On Tuesday, 11 September 2018 04:49:04 UTC+10, wrote:
On Monday, September 10, 2018 at 5:56:11 AM UTC-5, AS wrote:
On Monday, September 10, 2018 at 3:39:49 AM UTC-4, wrote:
One potential problem with multiple jets is if one starts while the other doesn’t but pumps fuel through.
Good engineering may help but there is a known issue there. (PM)

Tom - good point but each turbine has its own controller monitoring what it is doing. If it doesn't ignite and run within the set parameters, fuel to it is cut off.

Uli
'AS'

I have been using the twin Jet Cat P300's for a number of years on my homebuilt experimental HP-18J. I have to say that they can be troublesome, especially if they are not operated according to specifications. I have been using diesel fuel with 5% oil for the bearing lubrication. If you use Jet Cat oil or Aeroshell 500 turbine oil the bearings are reliable for one or two seasons tops. This includes a full summer of self-launches and sustainer starts at our mid-west altitudes. I attended Parowan and Dave was one of my helpers who graciously assisted me retrieve the glider from an out landing. That day the failure was pilot error and not the turbines. I had failed to plug in the CPU's prior to takeoff so the turbines and fuel were just extra ballast that day. During the rest of the event, I used the turbines for sustainer. They started consistently at the airport altitude of 6K and also at flying altitude when needed, sometimes above 10K. With the two turbines, I feel more confident that at least one will start when needed. I typically will do a start sequence for both turbines for a climb and shut one down if I go into sustainer mode to extend my fuel endurance. The problems that I have encountered over the years have been bearing failures, one glow plug ignitor failure and one EGT failure. The turbines are in my opinion good but not great. They still have their issues, two of them are better than one for redundancy and for my application of self-launch. In the mid-west I can take off with a 1K runway roll and climb at 500 FPM. During flight one or both can be started for another climb or for sustainer operations. Operation at high density altitudes and elevated temperatures greatly decreases the performance so I choose to use a tow and use them only for sustainer ops. Here is a link for short clip from our mid-west airport https://www.youtube.com/watch?v=xeQIRrZ7JCU I am currently putting a salvaged ASW-27 back together, I am happy enough with the twin turbines to incorporate them into the rebuild of the glider for the same self-launch and sustainer abilities.

Thanks,
Chris

Thanks Chris, always nice to get actual operating experience.

As I said earlier using larger than you think you need turbines is a great idea as you can operate at reduced power. Ask the airlines why they do reduced power takeoffs. Even the military sometimes derate engines a little to get better engine life although in a shooting war against a near peer adversary I think they'd remove the limits.
Also it is VERY important to minimise deployed engine drag. Done right with adequate thrust to weight the best rate of climb speed should be in the region 80 to 100 knots for most modern 15/18 meter gliders. If it is lower than this the drag increment is too high.
Installation can help prevent fratricide/fuel events in the event an engine has a RUD event and if you are going to run on one engine, they should be independently retractable OR the intakes should be blanked off if the engine isn't running as otherwise the bearings on the non running engine are windmilling and getting no lubrication. It also isn't too difficult to install a Halon fire extinguisher and adequate fire warning sensors. The German self launchers I've seen have a fire warning light on the panel - and no extinguisher.

As for the ASH25 that burned and crashed in Australia in January, we currently have NO IDEA what happened. ATSB is investigating although I do not hold any hope that anything definitive will be found. The wreckage was cleared away before the investigators found out what questions to ask and what to look for. Australian civil aviation is a disgrace.I've seen the images of it on downwind with flames coming out the top of the engine compartment area and the landing gear well.
We did have a similar event some years ago with a Stemme S10 where the fuel system let fuel drip on to the Rotax 914's turbo. Pilot and 4 year old son were killed. Petrol/Avgas (Flash point minus -43 deg)is way more dangerous than Jet A-1/diesel (flash point 38 deg C to 60 deg C).
The expense of self launch/self retrieve? - depends on your priorities. My wife and I drive an 18 year old Honda Accord, only driven by little old couple to airport on weekends :-). I'm not a car person and it has only 149,500 kilometers on the clock. We do live on top of the shop.
Yep, it is a lot of work to install the FIRST setup. Subsequent ones won't reguire the design time.
Winch launching has small margins where if things go wrong they do so very quickly and instant correct, corrective action must be taken. The BGA did look at it and managed to improve the record. I've only ever done a couple of winch launches but hundreds of auto tows from both ends of the cable. I have no interest in doing any more winch/auto tows.
We will know that self launch has come of age when the gliders do not even have a tow hook option. Progress will only come when enough people experiment with piston, jet and electric self launch. I'm not prepared to go the electric method just yet as the current high energy density battery cells are dangerous when shorted, damaged in any way, run down, or indeed suffering from hidden manufacturing defects. It has been 10 years since the Tesla Roadster came out and current batteries have higher energy density but the other defects are still there. Any battery technology announced now is likely 5 to 10 years from being able to be bought commercially. I'm willing to be proved wrong on this but electric gliders need both high energy density and high power density and reasonable cycle life(at least 200 cycles depending on cost)along with resistance to catastrophic failure that results in fire..

Mike