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Demo of the freighter's Mach tuck

Started by Hardy Heinlin, Mon, 6 Sep 2021 18:53

Hardy Heinlin

Hi all,

here are some steps to demonstrate the freighter's Mach tuck effect. It's a very subtle effect as the 747 is an extremely stable aircraft even at transsonic airspeeds.

The reference in this demo is PSX 10.142 or higher. You may want to install it if you haven't done it already.

Before we begin, please shake your joystick so that it's 100% centered and free of electric noise that may cause unwanted deflection signals. Or just go to Instructor > Preferences > USB and deselect the "USB on" checkbox.


Please go to Instructor > Situation > Load and load this situ file:

Basic 014 - Cruising at final cruise altitude.situ

On Instructor > Model > Airframe, under "Aircraft model", select "Passenger".

On the flight deck, on the MCP:
Select ALT HOLD
Select HDG HOLD
Under the speed window push the little brown SEL button to switch from IAS to Mach mode.
Select Mach .900

Place a stab trim indicator in one of your layout subframes, and zoom it to the max.

Wait until the aircraft has reached M.90.

Notice the trim is at 5.5 units when reaching M.90.

On Instructor > Model > Airframe, under "Aircraft model", select "Freighter".

Effect: The trim moves to 5.7 units.

This stab motion was not commanded by the Mach trim feature (the Mach trim is inactive when an autopilot is engaged). This was just the usual trim commanded by the autopilot. However, you can see that the freighter requires a 0.2 units nose-up adjustment to compensate for that tiny little Mach tuck effect.


Now let's demo the Mach trim feature which is independent of the autopilot.

On Instructor > Model > Airframe, under "Aircraft model", select "Passenger".

Notice the trim returns to 5.5 units.

Disengage the autothrottle and the autopilot.

Check that the aircraft is stable in level flight.

On Instructor > Model > Airframe, under "Aircraft model", select "Freighter".

Effect: The trim goes to 5.7 and the aircraft maintains its current level flight. Now this stab trim motion was commanded by the Mach trim feature. It was not an autopilot command.

The Mach trim will not move the stab trim again until you decrease your airspeed: When the speed gradually decreases to M.86, the Mach trim will gradually move the trim back to 5.5°.


Now we'll demo the pure Mach tuck without any automatic compensation.

Engage the autothrottle and the autopilot.

On Instructor > Model > Airframe, under "Aircraft model", select "Passenger".

The Mach trim system sits in the same electronic modules that compute the rudder control ratio and other things. We will deactivate them now to show the pure Mach tuck effect. These modules have many backup power sources; we'll have to pull eight circuit breakers:

Check that the aircraft (passenger variant) is stable at M.90 and 37000 ft.

Disengage the autothrottle and the autopilot.

Go to the overhead circuit breakers, and in row C, pull all eight CBs from number 10 thru 17.

On Instructor > Model > Airframe, under "Aircraft model", select "Freighter".

Effect: The nose drops (just a tiny little bit) because of the Mach tuck on the freighter's shorter upper deck. The Mach trim feature is inactive this time. So the freighter will descend, reaching ca. 300 fpm for a second, and will climb again and do some more phugoid oscillations and eventually stabilize about 150 ft below our original altitude and above M.90.

As mentioned above, the 747 is known to have no Mach tuck tendency otherwise. What remains is this tiny little extra effect on the freighter.


Regards,

|-|ardy


ASCTU744

How does a shorter upperdeck increase the Mach tuck effect?

B767300

Quote from: ASCTU744 on Thu, 22 Feb 2024 15:35How does a shorter upperdeck increase the Mach tuck effect?

I think it has something to do with the CG rather then the aerodynamic characteristics. A longer upperdeck should generate a shockwave at the exact  same place as a shorter one (if I'm not mistaken).

Hardy Heinlin

My understanding is that the aft downhill area of the upper deck profile generates lift.

On the freighter this lift point is closer to the nose, thus it has a higher momentum.

B767300

But Mach tuck pushes the nose down right? Wouldn't lift on the hump generate a nose-up moment?

Hardy Heinlin

QuoteWouldn't lift on the hump generate a nose-up moment?

Yes. It's stronger on the freighter. But not just for the nose-up momentum. It affects both the nose-up momentum in the normal airspeed range as well as the nose-down momentum at transsonic airspeeds, as far as I understand it.

When adding something that makes a great difference, removing that same thing also makes a great difference - just in the opposite direction.

When you place a 10 ton iron block on the flight deck, the nose will drop very far.
When you remove that same iron block from the flight deck, the nose will rise very far.
The momentum in both directions will be great.

When you do the same with a 1 kg block, the momentum in both directions will be small.

B767300

That does make sense, I suppose that the  CG doesn't have a whole lot to do with mach tuck.