Expo makes the controls less sensative around the nutral point....that is to say that a little movement on the stick will have less reaction from the plane or heli. Mostly useful for the beginner pilots who have a tendency to over control when learning. The nice think about the computer radios now is that you can experiment and find the settings that feel right for you. I used a lot of expo when I was learning to fly my helis. Not so much when I fly planes.
Helicopters don't really fly.......
They're just so ugly, that the earth repels them.
Too high of expo and you get a slow feeling response, where you have no much authority until the end. This can be bad, as full throws will happen more suddenly at closer to the end of stick movements, although not to the degree where there is no control until end stick movements. Typically you'll likely want some expo, although maybe not on slower planes/trainers. It helps smooth out flight of the plane, and allows easier control. You want max of 25-30% expo, or that is from what I have read. On planes I usually throw in 15-20%, and helis I use 0%
Expo also helps with scale flight, such as warbirds, allowing a smoother basic flight without the sacrifice of throw. On my UMX Pitts I have 20%, which allows simple flight despite the fact that it is set up with max throws. On full 3d planes, higher expo is often needed, especially when getting used to it, as it doesn't take much movement from surfaces to move the plane. On all my helis, I find no expo easier as it takes constant stick inputs to control and the less stick movement you have to give the quicker you can be. The guy at my LHS, who is a great heli flier, uses negative expo to allow quick movements closer to center stick. For planes, any amount of expo won't likely cause flight issues, just cause the control to be sluggish until more stick inputs. Negative expo could cause you to crash though, so you have to make sure in your settings that you aren't accidentally adding negative.
a) To lessen control at or near centre stick and make flight more stable but allowing sticks to move ... this becomes increasingly significant as the control throws increase for aerobatics etc.
b) To lessen interaction of stick movement across controls. We are human and not perfect, so when we give for example elevator command, on Mode 2 Tx's - it is common for a small amount of aileron to be given also as we do not move stick absolutely straight. The expo will lessen the effect this interaction has.
I have models with little or no Expo ... and others that have 60 - 70% ... as well as many with %'s in between. I usually start with 30% and then adjust after flight testing.
One of the areas that is really useful and I think many don't bother ... is on the rudder. This greatly helps on the ground when trying to keep a straight run ... often the steering nose or tail is over sensitive and we all get into the zig-zag routines ! Add Expo and see it reduced.
What are the pros and cons on using expo on TX settings
Several years ago, I had 35% expo on the maiden flight of a 3000 watt giant scale model airplane. That expo nearly cost the model on its first flight.
What happened? Quickly found out that the model required a lot of up elevator to maintain level flight. That was after running the elevator trim to full up. This placed the elevator command in the full expo region, where a slight elevator command resulted in severe changes in the flight of the model.
Getting the model back on the ground in one piece was a real problem. After re-adjusting the servo pushrods on the elevator, and getting the model trimmed out, the model flew very well.
After that episode, all of my maiden flights were done with zero expo, until the model was trimmed out.
Retired and the days are just too short, busier than ever!
Very good point. I've had a similar experience on two occasions. The first was with the Mini Switch, where it was trimmed...but the winter was getting colder and colder, bringing about physical changes to the plane. Inside it was good, and once I walked 10 minutes (more like 15 trudging through snow) in -15 to -25 C temps, the Mini Switch was no longer good to fly. I took it up and it wanted to go vertical. I believe there was 30% expo on the plane, so it wasn't easy to make it fly level, just bobbing constantly. Landing was not easy either, as it didn't want to, and with a plane wanting to just go up, stalling is going to happen at a lower airspeed. My second was actually recently, with the UMX Habu. Same issue of wanting to go up, but the issue was that the Switch, after I changed the motor, was able to go vertical, yet the Habu has practically zero thrust. With that scenario, it is easy to stall the Habu as it can't gain any speed going directly against gravity, or close to it.
I would say a con would be, expo can be confusing on different radios spectrum is one way and futaba is another. I still am confused when someone says "I have 80 percent expo dialed into the plane" In fact I am not sure I explained the last statement correctly. Also as said before its a personal choice on how a person flies so I'm not sure there really is a pro or a con.
Happy flying may your crashes be limited and if they are not limited let them be cool.
Whats even worse is when expo can be changed per direction and throw can as well. On the 9xr, if you are, for example, changing rudder throw, right rudder goes to right settings and left to left settings. I didn't realize this at first and was wondering why after playing with settings, right rudder was giving me a lot more throw and was snappier than left. Oddly, elevator and aileron set expo and throw both directions together, unless aileron stick is moved. The rudder always points right on the screen. Maybe I accidentally tap the stick during setup...it is annoying however. If a radio does this, make sure throws and expo are double checked, because it could end bad if one is not ready, especially on a stunt plane.