Speed Secrets – Volume 2
Considering all the things a person can do to go fast, what I feel gets missed most often is the attention to details I see on many of the planes on a typical race day. I’m not saying that any one of the little things makes that much of difference by itself, but when looked at a whole, I believe there is a cumulative effect at work here.
I will start at the nose of the plane, and work my way back in future articles. Probably the biggest hindrance to speed is the prop on the front of the engine. I will make the assumption that you have the proper size and pitch in place, but what about the prop itself? Is it old, chipped, dry rotted, and showing signs of stress? As APC props age, you will start to see small white stress lines out near the tips, and in my opinion, this is the time to get rid of them. Here is how I like to prepare a prop for racing. First off, I like to ream out the center of the hub from the back with a good tapered reamer. Ream the prop out so that you come close to, but not into the slightly larger ridge molded into the back of the prop. If you go to
www.darrolcady.com he sells prop rings for the Nelson engine that you place over the crank that will allow that little recessed ridge to sit perfectly on the spacer sleeve. This will give you a perfectly centered prop on the nose of your engine. At one point there were some prop rings floating about for the O.S. .46, but I am not sure at this point who has access to them. If all else fails, a simple rubber o-ring from the hardware store works fine in a pinch.
Next, I have 3 small sanding blocks that I made for preparing my props. They are about 6” long and made with ¾” x ½” pine. I have them set up with 150, 220, and 400 grit papers. If you look at the four edges of a new APC prop, you will see a ridge of plastic flashing that runs all the way around the prop. This flashing is legal to remove, as long as that is all you are removing, you are not allowed to rework the prop, just the flashing. I start with the 150 grit, and make roughly 10-15 passes along one side of the blade to remove the flashing at the TE of the prop, I then go to the LE with 150, and then the same with the other blade. It is important to make the same amount of sanding strokes with your block, or you will throw off the balance. After the 150, I do the same process with the 220, and then again with the 400. You need to be very careful at the tips, as the material gets very thin there. At this point, I take a damp rag and remove the sanding residue. At this point, the flashing should be gone, and you will have a very smooth LE and TE on your prop.
Now that the flashing is removed, it is time to check the balance of the prop. If the reaming, and sanding was done properly, the prop should be very close to balanced. I absolutely hate sanding on one blade for balance, so all I will do at this point, is take a sharpie (green is a good color for this) and mark the heavy blade. This is also a good guide to know that the prop has been properly prepared. When you get ready to install the prop on the engine I will set the piston at top dead center, and place the prop with the heavy blade opposite the engine head. Now that you have what I would call a properly prepared prop, how do you maintain it? After each and every heat on race day, I give the prop a very good look over. Look for chips, dings, and even broken tips from the plywood boards that we launch off of. I like to take a rag or paper towel and clean the prop after each heat as I inspect it for defects.
That’s it for this installment; stay tuned for future “speed secrets”!