Building a J-3 Piper Cub with an Electric Motor

[Cyrus]

I ordered all my parts for my new airplane today. It's a J-3 Piper Cub with an 80" wingspan.

Normally it runs on a .46 glow fuel motor, but I am going to run it on an electric motor instead. (Not an easy conversion process)

http://www.uploadfile.info/uploads/a3c80b0987.jpg

It's a fun hobby, except for the fact that I just spent about $700.00 and still counting on this thing...

 

[wolram]

very nice, how long will it fly electric?

 

[Cyrus]

I am using a $200.00 4200mAh Lithium-Polymer battery pack at 14.8V-DC with a max current draw of 84 Amps and a nominal discharge of 64 Amps.
(You have to be very careful charging these batteries or they will catch fire or explode)

With the motor and prop combination, I should get around 15-20mins in the air at half to 3/4 throttle.

(I will have to set up a bench test and run the motor and battery under varying loads <different sized props> and get actual numbers before I fly.)

The motor is rated for 500-900 Watts power and 1100W (for a short burst)

This is the motor:

http://www.allerc.com/images/Pictures/HP-Z4025-XX.jpg​

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A Glow fuel engine at a size of .46 would put out 1.23kW, and clearly shows how much more efficient electric motors are to their Glow counterparts. (With this battery, it should fly just as long as on a tank of Glow Fuel.)

(Once I have time I can give you a real post with loads of technical information if you want)

 

[wolram]

Amazing what they get out of a battery these days, i would have thought the motor would melt with that wattage.

 

[Danger]

You're going to have to lose a fair bit of weight to get off of the ground in that thing.

 

[Pengwuino]

You're going to have to lose a fair bit of weight to get off of the ground in that thing.

no he wont, those motors are ungodly powerful

oh wait a second, you're replacing a engine with a motor?

bye bye airplane

 

[wolram]

no he wont, those motors are ungodly powerful

oh wait a second, you're replacing a engine with a motor?

bye bye airplane

Pengwuino Are those wings or ears?

 

[Pengwuino]

wings :| duh

 

[rcgldr]

I fly radio control gliders (have a good slope site nearby with good updraft, so I don't need a motor), but follow what's going on with powered models at the local hobby store and the AMA magazine (American Modeling Academy), and the relatively new progress being made with electric models, the motors and li-poly batteries. Also I know some guys that fly F5B 10 cell and 27 cell type models (high speed 120+mph gliders that use the motors strictly for climbing).

80 inch wingspan on a powered model is really big. How much will the model end up weighing?

Do you plan on using gear reduction, or is the motor a lower rpm / high torque motor? Being able to run at 1100 watts (almost 1.5hp) even for a short run is really impressive.

 

[Rach3]

How big is this thing? What are the 900 watts for? Does it fly? I calculate those batteries will last only 4 minutes at 900W.

 

[Pengwuino]

How big is this thing? What are the 900 watts for? Does it fly? I calculate those batteries will last only 4 minutes at 900W.

Its for the motor, duh

And yah I am also figuring around 5 minutes of flying time if this is really 3/4 throttle, 64amps being full.

It'll probably crash and ill probably laugh for a day

 

[russ_watters]

I built one of them when I was in 8th grade (it is a lot cheaper when you get all the pieces as separate presents and have a birthday close to Christmas ). It is made entirely of balsa wood, so it took several hundred hours to build. IIRC, it has a 49" wingspan. It flew exactly three times:

-First was meant to be a high-speed taxi and just got off the ground for a few feet.
-Second, flight, it had the aeleron movement backwards and as soon as I got a little altitude and tried to turn, it did a wingover straight into the ground.
-Third flight, I got it in the air and everything was great, but being a panicky teenager, I tried to land it right away instead of getting a feel for it first, and I crashed it again.

Fast forward fifteen years, and it is still hanging on the wall in my parents' basement. Since I just bought a basement of my own, I've been emptying theirs a little at a time every time I go over. Today, I may bring it back with me and this summer I may try to get it flying again. It probably only needs minor repairs (tough plane), new batteries for the controls, and to take apart and clean the engine (.4 in^3 glow fuel).

Have fun - what other planes do you have? (I see a wing peeking out from under your worktable...).

 

[Cyrus]

I was having too much fun building the thing last night that I went to bed at 4A.M. .

So far I got all the control surfaces hinged. The ailerons are now hinged and centered with servos in place (Although I need to take them back out to attach servo extension wires so they can reach the radio receiver in the fuselage).

http://www.uploadfile.info/uploads/4e700826d9.jpg

Also, I have the elevator and rudder servo installed, with the rudder hinged and centered with the servo and pushrod. The tail wheel mount is also attached (the metal piano wire coming out the bottom of the rudder) I still have to attach the stab and then center it with the servo. It's hard to tell, but I also have the wing strut mount attached (that little piece jetting out the side of the fuse under the center window near the very bottom.)

http://www.uploadfile.info/uploads/ae6e51bc66.jpg

To Jeff:

I might get a glider myself some day, they look like loads of fun. As for the motor, it is direct drive. I did this because I do not want to loose power where the gears mate on the reducer unit. Also, with time those teeth will wear out, and there is a great chance they will break if the prop strikes the ground. These motors are rated in KV (NOT kilovolts, but RPM/Volt). My motor is rated at 560kV, which means I should be able to swing a 13x8 (13" diameter by 8" pitch) prop at ~7840 RPM.

Had I used a .46FX Glow engine, I would swing a 10x8 prop at 16,000 RPM. Quite a big difference, nearly half.

The model weight will be in the 6.5-7.5 lb range.

The motor is rated for 40~60A at max. efficient current and 80A for a max peak duration of 30 sec.

As I said, I will have to verify all these numbers on a bench test before I go flying. If I don't, I run the risk of over heating the battery and causing it to catch fire...and that would be bad.

To Russ:

If you want to get back into the hobby, I would recommend you hang that airplane on your wall and get yourself a proper trainer. If you want to fly it you will need to get membership into a local RC club that has a designated flying field and you will need AMA insurance. These airplanes are big, fly fast and have the potential to kill or maim people.

The wing you saw is uncovered and goes to a Ryan STA ~60". The other white wing is for a V-Tail Beechcraft Bonanza ~80".

 

[rcgldr]

As for the motor, it is direct drive.

There is a new type of electic motor where the inside is fixed and the outside rotates (plus a fixed can around all of that). These motors reach peak efficiency at much lower rpms, and are good for direct drive applications (they won't heat up as much at lower rpms). It your motor one of these? 7800+ rpm sounds like enough to keep the motor from cooking though.

The main reason for the gearing is to allow the motor to operate near it's peak efficiency range, but you probably already know that. My only experience is seeing what the F5B guys are running, which are geared motors with 16 or 17 inch props, with 15 or 16 inch pitch. The "27 cell" (32 volt) ones are outputting about 2.5 hp, and climb vertically at 120+mph, but are only on for 3 to 5 seconds at a time (LMR = limited motor run). 30 seconds of motor on time is enough for a contest. Powered models are different, needing continuous power (usally at least 1/2 throttle) most of the time.

Looks like your model will be very nice when completed.

Regarding gliders, the dynamic soaring record was pushed to 301 mph last November during Santa Ana wind conditions at Parker Mountain (actually the nearby ridge), in Los Angeles Crest Mountains. Link to that and other videos (the 223mph video gives a better picture, as the 301 run video is zoomed in a bit too tight).

http://www.slopeaddiction.com/thekids.html

 

[Cyrus]

I finally got the empennage completed. Getting those support wires all done up was a real chore!

http://www.uploadfile.info/uploads/b9596c5628.jpg


I have an outrunner motor, the outside cage spins.

 

[rcgldr]

Wow tail end really looks cool. How do you adjust the wire tension to get the tail end lined up properly?

 

[russ_watters]

If you want to get back into the hobby, I would recommend you hang that airplane on your wall and get yourself a proper trainer.

The plane is a sig Kadet MkII trainer. http://www.sigmfg.com/IndexText/SIGRC49.html

And I'll look for a club. I flew it before at my grandparents' farm - 65 acres of open space.

 

[Moonbear]

Boys and their toys. (Okay, yeah, I'm jealous! Those look fun! )

 

[scott1]

Have you ever tried putting a camera on one of those things and use it as spy plane?
I always wanted to do that(if i had one of those planes)

 

[rcgldr]

Have you ever tried putting a camera on one of those things and use it as spy plane?

It's been done, even with gliders and rc helicopters (the heli's are tough because of vibration issues). Some actually fly the models via a live camera feed from an aircraft.

 

[Pengwuino]

Have you ever tried putting a camera on one of those things and use it as spy plane?
I always wanted to do that(if i had one of those planes)

You can't put them on small airplanes. That is, cyrus's aircraft is normally engine powered so he will need to take off as much weight as possible so its probably not a good idea to put it on his. With engine powered rc aircraft however, it's awesome! Put little cameras in the nose and you can actually fly with it. Make a little spy plane out of them

 

[JasonRox]

I'd rather go with a scooter.

Still pretty sweet though.

 

[JasonRox]

You can't put them on small airplanes. That is, cyrus's aircraft is normally engine powered so he will need to take off as much weight as possible so its probably not a good idea to put it on his. With engine powered rc aircraft however, it's awesome! Put little cameras in the nose and you can actually fly with it. Make a little spy plane out of them

Wouldn't a helicopter be much better?

 

[Moonbear]

I'd rather go with a scooter.

I thought you didn't want us calling your scooter a toy.

 

[Cyrus]

Well, here she is almost ready to go. (Heh, I should not say that because I have to do a lot of motor and battery testing first)

I have not yet installed the motor, battery, BEC, ESC, Reciever, monuted the Canopy or the Engine Cowling.

http://www.uploadfile.info/uploads/7cd8ca1ce0.jpg

http://www.uploadfile.info/uploads/6fb9cb15d7.jpg

http://www.uploadfile.info/uploads/153f0e6c4c.jpg

http://www.uploadfile.info/uploads/f685db24c2.jpg

As for a camera, yes, I could put one on there but I don't want to. My airplane could easily carry a camera Penguino.

Penguino, an electric motor can out perform a Glow engine if set up properly.

I work with UAV's that have camera's on them, some are IR. An optical camera is about the size of 1/2 x 1/2 x 1/4 thick. The IR camera is bigger (2x 1.5 x 1.5) and more expensive (~$10,000).

I think putting a camera on a model is useless, it's just weight that serves no purpose.


To Jeff:

The wires on the horizontal and verticle stab are merely for looks; however, the support cross bars on the main wing are functional and act as structural members.

 

[Pengwuino]

Wouldn't a helicopter be much better?

Jeff says you have vibrational issues and I've never actually flown an r/c helicopter so i don't know.

 

[Pengwuino]

Well i'd like to see that baby fly on an electrical engine. I was always under the assumption cameras are too heavy... but i use to try my luck at the small r/c aircraft at like 100-200 watt motors.

 

[rcgldr]

Regarding the motor / prop setup, and assuming you get the AMA magazine, there were some articles a while back about power (watts) to weight ratios as well as some prop / gearing data based on a motor's KV value, and the thrust / speed you need for the model.

If the manufacturer includes this info in the kit, it would be very nice. So far, I've only seen this data included with RC electric heli kits.

 

[Pengwuino]

Speaking of which, i want to blow up GWS's factories. Those a-holes sell their A-10's with absolutely no way of flying on stock/recommended electronics.

 

[rcgldr]

better with a rc heli

Jeff says you have vibrational issues and I've never actually flown an r/c helicopter so i don't know.

It has been done. One thing is to do a really good job of balancing the blades, both weight wise and lift wise. Maybe a small supension system would help as well. The high end RC-helis have huge thrust to weight ratios, like 7 or 8 to 1, so they can afford to have a heavier, but smoother setup attached if they aren't going to do extreme manuevers. Because of the high thrust to weigh ratios, heading hold gyros that allow helis to fly sideways smoothly, fly bars to dampen roll and pitch rates, and the ability to fly upside down, backwards, and sideways, makes for some pretty impressive moves if the pilot can keep up with all the orientation issues. In this video, a Raptor 90 is flown, it's big, with about a 52 to 56 inch rotor diameter, but moves extremely quickly because of the high thrust to weight ratio.

rcheli.wmv

 

[Cyrus]

Speaking of which, i want to blow up GWS's factories. Those a-holes sell their A-10's with absolutely no way of flying on stock/recommended electronics.

I wouldn't exactly call those GWS's "model airplanes," they are more like novelty items to display in your trash can.

 

[rcgldr]

Penguino, an electric motor can out perform a Glow engine if set up properly

The motors have gotten more efficient, but the big breakthrough is the li-poly batteries, which provide a lot of energy for a relatively low weight. This makes the models significantly lighter, requiring less power for flight.

 

[rcgldr]

When it's ready, maybe you can have a buddy take some videos of the model in action and post them here. Waiting to see that cool looking model in action.

 

[Pengwuino]

I wouldn't exactly call those GWS's "model airplanes," they are more like novelty items to display in your trash can.

THREAD CLOSED!

 

[rcgldr]

THREAD CLOSED!

Nah, just the trashcans with the GWS models in them.