sweair From Sweden, joined Nov 2011, 1758 posts, RR: 0 Posted (1 year 10 months 2 weeks 4 days 8 hours ago) and read 3357 times:
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.
chuchoteur From France, joined Sep 2006, 720 posts, RR: 0 Reply 1, posted (1 year 10 months 2 weeks 4 days 2 hours ago) and read 3221 times:
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...
sweair From Sweden, joined Nov 2011, 1758 posts, RR: 0 Reply 2, posted (1 year 10 months 2 weeks 4 days ago) and read 3160 times:
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?
sweair From Sweden, joined Nov 2011, 1758 posts, RR: 0 Reply 5, posted (1 year 10 months 2 weeks 3 days 4 hours ago) and read 2906 times:
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?