CL Aerobatics
Frank McMillan, 12106 Gunter Grv., San Antonio TX 78231
Introduction
As the flying season reaches its peak, I thought I’d relate some experiences I had with a new version of my Caudron design. Most of the difficulties centered around the wing.
My trimming has been rushed at times by minimum flights, so I have a tendency to shotgun changes. What follows is an analysis of a quite lengthy frustration period. I hope it saves some of you problems.
Bench Trimming
With a new model, you should “bench trim” all the adjustable settings. If you’ve built a proven design, such as the SV series or the Cardinals, the settings for all the adjustable controls are well documented and you should use them prior to the first flight.
Before we start the process, remember that many factors are interrelated. Also, this is my opinion and you may not agree with it.
I would not recommend tweaking the flaps until you try other things. With all the newer methods of accurate wing assembly, it’s a good assumption that the wing is straight. Careful observation during the first flight is important.
Leveling the Wings
Make no mistake—leveling the wing is the foundation of a well-trimmed model.
You are looking for the trailing edge to be in line/parallel to the flying lines at normal level flight attitude, upright and inverted. If not, there will be a rolling tendency. To observe how the wing behaves, you must go inverted. The indication for insufficient tip weight will be the outboard tip up, upright and inverted. Too much, and the outboard tip will hang.
Another indicator of too much weight is the outboard tip continuing in a flight path as the airplane is turned (top of the hourglass).
There are various approaches to accomplishing wing leveling, depending on the problems. A few possible difficulties are balance, warps, flaps, elevators, and vertical center of gravity (CG).
The sequence below will get you in the least trouble and do the job.
Tip Weight and Vertical CG
The primary reason to adjust tip weight first is that it’s easiest to get a “ballpark” adjustment without touching your new model’s initial setup.
If the reduction of tip weight doesn’t get a linear correction, try adding some modeling clay to the bottom of the outboard landing gear. In this case, we are looking at making an adjustment to the vertical CG.
I mentioned looking at this aspect now just to get a feeling for whether a vertical CG adjustment is a candidate. If adding clay seems to make an improvement, try some more. Give it at least three flights.
Remember that you can do some bench trimming, but you have to fly the model to finalize a vertical CG solution.
The best implementation of a vertical CG solution is heavier or lighter wheels. Next would be to move the leads up or down (not forward or back). I'll come back to vertical CG for an important point later.
I mentioned linear change because that's the key point you look for in making adjustments. As I said before, everything is interrelated. If you have bench-trimmed and built your model correctly, you should get a linear change, as described.
However, if you get a combination of changes instead of a predicted reaction, some parameter is out of the viable range. Sometimes this will be subtle, but it can really frustrate your trimming.
The really tough problem to diagnose and correct is a vertical CG. Bench setup only goes so far, because you are dealing with moment arms/distribution of weight and centrifugal force. It is a dynamic.
With duplication of proven designs, no problem. But consider the older Classic designs, which had numbers based on really light engines (Fox .35) and heavy wheels. Put on a modern engine and the lightweight wheels, and you have a problem that has to be corrected.
The same goes for new designs with new equipment configuration.
Leadout Location
I've already discussed adding/reducing weights at different locations. Leadout location (up or down) is next. With the Classics, the probable shift will be up roughly 3/32 inch to 1/8 inch.
With the new designs, note that many of the plans call for the leadouts to be 1/8 inch below the centerline. This is with the thrustline 3/4 inch above the wing centerline. Newer thought has the wing centerline at 5/8 inch while keeping the leadouts 1/8 inch below center.
Once you've experimented with what we've discussed, you should have the model at a point that is very close to being in good trim. If the wings require adjustment, a slight flap tweak would be appropriate.
Elevators and Alignment
The elevators could be installed out of alignment. With the size of the new design elevators, even a slight misalignment could induce a roll as you turn the model.
You might say that can't happen, but it has. You need to check.
Wing Aspect Ratio
There does seem to be a correlation with ease of trim and wing aspect ratio. The higher the aspect ratio, the more sensitive to trim adjustment the wing will be.
Selecting the wing aspect ratio is a matter of working out a good compromise. The long-wing models fly really well when trimmed.
New Items: Spray Gun
The buzz around Texas is the exceptional bargain on a new spray gun.
Almost every year we see a new unit advertised by a discount company that looks like a major U.S.-manufactured item. Sometimes they are good copies and work reasonably well, for the price. This new unit is considerably better than that.
The Central Pneumatic Professional HVLP gravity-feed spray gun is of surprisingly high quality.
- The body of the gun is red and anodized, finished with large, knurled knobs controlling air, liquid flow, and pattern size.
- The gun is slightly larger than a traditional touch-up gun. The largest fan size is accordingly somewhat larger, but the adjustment can be all the way down to pinpoint size, showing extreme versatility.
- Reports from a number of modelers, including Al Rabe, Don Hutchinson, and Bill Wilson, are glowing. They say that the controls are linear and easy to use.
- Above all, the paint goes on very smoothly at all settings. When using the airbrush size settings, the gun performs like a high-quality airbrush, with smooth, controlled application.
- Careful examination of the spray head shows why the gun works so well. The head is heavy plated brass with many holes of various sizes that have specific functions as the pattern size is varied. Therefore, the head has versatility built in.
- One important parameter of good spray work is the correct air pressure at the gun. The longer the air hose, the more pressure drop. The gun comes with a pressure regulator that connects directly to the gun.
- The pressure used is roughly 20 psi. Users report that at that pressure there is minimum overspray, so more paint goes on the model.
The Central Pneumatic Professional is available nationwide through Harbor Freight Tools for approximately $50. I highly recommend it; it's a great value. On a recent visit to an auto-paint supplier, I priced what appeared outwardly to be an identical gun at $150.
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
