Getting your undercarriage settled correctly

[Beardie]

Hi all,

 

Currently building a few Roden 1/32 biplanes and wondering what steps y'all take to get the undercarriage properly squared off. When I was building in 1/48 I would typically assemble the undercarriage on the fuselage and correct and slight cant that occured by usind monofil rigging and tensioning it to true up the assembly. Roden instruct an approach of building the undercarriage as a separate unit and then offering it up to the fuselage which I have found on my first attempt has caused alignment problems and, between using EZline for the rigging and the larger size of the undercart, I don't have the facility to true it up.

 

I am not even sure to be honest just how true the assembly was on the real craft. Tips, techniques and general input welcomed on this subject

[Rob G]

The real aircraft would've been as perfect as the riggers could make. Firstly, professional pride, secondly, they tended to get attached to the aircraft and wanted to give it the best, third, ditto the pilot (yes, he's third - the aircraft was more loved as a rule, from what I've read), fourthly, seeing as the little jiggers weighed so little, any slight misalignment would have been been felt during taxying, possibly resulting in a crash during landing, almost certainly in a word from the pilot and the need for them to do it again.

I knew a chap who owned a fleet of Tiger Moths - despite being maintained by the same, well respected firm, they all flew differently. One was an absolute delight, three were OK and the last one was only ever flown when they needed her, because she barked. Everything was within spec, but all the little bits of variation added up to ick, and no-one liked her. Nothing dangerous, not even something that a new pilot would pick up on, just not right.

Not being familiar with Mr Roden's big biplanes, all I have are thoughts that you've probably already thunk. Use slow setting glue and support everything with Blu-tak or whatever as it sets, adjusting as required. Or, do it the way you usually would (damn the instructions!) and build it as you feel works best - kit makers don't always know the right way to do it.

[Beardie]

I take on board what you say Rob and I can agree wholeheartedly about rigging needing to be the best it could be, although I have read once or twice of a rigging line coming undone during flight. I wasn't sure whether this also applied to the undercart. I have been pondering it trying to figure out just how the undercart rigging functions and what it is required to do. The undercart should have a degree of bounce for landings and take-offs and the crossed rigging seems geared to stopping the gear struts from splaying excessively on impact with the ground although I would guess it couldn't take too much of a hard landing without busting. I would think that tensioning these wires simply to counter warp in the wooden undercart legs would possibly lead to imbalance in the tension on each side and increased risk of collapse. Obviously I am talking 'within reason' here not undercarts twisted so much that the aircraft performed drastic turn when trying to land or take off. As far as I can figure any slight tilt to one side or the other would little affect take-offs and landings because very quickly, due to their light weight and large wing area loading on the undercarriage on take-off would have been very brief from the prop being wound to tearing off down the field and landing I think you were much more at risk from bouncing all over the place on your bungee suspension.

 

 

[Rob G]

They would have made them as true as possible, no question. Light structures tend to move out of line a lot more easily and quickly than heavy ones, especially in a wind; a Camel only weighs 420kg, less than half a tonne. and these things were designed to catch the wind . Mass increases inertia, which increases resistance to change of direction (think of a motorbike vs a semi trailer.)  Watch the video I link down below and see how much effort the pilots have to put into maintaining a straight line in ideal conditions - rudders flapping all over the place. However, a half a degree on a full size a/c isn't usually visible to the naked eye, but it'll stick out like a sore thumb on even a large scale model. In the end, you do the best you can, and park it at an angle if you didn't manage to get it right (that's what I do, anyway )

 

The strength of most structures is based on the geometry of triangles.Here's a couple of (very bad) diagrams of how it works. First, simple bracing ie a farm gate (the brace can be anything suitable - timber, steel bar, wire rope) The weight of the gate is effectively transferred to the thick and strong gate post through the diagonal brace and the hinge.

 

 

Now, aircraft undercarriage.

 

 

Force applied at 1 will transfer through the axle to point 2, causing point 2a to want to swing up and out. That attempt to move will apply tension (stretch) to wire 3, which is secured at points 2 and 4. Wire 3 will resist the stretching movement and stay the same length, preventing the base of strut 5 from moving out (at point 2). (An imaginary line between the top of strut 5 and point 4 forms the base of a triangle)  When force 1 is removed, the whole system regains equilibrium and returns to even tension. If force 1 exceeds the breaking strength of any individual component, the whole system fails. Which is when the fuselage falls onto the airfield and the pilot has to buy the riggers drinks. The trick is to make the tensions between the various parts act together to strengthen the whole shebang. Anything under- or over-tensioned makes for a weak link and allows or forces undesigned strains into the system, which leaves it open to failure. Think of what happens to a tent when one peg is pulled out of the ground.

 

Properly set up and used, a timber structure is massively strong, as anyone who's sailed on a timber masted boat or crossed a timber bridge can attest; we're still using timber bridges here on parts of the sugar cane rail network, getting on for 100 years old. I would think that most u/c failures could be attributed to combat damage, or side loads exceeding their strength; WW1 aircraft often sideslipped into the field, for a lot of reasons, and if you weren't straight before you touched down... whump. The V-shaped strut pairs on each side are of course prevented from forward/aft movement by the bracing action that each has on the other - the diagonal wires could be replaced with wood, but weight, streamlining and adjustability makes wires much more practical.

 

In the modern world, triangles are our most used engineering form, especially for steel work. Bridges, crane booms, antennas, pedestrian walkways, scaffolding, all use the triangle as a bracing system. Next time you're out and about, have a look and see how much of it there is. Hope this helps (and isn't condescending. It's certainly not meant that way).

 

This link to Wikipedia may help with bracing.

 

https://en.wikipedia.org/wiki/Bracing_(aeronautics)

 

Suspension, such as it is, is supplied by either the natural flex of the whole airframe (timber has a LOT of give if constructed correctly), or in the later models, rubber bungee cords, rubber blocks or similar things arranged between the struts and the axle.

 

The opening 20 seconds of this clip shows how the axle is staying still while the port u/c strut moves up and down - the axle is essentially strapped to the strut apex with bungee cords. (And doesn't it sound glorious?)

 

https://www.youtube.com/watch?v=Hq78ZocOAkY

 

Here's a photo I found on Photobucket, showing how the bungees are wrapped around the axle to act as suspension - note how the bungees tie the strut and the axle together, thus holding the aircraft UP. The axle is ON TOP of the strut junction. If the bungees fails, the strut falls to the ground. (Great selection of images here too, for those who want to weather WW1 aircraft realistically.)

 

http://i1011.photobucket.com/albums/af234/Jamo_kiwi/Weathering WWI aircraft/ALBundercarriage_zps32f26d62.jpg

 

Larger aircraft such as the 0/400 and G.V etc, would most likely have used rubber blocks, similar to how we use springs in our cars these days.

I hope this helps. WW1 aircraft were certainly not easy or gentle to fly - if you managed to not get killed in training, you were doing well. Any more questions, ask away and I'll make up more stuff.

[Beardie]

I think perhaps that I should go back to using monofil for the undercart rigging to allow me to true it up more easily.

[TonyTiger66]

I learned a lot there @Rob G, very clearly explained.  .

 

The Camel in the linked video takes off in what seems like 15 feet and turns on a pin!

 

What a lovely aircraft.

 

Great thread chaps, helps to understand how it all works.

 

Best regards

TonyT

[Beardie]

most of the WWI scouts could take off in a very short distance. The Camels starboard turn rate is incredible due to the torque generated by the rotation of the engine. Not so quick to port though. This feature unfortunately also made it a killer of the inexperienced. The torque meant it was easy to send it into a spin and also meant that you constantly had to steer the aircraft against the pull of the torque to starboard. You can see the torque in that video when he is blipping the engine on the ground and when he is landing.

[Beardie]

I was actually thinking watching the camel land again that his undercart was canted slightly to port but I think again this is actually due to the torque of the engine.

[Brandy]

My method us to attach the undercarriage legs to the fuselage whilst it is propped up under the wings and level. I then position paint jars/plasticene or whatever else is necessary, or fits, to hold it in position as it dries. The axle is glued on last and the rigging will hold the struts firmly in position. Some undercarriage parts won't allow my drill and thread approach in 1:72 ( especially if I've used brass rod for the gear legs) so I then use stainless wire CA'd in place. This adds nothing structurally but the brass rod is itself inserted into drilled holes so it doesn't need any help to stay rigid.

Hope that helps!

 

Ian

[FZ6]

On 09/10/2016 at 1:59 PM, Beardie said:

I think perhaps that I should go back to using monofil for the undercart rigging to allow me to true it up more easily.

 

 

Roden arent the best fitting where undercarriage is concerned and the struts are thin and don't have the same positive fit compared to Wingnut Wings.

 

A couple of guys at the club make Wingnut Wings kits. The first guy uses monofilament and the other uses EZline or similar. The ones built with monofilament are much stronger whereas the ones with ezline wobble when the table is knocked at shows. I would use monofilament for the undercart to give it rigidity and consider pinning Roden undercarriage for strength if you can.

 

Regards,

 

Mark