CL Aerobatics
Frank McMillan, 12106 Gunter Grv., San Antonio TX 78231
EVEN THOUGH I've been in this focused part of the sport/hobby for many years, I am occasionally surprised by a situation. This time good procedures saved a potentially dangerous situation.
While attempting to start a piped .65 last flying session, the engine started. You may be asking what's so unusual about that; it's what they are supposed to do.
The day was lovely, with low humidity, temperatures in the mid 70s, and light winds. The airplane's engine was on its fourth start, but it had been sitting for more than 20 minutes, so it wasn't hot. I had flipped the engine with the battery connected, but it was dry and did not fire. I disconnected the battery and checked the engine, then I flipped it to atomize the charge. At that point the engine started. Remember, the battery was not attached.
Fortunately my friend and helper John Hill and I were holding the model as we normally do for a start. There was no problem, but you can easily deduce a potential for injury.
Many times we take things for granted or let our concentration wander. The wise approach is to establish a personal procedure that, if followed rigorously, will prevent accidents and accomplish the desired task.
Never assume that a status is in place or correct. If it's important to what you're doing, check it. If it's a safety factor, make sure it is in your procedure sequence. Finally, go over your sequence so that you are certain it will accomplish the task completely and safely every time.
Never flip an engine with fuel in it unless the model it is mounted in is held securely. Sometimes I hold a model casually in my hand and flip the propeller. Most times nothing happens, but certain conditions can cause the engine to fire on compression only. Be conscious of the fact that our engines have high power. Couple that with sharp-edged props, and you have a potential for severe damage if you are not careful.
The same holds true when you are ready to fly and are preparing to start the engine; it can fire instantly when the plug wire is attached if it happens to be resting on a compression stroke. This is rare, but it is better to anticipate the possibility than to be caught off guard and be injured!
Another safety factor is your flying lines. Be very cautious of crimped ends. The AMA rule book permits this style of making ends, but it is precise about how it shows the crimp executed. I strongly suggest that you use the wire-wrapping methods illustrated in the rules.
Crimped ends are susceptible to kinking or stress points within the crimps. Recently a friend decided to shorten a set of lines and used the crimp method to serve the ends. Fortunately, he was thorough enough to do a good pull test. The ends failed at approximately 20-pounds pull.
Let's talk trim again! I've discussed the following trim point before, but it was suggested that I revisit it because it can really affect the way a model flies—especially a potentially good one.
In general terms, the trim problem is not setting the control system and movable surfaces at a common neutral. If some component in the system is misaligned, the system will respond more to up or down control inputs. As you might be able to visualize, this certainly will result in nonlinear control.
Remember that what you are after in a control system is for the airplane to react equally up and down with equally applied line pressure. Implied in that is that the torque or hingeline to which the pressure issue addresses. The totality of this is grounded in each component of the system and how accurately it is assembled.
Before we get to the simple point I will make, let's take some of the esoteric geometries out of this discussion. Many feel, and have proven to their own satisfaction by drawings and math, that things such as 11° horns, perpendicular paths from horns to the bellcrank, and location of the bellcrank have an impact on control-line linearity.
They all have some relevance, but they have less the factor represented because of empirical control deflection maximums on the order of 25°. The factors mentioned come increasingly into play at the 25° deflection range, but does that really have great significance?
Recently I had a puzzling problem that turned out to be caused by improper control-surface alignment. The flaps were ever so slightly up with the elevators in neutral. To visualize the effect this had, think about my application of up-elevator in the transition of the Horizontal Eights. Because I had to apply a certain amount of up-elevator to neutralize the flaps before they began to generate the intended control, I had a differential.
If the misalignment is so small, does it affect the feel? You bet, and much more than you realize! It can also affect the way the airplane "grooves" in level flight, causing the pilot to constantly input slight up-control. And the airplane may appear to fly with a slight tail down-trim.
One reason I've chosen to use the "up" position as an example of control misalignment is that it affects the trim more than a "down" condition. The latter provides lift so that the airplane can be trimmed without noticing that there is misalignment. However, all the other effects are still there.
If your new model turns well in one direction but stalls when you try to make an equally tight turn in the other direction, it's a safe bet that the flaps and elevators are not properly aligned.
Bob Hunt had this problem with his Crossfire design. It stalled violently when he tried to execute an outside corner, but it turned inside corners very well. The problem turned out to be that the flaps were down rather significantly in relation to the elevators when at neutral. This meant that during inside corners the flaps were actually inducing lift early. In outside corners the flaps were producing significantly less lift, and the model would stall.
When Bob finally discovered his mistake and corrected it, the model turned well in both directions without stalling.
As you install your control system, check every item to ensure that it is in its neutral position. This is more important than being overly concerned about the other control setups mentioned. Even if you incorporate the "tricks," you still must have the flaps, elevators, and bellcrank set at neutral together.
New Items: By the time you read this column, the Brodak Mfg. Co. (100 Park Ave., Carmichael, PA 15320; Tel.: [724] 966-2726; Fax: [724] 966-5670) will be releasing two new kit offerings: the Olympic Mk IV and the Vector 40.
The Vector 40 is a famous classic 35-size design by Bob Gialdini. It is known for its twin rudders and superior flying characteristics. The Vector 40 is a new design by well-known designer Randy Smith and is intended to be a midsize airplane for up to .40 piped engines. Both kits exhibit John Brodak's usual thoroughness and attention to detail that make all his kits excellent values.
I have started the finishing procedure on a new piped airplane. As I was using the clear dope, I was reminded of how good the Brodak dope products really are. The clear is just that—crystal clear—and the colors are pure and vibrant, with more choices added continually.
This is my choice of finish, and I recommend it over automotive paints as the easiest and most user-friendly finishing system.
If you want to see yourself and your creation in Model Aviation, send pictures! MA
Transcribed from original scans by AI. Minor OCR errors may remain.
