Jet aircraft obstacle clearance, from Dole, figure 3.21
Choosing climb speed to get from low to high as quickly as possible is a chore we face just about every flight. The answer varies with weight, temperature, and even with wind. Most aircraft manufacturers give us a single speed or a few speeds that change with altitude.
Unfortunately there is no easier answer than flight test. After flying an aircraft for many years many of us get a "feel" for what is right and what is wrong for various conditions. When we leave the airplane that knowledge often leaves with us. Sometimes a "feel" is all we have. Most of my Gulfstream time is in the GV and G450, with less in the GIII and GIV. My conclusions?
What, no math? Yes, there is plenty of that, but the conclusion is pretty straight forward: your best rate of climb speed is faster than your best angle of climb speed.
What follows comes from the sources listed below as well as my comments and techniques shown in blue.
[14 CFR 1, §1.2]
14 CFR 25 does not mention VY at all.
Figure: Climb performance, from Air Training Command Manual 51-3, figure 2.21.
[Air Training Command Manual 51-3, page 152]
T = thrust available, lbs.
D = drag, lbs.
W = weight, lbs.
γ ("gamma") = flight path inclination or angle of climb
Of course some very high performance corporate aircraft, such as the G450, climb at angles where you cannot assume the inclination of the powerplant is negligible. Hence the section devoted to 1950's fighter type aircraft become suddenly applicable to us . . .
Figure: Climb performance, power available and power required, from Air Training Command Manual 51-3, figure 2.21.
RC = rate of climb, fpm
Pa = power available, h.p.
Pr = power required, h.p.
W = weight, lbs.
33000 is the factor converting horsepower to ft-lbs/min
Figure: Typical turbojet aircraft altitude performance, from Air Training Command Manual 51-3, figure 2.22.
Unlike Best Angle of Climb Speed (VX), there is no cut and dried way to find the best Rate of Climb Speed (VY) for a jet aircraft other than to flight test it. As pilots we are at the mercy of those who came before us. We learn from them by careful review of performance charts and, for the lack of a better term, folklore. GIV pilots, for example, have an intuitive understanding that the airplane climbs better a bit faster than the published climb speed of 0.75 Mach.
Gulfstream does not offer anything close to a VY chart, other than to recommend a best climb speed. The 0.80 M in a GV seems about right. The 0.75 M in a GIV or G450 seems a bit slow. We are left to study our aircraft in search of VY. . .
By all means fly your aircraft's published climb speed. But you might consider a bit faster and a pad of paper. If you have a typical flight profile with the same takeoff weight and climb altitude, consider flying the published speed and keeping a log of the following:
Once you've compiled fairly consistent data start all over at 0.02 Mach faster. If your time to climb improves you know your published climb speed can be improved.
Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapter 22.
14 CFR 1, Title 14: Aeronautics and Space, Definitions and Abbreviations, Federal Aviation Administration, Department of Transportation
14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation
Air Training Command Manual 51-3, Aerodynamics for Pilots, 15 November 1963
Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.