Vibrations By James Goss      When you first begin working with radio control airplanes you learn to do three basic things, protect your receiver by placing it in foam, wrap the fuel tank with foam to prevent foaming of the fuel, and to balance the prop. The foam isolates the sensitive radio receiver from vibrations that could shake the tiny components and maybe create a bad connection to the board or damage some of the components. If the fuel is allowed to vibrate it may create foam or bubbles and the engine might draw air instead of fuel. An out of balance prop will increase the amplitude of these vibrations so we try to balance it the best we can. The larger the prop diameter, the greater the amplitude of the vibrations will be. One sure-fire way to reduce vibrations is to use a prop that has less diameter with more pitch. The centrifugal force exerted by a prop is related to tip speed. Tip speed is a factor of prop diameter times pi (3.14), times the revolutions per second. This gives the tip speed of the prop in feet per second. So if you can reduce the tip speed you will also reduce the centrifugal force exerted by the prop.       After this initial introduction to the vibration rules we don't think much about the subject any more, we just build our planes and fly them.  You might balance a prop from time to time or you may not bother. On small planes, I know I have used many props without checking for a good balance first and they seem to work just as good as the props that I did balance. Most props come from the factory with an in the ballpark balance. The manufacturer could do a final balance and get them 100% in balance, but when you ream the drive hole so it will fit your engine's shaft it will probably be out of balance again. Your reamer can lead the hole to one side or the other so you will need to rebalance it again anyway.      Just because I said I couldn't see any difference in the unbalanced prop as compared to the balanced prop doesn't mean there isn't any. Using our sight, hearing and touch senses, we can't see any tiny changes of improvement when we balance our props, but I am sure it relieves a lot of stress from the systems components and the air frame as well. The new radio receivers today all use surface mount components, this means that the components have no leads coming out of them for connecting them to the circuit board. Instead the end of the component is soldered directly to the solder pads on the board. These are referred to as chip components and are much more rigid in nature than their counterparts. Take a resister for example that has axle leads for soldering it to the board. During heavy vibrations the resistor can actually physically move around due to the leads giving under a stress load. This can eventually cause the solder joint to break and separate from the board. Some manufacturers will glue the chip components to the board as well; this really keeps them in place, but makes them harder to remove when it is time to replace them.      Not having leads on the components also has another advantage; it reduces the lead reactance. In high frequency circuits even a resistor will also have some inductance and capacitance values due to the long leads. Not having leads on the components has minimized this design problem and has also reduced the size for circuit boards. The reliability of our radio equipment has been greatly improved with this technology and even though vibrations are still a great threat to our planes, the threat has been somewhat reduced as far as the receiver is concerned. The airframe is still very susceptible to the vibrations and probably always will be. I would like to run some experiments in the future in reference to which type of covering will reduce or absorb the greatest amount of vibrations. If everything goes as it should in the upcoming project that I am about to describe, I will have a tester that will check the amplitude of vibrations generated by the engine. I would be willing to bet that ultracote will dampen vibrations better than monokote because ultracote is a little thicker in its cross section than is monokote.  Fabric coverings should be even better but this is just a guess at this time.      As our planes get larger in size so does the vibration problems. Larger planes require larger engines and the engine is where our vibration problems originate. Each time the piston travels up in the cylinder it develops inertia and wants to keep traveling in that direction even though it is time for it to stop. This inertia then pulls the engine in that direction and the plane also must follow. When the piston travels down on the next half cycle the opposite happens and the engine and plane moves in that direction. This movement back and forth (oscillations) is what we know as vibrations and they are transmitted throughout the airframe. It is the same as anchoring a ten-foot rope to a fixed point and shaking the other end with regular motion, you will send oscillations (vibrations) down the rope. A long spring, instead of a rope, really illustrates the effect of transmitted vibrations. The balsa and ply used in our models are also a pretty good conductor of these oscillations. Most wood is a better conductor of waves than is air. The amplitude of these vibrations depends mainly on the size of the engine and the mass of the moving piston and rod assembly. These oscillations can be reduced a great deal by using isolation mounts for the engine, but some engine manufactures for large scale recommends not using them. The theory here is that the isolation mounts will certainly reduce the amplitude of the vibrations reaching the airframe, but the engine will be allowed to shake and vibrate even more by not being mounted on a rigid mount. This extra vibration could shorten the life of the engine.      How would vibrations shorten the life of a mechanical device such as an engine? Take the piston for example in a ringed gas engine, as the piston travels up and down in the cylinder it basically stays in the center of the cylinder. This is because the ring and its close tolerances will keep the piston centered. Vibrations reaching the piston will force it to one side or the other in the cylinder and cause undue friction at those points. This action will slowly wear the metal away and shorten the life of that engine. The same theory can be applied to any of the moving parts inside the engine that meshes with other metals. Ball bearings are another good example. They are very sensitive to vibrations and can actually have their balls become out of round, due to the unbalanced loads. This now adds even more vibrations to the engine chassis. In using isolation mounts you have to make the decision as to which is the most important, the engine or the airframe. In most cases I would think it would be the airframe.      The frequency of these vibrations is determined by the rpm of the engine. If the engine is running at 13,000 rpm the frequency will be 216.6 hzs per second. By each cycle having two alternations the airframe will receive 433 pulses of energy (shakes) each second. The amplitude of these vibrations will be determined by the mass of the piston assembly, and how bad out of balance the prop is. You may have noticed that your plane will tend to vibrate more at some speeds than at others. This is because your plane has a resonant frequency and when this frequency is reached your plane is vibrating at is maximum. Mechanical resonance is a lot like the electrical resonance of a series or parallel LC tuned circuit, in that minimum energy is required to sustain the oscillations at resonance. A good example of mechanical resonance is found in the following simple experiment. Connect four pendulums (Four strings with the exact same weight on each string) to a common beam. The beam can be about three feet long and the strings can be equally spaced on the beam. Pendulum A is 24 inches long, B is 30 inches long, C is 24 inches long, and D is 18 inches long. If one of them, such as C is set in motion, the others will begin to oscillate because a vibration in the beam couples them. Pendulum A will start to swing with the greatest amplitude because it has the same natural resonance, as does C. They have the same physical properties and energy between them will be transferred back and forth. Energy from A keeps C swinging and energy from C helps keep A moving. The other two strings, B and D will move, but with less amplitude than A or C.  This proves that objects with the same mass and length will definitely transfer energy back and forth.      You can think of your engine as being string A and the other parts of your plane as being the other strings. Any part of your plane that is the same length and mass as the piston and rod assembly would be in tune with your engine and receive the most vibrating energy. If no item on your plane matches the engines resonance, then the next closest item will vibrate the most, and then the next, and then the next. So some parts of your plane vibrate more than others. As the shake frequency increases from zero to max your airframe may vibrate a lot at one point, drop off and pick back up at another frequency. This is because of harmonics and each harmonic point will shake a little less than it did at its resonant frequency.  The airframe's mass is the main factor that determines its resonance. A larger mass would represent a lower frequency.  When the airframe reaches its resonant frequency this is when the glue joints are really tested for their strength of bond. If they are hard and brittle they may just break apart. I love to use ca glues but they are glassy by nature, the glue joint will not give, they just break. The size of most models is such that their resonance is somewhere between low to mid range speed. This is when they shake the most, as you increase the speed the vibrations will smooth down somewhat.      The resonant frequency of the airframe can be changed but it will require adding or removing mass to achieve the change. Usually adding weight because we have already built it as light as we could. In a parallel resonant circuit you add resistance to dampen the free wheeling oscillations. In mechanical resonance you add mass to dampen the oscillations. Adding mass is the same as adding resistance in an electrical resonant circuit. Nobody wants to add additional weight to dampen the oscillations so we live with it. It would be nice if we had some way to actually measure these vibrations. Like I stated above, it is not easy to determine if you have improved the situation by balancing the prop because you have no real reference to go by. It could be vibrating more after you balance the prop because you could be getting a false reading on the balance test. We need a vibration meter or (vibrometer I will call it) that we can simply place on the airframe and get an accurate measurement of the vibration amplitude that the airframe is being subjected to. This device would be a real benefit to modelers building both small and especially giant scale planes. I have not found a device such as this on the market that would be suitable for our hobby at a reasonable price. I have decided to build one of these devices in my workshop at home and do some serious experimenting with the effect of vibrations on model planes, and maybe find a way to reduce them.      The reason I have become so concerned about vibrations in our models, here lately, is because I have recently had a really, really close call with my new giant scale extra 300. From day one my engine, a 5.8 Brison, would cutout when mid to upper end rmp was reached.  I first thought it was a gas-related problem and the engine just needed more time to break in, so I spent a lot of time adjusting on the needle valves. After about eight or nine flights on the plane I decided to check everything from head to toe and sure enough something showed up. While measuring the voltage present at the electronic ignition, I bumped some wire in the kill switch wiring harness. The voltage at the ignition module would vary up and down, I knew I was in the right area. The radio activated kill switch is in series with the ignition battery, an 1800 ma battery, and controls the 4.8 volts to the ignition. The kill switch is a micro switch that is controlled by a servo arm swinging around and closing the switch. The switch was being closed and I thought it to be making good electrical contact, but the servo arm could be made to move a little more, it was not at the end on its range. The switch contacts were making good contact, but at high rpm the switch contacts were being bounced open and closed by the engine vibrations. Each time the contacts vibrated open, the ignition would skip. I think God that it was only momentary in nature and not completely open. This was a difficult problem to find because of the fact that it was working most of the time. After finding the culprit it only took about 10 seconds to reprogram my radio so the servo arm would fully press against the switch lever. Now the 5.8 runs the way it should, brute force is the best way to describe this engine. With a 26x10 prop it will take my 30-lb plane straight up and never stop.      I am now thinking about using a solid state switch for the radio controlled engine kill switch. I see that Jomar (EMS) has one for about $39. It works right out of the receiver and uses power mosfets for switching. Power mosfets have a very low on resistance, which creates a low voltage drop. No servo is required and it only weights a couple of ounces. Wrap it in foam and never worry about vibrations again, as far as the kill switch is concerned anyway. After experiencing this problem I am sure the solid state switch is the way to go. I could have lost my giant scale to vibrations and would have never known why. I might have rebuilt and did the same thing all over again. One thing is for sure; I have had a very good lesson in vibrations and plan to remember it for a very long time.      As stated above, I am now building an instrument that will measure the vibrations generated by a model airplane, or any other model that uses an engine for power. This device will let us know if balancing the prop really did help or hurt the vibration amplitude. It can detect areas on the plane that vibrates worse than others so we could spot reduce the vibrations. Maybe it will set a standard reference for me to use on my upcoming giant scale planes, so I will know before the first flight that something needs to be done. It has got to be better than nothing, which is what I have at this time. My next article, how to build a vibration meter, will be ready soon, so come back and check it out. The End

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