Simple Cub not flying

MasterEpic

New member
Hi, I have scratch built the simple cub. I am using A2212/10T 1400kv and I am using a 8060 prop. It has ailerons, the control surfaces work perfectly. But every time I throw it it death spirals to the floor and I am confused on why it does that and how to remedy it. The CG is correct and I am using the TATTU 11.1V 1550mah 3s Lipo, it weighs 60g and the plane weighs 600g and it has been built and cut properly according to the planes. The motor datasheet says that it should produce between 910 and 805 grams of thrust (the interval is due to the lack of an official datasheet but many of them online)
Please could you help me with this as it is my first RC plane I have built
Thanks
 

Foamforce

Well-known member
You mean it rolls one way or another, right? Side to side, not front to back?

The simplest thing is you could just have your ailerons and/or rudder backwards. When you move your left stick left, your rudder should move to the left (standing behind). When you might your right stick left, your left aileron should go up and you’re right aileron should go down.

Another thing it could be is torque roll. Try using less throttle to take off, and turn your motor on before your launch it.

You might just be overly aggressive with your control input and/or your control throws are too high. Check your control throws using the gauge and use the low throws initially. Also, try it without ailerons. The Cub flies very nicely without ailerons and eliminates a variable that makes it more difficult to fly.

Lastly, or maybe this should have been first, is check the alignment of all your control surfaces. The rudder should be perfectly straight with the vertical stabilizer and the ailerons should be perfectly straight with the top trailing edge of the wing. Adjust anything that’s it of alignment using either mechanical adjustment if possible, or using sub trim on your transmitter. Finally, do a glide test by plugging everything in and turning it on so that the servos are centered. Then toss it (moderate speed) toward a couch or some tall grass. It ought to glide with the nose slightly down, but not roll to either side. If it does, then you still have an alignment issue.

If you post a video we can help diagnose more. Post on YouTube and paste the link here. Good luck!
 

Tench745

Master member
Another thing to check: make sure your motor is spinning the correct direction and the prop is mounted with the numbers facing forward.
 

MasterEpic

New member
Yeah I have checked that the motor is spinning in the right direction, also the ailerons are perfect. Should the simple cub be able to take off from the ground as my plane goes a few centimetres off the ground at full throttle?
 

MasterEpic

New member
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Foamforce

Well-known member
Yeah I have checked that the motor is spinning in the right direction, also the ailerons are perfect. Should the simple cub be able to take off from the ground as my plane goes a few centimetres off the ground at full throttle?

Yes, it easily takes off from the ground.

The Cub normally uses a 9 inch prop, but that’s with a 1000kv motor. Since you have a 1400kv motor, you 8 inch prop seems right.

So it rolls forward but you don’t have enough power to take off? Double check that the prop isn’t reversed. As Tench said, the prop might be spinning the right direction, but it might be put on backwards. The numbers should be facing forward.

At this point though, you’re describing a different problem than the first one. A video would really help. It’s difficult to diagnose a problem from a short description.
 

Piotrsko

Master member
Judging by the term death spirals to the floor and tthe fact you don't mention having to rebuild it: you might be adding rudder input when using the throttle particularly if it turns left in the death spiral. In my case that's why I don't use rudder on my aileron equipped things, I was never able to coordinate my left hand and half the time I cant mess with throttle while flying
 

MasterEpic

New member
Yeah it was slightly
I tried throwing it and it crashed, I tried doing it from the ground and it would spiral towards the left but at full throttle and a push from the floor it would go about 10cm from the ground
 

Foamforce

Well-known member
That Cub looks too small, but it could be the video. Can you measure the wingspan? It should be 30.5”/775mm 38”/965mm.
 
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Foamforce

Well-known member
Ok, so I was watching the video closely with a friend and our conclusion is that you have either one or two problems.

The first problem is thrust. Lack of thrust, plus the high angle of attack (nose up) in the video is likely causing it to stall. Stalls frequently happen on the same side every time, so that could explain some or all of the roll issue. For example, when I had my first Storch, it was underpowered and I didn't get enough speed before pulling up and it kept rolling to the left and crashing, similarly to what we're seeing here. I don't remember mine <yawing> that much to the left though, so that's why I suspect there may be a second issue. In any case, if you don't have enough thrust, you need to fix that, so we may as well start there.

Possible causes of low thrust:
  • Prop installed backwards. I'm not talking about the rotation direction here, I'm talking about having the numbers (8060) facing forward on the prop. If it's on backwards, it will still produce some thrust, but a lot less. Check this first.
  • ESC not calibrated to throttle. To calibrate it, turn on your transmitter first. Put the throttle all the way to the highest position. Then plug in your plane. The ESC will beep. After it beeps, turn the throttle all the way down. The ESC will beep a few more times and it will be calibrated. Do that and see if you get more thrust.
  • The prop nut could just be loose.
  • The prop could be damaged. Check to see if it appears bent backwards a little. A friend's prop was slightly bent back and it substantially reduced thrust. Try replacing the prop and see if it makes a difference.
Lastly, to sort of test the thrust, hold the plane nose up and give it full throttle. It should have nearly enough power to support its own weight.

After the thrust, then I would start suspecting rudder/vertical stabilizer alignment. In the video it sort of looks like the tail kicks to the right before it rolls left. With the yaw/roll coupling on the Cub, a badly misaligned vertical stabilizer or rudder might cause that. The way to test that is to do a glide test. Just turn on the transmitter, plug in the plane, and throw it without giving it any throttle. It should glide, slightly nose down, without rolling. If it still rolls, then it's definitely an alignment issue. If it glides properly, then I'm guessing that you're just having a bad stall due to low thrust and too-high angle of attack.

Good luck! Post more videos!
 

Tench745

Master member
After watching that video a number of times (it's short so hard to get a real picture of what's happening) it looks to me like you 1) launch at too high angle of attack and 2) the plane seems to slow almost to a stop immediately on leaving the thrower's hand.

#1 can be a problem even when the plane is set up correctly. You want to give a good firm toss with the wing angled just slightly above horizontal.

#2 makes me wonder if the prop is on backwards, or the motor is spinning the wrong direction for the prop being used. If I'm understanding correctly, you say it will lift about 10cm from the ground with a slight push which seems to indicate that the prop is at least generating thrust, so it's turning the correct direction. But it should pull pretty hard, like Foamforce mentions. So I would definitely double check that your prop is on the correct direction (numbers facing forward.)
 

Foamforce

Well-known member
Good news and bad news, I have calibrated the ESC and tested it in the sports hall and it did this

That was just lifting up too soon and stalling. You need a lot more speed and pull up more gradually. You absolutely should not have to push it like that. You have a thrust problem. Try the other steps outlined above.