Balancing 2-, 3- and 4-blade props

Model Airplane News, Jul 2002 by Gierke, Dave

RPM REAL PERFORMANCE MEASUREMENT

A propeller converts the engine's torque into thrust. "Prop" is slang derived from the root word propeller, which means, "It breaks on landing." Seriously, though, it wasn't too long ago that propellers were primarily made of wood-a good choice, except that wood is vulnerable to breakage. Today, the hobby industry sells about 20 composite plastic props for every wooden one sold. This doesn't necessarily mean that wooden propellers are inferior; many fliers still consider wood to be superior in terms of strength and aesthetics.

Wood's high strength-to-weight ratio and ability to continually flex without fatigue are its main recommendations. Unfortunately, wood isn't the most stable material. Although manufacturers try to compensate for its inconsistencies (bending, twisting and warping), a wooden prop from a hobby shop still needs to be balanced before it's ready to fly. Composite plastic propellers are more stable in terms of maintaining their shape, but they can have problems caused by unsuspecting or careless modelers who drill them to fit the engine's shaft; an unbalanced propeller is often the result of faulty technique.

Balancing is the process of removing weight (material) in a systematic manner from specific surfaces of the propeller to bring it to a state of equilibrium while suspended from an accurate balancing instrument. Because propellers spin at high rpm, it's important to precisely balance them to avoid transferring damaging vibrations to the engine and airframe. Most propellers need to be balanced. Occasionally, you'll find one that doesn't require alteration, but you'll need a balancing instrument to determine this. Balancing techniques vary depending on whether the propeller is a 2-blade, 3-blade, or 4-blade unit. Here's how to balance each.

BALANCERS

Several types of balancers are on the market today, but I'm partial to the Robart High Point unit, which has been used in the industry for decades. The propeller balancing shaft is supported at each end by two large aluminum discs that pivot individually on Jewel-type axles. Capitalizing on the balancer's large mechanical advantage, no appreciable friction is transferred to the task at hand. The High Point unit doesn't need to be leveled to function flawlessly, either.

BALANCING A 2-BLADE PROP

Because balancing the common 2-blade propeller appears to be a straightforward matter, modelers sometimes take the process for granted; this often results in a botched job. The process is simple, but it must be performed correctly.

1 Enlarge the shaft hole size to fit your engine (see the sidebar, "Enlarging the prop-shaft hole"). If the hole is already too large, use another prop.

2 Place the propeller horizontally on the balancer to find the heavy blade (see Figure 1).

3 Next, place the prop on the balancer exactly in the vertical position, with the heavy blade at the bottom. If it rotates to a new equilibrium point, it has a heavy edge (see Figure 2).

Two methods are commonly used to balance the propeller. The first involves lightening the heavy blade until the propeller balances in the horizontal position and then removing material from the heavy edge (at the hub) to balance the unit vertically. Unfortunately, removing material from the hub weakens it, so to be safe, don't remove material from the propeller hub.

4 The preferred method is to lighten the heavy blade and the heavy edge at the same time without altering the hub. To accomplish this, remove material from the shaded area of the propeller as in Figure 2.

When you remove material from the heavy blade, sand, scrape, or file it from the front side, not the back side (this would adversely alter the prop's pitch). When you remove material from the front side of the blade, be sure to maintain its airfoil shape (see Figure 3). Last, don't remove stock from the blade tip and trailing edge; these are already thin and shouldn't be reduced any further.

5 When the propeller balances horizontally and hangs vertically with either blade in the down position, the balancing job is almost complete.

6 Wooden and composite plastic props should be final-sanded using a fine abrasive paper such as 220-grit or finer garnet. For wooden props, see the sidebar, "Finishing a wooden prop."

BALANCING A 3-BLADE PROPELLER

The method for balancing a 3-blade propeller is a bit unusual but is simple to do.

1 After you place the 3-blade prop on the balancer (see Figure 4), notice that one blade will probably hang low; this is the heavy blade, "H." The other two blades, "L" (light) and "I" (in between), will be positioned as shown in the diagram.

2 The next step is to lighten the front face of H until it allows blade I to hang straight down, becoming the new heavy blade (see Figure 5). The old heavy blade, H, is now equal in weight to the light blade, L.

3 The final step is to lighten the new heavy blade (I) until it sits horizontally (see Figure 6),

thus becoming balanced with L and H.

4 Finish-sand the 3-blade propeller as you did the 2-blade unit, and it's ready to run.