Question re forward cant on A320 nosewheel

[rooaaiast]

Having flown various pax aircraft, and now on A320, i have been puzzled why the nosegear is canted forward on the A320. It seems hard on the tyres on tight turns, and doesnt seem to do anything to improve tyre grip.
Anyone know the design reasons for this?
Thanks in advance.

[barit1]

Not unique to A320; Connies and several other types have used this. There's a prior PpRuNe thread (http://www.pprune.org/tech-log/359481-a320-family-nosegear-canted-forward.html#post4681102) (perchance you recognize the originator) on the subject.

[rooaaiast]

Bariti;
I do indeed,but since I see it every day, the question remains.
Strange that despite wandering off the relevant aircraft, no-one seems to have an answer.
It shall remain a mystery
Best Regards

[twochai]

Surely the reason is simply that the noswheel leg is canted forward (say 5 degrees?) to counteract the roughly 5 degree nosedown pitch angle of the fuselage during max effort braking, which puts the resulting loads into the fuselage at the vertical? A minor weight saving, perhaps.

[barit1]

I'd advance two possible motives:

1) Self-centering effect - due to the scrubbing force behind the steering axis tending to center the gear, as well as giving some shimmy dampening. The latter is especially true if the two NG tires (sorry, tyres...) are coupled via a cross-shaft. (I believe the CV240 etc. had this feature)

2) Possible envelope reduction in the retraction mechanism -> smaller NG bay.

Not sure if either applies to the A320 family, though.

[Pilot DAR]

I know for certain that reason (1) as presented by Barit1, is the case for many smaller aircraft (tricycle Cessnas) so I would think it valid for larger aircraft too.

[VinRouge]

Forward cant is used to get rid of unstable nosewheel shimmy.

Have to dig out my dynamics notes to give you the equations if you want, PM me and I will dig them out.

[Dakota435]

It's another way of achieving the required castering (self centring) effect.

Most large a/c get castering effect by having the strut vertical and offsetting the wheel axle aft so the bottom of the strut is like an L, like a shopping cart caster. This means compression loads are offset from the strut compression axis, so side loads are imposed on the strut sliding bearing surfaces when it compresses, which are added to drag side loads. Plus, side steering loads put a lot more torsion stress on the strut than if there was no offset.

Canting the strut (called steering rake angle) lets you move the axle to be in line with the strut and eliminate geometric side loads upon compression, plus less steering torsional loads and slightly less drag side loads (some of which now becomes compression load), while maintaining the castering effect. You can really see this on the Connie, which has several degrees of steering rake and the wheel axle in line with the strut axis just like a motorcycle, which I'll guess was done because the strut was so long. You'll notice the A-320 with its canted strut also has its tire axis right on the strut axis.

It's the same for motorcycles and bicycles etc., to create a castering effect with very little tire axle offset. With steering rake the pivot (steering) axis for the wheel intersects the ground ahead of a line extending vertically from the centre of the axle. How far ahead determines the castering tendency, so the larger the angle the stronger the castering effect.

Probably the best place to observe this is to compare road bicycles to mountain bikes, which have less steering rake to make them easier to maneouver over rough terrain. It's easy to ride a road bike hands off with its larger steering rake; a little more challenging for the mountain bike.

I think the big downside to this config on a/c with double nose wheels is that turning the nosewheel left/right causes the inside-turn wheel to move downward and aft and the outside wheel to move forward and up, so there is asymmetric tire loading that gets worse with increases in steering angle.

[On-MarkBob]

All good stuff proffered here but another reason is the mechanics of loading. I don't mean the baggage or passengers, but the mechanical loads exerted on the nose wheel particulary on touch-down. When striking the tarmac at speed the wheels are stationary and quite a load is imparted to 'spin them up'. By allowng a forward incline this force is directed into the oleo strut and not taken as a full load on the drag brace. This allows for a marginally lighter gear assembly. Every pound counts!

Bob.