H O F T E C
As time goes by you may attribute diminishing RPM to a tiring engine. Whereas that may be true, something else, quite hidden, may be putting on the brakes.
If you operate from a less than perfect surface with a metal prop, and you have a conscientious mechanic diligently removing nicks from the prop, as they should be immediately removed, the prop's aerodynamic parameters are being changed. Removal of nicks is very important. To see just how much a propeller contorts itself, you need to look at a running propeller at night, using a variable-frequency strobe light. You'd be amazed, and incredulous, at the standing waves set up on your very expensive piece of aluminum. If you did this, I assure you that you would never fly with a nicked propeller again. Just how many times can you bend a piece of metal back and forth before it breaks?
Back to finding the lost RPM. Engineers used up a year's salary to figure out the optimum geometry of your propeller – blade area, diameter, thickness, airfoil, tip shape etc. So now you take a file to it and change all of that. Try to understand the following diagram depicting the cross-section of a propeller blade:
The mean aerodynamic chord for this airfoil is parallel to the bottom surface, indicated by the arrow. When the engineers decided that the optimum angle of attack for your prop was going to be 4° they used that chord line from which to measure that blade twist.
So now you are going to file the nicked leading edge.
Notice that what has happened are several things: One is that the airfoil has changed angle of attack. That increases drag. The blade airfoil is no longer operating at its optimum angle. Having filed the leading edge has increased the angle of attack in the area where you filed. The second thing that has happened is that the % thickness of the airfoil has increased. That increases drag. Since the blade is just as thick, but now with a smaller chord, it is effectively thicker in the 500 mph wind it makes for itself. The third thing that has happened is that the narrower chord is operating at a lower Reynolds number. That increases drag. The Reynolds Number deals with the same phenomenon that lets longer boats travel more quickly through water than a shorter boat, same width, same power. Of course the worst thing with all this is that increases in drag are happening where their effect, due to the leverage they have way out near the tips, is most effective to fight the efforts of the engine.
So what should be done, and in most cases is not done, to minimize the effects of these repairs, is to also decrease the thickness of the blade in the repaired area. AC 43.13 - 1B/2A, Acceptable Methods, Techniques and Practices, our repair Bible, page 8-30, shows how important it is to do this work properly, and indicates what is typically done, wrongly, when just the leading edge is dressed. Mechanics tend to pay attention to properly dressing out the nick, making sure that the edges are well faired in along the leading edge. What then usually does not happen is that the thickness of the blade is reduced. You can convince yourself of this by looking at the paint in the affected area – chances are it didn't get touched. In any case, your mechanic should have the repair limits at hand for the particular propeller being repaired, just to make sure that minimum dimensions are maintained. AC 43.13 on p.8-34 gives some indication of the % reduction allowed as a function of blade radius – that could be up to 12%. That filing has to be done by a coarse body file to make sure no marks are left by the process. We even put chalk into the teeth to make sure no aluminum gets caught in the teeth which might scratch the finished surface.
Beyond all of the above, RPM is also reduced by vibration. A 6 ft diameter prop turning at 2400 RPM, generates about 6000 "G"s at the tips. In other words, a one gram difference in weight at the tip would generate an unbalanced force equivalent to 6000g, that is 6 kg, churning at 2400 RPM. As a consequence to this 'multiplier effect' of centrifugal acceleration, balance shops use coats of paint on the tips to do the final balance on props. Small wonder that engines remove themselves from airplanes when props lose 4” of tip. Imbalance is also the reason why wooden props should be parked horizontally. There is a price to pay for any imbalance, whether it is in RPM, decreased fuel mileage, loose rivets, instrument repairs, radio repairs, and just plain discomfort. That vibration costs money. And you were simply going to file out the nicks on one side of the prop!
Have the prop properly repaired, and have it balanced (once removed) after any serious filing. Balancing means both spanwise and chordwise adjustment. Having the prop dynamically balanced usually results in weights being added to the inside of the spinner backplate. That keeps the vibration from feeding back into the engine, but does not remove the source of the problem - imbalanced blades.
Why do only mechanics seem to know what is wrong with doing run-ups over puddles, dirt, stones and chunks of ice? If, on the other hand your prop has enjoyed all the proper maintenance and operation, and you are still looking for a 20 - 30 RPM rise, put a heavy coat of paste wax on the prop. You'll even feel that extra RPM before you fire up. It is not always the engine's fault!