Hello Pull what.
As Machdiamond stated, Vy is not a goal here, just a reference destination. When he writes:
No one said VY had anything to do with noise abatement, only that V2+20 is closer to it than V2+10.
I would add the qualifying statement
"at the same weight" because V2 decays with lower weights.
I should clarify a couple of things. First, this is not about noise
abatement, it's about noise
certification, two very different animals. Certification procedures dictated by the airworthiness folks are
certainly not the nicest way to please the neighbors at all. But they are standardized.
The GIII of which I write was originally noise certificated at its MTOM of 69,700 lbs at V2+10 which is 164 KCAS. The regs allow noise demonstrations to be flown in the window of V2+10 to V2+20 as selected by the applicant.
So, what's my dilemma? For some peculiar, but unrelated reasons, I need to re-certificate the airplane with a lower weight limitation. But I do
NOT want to re-do noise flight testing for two important reasons:
1. It costs way to much.
2. The airplane won't pass based on current regulations.
Back in the '80s, thrust cutback was allowed when measuring
sideline noise level (450 meters laterally from TO path at its noisiest). Even then, it just sqeaked by. Today, thrust cutback isn't allowed until deep into 2nd segment climb for the TO/flyover measurement.
But, surely you say, at lower weight, climb is better and noise is less, no? Not necessarily. Certainly true for TO and approach measurements, but
sideline noise
usually increases. As counterintuitive as that may seem, it is because jet transport noise is measured in an EPNdb scale which includes a
duration of exposure or time element. V2+10 for the GIII, say at 61,500 lbs, is 154 KCAS or ten knots less than the certificated airspeed. FAA/TCCA/EASA/UKCAA/ICAO et al will all suspect a noise increase of 0.1 EPNdb or more given the longer "dwell" time in the fly past.
Any such increase leads to a full scale noise testing program.
What to do? Fly at V2+20 which is now the original airspeed, hence no sideline issue. Hopefully the noise police would agree that represents "no acoustical change" (NAC) and I can move forward.
I guess that's a rather lengthy missive and perhaps more than you cared to read. My post here was to hopefully get some help about climb path performance calculations so that I can determine the TO/flyover altitudes 6500 meters from brake release, using different airspeed and weight combinations. The AFM is unhelpful in that regard, since it focuses primarily on operational V1, V2, Vr speeds and
net climb gradients. Machdiamond has been of great help, but looks like I still need to dig a little deeper.
Being able to figure out the geometry/TO profile the way Gulfstream first flew it (I posted the procedure above) would be a good start.
Felt good to get that written, even if no one reads it.
Bests,
Tom