Vess Balance Ring

Author: Eric Richardson

Author taking vibration data beside a red model airplane

VESS BALANCE RING

Reduced vibration
means lower
airframe fatigue

By Eric Richardson
cxdriver@gmail.com
Photos provided by the author
and as noted

The nemesis for our aircraft is vibration. After changing propellers recently, I had to enlarge the spinner openings to accommodate a flatter-pitch propeller. Modifying the propeller openings in the spinner raised concerns about the spinner balance and subsequent vibration. Most modelers routinely balance their propellers, but what about the entire rotating mass? With interest, I came across the Vess Balance Ring (VBR). It is a straightforward yet innovative way to balance the whole rotating assembly. As an RC enthusiast, engineer, and Lean Six Sigma Master Black Belt, I set out to see whether installing the VBR would significantly reduce airframe vibration.

The idea came from Professor Robert Vess, an aerodynamicist, who said, “It became apparent that the vibration due to the total rotating mass that is attached to the engine drive shaft (propeller, washer, bolts, and spinner) is simply an issue of the net center of gravity of these components not coinciding with the axis of rotation of the engine.”

The VBR is a CNC-machined aluminum washer that is similar to the OEM propeller washer and is used instead of a standard propeller washer. It comes in several sizes to match the various propeller bolt patterns of different engines. The VBR fits engines from 50cc to 170cc in size.

Before, with only the propeller balanced, vibration data was taken at idle and 50% power. Not having access to a vibrometer, I found a free smartphone app vibration meter, Seismograph by ExaMobile S.A., that is designed to measure seismic events. It measures acceleration in the X, Y, and Z axes in meters/seconds. In aircraft terms, these axes correspond to the longitudinal, lateral, and yaw axes, respectively.

Test Airframes

The Aeroworks Yak 55M has a 108-inch wingspan and uses a DA-120 engine, a Falcon 28 x 9.5 carbon-fiber propeller, and an aluminum spinner. The Extreme Flight 300 V1 has a 104-inch wingspan and uses a DLE-120 engine, a Falcon 28 x 9.5 carbon-fiber propeller, and a carbon-fiber spinner.

Blue and white model airplane on a grass field — Vibration data was also taken on an Extreme Flight 300 V1 with a DLE-120 engine and a Falcon 28 x 9.5 propeller.

Propeller balance assembly clamped to a workbench — The propeller balance assembly. Note that the balancer is clamped to the workbench.

Figure 01 illustrates the vibration data collection method by holding a phone safely on the airframe and recording data on the vibration app. For consistency, the data was taken aft of the cowling on top of the airframe. The phone was held as lightly as possible to minimize the truncation of vibration data.

Recall that the purpose of my experiment is to see whether there is a statistically significant reduction in vibration after installation of the VBR. For statistically skilled readers, the data collection method likely fails to pass a measure system analysis (repeatability and reproducibility) of the data, yet it proved sufficient for my purposes.

Installing the VBR

Following baseline data collection on both airframes, the VBRs were installed. To start the balancing process, I used a Hobby Fans Tru-Spin Prop Balancer

that is similar to the Du-Bro Tru-Spin (Cat. #499) to simultaneously balance all of the rotating components assembled as they would be when turning on the engine.

To accommodate the length of the larger spinners, I replaced the original Hobby Fans balance rod with a longer 3mm x 300mm stainless steel rod. Next, I assembled the spinner backplate, propeller, propeller bolts, and washers, replaced the OEM propeller washer with the VBR, and added the spinner.

After this rotating mass was assembled on the propeller balancer, as shown in Figure 04, I began the tedious task of balancing. The balancing process starts with making an alignment mark on the spinner, spinner backplate, and propeller to ensure realignment each time the assembly is taken apart.

Propeller and spinner assembly mounted on a balancer — After the spinner back plate, propeller, bolts, and the spinner were added, the author began the tedious task of balancing.

Following the VBR instructions (see Figure 06), I inserted one setscrew at a time. I reattached the spinner and checked for balance. Caution: When reattaching the propeller/spinner/bolts assembly on the airframe, keep the engine bolts in the same holes. I found the bolts and washers weighing anywhere from 7 to 9 grams each.

Close-up of the VBR ring and setscrews — The VBR with the setscrews installed. Notice the alignment mark on top of the VBR ring. This is for the Yak airframe.

VBR balancing instruction diagram — VBR instructions.

The variation in bolt weight could influence the rotating mass balance. I repeated this process several times until I achieved a “perfect” balance (see Figure 05)!

Sample Before Data

Figure 07 is a screen capture from the Vibration Meter app on the Yak 55M airframe. In the upper right-hand corner, the maximum vibration noted for the recording period of 6.1 m/s2 is shown. The vibration data collection was repeated after installing the VBRs in both airframes.

Vibration Meter screen capture for the Yak airframe before installation — A vibration screen capture before the VBR installation on the Yak airframe.

Figure 08 illustrates vibration data for the Extreme Flight airframe, with a maximum reading of 3.2 m/s2.

Vibration Meter screen capture for the Extreme Flight airframe before installation — A vibration screen capture before the VBR installation on the Extreme Flight airframe.

After Results

The after results—with ostensibly less vibration—are shown in Figures 09 and 10. I wanted to see whether installing the VBRs would make a statistically significant difference—not just “it feels better.” Hence, I ran hypothesis tests for two-sample means for both airframes. Much to my delight, I can report that the VBRs make a statistically significant reduction in vibration! Reduced vibration means lower airframe fatigue, improved equipment longevity, and lower noise. The price for the VBR is less than $30—a worthwhile investment!

Vibration data graph for the Yak airframe after installation — Vibration data with the VBR installed on the Yak airframe.

Vibration data graph for the Extreme Flight airframe after installation — Vibration data with the VBR installed on the Extreme Flight airframe.

At a Glance

Engine: DLE-120
Propeller: Falcon 28 x 9.5 carbon fiber
Spinner: Carbon fiber

Specifications

Skill level: Beginner
Weight: 26 to 43 grams
Construction: CNC-machined aluminum
Price: $25 to $30
Additional materials needed: Threadlocker

Test-Model Details (1)
Airframe: Aeroworks Yak 55M with a 108-inch wingspan
Engine: DA-120
Propeller: Falcon 28 x 9.5 carbon fiber
Spinner: Aluminum

Test-Model Details (2)
Airframe: Extreme Flight 300 V1 with a 104-inch wingspan