200 knots below Class B

[bigD]

So what exactly is the limiting factor in an "operational limit"? If there's not a structural or control problem, why is the limit there?

 

[GVFlyer]

At Flight Safety we watched a video of the Falcon Test pilots flying a Falcon 50. They got that thing darn close to Mach 1. Hands off the wheel with the autopilot off. No Mach-Tuck, Aileron Buzz or any adverse flying characteristics for that matter. I've been told that all Falcons actually exceed Mach 1 during flight testing. So as far as the Falcon goes, they could have a Mach .98 limit if they wanted. So I guess Mach .86 is just an "operational limit" as well.

But hey, Limits are Limits.

JetPilot500

This is unlikely. There are several ways that Vc is determined for jet aircraft: flutter speed, Vb + 1.32 Uref, 0.8 percentage of Vd, the speed at which you can accelerate 7.5 degrees nosedown for 20 seconds then recover to Vc with no more than 1.5 G's, or the speed at which you encounter a control reversal for example. The most common limiting speed, however, is 0.07 mach below the speed where compressibility effects are encountered (this speed may be reduced to a 0.05 mach margin if rational analysis for the effects of automatic systems are used).

So, if MMO is 0.86 mach the Falcon encountered something at 0.93 mach that determined it's design cruise speed.

On the production test cards for Certificate of Airworthiness, all production jets are required to be flown at some speed above MMO to prove their controlability should jet upset occur. Initially, this speed was 0.955 mach for the GV, then the FAA relaxed the requirement to the current 0.92 mach at which all GV's are flown. At MMO, the aircraft is trimmed for hands-off flight then the speedbrakes are fully deployed and the trims are noted. There can be no roll; slight nose up pitching is allowed.

Mach tuck occurs in all jets as the transition from subsonic to trans-sonic flight (mach crit)occurs. In normal unaccelerated subsonic flight the aerodynamic center of pressure is at 25% chord. This center moves aft as you accelerate until it reaches 50% chord in supersonic flight. As a resultant of this center of pressure moving aft, the nose tucks. The rate, degree and extent to which this occurs is largely determined by the Reynold's number for the wing.

No one would intentionally take a transport category jet designed for subsonic flight beyond 1.00 mach . Subsonic wings are designed using the Navier - Stokes equation and Computational Fluid Dynamics to be just that: subsonic. Supersonic wings designed for non-compressible airflow differ significantly in strength and design. On aircraft that do not have an all-moving tail the first thing that occurs at 1.00 mach is a shock wave forms up at 50% chord on the horizontal stabilizer which negates the effectiveness of any flight control operating behind it. Subsequently, pitch control is lost.

The GV went to 1.07 mach one time during developmental test, but it was not intentional. One of the stability tests that is done during "cert " is to simulate runaway trim. In the GV, the aircraft is accelerated to Vc at 51,000 feet (0.86 mach), trimmed for hands - off flight, next the trim is run nosedown for 3 seconds, and then the test pilot is allowed to recover the resulting maneuver. After demonstrating that you can do this point, the FAA is invited to come fly the point themselves and verify it. They are to neither "demonstrate exeptional strenght or skill" to do this. They do this part well.

On the day when the GV "Made the number" the FAA test pilot from the ACO was in the left seat to verify this runaway trim point. The jet was accelerated to 0.86 mach and trimmed for hands-off flight at 51,000 feet. The trim was then run nosedown for 3 seconds. All went well until the FAA pilot was told, "You got it, recover." He was reticent to pull back on the yoke at that altitude and speed. As a result the aircraft quickly accelerated to 0.99 mach. Seeing this, FAA pilot promptly announced to the Gulfstream test pilot, "You got it!"

We learned that day that the EFIS displays only show 0.99 mach as a maximum. When the Steely-Eyed Gulfstream Test Pilot pulled back on the yoke - nothing happened. The flight test engineers in the back advised him that their instrumentation in the back was showing 1.04 mach (the real time telemetry streaming to Gulfstream flight test operations was showing 1.07 mach). The elevators were ineffective because they were operating behind a well established mid-span shock wave on the horizontal stab. The GV has a fully movable emergency stabilizer, but it is not designed for supersonic flight. A good test pilot never does add-on testing so our SEGTP elected not to use the EMER STAB and rightly thought, "We are descending. As we do so temperature will increase and with it the speed of sound. If I just hang on the aircraft will go subsonic and I will regain pitch control." Which is precisely what happened in the high 30's.

GV

 

[AV8OR]

GV,

I don't know about every body else, but that was a GREAT read. Crap, either know your stuff or you fooled the hell out of me. Either way I was impressed. Schools out.

 

[750driver]

GV,

I don't know about every body else, but that was a GREAT read. Crap, either know your stuff or you fooled the hell out of me. Either way I was impressed. Schools out.

Either GVFlyer is the best BS artist I've ever seen, or he's a former mil/ civ test pilot. I'd bet on the latter of the two. Good educational post. I can always use an aerodynamics refresher

 

[Timebuilder]

Hey, that was neat!

I take it that a second FAA inspector took a ride to get the plane certified...

 

[501261]

Hey GV,

You should consider writing some articles for the aviation rags, that's some good stuff.

 

[Muddauber]

Aero 101

Nice post, GV.

 

[LrBob]

a guy i flew with just got a 90day susp for exceeding 250knot below 10-actually he was at 4000 just departed santa monica ca
he was doing 310 horn blaring all the way-atc called him and asked his speed he said 250 pulled the pwr back dropped 200ft and went right back to 300-the way they caught him was awacs was over head and alerted on him