On Wednesday, August 7, 2019 at 5:15:05 AM UTC-4, Paul Ruskin wrote:
At 20:30 06 August 2019, 2G wrote:

I have heard of far too many "failure to start" incidents
with jets to consider them a viable self-retrieve option. Just
consider it a bonus if they do start.

FES is far more reliable, assuming the battery fire
incidents are a thing of the past (there have been design
changes to the battery).

It's a bit more complicated than that.

Personally, I think it's a good idea to consider a start of any
engine a bonus, and to have a safe place to land if it
doesn't.

With the jets, if they are set up right they start very well.
Not all are set up right though, it seems. (It took a few
months to get mine set up correctly - it has been very
reliable ever since). Also, I know more FESs that have
ended up in fields than jets because they haven't had much
range after climbing.

With current battery technology the FESs have other
limitations too. My understanding is that full power is less
likely to be available on an even partially depleted battery.
So you can't perhaps do what you can do in a jet - climb
from low to a sensible height, then turn it off, and if
necessary do it all again a bit later. And again.

What was unexpected to me is that the FES owners I know
are using a higher decision height than I am with a jet. It
is the case that the jet takes ~40 seconds to get to full
power, but you know you have a start after 20 and having
the engine out adds little drag and workload. So in practice
you can start it at low key and go on with flying the rest of
a circuit - which puts my personal lowest start decision
height at about 500 ft AGL. (I've done it lower, but on
reflection think I was reducing my margins too much and
won't do it again). The FES owners seem to be using a lot
more than this due to lack of climb performance.

It's one of those interesting cases where there are several
different technologies to solve a problem - all have pros and
cons, but there's no clear winner. If you could double the
capacity of the batteries, then I think FES would win - but
as has been pointed out elsewhere, battery technology
moves slowly. So at the moment, it's a question of which
factors are important to you.

Paul

My sense, from talking to a number of FES users, is that The major benefit is search for lift, not just climb out at high power for the save. This uses very low power to search around for the thermal that makes the save and conserves battery. Doing at a bit greater height adds margin but also makes the lift found more usable.
Re battery technology. It will remain an evolution, likely not revolution. High volume batteries have a very few common packages. Example- the 18650 cell that is used in everything from lap tops to Teslas. A new battery needs to fit into the user product architecture, as well as the established production system, to get a viable user base. New cells are coming now that add about 10% more capacity while still handling high current loads.
FWIW
UH