Grove provides design details on Kitfox 7075 SPIRNG landing gear FYI

[rogerh12]

FYI on Grove spring gear:

Regarding the Grove 7075 spring gear questions (discussed earlier), and whether the Grove specified preformed gear is really appropriate for Kitfox 3 & 4 aircraft (some say it’s too stiff and won’t deform in a crash). I called Grove and got some insight into their design criteria for the Kitfox.

They size their Kitfox -4 -1200 gear for tail wheel operation. It is sized so that during a 3 G landing, which they qualify as a “hard landing”, but not a “crash landing”, the 7075 aluminum spring gear will be subject to a bending force maximum of 46,000 PSI. At this PSI level, the gear will flex, but not permanently deform.

For 7075 gear with their level of heat treating, the yield pressure is about 63,000, which would occur at what they guess to be about 5.5 G’s, a true “crash landing” where you want the gear to deform and absorb the impact energy. At this pressure, the 7075 will bend and permanently deform, but won’t actually break (that’s why the use it). Interestingly, using 7075 Aluminum allows them to make a lighter gear as the same size gear made of 2024 aluminum yields at 43,000 psi.

If bent badly, they can strengthen the gear up to 3 times, but after that it has permanently lost some of its spring and should be replaced (it could break on crash too).

I also asked them about this special spec Kitfox-4-1200 gear when used in Tr-gear applications. In this application, the mains are under somewhat less load then in tail dragger configuration, so this also supports my contemplation that in using this gear, at least in Tri-gear application, the gross weight of the Kitfox-4 could be safely increased to 1250 lbs if other modifications are also made (upgrading the tail feathers).

What do ya think?

Roger

[Dave S]

I think it sounds like this unit can be hard landed lots, but only crash landed 3X as long as a person limits their crash landings to 5.5 G

Sorry, Thought I needed to let you guys know I can occasionally come down with a case of demented humor. I know I normally take things too seriously.......

Dave S

[SkyPirate]

I'm liking the analogy with any gear a 3 g landing is pretty rough ,..for it to survive that and up to a 5.5 g landing "OUCH" that's a pretty good gear,.
something that has to be addressed when considering landing gear and the type terrain flying off of and on to ,.. is the tire choice,..the tire is in escents the shock absorber,..the landing gear is the spring,.. as with any application ,..ie air plane ,..auto,..etc bad shocks can brake a spring,.or make it too bouncy ,..but on the other hand, if your hitting the ground at 3 G's I garantee it's going to bounce or something is going to bend, regardless of the tire/landing gear choice,..
the lighter the aircraft ,..the more escential it is that you fly it all the way to the ground,...
It's like shooting a rifle weighing 6 lbs at cal. 300 magnum versus a 9 lb rifle at cal. 300 magnum ,..the guy squeezing the trigger on the 6 lb 300 mag is going to get bruised first

Chase

[rogerh12]

Chase;

One other thing Grove pointed out to me, regarding the tires. On landing, the spring gear flexes out, which stores energy from the landing, however, as the gear flexes back in it drags the tires back with it and scrubs the tires across the ground. This scrubbing absorbs and dissipates the energy of the impact (cool huh?). Therefore, tire selection is very important as tires play a multirole in in the landing gear. Also, he pointed out that if you land on glare ice, no dissipated energy (oh my aching butt)….. It’s all actually kind of complicated for such a seemingly simple physical system, or so Grove tells me.

Roger

[SkyPirate]

Makes sense to me Roger ,..the fulcrum ..center of the axle at the insection of the gear leg ,.. on the inboard side has nothing to reduce the action of the tire hitting the ground ,..so the gear flex's out as well as twists the nose of the tire away from center at the same time which would make the tire skid sideways slightly and reducing rebound providing the tire isn't too hard,..and the surface the tire is on has enough friction and will allow the flex to take place,.as you mentioned ,..on ice it would not.
this is true on all off set landing gear ,..just some exagerate the "toe out" flexing more then others.

Chase

[Russell320]

Did Grove give any advice on tire pressure or sizes?

[chefwarthog]

By the way, if I install Alaskan tires on a S.S. do I have to beef up the landing gear to 1550lb?

[SkyPirate]

Russ,.. the tire size would be your choice as to what fits your style of flying best,.inflation would be the manufacturer's recomended pressure for use.
Chef ,.I'm sure the stock gear on a model 7 SS would handle "alaskan" or tundra tires ,..

[Squirrelfox]

With my kit awaiting shipment I am still debating whether to certificate at 1550 or 1320. The question out there now is: Will the 1550 gross have negative effects if used at LSA gross wieghts? My thoughts are that the spring action might be to stiff. I have posted the Question to Grove but look forward to any debate.

[Dorsal]

Unless you have a need to be LSA 1550 is a clear winner in my book. I also have no concerns with my gear being too stiff.

[Squirrelfox]

Ah Dorsal...and that's the rub!

The only two things that LSA give are the use in case of medical issue. The safety thereafter that is seriously questioned. The other being possible expanded resale value with future aviators if not PSEL. Of course I have no desire to sell "my" airplane" and I'll keep fit forever though I'm approacing the seasoned age...and besides I seem to be getting a bit of Weight and balance creep.

[Av8r3400]

Why not build it for the 1550 gross, but only show the mtow of 1320 in the paperwork to be LSA compliant?

[RobS]

That would seem to make the most sense, as I believe you must meet the LSA definition at the time of the plane's original certification to have it qualify as LSA. If you certify at 1550, then it could never be brought back to LSA compliance if needed. Not sure about bringing the MTOW up to 1550 at a later date if needed, but that would seem doable??

[jtpitkin06]

Yes, you can change the maximum takeoff weight to a higher figure at a later date.

You can build your aircraft as E-AB, certify it with a takeoff weight of 1320 pounds and fly the aircraft with a sport pilot or above. If it meets the definition of LSA a sport pilot may fly it.



You may later go back and make a “major change” to increase the weight to 1550 with the FAA. It will require additional restricted flight testing of 5 hours. Once the weight is changed above 1320 only a private pilot or higher may operate the aircraft. You may never go back down in weight to make it LSA again as it will no longer meet the “continuously operated” definition of LSA. It may no longer be flown by a sport pilot.


Think of LSA as a one way revolving gate at the subway. Once you leave, you are outside and there is no hand stamp to return. Once the aircraft is certified for heavier weight, there is no return to LSA.


John Pitkin
Greenville, TX

[chefwarthog]

I will use my SS with (912uls or Viking fh110) mostly for back country flying hunting and fishing, I will carry some stuff in it, so what is the max gross weigth that this air plane can support?

To know if it will be more efficent to up grade my landing gear.

[Squirrelfox]

The max gross for the airframe per Kitfox is 1550 as non-LSA EAB. Other considerations come into play as wieght increase affects stall speed, etc. and we are the manufacturer that declares so to speak. Still, my plan seems to be to certificate LSA compliant at 1320 and build to 1550 as suggested. I'm awaiting a Grove reply regarding stiffness however. Besides, maybe the heavy gear will make my landings look good to the NAAB here at KCOI. (National Association of Airport Bums)

[rogerh12]

Regarding going from 1550 to 1320 post certificate.
Well, you could deregister the aircraft and then reregister it again as new "1320" LSA legal plane. But of course you would have to fly off another 40 hours. LSA rules are sorta grey though, exept for gross weight ,as it's on the paper work.. But by LSA rules, if your plane stalls at the legal 51 mph everyday, except one day when it stalled at 53 mphs ( for some reason), you can NEVER fly it as LSA again.. At least that's what the rules say. Will the FAA test fly your plane? No, but they will check the paperwork and if anthing is recorded that make the plane fall outside of LSA (like you wrote down the 53 mphs stall speed reading), it can't ever be flown LSA again.

Roger