Citation X supersonic?

[joema]

I saw an interview with producer/director Sydney Pollack, who is type rated in and flies his own Citation X.

He mentioned the Citation X has marginal supersonic ability in level flight, but the certification doesn't allow this, so that's why operators don't do it.

Is there any truth to that?

[AlphaWhiskyRomeo]

Probably. Fastest business jet out there and can touch 0.92 Mach according to it's official max. cruise speed, so I reckon they'd be a little play in that.

[PT6ER]

When I was working with Cessna on another program, I attended a presentation on the CX and they told us that during certification testing (in particular the rapid descent test used to qualify the aircraft in the event of rapid cabin pressurization) quite a lot of the structure was experiencing supersonic flow but they never mentioned a supersonic / transonic capability - even as a whisper with a wink...if you know what I mean.

Maybe one of you clever blokes could answer this for me - when does sub-sonic become transonic? Is there a recognized Mach number??

As for the CX, I've never seen two engines joined with such a nice fuselage :)

And it is bl00dy quiet too!

[Tarnished]

PT6ER

Sub sonic becomes transonic when the position of the shock waves begins to change from their subsonic position to their supersonic position. This varies between aircraft types but in the absence of a universal definition I would say 0.95 to 1.05M would be a transonic range. Shock waves rushing about up and down the length of the aircraft moving the centre of pressure around all over the place can give rise to some whacky rides particularly when anything other than in 1g S+L flight.

Tarnished

[PT6ER]

Tarnished
Much appreciate the explanation!
Thanks
PT6ER

[Mad (Flt) Scientist]

When I was working with Cessna on another program, I attended a presentation on the CX and they told us that during certification testing (in particular the rapid descent test used to qualify the aircraft in the event of rapid cabin pressurization) quite a lot of the structure was experiencing supersonic flow but they never mentioned a supersonic / transonic capability - even as a whisper with a wink...if you know what I mean.
Maybe one of you clever blokes could answer this for me - when does sub-sonic become transonic? Is there a recognized Mach number??
As for the CX, I've never seen two engines joined with such a nice fuselage :)
And it is bl00dy quiet too!

The emergency descent test is done at or below Mmo - it has to be, going faster would be cheating as you'd get down faster. So the fastest speed they should have seen during that test in 0.92 Mach, give or take maybe 0.01-0.02 for accuracy of the flying.

With an Mmo of 0.92, Cessna will have demonstrated a maximum demonstrated flight/dive speed - Mdf - of somewhere in the 0.97-0.99 range (depending on how they applied the rules; the margin required between mmo and Mdf can be from 0.05 to 0.07)

An aircraft is considered to be transonic when shockwaves start to form ANYWHERE on the plane; because the shape of the aircraft components can act to accelerate the local flow, this always starts to occur before the aircraft itself is at Mach 1.0. The goal of a high speed design is to delay the onset of transonic characteristics by careful shaping of the components - area ruling, thin wings, swept wings, deltas, etc., etc. An aircraft not designed for high Mach flight can experience transonic behaviour at rather low Mach numbers - down to as low as Mach 0.80 or so (such as the 'compressibility effects' that claimed the lives of many over-eager pilots in WW2 prop fighters) - while careful design may delay the start of shock formation to Mach 0.95. Typically I'd expect most (jet) commercial aircraft see 'something' at 0.85 and a big effect by 0.90.

[barit1]

If you're seated over the wing in a long aluminum tube, a shock wave may form around the fuselage about your station. And if the Sun is abeam the aircraft, the shadow of the shock wave can sometimes be seen on the wing. In gentle turbulence it will wander around a little - fun to watch.
:8

[chornedsnorkack]

Can Mdf be deduced from Mmo in this way?

I mean, the first passenger airliner to exceed the speed of sound was not Concorde. It was DC-8. Which reached M 1,012 in a shallow dive (in test flight).

If a subsonic plane were to demonstrate Mdf M 1,02 or M 1,05 or M 1,10, would it automatically be certified to have Mmo M 0,95-0,97 or M 0,98-1,00 or M 1,03-1,05 respectively?

The big problems with transonic range are:

drag increase will cause high fuel burn in cruise

drag increase might cause breakup in flight

the centre of lift moves back.

The last is a problem with dives - centre of lift backwards can cause the plane to pitch nose down deeper into dive.

Concorde had elaborate fuel pumping system to keep right centre of gravity in transonic range.

But DC-8 has somehow pulled out of a dive at M 1,012.

So... which subsonic planes other than DC-8 have demonstrated ability to exceed M 1,0 and recover?

[Mad (Flt) Scientist]

Can Mdf be deduced from Mmo in this way?

Yes it can. You are REQUIRED to demonstrate a minimum margin between MC and MD per FAr25.335(b)(2) of between 0.05 and 0.07. Typically Mmo will be the same as MC, and Mdf may be as high as MD (although it may be lower, it's likely it won't be by much, given the kinds of manoeuvres required to be demonstrated starting from Mmo which must not exceed Mdf). On our types (swept wing jets) a 0.04-0.05 margin between Mmo and Mdf isn't uncommon.

If a subsonic plane were to demonstrate Mdf M 1,02 or M 1,05 or M 1,10, would it automatically be certified to have Mmo M 0,95-0,97 or M 0,98-1,00 or M 1,03-1,05 respectively?
No. Because the Mmo-Mdf or MC-MD margin requirements are just one of many that must be met. For example, adequate handling characteristics must be demonstrated at Mmo, and also (but to a lesser degree) at Mfc (which is a speed somewhere between Mmo and Mdf). Those might limit Mmo anyway, regardless of demonstrated Mdf.

The big problems with transonic range are:
drag increase will cause high fuel burn in cruise
drag increase might cause breakup in flight
the centre of lift moves back.
The last is a problem with dives - centre of lift backwards can cause the plane to pitch nose down deeper into dive.
I'm afraid I can't conceive of a mechanism where a drag increase would cause structural damage. The engine mounts and associated structure can already transfer the full takeoff thrust loads into the airframe, and the thrust a transonic speeds is unlikely to be higher than that. Also a plane climbing vertically is decelerating at ~1.0'g', equivalent to very high drag in level flight, with no risk of structural failure.

Rapid and unpredictable changes in ALL aerodynamic characteristics are associated with the transonic range (unpredictable before you go there or unless you have some pretty smart wind tunnel or cfd work, that is); you're also able to get a pitch up as well as the nose-down 'Mach tuck' mentioned, as well as loss of control effectiveness (especially trailing edge controls like ailerons and elevators).

Concorde had elaborate fuel pumping system to keep right centre of gravity in transonic range.
I suspect that the fuel pumping system was as much for cruise trim drag reasons as for stability reasons.

But DC-8 has somehow pulled out of a dive at M 1,012.
So... which subsonic planes other than DC-8 have demonstrated ability to exceed M 1,0 and recover?

I wonder if that Mach number was 'real'. With typical pneumatic system lags and descent rates for that kind of dive one would expect the air data system to significantly OVERESTIMATE the aircraft speed. (Because there tends to be a larger lag in the static pressure, so you end up measuring the total pressure NOW and the static pressure from several thousand feet previously in the dive i.e. LOWER than it is NOW; an artificially reduced static will cause an overestimate of your speed.)
Not saying for sure it's wrong, but we had one of our types produce a very impressive Mdf number until we corrected it down for lags ....

[chornedsnorkack]

Yes it can. You are REQUIRED to demonstrate a minimum margin between MC and MD per FAr25.335(b)(2) of between 0.05 and 0.07. Typically Mmo will be the same as MC, and Mdf may be as high as MD (although it may be lower, it's likely it won't be by much, given the kinds of manoeuvres required to be demonstrated starting from Mmo which must not exceed Mdf). On our types (swept wing jets) a 0.04-0.05 margin between Mmo and Mdf isn't uncommon.


No. Because the Mmo-Mdf or MC-MD margin requirements are just one of many that must be met. For example, adequate handling characteristics must be demonstrated at Mmo, and also (but to a lesser degree) at Mfc (which is a speed somewhere between Mmo and Mdf). Those might limit Mmo anyway, regardless of demonstrated Mdf.

So, the conclusion is that Mmo can be used to give a lower bound of demonstrated Mdf - but not the upper bound. Citation having Mmo 0,92 M can be expected to have Mdf of at least M 0,97, but might have a Mdf higher than that, yet be only certified for Mmo 0,92 M.

[con-pilot]

So... which subsonic planes other than DC-8 have demonstrated ability to exceed M 1,0 and recover?

I have heard rumors for years that a Convair 990 belonging to a Mexican airline exceeded Mach 1 after loss of control trying to top a thunderstorm.

Before the Concorde the Convair 990 was the fastest civilian airliner.