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Old May 24, 2012, 09:07 PM
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Help!
Need inputs on (re)building Piper Cub

Hi folks,

Long-time lurker here, but this is my first post:

Someone handed me a never-used Piper Cub kit from General Hobby (http://www.generalhobby.com/piper-fu...low-p-185.html) and I set it up for an electric conversion with the following RTF configuration:-

6s 5000mAh LiPo
Turnigy TR 50-65B 350kv Brushless Outrunner (Eq: 4030/16 from Axi)
16x10 Zinger wood prop (you can see where this is headed)

Flying weight was about 8.6lb. I also tested that I was getting about 880-900 Watts at a theoretical ~8800 RPM.

First and only flight resulted in strong uncontrollable bank to the left and crash. Luckily I was able to bring it nose down, and the only damage was bent landing gear and a broken engine mount.

I can only point the crash to the effect induced by the oversized prop, all controls were working fine even after the crash, and the CG was carefully balanced. The plane did taxi straight down the runway and I could not see any other problems.

I understand that I possibly need a different prop. What do you guys suggest for this plane?

I also think I may not have put enough angling to the down and right for the motor mounting (it was perhaps a degree or so both ways). I'd appreciate some suggestions for this too. Importantly, is there any way to test this on the ground or by suspending the plane in air by some rope to check for torque effect?

I have flown the Radian, Radian Pro, and a couple of smaller foam warbirds. I understand that the Cub can be a handful, so any flying tips are also most welcome.

Thanks for your time,
Vijay
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Old May 24, 2012, 10:03 PM
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Joined Mar 2012
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Check for warps. ARF wings aren't always straight or consistent. People also sometimes botch gluing wing halves together and make something that resembles a propeller. I've seen a few cubs go in that way.

If you're not good at eyeballing warps from the backside, some 3' yardsticks or (straight) balsa strip stock taped under the wingtips and near the wing root will give you a good reference looking from the side regarding wing twists. With a 3' extension sticking out, even warps that are only 1/32 or 1/64" at the TE get amplified into something visible.
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Old May 24, 2012, 10:25 PM
Da' Cajun
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Lake Charles, Louisiana
Joined Jun 2009
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I've put together a couple of those Cubs from General Hobby. All were great flyers.
I doubt very seriously if the wing is warped at all.

And you could probably get by with a smaller prop but a 14" or 15" prop would still have some considerable torque too.

So, it taxied straight on take-off? how much of a take-off roll did it have?
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Old May 25, 2012, 03:15 AM
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Thanks for the suggestions, people.

I did look carefully for wing warp visually, but could find none.

Boogie, its good to hear you've had some experience with these Cubs. To be specific, the plane rolled down the runway straight (20 ft maybe 30ft) on providing full throttle, but veered and banked to the left on takeoff, with a 30 degree angle to horizontal, even with ailerons pushed to extreme right. I was able to bring it down while in this looping condition. Tried to bring it in level, but could not get the angle of descent as close to horizontal as I could.

I have setup the new motor mount angle downward and right at 3 degrees each. I also plan to put in a head-holding gyro to make the takeoff easier.

Do you think I should replace the 16x10, or simply use 60-70% throttle instead?
Any (other) thoughts welcome.
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Old May 25, 2012, 07:20 AM
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Having flown many sizes of Cubs, from what u describe sounds like it got airborne too soon. Cubs need a good long take off roll. I seen a lot of people goto full throttle as soon the start their takeoff roll. The cub then jumps into the air stalls and falls back to the ground. The trick to getting a Cub airborne is the use of throttle and rudder. Start your takeoff roll at a low throttle setting, gradually add throttle and rudder. If done right the plane will just about lift on its own somewhere around half to three quarter throttle. All of the Cubs I seen today are overpowered. Cubs are meant to fly off the wing and not the motor. Once the rudder takeoff and coordinated turns are master along with good throttle management you will find a Cub one of the most relaxing plans to fly. I usually fly mine from a chair and shoot touch and goes the whole flight. I can't remember when the last time I even went to full throttle on the Cub.
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Old May 25, 2012, 07:54 AM
Da' Cajun
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Lake Charles, Louisiana
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Quote:
Originally Posted by pdansalvish View Post
Having flown many sizes of Cubs, from what u describe sounds like it got airborne too soon. Cubs need a good long take off roll. I seen a lot of people goto full throttle as soon the start their takeoff roll. The cub then jumps into the air stalls and falls back to the ground. The trick to getting a Cub airborne is the use of throttle and rudder. Start your takeoff roll at a low throttle setting, gradually add throttle and rudder. If done right the plane will just about lift on its own somewhere around half to three quarter throttle. All of the Cubs I seen today are overpowered. Cubs are meant to fly off the wing and not the motor. Once the rudder takeoff and coordinated turns are master along with good throttle management you will find a Cub one of the most relaxing plans to fly. I usually fly mine from a chair and shoot touch and goes the whole flight. I can't remember when the last time I even went to full throttle on the Cub.
+1 what he said.

I have two Cubs right now (not from Gen Hobby) and neither of them even get to half throttle on take-off. Long, long roll out and a gentle climb out is the key.

The ones I've flown from GH were the same also.

Cubs are not hard to fly but will make you a rudder master. Be quick on the rudder but don't slap it around. Hold gentle pressure and let the corrections take effect as it builds up speed.
When the tail starts flying it will start to weather vane and little rudder will be needed after that. Let it roll on the mains until it starts getting light and starts flying on it's own.
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Old May 25, 2012, 02:17 PM
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Be sure that your motor is angled a little down and a little to the right, helps control tendency to yaw left and pitch up during takeoffs and when applying sudden throttle.
Start takeoff roll with up elevator to hold tailwheel down and aid steering, gradually increase throttle, as speed builds, let tail rise and control direction with gentle nudges of right rudder to counteract torque, and keep from veering left. Let model rise off at around 3/4 throttle and keep climb angle moderate. Give a little down elevator to countreract tendency of nose to rotate up and cause a stall, back off on throttle as needed to transition to level cruising flight. To land, reduce throttle to 1/2 and have nose slightly down, use RUDDER to keep wings level, (ailerons lose effectiveness at landing speeds), reduce throttle gradually until model's main wheels touch down and reduce throttle more until tail wheel touches down. Raise elevators to keep tail wheel down on runway and to aide steering. Keep some power on during landings, aids steering and control, I use coupled aileron and rudder most ot the time, makes flying scale models easier. Over-ride it or shut it off when needed or desired.
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Old May 25, 2012, 06:07 PM
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United States, FL, The Villages
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vijayfx35 - These folks must know what they are talking about, because I AGREE 100% with what they say. Good smart folks! The smooth application of power looks better too. I have 3D planes that can takeoff in almost no space, but I do more scale like departures. Just looks better, IMHO.
Don
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Old May 26, 2012, 11:00 AM
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Tucson
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My Seagull 80" Cub is the most difficult plane that I have ever flown. Weird points about this Cub are:
1. Must keep 30% down elevator to keep the nose down until it gets up flying speed
2. MUST use rudder to keep it straight on take off (it will try to turn into the wind, and roll)
3. Do RUDDER turns, with a little aileron to keep angle of turn nice.

The Seagull Cub will snap roll if you let it just lift off (the main wing loads before the tail has enough air to control the plane). When this happens, you get a strong bank in one direction, that is actually a stall and slow snap roll.

Maybe little foamie Cubs are stable, but Cubs that fly like the full-size plane have very demanding flying habits. You will learn real flying skill to get them into the air, and learn how to make them fly gracefully. Practice taxiing it and keeping the plane straight with the rudder, and keeping the nose down. It will build your flying skills.

(The generalhobby P-51 54 inch and 1600 watt aeolian motor and 3,000 mAh 6S LiPo and 190A speed controller was MUCH, MUCH easier to fly than this 80" Cub!! The P-51 is a flying brick, but it has lots of wing, goes exactly where you point it, and slows down wonderfully to land. It tracks true on the ground, and take off. I would suggest the P-51 before I would ever suggest a "low wing trainer" such as the Escapade.)
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Old May 26, 2012, 11:02 AM
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By the way, watching a plane fly on youtube is always wise. But the Cubs on youtube may be flown by guys who have advanced flying skills. Seeing their Cubs fly beautifully, just means that this is possible -- not that this is easy!
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