Siemens Schukert DIII - Electric Power - DJ?

[Don Eaton]

I am in the process of attempting to put together an electric power system for a 1/4 Siemens Schukert DIII (Dave Johnson) design.

I understand he is using a (gas) Brison 2.4 with a home made 4 blade propeller. From a previous post, I believe it is a 20 x 8 turning at about 5800 rpm.

If Dave or anyone is aware of any successful electric power systems that have been employed with this design and what their specs might be, I would be very interested and appreciative of the information.

Thanks
DE

[Madman]

[QUOTE=Don Eaton;321694]I am in the process of attempting to put together an electric power system for a 1/4 Siemens Schukert DIII (Dave Johnson) design.


Don

I have some experience with large WWI electrics and can help guide you. First what is your experience/knowledge with electrics? What is the weight of the gas motor and fuel system? I fly 1/5 scale electrics and a 20" prop is scale for them. At 1/4 scale what is the scale prop? With electrics at this size we tend to accept a lower RPM and go much higher in pitch to keep pitch speed up.

Stephen

[Don Eaton]

Hello Madman. Had some experience with smaller electrics but nothing this large. The Brison 2.4 that Dave Johnson uses weights 44 oz without the muffler, so I would guess the gas system would be in the area of 4.0 to 4.5 pounds total.

Been doing some modelling in Motocalc, (trial version - great program) and from what I can see the Hacker - A60 18L looks like the best option. A scale prop should be about 24 inches (scale is a four blade prop) and Motocalc give me a result of a 23x13 (2 blade) turning at about 4350 rpm. I can't seem to get a good result using a 4 blade prop in Motocalc. The Hacker A60 weights 28 oz. and the 9 Li-Po cells bring up the balance for a total power system weight of 55 oz. I was also told that Ni Cads or Ni Metal could be used to reduce nose weight for balance - so that case might be heavier.

I think Dave Johnson uses a 20 x 8 (4 blade) turning at 5800 rpm. It would be nice to use a 4 blade prop if possible.

Not sure of all the implications between what the gas system (which I understand is a great flyer) and what the electric system would be with a lower rpm and larger prop.

Thanks
Don.

[Madman]

Originally Posted by Don Eaton

Hello Madman. Had some experience with smaller electrics but nothing this large. The Brison 2.4 that Dave Johnson uses weights 44 oz without the muffler, so I would guess the gas system would be in the area of 4.0 to 4.5 pounds total.

How much weight did he have to add to balance it?

Been doing some modelling in Motocalc, (trial version - great program) and from what I can see the Hacker - A60 18L looks like the best option. A scale prop should be about 24 inches (scale is a four blade prop) and Motocalc give me a result of a 23x13 (2 blade) turning at about 4350 rpm. I can't seem to get a good result using a 4 blade prop in Motocalc. The Hacker A60 weights 28 oz. and the 9 Li-Po cells bring up the balance for a total power system weight of 55 oz. I was also told that Ni Cads or Ni Metal could be used to reduce nose weight for balance - so that case might be heavier.

Motocalc, and other similar programs are excellent to get you into the ball park and to determine differences between slight differences in set up (changing one variable such as prop, or number of cells.) I use it all the time for comparisons but it is not perfect and I always prefer real world numbers.

Looks like you know your stuff. The system you are talking sounds right in the ball park. Dave's system characteristics gives a pitch speed of about 45 mph, while yours is up around 53. His static thrust should be around 30+ lbs while yours should be about 20! This just shows what a large prop turning fast is putting out. Unless you want to be doing non scale tourque rolls down the runway either system is putting out way more thrust than you need.

If you are going to have to add a lot of weight anyways, might as well make it used weight. A NiCad/NiMH pack will cost a lot less than a lithium pack but weigh a lot more. Great for balance issues and if you are going to leave the pack in "dedicated for the plane" the reduced cost is easier to bear. Plus no lithium fire worries (I don't want to debate this issue as I don't know enough but most incidents seem related to operator inattention). NiCads can be charged in about 20 mins, NiMHs a little more. Li?? about an hour. If you are like me this will be a plane you will not want or have to fly every half hour or less but will want to have a little down time between flights to relax and yack.


I think Dave Johnson uses a 20 x 8 (4 blade) turning at 5800 rpm. It would be nice to use a 4 blade prop if possible.

Not sure of all the implications between what the gas system (which I understand is a great flyer) and what the electric system would be with a lower rpm and larger prop.

Thanks
Don.

About myself. I have been flying electrics for the past 15 years. 90% has been electric in that time. I do NOT say no to glow or pass on gas (old electric in joke) as I also have three YS 45s. I don't fly gas but who knows. All my electric planes fly on high end brushed motors. I like larger 1/5 scale WWI subjects and have been going the scale prop high pitch low RPM route. I do have 2 brushless motors for future projects and where I need the specific characteristics of brushless (very high speed or amp draws) I will use them in the future. I will be using one I have for a 1/5 WWI electric but mostly because I can reduce the cell count and decrease weight in a way over-built ARF. I need it to draw 60+ amps, not good for the life expectancy of a brushed motor no matter how high quality.

In your application I would choose an Astro 60 or 90 with high ratio gearbox on 40 cells. You can swing a scale diameter high pitch prop around the mid 3K rpms. This is what I have in the basement so that colours my thinking a lot. Since you need the weight and in my opinion you don't really need the massive thrust and a reasonable pitch speed this is my leaning. A VERY large factor in my thinking is cost. The power package you are proposing is about $550 US. Adding in the cost of the lithium batteries scare me. The brushed power systems can be found on various user groups in great used condition for about $200 to $250. I assume you are allready using Lithium cells (maybe an Astro 109 charger since you mention 9 cells) so you have some of this cost covered.

Just my opinions let me know what you think.

Stephen

[Don Eaton]

Some great advice - thanks Madman. Around the weight in the nose for balance I'm not sure what was required. The Model Airplane article refers to it but does not quantify.

As far as the Ni Cad' s or Ni Metal, I would like to go that way if possible. I know the cost would be a big difference. - I have a charger for Li Po, but I would need to upgrade if I were to go to 9 cells, whereas my existing charger would work on the Ni Cad' s or Ni Metal. I'll have to see if I can find out more on the balance issue above to finally decide.

In terms of the direct drive outrunner, I really picked due to a straight install vs. the gerbox issues - I need to possibly rethink based on cost difference.

Thanks for taking the time to look at this design.

Don.

[Madman]

Originally Posted by Don Eaton

Some great advice - thanks Madman. Around the weight in the nose for balance I'm not sure what was required. The Model Airplane article refers to it but does not quantify.

As far as the Ni Cad' s or Ni Metal, I would like to go that way if possible. I know the cost would be a big difference. - I have a charger for Li Po, but I would need to upgrade if I were to go to 9 cells, whereas my existing charger would work on the Ni Cad' s or Ni Metal. I'll have to see if I can find out more on the balance issue above to finally decide.

In terms of the direct drive outrunner, I really picked due to a straight install vs. the gerbox issues - I need to possibly rethink based on cost difference.

Thanks for taking the time to look at this design.

Don.

Well if you go NiCads or NiMHs nose weight may never be a problem. Wharever power plant you use you will be needing between 30 and 40 cells. They weigh about 2 oz each. If you have a charger which can handle NiXX cells keep in mind the number of cells. Very few chargers can handle this many. If your Li charger is limited to 8 cells then the Ni settings will probably not be above 24 cells or so. I prefer Astro Flight equipment and have one of their 112 Digital Deluxe chargers (as well as an older 112PK). The 112DDx will charge up to 40 cells at up to 8A.

As far as a gearbox equiped motor being more trouble then a direct drive one then that is a big fallacy. Many such motors now come from the factory with the gearboxes installed. However, choosing a suitable system now becomes an ever greater task as the number and variety of choices is vast. A suitable motor choice is made greatly more complex as the variety of gear ratios adds to the complexity. Helping you here is where I would really earn my keep!

A little about outrunners and geared in-runners. Out runners tend to be a little less efficient than in runners even given gearbox losses. Originally one of the ideas promoting out runners was the spinning case would aid cooling. However, the heat is generated in the windings which are burried inside the motor. In the in runners the windings are part of the outer fixed case so are closer to the surface aiding cooling. Also out runners are way heavier than geared inrunners. Not an issue for us and maybe a benefit. Of course as I stated excellent brushed systems can be gotten for half the price of these brushless systems. As a rule they are less efficient (unless compared to the cheapo Chineese crap) but the difference is minimal and with the high capacity batteries nowdays flight times are no longer an issue. Even with very old 1700s I regularly get 7 min flights which has always been my yard stick, electric and glow. Bump up to 2400s (the largest NiCads I have) and 11 min flights are realistic. Nowdays 3800s and 4300s are available in NiMHs. No matter which route you go for batteries they will probably be permanently fixed in the plane due to acces and CG issues. If you are flying Li now you probably know all the 'issues' with these cells and charging concerns. Not the best solution.

Now as an aside I have heard all the stories with Li cells and based on even very experienced electric pilots having serious consequences I have come to the conclusion it is probably not the Li cells which are the cause. I feel it is pilot comfort. Unless you take very painstaking care EVERY time you charge/discharge (flying or bench testing) something may go wrong and bite you bad. With all the automated chargers and the long and trouble free history of NiXX cells us pilots have become trusting and frankly lazy. When NiCads first came out they were prone to exploding as the primary charging method was to attach them to the charge battery with a specific length of wire (I think it was called a zip cord) and hold this for a set time. If the battery was only partially dischaged the cell would be over charged and explode. Also the exact charge rate and voltage were dependant on the correctness of the cord's characteristics. How far we have come. No wonder we now take charging forgranted.

As usual I have gone on and on. I hope this has been of some use and look forward to helping you work up a power package.

Stephen

[Madman]

Well I just looked over ALL my posts and found I am repeating myself and a little contradicting myself as well. Oh well. Its late and my two little boys (1 pilot in training (7 yr old) and 1 future pilot (4 yr old)) are distracting. Also I was watching my birthday present, Flyboys 2 DVD set, so just a little distracted.

Stephen

ps. Lots of views. I hope I am helping and informing some out there.

Don, You have made your mind up to go electric. This is certainly a viable option. I understand the vibration and clean-up issues are attractive in compairison. However I can't get past the cost of electric for this size model. This is me and what I think, is only what I think. I'm not here to cast dispersions on this technology or point out the cost difference to you. I am interested in why you and/or Madman have chosen this route, given the added expense and high failure rate of ESCs as I have been lead to believe. You are talking to someone interested in electric power, whom just can't justify the cost!

[Don Eaton]

I suppose the choice of an electric here really can't be justified purely on a cost basis. But the difference may not be as much as you think when considering other benefits of electrics.

A four stroke gas system here will cost between $300 and $400. NES have a Hacker A60 18L and a Pheonix ESC at $430, so the difference is primarily the cells.

Using Ni Cads or Ni Metal, I will require 30 - 32 cells, - Sanyo Ni M 2700 mAh cells can be obtained at 3.50 each so there is about an additional $100 -$110here.

If you have to purchase a charger then I guess that will really make a difference, however I do have two existing chargers each capable of 24 cells, so I thought I could make up two packs of 15 -16 cells and connect them in series. I don't think any small charging differences should make a lot of difference on Ni Metal packs here.

Don.

[Madman]

Sounds like a good deal on the motor and speed control. Where are you from and if in Southern Ontario are you going to Toledo this weekend? NSP will be there.

As you can see a high end brushless system is just a little more than a gasser. I am glad Don chimed in as I am not in touch with gasser costs. Now lets compare apples and apples. What infernal combustion system will swing a 23" to 24" prop? My buddy's got an RCV 120 which will swing a 20" prop in the 5000 rpm range, pretty similar to what we are talking and these are not cheap. I have no clue how big an engine is required to swing the larger scale props. Then the power is wasted if the RPMs are much more than 4K to 5K as you will be throttle back for the entire flight. Now compare the cost to the cost of a twin or multi cylinder (radial) 4 stroke. Electrics start looking pretty cheap!

Now if you are including the costs of batteries then electrics will blow glow and probably gas away. Yes, you read that right. That $100 (NiMH) to $300 (Li) battery pack will still be powering that plane years from now. I have packs still in use I bought when I started electric 15 years ago. Seemed like a big investment then but they really have paid for themselves after 2 to 3 years and from then on its just gravy. Yes battery technology marches on and nothing lasts forever, but, as I said I am getting 7 min flights from old tech 1700s. Time the average guy at the field. His 'always get 15 min flights' are really only about 7 mins. You get tired, or bored before too long. I know as I regularly time guys to see if it still holds true. Also MOST of my electric flights are longer than the average glow pilot. I know because I often have to wait for some slimer jock, who took off after me, to land his fuel critical plane NOW. After all with glow when the prop stops turning they just drop (big wink).

Seriously I have always liked being a little different at the field. I like to fly planes similar in type and size to those at a typical flying field. It used to be I was the only electric guy for clubs around. I felt a little like Keith Shaw answering tons of questions when I visited surrounding clubs. Now I am becomming just one of the guys. What attracted me to electric is noise (or lack therof) and mess (again none). The prospect of having to clean all those flying wires just doesn't do it for me. I would like to build a DH2 one day. Imagine trying to clean the mess off that! Plus starting is just a matter of throttling up. I also would not look forward to carfully removing a starter from the back end of a DH2 after start up, to say nothing of hand flipping it! After a few years flying glow I started having hearing issues. I think all the glow engines (my own included) have hastened this problem over the years. What I have come to appreciate the most after all these years of flying electric is consistency. Going to the field and putting in flight after flight without issues. No having to tune every time the barometer or humidity changes, no plugs to foul up a session, etc. And how about those fuel prices. Has the EPA started looking into our engines yet? Just wait for it! Can we say cataletic converters. Electrics are just plug and play. Yes you need to get the variables right and just like the glow guys I spend quite a few flights picking the best prop, or chaging the number of cells (think trying different nitro% fules). But once everything is right on the bench or at the field the flights are consistant. As for ESC failures the only times I have had issues with my electrics are when I knowingly exceeded the limits of the components. Oh that and inserting a battery pack and inadvertantly unplugging the speed control cable which just happened to have come loose from its holders. Didn't go flying that day, went home thinking how could my electrics have let me down only to discover something simple and feeling red faced (think pinched fuel line or binding throttle linkage). The only times I have heard of real problems are using cheap crap (read $10 Chineese rip offs) or running equipment at or over its limits, either through stupidity, but-headedness or ignorance.

In summation give me cheap, reliable, durable QUALITY electric most every time. Please note they are not for everybody and 5 years ago I would also have said not for every application but not anymore. Want proof? Check out BVMjets.com look at the electric violet fan and see just one PRODUCTION off the shelf application has taken us. Want to see turbine levels of performance check out RCgroups.com discussions on electric ducted fans. Woo Hoo!

Stephen