Goldsmith Trimming Chart

[Doug Pilcher]

Goldsmith Trimming Chart! A Must have!

Goldsmith Trimming Chart.pdf

[Doug Pilcher]

"Trimming Chart by Gareth Farrington based on Peter Goldsmith's Trimming Guide"

Trimming Chart 2.pdf

[Doug Pilcher]

This is copied from our old website under the "Technical" heading. It was written be Pete Goldsmith:

Peter Goldsmith: How to reduce your flying workload!
posted on March 02, 2009 15:02




One of my passions in life is to teach. After a long discussion with Mike Hurley after sharing with him how passionate I was to share my life’s aerobatic knowledge, we decided that a trimming article would be a great start. A properly trimmed model can reduce your workload in an aerobatic sequence by an enormous amount. I judged at the nationals this year and thoroughly enjoyed the experience. I was also impressed with the skills, especially in the lower classes when displayed by pilots trying to fly around a pour trim setup, I was truly amazes how tolerant people were. It drove me crazy to watch in fact. I remember bouncing out of my judging chair and saying to Mike, “boy” I need to help these people. Hence here I am typing away at my exciting pace of about 80 minutes per word <G> in an attempt to help educate the great skill base that exists in the US scale aerobatic scene.

Like all good input, it doesn’t come from me, but more a summary of 25 years of meeting people who shared their thoughts on trimming with me, then melting them down to put into some kind of legible format that most can understand.
Servo and control setup

3D, and precision model trim typically go against each other. What I mean by this is that when pilots set up their new 40% something, they go straight for the big rates, 35, 40, and in some case 50degrees of throw. Anybody thought how this can affect the servo resolution, and more importantly the servo power.

Scale aerobatic events typically allow a separate aircraft for freestyle. Between 80 – 100% of the results at a scale aerobatic event comes from the precision elements - known and unknown sequences.

Please don’t misunderstand me, I love to watch free style flying, and admire the talent of the pilots that do it well. It’s innovative and has been one of the main contributors to the growth of Giant scale aerobatic aircraft. My point is: Why not have your free setup specifically for the free event, and then have a precision setup for known and unknowns sequences. I know for me personally having a model just for free style will be something I aim to do in the future. Having said that, a good alternative is to bias your setup more toward precision, which will no doubt improve your sequence flying (especially if you’re frequently fly events).

With my Cap, I use 1” horns on all surfaces with the exception of rudder which use 1 ¼”. I have 28 deg on aileron, 32 on elevator and 35 on rudder. For me, this is a good compromise for precision and free flying with the bias towards Free.

With 1” horns, and 1 ½’ distance from control hook up, I am getting slightly better that 1 to 1 ratio. More importantly I am getting closer to the maximum servo power and control geometry available. With 1 ‘ horns my resolution is better, control slop is reduced, and servo wear is greatly improved. Another novel bonus is, I don’t need as many servos per surface. Give it a try next time you set up your aircraft, you may be surprised, in fact in some cases you may see no difference in control responsiveness by going to 1” horns, as with the better geometry you may be reducing your control blow back.

One of the biggest challenges I see pilots dealing with is surface blow back. With blow back, your snap exits will be all over the place, both entry and exits. Getting consistent flying is almost impossible. Every time your speed changes, you control response will change. Hmmn, I suspect a few lights just went on. Yep, could it be the fact that you consistently miss your snap exits is not due to your skills, but control blow back.

When setting up you servos, make sure you run the numbers. Do the math and figure out just how much power you are delivering to the surface. All servos are rated at inch ounces, that is at one inch from the center of the servo. A JR 8611 is 266 Inch oz on 6V. With a 2” horn they are only 133, and around 200 with a 1 ½” horn.

Years ago I was able to measure the forces on my Cap, believe it or not the ailerons required well over 30 lbs of force to deflect at 100mph. Now there are a lot bigger ailerons out there than mine. Please pay attention to this, it is crucial to consistent flying. If you have to use 1 ½” horns, or 2” horns, you will need more servos.
Sequencing your trimming

Many of the trimming articles I have seen over the years incorporate some kind of chart, graph - do this and this happens and so on. My concept of sequencing your trimming is simple. If you get your trimming out of sequence you will end up chasing your tail.

As with most things in this universe, for every cause there is an effect, same applies to trimming a model aircraft. So many times I see people test flying their aerobatic creation, flip the thing on it’s side and see what kind of knife edge mixing it may need. Thrust, CG, wing balancing and basic flight control trim can all effect knife tracking.

I can’t make this point more loud and clear, it is very important to trim you model in the correct sequence to make sure each adjustment has no effect on the previous adjustment.

You will see I have broken each element down, and there is an intentional order in which I recommend to trim a model. Model balance especially CG is the most important as differential, knife edge flight, and down line tracking will all be affected by the CG.

Next is dynamic balancing or wing tip weight. Next comes thrust angles, then aileron differential, and finally P mixing - knife edge tracking, roll coupling, down line track.

Oh…. if you change your propeller, you whole trim setup will change.
Balance

OK so…How do I know what is the correct CG for my model? If in doubt, read the instructions. Yep, chances are somebody from the TOC or similar has designed your aircraft and has determined over many flights where the CG should be.

For precision flying forward is better…but… too far forward can be a problem. I can not put in writing what is the best feel for each pilot other than simply stating - it is a feel thing. I can however, give you some symptoms of too far back and to far forward, plus some simple tests I do to check.

One of my favorite ways to determine the correct CG is spin entries. For precision flying I would expect you to be running between 12 – 15 deg of elevator throw. If you feel you need more than this, check your exponential it may be too high. Just as a starting point, 35% expo is what I call a linear feel. What I like to have with my expo is when at half stick, I get about 50% of the reaction of full stick, around 35 – 40% expo gives you this with modest control deflections. Ok, if that didn’t make sense - IF I have my stick at full travel, my aircraft will roll around 360-400 deg per second, about right for precision. Now when I only move my stick half way, I should be looking for 180-200 deg per second, make sense.

If when entering a spin, you model mushes, kind of slides into the spin with no real stall visible, you may be too far forward. Another sign of forward CG is excessive down elevator needed for inverted flight. This is not always the reason but is a sign.

Rear CG is probably easier to see for most pilots. Model is sensitive in pitch, unpredictable around the stall, climbs when on an inverted 45deg line are some I have noticed.

CG is mainly about feel, each person will have their preferences and each aircraft will have its preferences. Be patient, spend some time on this one; it’s important you are happy with your CG. I would recommend at least 10 – 15 flights before making the commitment to where it needs to be if it’s a new model.
Dynamic Balance

Ok, we are happy with our CG so the next thing is dynamic balance. This is really only relevant with wing tip weight as most other axis on a model aircraft are not affected very much by the dynamic effects of high g force loads. Having said that, your wings are. Just cause they both weight the same, and don’t carry any aileron trim doesn’t mean you can’t have a wing weight problem.

I have seen a myriad of ways to test for wing weight trim. Loop’s, pulling to vertical, and so on. My suggestion is to think about the sequencing argument. If you do loops, or pull to a vertical up line, the engine thrust can have an effect, yet we haven’t got to trim our thrust angles yet. Think about it, what could you do to check you wing tip weight in flight that will not be affected by thrust?

What I do is put the model into a vertical dive (minimum of 3 – 4 seconds) and pull a hard corner at the bottom. No matter where your wings are in roll, when you pull to level, the wings must be level. Check this concept with your stick plane. It really doesn’t matter where your wings are but as you pull to horizontal flight your wings must be level.

If you attempt to pull a hard vertical FROM horizontal, you must be absolutely sure your wings are perfectly level. I don’t know about you guys but I am not that good.

If you go from vertical TO horizontal, not only will the engine thrust have no effect but your wings can be anywhere as you are on a vertical down line. Now when you pull the corner, the aircraft may be pointing in a different direction than you planned, but that is ok, as long as the wings are level.

When I was told about this procedure several years ago, it made a lot of sense. The only thing I would recommend - pay attention to is make sure you only use elevator through the corner.

Don’t be quick to make a decision, have a friend observe the proceedings, do many pull outs, make absolutely sure before you move on to the next step of trimming.
Thrust angles

OK, guys, it’s time to put aside aesthetics and get that engine thrust correct. I sure see a lot of spinners perfectly lining up these days. One of the biggest deterrents to adjusting for the correct thrust angles is the spinner won’t line up any more. That’s true. Once again, when building you model, pay attention to the instructions, chances are somebody has figured it out pretty close.

What I like to do (or use to do) is test fly the model before I paint the cowl. Once I am happy with thrust, I can make the appropriate cosmetic changes to complete the model before painting. Setting up the correct thrust angles is fairly simple. Well… it’s simple to identify, harder to adjust.
Right thrust

Now we know our wing tip weight is correct, we should be able to, with confidence pull to some accurate vertical up lines. Number one issue with this is making sure your wings are level - don’t guess! - be absolutely sure your wings are level before pulling to a vertical.

I have seen people add unnecessary right thrust as they were not level when pulling corners, leaving a inside wing down (Normal human behavior) and the model would lean to the left.

What I like to do is to fly directly over head, into the wind, where I can clearly see my wings, then pull to a vertical up line. Ok, up we go, first 100’ is good, next hundred feet is good, moving through 500’, still tracking well, up over 1000’ now, still straight, coming up on 10,000, arrrrrh, I see it drifting to the left a little. Ok , Ok, I am being sarcastic, but I hope you see my point.

Most vertical up lines in patterns I have flown never exceed 1000’, well some do but it’s rare. You will never get your thrust perfect up to 10,000 feet. If your working at it, best you can hope fore is around 1000 or so feet.

Speed will have a huge affect of your thrust angle on a vertical up line. Entry speed, compared to under load after climbing to 100; will be as much as 30 – 40mph slower.

Now a great little tip I learned for making the adjustments:

After many pull ups you really need more right thrust, as you pass through 500’ you can clearly see you model drifting to the left. Apply some right rudder trim, and continue to apply it until it tracks straight. Bring the plan into land and check your rudder deflection. Use a protractor to see how many degrees of rudder you required for a straight vertical. What ever it is, divide it by 2 and that will be what you need to add to your right thrust. If you have 2 degrees of right rudder, you will need to add 1 deg more of right thrust. It works, it really does, and it works both ways, if you need left rudder (too much right thrust) you can use the same equation.

Differential

Aileron differential is one of the most important aspects of model trim. With the multiple point rolls on both up and down lines in today’s modern patterns, poor differential can be a real headache. The good news is it’s pretty easy to detect and adjust for axial rolls. The important thing at this stage of the game is knowing our thrust and wing weight is correct, we can proceed with our differential setups. You’re probably starting to understand the importance of trimming your model in the correct sequence. Each step compliments the last and should have no negative effects in trim.

Aileron differential is required when the drag of the down going aileron does not mach the up going aileron. Before we go further, I would like to interject here that you need to make absolutely sure you are not getting surface blow back. You will never get your differential correct if you are.

Blow back is easy to check for. Push to a vertical down line and roll to the right, stop rolling for a second, the roll again. The roll rate should be the same, if it is slowing then your surfaces are not reaching their intended throws. Another sign is you up line roll rate is faster than you down. Please, you need to fix this. Do what needs to be done:

1> Either increase you servo power or…

2> Improve your geometry by reducing the servo horn radius and/or…

3> Increasing the distance between the control horn pickup from the hinge line or…

4> If you have lots of cash, add more servos.

Whatever path you take, you can’t afford to have surface blow back as your flying will never be consistent. Most modern radios have a differential program. I have used both the ATV function or the Differential function and both work well.

I want you to use the same technique as before when checking for the thrust only this time, pull to 45 degrees, making sure you are either directly into the wind, or directly down wind. Using full aileron deflection, roll to the right. If the aircraft “walks” to the right, then you have too much down travel on your ailerons. If you roll to the right and the model “walks” to the left, you have too much up travel in your ailerons. Repeat this process to the left as well until you are satisfied your model is tracking true in the roll axis.

Like with the thrust angles, don’t expect your model to continue to roll for 5000’ on a string it just can’t be done. As per previous recommendations, go for the majority situation. With the correct differential on your model, you will be amazed how easy it is to do hesitations on lines.
P-Mixing

You will notice this subject is the last in the sequence but, for many, it’s the first thing they tackle.

I get phone calls all the time from excited pilots – “Pete! I just test flew my new Edge - it only has 8% Aileron mix and 4% knife-edge mixing”. Boy! I think they sure got to the details of trimming their model faster than I can.

The point I like to make throughout this article is to stick to the correct sequence. Only know after perhaps 10 – 15, perhaps 20 flights are we going to work on the mixing to fine tune our model.

What I would like to do is to break up this P-pix section into 2 sections. The first is the down line, torque offset or throttle offset mixing and second, the traditional rudder elevator/aileron mixing.

Most people have a fairly good understand of the later, rudder to elevator/aileron but not many are using throttle offset mixing. I have seen some only using the pitch compensation. What I would hope you all pay attention to is what your model is doing on a down line (at engine idle in the roll or yaw axis). It’s both these areas I see people struggle. One of the benefits of judging our events is you see a lot of strange trim situations. Remembering back at the Nationals, I can clearly remember models rolling on down lines and yawing off axis causing some strange looking down line rolls. It’s almost impossible to have perfect trim in roll at all speeds. All you can hope for is to mix out some compensation to help reduce you workload.

Both the yaw and roll axis, in most cases, have a bigger affect in you model tracking on down lines than any other situation. Imagine what the effects of a 5 degree error on every down line would mean. Over the length of the box, you can drift in or out by as much as 150’ and the same applies to the yaw axis.

Ever noticed how hard it is to get your wings level when approaching a pull corner with little time. With your model rolling and yawing at different speeds you will never be consistent. It is hard enough to be absolutely sure if your wings are level, let alone chase an out of trim situation. Good news is it’s fairly easy to compensate for.

Let’s do the roll axis first. You can do this 2 ways and both work well, in fact, I would suggest you try both to get the best input. Version 1 is to climb to a high altitude, simulating a typical top of the box altitude and fly directly over your head and into the wind. About 50 - 100 feet out from your self, push down. You are now looking at the plan view of the model. Watch careful to see if the model is rolling on the down line. Most models will roll slightly to the right. Personally I have never had a model that hasn’t needed a little left aileron mix on low throttle. I’m not saying it can’t happen but I personally haven’t seen it.

The second way to check for throttle/aileron mix is to fly along at level flight, medium height and reduce the throttle. Watch carefully and see if your model is rolling, chances are it will. Arrr, ever wondered why you always have to lean a little left aileron entering spins, or why your model always falls one way, perhaps it’s because your low power trim is not correct.

The second P-mix is the Throttle to rudder mix. Same deal, it’s hard to get you model to track correctly in the yaw axis at all speeds, you only hope is to apply a small amount of ‘left” rudder on low throttle. To check for this use the same technique as the Throttle to aileron fly above your self, directly into the wind and push down in front of your self and watch carefully, you will be amazed, especially at the start of the down line.

If you haven’t got any throttle offset to rudder, you are most likely flying around the problem and where I find it most challenging is in figure 9’s and vertical and horizontal 8’s. Any time you are using elevator and are off on the yaw axis, it’s a bad hair day. I can hear all you guys thinking, yes it’s true, your model perhaps could need a little rudder mix on low throttle, give it a try, you will be amazed.

I know of some fairly experienced modelers that use the same theory but reverse where the mix is. They use little to no right thrust on the engine but have right rudder mixed on full throttle. That works well to I’ve been told but haven’t tried it myself.

One thing you may want to experiment with in both these scenarios is where the mix is activated. I like to have the stick offset at least above half and let it progress from there, it seems to be the best balance, plus I am not getting a sudden mix input, it progresses more or less with the speed of the model. This will vary from model to model but try to keep the mix activation well above an idle setting. Hopefully you have noticed when working through the trim sequence each trim adjustment has complimented the next stage.
Rudder/Aileron Mix

In most cases, this mix only requires a linear P-mix. What I mean by linear P-mix is that you don’t need a progressive value to the mix (in other words: less at the start, more at the ends). Most modern designs, with the exception of bi-planes, are very close and only require a small amount of rudder/aileron mix. It’s fairly easy to identify.

Some like to put their model on knife-edge, but I like to just do flat turns, simulating rolling turn inputs. Remember play the numbers, rolling turns require more precise mixing than sustained knife-edge flight. In fact you don’t do much flying on your side at all, but you sure do a lot of rolling turns. So, I like to do the flat turn thing. Doing a simple inside rudder turn to the left, using left rudder, the model should just yaw, with no roll affect, if the roll rolls to the left, then you need to mix 2-5% right aileron to left rudder. . My cap is a little unique as it has adverse roll. When I apply left rudder the model rolls right, so I need left aileron mixed with left rudder. Repeat the process with right rudder.

Now what I want you to do is vary the speed in which you do you flat turns, if you find as you increase your speed, the mix becomes too much, you could be getting surface blow back. Man, sorry to keep harping about this but it is important. With insufficient rudder power, when you apply a P-mix for roll, or pitch for that matter, the mix will become too much as the rudder throw reduces due to aerodynamic pressure. I see a few lights going off again. Could this be why you have your mix perfect for knife edge, but you chase your aircraft all over doing rolling circles?.
Rudder/Elevator Mix

I think about 3 times in my entire life, I had a model that didn’t need rudder elevator compensation. Same as with the previous rudder aileron, start by doing a flat turn to the left and see what happens. If you model pitches down when rudder is applied then mix a small amount of up elevator, of if it pitches up, apply a small amount of down elevator. There are some cases, even without blowback, the mix value will not be exactly correct for all throttle settings. Don’t panic as with most modern radios you can use what is called a curve mix. This mix allows you to have multiple points along your mix curve to increase of decrease your mix value at different rudder inputs. My Cap is a good example of this. At low rudder throws, I only need 1-2% mix, but as the throw increases I need up to 10%. If I just have a 10% mix it will be to much at small rudder inputs, the curve mix is designed ot solve this problem.
Summary

One of the most common questions I get is what can I do to improve my aerobatic performance, my answer is unique, “I have been told so” but I hope you will at least consider the next few comments.

My advice is not to let the ego take over you goals - practice hard and focus on the fact there will always be somebody better than you and, in my experience, there always was. This may seem a little harsh but, in all my years of flying aerobatics, not excluding me in this comment, egos and overconfidence can be the biggest hindrance in a pilot’s ability to grow. Try to avoid letting your ego be your only motivation. Be objective, be humble, listen, watch and experiment, that’s what all the TOC and Masters pilots do. Sure, we all have egos, but at some stage of our lives our egos have let us down, we were humbled and forced to listen and be objective.

That’s about it, stay cool and hopefully we can catch up at the next aerobatic event.

Pete