not much more.  The exact orientation of the grain determines if this is harder or easier to maintain.

[silentblue1987]

It's all about aerodynamics as we all know.

The thinner the trailing edge the better, the more uniform the curvature the better, the smoother the surface the better, but increasing brittleness of the edge might stray you. Find a suitable compromise between thickness and brittle, not much you can do otherwise.

Sometimes when I was woodworking small chips would snap from the edge when you approached paper thin even with 200 grit sandpaper. So I used superglue on the area and it helped keep the chipping under control for a bit.

I wish I used epoxy though... Sands down much nicer and makes a smooth finish after painting.
(Like a boat hull, probably why they use it  )
Wood should absorb a good portion of it, making it possible to re-sand it down to near paper thin without chipping the edge. However one small nudge in the wrong direction can be disastrous.