Landing gear malfunctions

[Hardy Heinlin]

Good evening,

this topic also includes flat tires, hot brakes, autoskid failure etc. My first question is about partially extended gear on the ground:

For the case when all gear is down except the body gear, the manuals say: "... the airplane may tip tail down on the ground."

Here's a schematic:

Code Select Expand

Nose gear:     W

Wing gear:   W   W
Body gear:    W W

Assume the CG (center of gravity) be in a perfect middle position. In normal circumstances, with all gear down, you want some weight on the nose gear. By this weight the nose tire is pressed on the asphalt and the nose wheel steering has the control. Right?

That means: The wing gear is aft of the CG. If it were not, there would be no weight on the nose wheel. Right?

If the wing gear is aft of the CG, the body gear can be retracted and the aircraft won't tip tail down. Right?

Conclusion: It depends on the CG whether the aircraft will tip tail down. (Ground speed acceleration may influence it also.)

Conclusion of the conclusion: When the wing gear is not aft of the CG, the nose wheel steering is nearly ineffective. I can't imagine that such a weight configuration is permitted. So I conclude a third time: The aircraft may tip tail down with the body gear up only if the CG is behind the allowed aft limit.


(Tip tail down is modelled in PS1. I think there's a malfunction that keeps the nose and body gear retracted. When you land and decelerate, the aircraft pitches down below the horizon; when you release the brakes, the aircraft pitches up and parks at a tailstrike attitude.)


Cheers,

|-|ardy


P.S.: Of course, the body gear steering also wants to have some tire weight on the asphalt. Tricky. I think we're talking about very small CG variations. As the CG sits higher than the gear, the aircraft is in an unstable condition when the nose or body gear is retracted: If it rotates nose down, the CG, too, goes forward, producing even more nose down momentum. Vice versa when the body gear is up and the aircraft rotates tail downwards.

[Hardy Heinlin]

Another question:

All gear up with the naked belly on the ground, what would the radio altimeters say?


|-|

[John H Watson]

That means: The wing gear is aft of the CG. If it were not, there would be no weight on the nose wheel. Right?

This assumes the strut pressure in the body gear is not pushing up (i.e. not pushing the nose down). I don't know how significant this force would be.

The bogeys have tilt actuators. These would also act to keep the nose down, but I'd imagine the force would be somewhat less insignificant.

If the wing gear is aft of the CG, the body gear can be retracted and the aircraft won't tip tail down. Right?

Assuming the bogey tilt geometry allows it. The rear tyres on bogeys are changed by lifting the wheels off the ground using a jackpoint underneath the wheel axle (not the centre of the bogey), so there is tilt in both fore/aft directions. However, perhaps the strut will compress enough to allow the wheels to retract, even if the bogeys won't allow the required tilt angle.

Regards
JHW.

[John H Watson]

All gear up with the naked belly on the ground, what would the radio altimeters say?

An empty aircraft usually results in approx -6 feet. The fuselage is about 7 or so feet from the ground in this situation. Belly on the ground, this should give about -13~-14 feet.

This assumes the antennae haven't been destroyed I guess it would depend on the clearance generated by mangled gear/doors/engines in your scenario.

Also, I can't imagine the Radio Altimeters continuing to operate if you've lost all your engines (and, specifically, generators). If the APU was running prior to and after takeoff, however, you might be able to get the APU generators online.  

Regards
JHW

[Hardy Heinlin]

That means: The wing gear is aft of the CG. If it were not, there would be no weight on the nose wheel. Right?

This assumes the strut pressure in the body gear is not pushing up (i.e. not pushing the nose down).

If the strut pressure in the body gear pushes more up than the strut of the wing gear, then the wing gear is relieved and the weight of the aircraft is not equally distributed over the body and wing gear.

I assume it's almost equally distributed, so that both, nose and body gear steering, has good contact to the ground. Or is the purpose of body gear steering more for friction avoidance rather than for actual steering? I guess it's just supposed to avoid tire friction, while the nose gear is the actual steering gear.

I just wonder how that little nose wheel can be so effective when it has not much weight on it. Could the nose wheel maintain the aircraft heading when applying max thrust on engine 1+2? (Rhetorical question.)


Cheers,

|-|ardy

[Garry Richards]

All gear up with the naked belly on the ground, what would the radio altimeters say?

Ouch?

[John H Watson]

If the strut pressure in the body gear pushes more up than the strut of the wing gear, then the wing gear is relieved and the weight of the aircraft is not equally distributed over the body and wing gear.

If the CG is over the wing gear, then I assume the wing gear becomes a pivot point. I assume then, that most of the weight is on the pivot, leaving less weight on the body gear. The body gear is then free to push the nose down.

In your scenario, how far back is the CG going? As the CG shifts backwards, then the weight will become more evenly distributed over the four main gear, then the weight will start to shift towards the body gear (eventually the body gear will become the pivot point).

Or have I missed something?

Perhaps the guys who weigh aircraft can cut in here  :mrgreen:

http://www.airliners.net/photo/China-Airlines-Cargo/Boeing-747-409F-SCD/0222286/L/

[Hardy Heinlin]

The CG is somewhere between the nose and the body gear. It doesn't matter in your body-strut-push-up example. I think the wing gear is not a pivot if the aircraft sits on the nose and body gear. It has a pivot only when the nose and body gear are retracted; the wing gear is a pivot then.

Take a standing elephant and tell him he may want to stretch his aft legs. His aft part will rise. And that without a pivot in the middle. Even if he had a pivot in the middle, it wouldn't touch the earth, unless you tie him down with a chain between his belly belt and the ground. That would force him to lower his front legs when stretching his aft legs.

The wing gear is not mechanically connected with the ground, so it cannot help pushing the nose wheel down when the body gear strut goes up.


Cheers,

|-|ardy

[Hardy Heinlin]

:-) Actually, I forgot what my original problem was. It could be that there was no problem at all :-)

...

Anyway, here's another unusual attitude:





!-|

[John H Watson]

The CG is somewhere between the nose and the body gear. It doesn't matter in your body-strut-push-up example.

Trying to understand your fleshy animal analogy, but in the meantime...



Say the CG acts over the wing gear. The balance is held in check by the strut pressures of two body gears close to the wing gear and one nose strut pressure at the front of the fuselage.

Any movement of the CG away from the wing gear will be resisted by the struts of the other gears. This is what I was trying to say.

If the CG moves to the extreme aft, then the body gear strut resistance will be overcome and the nose will begin to rise. This can be seen by the amount of visible chrome on the strut.

When the gear compresses as much as it can, it becomes a hard pivot, rather than a soft pivot.

Regards
JHW

[Hardy Heinlin]

I think, if the aircraft sits on the nose gear and the body gear, whose strut is longer than the wing gear strut, then the wing gear is meaningless. It just hangs in the middle.

Take a playground seesaw. You can push the opposite end down by pulling your end up. This works only because the pivot in the middle is connected with the ground. Our wing gear is not connected with the ground. It lifts off when the body gear lifts off.

Edit: We could simulate a ground connection by putting a weight of 9000 tons directly over the wing gear station. But without it, the weight on the body gear is nearly as high as that on the wing gear. The wing gear is too light to counteract the up-push of the body gear. I think.


Cheers,

|-|ardy

[Jeroen Hoppenbrouwers]

You have to draw this with all forces and the sum of all moments to get any real idea.

[John H Watson]

I don't think anyone is trying to suggest that the weight of the aircraft is suspended on only the nose gear and 2 main gear bogeys.

What I'm trying to say is this:

Consider a balanced beam (plank) with irregular weights at different distances from the fulcrum (some of these weights may be negative - i.e. push up forces). How do we  know where the CG is? Of course, it's where the fulcrum is, otherwise it wouldn't be balanced. If the CG was directly over the wing gear strut, then this would be the fulcrum. I'm not saying, however, that the wing gear legs are longer than the other legs and that the other wheels are hanging in the air.

I'm sure Boeing designed an aircraft to have a roughly equal weight distribution on all 18 wheels/tyres. If there were perfect weight on wheel distribution, the CG would have to be somewhere between the axles of the wing and body gear bogeys. Not right in the middle, but just aft of the wing gear strut. Would it still be possible, then, to land with the body gear gear retracted with this formerly perfect wheel
/weight distribution? Probably not and Boeing is not really saying you should (even if the CG is slightly more forward with the wheels retracted (nose and body gear rotate forward when retracted))

A 744 pilot said in a PPRuNe thread a long time ago that the CG is normally(?) aft of the wing gear strut, but I haven't been able to confirm this yet.

Perhaps someone has a table which converts CG to a body station value?

Regards
JHW

[torrence]

I don't have a detailed answer to the whole cg question, but when I checked Boeing info on ground maneuvering I was reminded that the 747's body gear also helps with the steering (up to 13 deg) with the whole rig tied to the amount of nose wheel deflection.  So the nose wheel is not doing the whole job in normal situation.  The turning envelopes are shown in the charts as dependent on cg position, but no force diagrams given.

http://www.boeing.com/commercial/airports/acaps/7474sec4.pdf

Cheers,
Torrence

[Hardy Heinlin]

Thanks to you all,

never mind, the problem was already solved, as I said. Of course, it all depends on the CG (and dynamic momentum) whether the aircraft will tail tip down.

Also, in reality, the distance between the wing gear and body gear station is a peanut compared to their distance to the nose gear. Wing and body gear, along with their strut and tilt flexibility, almost merge to one common pivot.

Thanks also for answering the RA question. (Garry, you could send a >RA OUCH  message to the EICAS :-))


Cheers,

|-|ardy

[dutch57]

Hi Hardy,

Ra altitude with gear down and on the ground level is -6 ft, during landing just at maingear touchdown Ra-altitude is o ft. This is done to obtain cat 3b autoland certification with zero visebility.

The Ra alt system is calibrated to give zero feet at touchdown(think pitch attitude and location of Ra-antenna`s) this results in the -6 when on ground during taxi or parked.

I think when putting the plane down on its belly you coud add a few feet maybe it will result in  approx -12 feet.

sincerly Bob

Ps it will come back with the gear issue and tipping

[Hardy Heinlin]

Hi Bob,

that's good, I get -12 in my model, too. I just use sine functions and the result can only be logical.

I also get -12 when my model sits on the wing gear with the nose gear retracted. As the RA antennas are located by the Queen's breast, and since it's her breast that touches the ground when the nose gear collapses, it's the same RA readout as with all gear up.

One cannot see the millimeters anyway as the readout goes in 2 feet intervals.

But the nuances in the strut compression can be seen by looking at the runway geometry in the outside view, e.g. when changing from 200 to 400 tons.


Cheers,

|-|ardy


Edit: No, I get -14. It's due to rounding at ca. -13.

[John H Watson]

I also get -12 when my model sits on the wing gear with the nose gear retracted. As the RA antennas are located by the Queen's breast, and since it's her breast that touches the ground when the nose gear collapses, it's the same RA readout as with all gear up.

Hardy, the RA Antennae are aligned with the wing root (shiny metal part/leading edge) at "Station 910" or thereabouts. I don't know how you get the same value as belly down.

See photo at bottom of page

http://www.aussieairliners.org/b-747/vh-inh/vhinh.html

[Hardy Heinlin]

On that photo, I guess the antenna is 2 ft AGL. Without that obstacle below the nose gear it could be less than 2 ft?

I'm not saying that both scenarios produce the same physical height (they do not in my model). But the EICAS 2 feet rounding indicates the same number.


Cheers,

|-|ardy

[John H Watson]

Looking at the people, I'd say about waist height. The belly is flat in that area.

I didn't realise the webpage provided a closeup

http://www.aussieairliners.org/b-747/vh-inh/4630.039.html