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Reply to Seb's Thruster EFATO Question

Guest TOSGcentral

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Guest TOSGcentral

Hi Seb,


I wanted to deal with your main question as a separate issue as it has less to do with the Drifter v’s Thruster but far more on how the Thruster is operated.


In an EFATO situation the Drifter will inherently have it all over the Thruster before it even starts because of it’s lower frontal area and consequent better glide and accelleration. But let us take a closer look at the Thruster situation and specifically the two seaters as there is less drama in the single seat area.


The basic problem is a combination of the Thruster large cross sectional area, the very steep sided Total Drag Curve and the initiation airspeed when the EFATO happens. Now I have done a lot of practical research, plus proven it hundreds of times in practice, on the following so you can take it as damn near Gospel!


When I first began flying Thrusters I was appalled at the way the machine was operated and was told firmly by a Pilot Examiner that if I had an engine failure in one below 200’ agl then the aircraft would crash! I considered the first situation correctable and the second to be utter garbage!


The main problem resided not in the Thruster itself but in the manner the type was being operated. This was a tad generic to the early days of ultralighting and specifically the Rotax 503 motors that were fitted to them.


Again this is not a product entirely of idiots that felt the laws of physics and sound maintenance practice had been repealed overnight and so invited drama – it was the direct consequence of lack of adequate flying training and forcing issues by operating out of marginal airstrips coupled with total lack of knowledge on how to maintain an engine, resulting in the evil reputation all early ultralighting got as being bloody dangerous.


The Thruster two seater types (Glasshouse, Gemini series, TST, T300 and T500) that were equipped with R503 (or lesser motors in the case of the Glasshouse) had insufficient power reserves particularly when two-up. They did work (particularly when flown solo) but were conditionally ‘marginal’ on climb performance.


Add a confined airstrip or the stimulus of a big line of trees to cross and pilots slipped into Best Climb Angle rather than the slightly higher speed Best Rate of Climb scenarios.


Now this is the physics of the situation: At either Best Climb Angle or Best Rate of Climb you are still on the low side of the Total Drag Curve – any loss of energy is going to take you further up the low speed side, decrease your glide angle and increase the height your require to regain adequate airspeed.


If you get an abrupt and substantial power failure on a Thruster (whatever you are doing but especially if you are climbing) then you are going to lose 7 knots of airspeed, at normal pilot arousal reaction time levels, before you have the aircraft in an attitude where it can maintain flight energy in the glide.


If you started at an airspeed that is too low this may not mean that you actually stall but will zoom you into a high drag regime and an appalling consequent sink rate. If this happens at a height (say 200’ agl) then my Pilot Examiner was entirely correct – you are going to crash because you have insufficient height to regain adequate control and manoeuvring energy.


Before anyone rushes off saying “I am not flying a Thruster – they are dangerous” – what I have just written applies to any aircraft in principle but especially to high drag, low inertia ultralights. Which is basically why we are a movement apart from the rest of aviation (although closely allied with it) and should have adequate training to cover the consequences inherent in the breed of aircraft category!


OK, this is very solvable. It comes down to operation of the aircraft. You either have to have a suitably large and unobstructed area to cater to a sensible climb scenario or you should not be attempting a take-off!


These are the maths. For a Thruster two seater the best minimum energy required speed is 48 knots. Add to that the 7 knots pilot reaction and attitude change time and you require 55 knots to be totally safe. Any power loss gives you time and on-board energy to regain an attitude that will provide a sustainable glide flight energy. Plus you are automatically returning to best glide speed and the aircraft does this for you!


The R503 does not produce enough power to do that most of the time and give a pleasing climb rate. The advent of the R532, and very soon after the dual ignition R582, gave the power surplus required and with one of these motors you can sustain both Safe Speed Near the Ground AND get a better climb rate than an R503.


In this set of circumstances the Thruster is equally as safe as the Drifter. It depends entirely on how you operate – or have been taught how it should be operated.







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