sweair
Topic Author
Posts: 1816
Joined: Sun Nov 20, 2011 9:59 am

The Use Of Chevrons On Planes

Fri Jan 27, 2012 2:51 pm

I read an article about the test Boeing and NASA did in 2005 the QTD2 on a 773 with GE engines. It had chevrons on the nacelle and exhaust cone, the 787 with GE and RR engines only have chevrons on the nacelles and the 747-8 with genx2 has them on both nacelle and exhaust cone. What would be the reason for this? Is there a drawback to have chevrons on the exhaust cone? As genx1 and 2 is almost the same engine it is odd IMO.

Will this go on the 777X?
 
chuchoteur
Posts: 609
Joined: Tue Sep 12, 2006 9:17 pm

The Use Of Chevrons On Planes

Fri Jan 27, 2012 8:45 pm

you have two types of chevrons, they serve different purposes.

Essentially, the chevrons on the engine nacelles are known as "straight" chevrons. They are generally symetrical and flat.
Generally they are used for close-in noise, i.e. to lower engine noise in the cabin.

On the 787 there is a fairly logical use as composites tend to "resonate", so they are probably used to lower the energy hitting the fuselage (and lower the interior noise).

The chevrons on the exhaust (such as used on some A321 aircraft for example) are sometimes assymetrical, and "cut" into the exhaust flow. They are used to mix the air from the exhaust and the bypass fan, in order to reduce external noise (i.e. noise on the ground).

They do lower the engine thrust (through the creation of some drag).
The 787 should be a fairly quiet aircraft in terms of noise on the ground. On the 747, the engines are probably derated somewhat (4 engines vs. 2) and therefore the slight thrust decrease vs external noise benefit makes sense...

(in my opinion)

[Edited 2012-01-27 13:41:45]
 
sweair
Topic Author
Posts: 1816
Joined: Sun Nov 20, 2011 9:59 am

The Use Of Chevrons On Planes

Fri Jan 27, 2012 10:34 pm

Thank you for your answer. In the qtd2 they lowered noise in the frontal cabin by 15dB in the 773 aircraft, but the chevrons only help with noice behid the engine? BTW is this applyable to anyone in the aircraft industry? NASA dont hold patents right?

If this could save some insulation on the 777X maybe it will be possible to widen the cabin like B has said before?
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

The Use Of Chevrons On Planes

Sat Jan 28, 2012 2:34 am

Quoting sweair (Reply 2):
BTW is this applyable to anyone in the aircraft industry?

Yes. It's common on many types, just more visible on the latest ones.

Quoting sweair (Reply 2):
NASA dont hold patents right?

As far as I know, they do not.

Quoting sweair (Reply 2):
If this could save some insulation on the 777X maybe it will be possible to widen the cabin like B has said before?

The sidewall thickness isn't dictated by noise insulation. It's all about frame thickness and thermal insulation (which is really small)...it's about 95% about frame thickness.

Tom.
 
chuchoteur
Posts: 609
Joined: Tue Sep 12, 2006 9:17 pm

The Use Of Chevrons On Planes

Sat Jan 28, 2012 5:27 pm

Quoting sweair (Reply 2):
the chevrons only help with noice behid the engine?

Engine noise in the cabin is a bit more complicated to resolve than just using chevrons (or else everybody would be doing it).

Front end engine noise can sometimes resonate with the structure (for example on the early A320s without the fan frame forward modification, on take-off you'd get a pretty loud grating noise).

There is also a lot of aerodynamic noise coming off the door assembly...

Definitely in terms of cabin liner thickness, insulation is more dimensioning. With noise, you're better off addressing the root cause rather than piling on the acoustic liners  
 
sweair
Topic Author
Posts: 1816
Joined: Sun Nov 20, 2011 9:59 am

The Use Of Chevrons On Planes

Sat Jan 28, 2012 6:43 pm

Quoting chuchoteur (Reply 4):
Definitely in terms of cabin liner thickness, insulation is more dimensioning. With noise, you're better off addressing the root cause rather than piling on the acoustic liners  

Well the qtd2 article mentioned saving 600 pounds of insulation on the 787, I assumed it was because of the chevrons.. They talk about some lip infront of the fan lowering noise with 15dB.

Will we see more future engines with chevrons or less? A350? 777X?
 
chuchoteur
Posts: 609
Joined: Tue Sep 12, 2006 9:17 pm

The Use Of Chevrons On Planes

Sat Jan 28, 2012 7:06 pm

Quoting sweair (Reply 5):
They talk about some lip infront of the fan lowering noise with 15dB.

Will we see more future engines with chevrons or less? A350? 777X?

Inlet acoustics have got better, for example you're now seeing single piece fan nacelle liners (such as on the A380) that reduces any vibration and resonance that occurs at the joints.

You also have some front fan casings that are longer at the bottom than at the top, primarily for noise perceived from the ground.

It really depends, it's a case by case study as to whether the chevrons provide benefits...

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