What's this odd engine?

[Self Loading Freight]

If you look at this page, Pan Am advert (http://www.otisfodder.com/365days/archive/078.html) you'll see an ancient PanAm photo,



http://www.otisfodder.com/365days/archive/images/078.jpg

What is that strange device strapped to the back of the engine?

R

(PS -- the audio on that page is pretty fun, and the site as a whole is stuffed full of high quality weirdness. But mostly, I just want to know about that odd thing)

[calltheball]

I've been wrong before but it looks like an early version of a hushkit designed to make the engine sound quieter.

[Fresca]

This is a Hush Kit.

This one works by breaking up the jet stream into many smaller ones, thereby increasing the frequency of the sound it makes. The higher the frequency the shorter distance the sound travels, making it quieter when heard from the ground.

Maybe an engineer can give you a better answer.

One note (if I remember correctly): If one doubbles the speed of the jet stream the the sound increases fivefold. Hence the much quieter bypass engines.

[lomapaseo]

Back when I worked with the very first one of these in 1/4 scale wind tunnel testing in 1957 we called them silencers not hush kits :O

I always understood the engineers to be increasing the surface area of the shear zone between the high speed jet exhaust and the lower speed slip stream in other words better mixing.

[Self Loading Freight]

Thanks for the swift replies.

So, what happened to them, and when did they fall out of fashion? Can't help but think a fair amount of the puff out the back would be reflected into the fans causing non-optimal fuel to forward motion conversion.

Silencers would be nice!

R

[411A]

Yep, daisy type silencer....more than likely on the back end of a Pratt&Whitney JT4A...lots of thrust, lots of noise, and sucked up fuel like a Hoover....but very good performance at high altitudes.

OTOH, airfield performance...not so good at heavier weights.

[compressor stall]

They have not really fallen out of fashion - have a look up the clacker of a 744 engine - you will see a very curved loopy piece of metal designed to increase the surface area between the bypass air and the jet efflux. The greater the surface area, the less friction between the two air masses (one travelling faster than the other) therefore the quieter it will be.

I have a great photo of one somewhere...if noone else can link to one easily, I'll rummage in the attic for it and scan it in and email it to you.

CS

[Notso Fantastic]

It's a mixer to mix the pure jet exhaust into the airstream and hopefully reduce noise levels (not very well). As bypass engines came in, they helped integrate the hot jet exhaust into the cold bypass air. As modern engines have such high bypass ratios, what happens is the hot exhaust stays in a core within a shroud or tube of cool bypass air straight off the compressor fan, helping to prevent the shearing 'shock' of jet exhaust blasting straight into free air. That is why modern engines are so much quieter! Bring back the VC10 Rolls Royce Conways. Even the B737-200- when they take off you really know it!

[gas path]

Take a look HERE (http://www.aircraftenginedesign.com/pictures/RB211-524.gif) to see a modern day version. The cold stream duct extends past the 'cookie cutter' by a couple of feet. As has already been pointed out the idea is to increase the surface area of the shear force between the (fast moving) hot steam from the core and the (slower) cold stream from the fan.
I'm with Notso here, I say bring back the crackle of the Conway on an early morning departure, folks don't know what they're missing.:ok:

[Notso Fantastic]

We don't have them on RB211-524s, and to think about it, I don't really recall seeing anybody with them. They do cost a percent or two in fuel efficiency, and the market for modern turbofans is so competitive that the manufacturers don't seem to feel they want to compromise the fuel efficiency of their babies for the minor noise improvement offered by these mixers. I used to watch the 727s with these things taking off- if there was any noise 'improvement', I couldn't notice it. I suspect they may be slanted towards improving noise at particular frequencies only- I heard that noise monitoring was/is restricted to special frequencies only (I may be quite wrong), so their use may be more academic than practical.

[PAXboy]

NotSo - I think that's the case (about frequencies). Another thread (forget which forum) mentions that Airbus adjusted an aspect of the wing (A340, I think) to reduce noise levels on approach, so as to meet airport noise criteria.

However, the impact is that the machine burns a little more fuel than it might otherwise do. The Airbus wing is reputed to be super efficient and so they might be able to sacrifice the odd bit.

I recall discussing this with a pilot on a VS A340-200 whilst over the Sahara. Needless to say, that was in 2000. He commented that the wing was so good at flying that slowing down could be a problem and one had to get the speed down in good time on approach.

Early this year, SAA were taking delivery of their first 340-600 and seemed to learn about the desire of the wing to fly. Consider the hapless SAA capt delivering the machine at Johannesburg International. Press all lined up, cameras going as this beautiful machine arrives and ... he touches down and baloons.

SAA manged to pursuade the TV news not to run the film and they staged a 'training flight' the next day.

[gas path]

Notso... Take a peek up the ar$e end of a RR powered 744.

[Notso Fantastic]

Of course- there are small internal ones hidden away up there- my apologies! Bit like having large lumps hidden away up ones colon rather than on display (if you pardon the expression- and the thought). Thankfully I've reached the stage where I can 'delegate' the walkaround to others (especially when it's raining). Oh the joys of seniority in the airline business!

[Dan Winterland]

Notso, you don't have to bring back the RR Conway powered VC10. I live next door to Brize Norton and I can assure you that they are still very much around. Good job I'm deaf.

Pardon..........?

[Notso Fantastic]

Aye I love that crackle of a VC10 taking off first thing in the morning, or a Tornado/Lightning on full reheat! You can feel the base of your stomach vibrating, the ground shaking, the cows rejecting their calfs. Bloody greens are stopping all that these days. All you can hear now is the crescendo of trucks.......and kids with their tweaked motorcycle exhausts

[Self Loading Freight]

I can remember a family trip to Athens in the early 1970s (Clarkson and Dan Air, I suspect), where the Olympic planes were much, much noisier than anything else. Squeeky Greekies, my father called them. As I was still squeaking myself at the time (didn't hit double figures until the middle of that decade), I can't remember anything about them apart from that blaaardy great noise. Fantastic.

What might they have been, and why were they so much noisier?

R

[phnuff]

I remember living in Ealing and hearing Tridents taking off. Gee, they were noisy. Which engines did they have ??

[Notso Fantastic]

The 'Rodent', or to give it its other name, the 'Gripper' (as in 'Groundgripper') had Speys- very effective at turning fuel into noise. Being underpowered, climb angles were not large and the presence of the aeroplane could be very effectively shared! The BAC-111 had 2 Speys and could produce an earth shattering take-off. More than a few times I thought Concorde was taking off from Gatwick!

[Onan the Clumsy]

I've seen these on an Airfix kit I built a long time ago and I think they're still on the odd 727 here and there.

737s have a similar device, but as has been pointed out, it's a little harder to see it as they sit inside. I believe SWA had to put 'em on the 200s to get 'em in and out of DAL.

[ramsrc]

SLF

Early 1970's, Athens Airport, Olympic.

Probably their Boeing 720's. Powered by the P&W JT3D, (as was the 707), but less powerful and without water injection. Wonderful things, enough noise and smoke to make a NIMBY weep!

Unfortunately, I don't know why these seemed noisier than anything else that was around. Perhaps just because they had four engines?!?

Any memories as to what Aircraft you flew to Athens on? Perhaps the Comet?