Triangulation Trimming: Part 2

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In the last installment we laid out where we have come from in the evolution of our designs and schedules. Also, we talked about the increases in difficulty that are expected in our setups. I will lay the foundation for my trimming methods now and try to explain how to achieve a consistently good flying airplane without having a masters degree in radio mixing.

First, because this a trimming article aimed at pattern flying, we will assume you have a greater than average understanding of set up, radio installation, and slop free linkages. It is very important to do the shop work and precise radio set up before you even attempt to start flying and trimming.

Now, I’ve been doing this long enough and helped enough fliers in trimming their aircraft to know that trimming is an art. Some very good fliers have trouble disciplining themselves to allow the airplane to fly purely, without corrections or cheats, while trying to perform simple tasks in the trimming process. Learning how to trim an airplane and being able to duplicate that trimming procedure with another airplane can be laborious work, but in the end it is worth it. Here is the goal we are trying to achieve and to do it, these rules must be adhered to. What we want is for the airplane to fly itself. Yes, fly itself. You say, “Bryan, what do you mean?” Well, I’m glad you asked. Let me explain.

The airplane, absent our input, should do what we told it to do last, at all times, even in the wind.

The airplane, absent our input, should do what we told it to do last, at all times, even in the wind. That is when we know the trimming process is nearly complete. When you pull the airplane into a 45 deg up line you want the airplane to follow that line as long as possible without correction, absent even the elevator. The same holds true with a vertical up line or a 45 degree down line. No input at all should be required to maintain the line. That is why it’s important to be absolutely certain that you have hands off trimming on a straight horizontal line of flight. You must be certain that you have no adverse trim effects on any line that requires neutral elevator input. This is an achievable goal and should not be overlooked. It is the most important step in the analysis process.

A perfect setup will improve every maneuver in our schedule, from Sportsman to F3A, from stall turn to snap roll.

Let me start by saying you must be as honest with yourself as your skills allow. There is no cheating in aerodynamics. Compromises will always show their ugly heads somewhere down the line and usually where it hurts the most. The greatest percentage of the time it is in snaps and spins. Here is the first unbendable rule; when all else fails you must remember this. A perfect setup will improve every maneuver in our schedule, from Sportsman to F3A, from stall turn to snap roll. So if you think you are finished and you have a couple of maneuvers that are not quite as good as they used to be, or if you are having trouble hitting a particular maneuver, you either have to tweak your control throw/expo setup (something often overlooked after an adjustment) or you still have something out of whack! A real improvement will correct more than one maneuver. It will usually change, for the better, all maneuvers. So lets get started, shall we?

  • Bench Setup

The simple things have to be perfect, just like when trying to assemble a maneuver.

This cannot be stressed enough. You must be as precise as possible in your base setup. You must know where your actual starting point is. You need to be sure of your wing incidence in degrees, down thrust of the motor in degrees, and CG position. Another very important step is to know the exact throw, in degrees, of all control surfaces. What we are talking about here is the actual amount of deflection for every movable surface. We call this the control throw, or just throw, of the surfaces. Now, check, re-check, and duplicate every setting until you can be certain of these parameters every time. One of the biggest problems I encounter while helping pilots trim their airplanes is that they are not aware of the throw and incidence settings of their airplane control surfaces and wings. Many airplanes have been cut and twisted in order to fix a so called bad flying airplane. Some have even been redesigned due to bad diagnosis of an unexpected performance, such as finding that an airplane which will not snap or spin, when the only problems were incorrect throws and incidence or the CG was not correctly set.

Every airplane requires a slightly different amount of control surface throws for spins and snaps. That is why it is so important to set a base line of settings (incidences, throws, and CG) for all non-input flying such as level fight, vertical flight, and single input flight (like knife edge flight and inverted flight). Without these maneuvers being flown hands off, or at least flying with very limited input, it is impossible to have a base upon which to build your inputs for the complex maneuvers.

The small imperfections in trim will turn into big course deflections in the complex maneuvers, requiring large corrective inputs from the pilot. The simple things have to be perfect, just like when trying to assemble a maneuver. If your straight lines are not straight, then you have no foundation upon which to build perfect geometry. The less you have to apply course corrections to the airplane, the less chance you have of making an incorrect input. An error in basic things like alignment or surfaces traveling equally will always manifest itself further down the line, so make sure to pay close attention to every step in your bench setup.

  • You will need a repeatable incidence meter, and a repeatable throw meter.

…a meter reading is only used as a baseline in order to know where you began and to make adjustments according to what your airplane requires to fly true.

Robart makes a decent analog meter (most people don’t know this but this meter was designed and patented by Al Coomber, the owner of Central Hobbies. It is a great product.) If you have trouble seeing this meter you might want to use a digital inclinometer. However you must be very careful, most of the digital meters I’ve seen are heavy to the point of warping the surfaces, making the readings non-repeatable. If you choose to use one of these meters be certain you can adequately support the surfaces they are mounted upon. Bear in mind, these readings are only a starting point. Such precision is not mandatory, merely the repeatability of the meter. The best analog throw meter I’ve found and have used for years is the CRC meter, designed and built by Frank Capone. I flew against Frank years ago. He always had a well built airplane and a steady hand. After using his products all these years, I now know why he was so tough to best, it was the high quality workmanship he put into all of his airplanes. Budd Engineering sells one more throw meter of note. This is a laser unit, which can be a little more difficult to set up but is quite accurate. A Budd meter would be a shop tool, but a CRC tool is one that will lend itself to field use as well. For absolute accuracy the Budd meter is probably better, but for ease of use and portability, I use the CRC. Even so, a meter reading is only used as a baseline in order to know where you began and to make adjustments according to what your airplane requires to fly true. In other words, you will need to know where you started before you can make your next move. This will allow you to fix a poorly performing airplane. If you are going to be serious about trimming your airplane, you need both an incidence meter and a throw meter. So if you don’t have a meter, call Central Hobbies and purchase a USA built quality product, or acquire another unit of equal quality.

  • Patience is key

We all get a new airplane, make a flight or two, and say, “This is the best airplane I have ever flown.” Then, after about twenty-five flights (once we get settled in), we start becoming more aware of the pesky quirks that the airplane displays. Here is where we need to get busy.

Go home and check the basics again. At this point, pilots sometimes start getting creative with the radio to help “tame that dog”. If we look at the airplane surfaces, dissecting the information from the flight and where the surfaces rest, we can decipher what deficiencies are in the set up. Then, we can make improvements from precise repeatable measurements and by using feedback from the maneuvers after we have flown the airplane.

So, now, we get to the heart of my setup secrets, learned over twenty-plus years of building and flying. You should know where the zero line is on the airplane. If not, try to find it from the manufacturer or the designer. This is not a must, but it will be the base line for all measurements and you will need to be able to duplicate this zero measurement. It is very helpful. The dynamic between the wing incidence and CG is the most important adjustment in 95% of all trimming, even more than the use of exponential.

The zero line of the airplane is an imaginary line from nose to tail that represents the way the designer intended the fuselage to look in flight.  When this line is set perfectly level, the fuselage is in the desired attitude.  On my monoplanes I design the zero line into the baseline of the canopy for easy reference.  If your airplane does not have a reference, using the stabilizer (with elevators perfectly aligned to it) as a zero line will almost always be adequate.  The actual placement of the zero line, within reason, will not affect the trimming process.

The roots of all problems usually start right there. For your initial settings, adjust the wings 0.5 degrees positive to the zero line. Set the motor 0.5 to 1.0 degree negative, or down thrust, from the zero line and set right thrust of the motor 0.5 to 1.0 degrees right of the center line of the fuselage. Now, set the C/G at 25% of the MAC. If it is adjustable, set the stab at 0.0 degrees. Most likely, when we complete the process of trimming, it will have a slight positive incidence. These are a must to establish a baseline for my triangulation method of trimming. Any other settings and this method will not work completely. Picking and choosing which of these parameters you will follow will sabotage this trimming procedure to the point of confusion.

Adjust the wings 0.5 degrees positive to the zero line. Set the motor 0.5 to 1.0 degree negative… from the zero line and set right thrust of the motor 0.5 to 1.0 degrees right of the center line of the fuselage. Now, set the CG at 25% of the MAC. If it is adjustable, set the stab at 0.0 degrees.
Because we now use fixed gear, we have more unwanted drag and, as I wrote on the last page, no parasitic drag is good. You can figure that the fixed gear drag causes at least one to two percent of up trim measured from the normal total elevator throw, and this is where most of our trouble begins in the up lines. It is also the reason why the rear C/G is so prevalent on most setups, giving the unwanted effects in any maneuvers where the wing is unloaded (knife-edge and vertical).

…when you increase incidence, you will have to move the C/G forward

The way we overcome this is by increasing positive incidence in the wing. Most of the time, when you increase incidence, you will have to move the C/G forward. The forward CG increases the power and effectiveness of all control surfaces (incidentally, decreasing sensitivity of these surfaces). Since a positive incidence adjustment makes the airplane climb on horizontal lines, this is where we see the effect of the wing adjustment on the airplanes flight. We now need to give some down trim.

This trim adjustment will correct the canopy pull in verticals, up lines, and down lines. However, it can be a frustrating dance to get a perfect pull free down line. Remember this as a foundational rule: the wing is the most powerful and important adjustment. We want the stab and rudder to lift as little as possible. These are not trainers. We are trying to trim the attitude and flying direction of the wing. We want the stab and rudder to steer the wing and fuse not to be a predominant force lifting the tail like the tail of a foamie does (Now you know why a foamie requires so many mixes.)

Think about this: take some arrows, remove the weight from the front of these arrows, and shoot a dozen of them. See if you can hit any target or even get them to shoot twice in the same direction. You can’t. This is how your airplane acts when the CG is too far rearward. You have to constantly babysit it. That is why you see so many SFG`s (side force generators) on world championships model airplanes, some sport planes, and foamies. You have to add too many flow straighteners to keep the airplane on track with a tail-heavy airplane. Keep this in the back of your mind during the trimming process to keep the C/G from straying towards the tail.

Here are a few quick references to follow while you are trimming. These do not change, no matter what the setup on your airplane is and no matter who tells you otherwise. With these quick reference tips and my base line set up, you will have all you need to trim an airplane. Well, maybe you will need to throw in a little patience too!

  • If you need more than 1.0 degree down thrust in the motor to keep your airplane from going to the canopy in the up lines, then you don’t have enough positive incidence in the wing.
  • If you need more than 1.5 degrees right thrust in the motor to keep the airplane from pulling left in an up line, your elevator halves are off, your wings pins are loose, or you need to move the CG forward.
  • If you make a stab adjustment and it affects the way the airplane flies in the up and down lines, then the CG is too far back.
  • If your airplane requires a left rudder to throttle mix your airplane is very tail heavy, your airplane is crooked, or your wing incidence is under 0.3 degrees positive.
  • If your airplane goes to the belly on a left rudder knife-edge, move the CG forward. If it goes to the belly on a right rudder knife-edge as well, the same adjustment works.
  • If the airplane goes to the canopy on left rudder knife-edge, the CG is too far forward. If it goes to the canopy on right rudder knife-edge, the CG is very far forward.
  • If you have to use high rate rudder to do a snap of any kind the CG is too far back.
  • If you have to do any snap with no elevator move the CG forward.
  • If you have to do your spins with rudder and elevator only the CG is too far back.
  • Here’s a tricky one. If the airplane continues to rotate when exiting a spin either the CG is too far forward or the rudder rate is too high. However, it is a possibility that the CG is too far rearward. Use information gathered from other indicators to make the determination for a correct adjustment.
  • If the airplane does not rotate up onto the main gear during takeoff  (common problem) your CG is too far back.
  • If the airplane requires a greater amount of rudder and elevator to rotate to and maintain inverted flight, either the CG is too far forward or the wing incidence is less than the baseline setting.

There are many more examples, but we will stop here. It’s enough to get you on the right path. If your airplane has just one of these examples listed above, you are not finished and your airplane still needs work. However, I know from experience that if it has one then it’s sure to have some others as well. Remember, we said that one correct adjustment would fix more than one problem so keep this in mind while using this trim process. Also remember that the closer you set the wing incidence to zero degrees, the farther back you can run the CG for normal horizontal flying. Don’t forget to keep in mind that a 28-35% of MAC CG setting starts negatively affecting the demanding maneuvers in precision aerobatics, especially snaps and spins, in particular 1.5 snaps.

Conversely, when you move the CG forward it will feel nose heavy. The only way you can move the CG forward is by increasing the incidence in the wing, therefore taking out the up elevator trim induced by the forward center of gravity. It is important to remember that you wont have to use all that expo any more. This is the reason why most guys think they are nose heavy at first.

This is a good place to use less of the capabilities of our fancy radios and reduce your expo. These are guidelines you can use to establish a baseline of settings for your motor position, wing incidence and control surface throws. It comes from many years of designing mix free pattern airplanes. These settings and examples do not waiver on the modern designs we currently fly. The only exception is if you are flying the old style pencil fuses like the Patriot or the Typhoon with retracts or the skinny sport pattern models available for pattern beginners. You can usually fly with the CG at 28% MAC with these airplanes and the wing incidence rarely needs more than 0.5 degrees positive incidence. This is because of the wing sizes and reduced drag from the use of retracts.

With the older planes the snaps and spins require full rudder throw. This is because rudder power comes from the side area of the fuselage. If the airplane has very low side area, it will have very low rudder power. Our modern fuses can do a low rate rudder knife-edge loop, so there is a big difference in that dynamic with the larger side area fuselages.

Finally, lets start flight one. Begin by doing multiple trim passes with the airplane in level, upright flight. Make sure it flies from horizon to horizon with no need for any trim. Next, pull to a vertical up line and see if the airplane will go straight up with no input, rudder included.

Next, fly a perfectly straight down line, hands off, and see what the airplane does. After that, and be very precise, fly a left rudder knife-edge flight and a right rudder knife-edge flight. Do this a number of times to be sure you get the correct information. Make a note of what the airplane does, and, if you need to, have a friend record it for you.

This is where the fun begins!

Your airplane probably has these tendencies: the up line is O.K. or maybe pulls to the canopy a little at the very end, the down line has a canopy pull, the right rudder knife-edge flight is dead on, and the left rudder knife-edge flight has a belly tuck (because I know you didn’t move the CG to 25% of the MAC; it was too difficult and you needed to move too much stuff so you were going to try everything else first, just to see! Maybe it was too difficult to adjust the wing incidence, and you did not set it properly.)
This is where most airplanes wind up set. I’ve seen this too many times to mention. Please be patient, and use stick-on weights to get the CG forward if you have to. This step is very important.

the wing is the most important and powerful part of the airplane

If you have an airplane that has wings that cannot have an incidence adjustment, none of these settings will work. Do not even attempt this method. Wing incidence and CG together trumps everything else. Now we have to think about all the dynamics that come into consideration after the flight. Triangulate the information and use an adjustment or two that can fix all the issues. Because the wing is the most important and powerful part of the airplane, it has the most effect on the cause of the problems as well as the most answers for the fixes we are looking for.

We must remember everything else on the airplane is reactionary to what the wing requires and can be a tattletale. This is good! It allows us to use the information to fix an unhappy set up. Because the airplane goes to the canopy on up lines we know we need to adjust more positive incidence into the wings. Then, because the airplane goes to the belly on a left rudder knife-edge, we need to move the CdG forward until we get the tuck to stop.

You will see that moving the CG forward will make the control surfaces react more softly and predictably but more powerfully. It will feel like you added expo but have more control and power around the center.

After you get the airplane pulling a straight vertical line and the left rudder knife-edge is as close as you can to be perfect, only then should you adjust the stabs to match the elevator trims. Adjust the stab incidence until the elevator trim is centered. It will require adjusting then flying, perhaps more than once. You will also see that, with the CG forward, the stab adjustment has no effect on how the airplane trims. It only affects the center position of the elevator halves. Stab position is the least important trim effect in my process. It affects nothing except expo feel inverted.

You have been given all the information you will need to perfectly trim your airplane but it takes time, experience, and patience. We could have examined hundreds more examples, but you have already been given the most important ones with which to start. With this in mind, read and re read what is written here.

All the information here is for rough trimming. Remember that fine tune trimming is a whole other article.

I hope you give this a try in this off-season. It is worth the effort. You will be amazed that you bought in to all the hog wash that has buried itself into the lexicon of the trimming process, and that it kept you chasing your tail with misinformation and even false information with no improvement in the flight characteristics of your airplane.

Drop me a line and let me know how it worked for you. However, if you don’t try it don’t write to let me know it can’t work!

Until next time,
Practice with a coach
Bryan Hebert