F-35 landing gear

Design and construction
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BELA

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Unread post31 Oct 2016, 03:01

I did a search and couldn't find any info on this... apologies if it has been already answered, but could anyone hazard a guess on why the rear gear are angled forward like the are? they look to be a good 4 degree forward. could it just be a take off rotational issue?
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XanderCrews

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Unread post31 Oct 2016, 03:29

BELA wrote:I did a search and couldn't find any info on this... apologies if it has been already answered, but could anyone hazard a guess on why the rear gear are angled forward like the are? they look to be a good 4 degree forward. could it just be a take off rotational issue?



They were altered with the angle during the SWAT redesign. I know that much
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spazsinbad

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Unread post31 Oct 2016, 04:25

This old post has a lot of info about the F-35C gear however I'll attach a PDF page with full article.... You may find something here? http://www.scribd.com/doc/174844675/F-3 ... I#download

viewtopic.php?f=57&t=26634&p=282429&hilit=Ayton+complex+robust#p282429
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BELA

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Unread post31 Oct 2016, 06:07

That's great Info Spaz..don't know how I missed it. Thanks much.
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johnwill

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Unread post31 Oct 2016, 06:25

I can think of at least two reasons to sweep the gear strut forward. First is to change the distribution of load between the main and nose gears. By sweeping forward, the main gear contact point is closer to the CG, thus reacts more gear load proportionally than the nose gear. At landing touchdown, if the main gear contact point is moved aft, the airplane will rotate nose down faster, thus increasing the load on the nose gear and vice versa. Adjusting the sweep angle allows optimum balancing of main and nose gear load.

The other reason to sweep the gear strut forward is to reduce the bending load in the strut at impact. Here's what happens. At impact there is of course a large load on the tire contact point, perpendicular to the ground. Due to angle of attack, even an unswept strut is tilted back, so the impact load tries to bend the strut forward. But there is another load at the same time of impact, the load parallel to the ground that spins the wheel up to speed very very rapidly, called spin up load. That load tries to bend the strut aft, so impact load and spin up load tend to cancel the bending in the strut. That's good. By adjusting the strut sweep angle, the cancellation of strut bending can be optimized. Reduced strut bending results in a lighter strut and cylinder, along with less friction and wear.
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spazsinbad

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Unread post31 Oct 2016, 09:26

Thanks 'johnwill' - side view of Dutch F-35A mit wheels.
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BELA

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Unread post31 Oct 2016, 15:33

Johnwill,interesting info, but are you referring to the moving sweep /arc of a transitioning gear? I was referring to the angle of attatchment...the gear when in the weight on wheels config seems to have a built in forward angle to it.... kind of like the nosegear on an f-16.

Spaz.... that is a gorgeous side view pic. first time I have seen it.
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Dragon029

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Unread post01 Nov 2016, 15:08

BELA, he's talking about the same thing as you; he's explaining that the reason they have that angle is to make it easier to control pitch during take-off and pitch-rate during the end of aerobraking during landing (by having the main landing gear closer to the centre of gravity), and also so that as the F-35 lands, the forward angle absorbs the bending-back of the landing gear, as the wheels 'catch' the ground and go from 0rpm to 30rpm in a split second.
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BELA

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Unread post01 Nov 2016, 20:01

I thought so, the word "sweep" threw me off a little. Thanks to both of you.
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linkomart

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Unread post04 Nov 2016, 10:34

There might be another reason then the one johnwill suggested, it might be for commonality reasons with the F-35C.

I'm not 100% sure, but looking at pictures it seems like the landing gear on C is slightly less raked than on A. The placement of the main gear on an Airplane is dictated by the rearmost center of gravity, the contact point of the gear shall be on a line slanting about 15 degrees from the rearmost center of gravity. Put the wheel further back and the aircraft will have problem rotating at take off (thus giving a longer take off run). Put it further forward and the airplane might tip over when tugged by the ground crew.
On a naval aircraft things are a bit different, the airfield not only have the tendency to alter geographical location, it also heaves up and down and tilts. This means that the angle from the center of gravity needs to be more, rule of thumb says over 18 degrees.
landinggear.jpg

If I'm right, I would say that slanting the gear on A is an elegant solution to the commonality problem, the attachment point can be the same on A and C. It needs to be beefed up on C (or thinned down on A depending on witch reference you have) but that is a lot easier than moving load bearing structure.

my 5 cent.
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quicksilver

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Unread post04 Nov 2016, 15:27

What jw said.

The SWAT redesign, which affected all of the variants (and the manufacturing system to produce them), also changed weight distribution relative to the aerodynamic center of lift. The landing gear sweep helped accommodate those changes and resulted in an aircraft with improved takeoff and landing performance as well; AA-1 could not aero brake.
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linkomart

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Unread post04 Nov 2016, 15:48

quicksilver wrote:What jw said.

The SWAT redesign, which affected all of the variants (and the manufacturing system to produce them), also changed weight distribution relative to the aerodynamic center of lift. The landing gear sweep helped accommodate those changes and resulted in an aircraft with improved takeoff and landing performance as well; AA-1 could not aero brake.


Makes sense, Easier to rake the gear than to move the frames.
Do you know if the rake on the gear is the same on the A and C, or is it the same tip back angle on those?
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quicksilver

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Unread post04 Nov 2016, 16:10

A and the B are the same. C has virtually none.
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linkomart

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Unread post04 Nov 2016, 16:14

Killed two birds with one stone then....

Time for weekend...

Best regards
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johnwill

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Unread post04 Nov 2016, 17:55

Good points about different Navy requirements. One added requirement is tip-back resistance. On a carrier deck, the airplanes are often pushed backward with the tow truck and abruptly stopped near the deck or elevator edge. If the main gear is too far forward, the airplane may tip back and go over the side into the drink. Land based carrier suitability tests include demonstrations of tip back resistance at different weight / CG conditions.
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