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Piper J3 cub/3ch.

Old 04-02-2008, 05:18 AM
  #1  
lode
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Default Piper J3 cub/3ch.

I have a Piper J3 cub 3ch. plane. It has a 380 motor,plenty of power,but it turns over when launched by hand. I think the dehedrual(bad spelling!} is off. This thing wont fly at all. Good thing i flew over a soybean field for crash protection! Legally! Has anyone else had this problem? I was going to increace the wing-strut length to provide more angle on the top wing,the ? is,how much? THANKS!!
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Old 04-02-2008, 05:20 AM
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Grasshopper
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Hello lode and welcome to Wattflyer. Can you post some pictures of the plane or give us more information on which J3 Cub it is (who makes it)?
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Old 04-02-2008, 05:34 AM
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lode
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Default piper J3 cub

I have a piper J3 cub/380 motor. It barrell rolls upon hand launch,and EVERYTHING is set up.(3ch) I think its wing dehedral. How many degrees should a 37" wingspan have? It has alot of thrust,but wont fly. It is supposed to be stable at slow speed. Anyone else have this happen? Thanks!!
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Old 04-02-2008, 05:39 AM
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Originally Posted by Grasshopper View Post
Hello lode and welcome to Wattflyer. Can you post some pictures of the plane or give us more information on which J3 Cub it is (who makes it)?
GUANLI
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Old 04-02-2008, 05:41 AM
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Gualni
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Old 04-02-2008, 05:42 AM
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this is webtv and it's lousy! sorry
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Old 04-02-2008, 05:49 AM
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[email protected] would be the best way to respond. I just wanted to hear if anyone else had the same problem! THANKS!!
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Old 04-02-2008, 06:01 AM
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Lemme try this again! The box says GUANLI Item #401-2 27.075MHz.
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Old 04-02-2008, 06:15 AM
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http://www.diytrade.com/china/4/prod...3_RTF.html?r=0

how much did you pay for it? It could be badly out of trim or simply off. Look at the plane from all angles, make sure everything is perfectly lined up, if they aren't, you'll have to get them that way, one way or the other. Try a glide test to see if it glides correctly.
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Old 04-02-2008, 11:26 AM
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GreenAce92
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Is this the direct drive version? these are actually really weak, make sure your wings are straight horizontally, like the AOA(angle of attack) is the same maybe one wing is producing more lift than the other, the more dihedral you give the better the rudder steers.
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Old 04-02-2008, 12:58 PM
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Leo L
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Hi Lode,

Your problem is not the degree of dihedral! If the plane flew perfectly well straight, but was difficult to turn, then you would need to look at the dihedral. Your plane's tendency to barrel roll on take-off indicates that it is not developing enough lift and that one of the wings tips is stalliing.

Guanli is the chinese company that makes so many of the cheap knock-off planes that are sold all over e-bay. Some of their products fly OK, others are impossible to fly. Their quality control is terrible, so the same plane may fly decently for one person and not at all for another one. There are several threads here and on other forums like RCGroups and RCUniverse that address the problems of the "Non-PZ" J3. If I recall correctly, your plane has cheap little plastic wheels. The first thing that I would do is change the wheels to better, larger foam wheels and see if the plane will do a ground take-off. The second problem is that the plane is probably severely underpowered and can't generate enough lift to fly. Replace the motor that you have with the motor/gearbox from PZ, making sure that you use the "High Pitch" prop. Upgrading the motor to the Venom motor would be the next step. Finally, you can do a major upgrade and install a brushless motor and LiPo.
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Old 04-02-2008, 08:18 PM
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Part of the problem is going to be the weak direct drive setup, as GreenAce mentioned. Also, it could be that you have the plane horribly trimmed. The control surfaces, both rudder and elevator, should be level with the tail fin and tail wing, respectively, prior to your first flight. You adjust the trims using the little slider nobs on the controller that came with your plane. If you can't adjust the nobs far enough to trim the control surfaces to level, then you might have to do some manual adjustment on the length of the push rods. (This can be as simple as screwing or unscrewing the EZ snap connectors at the end of the push rod a bit and then re-attaching them to the control horns).

But I bet that you need to check your CG. You can do this by balancing the plane's wing on your fingertips. When the plane sits level on your finger tips, the point on the plane at which your fingertips are touching the wing is the CG. Normally (and in the case of the Piper Cub), this should be about 1/3 the distance from the front (leading) edge of the main wing to the back (trailing) edge of the main wing.

After doing some epoxy-heavy tail section repairs on my SDM Piper Cub (almost identical to the GuanLi Piper Cub, only with a gear-box and a much larger prop), it handled 5 kinds of squirrely, and wanted to dive down and left, just as you're describing. I checked, and the epoxy I added to the tail had shifted the CG over halfway back on the main wing. A little bit of weight and shifting the battery around to get the CG back to the 1/3-from-the-leading-edge-of-the-main-wing sweet spot, and it was flying stable again.

Remember this ever-so-true adage: "A nose heavy plane flies poorly. A tail heavy plane flies once."

I actually bought one of the GuanLi Jaguar Piper Cub's for my Dad to play around with (he liked the camo scheme), although I converted it to a brushless setup first. And man, do those little light things scream with an outrunner turning the prop. Didn't have any problems flying once I trimmed it, though.

Good luck, and do post back with some pictures if none of the above suggestions help with your problem.
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Old 04-04-2008, 05:14 AM
  #13  
lode
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Originally Posted by Leo L View Post
Hi Lode,

Your problem is not the degree of dihedral! If the plane flew perfectly well straight, but was difficult to turn, then you would need to look at the dihedral. Your plane's tendency to barrel roll on take-off indicates that it is not developing enough lift and that one of the wings tips is stalliing.

Guanli is the chinese company that makes so many of the cheap knock-off planes that are sold all over e-bay. Some of their products fly OK, others are impossible to fly. Their quality control is terrible, so the same plane may fly decently for one person and not at all for another one. There are several threads here and on other forums like RCGroups and RCUniverse that address the problems of the "Non-PZ" J3. If I recall correctly, your plane has cheap little plastic wheels. The first thing that I would do is change the wheels to better, larger foam wheels and see if the plane will do a ground take-off. The second problem is that the plane is probably severely underpowered and can't generate enough lift to fly. Replace the motor that you have with the motor/gearbox from PZ, making sure that you use the "High Pitch" prop. Upgrading the motor to the Venom motor would be the next step. Finally, you can do a major upgrade and install a brushless motor and LiPo.
The plane has plastic rims,and foam tires.about 1" the plane swerved to the right when taxiing on the ground when you goosed the throttle. It's a direct drive 380 motor. The props are flimsy,very! they break easily. It barrel rolled clockwise rear view. The trim should be OK,but even when I hit the elevator,nothing. Maybe I should replace the motor. Thanks for responding!
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Old 04-04-2008, 11:08 AM
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GreenAce92
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"A nose heavy plane flies poorly. A tail heavy plane flies once."
kinda funny you said that, i built a pusher plane which was way tail heavy and yet she flew like an airhogs plane, but i dont favor my plane dragging around its tail and flying at a really bad angle.
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Old 04-05-2008, 07:19 AM
  #15  
lode
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I think balancing the plane is the right idea.I NOW know that I have a crappy one,but I can use all of the parts. I will take the advice,and balance it. even with a weak motor,it should glide stable. When you hand launch,thats quick,it's up to the plane to maintain the velocity,in my opinion.I'll let you know what happens,and thanks again! I have a superstar 60 nitro plane that I learned on,but that is a whole different ballgame!That thing wieghs almost 6 POUNDS when loaded up with fuel!16oz. I like electric because of no mess!
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Old 04-05-2008, 04:52 PM
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Originally Posted by lode View Post
I think balancing the plane is the right idea.I NOW know that I have a crappy one,but I can use all of the parts. I will take the advice,and balance it. even with a weak motor,it should glide stable. When you hand launch,thats quick,it's up to the plane to maintain the velocity,in my opinion.I'll let you know what happens,and thanks again! I have a superstar 60 nitro plane that I learned on,but that is a whole different ballgame!That thing wieghs almost 6 POUNDS when loaded up with fuel!16oz. I like electric because of no mess!
If I could make a suggestion, you might want to consider doing a simple upgrade to the plane in order to make it fly worlds better. You can pick up a Tower Pro 2410-08 motor, ESC, and prop combo from HobbyCity.com for around $15. (If you decide to use them, be sure to register an account with them prior to clicking to add something to your cart, as you usually get a slightly lower price when viewing the items as a registered user.) In order to install it, you just remove the existing motor, cut down the motor offsets on the firewall, attach a small 1.5" x 1" x 1/4" piece of plywood to the firewall using epoxy, and then mount the 2410-08's mounting plate to that piece of wood (centered so that the shaft comes out of the cowling correctly) with a combination of Epoxy and small wood screws (long enough so that they penetrate both the wood AND the plastic firewall). The upgrade in power is unbelievable, and well worth the trouble, in my opinion.

You could get by with JUST that upgrade, as it will still work with your 600mah NiMH battery, although flight times will likely be pretty short (no more than 6-7 minutes, I would think). I would likewise pick up a small 2S LiPo (such as this one for ~$7) and probably get back up in the 10+ minute range if you don't go crazy on the throttle. Of course, you WOULD have to cut a little bit of foam out of the battery compartment in the GuanLi Piper Cub to make that battery fit, and then use velcro to hold the battery in place, but that's an extremely easy mod. However, you WOULD need to also buy a smart balancing LiPo charger. The cheapest way to go with that for now is the Esky balancing LiPo charger, which can be had from DealExtreme.com for $8.60 (free shipping!). Bear in mind that's a very basic and SLOW (charges at a maximum rate of 0.8A, and only one cell at a time) charger, and you have to either have your own 1.0A 12V DC adapter -OR- run it off a car battery. But it works well and gets the job done for VERY cheap.

Now I know you're probably thinking, "Why spend $40 upgrading a crummy plane, when I could just put that towards another BETTER plane?" Well, two things. First of all, the GuanLi Piper Cub isn't a completely crummy plane, it just has very poor electrics. The body is actually a nice little foamie build upon which quite a few nearly identical Piper Cub models are assembled. The basic structure of the fuselage and wing is a good design and, once reinforced with clear packing tape, can be very, very resistant to crash damage. Secondly, any of the electrics I've mentioned above are completely re-usable. In the event of the unfortunate (crash turns your foamie plane to snow), you can just salvage these parts and use them in another ARF plane with very little additional investment.

And, of course, there's something to be said about the experience you gain from doing a brushless upgrade. I learned more by simply going through that process than I had from any reading I'd done on the subject online.

At any rate, good luck with your little plane. Properly modified, I suspect it could be a very good little flier for you. Be sure to keep posting back around here with questions if you need some more help! The folks around here (not speaking of myself) are the friendliest and most knowledgeable community I've come across online. Hence the reason I tend to stay on this site now.

Take care, and happy flying!

EDIT: Almost forgot... ALWAYS remember to re-check and re-calibrate CG after ANY modifications to your plane. Going brushless/LiPo can move the CG towards the tail fast, especially depending on where the battery is. Good luck!
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Old 04-08-2008, 05:00 AM
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lode
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THANK YOU VERY MUCH!! I will post later when things get done! You are right,this is a nice group!
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