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Guest TOSGcentral

OK, I will attempt to shed some light on this area – but – you have to remember that I can only express an opinion and that I have no status.


This situation is (needlessly in my view) a lot more complicated than it need be and that is basically because of Regulation and that impacts types other than the Thruster.


So let us start with some basics. To my knowledge the Thruster two seater underwent only one type approval. That was on a Gemini X (that was little more than a Glasshouse and may actually have been one). The aircraft was proof loaded with bricks to around 6g Positive and 4 G negative absolute, without failing.


The basic ANO 95.25 (later CAO 95.25) models were based on this original proof loading (that I have the photographic evidence of in detail). To my knowledge the factory did no more formal acceptance other than probably getting their in-house CAR35 to approve the various structural changes, through the TST into the T300/T500, that primarily affected operational longevity of the airframe rather than any flight loading structural changes. This maxed out at 400 kg MTOW for the T500 at 400 kg.


However, the T300 and T500 are virtually the same airframe other than a rear fuselage enclosure, a boom change length to redistribute the Centre of Gravity, and shortening of the ailerons to accommodate the broader back of the aircraft aft of the trailing edge.


In my view, with the information at my disposal, any of the Thruster two seaters should be safe to fly at 400 kg MTOW – bearing in mind that the first Gemini wing spars were a lot stronger than the later models in terms of wall thickness.


So what is the problem? This basically seems to reside in the ultralight regulatory evolution of increasing MTOW weight. It appears that the various models of two seat Thruster were treated as different aircraft, in documentation terms, rather than the same aircraft evolving. For example, the T500 could be more accurately described (in airworthiness terms) as a Gemini Mk 4!


But that did not happen. The models of Thruster came out with regulatory changes and were documented as such – just as engine changes happened that were approved for the type. This has caused a great deal of needless confusion (eg a Gemini cannot – in the view of some people) have a 582 engine because it was not certified with one when there is no aerodynamic reason why it should not by the ‘book’, while 582s are the safest thing that could happen to a Gemini and go on almost identical airframes in terms of flight design – but that engine was not available then!). The Thruster factory were forging ahead fast and cared little for the past – nor what the future would bring for current owners.


So the main challenge is not what the basic design is capable of withstanding as much as increasing weight permissions via regulation allowed further complicance but that was constrained merely by a name! The aircraft itself is obviously capable of taking the flight loads – in fact more so as the later models had reduced control travel and surface area that assists in any flutter area. How strong it actually is is not known because that has not been put into a formal flight envelope. There was never any need because the aircraft was designed beyond the regulation at the time of inception, and future regulatory changes as where then known.


HOWEVER! Take care that we do not get things back to front! As myself, Chris and others have repeatedly warned – the permitted flight regime of an aircraft is fixed by design – not regulation! Because RAAus has a max permitted MTOW of 600 kg (or whatever it is now) does not mean all ultralights can fly to that limit – they are constrained by their design limits!


Similarly, an ultralight constrained by regulation to a weight limit, but below it’s design criteria, can expand in permission up to the limit of design.


What the actual design criteria of the Thruster actually is, is unknown, other than they can fly to 400kg MTOW. We have some insight from the Poms with the T600 and taking them to 450 kg MTOW required some pretty substantial modification. So the answer is somewhere between 400 and 450 kg.


What later owners are now facing is the legacy of a control movement that has never gotten it’s airworthiness act together (no fingers at you Chris) and has pursued a policy of ever more weight with out thought to the consequences on the overall movement, but built on essential freedoms of very simple aircraft where the delineation was quite clear cut and simple.


I am not putting this alone on RAAus’s shoulders because I believe the manufacturer’s are more to blame for consistently pushing weight limits to the maximum up to the point of making them unworkable in practical terms in the AUF/RAAus regime. From that point of view RAAus has been more than accommodating in trying to sort out consequent messes from a member’s point of view. So good on them for that! In the case of the Thruster the reverse seems to be true and the aircraft was designed way beyond the regulation in force at it’s inception.


Techman may care to add comment?







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Gemini MTOW


If I remember rightly, my old 503 powered Gemini 25-0044 had a MTOW of 385 kg and if you tried to fly at any greater weight than that it didn't want to lift off, so that was that. Even at 385, the rate of climb was only about 200 feet per minute, so you looked for thermals if they were around, and tried to stay out of sink.





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Guest TOSGcentral

582 v’s 503?


What we are actually looking at is safety management of the aircraft’s total drag curve.


The 503s worked for the two seaters but were marginal. To get performance out of the aircraft in the critical first phase of flight (climb out) the engine was not powerful enough to give sufficient airspeed for a concept called “Safe Speed Near the Groundâ€Â!


This speed on any two seat Thruster is 55 knots and is an inbuilt insurance policy that even a total engine failure would not result in a loss of control.


The 503 cannot produce this AND give an adequate climb rate. So pilots and instructors operated them at max climb angle (and presumably prayed). This was insufficient airspeed and an engine failure would take you either straight to the stall or at least to the very high sink rate pre-stall regime. So there were a lot of belly-in’s and any engine failure in that configuration below 200’ virtually ensured some kind of crash.


The grunt provided by the 532 and 582 motors allowed both safe speed near the ground AND a climb rate exceeding that of the 503.


It is as simple as that!


Personally I would not entertain any routine basic training in any 503 powered Thruster two seater – they are simply not safe for the situations that students have to be taken to for valid training.







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  • 3 months later...

Back when I bought my T500 I talked with the Robinsons, and the story as I remembered it was that the airframe was designed for 450kgs, but the regulations limited the 95.25 type to 400kgs, however the Ronbinsons weren't the original designers so this information is hearsay. I've also seen the photo of the Thruster frame hanging upside down with hundreds of bricks hanging off it.


I know my aircraft (T500/582 + spats+ doors with 2 bladed cruise prop) has negligible climb performance at ISA+10 when MTOW is ~420kg.


My personal opinion, is that with fairings, and Brolga prop, a T500/582 would be flyable at 450kg at ISA in smooth air.


The real issue is with turbulent loading, the huge Thruster wing area creates large stresses during gusts, which are only made worse by needing to fly faster because of the higher weight . The usable window between Vmin control and Vturb penetration closes up rapidly. If you have ever flown through a cold front in a Thruster and gone from stall to Vne during the gusts a couple of times a minute, you don't want to repeat the experience at MTOW.


At first blush you might compare the wing loadings of the Cessna 152 at +/-3g with the 6G thruster loadings and say the Thruster is good for 800kg at 3g. But bear in mind that a 30kt gust generates only 3g in a Cessna, but generates 6g in a Thruster, (due to the different wing loadings); and we all have to fly in the same lumpy air!


No one has mentioned the effect of c.g. yet , if it's wrong then MTOW is irrelevant.


Cheers, BobT



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