Model aircraft and noise abatement
Dean Pappas | [email protected]
IF IT FLIES, I'm interested in it! It's always been that simple. This column is dedicated to the notion that there is a great deal of commonality throughout aeromodeling, so that fliers in one group, say CL Combat, will have something to offer, in technique or practice, to another group, such as those who are looking for their first building project after learning to fly with ARFs.
The approach I'd like to take is to try to answer questions and observations that come from you—yes, you. The assumption is that your questions will almost inevitably come from a variety of experience and skill levels in all facets of the hobby/sport, and that in answering them some useful cross-pollination between the far-flung corners of the hobby/sport might take place. Accidentally we might actually come up with a helpful answer.
I use the second-person "we" because I intend to pull information together from the great many experts I have met through the years. These experts range from FFers to turbine-jet fliers to kit designers, and they are all just "plane" crazy.
With your help, the questions and problems we work on will be as varied as the wonderful resources we have at hand. Until then I will fill in by discussing subjects I think deserve to be general knowledge. I hope that won't last too long!
The First Link
In the last installment I mentioned how the tuned-pipe exhaust for two-stroke engines has evolved from the groundbreaking but finicky and unreliable CL Speed setups pioneered by the late Bill Wisniewski into several different uses.
In one important application the tuned pipe is useful for creating quiet. Yes, quiet: that tacit quality that helps protect our flying sites and maybe our sense of hearing as well. Let's cover noise abatement for a bit in this installment.
Let's look at the big picture. Why would you want to make your model airplane quieter? Some things in life were not meant to be quiet. I can't imagine a quiet Top Fuel dragster, nor do the words "speedboat" and "quiet" go together. Monster trucks wouldn't be monsters without the roar, and I can't imagine an RC pylon race or a CL Combat meet without the sound and fury. If you've never been to either of those two events, you must make the effort to get to one of each. Pylon, as with almost any form of racing, was created to be a spectator event, and CL Combat will make you feel like an ancient Roman watching gladiators. As I mentioned, some events just need the soundtrack that can only be provided by screaming engines! All those events have their fans, but, as with almost everything else, for everyone who thinks a Formula 1 engine revving 19,000 rpm is music, someone else thinks it is simply noise.
The problem is not limited to model aviation. At nearby Englishtown Raceway Park, home of one of the big national-class drag races, there are those who endlessly try to put a stop to the racing. Fortunately good management at the track and some common sense on the part of the local courts maintains the status quo. Who builds a retirement home right next to an airport and a drag strip and then complains about the noise? Only a fool! Still, most model airfields have neighbors, and some of them prefer the quiet.
Everything we do when it comes to making our airplanes quiet as part of being good neighbors is aimed at satisfying a reasonable person. Maybe someday I'll tell you about the time I got thrown out of a local park for a noise complaint while flying a glider. For now let's limit ourselves to useful talk.
The Pareto Principle—Fix the Biggest Problem First
The sound quality that annoys the neighbors most is the "snarl" of the propeller tips. Sometimes you'll hear it called propeller howl or a "ripping" sound, but if it is there you'll know it. When it is absent, the airplane's sound is decidedly softer.
Noise meters and property-line noise ordinances notwithstanding, the goal of simply being a good neighbor is best served by getting rid of the propeller-tip snarl and fitting a half-decent muffler. Taking those steps ahead of time can help avoid the irritation that comes with angry neighbors.
Propeller-tip howl usually carries much farther than the noise coming from the muffler. This is true for most sport engines above a .30 or .35 size and for almost all high-rpm racing engines.
Where is the dividing line? It depends on the propeller, so the critical Mach number isn't a hard-and-fast number. It is likely somewhere between 0.60 and 0.75 times the speed of sound.
You could calculate the tip velocity and divide by the speed of sound and all that, but I know a simple method. I call it the rule of 130, and Ed Izzo first suggested it in the early 1980s.
You multiply the propeller diameter in inches by the propeller rpm expressed in thousands, and compare the number to roughly 130.
- If the result is higher than 140, the setup is usually loud.
- If the result is less than 130, the setup is typically extremely quiet.
Examples:
- A 12-inch-diameter propeller at 10,000 rpm gives a product of 120. That's good.
- A 20-inch propeller at 7,000 rpm is bound to be loud unless throttled back a great deal.
It's time to pick a propeller that will fly at approximately the same airspeed as before the noise-reduction project while reducing the rpm and tip speed. You can try a propeller with the same diameter and add pitch. This will load down the rpm, while the added pitch will allow the airplane to fly as fast or faster, provided the available horsepower is adequate.
Instead of a 10x6 on a .40, try a 10x7 or even a 10x8 if the engine is particularly strong. Although the traditional propeller size for a .60 is an 11x7, you might try an 11x8 or an 11x9. This approach works well for light, low-drag airplanes, but it is not generally the best for heavy or draggy models.
The approach that tends to work better with many airplanes is to add a little propeller diameter and either maintain the same pitch or add just enough to get the rpm down to where the rule of 130 is satisfied. An 11x6 or 11x7 would do nicely on a .40–.45 engine, and a 12x8, 12x9, or 13x8 would harness the power of a .60 while making plenty of low-speed thrust and staying relatively quiet.
Why would we add diameter if we were trying to slow the tip speeds? It seems counterproductive because the bigger diameter would lead to a higher tip speed for any given rpm.
It turns out that propeller blades are wings. As with all other wings, you can get higher efficiencies with a larger aspect ratio (AR). The AR is the relation of the wingspan to the average wing chord. The CliffsNotes version is that the efficiency is better with a larger-diameter propeller because its tips are farther apart. This improved propeller efficiency can more than make up for the reduced horsepower the engine produces at the lower rpm.
Three- and four-blade propellers force you to pay an efficiency penalty, but they are useful when the engine rpm needs to be higher to suit the engine's running characteristics.
- Three-blade props typically end up with about 93% of the equivalent two-blade diameter.
- Four-blade props typically run at about 85% of the original two-blade diameter.
If you are faced with an engine that needs to run at too high an rpm for the propeller diameter needed to harness its horsepower, changing from a two-blade prop to a three- or four-blader might be the ticket.
Hushed Tones out of the Box
If your engine-and-propeller combination already meets the rule of 130, it's time to discuss mufflers.
The stock mufflers that come with engines these days are better than the fancy, sometimes expensive aftermarket items of a decade ago. The Evolution and O.S. AX-series mufflers are standouts.
I helped a clubmate set up an O.S. AX .55 with a 12x8 propeller. It turned that 12x8 in the mid-10,000 rpm range, pulled like a freight train, and whispered while doing it.
Four-stroke engines have always had a reputation for being quiet, mostly because of the reduced exhaust-note frequency. They fire only half as often as a two-stroke for the same running rpm. Even so, if you make horsepower you will make noise, and the newer four-strokes are finally coming from the factory with larger-volume, and even baffled, mufflers. Even here, simple improvements can be had.
Reduce, Reuse, Recycle
It is possible to spend all kinds of money on aftermarket mufflers, and some of them are quite good. However, if your engine came from the factory with a simple expansion-chamber muffler that has no baffles in it, there is still a simple, inexpensive option.
An exhaust extension makes the exhaust system longer, effectively "tuning" or filtering the harshest, high-frequency parts of the exhaust note. All you have to do to prove this is play trumpet with a long cardboard tube, such as the kind heat-shrink covering material is wrapped around. Cut the tube shorter, and the note will be higher and contain more high-frequency harmonics. In general, high notes are more annoying—except for the staccato howl of a racing engine!
This technique was used effectively on the Tiger 60 ARF that was reviewed in MA's Sport Aviator. A silicone exhaust elbow and 6½ inches of 1/2-inch inside-diameter tubing turned a noisy 20-year-old muffler into an extremely quiet exhaust package. The extension more than doubled the length from the exhaust port to the atmosphere. Although the exhaust note was dramatically mellowed, the horsepower was barely affected.
Larger engines respond beautifully to the large canister-type mufflers, sometimes even enjoying a horsepower boost. The mechanism behind this would take more ink to describe than we have left here, but we will go there if and when someone writes in looking for an explanation of tuned exhausts.
Similarly, four-strokes benefit from having a longer header pipe and then a muffler. Go to the local drag strip on "muffler-car night" and poke around. Some of those race setups make beaucoup horsepower! The header pipes on almost all the racecars are tuned to improve the torque in some desired rpm range.
In our case we are looking to boost the torque at rpm that will allow us to meet the rule of 130. That often means a header pipe that is roughly a foot long, made from aluminum, steel, or sometimes Teflon tubing. Teflon works better than you might guess. Clamp a piece with the right inside diameter around the header's stub that came with the engine, find a way to attach it to a small muffler, and you are in business.
Teflon is slippery, so it takes a fair bit of work to figure out how to lock the pieces together, but it holds up well once you get it. Industrial suppliers such as McMaster-Carr and Grainger stock the tubing.
There are other aspects to noise abatement, such as soft engine mounting, intake muffling, and shrouding the entire power train, but propeller noise and muffling take care of the lion's share.
Until next time, enjoy your time at the field or in the workshop, and please feel free to send in any questions or observations you might have. This column is all about making the remote corners of this hobby/sport work for you.
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
