The past few articles that I have written have been about aircraft set-up, primarily linkage installation and servo selection. The example aircraft that I will use in this article is the new 33% Matt Chapman Cap 580 from Great Planes. I have spent a lot of time flying this model and I am extremely pleased with the end result. When I think of a 1/3-scale model, I cannot picture one flying much better than this one.
When choosing a model, you must choose a design that you favor in looks, but more importantly, choose a model that has very linear flight characteristics. In this context, the term linear means the design does not require many mixes (e.g. Rudder to Elevator, Rudder to Aileron, Throttle to Elevator) to make the model fly true at all attitudes. However, nearly every aircraft needs SOME trimming, and some mixing/adjustment to get it to fly exactly how you like. What exactly do these mixes mean and what are they used for? I will walk you through these mixes and the different techniques to trim out your model within this article.
But first, lets talk about the Center of Gravity…
I have had the opportunity to spend a lot of time with the Cap 580 and have found it to have very good flight characteristics. The three that I have flown all use only one mix, which is a rudder to elevator mix. It uses 5% up elevator when rudder is applied.
Why are we talking about CG in the midst of an article on trimming? The percentage of a mix that is needed to make a model track true is directly relevant to the Center of Gravity of the model. An example being the following...Another Matt Chapman Cap that I have flown had a more tail heavy Center of Gravity and needed a larger mix value (about 10%). I then flew a third Matt Cap that had a more nose heavy Center of Gravity, and needed no mix at all. Before you start mixing away, make sure that you are happy with your Center of Gravity.
For my Center of Gravity, I always start off with the manufacturer's recommendation. Even if their recommendation seems surprisingly conservative, it is best to "play safe" and start where they recommend. Some models do not come with a recommended Center of Gravity, in which case I usually start by measuring between 25%-30% aft from the Leading Edge of the Root Wing Rib. This is a safe starting point for a conventional wing platform (an example being a double-tapered wing).
When flying Sequence with my 40% airplanes, or any airplane for that matter, I like to have a slightly “nose-heavy” Center of Gravity. Knowing the performance I want for patterns, I set my CG so I need to hold a slight amount of down elevator when the model is inverted. With this type of Center of Gravity setting, the model has of a more “locked-on” feel and does not wander. This is best felt in snap rolls. When I snap my models, they snap crisply. When the snap is unloaded (I begin the snap with applying hard elevator and then releasing the amount of elevator during the rotation once I apply aileron and rudder), the model tracks true. If the model were too tail heavy, it is likely that the airplane's nose would not be as locked in during the snap. Rather, it would somewhat “flail” through the snap roll.
However, for the avid 3D’ers, you may prefer a Center of Gravity that is more neutral. This will make your work-load easier as when the model is rolled inverted, you will not need to apply any down elevator. Also, it will make many 3D maneuvers such as the Harrier, the Wall, Blender, Torque Rolling, etc. easier to some extent as the airplane will want to remain in a “nose-high” situation easier than if the Center of Gravity were “nose-heavy.”
You may ask yourself, "why does John prefer to check for thrust on vertical uplines?" Simple. It is easier to see the tracking of the model when it is on a vertical upline. Plus, you are using full power (torque).
Again, make sure that you are flying your model on a relatively calm day. The reason being that when you perform this test and pull vertical, the model will want to “crab” into the wind. This would make it extremely difficult to check to see if you need up or down thrust.
While flying parallel to yourself and the runway, apply full power and perform a ¼ loop to a vertical upline. Then, release all inputs (put the rudder, elevator, and aileron sticks to neutral) and observe the flight tracking of the model. If the model tucks to the undercarriage, you will need to add up thrust. If the model pulls to the canopy, you will need to add down thrust.
The same test is performed to see if you need Right or Left thrust. Simply apply full power and perform a ¼ loop to a vertical upline (making sure that the model is tracking perfectly straight) and release all inputs while keeping the throttle at maximum. Then, observe the airplane’s flight path. If the model leans to the left, you will need to add right thrust. Similarly, if the model leans to the right, you will need to add left thrust.
Different modelers may have different approaches to checking and setting thrust. What matters most is that you have the model flying as consistently at all throttle settings, without any pitch reaction to applying power.
Changing the thrust of the airplane can be as easy as adding a washer where the motor bolts to the airframe. Please remember that proper trimming takes both time and patience. Expect to trim your model over the course of a few flying sessions.
Now, before we begin with mixes, and now that we have a CG we're happy with, let's trim out a model in regular flight. When I trim my airplanes out, I always trim at full power. I establish a base-line that is perpendicular to the runway (by base-line, I mean the altitude that I normally fly at) and apply full power. Then, I let go of the controls and make sure that the airplane maintains the same altitude and does not need any trim to keep this attitude as far in each direction as possible.
Once this basic form of trimming is complete, I then move on to any of several mixes that will help the model fly precisely. It is important to remember to only trim out a model (with the criteria in this article) on days were there is little to no wind at all. The reason behind this is simply due to the fact that wind conditions can alter the values you will need to use in the mixes listed below.
Remember that CG will affect mixing, and a model being out of trim will also affect your mixing. So be sure to get your CG and trim spot on perfect, on a calm day, before moving on.
One of the most common mixes used by aerobatic pilots is the Rudder to Elevator mix. When you roll the model into knife edge flight with a moderate amount of throttle, most models pitch slightly to the undercarriage. To correct this, we use a rudder to elevator mix that incorporates a small amount of elevator deflection (in this case, up elevator to keep the model tracking true) when rudder is applied. If the model pitches to the canopy, I would need to incorporate a rudder to elevator mix that incorporates down elevator when rudder is applied.
Let's look at what the mix is doing. If we have 3" of up elevator travel, and a 10% mix, this means that when full rudder deflection is input, the elevator automatically will move 10% of 3", or 3/10", to compensate for the tendency to pitch to the belly.
This is probably the second most common aerobatic mix, where the model is rolling when rudder is applied. Again, roll the model into knife edge flight with a moderate amount of power. Then, observe the tracking of the model. If the model rolls slightly when rudder is applied, we need to use a rudder to aileron mix to correct for it. We want the model, when rolled into knife edge flight, to track true, without the pilot having to 'work the sticks' to keep it on a perfect line.
When giving right rudder to hold the model knife edge, if the model rolls to the right, then the aileron input needed will be negative, to get the model to hold back to the left; if it rolled left, then the mix will need to be positive (the same direction -- right -- as the rudder), rolling the model back right to hold it's attitude.
I know this may sound strange at first, however, trust me, it works. Please read the following. Fly your model into a spin-entry altitude (in other words, gain a lot of altitude). Then, come to idle and push the nose straight down. Now, let go of the controls. Take notice as to how the model tracks. If the model pulls to the canopy, you will need to use a mix of Throttle (master) to Elevator (slave) that incorporates a slight percentage of down elevator when the throttle stick is at idle. If the model pushes to the landing gear, the reverse is needed (slight amount of up elevator when the throttle stick is at idle.)
You may now wonder why I do not do the "thrust" test on a downline. The problem by checking your thrust on a downline is that your engine is at idle (torque is not at its maximum).
To make a model fly perfectly, most airplanes require minor mixing. If you are using "too much mixing" to make your model track straight, the mixing values can affect other flight characteristics of the model. For example, say you are flying a rolling circle and you have a lot of rudder to elevator and rudder to aileron mix in existence. When you are at certain points in the rolling circle, the mixing that you have activated can actually "work against you" and make you work harder. Either find a way to reduce the mixing value (by CG change, etc) or turn the mix off during certain maneuvers by means of a switch. My models have little to no mixing, so this is not an issue.
The new Futaba 14MZ has a unique way of setting mix rates...at the time when you are in knife-edge and are holding the required amount of elevator to remain on a straight flight path, you can hit the assigned "memorize" switch and it will memorize the settings / percentages you are currently using for you. No more fiddling with percentages trying to figure out just how much you needed!
Most computer radios have the ability to perform the mixes we have mentioned above. Many radios have these mixes pre-programmed, or built-in. I know the JR 10X has a program that already has a Rudder to Elevator and Rudder to Aileron Mix that you only have to activate and plug in your percentage value. The Futaba 9Z and 14 MZ are similar. Check to see what mixing capabilities your radio has. If it has any Programmable Mixes that are not defined, you can establish the mix yourself by following this example:
Master = rudder
Slave = elevator
Rate = 10%
Link = ON (see note)
Trim = Off (we don't want rudder trimming to add elevator!)
Mix = ON
Switch = (do you want to experiment with this? if so assign it to a switch; if not, assign the switch to NULL, None, or -- , so that the mix is always in use.)
NOTE:When creating a rudder to elevator mix with two elevator servos, with some more basic radios, it is necessary to make two mixes (one mix to your elevator channel and one mix to your elevator slave channel). This would also be true for rudder-to-aileron with two aileron servos.
If you have any further questions about any of the topics we have discussed above, feel free to contact me via my RCGroups Forum under Team RCGroups. I will do my best to have a timely response within a day or so of receiving your posting.
Until Next Time, JPG
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