If It Flies ...
Dean Pappas | [email protected]
Rotating sonic booms and other noise sources
Also included in this column:
- Quiet the loudest part of the airplane
- Propeller efficiency
IF IT FLIES, it just might crash. I'm not referring to our model airplanes, although, sadly enough, it is sometimes true; sometimes columns crash. Okay, maybe this time I'm referring to a bad landing rather than an outright crash. There was an error in the October issue that needs correction.
I don't know exactly where it happened, but I do know how: pilot error. It's almost always pilot error, because the preflight inspection is supposed to catch things such as this.
The editor had requested that I shorten what had originally been submitted, because it would have filled roughly twice the allotted space. Yes, that was pilot error number one. So I shortened it in a hurry and skipped the final proofreading because these things always seem to happen when you're up against a deadline.
Putting things together in a hurry and not double-checking is a great way to cause a crash. I once beat up a good airplane just two weeks before leaving for the AMA Nats that way. Anyhow, more than a few of you wrote to call my attention to the dumb thumbs I had committed. Thanks.
The October column (near dead center of page 86) reads, "Your typical foot-long propeller turning in the neighborhood of 12,000 rpm has a blade tip speed that just exceeds the speed of sound. The propeller tip is moving at roughly 425 mph, compared to the nose of the aircraft." As it turns out, the blade tip is moving at approximately 425 mph (the actual calculation is slightly higher, and I rounded), but the speed of sound at "normal" temperatures is close to 767 mph. The propeller tip is moving along at nearly Mach 0.56, or 56% of the speed of sound, so it isn't exceeding the speed of sound. Oops.
With all the fervor of a National Transportation Safety Board aviation-accident investigator, I went digging for an earlier version of the computer document. There it was: "Your typical foot-long propeller turning in the neighborhood of 12,000 rpm has a blade tip speed of just over half the speed of sound. The propeller tip is moving at roughly 425 mph, compared to the nose of the airplane." At least it was right at some point, although it would have been good if I'd proofread just one more time. As I mentioned, it's almost always pilot error.
In the spirit of making lemonade with unexpected lemons, this started me thinking. I originally chose the example because propeller tip speed of Mach 0.56 is noise-friendly. I like to stick with noise-friendly examples when I can. The sound caused by the propeller tips as they approach and exceed the speed of sound is what causes that loud snarl or howl that typically characterizes a noisy airplane. I've heard many call it the "ripping sound." Whatever you call it, it sounds like raw performance to many of us, and it sounds like a nuisance to our flying-site neighbors. Nothing you ever do will satisfy an unreasonable neighbor. That's true both as an aeromodeler and as a homeowner, but only the unreasonable will complain when we take reasonable steps to reduce our annoyance factor.
This propeller-tip noise is often much louder than the engine's exhaust, and it can cause high-performance electric-powered models to be quite loud as well. This is because of the rotating sonic booms that emanate from a rotating propeller when parts of the blade (mostly out near the tip) go transonic or supersonic.
The shockwave and loud sound caused by an object that moves faster than the speed of sound (supersonic) is called the "sonic boom," but what does transonic mean and why is it important? I'll get there in a moment.
For most airplanes with engines sized .40 and over, the loudest noise source is the propeller, unless you've taken steps to change that. High-rpm racing engines, ducted fans, CL Combat engines, and the like tend to have very high propeller-tip speeds no matter how small the displacement. The next most prominent noise source is the exhaust, and then finally the carburetor and airframe.
Actually, airframe noise can vary wildly, depending on the airplane's construction. The important thing is that attacking a lesser noise source such as the muffler is not going to help much if the propeller tips are howling, so in general we start by dealing with the propeller noise.
When an airfoiled wing section moves through the air, the airflow around it speeds up locally. I wrote about that in the last column. That locally accelerated airflow can break the sound barrier even though the wing itself is moving through the air at substantially less than Mach 1.0. That local breaking of the sound barrier tends to start near the airfoil's high point, especially on the top surface.
Propellers that are carefully optimized for noise reduction can be operated at tip speeds that are close to Mach 0.6 (maybe slightly more) without severe transonic effects and noise. But for most purposes, we need to stay at less than Mach 0.55, as measured on the ground.
With a small allowance for round-off error, that corresponds to a 12-inch-diameter propeller turning at 12,000 rpm. This is actually close to the edge, as far as noise is concerned, because the in-air rpm is usually a bit higher than what we measure under static conditions.
Rather than calculating propeller tip speeds and comparing them to Mach 0.55 (767 mph, 1,125 feet per second, or 343 meters per second), we can use a rule of thumb. Multiplying the diameter in inches by the rpm (in thousands) for that 12-inch propeller at 12,000 rpm, we get 12 multiplied by 12, or 144. Taking 10% off for that in-air unload I mentioned and rounding off a bit, we get 130.
If either the diameter or the rpm increases, the tip speed will be higher and vice versa. If the diameter doubles and the rpm halves, the tip speed and the multiplied product will stay the same, so we can use simple multiplication and a rule of thumb to substitute for all that calculation. A figure of 140 is really the borderline. If the multiplied product is less than 130, the propeller will not howl unless the in-air rpm rises a lot, as in a full-throttle dive.
This rule works well in practice, and its effect is to set an rpm limit for any given diameter. Using the more conservative figure of 130, you see that a 10-inch-diameter propeller is limited to approximately 13,000 rpm. A 20-inch propeller must turn slower than 6,500 rpm to be quiet.
Now it becomes easier to understand why canister mufflers with their low-end torque-boosting characteristics have become popular on the big gas burners. Not only do they muffle the exhaust note well, but they also help the engines to breathe effectively at low rpm. Two-stroke exhaust scavenging is a subject for another day—maybe sometime soon.
A little 6-inch propeller can turn at close to 22,000 rpm without howling, but the exhaust will have a piercing note at that number. For a 10-inch-diameter propeller, the arithmetic is easy: about 13,000 rpm. For 11- and 12-inch diameters, the revolutions are limited to roughly 12,000 and 11,000 respectively.
Now that we know that we need to limit the rpm based on the diameter, we have a tradeoff to juggle. Let's say you have an airplane with a .40-size engine that was originally bought for Quickie 500 racing. Such an engine was probably intended by its manufacturer to deliver its best power and handling characteristics at fairly high revolutions.
It would do to keep such an engine propped for close to 13,000 rpm with a 10-inch diameter and keep adding pitch until the engine was loaded down to that figure. How much would it take? I can't say for sure, but although a 10 x 6 propeller is often used on a .40, it might take a 10 x 8 or a 10 x 9 on a really strong engine. Yes, these propeller sizes are available, even though many hobby shops don't stock them.
On the other hand, let's say that you have a .40 or .45 engine with a reputation as a torquer. Such a power plant might be happy in the mid-10,000 rpm range, where a 12-inch diameter is usable. You would then change the pitch to load the engine down to less than 11,000 rpm. The question remains: how much pitch does it take to load the engine to just less than 11,000 rpm with that big of a propeller?
You'll have to experiment. But if you can run enough pitch to get the desired airspeed, and the load produces the desired noise-friendly rpm, the greater efficiency of a large-diameter propeller will probably outweigh the slight power reduction caused by running the engine at reduced rpm.
The important thing is not the horsepower at the crankshaft, but the horsepower that comes out of the propeller. The difference is propeller efficiency.
Propeller Airspeed Values
This nomograph is not too useful for predicting airspeed because of rpm increase in the air and because similar pitch numbers stamped on different brand propellers don't always mean the same thing. It is useful for predicting the higher same-brand pitch that will be needed after a noise-friendly rpm reduction.
Rpm Vs. Diameter for Propeller-Tip Noise Reduction
This diagram encapsulates the 130 rule of thumb. Its purpose is to prevent the noise generated by transonic and supersonic propeller-tip airflow. Diameter and rpm combinations in the upper right make the in-air snarl or howl that frequently annoys neighbors.
A diagram (or curve) shows the rpm-and-propeller-diameter relationship that satisfies the rule of thumb of 130. If your rpm-and-diameter combination lies to the upper right of the curve, the propeller is likely to be noisy. If it is below or to the left, you are in the quiet zone.
For those who are mathematically inclined and the Algebra 1 students out there, the curve is a hyperbola. The formula for the "130" curve is:
Y (rpm) = 130,000 ÷ X (diameter in inches).
Kids, no matter what some adults will tell you, algebra can be useful.
Also shown is a nomograph for airspeed vs. rpm for different values of propeller pitch. The speeds it predicts are subject to substantial errors, because there is no explaining how some manufacturers come up with the pitch numbers they stamp on the propellers.
However, the graph is useful for figuring out how much you would have to change the pitch to fly at the same speed if, for example, you wanted to change the rpm from 14,000 to 12,000.
All you have to do is draw a horizontal line through the point where the diagonal pitch line (for the propeller you are using now) crosses the rpm you measure. As you move across that horizontal line, you can see what rpm would correspond to an increase or decrease in pitch. Then move to the diagram showing rpm vs. diameter to see if the new combination is in the quiet zone.
When you do this exercise, you may find several propeller sizes that "work" as far as noise is concerned. Start with the higher-diameter alternatives; they offer better takeoff performance.
Propeller efficiency
Propeller efficiency is better when the tip speeds are reduced and when the tips are farther away from each other, as in greater diameter. In that last respect, they are much like wings.
Glider wings have high aspect ratios because that makes them more efficient. How that works is the subject for yet another day. The aspect ratio of a wing is the ratio of the wingspan to the average wing chord (the distance from leading edge to trailing edge).
The reduction in tip speed is also an efficiency booster, because the drag losses on a part of the propeller vary with the square of the speed of that part of the blade. It all depends on how low an rpm your engine will run without lugging.
Although the "standard" propeller for a .60-size engine has been an 11 x 7 for as long as I can remember, 12 x 8s are great for most sport models. Most .60s will turn a 12 x 8 at roughly 11,000 rpm. If yours is stronger, try a 12 x 9.
Only the airplanes that need to fly fast, such as some warbirds, or aircraft you want to fly as fast as possible for fun, need to stick with the 11-inch diameter and 12,000 rpm. There you might need a pitch as high as 10 inches to keep the revolutions limited.
Expect the takeoff roll to be slightly longer than with an 11 x 7. But even though the model sounds quieter, it will go faster once it is "on the step."
I have been flying a Carl Goldberg Tiger 60 ARF with a muffled YS .61 turning an APC 13 x 7 Sport propeller, with great results. The engine turns that propeller right at 10,000 rpm, while my O.S. .70 Surpass turns it at just more than 9,000 rpm. The YS-and-13 x 7 combination is only as quiet as the muffler, because propeller noise is no longer an issue. Now I need to find a decent muffler to fit this engine that doesn't weigh a ton.
When you change props to reduce noise, consider these steps:
- Start by picking a larger diameter and lower rpm combination that keeps you under the 130 rule.
- Increase pitch as needed to load the engine down to the desired rpm.
- Experiment to find the prop that gives the desired airspeed while staying in the quiet zone.
- Prefer higher-diameter alternatives when possible for better takeoff performance.
It's time for me to sign off. Until next time we get together, have fun and take care of yourself.
—Dean Pappas
Transcribed from original scans by AI. Minor OCR errors may remain.
