Unequal Throw on Basic 4ch TX/RX

[forkastronomer]

Hello,

I recently bought two used Joysway Cub J3's (old 4ch foam Cub) at a good price. I am not an experienced fixed-wing pilot but I have played around with drones quite a bit so I know my way around the electronics.
The problem I am having is that with one of the tx/rx pairs, channel 1(mode 2) has unequal throw at the servo (not at the control surface). I have already verified this by changing the servo being controlled by ch1 and each time, I find far more cw rotation than ccw. I thought it might be something built into the transmitter, but when I tried the other tx/rx, all of the throws are equal as I imagined they should be. The transmitters (Joysway J4C01) are very basic 4ch with trim adjustment, no sub-trim or endpoint adjustment or anything else that would require a readout of some sort.
Does anyone have any ideas about what could cause this and how to fix it?

Thanks in advance,
fork

 

[Liam B]

Maybe the gimbal itself is being limited in how far it can travel? I'd say open it up and take a look if it's hitting anything.

 

[forkastronomer]

thanks for the response, I’ve already opened it up. That’s how I know its happening at the servo instead of at the control surface. No contact

 

[Indy durtdigger]

I had the same problem occur in a BNF Corsair earlier this year with both channels 1 and 3. My problem ended up being something hinky with the RX. After much cussing and discussing I swapped out the RX and everything worked just fine.

 

[Merv]

With ailerons, you want the up aileron to go up more than the down aileron goes down. If it’s the other way around, it can cause adverse yaw. You want more drag on the up aileron to help pull the around the turn.

 

[Tench745]

Do you know if your transmitter has a way of doing differential? It sounds like that's what is happening here, but if your radio has no provision for it, I don't know what could be causing it.

 

[forkastronomer]

With ailerons, you want the up aileron to go up more than the down aileron goes down. If it’s the other way around, it can cause adverse yaw. You want more drag on the up aileron to help pull the around the turn.

I get your point, but the ailerons are controlled by the same servo so in this instance, it would just bias the plane to roll in only one direction...

 

[forkastronomer]

Do you know if your transmitter has a way of doing differential? It sounds like that's what is happening here, but if your radio has no provision for it, I don't know what could be causing it.

It doesn't its a very basic 4ch. Only has trim adjustment and adjusting trim has no effect other than changing the center point (as it should)

 

[forkastronomer]

After talking with a friend who is a bit more experienced than I, we have two theories. Either the potentiometer in the tx is bad, or there is a problem with the rx itself. He told me that its more likely the potentiometer... any thoughts on that?

 

[Tench745]

After talking with a friend who is a bit more experienced than I, we have two theories. Either the potentiometer in the tx is bad, or there is a problem with the rx itself. He told me that its more likely the potentiometer... any thoughts on that?

It’s possible. You could spray some electrical contact cleaner into the pot and see if it improves things.

 

[FrankFly]

With ailerons, you want the up aileron to go up more than the down aileron goes down. If it’s the other way around, it can cause adverse yaw. You want more drag on the up aileron to help pull the around the turn.

Just to be sure I understand, Merv.

The aileron that moves up, causes that wing to go down. So you want more movement up, which will pull the downward wing "back" and "drag" your nose around the turn.

Makes sense.

 

[Merv]

Just to be sure I understand, Merv.

The aileron that moves up, causes that wing to go down. So you want more movement up, which will pull the downward wing "back" and "drag" your nose around the turn.

Makes sense.

Correct. You want the down wing, the inside wing, to have more drag than the outside wing. If the outside wing has more drag, you will have adverse yaw.