Best option for electric self starting glider

Dne sobota, 11. april 2020 12.02.19 UTC+2 je oseba jld napisala:
The propulsion efficiency has much more direct effect than pylon drag.
In fact, for Takeoff and climb performance, the overall glider drag has secondary effect compared to aircraft weight and available thrust.
Without fuselage interference and opportunities for larger props, the pylon mounted motor provides a significant efficiency advantage. This has a direct effect on climb performance in terms of climb rate and altitude gain.
Running large battery packs, beside running cooler, also provides better energy efficiency and installing large batteries in the wing reduces overall structure weight and provides better flying handling characteristics due to CG.

FES is a nice solution, especially for retrofit of existing gliders.
For a brand new design, there are opportunities for better optimizations.
Is GP at the optimum, probably not (yet:-), but they have certainly made good design decisions for both teh glider and the propulsion.

Actually efficiency of glider in powered mode is quite important:
1.Efficiency of FES powered glider in powered mode is the same as in glide mode.

FES powered glider, L/D 50 and 500kg T.O.W, minimum sink 0,5m/s:
-required thrust for level flight; 500kg/50=10kg which is 100N
-another 100N is required for climb rate 0,5m/s; 200N
-another 100N is required for climb rate 1,0m/s; 300N
-another 100N is required for climb rate 1,5m/s; 400N
-another 100N is required for climb rate 2,0m/s; 500N
-another 100N is required for climb rate 2,5m/s; 600N
-another 100N is required for climb rate 3,0m/s; 700N

2.Efficiency of most powered gliders with retractable systems is about half (or even worse) of the efficiency as glider. Note that pylon drag with its big pulley or motor on top is still there even if propeller is rotating .. Like flying with half opened airbrakes.

Pylon powered glider, with pylon extended, L/D 25 and 500kg T.O.W, minimum sink 1,0m/s:
-required thrust for level flight; 500/25=20kg which is 200N
-another 200N for climb rate 1m/s; 400N
-another 200N for climb rate 2m/s; 600N
-another 200N for climb rate 3m/s; 800N

Efficiency of propeller not depends just on its diameter but also how much thrust such propeller should create. Even with smaller propeller diameter is possible to achieve good efficiency when required thrust is smaller, for instance during level flight or more shallow climb rates and this is typical use of FES. As you can see above, with FES required thrust for climb rate 1m/s is 300N, while with pylon for 1m/s is required 400N. This seems to be possible to offset with higher efficiency of bigger propeller, but bigger propeller requires higher torque. With direct drive electric motors higher torque can be achieved only with bigger motor diameter, but this means even more drag. With more drag it would be required also more thrust from propeller, which means higher power, higher energy consumption, bigger battery packs, more weight. Another possibility is to install motor in the fuselage and transfer power with belts to pylon, but there are quite high loses in efficiency, more that most engineers imagine and so not in equation. Diameter of upper pulley is usually bigger than lower pulley, due to reduction ratio, which is not helpful from drag viewpoint. And number of propeller blades do not affect much to its efficiency. It is only helpful if prop loading it very high but is also possible to increase chord of propeller blades to some extent.

So 700N for FES with 1m prop diameter or 800N with 1,4m (or bigger) retractable system to climb 3m/s, seems to be in advantage of pylon due to better prop efficiency. However, when you put all factors into equation, result is almost the same.
However, in the range of lower power settings FES is much more efficient (100N vs 200N).
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.
I believe that in next months there will be more information about LAK17C FES.

BG wrote on 4/11/2020 10:17 AM:
On Saturday, April 11, 2020 at 9:04:39 AM UTC-7, wrote:
On Saturday, April 11, 2020 at 4:51:34 PM UTC+1, Martin Gregorie wrote:
On Sat, 11 Apr 2020 08:10:55 -0700, Muttley wrote:


Like to confirm a real world experience I had some years back while visiting El Tiro. I was there with my DG 800b took real interest in an Antares 20e that was tied down. I beleive the advertised climb from sea level in still air is 9000 ft. Well, sitting next to the glider was a hefty Honda gas generator. I asked the owner some question about his experience so far. First thing he said was he was disappointed. They found once the ground temperatures were above 100F they need to keep it in the shade before launch. If they left it ouside the batteries would over heat at around 1000AGL. Guess it� was designed for cooler climates like in Germany. Next I asked him how long it took to recharge the batteries from the generator.. He basically said all night long and consumed 10 gallons of gas. I doubt there are airports that would allow you to tap into their grid. El Tiro was using solar panels. On sites like Truckee where you can camp on the airport, you would become very unpopular in a hurry have that much noise. I have never seen an Antares fly from there, getting back into Truckee requires a climb to over 12,000 feet to safely get back in. The surrounding routes back in might have you starting the engine at 5-6k. Very marginal or impossible if you used the engine for self launch at the beginning of the day. In contrast the DG 800 will climb to well over 14k, personally in the winter I took off from sea level and climbed to 16k and it was still gaining 100-200 FPM. Recharging the gas tank takes less than 5 minutes for 5 gallons of fuel. When the battery's reach a much higher level of capacity and are able to be recharged in a different way, the electric glider will come of age, and i have no doubt they will. The backing to behind this push is driven by electric cars. Asking any one who owns a pure electric car and hear their stories on range anxiety. ONLY Tesla usiong super chargers do you see a charge time to drive time much less than one. My Volt is horrible! From a 110vac socket pulling 8 amps, it takes over 12 hours to get 40 minutes of driving time at 60 mph.

It is strange to hear "When the battery's reach a much higher level of capacity
and are able to be recharged in a different way, the electric glider will come of
age", as the number in service and on order increase quickly, and the pilots are
smiling broadly. The current batteries a quite sufficient for a lot of pilots,
especially for FES pilots.

I've flown my ASH 26 E for 25 years, so I have plenty of experience with the range
I use and want. Based on that experience (over 4000 hours), I decided the GP15
with the large battery would easily handle all but a few of the self-retrieve
situations I've encountered. Those few situations can be avoided by turning around
in flight a bit sooner, or accepting a car or towplane retrieve every few years.

Recharging the batteries may be the more difficult problem, but at the places I
normally fly, all have at least one of these choices:

- plugging in to the FBO sockets, RV campground power pedestals, or friend's hangars
- charging with my RV's generator, or with a portable generator
- taking the batteries to the motel for charging

Your judgement is based on how you operate, but your criteria are not universal,
as I have indicated. If we went by your DG 800 comparison, no one would be flying
an FES, and a pure glider would never fly from Truckee!

--
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

Not sure about a "competitive advantage" but you must be in
the running for a Darwin Award. Obviously Covid-19 holds no
fears for you.

Dave, I've already run my engine at 200ft more times than I can remember! Damn near every self-launch seems to involve climbing through 200ft with the engine out

To be clear, I haven't suggested doing anything dangerous. I'm exploring very reasonable operating limitations for a hypethetical motor glider that is orders of magnitude more reliable than a traditional one. Since a couple orders of magnitude is a VERY BIG difference, you really can't draw on experiences or practices from traditional motor gliders. It's a bit of a paradym shift so you really need to look at the math to get your mind around it.

My premise is this:

1) It is physically possible to install a redundant sustainer system on an existing FES motor glider.

2) The math suggests that there is room to improve reliability by several orders of magnitude.

3) With sufficient reliability, one could SAFELY operate over unlandable terrain, or lower decision heights. SAFELY means that the risk can be made very small relative to our other flight risks (stall/spin, mid-air, weather, etc.).

By the way, my cousin flies accross the ocean for work. My ex flies commercial heli tours over volcanos. My neighbor's airplane is certified to fly in known icing conditions. My aerial applicator buddies fly around at 5ft AGL with only ONE engine. I don't think it's very far-fetched to imagine an occasional trip down to 200ft AGL in a motor glider with redundant propulsion.. Unfortunately, existing motor gliders are just soooooo ****ty that we can't imagine starting the engine unless our fingers are crossed and we're at least 1,000ft above a landing field. Many accept this as reality, but I see this as an area for substantial improvement.

I don't pretend to be a techie on the subject of self-launch gliders, but I do have some personal experience to offer. I went to Lithuania in October 2018 to test fly and then buy a LAK17b Mini FES.
The demo Mini was not ready for flying when I arrived, so I made two flights in a LAK17B FES in 18 meter configuration that had been modified with a higher wheel gear and the more powerful generation III battery pak to give it self-launch capability. I believe this is the same FES system that is now being supplied with the new Lak17C FES.

I had never flown a motor glider before, but I found launching the LAK17B to be dead simple: twist the power knob to 4500 RPM, keep the tail wheel on the ground until lift off is achieved and then follow a launch trajectory that seemed to me to be very similar to my aero tow experience. I made multiple engine starts and stops in the air to get the hang of the propeller docking system and then landed.

On the final day I flew the Mini from the same relatively short paved runway. The take off was much the same, except a bit quicker. Baltic fall weather did not offer any thermals but I found the Mini to be exceptionally responsive and easy to fly – even nicer than my LS-6, which has the reputation of being a very friendly glider. The reason for my Lithuania visit was to determine whether the Mini had good xcountry capabilities, a notable problem with many 13.5 meter ships. When I pushed her, she did not fall out of the sky, but held her own. I later learned that the Mini has recorded several flights in excess of 700 kilometers – good enough for me.

I also spent a day touring the LAK factory. I was impressed with the well-equipped and organized factory floor and the obvious professionalism of the 50-odd employees. The place had the feeling of a hard working but friendly family. LAK has been building gliders for 50 years, and it showed.

My Mini has been delivered, but I've not flown it yet. Still working my way through the FAA certification process. I'll write a report when I've had some initial flight experience, but meanwhile, here's a cover piece I wrote for Soaring in February 2019: http://front-electric-sustainer.com/...ary%202019.pdf

Ron Gleason wrote on 4/11/2020 12:29 PM:
On Saturday, 11 April 2020 11:51:57 UTC-6, Eric Greenwell wrote:
Luka Žnidarši� wrote on 4/11/2020 1:14 AM:
Note that at LAK17C max power is 30kW. With such power climb performance are very good as there is no pylon drag.

The pylon mount allows a much greater choice in propellers, allowing it to
significantly exceed an FES in efficiency. The GP15, for example, uses a three
blade propeller, and the blade shape is not constrained by the shape of the nose,
or the need for low drag when the motor is stopped. The pylon is a single,
streamlined strut that will produce almost insignificant drag at climb speeds,
even when accounting for the open doors. I base that claim on how well my ASH 26 E
thermals with the engine partially retracted for cooling, a much dirtier object
than the GP15 pylon.

--
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

While classified as a CLass 2 Hang Glider, this machine fits the objectives you defined https://www.youtube.com/watch?v=G3qfW3ydZuY&t=178s

Yes they are ridiculously expensive

It appears to be a fine machine, certainly smaller/lighter (maybe not simpler),
but it's gliding performance is far less than my requirement: performance that
equals or exceeds an ASH26E. The price is close to my cost for a GP15.

--
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 Saturday, April 11, 2020 at 1:09:56 PM UTC-7, Luka Žnidarši� wrote:
Dne sobota, 11. april 2020 12.02.19 UTC+2 je oseba jld napisala:
The propulsion efficiency has much more direct effect than pylon drag.
In fact, for Takeoff and climb performance, the overall glider drag has secondary effect compared to aircraft weight and available thrust.
Without fuselage interference and opportunities for larger props, the pylon mounted motor provides a significant efficiency advantage. This has a direct effect on climb performance in terms of climb rate and altitude gain..
Running large battery packs, beside running cooler, also provides better energy efficiency and installing large batteries in the wing reduces overall structure weight and provides better flying handling characteristics due to CG.

FES is a nice solution, especially for retrofit of existing gliders.
For a brand new design, there are opportunities for better optimizations.
Is GP at the optimum, probably not (yet:-), but they have certainly made good design decisions for both teh glider and the propulsion.

Actually efficiency of glider in powered mode is quite important:
1.Efficiency of FES powered glider in powered mode is the same as in glide mode.

FES powered glider, L/D 50 and 500kg T.O.W, minimum sink 0,5m/s:
-required thrust for level flight; 500kg/50=10kg which is 100N
-another 100N is required for climb rate 0,5m/s; 200N
-another 100N is required for climb rate 1,0m/s; 300N
-another 100N is required for climb rate 1,5m/s; 400N
-another 100N is required for climb rate 2,0m/s; 500N
-another 100N is required for climb rate 2,5m/s; 600N
-another 100N is required for climb rate 3,0m/s; 700N

2.Efficiency of most powered gliders with retractable systems is about half (or even worse) of the efficiency as glider. Note that pylon drag with its big pulley or motor on top is still there even if propeller is rotating . Like flying with half opened airbrakes.

Pylon powered glider, with pylon extended, L/D 25 and 500kg T.O.W, minimum sink 1,0m/s:
-required thrust for level flight; 500/25=20kg which is 200N
-another 200N for climb rate 1m/s; 400N
-another 200N for climb rate 2m/s; 600N
-another 200N for climb rate 3m/s; 800N

Efficiency of propeller not depends just on its diameter but also how much thrust such propeller should create. Even with smaller propeller diameter is possible to achieve good efficiency when required thrust is smaller, for instance during level flight or more shallow climb rates and this is typical use of FES. As you can see above, with FES required thrust for climb rate 1m/s is 300N, while with pylon for 1m/s is required 400N. This seems to be possible to offset with higher efficiency of bigger propeller, but bigger propeller requires higher torque. With direct drive electric motors higher torque can be achieved only with bigger motor diameter, but this means even more drag. With more drag it would be required also more thrust from propeller, which means higher power, higher energy consumption, bigger battery packs, more weight. Another possibility is to install motor in the fuselage and transfer power with belts to pylon, but there are quite high loses in efficiency, more that most engineers imagine and so not in equation. Diameter of upper pulley is usually bigger than lower pulley, due to reduction ratio, which is not helpful from drag viewpoint. And number of propeller blades do not affect much to its efficiency. It is only helpful if prop loading it very high but is also possible to increase chord of propeller blades to some extent.

So 700N for FES with 1m prop diameter or 800N with 1,4m (or bigger) retractable system to climb 3m/s, seems to be in advantage of pylon due to better prop efficiency. However, when you put all factors into equation, result is almost the same.
However, in the range of lower power settings FES is much more efficient (100N vs 200N).
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.
I believe that in next months there will be more information about LAK17C FES.

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?

Luka,

I don't want to bother the group with too many details but please, don't present data in a biased way.

EFFECT of L/D:
The L/D of the glider only affects the thrust required for level flight.
Because we care about takeoff and climb, a large majority of the thrust produced is used for the work required to climb. This is independent from L/D, it is just about lifting the weight of the glider!
The equation can be simplified to : vertical speed (m/s) = glider speed (m/s) x Thrust (N) / weight (N).
Thrust being the thrust from the prop minus the thrust required for level flight.
Just to illustrate using your example:
With FES, to climb at 2.5 m/s requires 600 N. In the 600 N, only 100 N are dependent of L/D and 500 are dependent of weight of the glider.

PYLON POWERED GLIDER L/D:
You use L/D 25 for gliders with pylon installation.
This is too high for gliders with combustion engine which have L/D below 20 when pylon is extended.
This is too low for gliders with an electric motor on a single clean mast and a foldable prop. The drag penalty should not exceed 20-25 N at climb speed which means L/D should be at least 40 with your example of a 500 kg ship with 50 L/D when clean.
This means that when the engine is out, you still get a nice L/D allowing you more options, almost like with FES.

Back to climb performance, to climb at 2.5 m/s with a pylon would require an additional 25 N due to the drag.

Using your examples again, if you estimate the difference of climb thrust between FES and pylon to be around 100 N, there is a 75 N (100-25) advantage to the pylon mounted motor!
This will be more than that in reality since, beside prop diameter, the prop can be further optimized with a pylon installation (number of blades, shape, etc.).

Just a note concerning thrust. When you claim 700 N of thrust, it is probably static with 30 kw (not the current 22 kw motor). The climb thrust at 30 m/s assuming 25 kw continuous power is probably more around 500-550 N.

There are indeed pros and cons of FES versus Pylon but, if the goal is to allow high pressure altitude operation and high altitude gain (4000 m on a charge), Pylon has an advantage. If the goal is to use the motor more like a sustainer, then FES is a nice solution.

More important, a lighter glider (with high aspect ratio wing and low surface to maintain good performance / wing loading) will significantly improve takeoff and climb performance.

Regards.

Ps: Please don't use this type of dangerous suggestion: "Another big FES advantage is non problematic starting of engine above non landable terrain". You may decide to start engine at lower altitude with FES but it should always be above a landable area.

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!

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.

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