Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Posted (8 years 7 months 3 weeks 5 days 2 hours ago) and read 6092 times:
I've seen many photos that show probes/sensors inside the intakes of jet engines. They're located ahead of the first stage of compressor blades and are used for sensing inlet pressure (Pt2) for the Pressure Ratio Bleed Control system (PRBC) or they're a temperature sensor for the engine's Fuel Control Unit (FCU), according to a Tech/Ops discussion back in July 2002.
However, I haven't seen a photo before of a jet engine intake probe that has a NACA type inlet directly ahead of it, like the one in this photo of the #4 engine on a 747-4.
MD11Engineer From Germany, joined Oct 2003, 13334 posts, RR: 64 Reply 3, posted (8 years 7 months 3 weeks 5 days 1 hour ago) and read 6013 times:
As Mender stated, this sensor measures T2 (for the EEC). I assume that the NACA inlet like looking structure in front of it acts like a vortex generator to let the air mix thouroughly in front of it to break up the boundary layer, because the inlet cowl can be heated for anti icing purposes, so that you won´t just measure the temperature of the warm boundary layer, but the real temperature of the ambient incoming air.
Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Reply 4, posted (8 years 7 months 3 weeks 5 days 1 hour ago) and read 5984 times:
Thanks for your replies.
OK, so if I understand correctly, the metal probe is a temperature sensor (T12) for the NACA air inlet, which is used to cool an electrical fan, and possibly a generator, or CSD (what does CSD stand for?).
Here's a photo of a probe (without an air inlet ahead of it), in the engine intake of an older B727. It can be seen a the bottom of the intake.
Air2gxs From , joined Dec 1969, posts, RR: Reply 5, posted (8 years 7 months 3 weeks 5 days ago) and read 5957 times:
There is no NACA inlet in the nose cowl. The shape is similar, but it is a diverter (for lack of a better word). It shapes the air around the T12 probe so that the probe measures static air temperature rather than ram air temperature (wind chill). The T12 probe is strictly used by the EEC to help determine the optimum fuel flow for current conditions.
there are components on the engine that use fan air for cooling purposes, but that air is taken from somewhere behind the fan. It is then ducted to the various components or valves as necessary.
CSD is constant speed drive. Essentially a transmission that keeps the generator running at a fixed (constant) speed, regardless of engine speed, for frequency purposes. Fan air is usually used to cool the CSD oil through a CSD air/oil heat exchanger.
Mender From United Kingdom, joined Feb 2004, 224 posts, RR: 0 Reply 6, posted (8 years 7 months 3 weeks 4 days 21 hours ago) and read 5781 times:
A NACA duct is simply a duct designed to draw air into itself without creating a load of turbulence behind the duct. I does this because the divergent part of the duct creates a lower pressure zone into which the surrounding air is drawn. NACA ducts are used on all sorts of aircraft for all sorts of reasons. You can even see them on light aircraft to draw air into the cockpit for ventilation.
GE go to extreme lengths to maximise thrust and minimise fuel burn which is why GE use NACA ducts and other companies such as Rolls Royce simply hang the sensors out in the breeze.
The EEC (electronic engine control) uses T12 temperature and P25 pressure sensors to work out the MASS of air entering the engine. It uses air mass to calculate how much fuel the fuel control unit sprays into the combustion chamber. The fuel control unit is called a HMU on a CF6-80C2 (hydro mechanical unit) Note. T12 and P25 are not the only sensors that the EEC uses.
Incidentally, It's called a T12 sensor because it is fitted in staion (location) 1.2
Station 1.2 is fwd of the LP compressor, station 2.5 is between the LP and HP compressor, T3 is measured at the compressor discharge, T4.9 (EGT) is measured between the HP and LP turbine.
The generator is cooled primarily by a fuel/oil cooler which heats the fuel going to the HMU but it has a air/oil cooler which provides additional cooling at low power settings (low fuel flow). The air/oil cooler takes its air from the bypass duct aft of the fan. Some engines do use air to cool the EEC but I don't recall it being cooled on the CF6-80C2. It is on the RB211
TimT From United States of America, joined Jun 2001, 168 posts, RR: 0 Reply 7, posted (8 years 7 months 3 weeks 1 day 14 hours ago) and read 5653 times:
On a JT-8, that probe (installed at 5:30 facing engine) is a Ps-2 sensor. Basically, it sees ambient air pressure and it's counterpart, Pt-2 is installed at 7:30 and together they provide temperature and pressure sensing to the fuel control. Much the same for high-bypass engines
Mr Spaceman From Canada, joined Mar 2001, 2780 posts, RR: 15 Reply 8, posted (8 years 7 months 2 weeks 5 days ago) and read 5583 times:
I'm sorry for my delay in replying to your answers (I've been very busy lately).
>> MD11Engineer, Air2gxs, Mender, & TimT ......... Thanks guys for explaining that what appears to be a NACA inlet actually isn't, and that it's purpose is to make sure the T12 sensor that's behind it only measures static air temperature and not ram air temp (wind chill).
Thanks for all the other info too, about the CSD (contant speed drive), EEC (electronic engine control), the cooling methods used for the generator, CSD, EEC, etc, etc.