Adjusting Your Twin Engine By James Goss      The sound of a model airplane engine as it makes a low center field pass on a cool fall day is hard to beat. There are many things that keep me in this great hobby and sport, and I do think it is a sport and not just a hobby. The sound that an engine creates is an important part of the overall enjoyment of this sport. The high rpm audio report of a two-stroke is just at the frequency above that of its own four-stroke sound. Thats right, a two-stroke can also four-stroke, or at least we call it four-stroke sound. You know what I am talking about, the sound that the engine is making when you are peaking it out and it is running rich. It sounds as if it is firing on every other cycle and you lean it just a little bit more. Now you have that nice smooth two-stroke sound or high frequency whine that tells us that this engine is ready to fly your plane. I use the term high frequency; actually it is relative low in frequency. If a two-stroke engine is running at 13,000 rmp it will be producing sound waves in the range of 216.7 Hz per second. In the audio spectrum this is relatively a low frequency because the audio spectrum ranges from 20 Hz to 20,000 Hz per second. The audio spectrum is the range of frequencies that us humans can hear, and most people can't hear past 18,000 Hz, especially older folks.  As an example of low frequency being all around us, take the human male voice; during a conversation all of the sound waves that he produces will be less than 600 Hz.  So at this frequency of 216Hhz the engine produces the sound that I appreciate so much, the two-cycle audio report.      A four-stroke model engine runs at a reduced rpm so it has even a lower frequency audio. If running at 8,000 rpm it would have a sound wave in the range of 133 Hz per second. The term sound wave is referring to the acoustical pressure that is produced each time the piston travels up and each time it travels down in its cylinder. When the piston travels up it produces a higher pressure in the muffler, when it travels down it reduces the pressure in the muffler. These two alternations will produce one complete cycle of sound energy. This action sets air into motion that travels through the air and strikes our eardrum. So the 133 Hz is a soothing sound to our ears and many people like this lower frequency, I call it hum.  You will notice that the four-stroke sound will carry for a greater distance than will the two-stroke. This is because a lower frequency has a longer wavelength that is not absorbed by the surrounding environment as is higher frequency signals.  This means that you can hear a four-stroke engine at a greater distance than you can a two-stroke, assuming that they both have the same amount of amplitude to begin with. This would also hold true for a gas engine in the 6,000 to 8,000 rpm range. You got to love those gas engines!      There is only one thing that sounds better to me than the two-stroke or four-stroke engine, and that is more of them. Two engines have a very unique sound with both of them in operation. They generate what I call a “beating sound”.  If two engines in operation were exactly in phase, and also had the exact same frequency and amplitude, they would sound like only one engine running. Let me say that no two engines are exactly alike. The manufacture would like to produce all engines so they would be alike in every way, but this is impossible to achieve. No two things on earth are identical! Even in the precise field of electronics where components are produced by the millions, you find the same dissembler components that seem to be identical to each other. An example would be transistors, if you take two transistors with the same number, even a matched pair, and place them in a multivibrator to generate an electronic signal, one of them will start conducting more than the other and the generator will start producing an output. So nobody can build two engines that are identical in every way. One of the engines will be out of phase, run at a higher or lower frequency or have more amplitude than the other. What we perceive is a scrambled sound that varies in intensity as the engines sound outputs beats together. All modelers seem to love this unique sound that twin engines produce, but flying a twin can be more intense than flying a single engine. We are always in fear that one of the engines will go out and the plane will pull toward the dead engine and stall.      Over the years I have found that the best way to adjust the engines on a twin may not be the way you would think.  Now it would be ideal if both engines were running at the exact same speed, but this is hard to accomplish, especially with the smaller glow engines. The best approach is to start each engine and peak them out as if they were a single engine unit. Each engine should run at its on natural set point, and not be forced to run at a certain speed. Trying to force run an engine will almost always result in one of the engines quitting after its in the air. Forget about using a tachometer to match the engines rpm. Start one engine; adjust to its peak and then back off a click or two. Start the other engine and repeat the same procedure and you are ready to go. I have had very good success with this procedure and think it is the best way to fly a twin engine.      If a twin engine plane sounds that good will a three or four engine job sound three or four times better than the twin? They do sound better but not by much. Think about these questions, will two engines be twice as loud as a single engine? Will three engines be three times as loud as one engine? Will four be four times as loud and so on? I am sure you know that the answer is no to all of these questions. Sound amplitude will not be additive just because more than one source of sound is present. It's like having more than one source of voltage in a circuit with them connected in parallel and one being larger than the other. If you place a voltmeter across them it will read the larger voltage of the two. Our hearing is done on a logarithmic scale and we cannot detect multiple amplitudes of this nature. So we do know that several engines are running, but we just can't determine exactly how many by the amplitudes they produce. Multiple engines with all the harmonics they produce are well worth the effort.  I have thought many times about mounting several engines on test stands, get them all tuned and running ok and just set back and listen to the music.  But one important part would be missing from this otherwise ideal picture, the fear of an engine quitting.

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