Nerevar

I'm new to electric. Which means I know absolutely nothing about the electric power of RC aircraft.
I've flown RC airplanes over 20 years ago but they were glow engine powered.
I have read several posts which say 100watts per pound is needed - I have no idea what that means.
This plane is supposed to weigh 14-15 lbs and has a wingspan of 71.5". S

So my questions are;
What combination of parts is needed to convert a Great Planes PT-17 Stearman to electric?
Where do you get the parts? Maybe Tower Hobbies??
Any special concerns about changing from glow to electric?
Any comments or suggestions welcome.

Thanks

ElectRick

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Well, Ramon, I intend to find out.

I just ordered one of these for conversion to electric too. It's a project I've been toying with ever since I saw the plane advertised in the magazines.

100 watts/pound is the accepted rule of thumb for the power loading needed to achieve fully aerobatic power levels for an electric powered plane. 50 watts/lb is considered the minimum power loading to get an airplane to fly at all, 75 is roughly equivalent to an ordinary 2 stroke glow powered sport/trainer type plane, and it goes up from there. It is calculated by multiplying your battery pack voltage by how many amps the motor you use draws with your chosen gearing and prop. Volts X Amps = Watts. The voodoo is in determining how much power you need, and finding the right combination and type of motor, gearing, prop, and battery to make that power. It isn't simple at all.

I will say this--if you've never done anything electric, this will definitely NOT be a good way to get started in electrics. This plane is going to take some fairly expensive components to get airborne. I'd suggest you perhaps start with something a bit smaller so you can learn the ropes about setting up an electric powered plane. There is much to learn, and making a mistake with choosing or setting up a motor or ESC and burning up the motor, battery pack, or speed controller at this size can become very costly in a hurry. Most of what you will learn on a small model will carry over to larger stuff, only the size and cost of the components change.

Also, if you plan to use lithium polymer battery technology, it is VITAL that you learn how to care for these cells properly, as a charging or overamping mistake can burn up your plane or even your house very easily.

That said, my planned motor setup consists of a Hacker B50-10L/6.7:1 geared brushless motor, probably a 16-10 prop, a Jeti 77 ESC, and probably a 6S3P LiPo pack. The Hacker is what I have on hand, so that is what I'm using since Motocalc says it should work. An AXI 4130 brushless outrunner would be a good choice too, I think.

This setup represents about $400 in batteries, $300 in the motor, and $200 in the ESC. A lot of money can be saved by going with a geared brushed motor and NiMH cells, though. You could do it for probably less than half that figure by going that route. I have not looked at any brushed motor setups, however.

I would welcome other suggestions for power as well, as I am always open to ways to save money or do things more efficiently.

Rick

Nerevar

Rick,

Thanks for the info.
Needless to say, I am very interested in the results of your electric PT-17 and how it works out.

Ramon

P-51B

My Feedback: (5)

I have been thinking about this also.

I bought the kit with the intent of using gas or glow, but I am concerned about cooling of those while maintaing scale appearance. I have an AXI 4130-16 on hand that was intended for another project, and have been considering this, but I am not sure it will be enough for all that wing area.

Matt Kirsch

My Feedback: (21)

That "voodoo" as you refer to it, is simpler than you think.

When you're looking to do a "glow to electric" conversion, presumably you're looking for glow-like performance. It makes no sense to consider powering a plane with anything less than 100 Watts per pound if that's the case.

As far as the 100 Watts per pound, it means exactly what it says. For every pound of airplane weight, plan on 100 Watts of electrical power. A 15lb airplane will require 1500 Watts. It's simple math. This is all-up weight by the way, ready to fly. You can get a good estimate from the box the plane comes in, as they usually show a suggested weight range. Experience will tell you how "heavy" or "light" your planes come out compared to the recommended weight range, so you'll have a good idea where this one will go.

This whole Watts thing works because Watts is a measure of both electrical AND mechanical power, and they translate back and forth with no math involved. Electrical Watts are easily measured as well, so if your Whattmeter says a motor is pulling 1500 Watts, and you're not exceeding its rated capacity, you can be reasonably sure that most of it is getting to the propeller.

ElectRick

My Feedback: (2)

Based on calculations I've made from someone's figures in the ARTF forum here for this plane, it should weigh in the neighborhood of 11.25-11.5 lb., with radio, but engineless. He was reporting around 13 lb. AUW with a Saito 120.

I am not doing a 'conversion', per se. This plane will never see glow fuel. Additionally, I am not looking for performance like a P&W 450 HP powered airshow Stearman, but more prototypical of the 125 HP models the Navy used. After all, that's what this model is. I am shooting for 75w/lb. on mine. Being overpowered to climb vertically, hover, or zip around at unrealistic speeds is not what I'm after with mine.

I think the wing area of this plane would be a plus, not a minus, as it will provide lighter wing loading regardless of the motor used. I believe the AXI 4130 would be enough, as it's rated for planes in this weight range. More wing area just would mean better flight and slower landing characteristics, which is what a Stearman needs anyway. Too many people try to make a Pitts Special out of something slow like a Stearman or Tiger Moth by grossly overpowering it, IMO.

Matt, not to be argumentative, but for a person who has zero experience in electrics as the OP has stated, there is PLENTY of voodoo involved.

Nothing that cannot be learned, however. That's why I recommended starting with a smaller and cheaper project.

Rick

P-51B

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Just curious, has this conversion gone any further?

ElectRick

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Not really. I'm rethinking my motor setup, trying to decide if the geared Hacker is really the way I want to go, or if an Axi 4130 would be better. I am thinking the Hacker would be operating at or beyond it's limit for a plane this big and heavy. I also will need all the weight up front I can get, and the Axi is heavier.

I am gathering up other accessories like ESC and batteries while I debate over the motor. I probably won't get started in earnest until August. I plan first to partially strip the covering and redo it in the 'right' colors, like a Navy Stearman should be. [8D] Stripping the tail and fuselage will also give me a chance to see if there's any extra wood I can take out of the tail.

Rick

Tweet

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You know, for some reason this particular project continues to intrigue me. Maybe because I think the Stearman is the biplane's biplane and having a nice big one to cruise around with on a sunny day is really appealing.

You know, I can't really get the 4130/16 to work as I play around with Motocalc, batteries, and props. I am using a 4130 on a Kadet Senior on 5S, but that is a much lighter airplane.

Here is what I have come up with. How about a 5320/28 swinging a 19x10 prop? On 9S it will produce 221 ounces of thrust and draw about 49 Amps on an airplane weighing 240 ounces. If you go 10S you get 235 ounces and 52A. On 8S you get closer to scale performance at 193 ounces and 43 Amps. On 8S this is about 85 watts/lb. At 7S if you go with an 18x12 you are down to 143 ounces of thrust which is about 64 watts/lb. This probably provide very scale like performance, but perhaps a little too scale like. I prefer to throttle back a bit and have some extra power in reserve to get myself out of trouble when I need to.

On 8S with 4S (8,000 mah) you get more than 25 minutes of cruising around at 70% power, obviously you could go slower and fly longer.

I think the 5320/28 is a better choice than the 4130/16 including working it a bit less hard and I understand the Stearman needs weight in the nose so it might as well be useful.

Cool project. Has anyone actually done one?

Tweet

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P-51, I saw you post in another thread maybe going gas. What are you thinking? I think it would be a pretty nice E-conversion. I wonder if by properly placing the batteries you could eliminate the lead that many of the glow guys have to use?

ElectRick

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I asked Hobby Lobby about what they recommended yesterday and got the same answer about the 5320 being the way to go. Of course, HL doesn't sell any large geared BL motors, so this was their only offering. Most people wind up recommending too much motor anyway, in my experience. I specifically want scale-like performance, not a 3Der.

I got some E-Calc numbers from a fellow over on RCGroups tonight for various arrangements of prop and battery with the Hacker B50L and 6.7:1 gearbox. This motor will do it, but I think it's going to be working pretty hard most of the time. I like the idea of an outrunner, but these big AXIs are hard to get hold of it seems. I also have a big Inner Demon gearbox that i could strap a big Mega or something onto as well.
I'm searching the web and these forums for similar sized planes that have been successfully converted, to get ideas.

AFAIK, no one has done this conversion yet.

Rick

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ElectRick,

The motor itself won't give you the 3D performance, but more cells will. My guess is you could get ahold of a 5320/28 and then just need to decide how much voltage you will pump through it. For me, I think I would go with 8S. Give better than scale performance but not ridiculous and allow me to share battery packs with two smaller airplanes where I do want terrific performance (a Funtana 90 and a Goldberg Wild Stick 120). Again, I do think 7S would work as well, but would not do it myself...too marginal.

Anyway, I'm interested in this project and may have to go out and get the airplane to get the juices flowing. I'd like a larger Bipe. I was thinking of the lighter Tiger Moth, but I like the looks of the Stearman better.

Please keep us posted.

P-51B

My Feedback: (5)

I haven't decided yet, but yeah, I am leaning towards gas (not glow) right now. Primarily from the cost standpoint. The plane will likely get flown a number of flights per flying day, which would require a large investement in batteries...particularly LiPoly. Plus the motor and controller (I have the 4130, but it appears from what I am reading that it will take more than that). I also haven't done any large e-conversions yet (still working my way up).

ElectRick

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I'm also exploring another option for power. I have an Inner Demon 48 gearbox that I bought for no good reason some time ago. It will take either one or two motors. I am guessing two brushless motors along the lines of Hacker B50-10XL's would be well enough power, coupled to the gearbox.

Here's the cool thing. Balsa Products has a Feigao 540 sized brushless that they say is equivalent to a Hacker B50-11XL, for $80! I have one big 70 amp BL ESC already, would just need to buy another and the two motors. Heck, it may be enough with just one motor. I'd try it that way first, and add the second one if power was lacking.

Just another option for me to consider. This Stearman apparently is just outside the size and weight range of being relatively inexpensive to convert.

Ah well, it's only money.

Rick

P-51B

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Looking at the big axi's, I was leaning towards the 5330-18 as opposed to the 5320 series, what do you guys think of that option comparatively?

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The 5330/18 does 249 RPM/V as does the 5320/28. The 5330/18 has a slightly higher no load current and can handle more current. It also weighs 6 ounces more. It costs a few $$$ more. I think it just depends on if you think you'll need weight in the nose and what kind of airplane you might put it in later. It would probably be a nice choice for the Stearman which is a big heavy airplane, especially if you go with the higher cell count.

P-51B

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If anyone is interested, the axi's are on sale at Hobby Lobby through the 28th.

P-51B

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Apparently they ended the sale early, I went back over to the website a little while ago, and the axis are back up to normal price...so much for ordering the 5130.

Matt Kirsch

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Yeah, I think the sale was advertised "UNTIL July 28th," not through. That means it ended at 11:59PM on the 27th... Still, for a motor the size of a 5330/18, $250 is quite a deal. Just compare that to the Actros and big Hackers.

I jumped on a 5330/18 when I got the first notification of the sale. It came Wednesday. My plan is to convert this Kangke Waco that I reviewed for RCU a while back. It's similar in size and weight to the big GP Stearman. My plan is to use a 10S LiPoly pack, hopefully one of the new FMA SkyVolts when the finally come out so I can get the 3C charge rate. Probably an 18x8 to 18x10 prop, so the motor won't be working very hard.

P-51B

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I was going to order one, but I am travelling on business. I was waiting to order so I could be home when it arrived. I missed out. Or, Hobby Lobby missed my money due to there ad copy...depends on how you look at it.

Glad some people got them though.

ElectRick

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I managed to get in under the line and got myself an AXI 4130/20 for $109. What a great deal! [8D] I received it a couple days ago. It's a big motor. I just hope it holds together under fire--there have been many reports of these big AXI's shedding magnets when pushed a little.

I have also ordered the Feigao 5408411XL brushless with a planetary 5.2:1 ratio gearbox. MotoCalc says this combo on 8S2P LiPo will swing a 16-10 APCe at over 7000 rpm and make 78w/lb, pulling 47 amps at max throttle. Static thrust is 184 oz., and pitch speed is predicted at 67 mph.

I will test both motors in the plane, and use whatever works best. I am betting the geared Feigao will work better, but we shall see.

Rick

ramboman

The PT-17 is flyinf very well with cyclon 160 motor and 18"*10" prop; battery is 30*NiMh.
This is equivalent to a 1.60ci.
The only critical part for the conversion is building an battery box and access to it.

P-51B

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Care to share any photos of how you set your battery access up?

ElectRick

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I'd be interested to see that too, since I'm around this stage in my conversion (been foot dragging badly) where I have to think about how to do that.

Also, what did your all up weight come to?


Rick

ElectRick

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I figured I'd post an update in order to answer the original questions about the GP PT-17 electric conversion.

My conversion has flown, maiden flight was at SEFF 2006 earlier this month. I suffered an unfortunate inflight motor failure, but did get about six minutes of great flying before it failed. The motor failure forced me to make a hard landing in a rough plowed field, which caused some minor landing gear damage. Repairs are nearly finished and a replacement motor is in hand.

The plane flew with a geared Feigao 54084-12XL, with a 5.2:1 ratio and a 16-10 APC-E. I am using 2 TrueRC 4S2P 8000 mah LiPos, in a 8S2P series configuration using a premade harness.

The motor is more than enough power for this airframe. At a total AUW of only 13 lb. (3 lb. of that is batteries!), the plane took off smoothly and quickly. I flew most of the flight at half to 3/4 throttle, with the plane handling beautifully in the air. All things considered, I am quite pleased with how it turned out.

I did a conversion thread [link=http://www.rcgroups.com/forums/showthread.php?t=512454]here[/link]. If anyone has any questions, just ask.

Rick