sc_neo Posted August 13, 2017 Posted August 13, 2017 Hey guys, so how does the Gazelle's Turbomeca gas turbine settle on exactly 43500rpm and 387rpm rotor speed? Does it somehow measure the rpm from the drive shaft and then up/downs the fuel flow, which kinda sounds not very precise or does it somehow put ''the brakes'' on the drive shaft to keep it around 43500rpm?
FragBum Posted August 13, 2017 Posted August 13, 2017 Hey guys, so how does the Gazelle's Turbomeca gas turbine settle on exactly 43500rpm and 387rpm rotor speed? Does it somehow measure the rpm from the drive shaft and then up/downs the fuel flow, which kinda sounds not very precise or does it somehow put ''the brakes'' on the drive shaft to keep it around 43500rpm? Governor and control system and yes it controls fuel input, which is why you need to manage load on the turbine and exhaust gas temperature. Control is an illusion which usually shatters at the least expected moment. Gazelle Mini-gun version is endorphins with rotors. See above. Currently rolling with a Asus Z390 Prime, 9600K, 32GB RAM, SSD, 2080Ti and Windows 10Pro, Rift CV1. bu0836x and Scratch Built Pedals, Collective and Cyclic.
m.a.hristozov Posted August 13, 2017 Posted August 13, 2017 As stated above, altering the fuel flow is the primary way of changing the RPM of a gas turbine. The fuel control unit and/or governor monitors various engine parameters and adjusts fuel flow to maintain desired RPM. These systems also compensate for changes in the atmospheric conditions such as air density, temperature and air pressure.
joey45 Posted August 13, 2017 Posted August 13, 2017 It's actually done by witchcraft. The only way to make sense out of change is to plunge into it, move with it, and join the dance. "Me, the 13th Duke of Wybourne, here on the ED forums at 3 'o' clock in the morning, with my reputation. Are they mad.." https://ko-fi.com/joey45
FragBum Posted August 13, 2017 Posted August 13, 2017 It's actually done by witchcraft. :megalol: Control is an illusion which usually shatters at the least expected moment. Gazelle Mini-gun version is endorphins with rotors. See above. Currently rolling with a Asus Z390 Prime, 9600K, 32GB RAM, SSD, 2080Ti and Windows 10Pro, Rift CV1. bu0836x and Scratch Built Pedals, Collective and Cyclic.
Ramsay Posted August 13, 2017 Posted August 13, 2017 (edited) This :- Does it somehow measure the rpm from the drive shaft and then up/downs the fuel flow, which kinda sounds not very precise The engine has a governor to control fuel flow so speed is constant, whatever the load. or does it somehow put ''the brakes'' on the drive shaft to keep it around 43500rpm? As well as providing 'power', the engine is a big air compressor (+150 psi) - compressing the air requires power, this 'load' is used to stop the engine overspeeding at idle/off load. The Fuel Control lever in the cockpit controls the governor's speed setting. Edited August 13, 2017 by Ramsay i9 9900K @4.8GHz, 64GB DDR4, RTX4070 12GB, 1+2TB NVMe, 6+4TB HD, 4+1TB SSD, Winwing Orion 2 F-15EX Throttle + F-16EX Stick, TPR Pedals, TIR5, Win 11 Pro x64, Odyssey G93SC 5120X1440
Len62 Posted August 13, 2017 Posted August 13, 2017 Although I had a basic understanding about helicopters this thread, along with the videos, has furthered my understanding. Thanks!
sc_neo Posted September 1, 2017 Author Posted September 1, 2017 totally forgot about this thread, thx for the info. Especially the last graphic @Ramsay. Is that specific for the Gazelles turbine? Very informative. I don't quite grasp why there is this sudden spike in airspeed in the exhaust gas section after it has long past the turbine. Especially with no AB and only military power, why do we see a jump from 600 to almost 2000f/sec? Is that small narrowing we see at the end doing this?
Ramsay Posted September 1, 2017 Posted September 1, 2017 (edited) ... thx for the info. Especially the last graphic @Ramsay. Is that specific for the Gazelles turbine? Very informative. No, for simplicity - it's just a generic diagram. This is a Turbomeca Astazou II Astazou II : One axial compressor stage plus one centrifugal, annular combustion chamber, three-stage turbine The Gazelle has an Astazou XIV : Two axial compressor stages plus one centrifugal stage, annular combustion chamber, three-stage turbine. Gearbox integrally mounted on the front of the engine FragBum's linked video shows how contorted the gas flow can be in a compact helicopter engine. I don't quite grasp why there is this sudden spike in airspeed in the exhaust gas section after it has long past the turbine. Especially with no AB and only military power, why do we see a jump from 600 to almost 2000f/sec? Is that small narrowing we see at the end doing this? I assume it's the reducing diameter of the exhaust nozzle, but it is a generic diagram. This is a another generic diagram, this time of a non-after burning engine without a exhaust restriction or velocity spike. However the Gazelle uses the small Astazou XIV Turboprop for it's high power to weight ratio, to drive the shaft that comes out of the front of the engine, not the exhaust that comes out of the back i.e. any residual exhaust gas velocity is wasted power. Edited September 1, 2017 by Ramsay i9 9900K @4.8GHz, 64GB DDR4, RTX4070 12GB, 1+2TB NVMe, 6+4TB HD, 4+1TB SSD, Winwing Orion 2 F-15EX Throttle + F-16EX Stick, TPR Pedals, TIR5, Win 11 Pro x64, Odyssey G93SC 5120X1440
AlphaOneSix Posted September 1, 2017 Posted September 1, 2017 Also note that the compressor/gas producer/N1 RPM is not static, the number in the specs is at some specific power setting. It's the N2/power turbine/free turbine RPM (and thus the rotor RPM) that is important to keep in a relatively narrow range or at a specific RPM. The governor/fuel control on the engine will maintain the engine's N1 RPM at whatever level is required to maintain N2/rotor RPM at the proper speed by adding or restricting fuel flow.
Nerd1000 Posted September 2, 2017 Posted September 2, 2017 Also note that the compressor/gas producer/N1 RPM is not static, the number in the specs is at some specific power setting. It's the N2/power turbine/free turbine RPM (and thus the rotor RPM) that is important to keep in a relatively narrow range or at a specific RPM. The governor/fuel control on the engine will maintain the engine's N1 RPM at whatever level is required to maintain N2/rotor RPM at the proper speed by adding or restricting fuel flow. In the Ka-50 (and probably the Gazelle too) you can hear this happening. As you increase collective the sound of the engines becomes more more high pitched even though the rotors maintain the same RPM. The reason is of course that the governor is allowing the gas generator section of the engine to spin faster, supplying more hot exhaust to the power turbine and thus more torque to spin the rotors :thumbup:.
Weegie Posted September 3, 2017 Posted September 3, 2017 An interesting, if slightly off topic solution to the problem of balancing the N1/N2 revolutions, is to alter the energy transfer between them. This was done by altering the area of the second stage nozzle, it may have been used on other machines but GE is the only company I know of who utilized it on some industrial machine designs. What they had was an existing single shaft generator drive design so the compressor had a narrow RPM window due to its characteristics. What they wanted to do was to alter the design to drive a load compressor for gas & oil pumping applications, but the load compressor needed a wider RPM window than they could safely run the axial flow compressor to. This is what they came up with, a 2 shaft machine with the turbines mechanically separate but aerodynamically linked, sort of like a fluid coupling but the fluid was hot gas. By opening or closing the 2nd stage nozzle the energy transfer could be altered (a little), close the nozzle and more energy went to the N2 shaft driving the load compressor. Open it and more energy went to the N1 shaft driving the GT's axial compressor. The control loop was based on N2 RPM, controlling fuel flow and N1 RPM driving nozzle position. You can see the mechanism to drive the nozzle in this picture, the GT exhaust is nearest to us in the picture Apologies if off topic, I just thought it might interest some
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