Walker on Stunt
Paul Walker prepares his 2015 Predator for a flight on the way to his 12th victory in Precision Aerobatics at the U.S. National Championships. Bob Hunt photo.
You have the chart -- Now to the trimming process!
By Paul Walker
(EDITOR'S NOTE: This is the seventh installment of Paul's most recent trim flow chart update and series of articles. These articles were first published in Stunt News, the magazine of the Precision Aerobatics Model Pilots Association, and are reprinted here by permission of SN editor Bob Hunt.)
With the last installment, we finished the tour through the trim flow chart. This gives you a process to attack the trim adjustments on your killer machine! However, there are a LOT of variables to cover if you want to be thorough. This time we will discuss ways to shortcut some of this process, and general observations I have made through the years. This is all to help you streamline the trimming process.
Let's start with observations. Here is a list of several common issues I see with many CLPA pilots.
Starting with the first item, flying with too much tip weight is one of the most common problems in many planes. More than a decade ago I flew a plane that had just finished in the Top 5 of the Nats, and was appalled at how badly the plane hinged (dropped the outboard tip in corners) at even a soft corner. It was also very difficult to manage at the intersections of the consecutive eights. The point here is that this is so common that even a Top 5 pilot had this issue but was talented enough to work around it. I can't imagine how well he would have done if that issue was resolved! If a Top 5 pilot has this issue, it is easy for most to have this problem. Be very aware of this when you follow the trimming process.
The next issue is that too many pilots fly too nose heavy. Yes, it makes things feel nice and cozy, smooth and graceful, but too many over do this. It can increase the stick force too high and make the plane more difficult to fly in the wind. Partly for that reason, many choose not to fly in a good wind because they are concerned for the safety of their plane. And that can be for a very good reason. Be aware of this when trimming your plane.
Right behind this is the habit of wanting to fly too slowly. Yes, I know that it really looks cool when a real slow flight is executed perfectly. If I could fly consistently that way, I would. However this creates issues with flying in the wind. Many think that their reflexes can't take flying fast any more. I'm with you on that, IF the plane is out of trim. However, when in trim, flying fast is really not an issue. Reference a Skylark at VSC a few years ago that was flying at 4.2 seconds a lap. It really did NOT feel that fast, and was actually easy to fly. The slower you fly the plane, the more sensitive it will become to certain trim issues and make it feel like you need better reflexes. Work on the trim, and keep the speed up (relative to flying too slow).
Related to this is the fact that too many planes are simply too heavy! This extra weight forces you to fly faster to increase the dynamic pressure on the plane, thus giving more lift at the same angle of attack. It also increases the line tension, which many feel they like. Once again this issue creates issues with flying in poor conditions, like high winds or dead calm. It is easy to be fooled into thinking that a heavy plane really does fly well. I know as I have been there as well. Simply put, keep the weight under control to maximize your flight score. I'm not talking here about building a total feather, like Jason Greer's Impact at 56 ounces on electric, ready to fly, but keep the weight under control. More on that later!
I find that many pilots will accept an easy adjustment on their plane where a more complex combination will give better performance. My guess is that many don't know that things can be better, and think they have done a good thing. Well, they did do a good thing but now armed with this flow chart, they can find something better. Most experienced competition pilots will tell you that they stop trimming their plane with its last flight. It's a fact of life, so don't just accept a convenient trim.
I have also seen that many pilots can't see what their planes are doing wrong. This can range from the novice to the expert competition flier. This is when it is necessary to have a helper that can watch your plane and accurately communicate to you what is going on. It can also be beneficial to have an experienced pilot fly your plane to see what is wrong. At the same time, you can see your plane from a judges' perspective. The trick is to learn to see these fine points from the pilots' perspective, so you are in control of your planes trim. The only thing I can suggest is to spend some flights going through the motions of doing the pattern, but instead of total focus on the maneuver shapes and placement, spend that focus watching exactly what the plane is doing relative to the lines. Envision that there was a massless, infinitely rigid rod from your handle to the outboard tip of the plane. It is easy to envision what the rigid rod should be doing, but practice seeing what the plane is doing relative to the imaginary rod in local pitch, roll, and yaw. These excursions from the ideal position are a result of trim anomalies, and what they are doing tell you what trim is off. Try your best to envision this.
I can't tell you how many times I have heard that someone won't change things for fear of losing what they currently have. Somehow, if they can't mark where the current settings are that might be an issue, but measuring tip weight, marking LO position, marking the CG location, and measuring horn actuation points are not that hard to do. Mark or record all of these trim settings so if you change something and don't like it you can easily return to the previous setting. No excuses here!
I know many who refuse to take a knife to their plane to fix something that is wrong. I know as I have been there as well. It IS really hard to do that to your new pride and joy that you spent so much time making. You have to decide what is more important, a slightly higher appearance score or a much improved flight score. I was there in 2013 with my first Predator. I had to completely open the bottom of the plane to repair the control system due to a stooge mishap. It was repaired and still scored 19 appearance points a few months later at the Nats. If you have to cut into it, do it, but plan you work carefully.
After flying many other pilots' planes I find that there are a lot of pilots out there which could score better with some simple trim changes. This probably goes back to them not being able to see the trim anomalies. Fear not, if you think you should score better follow the flow chart to improve the trim of your plane, and your scores should go up as well!
I also find that most pilots don't have a thorough plan for finding their optimal trim. I trust that the flow chart presented resolves that issue.
Finally, how many pilots do you know who claimed that their plane flew right off the board? Yes, we all work hard to bench trim the new plane prior to flight, but I have NEVER had one fly totally without any adjustment. If you happen on one of these, I suspect you are in denial of reality. This procedure we have discussed for the last several months is what you would use to find that your pride and joy maybe isn't as perfect as you felt on those first few flights. I know because I have been guilty of that as well. Sometimes we just want our plane to be super and our overly positive attitude clouds our objective assessment of the plane. Be aware!
What is the point of these observations? If you find yourself in one or more of these buckets, take a truly objective assessment of it and then take steps to rectify the situation. This may involve extra work, but the result will be a better flying plane. If it doesn't need rectification, keep it in the back of your mind when making future adjustments. With time, you may find later that you really were in one of those buckets! An example would be that you fly nose heavy. Once you recognize this, try moving the CG aft some and fly it that way for a while. You might be surprised!
The following are some general guidelines for setting up your plane geometry and flight parameters. Setting up using these parameters will make the path through the flow chart easier as you will be closer to your desired trim at the start.
It still might seem intimidating to try to go through the chart and try every variable, and it can take a lot of time. However, there are some combinations of adjustments that shorten the process. They are as follows:
If you look carefully at the flow chart you will notice that the elevator to flap ratio shows up in many of the criteria. By selecting a setting and leaving it fixed in one position, it makes the process much shorter to go through. This then forces the CG to be adjusted to balance the squares and the corners. Once the CG is chosen, then the number of variables is not too significant. This assumes that you are starting with a good ratio between the two. The guidelines above should help that. This is in general the process I go through. I will select a variable to leave fixed and then optimize the others. At some point, I consider changing the original variable I fixed, and move forward with the process again.
This concludes the discussion on the flow chart. My goal was to make the process that I use available to anyone who would want to use it. There are also guidelines and simplifications that can be made to simplify the process and they have also been discussed. If you have questions on any of this, you can e-mail me a question on it.
The Trim Flow Chart
This page was upated Oct. 23, 2015