DIY:

DIY: Prop Balancer

_MG_5269

Not Every Prop Balancer is the Same

NOTE: This prop balancer functions by using two extra long razor blades which face straight up! USE EXTREME CAUTION when working with these types of prop balancers, as serious injury can result!

I recently had an unfortunate situation where some small rocks were sucked through my prop. On launch I attempted to get airborne too early, the cage settled, the prop at full throttle came close enough to the ground and that was it – I could hear sounds back there similar to a fork in a blender. Why I attempted to get airborne early is another topic I suppose, but in a nutshell it was a no wind day on a smaller wing then I’ve been flying lately, and I was getting tired of running – A simple formula that can make for a lot of work later on.

Anyhow, I lost a 1/2″ by 1/4″ chunk from the corner of one prop tip and there were several 1/4″ by 1/4″ dings inboard of the tip on the opposite side – Not pretty, but certainly fixable. And seeing as that I was faced with the choice of either fixing this prop or buying a new one for $230+ dollars, at least attempting a fix was the obvious answer. I wasn’t intimidated by the repair, I’d been building and flying model airplanes since the early 1980’s when they were still just shipping as balsa kits and seriously, the repairs necessary for the scope of this damage weren’t that in-depth. What did intimidate me a little though was the prospect of balancing the prop – Something I knew I’d have to do even with this low level of a repair job. From cycling, I have a sincere appreciation for balance in spinning things. And as a PPG pilot with a propeller spinning so close to my back, even the slightest out balance vibes shouldn’t be acceptable. Still, I didn’t have a balancer and with my work schedule, I couldn’t afford to wait for one in the mail if I wanted to fly again anytime soon.

So, I elected to read and study as much as I could in order to build a prop balancer myself. How hard could it be I wondered? In reality I discovered, not hard at all. In fact, I was very happy at how my balancer came out, so I would like to share my insights into it’s function and construction here. In it’s basic form it’s a metal dowel running through the prop hub, which is turning on top of two razor blades, one on each side of the prop hub. You could have the dowel resting on top of two bearings on each side, with the bearings attached to their own dowels. Although safer then razor blades facing straight up, that would require more extensiveness both in construction and parts needed. On that note by no means is this article the “be-all end-all” of prop balancers, as there is much more information out there and many other designs to choose from – In choosing to immolate one like mine utilizing razor blades, you are doing so at you’re own risk. But I am confident in this particular balancer’s function, as the results spoke for themselves – My re-finished and newly balanced prop spun smoother then when it was new!

In order to build a nice prop balancer, you’ll need basic tools, basic parts,
and basic skills. That’s all…

A Note About Hub Thickness: Double check the hub thickness of your prop – My prop is 1″ thick at the hub, so I elected to utilize a balance carrier width of 2″, giving me 1/2″ of space between hub sides and the balancer itself, at the top of the carrier section. The focus of this tutorial is tailored to a prop with a hub thickness of 1″; if yours is thicker you can make the appropriate material changes, although those modifications are beyond the scope of this tutorial. 

_MG_5245Materials Needed: Balancer Hub Shaft
Note:
You  can certainly go with bearings for your hub shaft. I simply went with bushings because my hardware store carried them, in spite of having to make 3 work as one single unit. Also, the smaller diameter dowel pin the better, as long as it’s steel and no smaller in diameter then 1/8″.  A smaller diameter dowel pin makes for less drag on the edges of the razor blades, and therefore more accuracy in your results.

Dowel Pin:

Luckily, available at my local hardware store
1 – 2 1/2″ x 1/4″ Steal Dowel Pin

Bushings:
Again, available at my local hardware store. You could essentially go with one 1″ x 1″ x 1/4″ or 1/8″ bushing or bearing, if you could find one.
1 – 1″ x 1″x 5/8″ Bronze Bushing
1 – 1″ x 5/8″ x 3/8″ Bronze Bushing
1 – 1″ x 3/8″ x 1/4″ Bronze Bushing
* Bushing measurements reflect width, outer diameter, & inner diameter

Materials Needed: The Rig
Note:  
The links below point to bulk purchases of each item, merely as a reference. Your local hobby shop should have individual pieces of the same or similar hard woods – It goes without saying but still… Even though they’ll have plenty of it, do not use balsa!
2 – Midwest Basswood 1/2″ x 3″ x 24″ – Vertical Carrier Arms
2 – Midwest Basswood 3/8″ x 2″ x 24″ – Base Center Beam
2 – Midwest Basswood 1/4″ x 2″ x 24″ – Base Side Support Beams
1 – Midwest Basswood 1/8″ x 6″ x 24″ – Blade Harnesses, Outsides
1- Midwest Basswood 1/32″ x 4″ x 24″ – Blade Harnesses, Inside Spacers

Other
– Long Utility Knife Razor Blades
– Wood Screws
– Wood Glue
– Clamps
– 150 & 220 Grit Sanding Paper
– Wood Paint of your Choice

Tools Needed
– Drill
– Drill Bit Set
– Screwdriver (Flathead & Phillips)
– Chop Saw or Hand Saw
– Exacto Knife
– Pencil
– Tape measure / Ruler
– Square

The Build
_MG_52481. Start by taking the 3/8″ x 2″ x 24″ – Base Center Beam material and cutting it in half. You’ll have 4 pieces of equal lengths from the original two. Glue all 4 pieces together with wood glue to get one 1 1/2″ x 2″ x 12″ Center Beam and hold together with clamps until the glue drys.

_MG_52492. Now take the 1/4″ x 2″ x 24″ – Base Side Support Beam material and cut that in half. You should have four pieces of 1/4″ x 2″ x 12″ now. Separate that into a pair of two pieces each. Glue each set of two pieces together using wood glue, resulting in two pieces now. Hold each set together with clamps until the glue drys.

_MG_52503. Mark the center of the 1 1/2″ x 2″ x 12″ Base Center Beam from step one on both ends of the beam. Mark the center of both 1/4″ x 2″ x 12″ Base Side Support Beams and line up the centers of all three beams to make an I-Beam as in the picture to the right. Make sure all pieces are flat on the ground, and very important: Make sure the Base Center Beam is turned to the side which has the 2″ width facing up (and flat on the ground). When satisfied, glue the structure together using wood glue and secure with clamps until the glue drys. Drill pilot holes in the outsides of the Base Side Support Beams and reinforce the joints with wood screws.

_MG_52524. It’s now time to attach the Vertical Carrier Arms to the Base Center Beam. Mark the center of the 1 1/2″ x 2″ x 12″ Base Center Beam from step one on both sides of the beam. Mark the center of both 1/2″ x 3″ x 24″ Vertical Carrier Arms and line up the centers of all three pieces to make the structure as in the picture to the right. Using a square, make sure that the Vertical Carrier Arms are straight up! When satisfied, glue the structure together using wood glue and secure with clamps until the glue drys. Drill pilot holes in the outsides of the Vertical Carrier Arms and reinforce the joints with wood screws.

_MG_52535. The next step in the game is to build the Carrier Blade Harnesses. The construction is simple and requires the Basswood 1/8″ x 6″ x 24″ Blade Harnesses Outsides and Basswood 1/32″ x 4″ x 24″ Blade Harnesses Inside Spacers. You will be cutting two lengths from each for a total of four pieces. The dimensions to cut are two 3″ x 2 1/2″ pieces of the 1/8″ x 6″ x 24″ basswood sheet and two 3″ x 2 1/4″ of the 1/32″ x 4″ x 24″ basswood sheet. Separate and laminate the four pieces together with glue so that you have two individual pieces as shown in the picture above, with a 1/4″ gap at the top of each – This is where the razor blades will be sandwiched between the 1/8″ x 3″ x 2 1/2″ basswood and the Carrier Arms.

_MG_52546. Glue the Carrier Harnesses to the Vertical Carrier supports with wood glue and clamps, making sure that the tops are all flush. Allow to fully dry before proceeding further. Note: Before gluing you’ll want to place the razors upside down for safety into the slots between the Vertical Carrier Arms and the Carrier Harnesses so as to ensure that there is the necessary gap in place required to hold the blades once the glue has set. Make sure that the blades move freely in this gap before the glue sets!

_MG_5268

_MG_52697. Add a nail or hook to one of the Vertical Carrier Harnesses to create a place to hang your package of Razors – When you’re done with the balancer each time, carefully remove the razor blades from their harnesses and place them back into this original package. Again, you don’t want exposed razors out in the open when not using the balancer – I cannot stress this enough! Tip: Add a small dab of WD-40 to the blades while in storage to prevent corrosion.

That pretty much completes your balancer. At this point you could add some finishing aesthetic touches such as the paint scheme of your choice – I figure you put this much work into it already, you might as well apply some nice paint for long term protection. Overall, I found this balancer achieved some pretty admirable results and was quite satisfied with it. Accordingly, I plan to cover prop balancing in depth in a future post.