Just had a low flyover of my house by a couple Air National Guard(?) helicopters. The window shaking thump thump thump from the rotor blades has me wondering; how much of the sound generated by smaller propeller driven, piston engine airplanes is motor and how much is propeller? Does the ratio of sound change at distance, does propeller noise carry further and better than motor noise?
Every once in a while I get an Ornge Air Ambulance helicopter flying overhead. These are large turbine-driven AW139 helicopters, but the vibration from the helicopter blades is very powerful and distinctive. So, yes, it’s the heli blades, not the engine, at least for the larger helis.
ISTM the engine provides a predictable hum or whine.
So, you have two distinct noises and can often distinguish the two noises (engine and propeller).
When Coast Guard helis go zooming by my place you can hear a distinct whine from their engine. The thump thump thump would be the propeller blades. Although I will say it seems more thump thump when they hover and more engine whine when they are moving forward pretty fast. Doubtless it will be different for different aircraft.
That’s called blade slap, and occurs when a blade passes through the tip vortex generated by the other blade(s). In the R22, I noticed it when I was carrying cruise power and in a slight descent.
Sorry, I meant a small airplane like maybe a cessna or similar. How much of the sound you hear on the ground while one is flying overhead is propeller and how much is engine? The helicopters just got me thinking about it
If you want a scientific answer with dB numbers for main rotor, and tail rotor, then this publication from NASA has good analysis from simulation, Boeing MD902 and Sikorsky S-76. There are about 7 different noises as shown in Figure 1. If you look at figure 14 for MD902, you will see the first peak (low frequency about 50Hz) coming from the rotor. Since the chart is logarithmic, think of multiplying by 10 to look at peak differences.
(PDF) Two-Dimensional Fourier Transform Analysis Of Helicopter Flyover Noise (researchgate.net)
As to what sound carries further depends on the frequency, in general (for the same initial intensity) the lower frequency noises carry further (you hear the bass from your neighbors music more than the guitar). Helicopters typically have a transmission that reduces speed to 1/100 of the engine of the main rotor. So yes, you will hear the main rotors further than the engine.
Well, the loudest plane ever was the US Thunderscreech and it seems the noise was mostly due to the propellers.
It caused men caught in the prop-wash to vomit or faint, and even triggered a seizure in one nearby engineer. It was so loud that the base commander, worried it was damaging the control tower near the runway, eventually demanded that the Thunderscreech be towed out to a dry lakebed on the other side of a ridge for engine tests.
…
The engine itself was not particularly loud; the intense noise came from the three-bladed propeller. Because it was running at a constant 2,100 RPM, the tips of the propeller blades were always moving at around Mach 1.18 (1,457 km/h; 905 mph) giving each its own sonic boom. Standing alongside the aircraft, these booms would come at a rate of more than a hundred a second, blurring together into a screeching roar. SOURCE
Supposedly the Soviet Tupolev Tu-95 (Bear) bomber was uncomfortably loud to be around or fly in because of its loud propellers.
I think most small planes (private aircraft) have engines smaller than most cars (or in the same ballpark). The engine itself is not especially loud.
For comparison, you could listen to a turboprop aircraft. No pistons, just a gas turbine making shaft power to drive a prop. So on top of the propeller noise, you only get the muted whine of the turbine (and none of the roar of jet thrust). Here’s a Piper PA-46-500 (which uses a turboprop) on takeoff roll:
Granted, this is not a fair contest, as this engine has 500 shaft horsepower and a Cessna-172 only has 160 horsepower. A slightly better comparison might be the Extra 300, an aerobatic plane that uses a piston engine with 300 horsepower:
For a gas turbine and piston engine of matched power, the pulsating output of the piston engine’s exhaust flow is pretty much guaranteed to generate more (and more obnoxious) exhaust noise than the turboprop.
I had no idea that was a gas turbine powered craft. for whatever reason I thought those were piston engine driven also. I made an assumption that turbine power wasn’t used on smaller (fixed wing) aircraft. I don’t know why I made that assumption, but it seems I did. Probably related to my track mechanic days in the army, I mentally associate turbine power with Big Heavy(tanks) first and helicopters and larger aircraft second.
Piston single engine airplanes have mufflers of about 1940s tech. The engine and prop noise are distinct but of similar overall decibels. On climb-out the engine is prominent and on descent the prop tends to predominate.
Fancy pants larger high speed retractable-gear singles might have turbine engines. In which case the turbine engine nose is qualitatively smoother, higher-pitched, and whinier, not impulsive compared to the prop noise.
Except for the Robinson, almost any helicopter, and certainly any law enforcement or military helo you find is turbine powered. For helos regardless of engine type, the rotor noise is likely the bulk of what you hear. With an honorable mention for the interaction between main & tail rotor flows, which in any case is not an engine noise.
Aside from all the other information posted. It may also have to do with RPM of the blades and direction of the air flow.
Airplane blades spin around 1500 RPM, helicopters up to 500 RPM. The frequency of the sounds will differ a lot. Frequencies propagate over distance differently. Airplane props will direct a lot of the sound energy horizontally. Helicopters may direct it more vertically.
And the Hughes/Schweizer/Sikorsky 300/TH-55, and the Enstrom, and the Bell 47, and the RotorWay, and non-converted Sikorsky H-34/S-58…
But yeah, most helicopters in commercial use (aside from training, rental, etc.) are turbine-powered.
It’s been years since I last saw a Bell 47 in the air. Likewise a -300. Not to say there are zero left, but not many. OTOH, I don’t live near a helo training school either.
I used to live in an area that had one ratty old H-34 doing urban aerial crane work. You could hear that distinctive engine noise seemingly 15 minutes before he hove into view. I moved from there 10-20 years ago depending on how you keep score & haven’t heard that nostalgic noise since.
I recently had the opportunity to take a ride in a DeHavilland Canada Turbo Otter (an Otter retrofitted with a Pratt & Whitney PT6 turboprop), and it didn’t sound significantly different than a piston-engined small airplane, from inside or outside. I’m guessing in their case, the majority of “airplane” noise was from the propeller.
If they’re the HH-65 Dolphins, those fenestron tail rotors do a lot to reduce their noise signature, so I’m not surprised you might be able to hear the turboshafts. In fact, the noise reduction of the fenestron tail rotors was a major selling/marketing point of the whole Eurocopter/Airbus Helicopter line when I worked there 20 years ago.
For your listening ( and sight ) enjoyment, here’s a clip of a restored Douglas DC6 taking off, beginning at just over the 1 minute mark. Approaching we hear a mix of engine and propeller sound, and then the Doppler effect of the deep propeller “buzz” as it passes us, followed by the deep throaty roar of 4 2500 HP piston engines.
The propeller blade noise seems to be mostly low frequency bass noise, while the engine noise is a much higher frequency treble whine.
So as you get older and your hearing declines, the engine whine will fade and the propeller blade noise becomes the dominant annoyance. Certainly true in my case, living about a mile from a major airport and an Air Force Base.
There’s a turbine powered Bell 47. I think there have been several conversions available over the years. I saw one take off at Slave Lake Airport - thing went straight up like a bat out of hell.
An even better example than a turbine is the new electric airplanes. Hardly any motor noise at all, all comes from the propeller. Here’s an example of same airplane model with piston and electric motor doing the same things:
I live under the flight path of an airport serving business jets and twin-prop planes like the Beech Super King Air. By far the loudest prop plane is the Paiggio Avanti. I assume this is because the props at the back don’t have the noise disrupted by going over the wings. Can any of the plane experts here confirm this?