Hi Greg,
I have done a fair amount of stress work (inc FE) on light aircraft landing gear. The steel leaf of the C150 is a very straight forward design using conventional materials and attachments - hence it is possible to get very accurate correlation between a decent FE model and a full scale test.
As with all things in the stress world, especially FE, your assumptions and methods will dictate the outcome.
The FE methodology you adopt depends on a) what you have available to you, b) what you need, c) the purpose of the analysis, d) how much time/budget have you got, e) etc (plus others.
I attached some pictures of a similar project l completed about 12-13 years ago. In that instance an aircraft (CFM Shadow, ultralight) had a landing gear that was originally designed for an aircraft mass of 396kg, landing ground reaction load factor of around 2.5. The designer used innovative materials (for the time) but was unable to control the quality of supply of some materials. Consequently the performance reduced, the aircraft mass grew and damages and failures started to occur. My brief was to design, test and certify (against JAR-VLA and BCAR Section S) a new main landing gear.
The design was developed using stress analysis and predictive FE modelling/analysis. The prototypes were tested against the regulations and the design was certified to JAR-VLA.
The tools needed were more extensive than you need (as the landing gear l designed was composite and hence had infinite variations in potential material properties. In summary the following steps were performed:
1. A thin shell FE model was created of the design. The FE model used elements of nominal length 20mm. Today l would use variable size, down to 10mm. The maximum thickness of my layups was around 12mm, so 20mm seemed reasonable.
2. Material properties were obtained from MSC Laminate Modeler software (a composites draping and layup tool now embedded within Patran. You can use metallic values direct from MMPDS (Mil Handbook).
3. The constraints and attachment structure for the aircraft were simulated in stages: a) initially assumed as rigid with a clamp hold down (like a C150), b) later the fuselage was modelled to enable the local structural compliance to reduce the stresses adjacent to the attachments. c) fully detailed clamp modelled in FE - straps, clamp bolts (inc pre-tension).
4) The first runs were made using a linear static solution (SOL 101 Nastran). This was OK for small displacements. However, the design needed to deflect around 220mm to achieve the right balance between a reasonable ground reaction load factor (3.0) and retaining reasonable prop clearance (it was a high boom pusher config a/c).
5) A large displacement FE solution was obtained initially using Nastran SOL 106, however, l dislike this code as the convergence criteria is weak and limiting when attempting to simulate dynamic performance (drop tests). These runs allowed the design to be partially optimised.
6) Dynamic performance was evaluated using LS-Dyna3D, this is a wonderful FE package/solver that is fully explicit. Sliding contacts, non-linear material models and extensive data recovery enable a full understanding of the potential performance to be made.
7) Having established the design criteria (geometry, laminate materials, layup and a ply by ply definition) l ensured the design was feasable for production. Small changes were incorporated.
8) The final design's static and dynamic performance was predicted using another iteration of analyis and hand calcs (detail stressing of the attachments, local fuselage reinforcements, etc).
9) Prototypes were manufactured and tested in lab conditions.
10) Predicted results were compared to the tests, some adjustments made to the FE models (in this case to the non-linear stress-strain curves of the composite parts) and correlation achieved (stiffness based to within +/-5% across the test range). The critical design criteria was proven to be reserve energy drop tests.
Advice for you:
1. Initially use a beam model and then a thin shell element model. Model the attachment system - do not over-constrain. If you want good stress correlation you may need to adopt a 3D mesh. If so ensure you use at least 3 elements through the thickness. Personally l would probably use 3D elements from the outset, but l have been doign FE work since 1984.
2. Use an FE solver than can cope with both material and geometric non-linearity. The C150 legs deflect over 300mm at ultimate loads, this is way outside ETB. I recommend LS-Dyna for drop tests simulation, but for a metallic structure Nastran SOL 106 will work or ABAQUS.
3. If you are trying to correlate to tests on a statically loaded aircraft (on its wheels) you will need to model the ground and the tyre/wheel. A LOT of energy is absorbed by the tyre. The sliding contact with the ground (even in a static test) are critical. To simplify things you may want to dis-assemble the gear from the plane and test separately. However, this will not achieve your goal for a drop test analysis.
4) Read and understand Pazmany's excellent book on Landing Gear design.
5) The C150 in the pictures looks like a typical high hours plane, be aware that the gear sags on high hours planes - so the measured performance will differ from a factory fresh version (if it still existed today).
I hope it all works out for you.
Regards
John W
