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View Full Version : “There are more things in Heaven and Earth, Horatio, than are dreamt of...


fdr
15th Nov 2022, 00:49
Aviation is a "mature" technological activity. What we do everyday is dependent on very simple, well established R&D, application and approvals. Not much changes, and we become moribund. We teach half truths and approximations, as they are good enough. We make assumptions and teach that at truths, as that is what we were told way back when. It's a mature industry.

Occasionally oddities arise.

Here is a photo of a takeoff run with two engines producing the same thrust, and behaving rather differently. The one on the right hand side is just the usual blender. The one on the left is proving some physics that seems to have been lost in translation. This is at low speed, when this effect is the least interesting outcome. Cruise, it is more than double that.

Question for entertainment;

does this break thermodynamic law? If not, why not?
Why does a CFM56, PW4060, GE90 etc have a Thrust Specific Fuel Consumption, TSFC that is really really good parked at the end of the runway, and yet burns twice as much fuel for a pound of thrust in the cruise, a point of more than curiosity, as a pure jet, J-85/CJ-60 doesn't... And when that is guessed, have another look at the photo below....


Anyone with an idea of what this does, feel free to PM me... :}

This is indeed strange behaviour from a "major kitchen appliance..."

For those needing to visit Specsavers, the left engine is N1: 91.8 EGT 839 (it has a measured offset of 25C at this point, it is actually 814C)
N2 of 96.2%, fuel flow of 1390pph. The Right engine is as born, N1: 97.0%, EGT 884C, N2: 98.4% fuel flow of 1675pph.

No bleed... no defects... the engines matched before this little test, other than the 25C offset. The video gives proof that the thrust is equal on these two engines.

So, to all rocket scientists, fleet managers, accountant types, in the vernacular,
"' ullo 'ullo 'ullo, wot's going on 'ere then..."

(hint: Schroedinger's cat is alive and well, I think living with Special Kherson Cat).

How much gas does the A380 burn in cruise?

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/606x1280/whatsapp_image_2022_11_14_at_15_58_23_e6391e797fb75d99892955 4141ded038f75fbb12.jpeg

BizJetJock
15th Nov 2022, 08:08
How are you measuring that they are producing the same thrust? There would not be much rudder required to compensate for this, so it could easily feel normal to the handling pilot.

TURIN
15th Nov 2022, 08:15
The right hand fuel tank is nearly empty, sucking in loads of water from the bottom of the tank, the FCU is compensating for the lack of power by upping the fuel flow. 😁 😁 😁

fdr
15th Nov 2022, 09:01
The right hand fuel tank is nearly empty, sucking in loads of water from the bottom of the tank, the FCU is compensating for the lack of power by upping the fuel flow. 😁 😁 😁

I'm burning gas out to do a W&B. I hate tossing gas out. Lateral balance is a factor for tracking on the ground. For this aircraft, a 1,000lb imbalance is quite unpleasant for the roll, but it acts to give a yaw into the heavy side. Ask how I know what a 1,000lb imbalance can cause.:}

fdr
15th Nov 2022, 09:13
How are you measuring that they are producing the same thrust? There would not be much rudder required to compensate for this, so it could easily feel normal to the handling pilot.

Ran a load cell from a yoke to the gear to determine the thrust change in this particular case, however, previously ran the engine in a test cell in April 2013 and got a whole lot better at 100% N1. The 100% is above the rated thrust limit, the FCU had sufficient authority, and the fuel pump had as well, to achieve that outcome. The outcome in this photo is a mild change. If you get bored, PM for a reading list to see what the CL, CD, CM, and L/D changes that can be achieved are, they are quite entertaining. This is an application of something that has gone well past the time that it should have been done. Apparently no one else was interested.

I started on propellers, and the very first T/O got a shift of this level. I was doing aero mods in other areas, and doing prop mods is not to be done without a reasonable amount of care. 20% thrust is actually very obvious. In this case in the photo, the engines are putting out the same thrust, Prior runs with matched N1s gave a constant demand to track straight. Airborne is easier to determine balance. Answering a question whether lift was altered by a wing mod, the test aircraft in that case was able to exhibit lift differences of 3 lbs at the wingtips, was a lot of fun to do that one, although did end up doing some vertical reversals due to some grumpy military airspace down range of the runs. Comparative shifts from the OPC are pretty straight forward, and in the right area of the world at least we can avoid Coriolis, geometric altitude gradients and all of the irritation that they cause.