Focus on Competition
Technically Speaking ...
Greg Hahn Technical Director [email protected]
There's nothing better than hearing a well-tuned model engine (or two) running nicely and pulling your favorite model through the air with ease.
Unfortunately, not everyone agrees that model engines make a pretty sound or should make a sound at all.
Airplanes, whatever their size and shape, are historically noisy. Since the Wright brothers, airplanes have needed maximum power production, and large restrictive mufflers not only cut down on noise but also cut down on power. Because of that, mufflers have never been very high on the designer's priority list. Even today, most piston-powered aircraft have small, inefficient mufflers if they have any at all.
During World War II, several airplane engines had nothing more than a 6-inch open stack on the exhaust, just enough to clear the cowling and that's all. Another theory is that airplanes fly high enough that the noise is controlled by distance and is only a factor when operating at or near airports.
We don't have either luxury; we fly relatively low compared to full scale and we're on the ground every 10 to 15 minutes at the same airport. We also operate more than one airplane at a time from the field, so if enough members are flying on a Sunday afternoon there is an engine running almost all the time for several hours.
Needless to say, no matter how quiet we are, it's going to annoy someone sometime, so we need to be aware of how sound carries and do everything possible to contain it and control it. Here at AMA we don't have specific set guidelines for noise limits at sanctioned fields. Mostly due to the variety of site locations and municipalities that govern these areas, one shoe will not fit all.
We are in the process of forming a committee to try to address this issue and get at least some type of workable number system that will work a majority of the time. The old numbers that were published back in the early 1990s stated 90 dB at 9 feet. There never was much adherence to that because 90 dB under perfect circumstances was almost impossible to attain. I know of a few Pattern fliers who did get those numbers, but only a rare few.
In the interim, here are a few items that may help you get your noise footprint down to a manageable level.
- Try a three-blade propeller. The most important thing to remember about airplane noise is that most of what you hear does not come from the engine; it comes from the propeller. What you hear when your engine unloads in the air is the prop tips going supersonic, so reducing engine RPM will limit much of that.
You might think a bigger two-blade prop will slow the engine down, right? Wrong. Yes, it will slow the engine, but now you have longer blades and the tips are actually going faster (physics). The answer is to add blades so you keep the diameter down while also reducing the RPM. The reason you see most of the big IMAC guys now using three-blade props is not for performance, but for noise control. If you don't know how to calculate for a three-blade prop, ask your engine manufacturer for a recommendation. A rule of thumb is to reduce diameter by 1 inch or pitch by 1 inch (or both) if needed.
- Use as large as possible canister-type mufflers. Just about all engines, glow- and gas-powered, sold on the U.S. market today are supplied with a large can-type muffler. Most of these supplied mufflers will do an adequate job of controlling the combustion noise coming from the cylinder.
Adding a piece of rubber hose to the outlet pipe can also help stop some of the ringing sound that can be produced by the muffler. The rubber hose doesn't have to extend past the pipe; just cover it like insulation.
- Soft-mount your engine. Often there can be quite a bit of noise coming from your airframe, especially if it contains a lot of fiberglass or is open structure covered in plastic film. When the engine is hard-mounted to the firewall, vibration will be carried back through the airframe and essentially act as a drum unless there is some type of absorption material to soak it up.
Many of the newer 30% and 40% aerobatic aircraft have foam turtledecks and other foam parts that absorb much of the vibration noise; if not, your beautiful machine will sound more like a drum kit than a purring kitten. There are several soft-mount systems on the market today.
By the way, to my knowledge all full-scale piston engines are soft-mounted. Take a lesson from the big boys.
I hope these few ideas will help you control noise at your field. It's very important that we stay on top of this issue because it is one of the biggest threats we have to saving flying sites. If we don't control it, our neighbors will, and it won't be pretty.
See you at briefing.
Transcribed from original scans by AI. Minor OCR errors may remain.
