Let's Make Robots!

HS-311 Servo

Hi everyone,

I was hoping someone could answer a question for me. I'm in a group that's making an R/C hovercraft and we want to save weight. I was wondering, what would be the smallest batteries we could use while still being efficient and cost-effective? I've tested it with AAs only to this point and so far it looks like it'll work with three of those.


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you can't hover without motion. I believe you would be better off to have a motor for propulsion, and, one for lift. It is my understanding that racing hovercraft use a single engine to minimize weight.

Do you have your main motor selected yet?

I have one of these little quadcopters. It seems like one of the motors from the quadcopter might be a good size for a small hovercraft. It's a brushless motor so it will need a brushless speed controller.

Here are some propellers that fit the motor.

I like HobbyKing's little HXT900 servos. I'd think it would be strong enough to use on a hovercraft rudder.

As previously suggested, I'd use a LiPo pack to power the thing. This one should do the job. HobbyKing also sells chargers for the LiPo packs. You can not use a normal charger on a LiPo or bad things (fire/explosion) will happen.

The speed controller has a BEC (battery elimination circuit). You can use the BEC to power the servo (don't power it from the LiPo pack directly). The BEC could also power the RC receiver.

If you used two motors, you could turn with differential thrust. If you used two motors (I'm not suggesting you do since one motor may be easier to use) you'd want to use counter rotating propellers to cancel out the torque. The propellers I linked to above include both kinds of propellers.

Fill free to continue to ask questions but try to give us enough information so we know what you're really asking.

The hovercraft is no more than 25 cm across and 40 cm long. I was mounting a couple cardboard fins on on the servo to serve as rudders behind my thrust fan.

Thanks for the suggestion JerZ. I totally forgot to mention what I was using the servo for. We've moved to a HS-55 servo now and we've mounted our motor onto it, deciding to go with vectorable turning instead of rudders. Would we be able to power this servo and the controller receiver (20mAh each I believe) with a couple large watch batteries outputting 3V, or would they not supply enough current?

Chris, try not to be so belittling when answering questions in a forum called "ABSOLUTE BEGINNERS". I believe it's called that for a reason.

Watch batteries would be absolutely inadequate for a project like this; LIPOs are the way to go. 

I'd suggest a setup very similar to an RC aircraft, using a 2 cell LIPO of 1000mAH or so giving 7.4V and a brushless motor, for which you'd need an ESC (electronic speed controller).  The ESC takes a standard servo control signal to control the motor speed.  You could also use an aero propellor for this, ducting off some of the down thrust for the vector steering.

A standard RC receiver will give you three, four or more channels for servo and motor control as well as regulating the LIPO voltage down to 5V for the servo supply.  The RC transmitter would be the costliest part of the setup, but you can find basic ones on eBay for pretty low cost.

If this approach appeals, I'll post back later with more detail when I have more time.

I missed the belittling part too.

It would also help if you provided links to the items you're asking about so we don't have to do a search for them.

I searched and found the HS-311 servo, but I'm too lazy to search for the latest servo so I don't know how much current it draws (since that information also wasn't posted).

Which "large watch batteries" are you thinking about (preferably with a link or at least specs about how much current they can supply). I think there are lots of different kinds. Most watch batteries are intended for very low current applications and I really doubt they'd work with a servo.

BTW, I second JerZ's suggestion.

They are light, produce gobs of current, and not too pricely compared to AA rechargeables. They require a Li-Po charger.

And yeah a relavent title and a better description, will get you better answers ;)

How in the world are we supposed to answer this? 

Is your hovercraft 2 inches across or 20 feet? Are the motors gas or electric? What size motors? What kind of current do they draw? Etc. Etc. Etc.