mach tuck ???

[walkthasky]

Im sorry if this is bad, but i have no idea what it is???

 

[asolo]

Well, I'm sure that other people will have better explainations for it, but without a book in front of me...here goes. Mach tuck is when the center of pressure on the wing moves aft due to the shock wave progresses. As a plane approaches the speed of sound, before the plane physically penetrates the speed of sound, the airflow over the wing starts to go supersonic. As the plane speeds up the shock wave will move aft and thats what causes the center of pressure to move aft. This will produce a pitch down tendency which only agravates the situation. Hopes this helps.

 

[Cornelius]

I think it may also have something to do with airflow separation after the shockwave which may cause the h-tail not to be as efficient.

Hey Walkthasky, what have you been up to at Commutair? I've been on reserve the last 3 days and pretty much sitting by the pool all day long dreaming of having a beer.

Peace.

 

[Booker]

I think it may also have something to do with airflow separation after the shockwave which may cause the h-tail not to be as efficient.

Hey Walkthasky, what have you been up to at Commutair? I've been on reserve the last 3 days and pretty much sitting by the pool all day long dreaming of having a beer.

Peace.

You and asolo gave two causes of Mach tuck. FWIW, I think asolo's explanation is the most commonly referenced cause. Indeed, the aft CP shift as local V increases above M1 causes a nose-down pitch tendency due to the movement of the pressure wave. And as you mention, the resulting flow-induced separation can cause the elevator to operate in turbulent, aerodynamically dead air, resulting in lost tail-down force. But also remember that the H-stab is an airfoil with locally high velocities. As such, it is possible that it has its own shockwave attached in front of the elevator. If this is the case, the elevator will again be operating in aerodynamically dead air, resulting in loss of tail-down force. Of course, the last case doesn't typically happen with an all-movable tail.

 

[Lindy]

Not too long ago, Learjet printed information regarding the Mach Tuck (any maybe the coffin corner too??). I don't have a copy, and if someone does, I would love to obtain one!!



The 20 series could get into a mach tuck. I've only seen it demonstrated in the sim (for obvious reasons) and neither my sim partner nor I ever recovered from a mach tuck.

 

[Cornelius]

Isn't that what the stick puller is for the early Lears? I think I remember seeing that somewhere.

 

[LR25]

In the 25 sim out at simu-flite we had it up to about .89 and I didnt notice a tuck.

Some of the flight instructors out there said there is almost no tuck at all in the 20 series Lears.

 

[want2fly]

What's coffin corner?

 

[501261]

There definalty is a mach tuck in a Lear 25, happens around .85-.86. I had a chance to watch this in person once, if they ever demate the wing of an old Lear. The factory (in this case the Tucson Service Center) has to recertify that all the mach warnings and aerodynamic characteristics are the same when the remate the wing with the body. So they take the plane out and test fly it to make sure the overspeed horn comes on at .78 and .82 (you have to listen to it for the whole test flight), then at .82 that the puller acts like it is supposed to. Then you turn off the stall warning (deactivating the puller) and continue up to .84 where you start to get aileron buzz. At around .85 the nose will drop down and you pull the throttles all the way back and recover just like you do in the sim. You've just been in a mack tuck, pretty lame if everything goes how it is supposed to.

The aerodynamic reasons for mach tuck have already been discussed. In real life the reason you learn about mach tuch is that it killed a few people back in the 60's. The old Lears are some of the few planes out there that can actually exceed Mmo in level flight. So what a few people did back in the 60's and not knowing anything about mach tuch is they took the Lear past Mmo until they hit mach tuck, most just lost altitude and recovered (other than an altitude deviation and a bruised ego, no harm done), but a few got the bright idea to pop out the speed brakes. This is what kills people, the mack tuck, will bring your nose down and increase speed, add speed brakes that in old Lears also have serious nose down tendency you're now going straight down.

I don't think there's been a recent case of someone getting killed because of mack tuck, though a Lear did disintgrate on a descent into El Paso a few years ago.

 

[proav]

Is this possibly?

I had a guy tell me that he was in a Lear and experienced mach
tuck and he extended the boards. When he did he claims that
the plane tumbled end over end and eventually he was able to
regain control. Is this possible or is he full of bull excrement?

 

[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.