Flight-Trimming No-Cals
by Michael A. Morrow
Here I show the step-by-step process by which I flight-test all my No-Cals. I have tried to keep the process as simple as possible, so as to produce the most reliable results. There are other methods, processes, and procedures used for flight-trimming No-Cals, or indeed any type flying model, that might produce a more finely tuned flying model, but this method should get you in the ball-park, and will put you ahead of those that do not use a consistent process for trimming. Soooo, on with the flight-trimming!


Flight Trimming No-Cals Made (relatively) Easy
COPYRIGHT 6/1/96 by Michael A. Morrow

I try to keep flight trimming as simple as possible. I do it in three distinct steps, in this order.

1) balance the model at 25% wing chord, and never touch the balance point again,

2) adjust the stabilizer for a nice flat glide, and never touch the stabilizer again,

3) make all powered flight adjustments with the thrust line.

I have followed this procedure with every model, and it works every time, if followed religiously, and there are no unintended warps in the structure. It is even the procedure I used to trim my Vought XF5U-1 Flying Pancake. There are slight differences depending on the model being adjusted, but they have to do with the model design, not the procedure.

I'll cover three different types of models as an illustration of the trimming process.

1) The first, and simplest type of model to trim is a model with balanced rubber: that means equal lengths of rubber in front of and behind the center of gravity. I have tried to design most of my models this way.

2) This section to be added at a later dateThe second type is a model with unbalanced rubber - that means more rubber behind the balance point than in front of the balance point (or vice-versa for a pusher). This is a common characteristic of models with very short noses, and includes long-tail models like my Cessna CR-3, Flying Flea, He-112, and Piper Skycycle designs.

3) This section to be added at a later dateThe third type of model I'll cover is a model that won't glide with the props in place because the props are too big and produce too much drag; specifically, my Vought XF5U-1 Flying Pancake.

So let's start with the simplest type of model - a model with balanced rubber, i.e., a model with equal amounts of rubber both in front of and behind the center of gravity of the model.


Flight-Trimming a Model With Balanced Rubber

Since the rubber is balanced on this type model, adding rubber will not change the balance point, so you can balance the model without the rubber installed.

Step 1: Balance the model at 25% of the wing root chord measured from the leading edge.

"Yeah, right!", I hear you say. "How do I find the 25% chord?"

Well, For a model with a constant chord wing, finding the 25%*** center of gravity is easy. Measure the wing chord at the wing root, divide by 4, and measure that far back from the leading edge. Voila! A 25% chord balance point. Use a ruler and measure it! Don't guess at it!

*** I use a point that is 25% of the wing root chord to insure stability. Some model builders use 28% or 30%, but I have found that the fun (ease of flying) seems to decrease dramatically for CGs beyond 25%, and decreases exponentially beyond 30%. This said, there are some very wide-chord models like the Cassutt racer which won't fly with a 25% balance point. Their balance point can be back at 30% or more. Models with very large stabilizers also fly better with a CG aft of 25% chord.
For a model that doesn't have a constant chord wing, it gets a little more complicated. Models with swept-back leading edges or swept-forward trailing edges require the CG to be measured on the Root Mean Chord of the wing, and then transfered straight across to the wing root rib.

To do this, you'll need to find the Root Mean Chord.

The Root Mean Chord is the chord on the wing where:

area to the left of Root Mean Chord = area to the right of Root Mean Chord

In the graphic below, that means A(1) = A(2).


I know two methods to find the Root Mean Chord.

Method 1) Use the quadratic equation to find the Root Mean Chord (not for the math-challenged)

Method 2) Use graphical anaylsis to find the root mean chord (the easier method)

Once you've found the Root Mean Chord, calculated the Center of Gravity (CG), and located it on the model, you are ready to balance the model.

At a later date I will show how to calculate the Root Mean Chord for wings with more complex shapes, like elliptical wings, or wings with multiple sections that are not alike.

 Most No-Cal models don't have a place to put pins in the wings to balance them, so I make a hole in the tissue on top of the fuselage at the balance point and hold the model up with a pin through the hole to check the balance.
To balance a model, I do the following:
a) Install the propeller

b) Do NOT install the rubber

c) Use small bits of clay to balance the model so that it balances at the CG located at 25% of the wing chord
I recommend NOT using a heavy prop to balance a short-nosed model, because . . .

Flying Model Truth Number 1:
The lighter the prop, the easier it is to flight trim the model.
This is because The Rubber doesn't Care Whether it Turns The Prop or the Model. It will do whatever is easiest.

If the prop is heavy compared to the model, it will turn (roll) the model, and you will have to use the flying surfaces to trim the model. All well and good as long as the rubber is turning the prop, but once the rubber runs down, those flying surface adjustments you made to compensate for the heavy prop take over, and the model spirals into the ground.

If the prop is light compared to the model, you won't have to use the flying surfaces to correct for prop torque, and when the rubber runs down, the model will continue to fly just as it did when the rubber was turning the prop.
Once you've got the model balanced so it hangs perfectly horizontal, it's on to Step 2.


Step 2: Adjust the stabilizer for a nice flat glide, and never touch the stabilizer again.

To trim a model properly, the stabilizer needs to be adjustable. I use a triangular slot for the stabilizer on all my models. The slot is the thickness of the stabilizer at the front. The bottom of the slot is flat, and the top of the slot angles up at about five degrees. With the fuselage blocked up vertically, and the stabilizer blocked up horizontally, I very carefully glue the leading edge of the stabilizer in the slot, and leave the rear of the stabilizer free for adjustment. I adjust the stabilizer angle by sliding a small stick of balsa forward and back between the horizontal slot member and the stabilizer itself.

To properly test glide the model, make sure you . . .
a) Test glide WITH the propeller installed,

b) Do NOT install the rubber, then

c) Adjust stabilizer for a nice flat glide, but not a "floating" glide - you don't want the model falling off in a stall once you add the rubber.

On to Step 3!
Step 3 Make all powered flight adjustments with the thrust line.

a) I set up all my indoor No-Cal models to fly left under power, so I start by adding a couple degrees of downthrust, and 2 to 3 degrees of left thrust.

There is a caveat to the "Make all flight adjustments with the thrust line" rule. On my low wing models, I add between 1/8 inch and ¼ inch of wash-in on the left wingtip to keep the wing-tip up under full-power launches.

b) Install the rubber motor. I generally use a motor twice the length of the hook-to-hook distance, and a rubber width consistent with the weight of the model. For the parameters I fly with, rubber size is directly related to the weight of the model. For the old Tan II, I was able to make up a graphical chart with the model weight on one axis, and the rubber width on the other axis. I would simply weigh the model, read the weight off the chart, follow it up to the graph line, and then read off the rubber width required to fly the model. Having a rubber stripper helped tremendously to get exactly the right width rubber for a model, as a difference of as little as 0.003 inches width could make the difference between great flights, and a model that wandered around like a drunken sailer.

c) Wind enough turns into the rubber to evenly distribute the weight of the rubber between the rubber hooks.

c) Stick a small straight pin in the front of the motor tube or stick to keep the prop from turning, and double check the glide to make sure the model is not stalling.

d) When you're sure the glide is good, un-pin the prop, and launch the model in level flight at the speed that the model wants to fly (which for a No-Cal, is a pretty gentle launch). Observe the model. It should fly in a fairly flat left turn.

If it stalls, increase the down-thrust.

If it dives, decrease the down-thrust.

Repeat as necessary.

Increase or decrease the left thrust to achieve a safe circle for your flying site. Models generally fly better and longer with a fairly wide circle, unless you've put too much wash-in in the left wingtip. There's always a compromise between a flight circle wide enough to give good flight times, and a flight circle small enough to prevent the model from turning right (and running into a wall) when the thrust decreases to the lowest levels at the end of a flight.

Copyright 6/1/96 - Michael A. Morrow
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