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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


The Prop Doctor looks at: surface finishes


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 (18.7/10).

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 necessary.

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 products. 

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! 

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