The Art Of Trimming

I WANT TO DISCUSS a topic that I refer to as "The art of trimming." I consider aircraft trimming an artform. I don’t want to lose your interest with the super-deep details of this, so I’ll cover the basics and you can choose how to best proceed. There are pilots who take a deep dive into airplane trimming, and some who do the basics and start flying.

What is the simple definition of a trimmed aircraft? It’s an airplane that can fly straight-and-level for several hundred feet without pilot intervention and little digital adjustment. It’s best to check your flying trim on a calm day with no wind.

What are the benefits of trimming? First and foremost, a true-flying airplane is less work for the pilot. You are not fighting or "chasing" it around.

Second, a trimmed aircraft is an absolute joy to fly. A truly trimmed aircraft will perform RC Aerobatics (Pattern) with much less effort than one that must be muscled into submission. This doesn’t make you look better; it helps you adjust your workload to what is important, making the maneuvers as perfect as possible so that you can enjoy how well your aircraft flies without darting around the sky and chasing it.

CG

Review the manual that came with your aircraft. You should have the center of gravity (CG) within the recommended range. Making the airplane severely nose-heavy and/or tail-heavy from this range will cause you to have trim issues that can only be resolved by attaining the proper CG. Try to get your airplane in the recommended range slightly nose-forward or level.

I have heard theories that more nose weight is better than a little, and some airplanes will practically fall on their noses when checking the CG. Unfortunately, a very forward CG will have negative effects on how your aircraft flies. The goal here is to get it in the manufacturer’s recommended range, and then make minor adjustments to get the feel correct. The diagram on the previous page shows an aircraft that was set up straight-and-level using a CG machine.

Alignment

Let’s check the alignment of the wing and stabilizer. This will affect the overall performance of the aircraft and is something to check on all of your aircraft, new and old. If you already check it during the build/assembly process, use this as a refresher.

Let’s look at how the stabilizer and wing align. You can see in the photo that the airplane is blocked up and sitting level. I placed a bubble level on the stabilizer and used some boxes to get the bubble centered.

Stand approximately 10 feet back and observe. Your stabilizer and wing should be equal in alignment. You can take some measurements to confirm your visuals. If you find that you’re not in alignment, adjust the base where the stabilizer sits to make things correct. If this is an airplane you have been flying for quite some time, you can add a shim to the wing cradle to make the adjustment. Start small and get it as close as you can.

Next, take a tape measure and confirm that your wing halves are in alignment and straight. I use a soft, flexible tape for this. Looking at the first figure in the diagram on the next page, measure the wingtip to the back center of the fuselage on each side. Adjust as necessary to get the wing straight (yes, I have redrilled mounting holes and adjusted wing tubes to fix these misalignments). Repeat the same method, measuring the wing to the stabilizer as you see in the second figure of the diagram.

If you are dealing with a prebuilt airplane and you have no way to remove your stabilizer, there are methods to make these adjustments. Consult someone in your club with this experience if needed.

I’ll mention that most modern airplanes that we fly are computer designed and laser-cut, and these issues are not what they used to be, but you always need to check. I have had issues with expensive ARF airplanes that were far more out of alignment than I would have imagined for the cost, so don’t assume that everything is good by the looks. Take some time and do your measuring.

Radio Trim

The most common method to trim your airplane is by adjusting the trim tabs on the radio for straight-and-level flight. Throughout my experience of flying and helping others, I have determined that using the radio trim tabs is skipped by many. I’ve been handed radios where the airplane darted hard left, climbed, and veered off course before I could get my fingers on the sticks. I immediately righted the aircraft and used the trim tabs to get it flying straight-and-level.

The first thing I do when someone wants to get started in Pattern is to try to observe the trim of his or her aircraft. Before I start walking a pilot through maneuvers, I ask him or her to let the sticks go so that we can observe how the airplane behaves. From that point, I start helping trim the aircraft with the radio.

Being able to fly and trim the radio at the same time can be challenging. It’s something that I encourage you to try. If you are not comfortable doing so, ask someone in your club. I’m certain you will find help. I still, even as of late, get asked to fly someone’s airplane to adjust the trim. I never mind doing this to help a fellow modeler out so that his or her airplane is more enjoyable.

Block up the airplane as shown and observe the alignment of the wing and stabilizer. You want it straight and true. Take measurements here to further confirm.

I’ve found that if the airplane measures out true and the surfaces are aligned, I generally don’t have to add much radio trim to achieve straight-and-level flight.

Final Comments

I covered what I think are the most important starts to trimming. There is so much more to dive into that will get us out of context. If you are interested in a deeper dive, I encourage you to purchase Bryan Hebert’s Triangulation Trimming Field Guide from CKAero.

Use these diagrams to make sure that the wing aligns with the fuselage and that the wing and the stabilizer align. You want them to be equal all around.

If you have a flat stabilizer, use a bubble level to make sure that the airplane is sitting level. This helps develop the datum line for your measurements.

Other trimming issues can be as simple as a linkage being loose, a servo not centering properly, surface warping, and/or alignment issues, as I have discussed. Take time and review the following before your next flying session:

• The CG is in the recommended range.
• The aircraft is in alignment.
• The linkages are correct.
• All of the servos are centered and return to center properly.

Good precision flying starts with an airplane that you are comfortable with, and one that is well trimmed. As I mentioned in my last column, any airplane can be used to participate in RC Aerobatics, so use what you are most comfortable with and work to get it flying better than you can imagine.

Until next time.