a simple servo question

this is a very simple question about servos.

what is the angular movement of a ‘arm’ type servo, none of the manufacturer spec sheets seem to give this out.

is it the same movement for all servos?



Hi Robert

the Hitec produce 2 powerfull analog servo arm :

HS-805 BB 90° rotation 24.7 kgcm 152g
HS-815 BB 140° rotation 24.7 kgcm 152g

Obviously as you knows, there is no need to go above 140°

Futaba : less powerfull for small boats

S 3801 125° rotation 14 kgcm 107g
S 3802 140° rotation 11 kgcm 73g

The digital servos can be programmed.


Try this link to Servo City. Click on any of the servos, and get the specs, including rotation, speed, torque etc.



While we are on the topic of servos - being an old “crystal” sailor, I started looking at the Spectrum radios from Hitec.

  1. Seems “surface” generally means a “pistol” style of transmitter, while “air” is a two stick like I am used to.

  2. Being able to adjust the servo “end points” is of interest - but do the servos have to be digital? If not, what spec am I looking for on the transmitters? I’m considering Hitec instead of Futaba, but am lost between the DX5e, the DX6, DX6i or the DX7 other than the number of channels which are rather unimportant given most r/c sailing is rather limited to two channels for most (not all) classes. To get servo endpoint adjustment, which model (or spec ability) am I looking for?

  3. I understand the Mode 1 vs Mode 2, but wouldn’t this change the stick functions anyway, if I plug the servo cables into opposite receiver plugs?

I’m finally tired of looking at frequency boards, only to find by the time I go back, grab crystals, and return to the water, some else has grabbed what I thought would be open. Thus the 2.4 ghz is of interest. Any comments or opinions appreciated. Thanks in advance.

Hi Dick,

Thre is no restriction in using the 2.4gig ‘air’ radios for surface use (just refers to the packaging).

We are using the 2.4gig radios extensively in our local IOM fleet (about 20 boats). They work great. . .no frequency conflicts.

We have a mix of mostly Futaba 6EX and Spektrum 6i.

The Spektrum 6 is designed for park flyers and has very limited range.

The DX5 is a simplified system with no programming capability.

The DX7 is a repackaged and updated DX6i with the second ‘slave’ Rx module in a separate box. . .no big deal.

I would recommend the Futaba or Spektrum DX6i. They work quite differently, but as a user you don’t need the details. The DX6i Tx uses drycell batteries. The Futaba uses a 9.6v pack.

Play with the programming on each and choose the one that suits you.

These are air-type Tx with two sticks.

You can use them with existing analog or digital servos. There is no need to program the servo. Just use the TX programming to set the limits and fine tune using the trims. (so long as the servo has the overall travel that you need - eg if you need a 180 degree movement for an arm servo, thw servo needs to be configured to do that)

They offer such features as ‘dual rate’ and exponential. These are useful for controling the rudder or even giving a fine tune on the winch.

I have used my Futaba with the RMG280 as well as the Hitec 785 drum winch and the latest high torque digital servos (used as an arm winch), and both digital and analog rudder servos. No problems at all.

There is a code change needed on the older RMG to set it up using the DX6i (which takes a bit longer to lock on to the Tx and was exceeding the timeout value on the RMG).

The little Rx ariels (2) can be inside the boat unless you have a carbon fibre hull.