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Landing Gear retraction

Posted: Wed Jul 29, 2020 11:47 am
by Rimsdale
Hi,

Correct me if I am wrong, but I am given to understand that when the gear is retracted into the wheelwell, braking is applied to stop the wheels spinning. If this is the case, can anyone explain why please?

Many thanks

Re: Landing Gear retraction

Posted: Wed Jul 29, 2020 1:47 pm
by GalaxyFlyer
It’s true due to gyroscopic forces as the gear retracting changes the plane of rotation.

Re: Landing Gear retraction

Posted: Wed Jul 29, 2020 1:55 pm
by DH106
As discussed on another thread, gyroscopic forces only come into play when the angle of the wheel axis changes during retraction e.g. sideways retracting gear, not forwards/backwards retracting.
Another good reason to brake during retraction is in the case of a damaged tire to prevent any flaling tread/debris damaging components withing the wheel well.

Re: Landing Gear retraction

Posted: Wed Jul 29, 2020 1:57 pm
by Starlionblue
GalaxyFlyer wrote:
It’s true due to gyroscopic forces as the gear retracting changes the plane of rotation.


To expand, if you apply a sideways force to a spinning thingy like the wheels you'll get quite the torque on the thing holding the spinning thingy, which in this case is the landing gear legs and struts.

To be clear, we can dispatch with some inoperative brakes so it is not a showstopper but it probably increases fatigue if you don't stop the wheels from spinning before retraction.

Since the nose gear retracts straight ahead (or backwards in some cases), there is no such issue and snubbers in the wheel well just stop the wheels once they are retracted.

Which leads to the question of how the Trident with its sideways retracting nose gear handled the issue. :D

Re: Landing Gear retraction

Posted: Wed Jul 29, 2020 3:04 pm
by GalaxyFlyer
On the C-5 , each bogie of six wheels rotated inboard (the forward pair were now facing toward aircraft centerline), the the strut folded as the gear retracted. Lots of torque in several planes. Inoperative braking required two minute wait before handle UP. Rare, I can’t remember it happening as the main cause would be anti-skid box failed.

Re: Landing Gear retraction

Posted: Wed Jul 29, 2020 4:03 pm
by WesternDC6B
Starlionblue wrote:
GalaxyFlyer wrote:
It’s true due to gyroscopic forces as the gear retracting changes the plane of rotation.


To expand, if you apply a sideways force to a spinning thingy like the wheels you'll get quite the torque on the thing holding the spinning thingy, which in this case is the landing gear legs and struts.
:D


Is thingine a specific engineering term,and if so. how does it relate to the doohickey?

Re: Landing Gear retraction

Posted: Wed Jul 29, 2020 5:43 pm
by Sancho99504
WesternDC6B wrote:
Starlionblue wrote:
GalaxyFlyer wrote:
It’s true due to gyroscopic forces as the gear retracting changes the plane of rotation.


To expand, if you apply a sideways force to a spinning thingy like the wheels you'll get quite the torque on the thing holding the spinning thingy, which in this case is the landing gear legs and struts.
:D


Is thingine a specific engineering term,and if so. how does it relate to the doohickey?

The exact same way doohickey relates to the whatchumacallit and thingamabobber......

Re: Landing Gear retraction

Posted: Thu Jul 30, 2020 11:15 am
by Legs
Rimsdale wrote:
Hi,

Correct me if I am wrong, but I am given to understand that when the gear is retracted into the wheelwell, braking is applied to stop the wheels spinning. If this is the case, can anyone explain why please?

Many thanks



Just to expand a little to the OP, tyres on the main gear almost universally get a quick dab of the brakes as one of the initial parts of the retraction sequence

The nose gear wheels, since they (usually) don't have any angular change and just fold straight up into the well, just get pressed against a snubber in the wheel well to stop them rolling.

Re: Landing Gear retraction

Posted: Thu Jul 30, 2020 11:41 am
by Rimsdale
Many thanks all!

Re: Landing Gear retraction

Posted: Fri Jul 31, 2020 10:41 pm
by LH707330
Also, most airliners (exception: some early 727s that then mostly had them yanked because they were not super useful) don't have brakes on the nose gear, so without the snubbers they'd just keep spinning.

Re: Landing Gear retraction

Posted: Sun Aug 02, 2020 11:08 am
by DH106
LH707330 wrote:
Also, most airliners (exception: some early 727s that then mostly had them yanked because they were not super useful) don't have brakes on the nose gear, so without the snubbers they'd just keep spinning.


+ the Convair 880/990, which had nodewheel brakes.

Re: Landing Gear retraction

Posted: Sun Aug 02, 2020 3:35 pm
by kalvado
Starlionblue wrote:
To expand, if you apply a sideways force to a spinning thingy like the wheels you'll get quite the torque on the thing holding the spinning thingy, which in this case is the landing gear legs and struts.

My feeling it will be primarily axle and bearings, not struts; twisting on strut and pitching force on the entire plane would be weaker effects..