On Friday, 7 September 2018 23:43:00 UTC+10, kinsell wrote:
On 09/07/2018 02:11 AM, David Salmon wrote:
At 16:58 06 September 2018, John Seaborn A8 wrote:
Good information on the pump. Can you expand on this a bit? What voltages
a=
re needed on the pump for a normal start on the M&D Jet? What makes sense
f=
or tuning based on fuel, jet vs. diesel? What is the best way to monitor
th=
e voltage to the pump under power?

I've been reading this long long thread, as I'm on a gliding holiday and
the weather could be better.
Something not mentioned, pertinent, but not to the reliability problem, is
noise. May not matter too much in the wide open spaces, but can be a bit
anti-social in the UK. We had a Shark jet fly from our club, and it could
be heard at 2 to 3 times the distance of a Solo engine. I have flown a bT
for 11 years, it has never failed to start, except for human error. Of
course one day it might fail, so advice given in this thread is good.
As to winch launching, which I have survived for over 50 years and into 5
figures, taking off with a wing on the ground is only for those with
suicidal tendencies. I have witnessed the result.
Look at the simulated videos on the BGA web site, under Safe Winch
Launching. No need to re-invent the wheel.
Dave


I mentioned both the noise and the fuel consumption. Seen two jets run
out of fuel on self-retrieves, even though neither had done a sef-launch.

And yes winching starting with a wing down is a terrible idea. Our club
had a two-place Grob drop a wing during launch, pilot didn't release in
time, and bent the fuselage in half.


A few of comments:
Nobody with the current jet sustainers has made the slightest attempt at noise reduction.
Landing out is rapidly becoming anti-social too. I've seen one piston self launcher run out of fuel and land out too. Lots more have had the engine fail to start and when the piston engine is extended the L/D becomes not much better than a Cessna with an engine failure.

It is 2018. Why are we even talking about winch launching?

If you want more than 100Km range you are better off to replace the extra fuel with a second engine and just climb fast, then shut down and retract the engines
and use the excellent glider L/D to fly further. Alternatively use a bigger engine so the thing has a decent climb rate. the downside of that is if you must fly level, jet engines have poor SFC when throttled below about 70%. Two engines have advantages as you only need one to avoid an outlanding. If the start failure probability is 2% failing to get one out of two running is 1 in 2500.
Two also means it self launches. You need a thrust to weight ratio of about 0.13 or better, optimally 0.14 to 0.15 and actually full thrust ratio of around 0.18 or so which allows de-rated operation. Some attempts have not had this and I've been amazed at the projects that have been done where no estimated performance calculations were done. It isn't quite as easy as you might think as the best rate of climb for an optimal jet self launcher is in the region 90 to 100 knots.
It is also important to minimise the drag of the extended engines as this can have large effect on achieved rate of climb at these higher airspeeds. Gliders are very slick and it doesn't take much to make them perform worse.
I have a nice spreadsheet that lets you enter the glider mass, polar, jet thrust, temperature, pressure, runway surface, slope, extended engine drag increment and gives rate of climb vs IAS and liftoff distance and to 50 feet at 1.3Vs. What sketchy reports I've got on the jet projects' actual performance seems to validate it.

Mike