Hi Friends,

Just wanted to know if there is any problem in connecting different rated servos at the ends of Y-connector ?
Rudder servo unable to bear load of nose-wheel steering, therefore contemplating using a separate servo for nose wheel thru Y- connector. (Not going for more power servo for rudder as installation problems are there).

Please advise. Thanks

Quote from: Aeroresurrect on December 22, 2020, 06:52:30 PM
Hi Friends,

Just wanted to know if there is any problem in connecting different rated servos at the ends of Y-connector ?
Rudder servo unable to bear load of nose-wheel steering, therefore contemplating using a separate servo for nose wheel thru Y- connector. (Not going for more power servo for rudder as installation problems are there).

Please advise. Thanks

Initial, off the cuff, response.
The current draw from the Rx will be the sum of the current drawn by each servo, irrespective of whether they are equal or not. If the Rx can cope, different servos on Y lead should not be a problem.

NEEDS MORE THOUGHT THOUGH...

Adding on to what Iyer sir has said, make sure the BEC on your ESC can fulfill your aircrafts entire power requirements!

If you are using a generic non branded ESC there is a chance the BEC might fail on you!

Consider this scenario:
You a have 4ch aircraft with fixed landing gear having 4 analog servos (9g) 2 for the aileron, 1 for the elevator and 1 for the rudder. In this configuration the load on your ESC BEC is normal and nothing to worry about! Most generic ESCs can handle this!

But now take this same plane and add steerable landing gear, you now have a slight increase in aircraft weight and also need a servo powerful enough to steer this aircraft in various runway conditions i.e road, grass or mud so this servo would need to be different let's say a 12g servo which has a different current draw compared to the four 9g servos already used in the aircraft, So now your BEC has to power up all these 5 Servos ! So each time you give input to the rudder servo you are also giving input to the nose steering servo since they are Y connected.Most generic ESC will struggle to provide stable power output to the RX in this case which leads to the RX shutting down for a split second (brown out).

We've so far considered the servos to be analog, so if you use digital servos then your current consumption is gonna be higher.

So the point of this entire scenario was to make sure your RX can handle the current requirements of the servos (as pointed by Iyer sir) and that your ESC BEC can supply the required stable output.

Regards,
Mohammed Naveed

Thank you, Iyer Sir and Naveed for your helpful responses.

First, Iyer Sir:
My initial (off-the-cuff) conclusion somewhat coincides with your response. But since I propose to use a heavier (meaning larger current) servo and a lighter servo at a Y-extension, a possibility is there that the lighter servo might get power-starved because of the heavier servo. The heavier for nose-wheel steer while the lighter for rudder. I have got to check the situation extensively on ground trials etc. The argument going in favour is that once aircraft is off the ground, the nose wheel load will lighten and may not pose a problem for rudder functioning in flight.

Now for Mr Naveed :
The aircraft is nitro powered and already has 5 servos . The intended 6th servo is for the nose gear. Therefore, the constraints here are the receiver battery and the current handling capacity of the receiver. The 5 servos are 17g while for the nose wheel, I intend to add 25g. Battery is 2800 mAH, which I guess is sufficient. Now in case the receiver is unable to handle the enhanced current, I propose to feed power to the nose wheel servo separately. I saw a schematic used for large sized gassers.
Ultimately, as I understand, whether inside the receiver or outside it, the power bus is common and all servos are in parallel connection.
If any other viable alternative is there, I'd sure give it a thought.....

Thank you once again, and my regards  to you both!

@aeroresurrect,

Further inputs:

1. I have 2.5kg, 40 powered model with 4 Futaba standard size servos with nosewheel connected to the rudder servo. Never faced any current overdraw issues. It has a 5 LED voltage display on the side of the fuselage. With the 1300mah NiMH fully charged, all 5 LEDs are lit.
A few years ago I noticed that if 2 or more controls are moved at the same time, the first and second LEDs go out. So I retired the battery, which was 10 years old.
Then for a while, I used a 2s lipo via an ESC that had a 3A BEC. Till one day I merely turned the switch off, and forgot to physically disconnect the lipo. The lipo died overnight presumably due to the current drawn by the ESC, even when everything downstream was switched off.
Finally got another 1300mah NiMH.
In a nutshell, I doubt if you'll have any problem.

2. If the load of the two servos on the Y lead is too high, it will cause a voltage drop.
BOTH servos will see the same voltage/ voltage drop.
I don't think the larger servo will drown out the smaller one.

3. Why not just connect the rudder to the nosewheel servo? 

Regards

There is an alternative method!
You could connect the nose wheel servo to CH5 and then mix CH 5 with CH4(Rudder), so when ever you give rudder input the nose wheel servo also gets the same input.
This method will be simple to do.

Yes, Naveed
That way a y-connector will be avoided, But still a separate servo is required....

Wouldnt it be better to have dedicated steering servo?

Quote from: Mohammed Naveed on December 25, 2020, 02:03:58 PM
Wouldnt it be better to have dedicated steering servo?

Most cases it's overkill. If needed, can put a small kink or loop in the steering pushrod to absorb shocks. However, I've never needed one

@ Naveed, Dedicating a servo for steering without using Y-connector would indeed be a luxury that would eat up a precious spare channel of my transmitter (which could be used otherwise in future). But nevertheless, the best way to de-link rudder and steering.

Sanjay, I will surely try every possibility to go the conventional rudder-steering arrangement before any other arrangement. As earlier KK Iyer sir had suggested, going for a bigger servo shall be diligently explored !