More than 15 years ago the Cessna Citation X pushed through the Mach .90 barrier to become the fastest civilian airplane with an Mmo (maximum Mach operating speed) of Mach .92. At the altitude where business jets typically cruise above 36,000 feet Mach .92 equals about 528 knots true airspeed.
When Gulfstream developed the G650, the largest cabin and longest range business jet, and the fastest, it upped the ante giving the airplane an Mmo of Mach .925. That put the maximum operating speed of the G650 about 3 knots faster than the Citation X.
Cessna is developing a new, stretched version of the X and changed the name to the Ten. The X is the Roman numeral for 10, but not everyone made that connection, so this time Cessna is spelling it out. The Ten has a longer cabin, and new touch screen flat glass avionics from Garmin, and more efficient versions of the Rolls-Royce engines. And it has winglets.
Most of us expected Cessna to try to find a way to eek out a little more speed to reclaim the fastest business jet title, and the company recently announced that it plans to certify to an Mmo of Mach .935, which is .01 Mach faster than the G650.
Obviously, this is a game of bragging rights because .01 Mach is less than 6 knots of true airspeed which will be hard to find even on a long trip. But still, bragging rights are important. And both companies are bumping up against the unyielding sound barrier at Mach 1.0.
Mmo is a certification requirement for jets and is the indicated Mach operating limit. Jets have two red line airspeeds—Mmo, and Vmo. The Vmo is an indicated airspeed limit and is set primarily by the structural loads on the airplane. Indicated airspeed is a measure of air pressure the airplane experiences and that air pressure stresses the airframe so there must be a maximum indicated airspeed.
However, at higher altitudes where air density is low, indicated airspeed is also low, but the effects of Mach—compressibility—become the limiting factor. Even though an airplane is flying well below Mach 1, the speed of sound, the air accelerating to pass over the wings, tail and fuselage does reach Mach 1, or faster. At Mach 1, or actually a little slower, a shock wave develops which creates high drag, but can also affect flying qualities.
The Mach shock wave initially forms near the leading edges of the wings and tail, or over the canopy of the fuselage, and then the wave moves aft as the airplane accelerates. The lifting characteristics of the wing and tail change with movement of the shock wave so flying qualities and stability can also change. When an airplane flies fast enough the Mach shock wave can move aft over the flight control hinge lines, or onto the flight control surfaces and that can cause very serious control problems in an airplane not designed for that speed.
To give pilots a safety margin the FAA requires new jets be tested to a speed called Md, which stands for maximum Mach demonstrated dive speed. Md must be faster than Mmo so that if the airplane is momentarily upset and noses down and accelerates past Mmo, the pilots know that nothing bad will happens while they regain control of the speed. In general, Md must be .07 Mach higher than the design Mmo. There are some analytical ways to get the difference between Md and Mmo down to .05 Mach—including the use of automatic devices to slow the airplane—but Md will need to be at least .05 Mach above the certified Mmo.
But Mach 1 is there as a cap on Md speed. It requires special permission and procedures for a civilian airplane to exceed Mach 1, and it’s unclear that the FAA would approve a Mach dive test faster than the speed of sound. Both Cessna and Gulfstream dance around the question of how fast their jets went during dive tests, but even as good as their test pilots are, it’s impossible to believe either airplane was able to dive to the required Mach .99 and then slow before hitting Mach 1. But that’s their story and they are sticking to it.
So with Cessna aiming to certify an Mmo of Mach .935 there is not much room left for the dive test and a faster Mmo. Can Gulfstream squeeze in with an Mmo of Mach .94. It will be tough under the rules, but they could probably match Cessna’s Mach .935.
Which airplane is fastest? Well, the G650 has a provisional certificate and first deliveries will be made later this year so its Mmo of Mach .925 is already in the bank. Cessna is confident it can certify to its announced speed, but that is yet to come.
As for which has the real world usable speed advantage that goes to the G650 hands down. Gulfstream test pilots flew a G650 for 5,000 nm at a constant speed of Mach .90. No other jet has flown that fast that far except for a few dashes by military airplanes that refueled in flight. The Citation Ten expected maximum range is 3,242 nm and that comes at a leisurely Mach .82 cruise speed so you can see which airplane will get you a long way the quickest.
But there are those bragging rights and we can all enjoy watching Cessna and Gulfstream look for hundredths of a Mach while knocking on the Mach 1 door.