Why aren't piston aero engines silenced?
Discussion
Genuine question, as I know next to nothing about aviation.
I'm working from my home office in mid Staffordshire and I look out over the Trent valley. I'm guessing this is a flightpath, as during the day, I tend to hear all sorts of stuff going over, at various speeds and altitudes.
As one particular light aircraft flew overhead, it occurred to me that its engine was pretty much un-silenced. It was obviously a lighweight, as the engine sounded like a couple of angry wasps in a tin can and it took ages to fly past. Loads of other stuff goes past during the day and night and it seems that the lighter the aircraft, the less it is silenced (although that could easily be down to the bigger stuff flying at greater altitude).
Anyway - it got me thinking (better than working...) Given the huge pressures on noise polution for road traffic, why are aircraft allowed to roam around without much, if any silencing?
TBH, it doesn't bother me much, especially as I occasionally get to see/hear some properly old stuff as well. For some reason, I'm happy that Merlin V12s aren't silenced...
For road vehicles, I was always under the impression that no exhaust was a bad thing, as exhaust port scavenging can only be achieved by a 'tuned' length of primary exhaust header. I can't see why an aero engine would be any different in this respect.
I'm working from my home office in mid Staffordshire and I look out over the Trent valley. I'm guessing this is a flightpath, as during the day, I tend to hear all sorts of stuff going over, at various speeds and altitudes.
As one particular light aircraft flew overhead, it occurred to me that its engine was pretty much un-silenced. It was obviously a lighweight, as the engine sounded like a couple of angry wasps in a tin can and it took ages to fly past. Loads of other stuff goes past during the day and night and it seems that the lighter the aircraft, the less it is silenced (although that could easily be down to the bigger stuff flying at greater altitude).
Anyway - it got me thinking (better than working...) Given the huge pressures on noise polution for road traffic, why are aircraft allowed to roam around without much, if any silencing?
TBH, it doesn't bother me much, especially as I occasionally get to see/hear some properly old stuff as well. For some reason, I'm happy that Merlin V12s aren't silenced...
For road vehicles, I was always under the impression that no exhaust was a bad thing, as exhaust port scavenging can only be achieved by a 'tuned' length of primary exhaust header. I can't see why an aero engine would be any different in this respect.
I grew up within a decent Tiger Woods drive* of the end of the runway of an International Airport, things have improved dramatically and drastically re: noise. I used to know what the weather was by the aircraft take-off sounds, particularly a BAC1-11 at full chat informed me that it was somewhat blustery. A merchantman, or an ATP, struggling to get going often meant the frost had hit, hard. The Russians were also...interesting.
*I am, of course, exaggerating, it was just over 600m.
rewild said:
Some do.
But weight and size are the main reasons.
Most noise on prop craft comes from the prop, so exhaust silencing wouldn't make as much difference as you'd think.
A couple of the PA-28s at Waltham had silencers when I was learning to fly (nearly 20 years ago now...), but as you say, the prop generally what causes noise issues - not helped by those who insist of going fully fine early downwind in the circuit and needlessly making a hell of a racket in the process. But weight and size are the main reasons.
Most noise on prop craft comes from the prop, so exhaust silencing wouldn't make as much difference as you'd think.
Again - apologies for the lack of knowledge - just idle interest
I know enough about car engines to know that a normally-aspirated car engine NEEDS an exhaust manifold and header pipes to function correctly and efficiently. It's all about exhaust-port scavenging, where the pulses in the pipe actually help with the extraction of the gasses from the cylinder.
Are we saying that the stubby little headers on an aero engine are enough to meet this need?
A bit more Googling suggests that planty of light aircraft on the continent are silenced, but in the UK, the costs of CAA approval / certification are punitive (ie as much as the cost of the plane for some lightweight stuff)
I know enough about car engines to know that a normally-aspirated car engine NEEDS an exhaust manifold and header pipes to function correctly and efficiently. It's all about exhaust-port scavenging, where the pulses in the pipe actually help with the extraction of the gasses from the cylinder.
Are we saying that the stubby little headers on an aero engine are enough to meet this need?
A bit more Googling suggests that planty of light aircraft on the continent are silenced, but in the UK, the costs of CAA approval / certification are punitive (ie as much as the cost of the plane for some lightweight stuff)
wisbech said:
Yep - the main way that jet aircraft meet noise regulations is by getting high fast - because the noise is measured on the ground... The wavy nacelles on new generation aircraft also help a bit, but taking off like a bat out of hell works best
I know this is probably taking the thread OT a bit, but has anyone else found that some of the A320 NEOs sound louder on the ground when on approach than an equivalent with the older engine?I don't think the difference is as pronounced with the 737-800 vs the Max, oddly enough.
I know measuring with a sound meter would probably show the NEO as quieter, but it seems to sound louder, maybe a more bass-heavy engine note.
Simpo Two said:
Eric Mc said:
Silencing also causes a power reduction.
Sooo... if cars didn't have silencers, they'd be more economical and so make fewer carbons!Modern car engines are quite a bit more sophisticated than some piston aero engines when it comes to mixture control etc. Many light aircraft flying today are over 40 plus years old too - so many aero engines you hear today date back to the 70s and earlier.
eharding said:
- not helped by those who insist of going fully fine early downwind in the circuit and needlessly making a hell of a racket in the process.
That's just what instructors get people to do - get your downwind checks done early, including prop fully fine. More experienced pilots may choose to delay that action, if they ever think beyond what their instructor has taught them. Certainly one flying club that I was a member of had a CFI who wanted everything done his way. I recall him once having a go at me for landing flapless shortly after I moved to this club, and telling me that "that's not how you land our aeroplanes". I took great delight in pointing out to him that actually it was my aeroplane, and the club was paying me for it by the hour so I'll fly it however I choose. This was also the CFI who didn't like people leaning during taxying, and wondered why they had so many plugs sooting up. But I digress!rodericb said:
I live near a small airport and hear some single engine aircraft go overhead which pretty much let out a mild hum. Flying under VFR I would guess as they're not high up. They are amazingly quiet. I don't think I can hear them when I'm inside.
I'd guess that they're Rotax powered. I've not flown much Rotax engined stuff, but it always seems much quieter on the ground than Continental / Lycoming engined aircraft. I recall the Tecnams that were operating out of Wellesbourne being much quieter than the PA28s. I only flew the Tecnam once - no matter what I did with the seat adjustment my head was almost rubbing the canopy. This is probably great motivation to land very, very smoothly though.
Nigel_O said:
As one particular light aircraft flew overhead, it occurred to me that its engine was pretty much un-silenced. It was obviously a lighweight, as the engine sounded like a couple of angry wasps in a tin can and it took ages to fly past. Loads of other stuff goes past during the day and night and it seems that the lighter the aircraft, the less it is silenced (although that could easily be down to the bigger stuff flying at greater altitude).
Anyway - it got me thinking (better than working...) Given the huge pressures on noise polution for road traffic, why are aircraft allowed to roam around without much, if any silencing?
Microlights on a permit to fly are all silenced and until ( I think) 2019 actually had noise limits (testing was scrapped due to it being largely pointless as they're all pretty quiet and the first person to get annoyed about it being louder than it should be will be the pilot)Anyway - it got me thinking (better than working...) Given the huge pressures on noise polution for road traffic, why are aircraft allowed to roam around without much, if any silencing?
Older 2 stroke stuff is certainly more noticeable, partly because the sound they make is more obvious, partly because they tend to be slower so it's there longer and partly because they are actually just louder.
Slightly heavier stuff tends to be a lot louder due to being from the 70s when nobody really cared, if half the cars you saw were 50 years old you'd think cars were really loud too
To be properly offensively noisy though requires a helicopter.
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