From Grant Potter
comes this month's winning question. What are the effects
of blade surface finishes on Speed, Team Race and Combat props.
We receive props from Russia with Poly-urethane finishes -
is this an advantage or just a selling point? If at all how
much would our times be affected by Poly-urethane, finely
sanded and bastard file finishes on our props?
Wish I new Grant, but thanks anyway for the
question. Also the feed-back on my Good-Year prop was appreciated
Talking about surface finishes makes me think
of the old Tante-Ju Junkers-52 transport of WW11 and its ilk.
This aircraft was covered with corrugated sheet, just like
the sheet-iron roofing on my father's old house in Gumnut
Road, West Pennant Hills. I gather they both flew about the
same, especially when my mother was raging about the balsa
dust and dope fumes.
Even though the Ju-52 corrugations ran front
to back, drag was high. Problem is, air does not run front-to-back
on a wing, but manages to flow sideways as well, for all sorts
of reasons. These include tip vortex roll-up, distortion of
the flow due to the pressure field of the fuselage and the
engines, and instabilities in the air itself.
So I guess this might not work too well on propellers
either. In the case of combat props, at high angles of attack
there is also radial flow in the reduced pressure region behind
the high point of the airfoil. Well, since corrugations don't
work, perhaps the prop surface should be smooth!
Also I noted recently that some F1C fliers (F/F
power) are going to great efforts to make their prop tips
nicely polished. They are pushing Mach .8 at the tips, so
I guess they are worried about shock waves as well as normal
skin drag. If the surface is rough, multiple small shocks
may form instead of one large shock wave. I don't think anybody
knows if that's good or bad. It's the same argument as for
rounded and square tips at high Mach numbers. Maybe you should
have one of each!
The Poly-urethane finishes on the Russian props
worry me a little. The paint is beautifully applied, indeed
I could not tell from looking that they were in fact painted,
the gloss is that good. However, it seems to me that the paint
must alter the airfoil profile, which could lower the L/D
slightly. The props don't appear to be made from machined
moulds, so this may be a bit spurious anyway. My own props
have the finish of the polished Aluminium from which the prop
airfoils are accurately machined: it is not recommended that
they be painted, except perhaps where balance rework has been
In more general terms, the efficiency of a propeller
is affected by the L/D of the airfoils which form its surface,
but that parameter is only one of several that affect the
efficiency. In most cases, I don't think you would be able
to detect the difference in performance between paint, fine
sanding and bastard file finishes. The exceptions would be
where tip speeds exceed M=0.7, as in F1C, F2A, F3D etc.
I generally finish mine with 1200 wet-and-dry
paper, mainly to stop ripping up my flicking finger on the
trailing edge. In the case of the paint finishes, I would
refinish these with a small sanding block and 1200, topped
off with a polishing rub of T-cut. Many years ago, when I
still had muscles in my right arm, I had good results in chuck
glider by polishing the wings. The wing was painted with 2-part
Estapol, cut back with 600 and polished with Brasso. This
gave a big lift in performance in the case of the low-Reynolds
number chuck glider regime. Such a gain might not be observed
at higher Reynolds numbers of I/C propellers.
To finish off, I think it is probably a first
priority to get the airfoils on a prop right before worrying
too much about the surface finish. Smoothing with 1200 is
probably good enough. The L/D of an airfoil is strongly dependent
on its profile and camber. Getting these points right puts
you ahead of the opposition very quickly. My NACA airfoils
generally kill the crude airfoils on competing props. I have
seen a reworked Gillott F2C prop with the high point at 60%.
Basically, the airfoil was on the prop backwards! That setup
has 4 times the drag of the correct airfoil, so do try to
get it right.
There you have it Grant, perhaps a few experiments
may be in order. Either way, you have won $30 worth of Supercool
To any of my readers who happen to get to Perth,
do give me a call, come and visit: I'm on (08) 9247 2481 and
Rosemary is pretty good on the Leek and Potato soup!