Sure is a good reason for the Rotax heavy spring then, as the accident to Jon revealed PDQ. But in that case of course the proud owner in the early stages of flight testing his creation got too clever & fiddled with reducing the tension in an important functional spring.
Trouble is in my engineering experience springs eventually get tired, or break where the stress build up is greatest, even wearing through where the often sharp edged load is carried. So to rely on one doing my job of opening the throttle, & hidden in two places under the cowling for years on end with perhaps some form of annual inspection, is a risk I'm not keen to take
The Rotax standard set up IMHO (minor admitttedly) is thus a risk: it relies purely on the integrity of a spring to open the the carb up when the cabin throttle lever/rod releases the cable. If the cable has a jam, sticks or otherwise throws up a 'wobbler' all you the pilot have is that spring to overcome it.
With it reversed to a conventional set up i.e. pull cables to open (e.g. my Rans S6-116 with Rotax 80 h.p. twin flex 'Bowden' type cables plus one junction box, to operate the two carb's. & successful for over 12 years) then within reason the pilot can apply sufficent pull through these cables in tension to ensure any untoward obstruction in the system can be overcome. The spring shuts the throttle just like almost all the other planes have.
Seems to me the conventional system has a lot going for it and positive opening via a cable (or better I guess the solid push/pull piano wire) is tried & tested.
Cables and throttle linkages of any type push/pull solid or stranded wire can fail and cause you a problem in flight.I personally don't care for the solid wire over the stranded because the carbs are a PITA to sync compared to a more flexible wire. The nice thing about the way Rotax has the spring to go full open on the carb and when using a thin flexible strand throttle cable is the carbs are easy to sync and tend to stay in sync better because there isn't any friction inside the cable sheath with thin strand wire, the throttle rarely has any issues, the thin strand wire doesn't hang up in the cable sheath and if anything from the throttle lever right up to carb fails the carb goes wide open for power and not back to idle that becomes more or less worthless. If one cable totally fails and one carb goes wide open then advancing the throttle linkage for the other carb lets it go wide open then you can fly where ever you want before landing or if it was on a take off or landing going wide open is more desirable than shutting down to idle. As far as the springs they tend to last for the life of most engines so long as they aren't exposed to a lot of moisture to rust, but inspecting them is easy and replacement is cheap and quick.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
Mike Brown,
Trouble is in my engineering experience springs eventually get tired, or break where the stress build up is greatest, even wearing through where the often sharp edged load is carried. So to rely on one doing my job of opening the throttle, & hidden in two places under the cowling for years on end with perhaps some form of annual inspection, is a risk I'm not keen to take
Mike:
I agree. The throttle springs should be inspected frequently and replaced often. The Bing 64 carb has been used widely on BMW motorcycles and many of the "Rotax" parts are available through BMW motorcycle dealers at 10 to 20% of the Rotax cost. This spring is one such part should you be so inclined.
Mike
The tiny plastic inserts that hold the spring on each end are a must. In a high vibration enviornment, they can be a real asset to prevent spring eye wear.
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