CL Aerobatics
One of the really enjoyable aspects of our hobby/sport is passing along the knowledge to make our wonderful models fly well. Often, that can be a difficult exercise because of the many variables involved.
While working with several good friends recently, the light bulb burned brightly about why we were having problems making the power package work the way we wanted. Of course, the answers were in the details.
I'm going to discuss our lessons learned and pass on some hints that can help make your latest model work.
So many times I've said to friends, "There's magic in props." That's still true, and even more so as we get more and more power, along with wonderful engine-run characteristics. There's no mistaking a good engine run.
Listen to the top fliers' airplanes as they break ground on takeoff and drop into a deep, throaty run. Pay attention to the way the engine "cycles" in the maneuvers. Does it come on and off under load, and does it do it quickly?
All this is an indication of matched load, airplane, and propeller.
The good runs seem to elicit questions about what settings are used—especially the propeller. However, the answers to those questions have often created as much confusion as not knowing the answers.
I'm going to focus on the propeller because that is where many problems arise. The parameters we deal with are obviously type and manufacturer, number of blades, blade airfoil, profile, and pitch. Most of these parameters are easily measured and repeatable.
However, what about pitch? You might say that should be no problem because you've got a pitch gauge and can measure the pitch exactly. You can duplicate the profile that you have seen work well.
That isn't necessarily so!
Consider the situation I've been in many times. I've given the exact settings to friends, they've set their propellers up the same, and the profile didn't work—even acceptably.
Most times when that has happened, I thought something else was wrong. After all, a number of variables can affect the quality of run. My friends probably thought I was nuts or just misinforming them.
The first clue that something strange was happening was that I could take one of my propellers, put it on any of their airplanes, and it would work.
Was the problem solved? Not really, partly because my propellers were flying away on other people's models, and my propeller supply was diminishing!
But we didn't get to the root of why the propellers wouldn't work.
Recently I happened on what needed to be done when I compared pitch-gauge readings with the same propeller at the same time on different gauges.
The readings were the same out to the number "six" station (with a Prather gauge), but diverged until the "twelve" station was reading almost an inch more on my friend's gauge.
So when he set the pitch I was using—3.9 on my example—he had 4.9. Thinking back, his engine ran as if that's what he was using.
Now that we've discussed the problem, what caused it and what can we do? A close visual inspection of both gauges didn't show anything apparent, yet they read differently. The silkscreen markings were identical.
We concluded that the holes for the graduated plate on his gauges were slightly different from mine. If you look carefully at the graduations, you can see that it doesn't take much to throw the readings off.
Before you go off complaining about your defective gauge, there is probably nothing wrong with it. Consider that you care about knowing what the working setting is and being able to duplicate it—not necessarily the absolutely accurate pitch number.
I suggest that you calibrate your pitch gauge to a working propeller or another person's gauge that has a working setup. This is not a difficult procedure, nor is it a hard-and-fast rule.
Put a propeller that works on the gauge on which it was set. Take the readings at various stations (the even stations from number "six" out on a Prather gauge).
Lift the slider block and propeller, transfer it to your gauge, and take the readings at the same stations. Marking your settings at this point is a good idea.
Note the difference in relative terms. Now you should be able to duplicate that propeller and change the pitch profiles. You will also be able to compare your new readings with the gauge to which you calibrated. You should also compare your gauge with a third gauge.
What I've discussed in theory can be duplicated on all the instruments we use, including tachometers. These are generally not precision instruments, and you can expect repeatability and "reasonable" accuracy.
By calibrating and comparing your devices and the physical readings, you should have a good correction factor for your propeller work.
Tachometers
It is common for different brands of tachometers to read more than 300 rpm difference. When I have flown with my friends, we have compared readings on the same airplane at the same instant and noted disparities.
As with the pitch gauge, the relevant point is the reading on your tachometer when you have the engine running as you like, then use that for future reference. You care more about repeatability than absolute accuracy.
I've had more than my share of tachometers throughout the years. I've had the one I consider the best for several years, and I have mentioned it in this column. It's the TNC digital optical tachometer.
The standard version reads to 100 rpm with an option to 10 rpm. The accuracy is to one rpm. It is temperature-compensated and reads extremely well in low-light condition. It also has crystal-controlled circuitry, which accounts for its accuracy.
Contact TNC Electronics at 2 White's Ln., Woodstock NY 12498; Tel.: (845) 679-8549.
A friend called and inquired about how I was running my PA .65. He asked which propeller I was using, rpm, how much fuel—the usual questions, which I already mentioned. I passed on the settings I was using. Part of that conversation was relevant to my discussion here.
When I relayed the propeller diameter, he said that in his area—Southern California—the engines would not pull the load I was swinging, 12½-inch-diameter versus 12¼-inch three-blades. (Small diameter increments make large changes in disc area, hence load.)
It is difficult to compare exact readings because there are other factors. In this case, the venturi was not big enough so the engine was not putting out the power it should.
But the basis for comparison still goes back to the quality of run. Each flier has to get to that. Remember that the settings I shared are relative to that measuring device.
After this discussion, hopefully you've got a better handle on why you may be having problems. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
