rotorblade finishing

While there are many ways to finish AeroBalsa RotorBlade Airfoils™, one of the more common methods is with iron-on film. As with other model structures to which it is applied, using a covering such as Monokote™ or Ultracote™ makes the rotor blades stronger. The following procedure is to be done after your rotor blades are cut to length and the plywood mounting plates have been added (if your application calls for them).

Please read through these instructions entirely before starting. As with any aerodynamic object, the goal is to achieve a rotor blade surface that is as smooth as possible for greatest performance.

Before starting, you will need a few things as described below:

Thin CA
220 grit sandpaper
Wax paper for your work surface and rubbing the CA into the balsa
Iron-on covering of your choice, such as Monokote™ or Ultracote™
Typical covering tools

NOTE: The process described here does not include adding tip weights. While some applications may need them, others do not. You will have to decide what is best or commonly done for your application.

STEP 1: SAND THE ROTOR BLADES (OPTIONAL)
While we make every effort to produce airfoils that are as smooth as possible, variations in grain and wood density sometimes results in a surface that could use a little sanding. If you desire, lightly sand the rotor blades to your liking.

Fig 1

Fig 2

STEP 2: STRENGTHEN THE TRAILING EDGE
Start by applying thin CA to the trailing edge. The trailing edge on our airfoils is thin, as in 0.015”, so it makes sense to strengthen it prior to covering. (Note: We have customers who do not apply CA… and just cover the blades as they are. You will have to decide what is best for your application.) CA application is shown in figure 1. Note that the rotor blade is laid on top of a piece of wax paper. Run a bead of CA down the entire length all at once, and work it into the balsa by rubbing it with a small piece of wax paper on your finger as shown in figure 2. Try to keep the CA to a uniform width. On the 2 3/8” rotor blades shown, the CA width is about a ¼ to 3/8” after rubbing it in. You can narrow this down a bit for smaller chord sizes if desired.

Fig 3

Fig 4

Once the top side is done on all the blades you are finishing, flip them over and repeat the CA application on the bottom side. See figures 3 and 4. Note that in figure 4 I’m holding the leading edge up to keep the trailing edge down on the work surface. You will need to do this to support the trailing edge while rubbing in the CA.

Figure 5 shows the CA on the rotor blade.

Fig 5

Fig 6

Allow the CA to dry and harden sufficiently. Once it is dry, use 220 grit sandpaper to lightly sand the CA’d surfaces. Use extreme care when sanding. The CA’d balsa is very hard… especially compared to the relatively soft balsa next to it. Avoid sanding too far beyond the CA’d area. If the sandpaper is allowed onto the soft balsa it may create a divot or low spot.

When sanding, make sure to scuff the surface of the CA’d area completely. This will provide the best possible adhesion with your covering.

You will again need to hold the leading edge up to keep the trailing edge down on the work surface when sanding the bottom side as shown in figure 7.

Fig 7

Fig 8

STEP 3: MARK THE STARTING POINTS FOR COVERING
The next step is to mark the rotor blade for where the covering will start. For the 2 3/8” chord blades shown, two tick marks were made 3/8” up from the trailing edge on the bottom of the rotor blade.

Important: By placing the starting edge of the covering in the undercamber, you avoid having it shrink and try to bridge the undercamber as it would do if the seam was placed elsewhere. This method allows the covering to stay down on the contour of the balsa during and after ironing it on.

Fig 9

Fig 10

STEP 4: PRECUT YOUR COVERING
After making the marks on the rotor blades, precut pieces of covering for all the blades you are finishing. For the 2 3/8” chord size, cut the covering at a width of 5”. This allows an overlap of about 1/8”, as shown in figure 14.

For the length, you will need to decide if you want to iron the covering around and onto the ends of the rotor blade, or simply trim it off flush with the ends. Depending on your decision, you need to cut sufficient length covering to do the job.

STEP 5: COVER YOUR ROTOR BLADES
Figure 10 shows the covering’s edge aligned with the tick marks. Start ironing the covering on, taking care to keep it aligned with the tick marks. Continue down the length of the trailing edge until it is all bonded.

Now pull the covering around onto the top side of the rotor blade. It should look like figure 12.

Fig 11

Fig 12

Hold the rotor blade while pulling the covering taut, and start tacking down and ironing on the top side. Keeping the covering taut when starting this step will ensure a crisp, clean trailing edge. See figure 13.

Once the covering is on the bottom of the blade, you should have about 1/8" overlap as shown in figure 14.

Fig 13

Fig 14

Continue ironing the covering until complete. Trim any excess off ends. Your rotorblade should look like figure 15.

Figure 16 shows the overlap after the rotorblade is finished.

Fig 15

Fig 16

STEP 6: CELEBRATE AND AUTOROTATE!
Your completed set of rotorblades should look like figure 17.

Fig 17

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