Best option for electric self starting glider

jfitch wrote on 4/11/2020 6:54 PM:
Despite all the hand wringing, in over 250 self launches and a few retrieves or relights in my ASH26, I have experienced only one failure to start, and that on the first ground check after the winter layup. I have had a very few incidents of unscheduled maintenance, in every case caught at annual or during preflight, and none of which would have resulted in a failure to start. I.e, 100% reliability. Nevertheless, I consider starting the engine outside of sure and easy glide to a known landing site foolhardy and know many pilots who have done so resulting in considerable drama.

My experience with my ASH26E is essentially the same as Jon's, but with about
700-800 self-launches and over 200 in-flight restarts, with only one start
failure. And, I also always have a field within an easy glide before attempting to
start the engine.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

"Part of the problem is that the pylon is located behind the prop, rather in the free stream. Increased thrust will cause higher pylon drag, even if you maintain the same airspeed"
== You should look at modern pylon solutions with clean single pylon and pusher foldable prop.
== FES is not free stream at all, due to fuselage interference.

"Still, neither configuration has a clear advantage for all missions. However, if I ever switch to electric, it will likely be for increased reliability. In this regard, FES would be the clear winner. They pylon in my ship ....."
== again there has been lessons learned over time since ASH, DG, SH and Lange designed their propulsion systems. Pylon designed for electric pushers can have much less components and demonstrate much better reliability.
== A possible advantage of electrical solutions is that it becomes practical for glider manufacturers to run automatically, in advance to users, many cycles on their prototype to identify early design issues and reliability risks. You can run 1000 cycles in a few weeks and get more cycle than any pilot will do over many years.
== If your mission does not require to climb 1000 m AGL before starting your task and you don't have the expectation to have the energy to come back home even when 100 km away, then FES is a solution. Otherwise, Pylon mounted electric pusher is probably the way to go.


"Nevertheless, I consider starting the engine outside of sure and easy glide to a known landing site foolhardy and know many pilots who have done so resulting in considerable drama"
== Amen

A couple of points: -
(i) On the Antares 20E the pylon is in front of the prop.
(ii) "Bricks" have various glide angles: The Antares20E is 30:1
with the engine up and the prop windmilling so better than
many basic two seat trainers.

On Sunday, April 12, 2020 at 1:00:05 PM UTC+1, Dave Walsh wrote:
A couple of points: -
(i) On the Antares 20E the pylon is in front of the prop.
(ii) "Bricks" have various glide angles: The Antares20E is 30:1
with the engine up and the prop windmilling so better than
many basic two seat trainers.

Why would the pylon being in front or behind the prop make a meaningful difference? Whichever way, you're either sucking or blowing air through it at a speed proportional to the thrust you're trying to make?

On 4/10/20 9:46 AM, Magnus wrote:
On Wednesday, April 8, 2020 at 5:14:54 PM UTC+2, wrote:
Hello Magnus,

I can't respond to flight characteristics of the two self-launchers you mentioned (LAK-17C and GP-Gliders JETA). But, I can say that I sat in a JETA and also have JETA serial number 5 on order.

The JETA MTOG, per the provisional flight manual, is 525KG (1157 lbs). Below are the advertised self-launch numbers with the larger battery:

a) 5 x takeoff and climb to 800 m (2,600 feet) altitude, or
b) 1 x takeoff and climb to 800 m altitude + 150 km (80 nm) autonomy, or
c) 1 x takeoff and total climb to 4 500 m (14,700 feet)
d) my number: aero tow and 185 km (100nm) autonomy based on 20 nm/launch.

Climb rate: 3,7 m/s (728 fpm or 7.2 knots)

takeoff distance: 180 m (590 feet)

I visited the GP-Glider factory near Krosno, Poland (jokingly to make sure that they weren't operating out of a tent). I was happy that they were doing things correctly. A visit to the LAK factory would have given me a balanced experience, but I was not able to do that.

Raul Boerner
LS-6BWL

Hi Raul,
When is your delivery of the Jeta? I am really interested in your experiences. Where are you flying?
According to the max wing loading of 60/37 kg/m2 the MTOM is below 475/285kg UL with a wing area of 7.77m2 - this also is indicated on the website. I suppose the take-off distance is without ballast but it is still good. How/where do you register it as UL?


I'd be interested in hearing when the order was placed, what the
original delivery date was supposed to be, and now when Raul
reallistically expects to get it.

Why would the pylon being in front or behind the prop make a meaningful difference? Whichever way, you're either sucking or blowing air through it at a speed proportional to the thrust you're trying to make?

Indeed front or behind does not make a lot of difference.
The cross section and shape of pylon is what matters.
The point of having the pylon in front is mainly to use foldable prop which lowers drag significantly when engine is not operating.

I would think that electric motors have a very real advantage in not
needing much, if any, warm up time.Â* Whereas my Stemme starts reliably
every time, after a long cold soak it requires a lot of time to get the
oil temperature up to a level where I'm willing to advance the
throttle.Â* I feel that I therefore need to be more considerate of what
is below the aircraft...

On 4/11/2020 7:54 PM, jfitch wrote:
On Saturday, April 11, 2020 at 5:16:32 PM UTC-7, David Shelton wrote:
Are you sure this analysis is correct? It does not match my understanding of aerodynamics. Yes, the engine pylon will add drag and reduce L/D by some amount. However it is parasitic drag, and once overcome at climb speed need not be overcome again and again as climb increases. Lift and drag at climb speed remain constant regardless of climb rate, once airborne in unaccelerated flight. Additional thrust is put directly to work as climb rate. In your example 100N extra would be required to push the pylon through the air, but every 100N above that contributes the same to climb on either ship. Where else would the energy go?
Part of the problem is that the pylon is located behind the prop, rather in the free stream. Increased thrust will cause higher pylon drag, even if you maintain the same airspeed.

Trim drag may be another factor. The pylon configuration has a substantial pitching moment. More thrust will require more trim drag.

Still, neither configuration has a clear advantage for all missions. However, if I ever switch to electric, it will likely be for increased reliabiity. In this reguard, FES would be the clear winner. They pylon in my ship is a Rube Goldberg system with a manual crank, chain, gear rack, gas strut, micro switches, hinged doors, cable stay, prop brake system, a lock to keep the engine seure while retracted, and a little mirror so I can see what the hell I'm doing. It's kind of amazing they figured this out before CAD!
Surely the increase in pylon drag due to increased thrust is at least equaled in the FES, where the prop wash is blown directly over the fuselage, wing root, and enpennage. But in either case it is no where near the total drag of the aircraft. I agree the FES is mechanically simpler.

Despite all the hand wringing, in over 250 self launches and a few retrieves or relights in my ASH26, I have experienced only one failure to start, and that on the first ground check after the winter layup. I have had a very few incidents of unscheduled maintenance, in every case caught at annual or during preflight, and none of which would have resulted in a failure to start. I.e, 100% reliability. Nevertheless, I consider starting the engine outside of sure and easy glide to a known landing site foolhardy and know many pilots who have done so resulting in considerable drama.

--
Dan, 5J

On Saturday, April 11, 2020 at 8:06:30 PM UTC-7, Eric Greenwell wrote:
On Saturday, April 11, 2020 at 1:09:56 PM UTC-7, Luka Žnidarši� wrote:
...
In case of FES self-launch, I feel much safer on critical altitudes till 50m,
as in case of motor failure there is still available pure glider efficiency to
perform a turn back if required. With retractable systems you end up flying a
brick, where you can hardly afford turning back without risk of spin entry.
Another big FES advantage is non problematic starting of engine above non
landable terrain. With retractable systems you should never try to start engine
without landing field below, as in case that engine do not start and cannot be
retracted for any reason, you end up flying a brick. Clearly propeller
clearance is advantage of pylon, but with pylon there are more problems related
to take-off in side-wind conditions. With FES there is much better rudder
efficiency. It is hard to compare FES with retractable systems, as they are
very different in also in flying style, and all pilots will prefer one over
another.

One: My ASH26E is hardly a "brick" with the pylon and gear extended, and I can
turn around to land back with just 200' AGL just as well as an unpowered glider
(for which I always used the 200' AGL criteria if the tow failed). I tested
that years ago - it loses very little during a 180 degree turn. A friend found the
same thing for his DG 400.

Two: It is true the FES pilot will have a better L/D after a failed start than a
similar engine/pylon glider (like my ASH26E) with a failed start AND a failed
retract. It is not necessary to be over a field, but only within gliding reach
with the mast extended. It is the same decision process for either propulsion
type. Unlike the ASH26E, the GP15 can risk a start almost as far from the landing
area as a comparable FES glider. Because mast is streamlined like an airfoil, and
the propeller folds in-line with the motor, it has much less drag than the
conventional "engine on a stick".

Three: I am not aware of any crosswind problems for engine/pylon gliders caused by
lack of rudder efficiency. The propeller air goes directly past the rudder from
the nearby propeller on the pylon, increasing the rudder's effectiveness at least
as much as the FES, where the propeller is much further from the rudder.
Regardless, it is not the rudder that is used to keep the glider going straight
down the runway in a crosswind, but the steerable tailwheel (every glider should
have one). On my ASH26E, I hold the tail wheel on the ground until about 25 knots
airspeed, then lift it off by moving from negative flaps to positive flaps (soft
fields may need a different technique).

Four: Off course, pilots will generally prefer one system over the other, but I
think some (many?) will find the GP15 much closer in operation to the FES gliders
than to the gasoline "engine on a pylon" gliders like my ASH26E. From clicking the
"mast up" switch to full power on the GP15 is 5 seconds; retraction is about 3
seconds. Compare this to my ASH26E, with about 20 seconds to initial power, and
about 30 seconds to retract partially for cooling, then another 1 to 4 minutes to
full retraction after cooling.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1


I find a huge difference in cross capability in my DG 800 on pavement vs. dirt or grass. You need help from the wheel on the ground to prevent weather vanning. I find the fringe turbulent thrust from the engine is not so effective on the rudder, standing behind the glider you fill most of the prop thrust is up near the elevator anyway. The pylon thrust line lifts the tail and reduces traction on the rear wheel. I have lost track of how many aborted takeoffs I have had in strong cross winds, while gliders being towed were doing just fine off dirt and grass..

BG wrote on 4/12/2020 9:45 AM:


Three: I am not aware of any crosswind problems for engine/pylon gliders caused by
lack of rudder efficiency. The propeller air goes directly past the rudder from
the nearby propeller on the pylon, increasing the rudder's effectiveness at least
as much as the FES, where the propeller is much further from the rudder.
Regardless, it is not the rudder that is used to keep the glider going straight
down the runway in a crosswind, but the steerable tailwheel (every glider should
have one). On my ASH26E, I hold the tail wheel on the ground until about 25 knots
airspeed, then lift it off by moving from negative flaps to positive flaps (soft
fields may need a different technique).


I find a huge difference in cross capability in my DG 800 on pavement vs. dirt or grass. You need help from the wheel on the ground to prevent weather vanning. I find the fringe turbulent thrust from the engine is not so effective on the rudder, standing behind the glider you fill most of the prop thrust is up near the elevator anyway. The pylon thrust line lifts the tail and reduces traction on the rear wheel. I have lost track of how many aborted takeoffs I have had in strong cross winds, while gliders being towed were doing just fine off dirt and grass..

I don't have any personal experience with the DG 800, but your description shows
it's very different in some respects from my ASH26E. I had no idea it was so bad,
or I would not have made such a general statement.

What measures have you tried to improve the soft field, crosswind takeoff? Are
your takeoffs unassisted? If so, have you tried placing the downwind wing on the
ground to help counteract the weathervaning from the wind?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

On Sunday, April 12, 2020 at 10:19:01 AM UTC-7, Eric Greenwell wrote:
BG wrote on 4/12/2020 9:45 AM:


Three: I am not aware of any crosswind problems for engine/pylon gliders caused by
lack of rudder efficiency. The propeller air goes directly past the rudder from
the nearby propeller on the pylon, increasing the rudder's effectiveness at least
as much as the FES, where the propeller is much further from the rudder.