Moving the axle forward

[herman pahls]

I could land shorter in my Model 4 Kitfox if I could brake sooner and harder.
I saw this gear extension on a Cessna 140 and am considering trying the same idea on the Grove gear.
I can usually land in 200 feet but am wanting to get in shorter at times.

I have 2 concerns regarding this modification.

1. Would the Grove gear legs, the attach brackets to the longerons and the longerons be up to the loads?

2. Without doing the calculations, any idea how much the weight and balance will be effected moving 29" Bushwheels with Matco brakes forward 4"?

I would appreciate any ideas or comments before I waste any time trying this.

Years ago when I was considering a Just Highlander (which are too expensive for me), I was told by Troy the owner of Just that the main reason Highlanders land shorter than Kitfox's is the the gear is pushed forward placing more weight on the tail.
I have been told that Highlanders weigh 120 pounds on the tail which is close to twice the weight of a Kitfox Model 4 tail.

Thanks
Herman


http://i813.photobucket.com/albums/zz54/hermanpahls/2DF96A27-6814-42E4-9083-7EBA7CDAE898_zps8ki3jrr7.jpg (http://s813.photobucket.com/user/hermanpahls/media/2DF96A27-6814-42E4-9083-7EBA7CDAE898_zps8ki3jrr7.jpg.html)

[jiott]

Herman,

This mod would definitely put a torsional twisting load on the Grove gear. Probably not a problem except for the occasional hard landing. If it was me, I would send a dimensional sketch with your anticipated worst case loading to Grove and ask for an analysis. They may charge something for the calculations, but then you will know for sure.

As far as W&B, it should be fairly easy to treat the wheels and tires as loads at the associated arm. Subtract the existing moment and then add in the new moment. The Kitfox website spread sheet makes this sort of thing easy to do. You will first need to level the plane in flight attitude and measure the existing CG location of the wheels and tires with respect to the datum (wing leading edge at the root) to get the arm. The new arm would be 4" forward of this. Probably will need to remove one wheel/tire assembly to get its weight. You probably already know all this, but I don't know any easier way.

After all this, assuming the Grove gear can handle it, there will still be an unknown, and that will be-can the Kitfox attach brackets and longerons handle the increased twisting loads? No doubt the reserve strength for hard landings will be reduced.

Jim

[herman pahls]

Jim
Thanks for your ideas and I will contact Grove.
I was also considering asking John at Kitfox for his opinion.
Herman

[mscotter]

Yes, definitely get this change in front of landing gear and/or airframe manufacturer's eyes. As mentioned, you're changing the loading on the landing gear. For that matter, you're changing the loading on the landing gear attachment points too.

[cgruby]

Another thing one should be prepared for, moving the gear forward has an additional destabilizing effect on directional control, i.e., ground loops are easier to establish and control. The effect can be such that a docile airplane turns into a monster, or it can be hardly noticeable.

Cheers,

[Monocock]

Forgive me for saying, but surely if you're having to brake so hard that you risk nosing over, then you're either landing in a strip that's simply too short, or you're coming in too fast.

Modifying the airframe to enable you to stamp on the brakes seems rather excessive.

Or maybe I'm just misunderstanding your point.

[jiott]

Chuck, maybe I'm wrong, but I thought that a longer wheelbase would have the effect of Stabilizing the airplane with respect to ground loops rather than the other way around.

[Monocock]

It's more the fact that you're effectively shifting more of the overall weight behind the main axles than in front of them, therefore you have more rearward mass trying to overtake the forward mass.

[jiott]

OK, that makes sense. So you have two opposite things occurring: More weight aft of the mains is destabilizing but a longer wheelbase is stabilizing. I can see why it may be hard to predict what the overall effect will be, as Chuck said.

[wannafly]

Im thinking if you can land and stop in 200 feet you are just fine.:) Why complicate things. I would wonder if the tail wheel, springs, frame can take the extra load on the back end?

[cgruby]

OK, that makes sense. So you have two opposite things occurring: More weight aft of the mains is destabilizing but a longer wheelbase is stabilizing. I can see why it may be hard to predict what the overall effect will be, as Chuck said.

Back in my heyday building experimental airplanes, I saw a lot of guys who moved their gear forward to eliminate the nose-over tendency just to find they had created themselves a real monster to control. A good example, look at a sailplane, the wheel is just about on the cg. Did you ever hear of anyone ground looping a sailplane?

[Dusty]

Has anyone measured the position of the axle with the bungy to highwing cabane conversion? I suspect it may be slightly forward.
No adverse handling!
Better coordination between brakes elevator and throttle would be the best prevention from nose over!
I don't have the power of a 912 but a rearward position for me would be better as I have trouble getting the tail up before rolling and need nearly 2/3 throttle hard breaking and stick forward progressively applied on landing to keep the tailwheel up in the rough.

[Esser]

Sailplanes have a rudder the size of a sheet of plywood too. Many times they have a skid or forward wheel to fall onto when the brakes are applied.

[herman pahls]

I appreciate the replies and concerns about ground looping and nosing over.
Most of the landings I do is a balance between the brakes and the elevator, to stop as short as possible to practice for the type of flying i enjoy.
Take a look at minute 2:30 and 4:35 in the link below to see the type of flying I do (that is not me flying the cub) and why I want to move the gear forward to be able to brake sooner and harder.

https://www.youtube.com/watch?v=ucdb0TKu3rks

I was looking for structural advice regarding extending the Grove gear forward like the photo shows in post 1 as is commonly done on Cessna 140's to be able to brake harder and sooner.

[Av8r3400]

The leverage would be somewhat greater trying to bend the side door trusses.

This is what I did to strengthen this area on my IV project.

http://www.avidfoxflyers.com/uploads/monthly_01_2013/post-36-13595151980036.jpg

I would suggest if this area hasn't been strengthened on your plane already, it needs to be. An old trick for the Avid guys to help their bungee truss under the seat, was to cut plywood triangles to fit snugly inside the truss webbing, then hysol them on place. This will work here as well if welding is not an option.

[herman pahls]

Larry
I am sure your "Swiss Cheese" reinforcement would take care of my longeron concerns with extended and forward swept Grove gear.
How thick is that plate and any idea the weight added to make the reinforcement.
What style of gear will that airplane get?
Thanks
Herman

[Av8r3400]

That plane (the Mangy Fox) is getting a custom Grove spring that's 2" taller and 3" wider.

The webbing is made from .032" sheet steel stitch welded in place. Overall weight of that mod was about 3 pounds.

[Paul Z]

Larry are you planning on adding the wing slats, and practicing landing similar to the Super STOL guys. That definitely improves the strength of the landing gear & frame! I guess your planning on some smack down landings. :D

[Av8r3400]

It won't be "just" any old Kitfox when I'm done with it...

[HighWing]

Has anyone measured the position of the axle with the bungy to highwing cabane conversion? I suspect it may be slightly forward.
No adverse handling!
Better coordination between brakes elevator and throttle would be the best prevention from nose over!
I don't have the power of a 912 but a rearward position for me would be better as I have trouble getting the tail up before rolling and need nearly 2/3 throttle hard breaking and stick forward progressively applied on landing to keep the tailwheel up in the rough.

Dùsty,
That is an interesting question. When we designed that gear our intent was to give the airframe a 2" rise at the firewall and wider stance. The purpose of the rise was for better angle of attack on the take off run.

This was all done while the airplane was in flying attitude so the gear when the fuselage is rotated back into three point does move forward a bit relative to the aft rotating fuselage. I just measured mine and when in level flight attitude the leading edge of the wing is about 2-3/4" forward of the center of the axle. When in three point, the wing leading edge is about 9-1/2" aft of the leading edge. It would seem that this would add a bit of weight at the tail wheel in three point attitude.

The factory cabane gear has a profile that suggests they lengthened the gear leg while in three point while maintaining the fore/aft location of the axle. This is just supposition on my part, however.

We have always maintained that installing our gear did not make the airplane a "Bush" plane, but was intended to make the airplane easier to handle on improved strips and to eliminate the need for periodic bungee replacement.

It has long been my thought that the frequent stories of original bungee gear collapse in the early days was a design feature. The airframe was designed to be light weight and the gear, by design, the easier component to replace in case of stress damage in a hard landing. I think Larry is trending in the right direction. All the gear alternatives are quite robust. It is the air frame in the attachment area that typically remains the same while we ever increasingly challenge it. Again just one guy's opinion.

[cgruby]

I mentioned earlier that moving the gear forward could produce bad handling characteristics, and afterward saw a picture of a Grove installation where the legs are mounted forward of the center set of lugs on the lower longeron. That's certainly further forward than an inch or two. I'd say go for it.

[Monocock]

Yes, it uses the front lugs, but the axle points are pretty much exactly the same. Don't forget, on the standard gear the front of the leg (middle lug) goes straight down. It's not equidistant between middle and back lugs.

Same aircraft, different gear, same axle positions.

http://s6.postimg.org/bjufozq1p/image.jpg (http://postimg.org/image/bjufozq1p/)


http://s6.postimg.org/nzr5iqjdp/image.jpg (http://postimg.org/image/nzr5iqjdp/)

[cgruby]

Hmmmmm...............