In the jetliner biz we are getting aircraft engines quiet enough now that most of the noise on approach is just the plane passing through the air. The manufacturers are working hard to further reduce that by careful testing of flap designs & landing gear shapes to minimize turbulence.
Nevertheless, there is a lower theoretical limit to how much noise even an unpowered plane can make as it continuously disturbs the air to hold itself up. And there is a much higher practical limit as well.
On takeoff & climbout you have the additional challenge of providing the power to accelerate the aircraft & to force it to climb. The only way you can apply that power usefully using today’s technology is to move air aft, thrusting the plane forwards. Until / unless we develop anti-gravity, that action / reaction is the only vailable technique.
Any mechanism we use to force air aft introduces turbulence, aka noise. Certainly reduced noise can be designed for: a 1980s 757 produces barely 0.1% of the noise of a 1960s 707, while otherwise being almost identical in size, weight & speed.
For typical aircraft which spend most of their time at high altitude, distance to the ground provides a great natural muffler at little or no cost. In fact the 757, just by climbing much better than it’s predecessor 707, produces a much smaller noise impact footprint even before you consider the reduced engine noise at the source.
There are in fact mandated noise standards which are getting tougher every few years. Aircraft are expensive capital assets with 30+ year economic lives, so the standards can’t move too fast without greatly disrupting the industry economics. Check out 14CFR36 for more details.
You can do that for one specific location in space, but not in all directions. For that, you would need your speakers to be located exactly where the noise source is. Otherwise you will in some locations lower the noise, and in others raise it.
Low wing loading and low speed. If we had a plane that weighed as little compared to the wing size and travelled as slowly than we could make it as quiet (or close).
Well, for perfect cancellation everywhere, your speakers need to be in exactly the same place as your source. But as long as the distance between the speakers and the source is less than half a wavelength, you won’t get any constructive interference anywhere, and if the distance is significantly less than half a wavelength, you can get significant destructive interference everywhere. That said, though, aircraft noise, like most noise, has a broad range of wavelengths, and I’m not sure what wavelengths would be most significant.
This is absolutely right, but as aircraft noise tends to cover a very wide range of frequencies, some of which are high enough that the wavelength is very small, it tends not to be practical.
Sikorsky was working on a stealth helicopter a while back that had deflector flaps on the rotors to soften the “wap - wap - wap” of each rotor slapping into the wake of the previous one.
From what I recall, it used something like 20% more power for the same lift (which is huge figure for aeronautics), but was much quieter.
Probably we’ll only get something really quiet when someone invents a practical ornithopter.
A modern composite sailplane is very quite in “clean” configuration (gear up, flaps neutral, spoilers closed) and between stall and a little over best L/D airspeed. (say 45-60kt range) Older metal and/or fabric gliders are fairly noisy by comparison, but if one overflew you at 1000’ you probably wouldn’t look up unless he was flying at well over best L/D.
Dropping the gear, flaps, and using the spoilers generate some noise, but near silently landing gliders are still a serious hazard in locations where people need (or think they need) to cross the runway.
Noise increases substantially with airspeed. At Vne (redline) it can be quite loud.
Was this on final, or a high-speed, low level (usually crosswind) pass followed by a climb to 1000 AGL to actually enter the pattern? Such “race finish” passes are not uncommon. Achieving 800-1000’ on the pull up requires near redline speed for the low pass. If it was on final, and it was passing you, then there is a good chance the pilot had come in high and/or hot and had full boards out to avoid overshooting the end of the runway, or a long pushback at least.
Stirling engines have poor power to weight and size ratios. The “exhaust” is recycled, so they don’t have exhaust noise, but you’d still have a burner. Trying to make the engine light and compact would require a high power burner, that would probably need a noisy blower to feed air to it. Then there is the problem of transferring the engine’s shaft power to the air. Propellors generate a LOT of noise. Depending on the theory of lift you subscribe to requires that each propellor blade generate vorticies, low pressure zones, high pressure zones, or at least a “sheet” of wash. Since the blades are rotating, whatever is generating the lift will be periodic in nature, and that will consiquently generate a large accustic signature.
There has been debate over Bernoulli vs. Newton. When I was growing up (I started looking into the theory of flight when I was in single-digits) and when I got my pilot’s certificate, Bernoulli was taught. So naturally when the issue was raised here a few years ago I supported it. I and others basically got our hindquarters handed to us.
I have the latest printing of The Pilot’s Handbook Of Aeronautical Knowledge. It says that both Bernoulli fluid dynamics and Newton’s Laws of Motion are responsible for flight. I’ll see it I can dig up a quote.
Cheap? You price one of those things lately? ::: ouch ::::
Know a jet jockey that flies a Saberliner that cost them about $750,000 way back when. To get equivalent machinery at the time new was going to cost them $2,500,000. The new machine shortened the normal run by 10 minutes as determined by folks who flew them on the same route.
The old Saberliner used a lot more fuel. It would still have taken of many years of usage at the rate they flew to make up the cost difference in fuel. And the new planes were much smaller inside. To get the same cabin would really up the cost.
I recently saw a mint-condition kit for a couple thousand dollars. I saw a flying one for sale a couple of years ago for $25,000. About the cost of a new car. And the Q2 with the VW Revmaster engine, according to the link, gets 44 mpg. (Or 60 mpg if you slow to economy cruise.) And they’re really pretty.
