# Is it possible to build a practical powered aircraft that operates silently? **URL:** \ **Category:** Factual Questions\ **Created:** [May 1, 2006, 10:57am UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932 "2006-05-01T10:57:52Z")\ **Posts on this page:** 17\ **Page:** 2
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 2, 2006, 12:38pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/21 "2006-05-02T12:38:10Z")
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. ---
**Author:** ![The\_Controvert](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@The\_Controvert](https://boards.straightdope.com/u/The_Controvert)\ **Post date:** [May 2, 2006, 4:00pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/22 "2006-05-02T16:00:39Z")
Using [Airwolf](http://en.wikipedia.org/wiki/Airwolf) as inspiration… maybe it’s possible to have loudspeakers broadcast anti-noise to make the aircraft quieter? ---
**Author:** ![Elendil\_s\_Heir](https://avatars.discourse-cdn.com/v4/letter/e/7cd45c/32.png) [@Elendil\_s\_Heir](https://boards.straightdope.com/u/Elendil_s_Heir)\ **Post date:** [May 2, 2006, 6:03pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/23 "2006-05-02T18:03:42Z")
[QUOTE=RealityChuck] Mr. Memory knows. Am I right, sir? [/QUOTE] From _The Thirty-Nine Steps_, I presume? Nice. ---
**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\ **Post date:** [May 2, 2006, 6:26pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/24 "2006-05-02T18:26:07Z")
[QUOTE=The Controvert] Using [Airwolf](http://en.wikipedia.org/wiki/Airwolf) as inspiration… maybe it’s possible to have loudspeakers broadcast anti-noise to make the aircraft quieter? [/QUOTE] 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. > [@scr4](#): > > Owls manage it. Is that because they create less turbulence than other birds? > > Maybe what we need is a giant genetically-engineered owl. 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). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 2, 2006, 7:12pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/25 "2006-05-02T19:12:31Z")
> [@](#): > > 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. 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. ---
**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\ **Post date:** [May 2, 2006, 9:13pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/26 "2006-05-02T21:13:31Z")
[QUOTE=Chronos] 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. [/QUOTE] 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. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [May 2, 2006, 9:51pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/27 "2006-05-02T21:51:55Z")
[QUOTE=flight] 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. [/QUOTE] How long are the wavelengths in question? ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [May 3, 2006, 2:29am UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/28 "2006-05-03T02:29:01Z")
[QUOTE=Broomstick] I’ve flown a glider. They’re not silent. Quite a bit of noise is generated by their motion through the air (as previously noted) [/QUOTE] Depends on the glider. High-performance sailplanes tend to be rather quiet. (Since noise represents energy lost, this is scarecely surprising.) ---
**Author:** ![Thrasymachus](https://avatars.discourse-cdn.com/v4/letter/t/ce73a5/32.png) [@Thrasymachus](https://boards.straightdope.com/u/Thrasymachus)\ **Post date:** [May 3, 2006, 2:59pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/29 "2006-05-03T14:59:09Z")
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. ---
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [May 3, 2006, 6:36pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/30 "2006-05-03T18:36:56Z")
It is very dificult to search for ‘silent’ and ‘practicle’ at the same time. Same for wanting to go ‘fast’ and ‘cheaply’. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [May 3, 2006, 6:49pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/31 "2006-05-03T18:49:52Z")
[QUOTE=GusNSpot] Same for wanting to go ‘fast’ and ‘cheaply’. [/QUOTE] [180 mph, 44 mpg](http://www.quickiebuilders.org/cgi-bin/smartfaq.cgi?answer=980714241&id=980713619). 😃 ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [May 3, 2006, 9:20pm UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/32 "2006-05-03T21:20:18Z")
[QUOTE=Sunspace] Even gliders are loud? I am amazed. I would have thought that the engines on a plane would be much louder than any air noise. [/QUOTE] 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. [QUOTE=SuperNelson] I’ve been passed by a glider on close approach - and they are LOUD up close. Granted, we didn’t hear this one until it was within a few hundred feet, but it got really loud really fast. Wing noise will be a concern. [/QUOTE] 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. [QUOTE=Sunspace] Speaking of engines, though: I have read hints that Stirling-cycle engines are [very quiet](http://en.wikipedia.org/wiki/Stirling_engine). Perhaps they could be used in place of other piston engines inj a plane? [/QUOTE] 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. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [May 4, 2006, 3:20am UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/33 "2006-05-04T03:20:40Z")
[QUOTE=Kevbo] 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. [/QUOTE] Bolding mine Thanks! But… “depending on the theory of lift”? You mean we’ve been building airplanes for a hundred years and we don’t know why they work? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [May 4, 2006, 3:50am UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/34 "2006-05-04T03:50:52Z")
[QUOTE=Sunspace] But… “depending on the theory of lift”? You mean we’ve been building airplanes for a hundred years and we don’t know why they work? [/QUOTE] 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. ---
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [May 4, 2006, 4:13am UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/35 "2006-05-04T04:13:05Z")
[QUOTE=Johnny L.A.] [180 mph, 44 mpg](http://www.quickiebuilders.org/cgi-bin/smartfaq.cgi?answer=980714241&id=980713619). 😃 [/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. Airplanes is strange business… ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [May 4, 2006, 4:14am UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/36 "2006-05-04T04:14:04Z")
From [_The Pilot’s Handbook Of Aeronautical Knowledge_](http://www.faa.gov/library/manuals/aviation/pilot_handbook/), Chapter 2: > [@](#): > > The construction of the wing, so as to provide actions greater than its weight, is done by shaping the wing so that advantage can be taken of the air’s response to certain physical laws, and thus develop two actions from the air mass; a positive pressure lifting action from the air mass below the wing, and a negative pressure lifting action from the lowered pressure above the wing… > > [If] all the lift required were obtained merely from the deflection of air by the lower surface of the wing, an airplane would only need a flat wing like a kite. This, of course, is not the cast at all; under certain conditions disturbed air currents circulating at the trailing edge of the wing could be so excessive as to make the airplane lose speed and lift. The balance of the lift needed to support the airplane comes from the flow of air above the wing. Herein lies the key to flight. The fact that most of the lift is the result of the airflow’s downwash from above the wing, must be thoroughly understood… > > As seen in the application of Bernoulli’s theorem to a venturi, the speedup of air on the top of an airfoil produces a drop in pressure. This lowered pressure is a component of total lift. It is a mistake, however, to assume that the pressure difference between the upper and lower surface of a wing alone accounts for the total lift force produced. > > One must also bear in mind that associated with the lowered pressure is downwash; a downward backward flow from the top surface of the wing. As already has been seen from previous discussions relative to the dynamic action of the air as it strikes the lower surface of the wing, the reaction of this downward backward flow results in an upward forward force on the wing. This same reaction applies to the flow of air over the top of the airfoil as well as to the bottom, and Newton’s third law is again in the picture. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [May 4, 2006, 4:18am UTC](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932/37 "2006-05-04T04:18:33Z")
[QUOTE=GusNSpot] Cheap? You price one of those things lately? ::: ouch :::: [/QUOTE] 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. 🆒 [Previous page](https://boards.straightdope.com/t/is-it-possible-to-build-a-practical-powered-aircraft-that-operates-silently/354932.md?page=1)