The Mystery of Planks

A brief “how to” guide to fine tuning plank style slope planes.

One of the reasons why fine tuning a plank can be so difficult is that most people don’t realize that they are completely different animals than conventional planes.  Planks are “tailless” planes where the elevator is mixed with the left/right controls.  Because  the up/down and left/right is mixed, the control surfaces are called “elevons.”  The term “elevon” comes from mixing the two terms “elevator” and “ailerons” (up/down, left/right).

The second reason why fine tuning a tailless plane can be difficult is that the two important adjustment areas, CG and elevon throws can work against each other.  In order to successfully fine tune a tailless sloper, it is important to make your adjustments with the understanding that both areas (CG and throws) must work together. 

Before we begin, a word about individual preferences and flying styles.  It is seldom the case that two identical planes are set up exactly alike.  This is largely due to the fact that each pilot has their own preferences and flying styles.   Some pilots may like the CG to be slightly back from the manufacturer’s recommendation, giving the plane a slightly tail heavy configuration and making it more lively and less stable.  The reverse is true as well.  Some pilots would rather the plane be more nose heavy and thus slightly more stable in the air.  Regardless of your preferences, it is best to start out your first few flights using the recommendations given by the manufacturer.  If you are going to error on any side, error on the side of the plane being slightly nose heavy.  This will allow for more stability during those first few flights.

Now that we better understand the relationship between elevon throws and CG, let’s begin to set up our plank.

Begin by setting the CG and reflex as the manufacturer has recommended.  Typically the CG is measured from the leading edge of the wing at the root.  For example, the Kaze 40 has a suggested CG of 1 3/4”. 

Once you have the CG at the suggested position, it is time to make the initial settings of the elevons.  For the Kaze 40, the suggested settings are as follows.

The figure below is an example of how to correctly measure the throws of your elevons.  In this case, the throws are for Up/Down (pitch).

Test Flights and Fine Tuning

Merely getting the CG at the manufacturer’s recommended point and the throws measured correctly is not enough.  Final adjustments of your plank will be based on flight tests.  How the plane reacts during these test flights will determine the final adjustments or what we will refer to as “fine tuning” your plane. 

Once you have your plane in the air and under control, take it up to a safe altitude.  Begin by diving the plane at approximately a 45º angle.  Release the control stick and observe how the plane reacts.  Your plane will do one of three things.  It could immediately arc back up, it could remain diving at a 45º angle with no change or it could dive at even steeper angle or what is called “tucking under.”

A plane that immediately arcs back up means that it is nose heavy

A plane that increases its dive angle or tucks under it is tail heavy

A plane the maintains its angle of attack without change has the CG set correctly.

When adjusting the CG, do so in small increments.  It is best to start out nose heavy and continue to move the CG back than it is to start out tail heavy.  It may take several dive tests to get the Center of Gravity exactly where it belongs.  But once the plane is balanced correctly, it should perform as intended.

Dive Test

Here are some general guidelines to fly by...

1. 1)If the nose of your plane is bobbing up and down rapidly, this is known as “Hyperstalling.”  Hyperstalling is an indication that      your plane is nose heavy.
   

2. 2)As you move the CG back in increments, you will notice that your plane will fly faster and be more responsive.
   

3. 3)Typically, the more you move the CG back the less elevator throws you will need.  If you find that you are moving the CG back considerably, be sure that you are reducing the elevator throws.
   

4. 4)As a general rule, if a plank is properly balanced, it will require very little “reflex.”  If a plank type plane is flying with a lot of reflex, it is almost a guarantee that it is nose heavy.
   

Typical Approach to Setting Up A New Plank

1. • Center all servos and transmitter trims.
   

2. • Make sure that all End Point Adjustments are at 100%
   

3. • Mechanically set the linkages so that they have the right amount of aileron throw.
   

4. • Using the transmitter (radio), dial down the elevator throws to about 15%.

Some pilots like to add a little more down throw to make cleaner outside loops.  So if you have the “up” at 15%, you could have the “down” at around 18%.

Thanks to those that contributed to the contents of this article.  If you have any corrections or additions that you would like to make, please email Future Slope Designs at:  futureslopedesigns@mac.com

-Paige

Please note that the above settings can vary depending on both the slope plane being set up and the skill level of the pilot.