Hello guys. I have just ordered a b17 from wing mfg and am planing to go electric instead of the recomended .25 -.35 2 stroke or .40 4 stroke.The plane has a 96 inch span and is supposed to weigh 14 to 17 pounds dry with glow motors installed. My question for now is what is the proper way to set up a 4 engine plane for electric? Should each motor have its own battery? Should I run inside and outside motors counter rotating to counter act torque?What sized motors? Geared or not geared? Alright, so I have a lot of questions and any help would be greatly apprecieted. Also Greg, You gave me some advice on a small p51 conversion I'm doing and it's going great. Everything fits and it's nearly ready for the maiden.

I have built one of these as glow (four OS30 four strokes); I started a short build thread a few years back. Maybe some of the pics can help you out. I sold it and have been wanting to build an electric ever since. The glow took several pounds of nose weight; so I have been thinking of putting the batteries in the very nose of the plane with the esc's on each firewall. I think some decent looking air inlets could be made from the cheek guns of a b17G, and the exit could be the waist gun openings or the opening at the top rear of the fuselage "bump". Counter-rotating would just be a matter of finding the right props; but I think it's not needed. The batteries could go in the fuel tank locations also but you'd likely have the same balance issues I had.
Use very very light materials and techniques in the tail of this one!! I'd suggest 1/16 or maybe even 1/32 balsa to skin the stab, and use pull-pull cables for the controls. I ran internal control horns for rudder and elevator with a hatch on either side for access. Did split flaps, retracts, fiberglass finish & the whole works!
instead of the dowel & bolt approach to attaching the wing, I would suggest four bolts which would allow the wing to be dropped straight down into position rather than angling for the dowels... there is a decent amount of fairing/"fuselage" attached to the bottom of the wing which gets in the way of "tipping" the wing in with a dowel attachment.
The laser formers from wing make life a lot easier.... and the dummy engines that Dan has will work great as well.

I'm just finishing up a Wing FW-190 which is set up for electric. What are you planning to use for motors in your b17?? I'm thinking of axi 2820's, or maybe the Atlas motors that Hobby Lobby sells for a bit less $$. We don't really need ultra-performance for a bomber! Maybe I'll have to start on my secong B17 sooner than I thought

I haven't converted a four motor, but have done a few twins on electric. You will need a seperate ESC for each motor if you are going brushless. You can use one battery if it can handle the amp draw rate. At around 15 lbs. you will need about 1800 to 2000 watts to fly the plane well. If you keep the motors sized right you could probably go with one battery maybe a 4S or 5S pack. You should be able to use something like an E-Flight Power 15 or 25 for each motor and get great power. You will need to put an amp meter on the plane and see what it's pulling at full power. One battery would make it easier as you would have less wiring issues. It would be good to counter rotate the two motors on each wing in case you loose one side for some reason, you may still be able to land the plane. So far, I haven't ever lost an electric engine on my twins. I have lost both of them becuase I flew too long, but that's no problem if you have enough alt. you can still land.

Keep us update on your progress. I would love to convert a B17, B29 or B24. I have done a B25, P38, OV10 and A10 so far.

Should be a fun project. Good luck!

I have just started my Wing B17 but my mind is rushing ahead regarding electrification. i sure don't want four IC engines.

There are many ways to go about the set up. I suspect that having four self-contained power pods is not ideal. Let's think in terms lateral power symmetryand yaw control. Also we want to land with a couple "turning" after batteries start to peter out.

Try this. Two motor systems. One for inboard engines and one for outboard. Place the single battery (you could use two, one for each system) in the forward part of the fuselage. The battery would feed two voltage cut-off units , one for each system. These units would each feed two ESC's all near the battery in the fuselage. Tri-wire bundles run out the wing panels to their nacelles. It is my understanding that we should not place the ESC's just behind the motors because of interference problems.

I believe I shall select bigger motors inboard than out. Set things up so that the outboard motors cut off first etc and so forth.

Respectfully,

Nathan Rambo (Aeronately)
Camarillo, CA.

I notice that my above forum note has seemed to stop all other replies. Is anybody active?

Aeronately

Nate Rambo

Not a B-17, but I'm converting a P-61 that I flew some time back on 2 OS 19's. One of my concerns has been remote placement of batteries (for CG purposes) and what to do about extending wires. I had seen some discussion of whether to extend battery wires or motor wires. i.e., do we put the speed control near the batteries or near the motor? Some folks are adament about which wires to extend or not extend. I contacted the manufacturer of my motor/esc and he said that opinion varies on this subject. His experience was that either way will work OK.

Another issue is whether to utilize the BEC from both speed controls to feed the receiver or to use only one BEC or even to eliminate the BEC's and use a separate battery pack for the receiver. My source (Innov8tiveDesigns, aka Scorpion) said he has tested both BEC's feeding the receiver and had no problems.

If I could get reverse pitch props of the correct size I would go with counter-rotating on the motors. But finding props of the right size is difficult, so I'm planning on turning both motors in the same direction.

Nate, if you're in Camarillo I'd make the trip to see your B-17. Keep us posted.

Dick
Arroyo Grande, CA

I don't think you will have any issues if you place the ECS behind the motors. I have them setup like this on several planes without any problems. Two battery packs would give you lots of flight time. Use a seperate UBEC and battery for the receiver or a NiCad receiver pack. Keep the wings light and place the batteries in the main fuze.

Good comments, gentlemen.

As far as my B17 goes .... it proceeds slowly because I am leaning. And I must think thru each step which is very time consuming at my age. Details make the foam modeling process slightly different from that used for man-carrying airplanes. Whether or not I can use foam for this model remains a question. We'll see.

My first fuselage made of white polystyrene foam (shown in the foto) is unsat. Pretty crude. I have started a new fuse made with blue foam. It really sands nice and will be far superior. The old fuse will be used as a learning tool for covering with glass.

Dick, remind me to keep you posted.

Nate Rambo
Camarillo

Okay Folks: Should we call this a foamy? I guess. The foto shows the status of my fuselage / tail group. As you can see it is mounted in the rotational building jig . Not much balsa ..... blue foam and glass. The foam/glass work has been unbelievably slow compared to old fashioned balsa-based methods. I'll be starting the wing soon and I expect construction to go faster. Much faster.

I am expecting 600W to be satisfactory take-off/climb power. Picked up a pair of motors and three-bladed props to experiment with. The motors are from Hobby City. Turnigy 2231/20 (kv1050). Any comments.

Nathan Rambo

Gee. Gosh:

I feel very lonely. I guess nobody reads this topic. Sniffle ... sniffle.

Well the B-17 construction is progressing. Hope anybody who might be watching gets something out of these fotos showing the outboard nacelle built for electric power. Planking isn't complete so you can see the structure. It is built on a full height ply keel.

Also note that the outboard wing panels are removable so she goes into my Toyota and sits on the gear. Assembly is now quick and simple.

Resp. Nathan Rambo[img][/img]

Nathan,
Don't feel lonely, I check this thread on a regular basis looking for updates. I have a Wing Mfg B17 NIB which I plan to do in electric, as a person with no electric experience I am counting on you guys to lead me through it.

Please keep the updates coming.

Cheers,

Steiny

Okay, Steiny.

Welcome aboard and good luck on your B17 project. It's lonely here and I am glad to have some company. Keep us posted as you proceed.

Nathan

Nathan,
As I have a couple of other projects to finish up first so I will likely not start framing up the B17 until this winter some time. How far can I go in the framing up process before deciding what batteries and motors to use? Should the battery or batteries go in the wing or fuselage? Do you have to use different construction techniques for brushless motors?

I've been building from plans for about 10 years now so I am comfortable with all the traditional techniques (fully sheeted warbirds in fibreglass etc.) associated with glow engines, but I am just not sure what has to be done different for electrics. I assume weight saving is important so fibreglassing is probably out? Do you just use Monocote?

I am looking at other threads for electric construction techniques but any advice will be welcomed.

Cheers,

Mark

Mark:

Sure, build light and use Monokote if you like. Don't copy my techniques of glassing etc. My ship is too heavy.

My ship is framed. To do this I knew that I wanted to use outrunner motors so I installed radial motor mounts. I also knew that I would need the batteries (LiPos) in the fuselage nose for CG purposes (just as you will). That meant I needed three wiring tunnels in each wing panel to carry power to the nacelles and aileron servos. That's about it.

That should be all you need to know until you get into building. You have lots of time until you need to make final decisions.

Nathan Rambo

Nathan,

You mentioned that your B-17 was pretty heavy. Do you have a projected weight yet ? Also, what is the wing area of your plane ? The weight of the final finish is always a shock to me, no matter how many times I get that shock.

I've now flown my P-61 one flight with electrics. Everything went OK except that the top speed was disappointing. After a bit of thought and some consulting with the motor manufacturer I think my prop pitch was too flat. Takeoff acceleration was great, climb was OK, but the plane wouldn't gain any speed beyond climbing speed. I've replaced the 9X4.7 props with 9X7 and this should do the trick even though I'm dangerously close to maximum amperage on takeoff. Kind of like trying to take off in high blower. You may have this same problem as electric motors don't behave quite the same as wet motors with regards to winding up as the prop unloads.

I haven't made a second flight yet primarily due to lack of a good flying site. Airplane needs a paved runway. A friend got me onto Vandenberg AFB for the first flight. They have a great runway, but it's always windy there and it's a crosswind to boot. Do you have a decent flying site in the Camarillo area ?

Dick Fischer

Dick:

I would be interested in the specs for your P61. I'm always looking for Watts required , weight, wing load etc.

For info, a gentleman I know at church flew night intercept with a P61 in Korea. During WWII his squadron flew old Beaufighters.

We have a fairly nice flying field in Camarillo. Our site has many improvements like sun shaded pits, out houses etc. The runway is only 400 feet long which really keeps me on my toes. The wind can get crossed often enough. You are always welcome as my AMA guest if passing thru but I doubt that our field is good for your P61.

In about 30 days or so I should know what my ship will gross out. Probably about 13#. That is 13#/ 3.8 sq.ft =3.4#/sq ft. Ugh! I am seriously concerned that this thing has been a big waste of time and should better be used as a wind sock. We shall see.

At this time I'm thinking of 13# * 60 W/# or 800W minimum TO power. Probably try for a KW max available power.

Nathan Rambo

Nate,

I'll check out the numbers on my P-61 and post them for you. It's all written down, but my mind doesn't hold things accurately enough to do it from memory.

Your comment about the wing area on the Wing B-17 really caught my eye. From the photos, your airplane looks quite large and I assumed that it would have much more wing area than you said. I did a search on the internet and turned up a kit description showing wing area as 1125 sq. in. If your airplane is 1125, that's 7.8 sq. ft., which would yield a wing loading of 26.6 oz/sq. ft. Hope you just had a typo in your wing area.

This whole business of motor/battery/prop selection with an electric is sort of perplexing to me. My ultimate goal is a Stafford B-24 on electrics, and I electrified my P-61 primarily as a learning tool before stepping up to the B-24. Here are a couple of things I've learned so far:

1. Watts per pound doesn't tell the whole story. Faster airplaes (i.e. higher wing loading) need more watts per pound than light weight airplanes. I can explain why, but it's a subject unto itself. The good news is that bombers don't need to be aerobatic, so that relieves the power requirement quite a bit.

2. Prop pitch seems more critical on an electric than on a gas motor. That's because gas motors are usually running as a speed that is less than their maximum horsepower point and when you take off they wind up a bit more and produce slightly more power. An electric motor, on the other hand, develops less horsepower (wattage) as it winds up and unloads. So with an electric model that travels fast enough to unload the prop, you get the effect of throttling back as you attempt to gain speed in flight.

3. There is a lot of merit to knowing what your watts, amps and RPM are in flight. I just bought one of those Eagle Tree recorders for the P-61, but have not yet flown with it installed. This device will tell whether I am close to burning out the motor or speed control on takeoff and how much I'm unloading in level flight.

On the first flight of the P-61, the model seemed very slow and I thought I was terribly underpowered. I couldn't throttle back at all and maintain level flight. After about 3 minutes I landed, thinking my batteries might die soon. But discharging the batteries after landing showed that I had only used about 20% of my capacity. That means the motors simply weren't drawing sufficient amps to propel the airplane like I expected. You can make your motors draw more amps by increasing prop pitch, but then you run the risk of blowing the motors or speed controls on takeoff when the props are temporarily lugging. More work to be done here for sure.

4. My P-61 has separate batteries and speed controls mounted in each nacelle. Then the two speed controls are hooked through a Y-harness to the throttle channel in the receiver. The batteries are as far forward in the nacelles as I can get them (actually, under the motors). The CG worked out fine because the batteries and motors are such a large percentage of the model's weight. I do have a "crossover cable" between the two batteries, effectively putting the batteries in parallel. More on this later.

Dick

Dick:

You are absolutely right. What was I thinking? I can't believe what I wrote. I have had some really serious personal problems lately and don't have my mind (?) on this toy. At 78 I obviously have moments of utter stupidity. Sorry.

A soon as I have time I will go through your mssg thoroughly.You are way ahead of me on these electric conversion thoughts. (Please don't don't bother to explain the everyday aero theory; I really do have the credentials and experience. ) I am really interested in electric versus IC factors and other points you bring up in your mssg. Later. Thanx.

Nate

Hi Nate,

Here are the specs on the electric P-61:

Span 54 in.

Weight 65 oz/ (4.06 lbs.)

Area 3.13 sq. ft.

Wing loading 21 oz/sq. ft.

Motors (2 ea.) Scorpion 2212-22 Rated at 200 watts max

Speed control (2 ea.) Scorpion 25A

Props First flight with 9 X 4.7 SF (Not enough pitch)

Although the motors are rated at 200W each, the motor chart from Scorpion says I was only developing 140 watts each at takeoff due to the incorrect prop size. Based on remaining battery capacity after the flight, I think I was only developing around 100 watts per motor in flight. That's clearly not enough.

On the first takeoff, I had a power-to-weight ratio of 69 watts per pound. In flight I was probably more like 49 watts per pound. That power level produced a good takeoff and a reasonable climb, but the speed did not build up once I leveled off. I could not loop out of level flight, for instance. I don't need jet-like performance, but I'd like it to look more like a fighter than a cargo plane.

On my next flight I will use APC 9X6 E props, which will give me about 83 watts per pound for takeoff. The Eagle Tree flight recorder should tell how many watts I'm getting in flight.

The information from my first flight should give you an idea of the MINIMUM power-to-weight ratio that would work on your B-17. I'll give you a report on the results of my next flight.

Dick

Okay, Dick:

Before we go further, do you have any guidance on what wire size to install? I can't seem to contact of the techs at Castle. With the ESCs in the fuse I have 30 inch 3 phase wire runs to each outbds. I would like to avoid serious IR drop.

Your selection of APC 9-6E props for your 61 sounds like it sure should help. The 9-4.7s sound better for 3D foamies. Let us hear what data you get with the 9-6Es. Hope that the TO current draw is acceptable. I'm not beyond clipping tips.

My 17 is "on the gear" and right now I'm up to my ears in completing the last two (inbd) nacelles. From then on it's just countless details and cosmetics. At that time I will switch what is left of my mind (?) to the electric aspects of conversion.

Oh, be still my heart. Nate

Hi Nate,

I'm glad to hear the status of you B-17. That's really a big and complicated airplane.

With regard to your questions about wire sizing, I can only plead insanity about all things electrical. You have already plumbed the depths of my electrical I.Q. But the good news is that I get all my electrical/electronic help from a friend and fellow modeler in Moorpark who is an EE. I've already spoken to him and he said he would be happy to talk to you about the B-17. His name is Lane Tufts and he is at 529-2359. Is it possible that you once worked at Mugu? Lane thought he recognized your name from some time back before he retired there.

Although I'm an electrical layman, the question of long wires has intrigued me because of my B-24 project and of course the P-61. Here are some random thoughts that I've considered for my very similar project:

1. No matter how large the wires, you will get at least some voltage drop over the 30". The loss in wattage will be I(squared) X R. You can make R pretty small by using a big wire size, but you will pay a price in wire weight. If you save weight with smaller wire, then you will need to make up for the loss in power by installing bigger batteries and bigger motors. So no matter which way you go, there will be a weight penalty to be paid. In round numbers, you will probably need about 14ga. wire. You could at least check out the weight of the appropriate amount of 14ga. for estimating purposes.

2. You might choose to reduce current by installing smaller motors in the outboard nacelles. Then compensate by installing larger motors in the inboard nacelles, where the wire runs would be shorter. This has other operational advantages in the area of reducing thrust assymetry, but it offends my purist desire to have four equal motors, just as Boeing did.

3. As I understand it, you need to put the batteries in the fuselage for CG purposes. Are you certain this is necessary? Maybe the heavy leads out to each nacelle (and the slightly larger batteries and motors) would weight more than the ballast that would be required in the nose if you carried the batteries in the nacelles? I went through this trade-off with the P-61 and ultimately decided to put the batteries in the nacelles. My batteries amount 16% of the weight of the airplane, so they strongly affect CG. Also, the batteries weight the same on takeoff or landing, so you don't have a "zero fuel" CG to consider like you would with gas engines. As it turned out, my CG came out OK with no ballast required. Even if you do need to install some ballast, a lighter plane will require less lead for a given CG adjustment.

Hope these thoughts will serve to stir your imagination. Don't hesitate to call Lane. He's a very practical electrical guy and also very much of an airplane enthusiast.

Dick

ESC Location:

Finally connected with Castle's tech adviser. He only spoke "small silhouette foamy" but I did get this info. Castle advises that an ESC should not be more than 18 inches from the battery.

What this means for our B17s is that the outbd motor ESCs must be in the fuselage with the batteries. I'm putting all four ESCs in the fuse.

Nate Rambo
flyernate@ verizon.net

I've got a 70" wingspan B-17 from EAM and it's powered with four outrunner motors and four ESCs. The ESCs are are located in the nacelles and are soldered directly to the motor leads with 4" wires. The ESCs battery leads are wired in parallel with the inner length of wire being about 12" and the outer lengths of wire being another 10". The battery packs are then connected to a three way parallel connector which ends in a single Y at the wing roots. The total length between the battery and outboard ESCs are about 26". The motors draw 20 amps each at full throttle. I'm using 16 gauge wire and I've had no problems with glitching at all. And since I'm using 3s, I'm also using the ESC's built-in BECs. Those are connected with similar lengths of servo extensions, also no problem.

I ran a test of the above motors with wire lengths of approximately 36" before I installed the system in the model and it worked fine. No glitches at all. The wires even run together with the ESC, aileron servo and retract servo leads and there's no problems at all.

My 8 lb B-17 flys with great authority on about 800 watts. I wouldn't want any less power.

Good post Thunder 21. Lots of good data there. Thanks for the help. Stay aboard please and lets hear more. Some of us would like to know how the 800 W was measured, etc.

I did some number crunching. If your 70 inch EAM weighs 8#. That would be equivalent to our 96 inch Wing 17s weighing over 20#. Your 800 watts is equivalent to our Wing 17s requiring 2 KW. I guess that this is a case of weight and the resultant induced drag really eating up the power required to fly. Wow!

I suspect that this thread is going to help establish some across-the-board techniques for knowing and equating what true power is required for various designs and how we should best go about estimating new requirements. I believe that Dick Fischer (and his airborne data recording techniques) will be a great help in this area.

Nathan Rambo

I measured the power with an Emeter. It's a tool which plugs into the battery side of the ESC and then you plug the battery into the Emeter. I adjusted the varioprop pitch on each prop so that each motor was drawing 20 amps at full throttle. The voltage ended up being about 11 volts for about 220 watts. The entire setup drops the voltage on the three batteries a little more to about 10.8 for a total max wattage of 860 watts. That's definitely more power than I need. The model flys like a big fighter and will roll and loop at much less than full throttle. Still, the power is nice because the model will stall easily due to the relatively high wing loading of around 30oz/sqft. I believe the weight of mine isn't quite 8lbs. It's probably closer to 7.5lbs.

I'd be amazed if the 96" B-17 could come out at less than 15lbs RTF. That means at least 1500 watts for sport performance. I'd imagine even that weight would require keeping the tail very light and moving everything as far forward as possible to reduce the weight required to balance. I guess if the model is not sheeted in the tail it would be possible to come in at less than 15 lbs. My plane is definitely heavier than it could be. The fuselage is fiberglass and the wings and tail are balsa sheeted and finished with polyurethane and primer.

Here's my build thread if you need any more info:
http://www.rcgroups.com/forums/showthread.php?t=536562