Aircraft Engine Tests - Questions

studentflyer · Sat Oct 30, 2004 8:36 am

Hi all, I have recently noticed that new engines being tested for new aircraft uses older test aircrafts, such as the GE90-115B done on a 747-121, and RR Trent 500 & 900 on a 340-311.

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Now, my question is: Why are these engines placed in engine no. 2 position? I know the wing fairings are stronger near the fuselage, as they are near the centre-wing-box. But, why don't they place it in Engine no. 3 position? Has anyone seen engine tests that are placed in engine no. 3 position?

Also, does anyone know which engine was tested in this similar manner?

Sorry if this topic has been asked before, but I couldn't find it in the search.

Thanks,
AK

air2gxs · Sat Oct 30, 2004 8:44 am

An inboard engine is used in order to minimize the moment-arm of the engine being tested. Imagine a GE90 producing full thrust while installed in an outboard position without the same thrust being produced on the other wing.

Once you modify an aircraft to do this type of task, you'll use the same aircraft. So, the #2 position will be used on that particular test-bed. Why mess witht he other positions?

Why is the #2 position chosen over the #3? Who knows. Tradition, maybe?

Santhosh · Sat Oct 30, 2004 9:32 am

Air2gxs

Sir how is the vibration in N1 and N2 measured? Dose they use accelerometers or other kind of transducers to sense this. Also what units are used to represent the Vibration in engines and what values does it usually reach?

George

air2gxs · Sat Oct 30, 2004 11:38 pm

I can only speak for a few engines, but N1 is usually measured with a transducer that either counts fan blades (JT9D) or counts "teeth" on the fan shaft (CF6 & CFM). N2 is usually from a small tach generator mounted to the gearbox.

Vibration is measured using accelerometers. Fromthe B767 AMM:

With engine operating, the engine accelerometers generate signals proportional to engine motion in a radial direction. These signals are received by the AVM signal conditioner, where they are converted to signals suitable for the EICAS computer. Signals are then sent to the display unit from the computer where vibration displacement or velocity is read in units.

Santhosh · Sun Oct 31, 2004 3:34 am

Sir thankyou very much for the information regarding vibration and its measurment. Sir could you please tell me what is the difference between the CF6 and CFM engines?

dl757md · Sun Oct 31, 2004 3:53 am

how is the vibration in N1 and N2 measured? Dose they use accelerometers or other kind of transducers to sense this. Also what units are used to represent the Vibration in engines and what values does it usually reach?
The accelerometers are usually a piezoelectric type. Here's a link about how piezoelectric accelerometers work. http://www.sensorsmag.com/articles/0204/27/main.shtml
The accelerometers are mounted either on a bearing housing inside the engine or on the fan case, depending on the engine. The signals are processed in the AVM (rotational speed is factored in) and sent to the EICAS computer which displays them on the EICAS display in the cockpit. Most of the displays I've seen max out at 4. This scale is arbitrary in that is there is no quantifiable unit. It is a reference. The AMM will tell you what readings you have to reach before the vibration has to addressed. Generally anything above a 2 needs to be rectified. There's not much you can do to fix N2 vibes without an overhaul but you can balance the N1 fan on the wing.

Dl757Md

air2gxs · Sun Oct 31, 2004 6:05 am

Getting into a lot of technical stuff between the CF6 & CFM(56). CF6 is a large, high by-pass fan engine mounted on a variety of aircraft including B767, B747, MD11, etc. The CFM is a smaller high by-pass fan mounted on aircraft such as DC-8-7x and B737-300 & above I believe.

studentflyer · Sun Oct 31, 2004 9:22 am

Also, CF6 is made by General Electric, whereas the CFM is a joint project between General Electric and SNECMA Moteurs. CFM56s are also the only engine used in Airbus A340-200 and 300 if I'm not mistaken.

Anyway guys, thank you so much for the reply to this thread. It has certainly increased my knowledge Smile

. Thanks Air2gxs, and Dl757Md. To Santosh, thank you for asking such a question. It has certainly helped me with a lot more information.

Regards,
AK

Klaus · Tue Nov 02, 2004 1:26 am

I guess the reason for the #2 position could due to logistics: Even a flying testbed will have ballast and test equipment to be loaded and unloaded; Obstructing the cargo access doors on the starboard side either by the big engine itself or by the ground support or maintenance infrastructure might be impractical...

oly720man · Tue Nov 02, 2004 1:47 am

Pratt's B720 (N720PW) has the #3 position

From here
http://www.pw.utc.com/pr_082100.asp

studentflyer · Tue Nov 02, 2004 10:29 am

Oly720man,

Thanks for providing the picture of the B720 with engine tested at #3. Now I know that engine test does not necessarily be at position #2. Thanks for the picture Smile

Welcome to my RR list Smile

2H4 · Tue Nov 02, 2004 11:41 am

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2H4

Buzz · Tue Nov 02, 2004 10:14 pm

Hi StudentFlyer, Buzz here. And maybe it's tradition. On the 747's there are hard points to bolt a 5th pod between the #2 engine and fuselage.
Back when the 747's were new, somebody was wondering how to haul an extra engine to a sick airplane. Sure, you could try to find a large cargo plane, or maybe you can bolt one on... So the 5th pod (4th pod on a DC-10, and i don't know L-1011's very well) was developed.
There's a short pylon, and in the kit there's a dome to cover the core of the engine. Fan blades are removed and stowed in a box, carried as freight.
Anyway, i'm thinking it's tradition.
g'day

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