Servo functionality questions:

[CookieMan]

Hello mates,

Mates, forgive me if these questions are answered elsewhere. If they are, please do show me the way there...

I WANT to learn about how these 'servos' are judged and on what basis they are judged to suit the need of the Helicopters, we fly.

My questions:

1) What is a 'digital' servo and what is a 'high-voltage' servo?
2) What does the following information mean, in terms of servo performance:

JR DS8717 Digital Cyclic Servo SPECIFICATIONS:
Torque - 160 oz/in @ 4.8V
Torque - 196 oz/in @ 6.0V
Speed - 0.09 sec/60Deg @ 4.8V
Speed - 0.07 sec/60Deg @ 6.0V
Wieght - 69g

Futaba S3157 SPECS:
Torque: 21 oz-in (1.5 kg-cm) @ 4.8V
24 oz-in (1.7 kg-cm) @ 6.0V
Speed: .10 sec/60° @ 4.8V
.09 sec/60° @ 6.0V
Size: .9 x .62 x .9" (21.8 x 15.7 x 23.2mm)
Weight: .3 oz (8.3g).

On what basis, from above information, can you judge if the servos can be used on different choppers?

Is there like a thumb rule for understanding the functionality of the servos?

Also, on a slightly different topic,
What is the "drive Gear" on a chopper kit and what does the following Drive gear ratio:1:13.5:4.68/1:12.46:4.68 mean?


Thanks a lot folks, in advance...

Cheers boys (and girls),

[pchristy]

Much of the terminology used with RC equipment is very misleading! In one sense ALL our servos are digital, since the motor control is either "On" or "Off" - and nothing in between. In an analog servo, the motor is fed a varying voltage depending on the movement required. These haven't been used in model control since the late 60's!

However, the term "digital" has been corrupted to describe a new type of servo which uses the same kind of technology used in motor speed controllers to drive the motor. Basically the motor is driven at a fairly high frequency - typically in the low KHz range, rather than at the frame rate of the transmitter - which is typically about 50-60 Hz, as with ordinary servos. This - combined with some other technology advances - provides a much more powerful and efficient servo - at a price!

Most servos are designed to run on a 4-cell Nicad or NiMH battery, which will have an on-load output of 4.8 volts. Some may be driven by a 5-cell battery which will give 6 volts. Some newer servos are designed to be driven directly by a 2-cell LiPo battery, which will have an output of around 7.2 volts - maybe a bit higher. These are "high-voltage" servos and usually command a premium price!

DO NOT try and run 4/5 cell servos directly off a LiPo! They won't last long! You will need a voltage regulator of some kind to reduce the voltage to something the servos can take, if you want to use LiPos for the radio gear.

Torque: This the the amount of twisting power - or pull - that the servos can deliver. Notice that it rises with voltage.

Speed: How long the servo takes to move by a given angle. Usually the faster the servo, the lower the torque, and vice versa - unless you spend more money!

Weight: Self explanatory!

In very general terms:

For a tail-rotor servo, you need very high speed, but not much torque. If the gyro has a high frame rate option, you will need a "digital" servo to take advantage of it.

For cyclic servos you need medium speed and medium torque.

For collective servos, you need high torque.

If you use electronic collective/cyclic mixing (as on most modern machines) you need three closely matched (identical) medium speed, medium torque servos.

Of course a lot depends on your flying style! Full on 3D or F3C (competition aerobatics) is much more demanding on servos than training or scale flying. The bigger and heavier the machine, the more power will be required of the servos.

This is a very rough and general description, but should give you the idea!

--
Pete

[CookieMan]

Thanks a bunch Pete mate. I did understand what you tried to explain... Just made life a little more 'smoother'

Cheers mate..

[CookieMan]

Oh and any input on the 'drive gear' stuff, guys?

Cheers guys,

[pchristy]

Oh and any input on the 'drive gear' stuff, guys?

Cheers guys,

Its basically the ratio between the pinion on the engine shaft and the big gear at the bottom of the mainshaft - assuming a single stage reduction. A few machines use a two stage reduction.

Most 60 size machines will have a ratio somewhere in the region of 9.8 to 1 ie a 98 tooth main gear and a 10 tooth pinion. 90 size machines generally use an 11 tooth pinion, as they develop their power at slightly lower RPM.

You will often see this expressed as 9.8:1 or 98:10 or maybe 98:11.

Sometimes a third figure is added, and this is the ratio to the tail rotor, ie 11:98:xx, where 11 is the motor pinion, 98 is the main gear and xx is whatever number of teeth the tail drive has. The tail rotor gearbox itself is generally (but not always) 1:1.

A higher speed tail will be more responsive, but will also absorb more power that could be going into the main rotors. If its driven during the autos, it will also eat into the auto reserve.

As with everything else, these gear ratios are a compromise for whatever style of flying you adopt!

--
Pete

[CookieMan]

Thanks mate... Never thought those (ratio) numbers had ever somuch meaning to it! Well thanks for making my life a little less complicated mate.. I learned something new..

Cheers mate!