WingedMigrator
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Effect Of Bogie Tilt On Landing

Sun Aug 26, 2012 3:36 am

Conventional wisdom holds that aircraft with forward-tilted main landing gear bogies (767, A380) where the front axle touches down first will land "harder" than aircraft with rearward tilted, trailing MLG bogies (A330, 757) where the rear axle touches down first.

That sounds nice in theory, but is there a good physical explanation?

The MLG bogie articulation is for all intents a pinned joint, so which way it rotates should not matter one bit given the enormous loads involved. The only plausible explanation that I can think of is that the total stroke (between first wheel contact and strut compression) is longer for a rearward-tilted MLG bogie since the tilt angle can be much steeper... would that extra fraction of a second provide enough feedback for the pilot to better "finesse" the landing?

Or is this another aviation myth that people swear up and down is true? Physics please, no swearing it's true!
 
ak907
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RE: Effect Of Bogie Tilt On Landing

Sun Aug 26, 2012 4:57 am

I'm with you, the landing gear strut itself should not compress until all wheels are on the ground. The plane lands with a nose up attitude and a forward tilting bogey will be more level to the ground than an aft tilting bogey. For example, the plane lands with a 5 degree nose up attitude, and the fwd tilt bogey is also at 5 degrees. All wheels will be parallel to the ground. An aft tilted bogey on the other hand will have a 10 degree tilt relative to the ground.
 
pwdalmech
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RE: Effect Of Bogie Tilt On Landing

Sun Aug 26, 2012 5:40 am

On most aircraft the tilt or angle position of the bogie is dictated by the position the gear is stowed in the wheel well. The landing gear must retract in a configuration to fit in the wheel well.
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chimborazo
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RE: Effect Of Bogie Tilt On Landing

Sun Aug 26, 2012 11:36 am

When the wheels touch down there is a large transfer of energy in a short time to spin the wheels up from 0 to circa 150mph. That energy is transferred from the wheels through the bogie into the strut.

On a bogie with front wheel hanging low the action of the wheel spinning up tends to pull the bogie down onto the runway, biting hard is the best way I can think to describe it. That force goes straight into the strut as a bending moment.

On a bogie with rear wheels hanging low I would think it would be a lighter touch as the pivot is in front of the wheel and so if the bogie rotates it will tend to lift the weight from the wheels.

Effectively, front wheel first on a twin or triple axle, the bogie has no choice but to bite down as the forward acceleration of the wheel tends to rotate the bogie down.

That's how I've visualised it. With no crabbing angle they probably feel the same, but in a crab, the front axle down will tend to try and rotate to the actual vector. Similar to a shopping trolley, the wheels prefer to trail when the force is not not straight. With rear axle low I would imagine this force is lower as the bogie can 'lift'.

No direct experience of either type to compare as I'm not a pilot. Also, anecdotally, I've travelled on 777s a lot and nearly always found the landng has quite a "lurch" as the mlg gets fully planted. Have always put that down to the length of th 3 axle bogie: relatively higher side loads for a given crab angle compared to a twin axle bogie. A380 inboard gears would I imagine get this too but the effect is less as the track is much narrower compared to T7 mlg.
 
Okie
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RE: Effect Of Bogie Tilt On Landing

Sun Aug 26, 2012 11:56 pm

Try

http://www.youtube.com/watch?v=uvbZe87zQ4g&feature=related

Not much of a youtube fan myself, this was posted on Civil Av a while back, but these landings are worth watching.

Notice rudder involvement, anti-skid bouncing the gear, spoiler/lift dumper deployment.



Okie
 
bueb0g
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RE: Effect Of Bogie Tilt On Landing

Sat Sep 01, 2012 9:44 pm

Quoting ak907 (Reply 1):
For example, the plane lands with a 5 degree nose up attitude, and the fwd tilt bogey is also at 5 degrees. All wheels will be parallel to the ground. An aft tilted bogey on the other hand will have a 10 degree tilt relative to the ground.

No, it doesn't work that way... the tilt of the bogie is independent of the aircraft's pitch angle. If a bogie tilts forwards 5 degrees, it will be 5 deg forwards whether the pitch angle is 10 degrees nose down or 30 nose up. The same goes for backwards tilting (this obviously ceases to be true at very high nose pitch angles). So no, the wheels will not be parallel to the ground.
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Jetlagged
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RE: Effect Of Bogie Tilt On Landing

Sun Sep 02, 2012 7:42 am

The only adverse effect landing with a forward tilted bogie can make is that the wheel touches down in front of the strut. If the aircraft is slightly crabbed this will create a yawing moment tending to increase the crab angle, i.e. directionally destabilising. But the rear wheels will touch very soon afterwards so if this has any effect it will not last long. Clearly this isn't a significant problem for the many 767s flying around. However the 777, which also needs the gear to be tilted forwards during retraction and extension then has the bogies tilted rearwards for landing so the rear wheels touch first. So perhaps that extra system complication was worthwhile in terms of the touchdown dynamics?

Quoting bueb0g (Reply 5):
No, it doesn't work that way... the tilt of the bogie is independent of the aircraft's pitch angle.

That's not correct. The bogie is held in position, relative to the strut, by hydraulic pressure so it will always be at the same angle relative to aircraft pitch too.
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my235
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RE: Effect Of Bogie Tilt On Landing

Sun Sep 02, 2012 9:23 am

It's really the same reason landings are softer if you don't land on the nose gear and bounce I think. The landing gear should have "flare" to it.
 
rwessel
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RE: Effect Of Bogie Tilt On Landing

Wed Sep 05, 2012 7:13 pm

Quoting Chimborazo (Reply 3):
When the wheels touch down there is a large transfer of energy in a short time to spin the wheels up from 0 to circa 150mph. That energy is transferred from the wheels through the bogie into the strut.

On a bogie with front wheel hanging low the action of the wheel spinning up tends to pull the bogie down onto the runway, biting hard is the best way I can think to describe it. That force goes straight into the strut as a bending moment.

While locally a fair bit of energy is involved in spinning up a wheel, it's a pretty small amount relative to the mass and kinetic energy of an airliner. Assuming a 1.5m diameter wheel massing 200kg (and assuming it's a solid disk, which it's not), getting spun up to ~450rpm (~150kts), the total energy required is only about 60kJ. For comparison, a 100t aircraft at the speed will have kinetic energy around a quarter of a billion Joules. Even spinning up four wheels in that situation would only scrub off a thousandth of the kinetic energy of the aircraft.

Quoting Jetlagged (Reply 6):
The only adverse effect landing with a forward tilted bogie can make is that the wheel touches down in front of the strut. If the aircraft is slightly crabbed this will create a yawing moment tending to increase the crab angle, i.e. directionally destabilising. But the rear wheels will touch very soon afterwards so if this has any effect it will not last long.

No - only if the wheel is ahead of the CG of the *aircraft* will it tend to increase the yaw. If the wheel is ahead of the strut, but behind the CG, it will produce an anti-yaw force on the aircraft as a whole, but a definite torque in the other direction on the strut.

I don't know, but while mains are always mounted behind the CG on a tricycle gear aircraft, it seems plausible that the forward-most main wheels on at least some aircraft could actually be forward of the CG (in which case they would generate a pro-yaw force if they touched down first*), particularly with exceptionally long bogies like on the 777, or on configurations with "extra" bogies, like on the 747 or A380.

In any event it would be a minor force – it’s not that hard to tilt the bogies (absent some locking mechanism, like that used, *on takeoff*, on the longer 777s), so the pressure on the pavement when only one set of wheels is down is modest, hence the maximum amount of force they can generate is small as well.


*obviously not the case with the 777, 747 or A380 used in the rest of the example
 
WingedMigrator
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RE: Effect Of Bogie Tilt On Landing

Wed Sep 05, 2012 8:07 pm

Quoting Chimborazo (Reply 3):
On a bogie with front wheel hanging low the action of the wheel spinning up tends to pull the bogie down onto the runway, biting hard is the best way I can think to describe it.

The "biting hard" is reacted by an upward force on the aircraft through the strut. Could that really be felt as a harder landing?

The only difference from "biting hard" is that the wheel is spun up more quickly (due to greater normal force), but the total impulse--force times time or moment times time--imparted to the aircraft once the wheel is spun up is the same regardless of timing, and therefore regardless of front or rear axle first. With this in mind, I'm not sure how this "biting" phenomenon would make a big difference.

Quoting okie (Reply 4):
anti-skid bouncing the gear

Oh man, that really begs for a better control system... those bogies are bucking like crazy!

Quoting Jetlagged (Reply 6):
The only adverse effect landing with a forward tilted bogie can make is that the wheel touches down in front of the strut. If the aircraft is slightly crabbed this will create a yawing moment tending to increase the crab angle, i.e. directionally destabilising.

Not if the front wheel is behind the CG of the aircraft-- which it clearly always is. While there is as you describe a toe-in moment about the strut, the moment about the CG is in the opposite direction because the CG is forward of the gear.

Quoting Jetlagged (Reply 6):
The bogie is held in position, relative to the strut, by hydraulic pressure so it will always be at the same angle relative to aircraft pitch too.

That could be true of the 773ER (for tail clearance reasons), but is it generally true of all airliners? I would think a simple spring would do the job of properly positioning the bogie for retraction.
 
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Jetlagged
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RE: Effect Of Bogie Tilt On Landing

Wed Sep 05, 2012 10:47 pm

Quoting WingedMigrator (Reply 9):
Not if the front wheel is behind the CG of the aircraft-- which it clearly always is. While there is as you describe a toe-in moment about the strut, the moment about the CG is in the opposite direction because the CG is forward of the gear.

It doesn't matter whether it's ahead of the CG or not. There is a destabilising turning moment relative to the strut, which will then be transmitted as a turning moment to the aircraft.

Quoting WingedMigrator (Reply 9):
That could be true of the 773ER (for tail clearance reasons), but is it generally true of all airliners? I would think a simple spring would do the job of properly positioning the bogie for retraction.

Well it's certainly a hydraulic actuator on the 747 and 767 (and of course the 777), the bogie is hydraulically moved relative to the strut. A330 and A340 bogies are also hydraulically aligned. In each case the bogie must be in a given position to extend and retract the gear, so a spring simply isn't positive enough. It's nothing to do with 773 tail clearance either as when on ground the bogie is held level with the ground, a hydraulic strut would not resist the aerodynamic forces at rotation. Simple springs in landing gear is something you can only get away with in light aircraft.

This video shows a 777 gear swing and how bogie tilt changes during extension and retraction. When the gear is down and locked it tilts rearward. When UP is selected the bogie rotates to tilt forwards. This would not be possible with a simple spring.

http://www.youtube.com/watch?v=hJ3i_QNYASY

Even if a spring was used it would still push the bogie onto an end stop and so it would hold its position relative to the strut and therefore relative to the aircraft.
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WingedMigrator
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RE: Effect Of Bogie Tilt On Landing

Wed Sep 05, 2012 11:02 pm

Quoting Jetlagged (Reply 10):
There is a destabilising turning moment relative to the strut, which will then be transmitted as a turning moment to the aircraft.

Not quite. If the left gear's left front wheel touches the runway while the aircraft is yawed to the right, the moment about the strut axis will be clockwise (viewed from above) but the moment about the aircraft CG will be counter-clockwise, straightening out the aircraft. Moments are not "transmitted" like forces... they depend on the reference point.
 
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Jetlagged
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RE: Effect Of Bogie Tilt On Landing

Thu Sep 06, 2012 4:16 pm

Quoting WingedMigrator (Reply 11):
Not quite. If the left gear's left front wheel touches the runway while the aircraft is yawed to the right, the moment about the strut axis will be clockwise (viewed from above) but the moment about the aircraft CG will be counter-clockwise, straightening out the aircraft.

Your're thinking of the wheel as creating a moment about the CG, which it does of course. I'm looking at the wheel moment about the strut and the strut moment about the CG separately. Just as valid and the same total effect. I mentioned the wheel moment about the strut in isolation because that's the moment which changes depending on whether the front or rear wheel touches first, so that's what makes the difference to the ground handling.

Touching down front bogie wheel first is just like moving the main gear forward a few feet. It reduces the directional stability of the gear until the rear wheel touches and balances things out. Landing rear wheel first increases directional stability. The effect only lasts briefly. Only a 767 pilot could say if it's noticeable or not. However as Boeing took the trouble to make the 777 gear touch down rear wheel first it must have a beneficial effect.

Quoting WingedMigrator (Reply 11):
Moments are not "transmitted" like forces... they depend on the reference point.

Moments are transmitted and add up, just like forces. Structures 101. If you apply a yawing moment to a rigid aircraft structure at the wingtip the effect on its motion is exactly the same as applying the same yawing moment at the CG.
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tdscanuck
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RE: Effect Of Bogie Tilt On Landing

Thu Sep 06, 2012 6:12 pm

Quoting WingedMigrator (Reply 11):
Moments are not "transmitted" like forces... they depend on the reference point.

I think you're thinking of force-distance pairs (which give rise to moments). The moment you get from a particular force does depend on the reference point. But an applied moment (which already includes both the force and the arm if it's moment due to a force) absolutely does "transmit" just like a force. You can also directly apply moments (aka torques) which don't have a reference point and do "transmit" just like forces.

Tom.
 
WingedMigrator
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RE: Effect Of Bogie Tilt On Landing

Thu Sep 06, 2012 6:37 pm

Quoting tdscanuck (Reply 13):
But an applied moment (which already includes both the force and the arm if it's moment due to a force) absolutely does "transmit" just like a force.

Then in my example from reply 11, would the clockwise moment in the left strut result in right yaw of the aircraft? Jetlagged clearly seemed to think so in reply 6 ("a yawing moment tending to increase the crab angle, i.e. directionally destabilising"). Upon re-reading this, I think I understand now that he was describing only the direction of the strut moment, and not the resulting motion of the aircraft.

My point was that the clockwise moment reacted by the strut is overwhelmed by the counter-clockwise moment that the force transmitted through the strut applies about the aircraft CG. The result is left yaw of the aircraft, or a reduction of crab angle, regardless of bogie tilt or which axles touches down first.

Quoting Jetlagged (Reply 12):
Touching down front bogie wheel first is just like moving the main gear forward a few feet.

That I agree with.
 
tdscanuck
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RE: Effect Of Bogie Tilt On Landing

Thu Sep 06, 2012 6:54 pm

Quoting WingedMigrator (Reply 14):
Then in my example from reply 11, would the clockwise moment in the left strut result in right yaw of the aircraft?

Nope, it would result in left yaw like you said. It's not really an applied moment, it's an applied force (which is where I think the confusion is coming from). The resulting moment about the strut is different in amount and direction than the resulting moment about the CG.

Quoting WingedMigrator (Reply 14):
Jetlagged clearly seemed to think so in reply 6 ("a yawing moment tending to increase the crab angle, i.e. directionally destabilising"). Upon re-reading this, I think I understand now that he was describing only the direction of the strut moment, and not the resulting motion of the aircraft.

I had the latter interpretation (just talking about the strut).

Quoting WingedMigrator (Reply 14):
My point was that the clockwise moment reacted by the strut is overwhelmed by the counter-clockwise moment that the force transmitted through the strut applies about the aircraft CG.

I agree, but we're getting pretty fuzzy between forces and moments here. For an analysis like this, it's much easier to treat the airplane as a rigid body and ignore forces & moments within the strut...the aircraft is getting a off-CG force applied to the gear (there is no applied moment), causing a resulting moment and force on the aircraft.

Tom.
 
rwessel
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RE: Effect Of Bogie Tilt On Landing

Thu Sep 06, 2012 7:08 pm

Quoting Jetlagged (Reply 12):
Touching down front bogie wheel first is just like moving the main gear forward a few feet. It reduces the directional stability of the gear until the rear wheel touches and balances things out. Landing rear wheel first increases directional stability. The effect only lasts briefly. Only a 767 pilot could say if it's noticeable or not. However as Boeing took the trouble to make the 777 gear touch down rear wheel first it must have a beneficial effect.

I agree - but so long as the front wheel on the bogie remains behind the aircraft's CG, it will produce force contributing to positive directional stability, if somewhat less of that than if the read wheel had touched down first. But you originally said: "The only adverse effect landing with a forward tilted bogie can make is that the wheel touches down in front of the strut. If the aircraft is slightly crabbed this will create a yawing moment tending to increase the crab angle, i.e. directionally destabilising." which is not true, unless that forward wheel is in front of the CG.
 
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Jetlagged
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RE: Effect Of Bogie Tilt On Landing

Fri Sep 14, 2012 12:10 am

Quoting rwessel (Reply 16):
agree - but so long as the front wheel on the bogie remains behind the aircraft's CG, it will produce force contributing to positive directional stability, if somewhat less of that than if the read wheel had touched down first. But you originally said: "The only adverse effect landing with a forward tilted bogie can make is that the wheel touches down in front of the strut. If the aircraft is slightly crabbed this will create a yawing moment tending to increase the crab angle, i.e. directionally destabilising." which is not true, unless that forward wheel is in front of the CG.

I said it was destabilising, i.e. less directionally stable than if the rear wheel touches first. I didn't say it was unstable (which it would be if the wheel was ahead of the CG).

Quoting WingedMigrator (Reply 14):
My point was that the clockwise moment reacted by the strut is overwhelmed by the counter-clockwise moment that the force transmitted through the strut applies about the aircraft CG. The result is left yaw of the aircraft, or a reduction of crab angle, regardless of bogie tilt or which axles touches down first.

All this is true, but having the initial ground contact point forward of the gear strut makes the directional stability temporarily worse.
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