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Written by Jay Smith Du-Bro’s Terry Dunn shares his knowledge about propeller balancing As seen in the September 2020 issue of Model Aviation.
shares his knowledge about propeller

One of the most popular items in Du-Bro’s lineup is the Tru-Spin Prop Balancer. We receive a lot of questions about the different ways that it can be used. Most people want to know the correct way to balance standard, two-blade propellers for model airplanes.

Vibration from an unbalanced propeller can cause many problems. A short list includes premature wear of motor bearings, failed electronics, and airframe fatigue. I am more than happy to provide input on this topic.

Get Set Up

No matter what type of propeller you are balancing, it is very important to get the Tru-Spin properly configured. Place the base of the balancer on a flat, level surface. Be sure to set both side plates to the same height. A handy trick for doing this is to temporarily place a scrap piece of wood (such as a 1 × 2-inch piece) between the base and the side plates to match the height.

The Tru-Spin Prop Balancer is extremely sensitive. Even small air currents can cause a propeller to spin and will make a good propeller appear unbalanced (or vice versa). Turn off any fans in the room and make sure you’re far away from air conditioning or heating registers. You might even want to hold your breath while testing the propeller (seriously).

Next, make sure that the propeller is ready for balancing. Verify that the mounting hole is free of any burrs or deformations. Also remove any molding flash on the hub or blades of a nylon propeller. A few swipes with 200-grit sandpaper will usually clean things up nicely.

Always use a propeller reamer if it is necessary to enlarge the mounting hole. Unless you are a machinist, using a drill bit to enlarge the mounting hole is just asking for irreversible trouble.

Many applications require you to use adapter rings in the hub to reduce the diameter of the mounting hole. Make sure the adapter is also free of burrs and install it in the propeller for balancing.

Balancing Nylon Propellers

One cone of the balancing shaft is supported by a spring. The other cone is held in place with a short piece of fuel tubing. Place the propeller on the balancing shaft so that the narrow ends of the cones support the propeller’s mounting hole. It does not matter which way the propeller faces. The spring on the balancing shaft should be slightly compressed.

Set the balancing shaft between the side plates so that it can rotate on the aluminum wheels. Hold the propeller with the blades horizontal then release it. If the propeller is unbalanced, the heavier blade will rotate downward. I use a marker to make a discreet dot on the back of the hub to note the heavy blade. Otherwise, it is easy to lose track.

Many people argue over the best way to correct an unbalanced nylon propeller. There are several good methods. I have always had success with adding tape to the lighter blade. Common, 3/4-inch wide Scotch Brand tape works in most situations and is nearly invisible.

Start with a piece of tape approximately 1/4-inch long. Gently place the tape on the back of the lighter blade, approximately halfway to the tip. Leave part of the tape overhanging the leading edge (LE) so that you can easily remove or reposition it as necessary. Recheck the balance of the propeller.

If the blade is still light, carefully reposition the tape closer to the tip. Move the tape closer to the hub if the blade is now heavy. Start over with a larger/smaller piece of tape if the propeller will not balance with the tape placed close to the tip/hub.

Repeat this process until the propeller does not rotate on the balancer after you release it. A well-balanced propeller will stay in place when released from any orientation.

After you have found the correct size and position for the tape to achieve balance, reposition the tape so that it completely adheres to the back of the propeller blade. There should be no overhang at the LE. Press the tape down firmly with your fingernail.

Double check the balance of the propeller with the tape in its final position. Use a marker on the rear of the hub to indicate that the propeller has been balanced and is ready for use.

Balancing Wooden Propellers

The method for checking the balance of wooden propellers is the same as previously discussed for nylon propellers. The only difference is how to correct unbalanced propellers. Again, there are many valid options. I prefer to remove material from the heavy blade.

Use 200-grit sandpaper to lightly sand near the tip on the back side of the heavy blade. Recheck the balance of the propeller often.

You do not want to remove any more material than necessary. Do your best to maintain the original contour of the blade. It is an airfoil after all. You might want to switch to 400-grit sandpaper as you near the balance point.

Many wooden propellers have a varnish on them. Sanding will obviously remove the varnish. It is not typically necessary to replace the varnish, but if you do, be sure to recheck the balance afterward!

Further Information

Many of the product pages on the Du-Bro website include videos about how to use them. The company has also created the "Du-Bro 101" series of videos that illustrates a range of common modeling tasks. We are always adding more videos, so let us know what you would like to see.

SOURCES:

Du-Bro

(800) 848-9411

www.dubro.com


9 comments

Good information for the beginner.

I fly giant scale, and find spraying clear polyurethane paint on the light blade works well and looks good. If the heavy blade is off very much then over sand and use paint. Randy

The method shown will statically balance a propeller but leave it unbalanced dynamically. Dynamic balancing is also important. That is why they spin automobile tires when they balance them. Please tell us how to dynamically balance our propellers.

Food for thought. One should be aware that by removing weight from the heavy blade by sanding, the prop will be statically balanced but dynamically unbalanced because by sanding the heavy blade the physical shape of the blade is changed in varying amounts depending on the degree of unbalance. 2 different airfoils creates vibration . Additionally when the protective finish is removed from the heavy blade, moisture in humid climates absorbs into the unprotected wood changing the balance. The solution for horizontal balance is simple. Simply spray the light blade with clear lacquer. Let dry, recheck balance and spray another coat if needed. This works on wood, glass and carbon props.

Additionally the process described in the article only covered horizontal balance . A prop can be horizontally balanced but not vertically balanced. It is necessary to perform vertical balance and prop tracking to complete the prop setup process.

1. I didn't know that the spring went between the stop and the first cone. I had been placing it between the piece of fuel line and the 2nd cone. Propellor will be better centered your way.
2. It never occurred to me to use tape for balancing. I've just been sanding near the tip. I like the tape idea.

Hi Gil,
I have no idea weather it will actually make any difference at all, however my instructions show mounting the spring towards the middle. The way you originally mounted the prop. There would be less flex in the shaft, the closer to the support? Food for thought.

After you balance the tips, you must start the very delicate task of balancing the hub. Stand the prop vertically and see if the hub pulls one tip down. Flip the prop and repeat until no side is favored.

Need information on 3 & 4 blade also. Also occasionally as on late model Spitfires, 5 blades.

That was horizontal Balanceing what about vertical balance????

I usually take this one step further and balance the prop vertically (my OCD kicking in). Since the Dubro balancer is so sensitive, it can detect slight variations in weight at the center of the prop hub. Sanding off material on the heavy side of the center will bring the prop into vertical balance.

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