I'm constantly asked about what servos to use in aircraft and helicopters.
The majority of models flying in this country are suffering from the choice of servo used.

You cannot over power a control surface.

Models that feel spongy, lacking a crisp sharp feeling are suffering from control blow back.

This can be down to the linkage used, but is probably the servo struggling to hold the surface in the position your control input is requesting.
The control systems on the latest 3-D helicopters and aircraft are using longer servo arms, which reduces the torque available from the servo you have chosen.

A 6.0kg servo with a 20mm servo arm is reducing the torque to approx 3.0kg as all figures are quoted at 1cm (10mm) radius.
JR 591 servos are a budget servo offering 5.0kg, using them in any accurate flying sport or pattern model is a complete no no. They are sloppy and have poor resolution.

I'm using some in my Graupner ME108 and they work ok as it is a model that is flown in a scale manner-the full size 108 would not be crisp in it's control feel.

Any sport model with a 40 or larger motor really deserves a decent servo set-up. it pays long term.

40-50 Sports aerobatic model like the new Great Planes Venus 40.

Ailerons JR811 4.0kg digitals
Elevator minimum JR 811-ideal JR8231 6.0kg digital.
Rudder minimum JR8231 digital, better still with JR 8411 11.0kg digital!

Fun Fly type models with large cord ailerons use JR 8231 on Ailerons as well.

60-90 Pattern like Flair Swallow.

Ailerons JR 8231 digitals
Elevator min JR8231 6.0kg ideal JR 8411 11.0kg digital.
Rudder JR 8411 11.0kg digital.

30 size helicopter (Raptor 30)

Collective Pitch JR 8231 6.0kg Digital
Cyclics Minimum Jr811 4.0kg Digitals-Ideal JR 8231.
Tail Servo Minimum JR 810G-ideal 8700G

60 size CCPM Fury 60-90.

3 x CCPM servos Minimum JR 8231 3 x 6.0kg ideal JR8301 0.19 6.5kg digitals
Throttle JR 8231 (good resolution)
Tail JR 8700G with JR 5000T gyro.

email any requests for servo set-ups.