L-39 型柔性支架用于 Freewing Albatros 飞行器的鼻部装备

L-39 Flex Brace for Nose Gear on Freewing Albatros

2023-06-25 18:07:39

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Summary

CAUTION: This design has been bench tested ONLY! If you install and fly it you may be the 1st to do so...

This Flex Brace is an alternative to the typical rigid nose gear supports. It makes use of the elastometric properties of polymers and the shape forming capability of 3D printing. The web thickness of the Brace 'spring' and type of filament will both affect the stiffness/reaction force. Based on my observations, the spring seems to have a non-linear force response which increases with deflection.

Design intent is to better distribute the aft and lateral forces on the nose gear between the 1) gear primary structural support , 2) fuselage foam on the perimeter of the brace

Potential benefits include
1) Improved nose gear & attachment durability
2) Reduced damage to fuselage foam on the perimeter of the brace compared to typical rigid supports
3) A two stage lateral spring stiffness to reduce the likelihood of retract force overload due to minor misalignment

CAUTION: Consider operating temperature range when selecting filament. Some polymers become brittle at low temperatures and could fail with 'normal' deflections. Some soften with increased temperature which could affect performance.

The design is intended to be adhered to the fuselage on the entirety of the Brace perimeter. Select an adhesive which is appropriate for the foam and filament. My preference is FoamTac if the filament is compatible. White Gorilla Glue is my 1st alternate.

Print Settings

Printer:
Bambu Labs X1 C
Supports:
No
Notes:
Polyamide (PA, Nylon) is the filament which seems to work best for me on this application

I got acceptable results with two types of PA. Carbon fiber (CF) is much stiffer than non-reinforced PA. A much 'thinner' spring thickness can be used with CF. It seems PAHT-CF with a 1.40mm thick spring is similar to CoPA with 1.60mm immediately after printing. However, 6 to 10 days later the CoPA braces seemed to soften dramatically. I suspect this is caused water absorption from the air. The PAHT-CF braces are not similarly affected.

I am testing thicker 'springs' in CoPA at present.

1) Bambu Labs PAHT-CF filament - 1.40mm, 1.35mm & 1.30mm brace files
2) Polymaker CoPA filament - 2.00mm, 1.80mm brace files

The 1.60mm brace file is not recommended for either filament above, but is offered in case you wish to try it with another material.

Both filaments were placed in a forced air dryer @ 70C for ~12hrs prior to printing.

I bench tested several other filaments including PLA, ASA, PC & PETG-CF with varied results.

User Guidelines

Flex Brace files provided have varied web thickness in the 'spring' portion of the Brace. The file name include the STL file thickness in mm eg 1.60, 1.40. Printed thickness may vary, but should be proportional for the same printer/filament/slice. These are provided so the user may tailor the Flex Brace to their preferences. eg filament material, overall stiffness

To evaluate the stiffness of the Flex Brace, lower the nose gear and apply lateral and aft force. Observe the relationship of force to deflection for both the plastic nose gear mount and the gear strut @ brace height. Now raise the gear and slide the Flex Brace into position without adhesive. Lower the gear and again observe the relationship of force to deflection. The deflection of the fuselage foam should be closely observed. If the foam deflection seems excessive, select a softer brace configuration. If the foam deflection seems inadequate, select a stiffer brace configuration. If you have a rigid nose gear support available, you may want to compare it in the same manor. Keep in mind that once glued, the Brace will have additional contact with the foam to better distribute the forces.

Consider that many fly successfully with the stock configuration (no supplemental support) Historically those with a nose gear support have used a fully rigid version. Any one of these may be the right option for you.

At your final selection, recommend printing a pair and using one for strength testing prior to installation. Trial fit into plane to verify the dimensions look correct relative to the strut. Remove and manually deflect the elastic portion as the gear strut would. Listen for popping and look for cracks. Reinstall in the plane to verify dimensional stability. It should look the same as the 1st time. If a gap opens up between the brace and the strut, the print failed the strength test. Best to resolve the strength issue before gluing it into you plane. Many of the other filament types I bench tested failed this test. The PA filaments cited above performed well consistently for me.