There needs to be a reason for each setting in your transmitter.
High aileron rate (on a 3D plane) is usually chosen for low speed maneuvers where the ailerons aren't very effective, such as harrier rolls, or holding stright in a hover. Personally, I use roll rate in a harrier roll to determine my aileron high rate, so I can hold full aileron control in a harrier roll and concentrate on the other controls. Low aileron rate is typically set for all higher-speed rolling maneuvers, and a lot of competent pilots use a setting that gives about 1 roll per second, which is a nce, manageable rate for precision maneuvers.
High elevator rate is really important. On 3D planes, we typically want at *least* 45 degeees of elevator throw on high rates, so that we have enough throw to actually stall both wing halves on command and fly 3d maneuvers. If the high-rate elevator setting on a 3D plane is set at some lower value, 25-35 degrees, many 3D planes won't 3D, but will instead snap and spiral out of maneuvers. You also have to pull the stick like you mean it, to ensure that you actually get the 3D stall that you want. A weak or slow pull can also cause the spiraling or snapping out on some aircraft.
Low elevator rate is also critical. We typically want a low value, like 15 degrees, that is totally safe at all speeds, and a hard pull results ONLY in a partial loop, never a snap roll. We also expect to land on low elevator rate, and it need to be low enough that the pilot does not overcontrol and cause the airplane to porpoise up and down during final approach. The lesson to be taken here is "low rate usually needs to be lower".
Rudder can't be ignored either. High rate rudder should be enough to allow the rotation we want for flat spins, stall turns, blenders, etc. However, many airplanes develop a lot of bad habits in that last several degrees of rudder throw before the rudder hits the elevator half. If you ever have an airplane that develops too much coupling in high-alpha knife edge flight, try reducing the rudder throw a bit on high rates. You might find that you don't give up any noticeable KE performance, but a lot of your coupling goes away.
Low rate rudder may only be for landing or cruising around, or a pilot may choose it very precisely for point-roll maneuvers. Personal preference.
First off, make sure your expo is going the right direction. We want the kind of expo that makes our controls LESS sensitive around the center stick position. Most transmitters that have the EXPO function also allow you to do it the other direction (have you ever actually seen anyone use expo the other way? I haven't, but the radio wil let you do it).
Expo basically has two purposes:
1.To make a high rate control more flyable without the danger of over overcontrolling.
2.To make low rates and high rates feel more similar around the center-stick position.
High-Rate ExpoExpo is a very personal thing. Some people tend to overcontrol on high rates, and therefore tend to use a higher expo value (60, 70, or 80%) other people tend to undercontrol on high rates and use less (20, 30, or 40%). The first time you flip a new airplane onto high rates, you'll know if you have enough expo or not. If you find it difficult to keep from overcontrolling the airplane on high rates, no matter how small a stick movement you make, you can bet you don't have enough expo.
Keep it straight in your mind which control you're working on. If you find the controls feel sluggish around center on high rates and you feel sort of disconnected from the airplane on high rates, you probably have too much expo on those controls.
As long as you have a low enough low rate set before you fly, you can flip to high rates, see how it feels, and flip back if it's not right. Low rate is your safety position. For this reason, many pilots prefer to program their radio to have all 3 rates (aileron, elevator, and rudder) controlled on one switch. Makes it easy to get back to the safety position in the air.
Low-Rate Expo: I set my low-rate expo values at an arbitrary value to start (for me, 25%) and tune them after I have set my high rate expos. I tune them so that, at low stick movements, I can flip the rate switch back and forth and the surfaces don't change position much. I want the controls to feel about the same near stick center on both low and high rates. Then, as I move the stick to the end of its travel, the rates feel very different from each other. For me, this keeps the amount of switch-flipping to a minimum during flight. This is a personal preference thing, and other pilots have different purposes for low-rate expo.
Specific Expo Settings:
There are a couple of expo values that, if set correctly, can be really helpful:
For hovering, if you are having problems, take note of how the airplane falls out of hover. If the airplane tends to slowly fall to the right or left, and you can't seem to get it back in time, try reducing your ridder hi-rate expo.
If, instead, the airplane is always dancing back and forth to the right and left too quickly and is never stable, try increasing your rudder hi-rate expo.
For harrier, take note of two things - If you have a very high expo setting (75%+) on your elevator hi-rate, then your elevator is very gentle around the center stick position, but is very sensitive at the end of the stick travel. It may be too sensitive in maneuvers where we're flying with a lot of stick but still need to be precise, like the harrier. If the plane jumps around in pitch while flying the harrier, and in general seems too sensitive, be sure to try the Expo both more AND less before deciding which is better.
Rudder is very important in harrier. When you turn in harrier, it's important to be very gentle on the rudder, or you will cause the plane to very unstable in harrier. One of the leading causes of unstable harrier performance is a too-sensitive rudder control which causes the pilot to overcontrol the rudder. If your plane isn't stable in harrier, try more hi-rate rudder expo, so that you can add just the tiniest bit of rudder and be smooth and gentle on the sticks...this will really improve your harriers.
The only way to have perfect transmitter settings for your plane is to fly it a lot. As you fly, you will learn the aircraft and get more comfortable with it, and you will improve your settings based upon your experience.