Area-51
Joined: 03 May 2021
Posts: 414
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 Posted: Thu Jan 23, 2025 10:04 pm Post subject: The Obsidian Files - Mono Classic Undercarriage |
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In this chapter of the obsidian files we investigate the undercarriage of the Mono Classic and what the numbers returned.
The study came about through a primary request to review the possibility of fitting a 75" propeller that had been offered for trial, and an 8.50x6 tyre... The short answer to both these opportunities was, "not without raising the nose of the aircraft while on the ground and redesigning the undercarriage swing arm". So the swing arm got completely redesigned and results stress tested.
Results showed that at AUW of 650kg the OEM swing arm, at 4.2kg mass, was able to yield separately at 0.5g lateral and 2.5g vertical, and able to yield at 0.5g lateral and 1.5g vertical combined.
In contrast a redesigned 4041-T Chromoly tubular arm in 2.00mm wall thickness, at 4.7kg mass, was able to yield separately at 1.5g lateral and 2.5g vertical, and at 0.5g lateral and 2.5g vertical combined... The same design utilising a 3.00mm wall, with a mass of 7.0kg, was able to yield 2.5g lateral and 4g vertical. The design was rolled back to 2.00mm wall thickness.
Results indicated similar numbers across the OEM and substitute design. A lighter 1.65mm wall thickness was trialed and although weighing 3.2kg failed stress testing at AUW of 650kg plus 2.5g vertical loading. Overall the tubular chromoly design displayed more robust results than the OEM unit.
Fitting of an 8.50x6 tyre requires lateral clearance for 200mm width and 560mm OD of the tyre. Utilising the same swing geometry as the OEM arm and trimming of the aircraft belly skin forward of the centre tunnel bulkhead panel is required for retraction. Attention to the throttle sector arm cover may be required also. The redesigned tubular swing arm provides retraction stops to mitigate a wheel up landing scenario.
Utilising an 8.50x6 tyre on the classic mono allows for fitting of a 75" propeller. With the tail wheel down a propeller tip to ground clearance of 8.0" is achieved. With the aircraft in a Take Off Attitude with tail wheel raised off the ground propeller tip to ground clearance is 6.0". These dimensions have been calculated with allowance for tyre squish.
Although the above clearances look appealing they will be reduced considerably when a momentary rut combined with an over forward pitch is seen. Running the above 75" prop option would be for the experienced high time mono pilot only.
That's all for this episode of the Obsidian Files today...
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| Description: |
| Chromoly tubular swing arm for accomodating 8.50x6 tyre |
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50.06 KB |
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| Stress testing to material yield to arrive at "Hard Landing Limitation" figure. |
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51.6 KB |
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| Stress testing the OEM swing arm for a "Hard Landing Limitation" figure |
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39.34 KB |
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| Comparing the sizing of 75" diameter propeller to 63" size unit |
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63.67 KB |
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51.98 KB |
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47.61 KB |
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| 8.50x6 chromoly tubular swing arm mechanism |
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142.81 KB |
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| Trimming of the belly skin forward of the bulkhead |
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118.88 KB |
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52.71 KB |
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| Taxi attitude and 75" propeller tip to ground clearance - 8.0" |
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47.52 KB |
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| Take Off Attitude and 75" propeller tip to ground clearance - 6.0" |
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52.6 KB |
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40 Time(s) |
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