Pun Jet, uncontrollable rolling

[Laser411]

So I built the Pun jet, admittedly with cardboard which all seemed fine. The issue is that when I throttle it up (Even while just holding it), the whole plane wants to roll the opposite direction of the motor torque. I tried flying it anyway, to see if maybe it was a behavior that didn't happen in the air, but it did. Even with full roll in the opposite direction, it still rolled just much slower.

This is clearly the motor torque and/or air friction from the props that is forcing the plane to roll, but how is it the design flyable like that? I would like to build it again, but I would like to figure out how to fix the issue.

 

[PsyBorg]

If you copied the plans from FT exactly I would take a guess that your motor is way too big or you are over propped as the originals seem to fly pretty good.

Pictures of the build and the electronics specs would better help people figure out what is going on to help you.

 

[Laser411]

I was using raycorp 5045 3 blade props with a 2280kv motor on a 3s lipo. The build has been destroyed unfortunately so pictures won't help.

The motor and battery is the exact rating they have on the plans, the prop is a little smaller but has 3 blades. The power pack that is recommended has a 2280kv motor and a 6x3 2 blade prop.

I felt it wanting to rotate while holding the plane and throttling up, it was pretty significant, so i had a pretty good idea it was going to spin when launched but tried anyway. How does this (and other) single motor models deal with the rotational torque from the motor?

I know with quads and dual propped models, the other motors spin opposite directions, and changing the speeds of opposing motors allows quads to yaw. So I'd imagine these forces have to be dealt with somehow, am i missing something?

 

[JasonK]

the pun jet recommends a 5x4 (which I do recall a few of the minis ask for a smaller prop then what comes in the A pack). The other thing, with it's size, if you blip the throttle when the plane doesn't have any speed, it is going to induce quit a bit of torque and not have the airflow to counter it.

Where your throws setup the same as in the build video? IIRC, they said it had some very extreme response/controls.

 

[Laser411]

Maybe my throttle was too high but I don't know how the airflow would counteract this much torque. It quite literally spun about 8 or 9 times before hitting the ground.

My throws were about what it was in the video but i never made it to the point i could really use the controls. It always wanted to rotate left, if i held the roll all the way right, it would roll left about 2 times before hitting the ground.

In general, is cardboard a suitable replacement for foam? It came together nicely, had rigidity, and the weight didn't seem too much but i don't know how it compares to foam board.

 

[Merv]

In general, is cardboard a suitable replacement for foam?

Yes, cardboard makes a good plane, foam board is better, but cardboard is good. Here is a FT video of a cardboard plane.

 

[Tench745]

As far as the prop is concerned, you added a blade and upped the pitch from what was recommended. Both of those things increase the prop's resistance to turning. If you have a similarly sized 2-blade, try that and see what happens.

 

[Laser411]

I will make another and report back when i get the chance

 

[Paracodespoder]

Is this your first plane?

 

[Laser411]

First home built plane, yes.
First plane in general, no.
I have 3 other planes and 3 quadcopters

 

[Paracodespoder]

what would you say your skill level is? The pun jet is known for being a rascally plane that requires a skilled flier to tame. Could also be you need to put some thrust angle into it.

 

[Laser411]

With overhead wing designs like a cub, i would say high intermediate and just shy of expert.

For Jet designs, high beginner, low intermediate.

I do not believe this is a piloting error though.

Here is the scenario:

Throttle 50%
Roll axis to the right (left aileron down, right aileron up)
Plane rolls left about 2x in about 3 seconds immediately after hand launch before hitting the ground

Throttle 50%
Sticks neutral
Plane rolls left about 8x in about 2 seconds immediately after hand launch before hitting the ground

Throttle 100%
Roll axis to the right (left aileron down, right aileron up)
Plane rolls left about 4x in about 3 seconds immediately after hand launch before hitting the ground

There is no scenario in which the plane doesn't roll left unless i have the throttle off when i throw it.

I think Tench might be right with the propeller, but I am generally having an issue trying to understand the physics of why it would be different with a larger, less pitched, 2 bladed prop. To get the same thrust, i would have to spin that one faster no? Wouldn't that have the same rotational torque effect?

 

[JasonK]

well

• your going to get less thrust with less props / lower pitch / smaller blades
• rotational inertia is based on the amount of mass and the distribution of that mass
• given that we can't see your plane, we can't tell if there could have been other contributing factors.
• when people are flying 3D, torque rolls go much faster one way then the other (because of the imparted torque from the prop back on to the main air-frame)
• stabilizing surfaces are more effective, the more air going across them.. the PUN Jet is a pusher, so the only airflow you get is from forward velocity

also, how are you throwing it? how fast is it going when it leaves your hand?

 

[Laser411]

I didn't really think this through, as i took pictures before it was flown.

 

[JasonK]

you have your prop on backwards. the numbers always face in the direction your plane is going.

 

[Laser411]

You know, i debated that with myself for like 15 minutes. Obviously flipping it would have reversed the thrust but putting on an identical prop but one designed for the opposite rotation worked. I had assumed it wouldn't have mattered.

I just tested it on the little that is left of the plane and sure enough, no rotation issues. It was still uncontrollable because it has no nose section and is bent pretty good but it was clear that the issue was resolved as i made it about 30 feet without crashing and it was ALMOST controllable.

Why would the rotational torque be so bad with a backwards prop?

 

[JasonK]

backwards prop had more drag and less thrust.

 

[Laser411]

Ok, that clicks pretty well with me. Thank you for your help, I think this plane will do just nicely once built again. It sure was quick too.

 

[leaded50]

look at the outer ends on the prop.. it shows its wrong way, and if checked the blade profile, you will find its now thinnest forward,..not opposit as should be... remember how a wingprofile is... gets thicker the first 1/3 approx, then thinner.

 

[Merv]

The backwards prop could have caused the problem with the first plane. More drag & less thrust.