stinsonjr

If the Citation X can cruise at .92mach, what is the secret to the extra .08?

I read here that Cessna has a picture of the X breaking the sound barrier during a test, and would love to know if this is a rumor or true. Also, what keeps the X from going mach 1 in cruise?

Planespotta

Because you can only go that speed above FL600, and the X can't go that high.

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F16Driver

For certification purposes, an aircraft needs to be able to handle speeds x% (I don't know the actual percentage) over placarded Vne. This is why Cessna took the X to supersonic. It's nice to know it can handle it.

Engines are also detuned. Even so, the X would have to be in a slight dive to bust mach.

F16Driver

You can go supersonic at any altitude.

Cubdriver

It is true and I will try and find a published photo of the prototype airplane. The photos do not show breaking of the sound barrier as such, but several of the test pilots used at the time of certification confirmed it went faster than sound and are on record saying so.

Certification requires a design to be able to fly M.7 over whatever it wants to be certified at, so the cruise speed of the Ten was actually set by subtracting M=.7 from M=1 to get M=.93.

As far as how the airplane differs from other designs there is quite a bit of published material on this already, but most of it is included in broader discussions found here and there. This is as it should be, since most of the features that make the Ten fast were already being used by various aircraft at the time it was designed. For light reading you could see-

Airliners dot net Citation Ten article
Wikipedia Citation X article
The Legend of Cessna by Jeffrey L. Rodengen; Third Edition. Write Stuff; Fort Lauderdale, 2007. Chapter 18: Citation Ten-Fast Track to the Future.

Some features that help the Ten go faster than other jets in its class are:

*supercritical wings, refers to the shape of the wing cross section (airfoils). Allows the the drag curve stay low closer to the speed of sound but at a cost to low-speed lift.
*wing sweep of 37 degrees, has a similar effect as supercritical wing by pushing drag divergence point higher.
*swept flight surfaces on the tail reduce drag.
*area rule reduces drag by smoothing the rate of change of the longitudinal cross section area of the total airplane. To do this the belly fairing was carefully shaped and the fuselage is concave at the pylons.
*CFD wing design. Boeing pioneered CFD codes for the 777, which occurred at the same time as the Ten and Cessna enlisted their help.
*thrust to weight ratio was set high using specially designed Rolls Royce engines.
*high bypass ratio engines increased engine efficiency, needed to offset the higher thrust requirements.
*use of slats and increased flap area allow greater flexibility in surface area and camber. In turn, the wing shape can be tailored better to reduce drag at cruise speed.


These are just a few key features that come to mind. I am sure others can add more. The full discussion really involves getting into aerodynamics and there are lots of books on the subject from an academic standpoint, pilot standpoint, and for casual reading.

-Cubdriver

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stinsonjr

Thank you Cubdriver - I was thinking of you when I wrote this post, knowing that you would have an excellent answer. How much would they have to "tweak" the X to make it cruise at Mach 1?

Senior Skipper

Are you absolutely positive about that? 0.7 is a hell of a lot. Not challenging you, just wondering if it's a typo?

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Senior Skipper

FWIW, the X isn't the only one that can go supersonic. Older designs like the 747 and 727 have gone there during flight testing. I don't think it's a part of the normal pre-delivery test though, more likely at the certification level.

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Mitragorz

I'll challenge it!

Maybe you meant M0.07? Subtracting M=.7 from M=1 is actually M=.3

Planespotta

Of course you can. It will only be legal, though, above FL600.

EDIT: This is what I've learned from the FARs, so it's probably different for military flyers in MOAs and other restricted areas.

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