F-35 landing gear

Unread postPosted: 31 Oct 2016, 03:01
by BELA
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?

Re: F-35 landing gear

Unread postPosted: 31 Oct 2016, 03:29
by XanderCrews
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

Re: F-35 landing gear

Unread postPosted: 31 Oct 2016, 04:25
by spazsinbad
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

Re: F-35 landing gear

Unread postPosted: 31 Oct 2016, 06:07
by BELA
That's great Info Spaz..don't know how I missed it. Thanks much.

Re: F-35 landing gear

Unread postPosted: 31 Oct 2016, 06:25
by johnwill
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.

Re: F-35 landing gear

Unread postPosted: 31 Oct 2016, 09:26
by spazsinbad
Thanks 'johnwill' - side view of Dutch F-35A mit wheels.

Re: F-35 landing gear

Unread postPosted: 31 Oct 2016, 15:33
by BELA
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.

Re: F-35 landing gear

Unread postPosted: 01 Nov 2016, 15:08
by Dragon029
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.

Re: F-35 landing gear

Unread postPosted: 01 Nov 2016, 20:01
by BELA
I thought so, the word "sweep" threw me off a little. Thanks to both of you.

Re: F-35 landing gear

Unread postPosted: 04 Nov 2016, 10:34
by linkomart
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.

Re: F-35 landing gear

Unread postPosted: 04 Nov 2016, 15:27
by quicksilver
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.

Re: F-35 landing gear

Unread postPosted: 04 Nov 2016, 15:48
by linkomart
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?

Re: F-35 landing gear

Unread postPosted: 04 Nov 2016, 16:10
by quicksilver
A and the B are the same. C has virtually none.

Re: F-35 landing gear

Unread postPosted: 04 Nov 2016, 16:14
by linkomart
Killed two birds with one stone then....

Time for weekend...

Best regards

Re: F-35 landing gear

Unread postPosted: 04 Nov 2016, 17:55
by johnwill
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.

Re: F-35 landing gear

Unread postPosted: 04 Nov 2016, 18:13
by linkomart
johnwill wrote: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.


What I was trying to say in my earlier post, but clearly one of us has the better ability to express oneself.
Thanks for your insights and ability to share. It is much apreciated.

Regards

Re: F-35 landing gear

Unread postPosted: 10 Nov 2016, 14:10
by spazsinbad
"Description A U.S. Marine Corps Lockheed Martin F-35B Lightning II (BuNo 168059) on the runway during the first short take-off and vertical landing mission at Eglin Air Force Base, Florida (USA), on 24 October 2013. The milestone training mission was flown by Major Brendan M. Walsh of Marine Fighter Attack Training Squadron 501. Walsh qualified in vertical landing operations at Marine Corps Air Station Yuma, Arizona (USA), in preparation for this mission. Date 24 October 2013 Source U.S. Air Force photo 131024-F-oc707-001 from the Eglin Air Force Base website
Author U.S. Air Force photo by Samuel King PHOTO: https://upload.wikimedia.org/wikipedia/ ... B_2013.jpg (2.1Mb)

Source: https://commons.wikimedia.org/wiki/File ... B_2013.jpg

Re: F-35 landing gear

Unread postPosted: 15 Nov 2016, 19:09
by spazsinbad
Super Hornet F/A-18E/F 'tipback' angle info from a large PDF F/A-18E/F Configuration Baseline 02 Apr 1991: 130Mb

http://www.filefactory.com/file/26rjl6h ... DPI%29.pdf

Re: F-35 landing gear

Unread postPosted: 15 Nov 2016, 21:24
by BELA
Ok, thats another term that is new to me...."tail down angle" are they referring to the anhedral of the tail? .... or incidence?

Re: F-35 landing gear

Unread postPosted: 15 Nov 2016, 21:59
by linkomart
BELA wrote:Ok, thats another term that is new to me...."tail down angle" are they referring to the anhedral of the tail? .... or incidence?

Tipback angle is the angle when, balancing on the maingear, the Aircraft will tipover backwards and land on the tail. Tail down angle is the angle when, balancing on the maingear, the tail will hit the ground. Note:
Tipback is measured with landinggear in normal loaded position (approx 2/3 compressed is normal) Tail down is with gear fully extended.

Re: F-35 landing gear

Unread postPosted: 15 Nov 2016, 22:05
by linkomart
The angle is measured with the centre of gravity in the rearmost position....

Re: F-35 landing gear

Unread postPosted: 15 Nov 2016, 23:46
by count_to_10
Interesting, particularly given the F-18's noted difficulty getting it's nose into the air. I wonder if it would pay to make the main gear capable of moving backward while taxiing or parking, but forward for takeoff.

Re: F-35 landing gear

Unread postPosted: 16 Nov 2016, 00:09
by BELA
linkomart wrote:
BELA wrote:Ok, thats another term that is new to me...."tail down angle" are they referring to the anhedral of the tail? .... or incidence?

Tipback angle is the angle when, balancing on the maingear, the Aircraft will tipover backwards and land on the tail. Tail down angle is the angle when, balancing on the maingear, the tail will hit the ground. Note:
Tipback is measured with landinggear in normal loaded position (approx 2/3 compressed is normal) Tail down is with gear fully extended.


Ok got it. Thanks again Linkomart

Re: F-35 landing gear

Unread postPosted: 16 Nov 2016, 01:31
by spazsinbad
"...F-18's noted difficulty getting it's nose into the air..." info on change to nose strut to follow from same source above.

Re: F-35 landing gear

Unread postPosted: 16 Nov 2016, 02:46
by spazsinbad
Attached 19 page PDF showing content above and then very detailed drawings which can be Zoomed to see detail below:

http://www.filefactory.com/file/26rjl6h ... DPI%29.pdf (130Mb)

Re: F-35 landing gear

Unread postPosted: 16 Nov 2016, 05:55
by johnwill
count_to_10 wrote:Interesting, particularly given the F-18's noted difficulty getting it's nose into the air. I wonder if it would pay to make the main gear capable of moving backward while taxiing or parking, but forward for takeoff.


Evidently that is an unnecessary complication. The combined effects of inward deflected rudders, swept horizontal tails, and nose gear compression are enough to force the nose up. Of course, the NG compression is effective only for catapult takeoffs. What really hurts them is the forward CG location, since they never took full advantage of the aft CG capabilities of the fly by wire system. The E/F CG was moved back a bit, but is nowhere near the aft CG of the F-16.

Thanks to spaz for posting the E/F development .pdf. Some very interesting content. Notice the inlets are simply enlarged A/B/C/D round-ish rather than the eventual E/F rectangular ones. Similarly, the LEX looks like an enlarged version of the original rather than the much larger production E/F version. Interesting also is the -414 engine is the same as the -412 planned for the Navy A-12 Dorito plus an afterburner.

Re: F-35 landing gear

Unread postPosted: 16 Nov 2016, 07:29
by spazsinbad
'JW' was wondering if something like what is happening with the F-35C nose gear is happening on the Super Hornet NG?
Complex & Robust
Flight International F-35 Special ? 2014? Magazine

"Mark Ayton explains the highly complex landing gear systems used on the F-35...

...The nose gear of the CV variant is a dual stage gas over oil cantilever strut with a staged air curve that provides a source of high energy, which helps the aircraft to achieve adequate angle of attack when released from the catapult during take-off from the aircraft carrier....

...There are two reasons for having a staged shock strut for the nose gear on the F-35C CV variant.... The second is to store energy gained from the compression of the strut under the high pressure effect of the catapult. When the catapult lets go of the launch bar, the energy is released, providing a rotation that helps achieve the angle of attack necessary to get off the deck.

Similarly when the aircraft hits the deck on landing the strut is compressed and energy is stored to help rotate the aeroplane and get it back off the deck if the arrestor cables are missed and a ‘go-around’ or ‘bolter’ is required. Bolter is the term used when the aircraft’s tail hook misses the arrestor cables on the carrier deck forcing the pilot to go around for another landing...."

Source: No longer available at former URL - download available here: download/file.php?id=23692 (PDF 1.44Mb)

Re: F-35 landing gear

Unread postPosted: 16 Nov 2016, 08:06
by johnwill
Exactly the same functionality for both airplanes. As far as I know, all Navy catapult launched airplanes that use nose gear launching use the same process. Bridle launched do not, because they do not compress the nose gear. Even the F-111B I helped test 48 years ago had the same compressed strut design.

Notice the launch bar is connected to the shuttle at a 45 deg angle, so when the the bar pulls on the gear it is pulling forward and down. Look at the F-35C videos and you will notice the airplane nose drops abruptly and oscillates a couple of times as the airplane starts to move. The strut has compressed the air/oil in the cylinder until it hits bottom, then the tire is compressed and causes the oscillation. When the shuttle releases the launch bar, the tire expands and the shock strut is extended, both of which force the nose of the airplane upward to get the desired AoA.

Re: F-35 landing gear

Unread postPosted: 16 Nov 2016, 08:09
by spazsinbad
OK - thanks 'JW'.

Re: F-35 landing gear

Unread postPosted: 16 Nov 2016, 13:37
by spazsinbad
Aha. More on the nosewheel lift off issue - caused by unrealistic tipback requirements 4 page excerpt attached:
F/A-18 Flying Qualities Development
25 Mar 1983 J.M. Abercrombie; McDonell-Douglas Corporation

"...Nosewheel Liftoff
Figure 1 illustrates the first problem encountered in the flight test program which was experienced on the first flight: poor takeoff rotation characteristics -- high rotation speed and, as a result of a very brief time between nose gear liftoff and main gear liftoff {not much time for pilot reaction} a characteristic that was described as "explosive". The cause was two-fold. First, a main gear well aft of the center of gravity (to satisfy the unrealistic tip back requirement to be able to apply full brakes rolling backwards downhill on a 5 degree sloping deck without bumping the tail on the deck). Secondly, a horizontal tail that was virtually stalled at full airplane nose up deflection at takeoff attitudes. This problem was anticipated by us in the flying qualities business (as a matter of fact, our pre-first-flight simulations were virtually identical to actual flight experience) but it was decided to await flight verification before making any major changes...."

Source: http://aviationarchives.net/F-18%20Flyi ... opment.pdf (8.6Mb)