Jab under cart

[Bruce Tuncks]

Thanks for cutting the leg. It shows that Oscar was right, and I have learned something new about fibreglass. It is a much more variable material than aluminium alloy, and a lot safer in that failures tend to be more gradual.

 

Nice to hear all is well now.

 

 

[seb7701]

Thanks heaps for that Ken - really appreciated. Dying to get my new legs on, but might not be until next week now. Tell me, the flocking that I was interested in was the flocking of the saddle to the leg. The tech manual suggests a dry fit to mark position of saddle on leg, then flock the saddle to the leg (24hrs drying time??), then tighten everything up. Was this how yours went? I am curious as to the need for flock on the saddle (aka clamping plate)??

 

 

[jabiruken]

Thanks heaps for that Ken - really appreciated. Dying to get my new legs on, but might not be until next week now. Tell me, the flocking that I was interested in was the flocking of the saddle to the leg. The tech manual suggests a dry fit to mark position of saddle on leg, then flock the saddle to the leg (24hrs drying time??), then tighten everything up. Was this how yours went? I am curious as to the need for flock on the saddle (aka clamping plate)??

No, I just used flap disc on sides to ensure the leg sat inside saddle. Not sure what flocking saddles in would achieve as they firmly hold leg in place, I understood flocking axle in place once alignment was done. There was no flock on my saddles but was evidence on my axles. I figured the inboard rubber mount was allowed to flex and having rubber on top of the leg at saddle would also compress to a degree to absorb arrivals. I can see heavy arrivals flexing leg forward, but the result will stretching aft saddle bolt.

 

 

[Bruce Tuncks]

Here's what I did ( 16 years ago and it is still ok),

 

The legs were sanded at the clamp positions. Then the clamps were covered with plastic ( packing tape) for mold release.

 

Then the legs were coated with flock and the clamps installed so that the excess flock was squeezed out. After curing, the clamps were undone and the excess flock was trimmed off before reinstalling the clamps.

 

 

[jabiruken]

Here's what I did ( 16 years ago and it is still ok),The legs were sanded at the clamp positions. Then the clamps were covered with plastic ( packing tape) for mold release.

Then the legs were coated with flock and the clamps installed so that the excess flock was squeezed out. After curing, the clamps were undone and the excess flock was trimmed off before reinstalling the clamps.

The issue I had was that the legs were to wide to fit inside clamp. So had to be touched up to fit, onced clamped up they appeared to sit well in clamp once secured. So, I just left them. I guess the idea of flocking the saddle was to ensure full distribution over the width of leg, but mine appear ok

 

 

[Bruce Tuncks]

My guess is the last wrapping bit was done by hand so they are all slightly different. In my case the legs fitted in but were bearing in the center so there was potential for fore and aft movement of the leg.

 

A regular check is to hold the plane still and try to move the wheel fore and aft. I reckon this was the weakness in the old legs which made them change to the newer design.

 

 

[johnm]

on any of these 300 kg planes (particularly high wing) its easy to get one person to lift the wingtip .................. thereby lifting the wheel off the ground and another person to check if the leg is secure ?

 

 

[jabiruken]

on any of these 300 kg planes (particularly high wing) its easy to get one person to lift the wingtip .................. thereby lifting the wheel off the ground and another person to check if the leg is secure ?

The issue is fore and aft movement or the leg rolling inside saddle clamp on heavy arrival. Movement would also screw up camber and toe. Creaking and groaning from under cart whilst taxiing would be a good indercater that something is up.

 

 

[Bruce Tuncks]

You don't need another person to do this check. If you don't have wheel pants, holding the strut and pushing back on the wheel with your foot will do the check. I do it like this. With wheel pants, you could push on the leg as low as possible.

 

I also think you could apply the park-brake and try to push the whole aircraft to and fro.

 

 

[Bruce Tuncks]

johmn, I have a recollection that you are not supposed to lift a Jabiru by the wingtip, but I couldn't find the reference when I looked.

 

At Gawler, we have had a Jabiru fall onto a wingtip with no damage but a Technam with a similar mishap needed a new wing.

 

If the Jabiru can fall on a wingtip with no damage then the lift you propose is probably quite safe. Maybe somebody else knows more about this.

 

 

[Oscar]

If you are lifting on the tip moulding rather than the wing skin, where the load is carried by the spar) I can see the inadvisability. However - the main wing structure would easily carry the weight being reacted at the opposing main wheel.

 

 

[Yenn]

Is the spar designed to carry the load of lifting a wheel off the ground with a wingtip?

 

Most wing spars decrease in strength as the flying loads reduce away from the centre of the aircraft. Lifting a wheel off the ground imposes more load on the spar than normal 1G loading because the far wheel is still on the ground and you are lifting all of the fuselage.

 

I wouldn't try it with an RV or the Corby I have built. The Corby spar is less than an inch deep at the tip, although it is designed to take tie down loads.

 

 

[facthunter]

You have a lot of leverage with a reasonable wingspan plane. A wide track U/C would rule it out but the other wheel will probably be taking 70% or more of the aircraft's weight, in the average situation. The nose or tailwheel will take some too. Wingtip areas may be down on strength as they don't need to carry much load but they are still designed to take around 5 times the S&L weight in the form of lift from the surfaces.. Point loading is not part of the design. Even sandbagging an inverted wing is not correct from the load simulation aspect either. Where the strut is fixed is one of the strongest parts of that type of wing. Nev

 

 

[Oscar]

The Jab wing is untapered and of constant section. It is also supported by a lift strut. The Jab is certificated at around +4G. The mainspar tapers in thickness outboard - but not in depth.

 

If you can't lift a Jab by the outer wing skin and have the wing not even noticeably deflect, then run away from it as soon as you have put it down - it's seriously damaged.

 

 

[jabiruken]

The Jab wing is untapered and of constant section. It is also supported by a lift strut. The Jab is certificated at around +4G. The mainspar tapers in thickness outboard - but not in depth.If you can't lift a Jab by the outer wing skin and have the wing not even noticeably deflect, then run away from it as soon as you have put it down - it's seriously damaged.

I have all ways lifted my wing after flight so to tuck the under carriage under as instructed by jab factory many years ago with my lsa 55, the idea was to prevent splayed leg syndrome after you push plane backwards.

 

 

[Bruce Tuncks]

Oscar, I really like your posts but this last one has me puzzled...could you please explain more?

 

In the meantime, I understand the factory saying not to lift at the wingtip for liability reasons, but I still can't find the reference.

 

Maybe I imagined it.

 

 

[Oscar]

Bruce: I can only measure the wing I have here: an LSA55. But I suspect there is not a huge difference in ratio betwen all Jabs. And my maths is not exactly world-class, so forgive me my errors..

 

I don't have the wing loading figures for the LSA 55, but the J120 is fairly similar. I believe the J120 wings are made from the last of the LSA55'variants' - perhaps the SP470?

 

The wing loading on the J120, is quoted at 63.5 or so KGS/m/2 at Gross weight of 500 kgs. . Since the wing chord is almost exactly 1000, then every 100 mm of span is carrying 6.35 kgs. BUT: when lifting a wing in the hangar, you subtract two x people at 85kgs: Gross weight of about 330 Kgs. = about 42 kgs/m/2, or 4.2 kgs/100mm

 

The lift strut attachment point on my wings, is at 1550 out from the root attachments - or almost half the distance to the tip rib at 3200. Now, when the lift strut is taken into consideration, the 1G loading at empty weight for the last 100mm of the wing span, would be about 4.2 kgs x say 16: about 67 kgs. ONLY the spar and wing skin outboard of the lift strut attachment point is in shear: inboard of that, it is in compression.

 

Now, for a hangar lift against the 'outboard' main wheel, I reckon Nev's estimate of about 70% of the aircraft weight is a good guide: so let's call the force required on the lifting wing tip rib is: 67 /.7: about 95 kgs. I am damn sure I can't lift 95 kgs using just my arms, but I think I can get a wheel up by getting my shoulder into the job.. Try yours: can you lift the inner wheel off the ground by pushing up at the tip rib? If you can then I reckon my estimate of around 100 kgs force is in the ballpark.

 

Now, Jabs are certified/certificated to at least +4G - so that last 100 mm of each wing is lifting about 25kgs. ( they are supposed to have a 1.5 x safety factor : 37kgs, and a claimed +7G structural limit:: about 42 kgs) Just at the .normal flight load' limit of 4G at MTOW, that means a force capability of 37 x 16: 555 kgs - distributed evenly from the outside wheel to the inside tip rib - NOT 555 kgs of fuselage!!!. It sounds CRAZY, but I think you'd need a real engineer to do the maths before you just laugh it off.

 

If you want the best informed opinion, the man to ask is Alan Kerr - he did the structural justification on all the early Jabs.

 

 

[jetjr]

The wing tip is a separate piece, doesn't look very structural.

 

Id estimate weight to lift a j200 by the wing just inboard of tip at around 40-50 kg effort

 

 

[Oscar]

The wing tip is a separate piece, doesn't look very structural.Id estimate weight to lift a j200 by the wing just inboard of tip at around 40-50 kg effort

Yep, don't lift by the tip extension, they're hollow boxes, but by the tip rib (at the end of the aileron.) Preferably right under the mainspar ( about 40% aft of the leading edge.) I think I made some incorrect assumptions in my maths above ( I never trust my maths, to be honest) and overstated the load on the last 100mm 'strip' of wing, but about 50 - 60 kgs sounds about right to me.

 

 

[jetjr]

For lifting aircraft for maintenence I bought a cheap engine crane, super cheap on special, and made a u shaped hook which fits over from to wing and lifts in the middle just outside strut

 

Quick and easy and wheels allow movement

 

 

[Bruce Tuncks]

My Jabiru weight 235 kg empty, and by taking moments about the offside wheel, I calculate that about 47kg of lift is required to lift the nearside wheel. So we all agree about the lift.

 

Nice idea about the engine crane jetjr.Those cranes are real cheap and could just save your back from a megabucks injury.

 

I've been thinking of getting one.

 

 

[seb7701]

Well, haven't had a chance to put my legs on yet, as I'll need a hand but did notices an interesting circular kinda deal between my old legs and the brakes. Does anyone know whether there is usually something between the bare gear leg and the brakes? In my case the spat bracket is the first item, but wasn't sure about how the circular spacer thingy got there?? I hadn't previously looked, but thought that there wasn't any washers etc required (unless for camber adjustment...)

 

As per the first photo, the camber alone from my droopy legs explains a lot about what I felt were bad manners during the landing phase!!!

 

 

 

 

[jabiruken]

Well, haven't had a chance to put my legs on yet, as I'll need a hand but did notices an interesting circular kinda deal between my old legs and the brakes. Does anyone know whether there is usually something between the bare gear leg and the brakes? In my case the spat bracket is the first item, but wasn't sure about how the circular spacer thingy got there?? I hadn't previously looked, but thought that there wasn't any washers etc required (unless for camber adjustment...)As per the first photo, the camber alone from my droopy legs explains a lot about what I felt were bad manners during the landing phase!!!

My legs looked worse than yours, They where scrubbing the inside of the tyres at an alarming rate. as for the washer? Nothing Found in my build manual stating there ever was one, it appears to be brake only fitted. My build log states that my shim style washers where fitted in the same place as your washers sometime after build in an attempt to slow tyre wear. Interesting to note that there is a huge math equation noted for saaa regarding the new geometry changes this had on aircraft as rims were also widened with 20 mm spacer at same time.

 

 

[seb7701]

Thanks Ken - couldn't see anything in the manual, but did wonder.

 

As for tyre wear, well, my tyres went from photo 1 to photo 2 in 20hrs.... The shots are fuzzy, but you get the idea!!

 

 

 

 

[jabiruken]

Thanks Ken - couldn't see anything in the manual, but did wonder.As for tyre wear, well, my tyres went from photo 1 to photo 2 in 20hrs.... The shots are fuzzy, but you get the idea!!

Looking at the photos I see your right leg bowed in the same place as mined, the left (pilots side) looks ok. In the end my brake rotors would touch ground on some arrivals, (they are slightly larger than the factory). I would check carefully the bolts and mounting points on aircraft upon removal as I suspect there is movement in your saddles. Also be aware factory bolts sent aren't always correct, there appears to be three different lengths. Unfortunately I had to pack some bolts with up to 4 washers, most of my original bolts where shorter than those supplied. I did mine on my own and yes it was a bit of a pain, especially when I stuffed up and had no one else to blame.