A friend and I are having a discussion regarding servo speed. He claims that a faster servo will give better control. I say that it will just require more Expo to have the same controllability. We are talking about use in a 3D plane that already requires a good amount of Expo.
Phillip

Hey Phillip... I have to watch this thread.... I started thinking that also... On
my Showtime, I have the fastest servos that I have, And I guess they are slow at
18sec. And it could be the plane.... But I find I'm using more expo, and going to
low rates more often, where as I did not before..... Not sure if it's good or bad

Faster servos will give more instant response, which can be helpful when doing very fast point rolls, stopping snap rolls instantly, and other maneuvers where quick reaction time is important. For that purpose, a fast servo is important. However, using expo serves a different purpose, and having excellent-centering servos serves yet another purpose. For the best control, you need a fast, powerful servo that centers well, and using the right amount of expo for your taste.

My question is: what's the point of using a faster servo when you could achieve the same thing by reducing Expo?
Phillip

Phillip, expo only changes the amount of servo travel at different control stick positions NOT the speed at which the servo travels. A faster servo will more closely match (at the control surface) the RATE at which you move the sticks.

LuvBipes,
Thanks for the good explanation. Hypathetically, if a faster servo is installed, would the "felt" effect be the same as speeding up Expo. I am using the term "speeding up" because JR and Futaba have opposite definitions on what increase and decrease Expo are.
Phillip

I remember Chip Hyde posting .18 sec is enough speed for him, so that is more than enough for me

I don't think so. Picture this: you're on final landing approach flying on low rates, suddenly a hard crosswind gust hits the plane resulting in a violent bank to the right. You find yourself scrambling to level the plane and regain control and you have to do it NOW before the plane either stalls or a wing tip hits the ground.

Remember you are on low rates (as most of us are on landing), so having too much throw is not an issue. You bang the stick to the left to correct - whether or not you catch it in time will depend on your skill and reflexes but ALSO on the servo's response time. IMO is where a fast servo will save your bacon.

It's been my experience that fast servos (I run HS5925s on 6V and they are *fast*) make a plane feel much more resonsive and "tighter" than slower servos.

Even when hovering, fast servos allow you to use large inputs of short duration -- which in some aircraft, make for more stable hovering.

Same goes for high-alpha rolling harriers, etc -- if your servos are slow it can be hard to get enough rudder/elevator deflection at the right time )with a small model) to allow sufficient control to do rolling horizontal figure-eights for example.

There have been more than a few occasions also, when having very fast servos have saved my plane (phew!).

However, for your average sport flying (as opposed to 3D) where control inputs are usually quite small, even a slow servo (0.23) will keep up with most people's thumbs.

Flying 3D though, often requires quite large and rapid inputs -- that's where fast servos shine.