mach tuck ???

[501261]

Proav,

That scenario is certainly possible, when you extend the boards at mach tuck. However, I think that just about anybody who has earned a LRJET type in the last 30 years knows not to pop the boards out. It seems to be the first thing they teach you at Lear school.

So the guy is probably full of it, here are the reasons:
1. It's hard to get into mach tuck
2. The overspeed horn is constantly chirping above .82
3. You have to turn off the stick puller (against AFM)
4. Most normal pilots will slow down when they get aileron buzz (its pretty frightening).
5. Even the stupidest LR captain knows not to pop out the boards at mach tuck.

 

[Booker]

What's coffin corner?

Basically, it's the altitude where Mmo and Vs are equal. Vs remains the same with altitude, but Mmo expressed as IAS decreases with altitude. (A given IAS results in a higher TAS, which is what compressibility effects are based on, at higher altitudes.) As such, any increase in speed will result in a shock stall, and any decrease in speed will result in a generic aerodynamic stall.

 

[Future SNA]

X planes....

If you ever get to watch any sort of documentary on the making of the x-planes, they talk about in detail mach tuck. Was experienced early on in the initial testing of the x-planes, and also during high speed strafing runs in WWII with the Hurricane(??). Of course it was the British, who later realized that mach tuck was caused by the airflow being virtually cut off over the tail during transitional mach speeds. On the X-1, as well as in the Hurricane, is was during transitional speeds, closer to .98-.99. Anyways, the British then designed the "flying tail" to counter this, the Americans stole the idea, and thus broke the sound barrier. I know this has nothing to do with the LJ, but some neat history on how it all began.

 

[Timebuilder]

On the 35, there is a "mach trim" function. When it fails, you must reduce MMO to .77 or .74, I can't say for sure.

It's only the second cup of coffee....

 

[trainerjet]

Timebuilder,

I recommend that you wait until 10:44am, AFTER that 3rd cup of coffee, when both eyes are wide open, to post on this board. Then you will always be sure of what you have written.

 

[Timebuilder]

HA!

I'm not sure if this is sleepiness or a case of CRS.

In any event, I don't have the manual here, so I hope that someone who CAN remember the limit will help.

 

[avbug]

In the Learjet series, the reason for the autopilot restrictions above .74/.78 isn't tuck, it's a reduction of stick force beyond that acceptable for certification. Tuck in the airplane is very mild, and begins to occur in the region of .85, but isn't a factor in normal operations.

Mach trim is a feature of the autopilot (AFCS) that uses pitch trim via secondary servo to counteract the increasing nosedown tendency, and is part of the autopilot function during high speed flight.

The learjet doesn't come anywhere near coffin corner. In fact, it's really rather insignificant for most turbojets, certainly most business jets. Nobody goes high enough.

Tuck was a fatal factor in a number of high speed diving tests of piston driven fighters during and following the second world war. Associated with tuck charactaristics were buzz and flutter regions; flutter being destructive, buzz being annoying and disconcerting, but generally not dangerous. However, also associated with buzz, which is pronounced when applying G's during recovery, is a hardening or resistance in the controls, also referred to as "freezing" of the controls. Control reversal or lock is also possible in aircraft without "nonreversible" controls. This is not found in the Learjet.

Aileron buzz may occur above .85 when a G load above 1.5 or so is applied; this is not dangerous, or particularly noticable generally, but can be come pronounced as the G load is increased.

Application of spoilers at speeds in excess of Mmo can create a significant nose-downpitching moment, will likely disconnect the autopilot. Stick forces have already become light enough to fall below certification standards once above .78, and above .83/.83, the aircraft can experience a rapid pitch down or departure with spoiler application during autopilot flight. Disconnecting the autopilot, the pilot is then faced with an out of trim situation in which control forces are very light and the aircraft is very touchy, and already in an overspeed condition. For that reason, during an overspeed, the pilot is not to deploy the spoilers.

Instead, power is reduced and the wings levelled, and if drag is required, the gear is extended while adjusting pitch trim. In the learjet in particular, a significant amount of trim is required to counteract spoiler application, and spoilers should never be applied while the autopilot is engaged.

 

[LR25]

When I was at Simu-Flite awhile back, some of the LR instructors said during early certification of the Lear they were only going to certify the airplanes to .78 only.

That was the devolopment of the AFCS for .82.

They also said that tuck was almost non-existent in the normal operating range.

 

[profile]

Avbug-

Interesting that the spoilers cause a pitch down on the Lear. That happens on some airplanes due to disruption of flow to the horizontal stab, but it would seem that with the T-tail that is less likely in the Lear. Absent that, intuitively it would seem that the spoilers would move the CP forward in the vicinity of the spoilers themselves. On highly swept wings the wing outboard of the spoilers, toward the tips, would then tend to lead to a forward pitch moment in that scenario, but are the Lears in that category, perhaps I haven't looked that closely at them?

Do the Lears have stabiler trim or elevator trim? Are all the controls unpowered/augmented?

 

[Starcheck]

mach tuck

I believe the last comments on center of pressure are correct. Though on the Learjet, when spoilers are extended the deflected air affects the tailplane and creates a significant tail-up/nose down pitching moment. Many of our Learjet 35's now have the Avcon (delta) fins, and the pitch change is very benign.

All the 20 and 30 series Lears have a fully trimable tailplane. The primary flight controls are all cable/manual type.

The ailerons are the only "augmented" primary flight control. Each aileron has a servo tab mechanically linked to assist in aileron movement.