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Old Mar 02, 2012, 02:34 PM
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United States, TX, College Station
Joined Jan 2012
7 Posts
Help!
7 x 7.4V 5.0A Servos on one Receiver...

I have recently run into a small problem for my undergrad research project.

We are building a "Morphing" wing plane which will allow us to change the sweep, dihedral and incidence angle of the wing while in flight. We are going to use two receivers and radios, one person flies the plane normal, the other controls the two extra channels for sweep, dihedral. The incidence angle will act as our Ailerons.

To do all this on a 10lb plane we need some BEEFY servos. The only thing we have found so far that will work for us are some MASSIVE 611 oz-in servos (http://www.troybuiltmodels.com/items/HS7980TH.html) that draw 7.5V and 5.0A under load.

The configuration will be 2 servos per wing for dihedral, and 1 for incidence, per wing, plus one for sweep.

This is what the layout will look like.



My main concern is the servos used in the dihedral which will have to run on a y-harness will draw 10A of current! In total this is going to wreck the battery/receiver.

I was planning on using an AR6200 2.4Ghz which allows 3.5–9.6V but I know it will draw too many Amps. We have a Futuba T7C and a Spektrum DX6i.

So here is my question. What do I do??? Is there some way to power the servos directly from the battery while by-passing the receiver?

I would be very grateful if someone could help me out. =D
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Old Mar 02, 2012, 03:02 PM
Balsa addiction since age 3
ScottSails's Avatar
Upstate NY
Joined May 2005
1,264 Posts
Quote:
Originally Posted by TAMU View Post
I have recently run into a small problem for my undergrad research project.

We are building a "Morphing" wing plane which will allow us to change the sweep, dihedral and incidence angle of the wing while in flight. We are going to use two receivers and radios, one person flies the plane normal, the other controls the two extra channels for sweep, dihedral. The incidence angle will act as our Ailerons.

To do all this on a 10lb plane we need some BEEFY servos. The only thing we have found so far that will work for us are some MASSIVE 611 oz-in servos (http://www.troybuiltmodels.com/items/HS7980TH.html) that draw 7.5V and 5.0A under load.

The configuration will be 2 servos per wing for dihedral, and 1 for incidence, per wing, plus one for sweep.

This is what the layout will look like.



My main concern is the servos used in the dihedral which will have to run on a y-harness will draw 10A of current! In total this is going to wreck the battery/receiver.

I was planning on using an AR6200 2.4Ghz which allows 3.59.6V but I know it will draw too many Amps. We have a Futuba T7C and a Spektrum DX6i.

So here is my question. What do I do??? Is there some way to power the servos directly from the battery while by-passing the receiver?

I would be very grateful if someone could help me out. =D
Sure, power from a direct connection to the battery for the servos ( battery V+ does not get modified by the receiver) and just use the servo signal from the receiver (ie, remove the power wire from the servo connector at the receiver). Just make sure you have right size wires to the battery and the battery can handle the load. If using separate batteries for receiver and servo's, make sure grounds are tied (but not +).
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Old Mar 02, 2012, 03:47 PM
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United States, MA, Bristol
Joined Sep 2009
1,561 Posts
Quote:
Originally Posted by TAMU View Post
I have recently run into a small problem for my undergrad research project.

We are building a "Morphing" wing plane which will allow us to change the sweep, dihedral and incidence angle of the wing while in flight. We are going to use two receivers and radios, one person flies the plane normal, the other controls the two extra channels for sweep, dihedral. The incidence angle will act as our Ailerons.

To do all this on a 10lb plane we need some BEEFY servos. The only thing we have found so far that will work for us are some MASSIVE 611 oz-in servos (http://www.troybuiltmodels.com/items/HS7980TH.html) that draw 7.5V and 5.0A under load.

The configuration will be 2 servos per wing for dihedral, and 1 for incidence, per wing, plus one for sweep.

This is what the layout will look like.



My main concern is the servos used in the dihedral which will have to run on a y-harness will draw 10A of current! In total this is going to wreck the battery/receiver.

I was planning on using an AR6200 2.4Ghz which allows 3.5–9.6V but I know it will draw too many Amps. We have a Futuba T7C and a Spektrum DX6i.

So here is my question. What do I do??? Is there some way to power the servos directly from the battery while by-passing the receiver?

I would be very grateful if someone could help me out. =D
I would be surprised if that servo drew 5 amps under load. 5 amps is likely the lock/stall current. The RC sailing crowd use servos like that all the time for sail servos. I have used the Hitec servo just below that one in torque, 486oz-in at 7.4 volts. I believe it has a stall torque of right around 4.5 amps.

I looked on the Servocity site and they didn't list any current specs for your servo yet, but it will be ALOT less than 5 amps.

Standard servo wire and connectors are typically only good for 3 amps or so.

You need to get a data logger and load up the control surfaces to what you think they will see in flight.

I have stalled my sail servo on a couple of occassions when it was nuking out(windy). I have also fouled my sheets and stalled the sail servo for a couple of minutes. Surprisingly it didn't burn up the receiver, just killed the battery.

I run all my high current sail servos through a Y harness with no problems.
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Old Mar 02, 2012, 04:48 PM
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United States, TX, College Station
Joined Jan 2012
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Well that's really good to know! So the lock/stall current is the current the servo draws when it's maxed its travel? The servos responsible for the dihedral will be under constant power since they need to be able to take all the flight loads, but they will most likely be at their zeroed location so less amp draw? Sorry I don't know much about sail planes.

If this is the care I should not worry about frying my AR6200?

Also I'm currently looking at a 4200mah battery, if I draw about 20amps I should get 12 min of flight time.
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Old Mar 02, 2012, 08:26 PM
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United States, MA, Bristol
Joined Sep 2009
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If the force you're trying to move is momentarily too great for the servo to move and the servo stops, that is lock/stall. It can happen at any point in a servos travel. If the servos are sized properly and you don't have any mechanical malfuctions you will not have to worry about stalling the servo.
You still may have issues using 7 high torque digital servos. I've never used that many high current servos with one receiver. You may want to ask this question in a large scale aircraft thread if you don't get any answers here.
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