Post by cuban8 on Nov 28, 2016 20:35:45 GMT
From a model flying forum............The original thread
Expo is a wonderful tool but is very often misused and the way it operates is misunderstood.
In general, it is better to set up the model with no expo and suitable rates for a test flight. I tend to ignore instructions these days, but if the instructions have a high and low recommendation I take the low setting, then set my rates 10% or so below that for low rates and 10% above it for high. This guarantees me adequate control without it being excessive.
Once in the air I make up my mind if I need more or less rates before going any further.
Elevator rates are usually easy to work out. If its super pitch sensitive then take some movement out. This is especially true for warbirds where elevator is normally really sensitive.
Ailerons are also easy and irrespective of what model I am flying I set them by running in at full power, pointing the nose up 45 degrees and using full stick travel do a roll. I then reduce or raise my rate to get to my desired rate of roll. In the case of a warbird, this is say 10-20% faster rate of roll than scale. Now that is set, if I find the model is sensitive on ailerons at normal flying speed I add 10% expo and bosh. All sorted.
Where I have seen people go wrong is they have a really sensitive model, with excessive rates and then slap 20 or 30% expo on to calm it down, sometimes more. They then fly around using only 20% of their stick travel. In this instance, they would be better off cutting their rate at least in half and ditching the expo. By using more stick travel, you automatically make the input smoother as a larger deflection of the stick results in a smaller deflection of the servo.
On most of my models I regularly end up at the full deflection of my sticks during a flight, especially on my importune who's ailerons are soggy to say the least.
The problem with using high expo is the sudden increase in control surface movement for a given stick input. Without expo you get gradually decreasing control deflection for a given input due to the arc our linkages follow as the servo moves. This can be quite handy as the effect on elevator would be to give you more delicate control when the stick is fully aft. This would be very helpful when holding off to land.
If you take the opposite, add some expo and then try the same. As you pull back, you get an ever increasing amount of elevator which often leads to a greater than intended pitch up. This is corrected but often results in greater than intended pitch down...and so the cycle continues. When it comes to flare, the rapid increase in elevator deflection can easily pitch the model beyond stall and everything ends up in a crumpled heap.
I do understand the comments about using it to prevent elevator/aileron interaction on mode 2, but I also dismiss it as I fly mode 2, and I did an experiment with a friend some time ago where I had my setup on one rate setting and his (20% expo) on another, he said he preferred his and asked me to reset my one to his. I did, but actually reset his to mine. He then flew waxing lyrically about how this was much better than on my setting....I smirked and only told him what I had done after he landed. He was quite surprised and now never uses expo on anything.
So long story short, it will only take a flight or 2 to get used to being more accurate around stick centre, and for Geoff with his rudder (and anyone else on any other surface) I would say to ask yourself 'do I ever need to use full stick deflection?' if the answer is no then reduce the rate first, don’t add expo right away. If the answer is 'yes I need full rudder for a stall turn' then great, add a little expo if it helps you on the ground.
I also tend to setup different rates for take-off/landing on some models to suit their behaviour at low speeds. Again, its not uncommon for me to touch down with full elevator applied, and even on a warbird I have no fear of a stall because of the setup I have used.
Expo is a wonderful tool but is very often misused and the way it operates is misunderstood.
In general, it is better to set up the model with no expo and suitable rates for a test flight. I tend to ignore instructions these days, but if the instructions have a high and low recommendation I take the low setting, then set my rates 10% or so below that for low rates and 10% above it for high. This guarantees me adequate control without it being excessive.
Once in the air I make up my mind if I need more or less rates before going any further.
Elevator rates are usually easy to work out. If its super pitch sensitive then take some movement out. This is especially true for warbirds where elevator is normally really sensitive.
Ailerons are also easy and irrespective of what model I am flying I set them by running in at full power, pointing the nose up 45 degrees and using full stick travel do a roll. I then reduce or raise my rate to get to my desired rate of roll. In the case of a warbird, this is say 10-20% faster rate of roll than scale. Now that is set, if I find the model is sensitive on ailerons at normal flying speed I add 10% expo and bosh. All sorted.
Where I have seen people go wrong is they have a really sensitive model, with excessive rates and then slap 20 or 30% expo on to calm it down, sometimes more. They then fly around using only 20% of their stick travel. In this instance, they would be better off cutting their rate at least in half and ditching the expo. By using more stick travel, you automatically make the input smoother as a larger deflection of the stick results in a smaller deflection of the servo.
On most of my models I regularly end up at the full deflection of my sticks during a flight, especially on my importune who's ailerons are soggy to say the least.
The problem with using high expo is the sudden increase in control surface movement for a given stick input. Without expo you get gradually decreasing control deflection for a given input due to the arc our linkages follow as the servo moves. This can be quite handy as the effect on elevator would be to give you more delicate control when the stick is fully aft. This would be very helpful when holding off to land.
If you take the opposite, add some expo and then try the same. As you pull back, you get an ever increasing amount of elevator which often leads to a greater than intended pitch up. This is corrected but often results in greater than intended pitch down...and so the cycle continues. When it comes to flare, the rapid increase in elevator deflection can easily pitch the model beyond stall and everything ends up in a crumpled heap.
I do understand the comments about using it to prevent elevator/aileron interaction on mode 2, but I also dismiss it as I fly mode 2, and I did an experiment with a friend some time ago where I had my setup on one rate setting and his (20% expo) on another, he said he preferred his and asked me to reset my one to his. I did, but actually reset his to mine. He then flew waxing lyrically about how this was much better than on my setting....I smirked and only told him what I had done after he landed. He was quite surprised and now never uses expo on anything.
So long story short, it will only take a flight or 2 to get used to being more accurate around stick centre, and for Geoff with his rudder (and anyone else on any other surface) I would say to ask yourself 'do I ever need to use full stick deflection?' if the answer is no then reduce the rate first, don’t add expo right away. If the answer is 'yes I need full rudder for a stall turn' then great, add a little expo if it helps you on the ground.
I also tend to setup different rates for take-off/landing on some models to suit their behaviour at low speeds. Again, its not uncommon for me to touch down with full elevator applied, and even on a warbird I have no fear of a stall because of the setup I have used.