Dimples on a propellor to improve aerodynamic performance
by Bill Adkins
Jerry asked me for a couple paragraphs on dimpling efforts. I figured you'all should get the whole schtick so here it is:
Had to purchase a prop some years ago as I had split my home-made at 'Sunken Lunken". That should date me! I'll not mention the make as it is no longer a Aymer-Demouth. Some trees jumped up during landing a couple months ago so it needed a bullet-proof leading edge and sanding the dents out. Leading edges have proved impervious to dings for some years now on my props. So my son and I decided to try the dimple-thing while it was in the shop.
Why: I was in on the vortex generator testing done some years back in Langley's 20x20 Laminar Flow Control Tunnel. Thousands of shapes and positions were tried. Results were that they do re-attach flow, but not all shapes are very good and certainly not for all velocities. Results were published and are availble as are all public funded research. As this tunnel goes to a bit over 100mph and golf ball go a bit faster I thought it just may have merit. I'm now retired and as 'Experimentation is the mother of invention and lazyness is the father' figured I'd try it.
How: Some things are pretty obvious such as you do not put one trip directly in front of another. The leading edge of tape IS a trip. A stripe of paint on the leading edge of the wing can ruin your entire day, as has been proven. Using 1/10th inch graft paper I marked the intersection lines of one block, moved to the left and below for another and so forth till I had four rows. Then back to the top and left for the next row and so forth. Then cut the paper into strips and using a pin punch transfered the pattern to the prop. Found the high point on the prop with a caliper every two inches and connected them into a line paralell to the leading edge. Surprised to find this line was not a wpe1B.gif (1652 bytes)smooth curve. Smoothed it out and proceded, putting two rows above and two rows below that line. Looking back I probably should have used only three rows! Now to cut them accurately! Put a 3/8th inch burr into a micro-adjustable countersink tool and began burring out dimples with a air drill. Each dimple is .1285 deep and .1865 in diameter. As the prop got thinner I raised the burr .0046 per row for the last 5 rows and but three rows deep near the tip. Obviously less diameter but who needs to measure so accurately?
Result: Two test flights a week apart, so far. First at 3000ft and the second at 6000ft. Recorded static RPM, t.o.RPM, climb RPM, cruise RPM and max RPM, plus all other parameters for which I have instrumentation. I used aircraft mounted instruments because all previous data were taken with them. NO MARKED IMPROVEMENT NOTED. RPM's may have been a bit higher, but NO INCREASE in speed at any point. One fellow EAAer on the ground said it sounded louder on take-off. Am going to try again at 10000ft to see if it improves my 4.6 gph fuel burn at 150mph cruise. If so, I will report.
Do I regret doing it: Certainly not! Prop is holding together, looks great with all the white dots on it, a conversation starter and how else would I (we) know? Not the first dumb thing I have done and for sure not the last! Now back to finishing my three- blade which has been in the 'mill' for the last 5,6 or 7 years.
Remember: Re-attachment of flow over a surface can be done with a lot of 'things' including SOUND, BUT NOT AT ALL VELOCITIES. Ask no more about that one!