The complex subjects of vibration and aircraft structural dynamics may be studied in aeronautical libraries since enormous work has been done in full scale aircraft development. This article is only to introduce the reader to the subject in model aircraft development. The data presented are strictly idealized and theoretical.

Reciprocating and rotary devices required considerable design and manufacturing care to minimize vibration. The one cylinder engines we use in Giant Scale model aircraft certainly provide a real challenge to the builder and flyer regarding the effects of vibration. Even those engines designed specifically for model aircraft require careful attention to the effects of vibration on airplane and equipment. Imperfectly balanced propellers also add to the environment. What to do! The customary approach is to screw the engine firmly to the airframe and use rubber vibration isolation to reduce vibration transmitted from the airframe to important equipment such as the fuel tank, servos, batteries, and receiver. This works reasonably well except that it is difficult to identify and isolate everything susceptible to vibration such as control surface hinges, clevises, push rods, and other unidentified parts until they have failed due to vibration. Primary and secondary airframe structural parts are even susceptible to vibration induced failure.

What additional things may be done to reduce the vibration environment? It would be nice if engines and propellers could be more perfectly balanced. Also it would help if the impulsive torque of the engine could be reduced. Each time the cylinder(s) fires a torque impulse is present as a source of torque vibration. More cylinders alternately firing will improve this problem. Also, a throttle coupled spark advance will smooth the impulse a little. However, the firing torque impulse generally results in only an unattractive shaking of the airplane at low throttle settings and is generally nondestructive. Other components of the vibration environment generated by the engine-propeller system can be destructive, especially at the higher throttle settings. The relatively simple propeller balance problem provides an eye opening view of the nature of the threat. The centripetal force equation is:

F = Wv(2)/gr

Where: F = Force in pounds, W = Weight in pounds, v = speed in feet per second (2 pi r x revs per sec.), g = gravitational constant (32 ft. per sec.(2)), and r = radius of path curvature. If we assume an engine R.P.M. of 8000 (133.3 R.P.S.) for a 24 inch diameter propeller and a .001 pound unbalance at the tip of one blade we have an unbalanced force of:

F = .001 x (6.28 x 133.3)'/32 x 1 = .001 x 701127/32 x 1 = 701/32 = 22 pounds

Although we balance our propellers somewhat better than this, the potential for generating enormous vibration from this source is obvious. Two things are apparent. First, do a good job of prop balancing. Second, keep engine R.P.M. down by using "more" propeller since any unbalanced force is proportional to the square of engine R.P.M. This will also help reduce the effects of engine unbalance because it also generates forces that are the square of engine R.P.M. Figure 2 shows the nature of square law generated forces F.

Engines and propellers will never be perfectly balanced. Accordingly, vibration isolation as done customarily on servos, receivers, etc. will continue to be necessary. I believe that additional vibration isolation at the source (engine) is very helpful in improving the reliability of airplane equipment by reducing the amount of vibration the airframe receives from the engine. I have designed my own engine shockmounts for several years since I could not find anything suitable for my applications. l use a simple silicone rubber isolation system that is somewhat the equivalent of a spring isolator with friction damping. Grossly oversimplified it looks like figure 1. The vibrator force of the engine is transmitted to the airframe approximately as shown in one of a family of curves KX/F idealized in figure 2. The important part of figure 2 is the higher R.P.M. portion of KX/F showing a reduction in the amount of force transmitted from the engine through the isolator to the airframe. The possible additional resonance energy at low R.P.M.'s transmitted to the airframe by the engine resulting from the isolator system parameters is not of major importance. This is true because the input vibratory force F is low at low R.P.M.'s since it follows the square law as discussed above. The isolation afforded at full R.P.M.'s is well worth the slight undesirable resonance around idle R.P.M.'s even when using a very simple vibration isolator. A resonance (s) usually occurs somewhere in the higher R.P.M. range with a hard mounted engine. Any amplification of vibration in an area where the greatest magnitude of vibration is to be expected is obviously undesirable.

What does a simple silicone rubber vibration isolator installation look like? Figure 3 shows the installation I have used for several years on POGO, OLE TIGER, FAR-CAM, and FLOYD BEAN SPECIAL airplanes in conjunction with Q35, Q40, and Z38 engines. The system is composed of four silicone isolators. With reference to figure 1, the "spring" is from the compression of silicone rubber and the friction damping is from silicone internal friction plus rubbing if the interfaces of metal, wood, and silicone. Spring rate and friction damping are both inseparably affected by the preload that is adjusted by the tightening of the mounting bolts. The damping effect, with reference to figure 2, changes the magnitude of the resonance as represented by the two idealized curves. Additionally, as the silicone is compressed by tightening the mounting bolts the spring rate (K) is increased causing the resonance to shift to a higher R.P.M. Accordingly, some "tuning" is possible but unnecessary. I just simply tighten the bolts enough for a very slight compression of silicone parts.

Enough of the theory! How about the practical side of this vibration isolation system? It is easy to install. I have used only 1/4 inch ply firewalls with small ply discs glued to the firewall to make it 1/2 inch thick where the vibration isolators are located. This firewall weight reduction is permitted due to lower vibratory forces reaching the firewall. It has a long life. Although there is some small amount of wear due to rubbing friction, I have never worn out a set of silicone rubber mounts even though I fly about 15 hours per year on each of two or three planes. Silicone parts should probably be replaced at about 50 hours. It requires low maintenance. I inspect and tighten the bolts each year, It doesn't reduce engine power output. The FAR-CAM at 15 pounds with a Zenoah 38 would fly vertically indefinitely. It probably improves airplane reliability. I've never had a vibration induced crash in about 300 hours of flying. It gets the attention of the safety inspectors. I like to watch the face of our IMAA safety inspectors when they check the engine for security as they twist the propeller blades and the tips move back and forth about 1/2 inch. It was designed for the Q35 engine with the Bennett 4 way engine mount.

I have used the mounting system with the Tatone prong type mount for the Zenoah 38 engine. I have also used it with the Zenoah 23 engine in my new TIGER II airplane with equal success. Other engine sizes and type of engine mounts could possibly be accommodated with careful design and experimentation. Vibration isolator parts may be obtained from:

A H S Manufacturing Phone (517) 433-0752.
13 N. Main St.
Centerville, Ohio 45459.