Forum menu
Kryton - the Jetstream is always there - it’s just the fact that it moves up and down a bit that changes.
The strong Jestream winds over the Atlantic tend to flow from West to East. To avoid the strong headwinds and save fuel when heading to the US, we are normally planned to avoid the jetstream, whereas flying home we aim for the strongest tailwinds.
One of my best flights home from Chicago was right in the middle of a 180mph jetstream carrying me home super fast! Not a bump in sight.
Basically - it will be no different to any other flight.
@dantsw13 - have you been on a bombardier cs300?
Do you know why the engines sound like they are screaming? Swiss use them on the Manchester-Zurich route and they're really annoying. Heading back out there next month again..🙉🙉🙉
… or failing that, go see your doctor and take half a Valium before you get on board. My ex was in the same position as you and successfully got a prescription for like two pills rather than a large pack of them.
I’m imagining OP off his face on pills and free spirits re-enacting ‘The wolf of Wall St’ Lamborghini scene when he gets over to the other side 🙂
Elshalimo - never been on the C series (now known as the Airbus220) but new modern engines do tend to make a more whiney noise.
@dantsw13 - thanks.
They scream like a banshee when they're climbing to cruise altitude and it takes a long time for them to calm down whilst cruising. I was really worried the first time I flew on one!!
I'm a nervous flyer and they also seem very manoeuvrable, or maybe the Swiss pilots just like throwing them around?
The bombardier utilises P&W's latest geared turbofan engines. Engine noise comes from a number of sources - buzzsaw from the fan, usualy that raspy sound you hear during initial climb when fan speed is high and air density is also high and the blade tips are going transonic, turbine noise and bleed valve noise: at low altitudes the compressors are pulling in too much air so some of it needs to be dumped overboard so bleed valves are open during take off and most of the climb.
In order to achieve the significant strides in fuel efficiency engines have in the last 20 years they have become much higher in their bypass ratio - that is why the fan diameter has increased so much over the years. The larger the fan the slower the turbine (speed limited to fan blade tip speed), the lower the turbine noise. However if you slow down the turbine too much as you grow the fan size you loose turbine efficiency and negate the fuel efficiency benefit of the higher bypass ratio and larger diameter fan.
So the new P&W geared fan features a reduction gearbox enabling the turbine to spin much faster while the fan can turn much more slowly. A good portion of the turbine whine noise will be the faster spinning turbine and probably some bleed valve noise as the faster turbine will require more air to be dumped from the compressor at take off and climb. Remember the old 737-200's with their turbojet engines? they whined like a banshee as they were turning much faster than modern day high bypass ratio engines. In the strive for better fuel efficiency we're returning to faster spinning turbines.
The screaming sound is good...turbines like to scream. Its when they don't scream you have to worry!
I do wonder whether it’s the noise muffling technology, to reduce noise footprints around airports, leaves behind that high pitch.
No muffling technology. Muffles are heavy, draggy and sap efficiency. Primary engine noise reduction is done by optimisation of the compressor and turbine....remember the old WW2 air raid sirens? That is an example of the opposite of design we're trying to achieve with gas turbine engines - so the number of rotating turbine and compressor blades vs.the static turbine and compressor vanes impacts noise massively due to the interactions of the gas flow within the engine, also the shape of the ducting within the core and a lot around combustor design. The rest is around the nacelle design to shield noise utilising noise suppression materials and ensuring the nacelle covers areas where turbine and compressor noise can escape - like the bleed valves. That is why 777's with GE engines and 787's with GEnx engines make such a racket on start up - they have very long engine cores and the nacelle doesn't do such a good job of covering up the bleed valve noise. You can't extend the nacelle as that adds weight and drag...so its the classic engineering quandary of balancing competing attributes - noise, fuel efficiency, weight etc.
Ultimately though engines are much quieter than they used to be they're always going to generate some noise. And though there are strict regulations around how much noise the engines generate externally there are none to regulate the noise you hear in the cabin. You can have an engine installation that is pretty quiet externally, but noisy as hell in the cabin - thought a lot of this has to do with the design of the fuselage of the aircraft and the attention to cabin noise the airframe makers invest.
Ultimately though apart from take off the majority of the noise from an aircraft is from the airframe and not the engine. Flaps and undercarridge makes a lot more noise than engines on landing.
Wobbli - you’re obviously far more technically minded than me!!!!
And this ladies and gents is why we love STW so much. sometimes.
Although the pilot just confessed to not being technical (mild exaggeration) 😉
By the way, the CS300/A220 is a nice plane to fly on in economy.
Roomy, comfy and Swiss always give you free chocolate!!
Wobbli – you’re obviously far more technically minded than me!!!!
Not at all, it's just my job and I have access to the real clever bods who design them. I talk to pilots and airlines alot time about engine technology and how they can operate engines to be more mechanically sympathetic and eek out more fuel performance for their airline accountants. The real geniuses are the guys and girls actually designing this stuff and their intention is to make them invisible to the pilot - all you want is thrust when you demand it and that's what we're trying to achieve. It really is on the edge of physics and our technical understanding. A gas turbine is a classic example of something that really is such a simple thing that has been made extremely complicated in the eternal pursuit of efficiency. I'm just a failed engineer who loves to talk about engines.
Hopefully Kryton has a good flight today .
*Like*
I questioned a united airlines that was going to San Fran from Heathrow. The reason being his flight plan was very unusual because he was flying out over East Anglia towards norway. Normally it's via Scotland.
I'm paraphrasing but his reply was, "some smart ass in flight planning filed the route for us. We doubled checked it but they're right. Because of the winds they've saved us 1.5hrs!"
Track miles it was longer to go via Norway and the pole but there was a hoofing southerly wind up over norway.
Anyway, I'll put my anorak back on.
Just to clarify regarding engines:
In the Trent 700s on my current steed (and like their 1970s predecessor, the magnificent RB211s on a previous steed) we have acoustic lining in the intakes to reduce the noise of the compressors. Acoustic lining is present in most, if not all modern airliner intakes. It looks like the stuff in the old style phone booths with lots of holes in it. It deadens some of the high pitched sounds coming from the compressor.
Pretty much all engines will have their 1st stage RPM limited by fan tip speed (as do turboprops) to avoid any supersonic effects. Part of an intake's function is to reduce airflow rates so the engine is always using subsonic air (think concorde intake ramps). Small engines will have similar tip speed as to, say the Trent, which is large, but the fact the RPM is so much higher will result in higher frequencies, hence the higher whine. There's no getting round that one.
Regarding airflow and bleeds to dump any excess: Any "excess" simply goes down the bypass and shields the exhaust gases from the core, reducing the shear noise, the roar that all jets make. They do tend to have variable inlet guide vanes (IGV) which control the amount of air going into the core, but there are no 'bleed valves' as such in normal (non-starting) operation (as there can be bleed valves used there), other than compressor bleeds but those are for pneumatic systems rather than flow control.
Fighters sometimes have flaps near the intake; these open to at slow speed to allow higher mass flow rates, but close at normal speeds, but these don't 'bleed' air out.
The latest Trents have a jagged rear on the bypass duct; this induces turbulent airflow, reducing the shear experienced by the air and therefore noise.
It's been a very very long time since airliners have been using Turbojet engines; they were low bypass turbofans in the early days (727s) and noisy, and became higher bypass as technology improved (L-1011s).
Don't start me on Geared Turbofans! Nothing wrong with a Triple Spool High Bypass engine!
Well the original question was around why the P&W GTF on the Bombardier was so noisy. Whilst the bleed valves dump excess air into the fan duct that air is very noisy - its very high pressure being squirted through a valve, and when the bleed valve is open you also get alot of noise from the actual compressor leaking out which is hard to suppress. The engine nacelle does suppress most of this noise but some are better than others at it. The RR engines are very short in length due to their 3-shaft architecture so the bleed valves are nestled well within the engine nacelle so do a good job of suppressing noise. Engines with 2-shaft architectures are much longer and the bleed valves tend to be positioned closer to the exit of the fan duct therefore the noise suppression isn't as good. The P&W GTF has a faster spinning core which will generate more noise to begin with anyway so the engine nacelle has a tougher job dealing with that so why that engine will probably sound like it does. Never flown on one so can't comment. But I'm not sure how good the Boeing 787 Chevron system is (it's a Boeing design not an RR or GE design). They don't have it on the 777X and didn't have it on their proposed NMA and the 737MAX doesn't have it either, so either it is a design that has to be considered from day 1 along with the whole system to get the benefits or it isn't very effective at all. The old RB211-524 had the pastry cutter on the core exhaust which attempts to do the same thing as the Boeing chevrons. Effective but at a sizable weight penalty.
But I'm a 'Trent man' too and triple spool is the business....but if us in the aerospace industry truly want a sustainable future we have to strive to continue and accelerate the trend from the past 50 years in continual fuel efficiency improvements, and the only way we're going to do that is to increase fan diameter. On the latest big Trents the LP Turbine (which drives the fan) is already getting so big and heavy it's getting hard for the airframers to physically design wings and pylons to take them. If we want to reduce the size and efficiency of the turbines while increasing the fan size and bypass ratio then unfortunately even the 3-shaft architecture has reached the end of the road. We're at bypass ratio's of 10:1 on the latest engines (over double that of the Trent 700 engines) and we need to be heading to bypass ratio's of 15:1 or more for the next generation of engine to get the efficiencies we need. On the big wide body engines that means fan diameters of around 135 inches vs the circa 97 inch of the mighty Trent 700.
Hopefully Kryton checks back in to this thread before his flight - should certainly help him sleep!! 😎
Hopefully Kryton checks back in to this thread before his flight – should certainly help him sleep!! 😎
Are you saying this isn’t riveting??!
Well...........
Hopefully wandering around downtown with an Oriels hat on.
It started well. I got a CBT session Thursday night which bolstered my confidence then checked in Friday afternoon to get a free upgrade to a Premium bulkhead seat...
...then woke at 3am Saturday ejecting fluids from every orifice and haven’t eaten since. Currently at home with a fever and even my skin & hair hurts. Will be seeing a Dr tomorrow.
Flights have been setting speed records US-UK
Feel for you Kryton, anxiety is a ****
Sorry to hear that Kryton. Get well soon.
I think you need to change the thread title to Gastroenterologists of STW - help please!!
Speedy recovery...
Wobbliscott is absolutely right in what he says. There are multiple bleed valves besides those used to tap off IP and HP air for anti-ice and air conditioning (if not on a 787).
The Trent 1000 has at least 10. Rolls Royce state that "during engine operation certain bleed valves will open or close at pre-determined shaft speeds to assist engine handling [and] control the volume of air flowing through the engine compressor systems".
I think you need to change the thread title to Gastroenterologists of STW – help please!!
I’m not one to go off sick easily but still unable to keep food down, I went to the docs today and got diagnosed with viral gastro, a mild case of flu on top and fatigue. Signed me off for a week and told me to rest and spend the next 2 days on water only.
Incredibly bad timing for the trip as there’s some important stuff going on at work and all I can do is sit around bored near the bathroom with my headache and hurty skin.
Anyway, I quite like the reading about engines in the short bursts I can manage, thumbs up and thanks for the get wells.
Flaperon
There are multiple bleed valves besides those used to tap off IP and HP air for anti-ice and air conditioning
I didn’t say otherwise, however I accept I don’t know about the P&W setup, and indeed there are bleed tap-offs on the 700 to manage core flow for stable operation..... I stand corrected (I thought it was the function of the IGVs)!
You're both right, it is indeed the primary function of the variable inlet guide vanes (VIGV's) to manage the compressor handling, but at low altitudes where the air is dense bleed valves are needed on all gas turbine engines to prevent compressor stall and surge. Engines are not optimised or sized for their maximum thrust at take off...they're optimised for cruise as that is where they spend most of their working life. So at take off you're just working the machine harder and you can't build a machine that is optimised for two points of its operating envelope so you have to make compromises and bleed valves are just that.
The RB211-535 (which powered the 757) actually dispensed with VIGV's altogether and managed compressor handling purely from bleed valves (well it was actually a circular band) and did away with the complexity of the VIGV system altogether at the expense of a bit of efficiency - you ideally don't want to dump air from the compressors as you've invested expensive fuel to compress the air only to dump it overboard without earning you any revenue. But complexity and reliability are important attributes as well. No good having a super efficient engine if it's always going tech and causing delays and cancellations. This was one of the secrets of the 535's unbelievable reliability back in its day, which is still impressive today and was orders of magnitude better than engines of its generation.
But modern engines use pressurised fuel to actuate the VIGV mechanism and electronic control systems to manage control, which is far more reliable than the old and problematic pneumatic systems of previous generations of engine like the 535, so VIGV system reliability is no longer a problem.
But yes, though the bleed valves are only open for take off and part of the climb phase air is being siphoned from the compressors all the time for various things - anti ice, turbine cooling, turbine blade clearance control, engine sealing, cabin air (i.e. the air you breath comes off the engine compressors)...and the designers are always looking for ways to make the air siphoned off work harder to minimise it's use and eek out more and more efficiency from the engine - continually chasing fractions of a percent of efficiency.
Kryton - sorry to hear about your bought of illness. I hope it wasn't brought on by concerns about your up and coming flight.
It's all getting very techy on the engine front. As Kryton is spending some time on the throne, my contribution to this thread is to point out that when you flush the toilet, it's the low pressure outside that sucks the contents out into the tank. Apparently when the aircraft is on the ground (and also presumably at low altitude) a pump is used to create a vacuum in the tank. I was going to say compressor, but what's the opposite of a compressor?
Expressor?
On the basis the fundamental workings of a jet engine are suck, squeeze, bang, blow (though technically not correct but a close enough approximation), the compressor part is squeeze, so the opposite of the compressor bit is the blow bit, which is the turbine. Which is fitting as that part on the throne can be very hot too, especially after a good Jalfrezi.
Please lord, won't anybody think of Kryton? 🙁