« Reply #3 on: Feb 05, 2019, 12:27 pm »
Nice piece of bed time reading! ;)

Memories are BETTER than Dreams---"Capn" FLINT

« Reply #2 on: Feb 04, 2019, 7:31 pm »

I did a lot of work on this during the preparation of the MCA Hovercraft Code of Practise, and the result was a clear divergeance of the fan speed limits for racing and cruising use. The HCGB use fans well beyond the manufacturers recomended limit, at very high stress levels. This is justified because they apply many additional safety controls in race events, such as keeping people well clear, and they accept a failure rate that would be completely unacceptable in cruising use.

The HCGB allow fan speeds up to 168 m/s, whilst the MCA Hovercraft Code of Practise (ie construction regs for commercial light hovercraft) allow 133 m/s - much less. This value was a compromise, we recommend that fans are used at the manufacturers maximum speed limit (110 or 120 m/s depending on fan type). This has a huge impact on the stress of the blade and hub - see attached graph. What this shows is that at 168 m/s the fan is being used well above its safe fatugue limit, and failure is inevitable - its just a question of time. Since racing craft are get very few hours per year, they are able ot get away with these high stress levels.

What this means is we are using our fans at just 40% of the stress of the same blade in a racing craft - we are well into infinite fatigue life, and we have reserve strength for unexpected events such as a wave strike. To set it into context, the Britsh Hovercraft Regulations CAP 458 (large commercial craft), British Microlight Aircraft Association (TIL 011) and the Civil Aviation Authority (and us!) all require a static safety factor of 3, whilst the HCGB (WHF010) require a static safety factor of 2. Once fatigue is taken into account, the HCGB limit results in a negative safety factor, which is only acceptable due to the short duration well controlled environment of racing.

Ian Brooks
Gloucester, UK

« Reply #1 on: Feb 04, 2019, 9:56 am »
By way of dissemination of safety news, the HCGB has banned the use of this male to female hub joining due to three failures.
Perhaps we should all "bear this in mind" but from my slowly diminishing memory bank, i "think" our cruising recommended rev/load limits are lower than the racing fraternity, as indeed our power transmission and changes in such are much more mundane, i.e. cruising, rather than repeated throttle blipping, which is also bound to load the fan blades also.
When next skimming across the smooth loch -----think smooth meditational Huuuuuuuummmmm,
rather than an angry bee that has landed on a hot plate ZEEZIZIZIZEE
Just passing this on.  ;)

« Last Edit: Feb 04, 2019, 10:02 am by Nick Flint »
Memories are BETTER than Dreams---"Capn" FLINT