I have a smaller aircraft of size T40 hobbiest plane. I need a little details about the actuators to be used for control surfaces may be linear or rotary.

I have no idea of its selection and installation as I want to increase the size of this plane. I know this is too much general but give me what you have in this regard.

From : Don Stackhouse

Nearly all servos today are rotary. The amount of rotation either side of neutral varies, but typically they are 45 degrees, with some going 60 degrees each side of neutral.

Calculating servo torque is tricky. There are a lot of factors involved. Most folks just use past experience, rules of thumb (like "for a model this size I usually use this type of servo"), or if they don't have that experience, they ask a friend who does. Most of the time the experience and rules of thumb methods work OK, but sometimes they don't.

There are a number of calculators available for helping determine the aerodynamic torque generated by a control surface. Most of them use simplified methods and assumptions for determining the air pressures on the control surface, so their predictions may not be entirely reliable, either. Short of doing some CFD (Computational Fluid Dynamics) studies or wind tunnel tests, you're not going to get a 100% reliable prediction. Still, these calculators will give you a better answer than the old "That looks about right" ("TLAR") method.

One fairly simple servo torque calculator, written by Chuck Gadd, can be found on-line at:

http://www.csd.net/~cgadd/eflight/calcs_servo.htm

Of course on top of its predictions, you also need to add some safety factor for uncertainties in your numbers, as well as an allowance for friction in the linkages and hinges, G-forces in the linkages during things like touchdown on landing, etc..

Since you do have an equivalent application of a smaller size that appears to be working OK, you can use that as a basis for calculating the required servos for your larger model. Take the numbers for the control surfaces on your trainer and plug them into the calculator. It will give you its prediction for what your trainer requires. Now, plug in the numbers for your scaled-up model to get a prediction for its servo torque. Divide that number by the first number to get a ratio of how many times bigger your servos need to be for the bigger model. For example, if the calculator came up with 30 inch-ounces torque for the trainer, and 60 inch ounces for the scaled up model, then 60/30 = 2, so you should use a servo with twice the torque. If your trainer actually uses 42 inch ounce servos, then you should look for servos for your bigger model that have at least 84 inch-ounces of torque.

Note, those numbers are just made up ones for this example. You will need to figure out what actual numbers apply to the models you are working on.

The other thing to consider on really large models is reliability. Really big models often have such high loads that there aren't any really practical servos readily available of that size. Also, the idea that a single servo failure could result in loss of control of a 50 pound model isn't too encouraging either. For these reasons, "giant scale" model builders often use multiple servos on each control, either by combining the servos through a lever mechanism, or by dividing the control surfaces into sections (such as the left and right sides of an elevator) and using one servo on each section. That way, of one servo dies, you still have some control available through the other one.

Don Stackhouse
DJ Aerotech