Found out from futaba that test data shows each port on a hub can pull 3 amps. So I decided to hold of on my large jets and planes until the beef up some things.

Its not as bad as it sounds. Servo connectors in general have that limit. The limit however is for CONTINEOUS current. More than that will start heating the contacts up. But our servos draw this type of current only in short burst so we get away with it on high torque digitals. BTW those "hubs" are just like a multiway Y lead. All you need for the feed is uprated power wire, never a bad idea anyway. In effect the reduced connector count as opposed to extension leads as a solution, is fewer points of failure and lower power use/less voltage drop. ( with uprated power cable )

This is what robbe has done and I think I may go this route

When I think about a jet and the amp draw maybe okay to use futabas sbus, 8711 dont pull that many amps even on the elevators. what would probably be the highest is where the sbus cable with 6 servos plugs into the receiver what kind of load u get there.

Does having a pack closer to the high drawing servos make a difference? You can plug a pack right at the hub. I ordered two complete Sbus set ups for my skymaster mig 29 and bobcat. And the new 18mz :-) On the mig, I'm gonna run one 3' extension to each side and have a pack at that hub. I was told 10 amp max draw, not 3????

I was told each spot u plug a servo in can handle 3 amps, plugging a battery in at the hub would prevent the servos from pulling to many amps from your receiver causing a possible lockout. In my large 100 cc plane I had one battery just for the receiver and another large pack that was isolated fromt he receiver to prevent voltage drop. The problem with Futaba is one there lack of instructions for the products and two lack of detailed info on new products. I still love my 14mz just took hours of playing to figure it out the book was useless.

who knows i thought a regular servo end could handle 10 amps but??????????Im sure the system is fine I would just like numbers to prove it before I put the stuff in a 15k project is all.

Not sure about the 8711's, but the 8411's are capable of pulling up to 5amps each, been proved over and over again on large 3D petrol airframes ask QS when he stuffed his Bulldog after browning out his Spektrum, first real high profile crash of that system that was fully investigated, but it did highlight the amp draw when you start banging the 3D surfaces from end to end, not that we do that with our jets, but at high speed I wonder what the load is? I bet its more than we think.

Mike

More RCU urban myths !

Its actually a lot LESS than you think. Weatronics records it all at .1 second intervals.

Masses of accumulated data on a number of large jets show the exact current draws at each and every servo position at the whole range of GPS speeds for the entire flight.

Average current for an Airworld Hawk with numerous 8711s is around 3 amps.

Doubt if even the Futaba 18 does that ?

So with a Weatronic 12-22 or larger receiver, one can create a graph to plot a given servo position, total current, battery voltage and GPS speed (even IAS with the link Vario connected) to show EXACTLY the situation.

Just ONE of the reasons I have converted to the Weatronics system, in many aspects, particularly precise data recording, its vastly more capable than any other system I know of.

David.

I think there is a lot of guff talked about servo amp draw on jets. As most of you will know, current draw issues with digital servos is a serious consideration with flybarless helis, particularly large ones. Big 3d planes can also draw a lot of current. interestingly, the biggest draw, short of a completely stalled servo, is when you get rapid direction changes of the servo, something that you have a lot in helis and 3d planes but nowhere near as often in jets. Jets do of course travel faster, so the air pressure at the control surfaces is going to be higher, but then jets should be using servoswith torquecapable of dealing with the sort of demands that this places on them so that they do not stall.

My suspicion is that most jets, in a normal flight regime, pull no more amps than a high performance glider, provided the control linkage geometry has been set up correctly and the servos are well matched to the size and performance envelope of the jet and the specific control surface they are having to move.

Hi David

are you saying taking it s an average over all the servos in the Hawk? If so does it also log the peak current to those 8711's individually?

Jason

I like weatronics stuff but that's another big cost I choose not to add to my jet or large planes. However having one system to test with would be nice but just not in my budget at the moment. Maybe someone with a weatronics can test out a SBus system in a jet and give us real data.

Thats just about right. Many years ago I used el cheapo JR507 servos on a kangaroo's elevons, ie one servo per side. I certainly had good control authority but you could feel them go mushy at the bottom of a loop. So they were on the edge of marginal. Sopposedly they had 3.5Kg torque. So I think a digital with 10Kg/cm torque is probably OK on regular control surfaces. But on flying stabs and flaps thats another story.
8711's are overkill on most jets IMHO. Yet I still use them.....
Andre

In my view there is a massive tendancy to overdo the size of servos we use

The peak currents are much higher than that. The Weatronic averages the spikes out. I used a Powerbox Royal in my CARF Pitts S12 with 8 x 8511 on the ailerons, 4 8911 on the elevators, and 4 8511 on the rudder (yes I know, more servos than on a jet ), the peak currents on the ground was around 30..40 amps (small spikes). No surface was binding, all servos was programmed to give no resistance over the full travel.

If you are going to put battery packs "all over the plane", how about power on/off, you would have to walk around your plane and switch off the extra packs (and we usually don't want heavy packs at the back of our planes) ?
Sorry, I just don't seem to understand the "single wire does it all" concept, pull 10..20 amps in spikes on a single cable => lots of voltage drop

One cable to each servo => more redundancy/distributed load/constant voltage.

Agree, I have seen spikes like that as well. Which brings us to the real issue, brownouts....

All interesting stuff, but the fact remains that you can pull a lot more amps than you think when changing a surface's direction even if only momentaraly, avaraging out those peaks does not give you the data you need, some batteries have too high an internal resistance to allow the power needed, using A123 batteries goes a long way to helping this on a simple sport jet, I have no boubt that the Weatronics and Power Box's have their place in the high end airframes, but I and 95% of the rest of us don't scale to these heights.

On my 2.5M 3D Yak, if I wack the sticks into the corners and stir it all around the peak reading on my Watt meter is over 15amps that is stationary on the ground, and none of the surface's are binding or the servo's reaching the end of travel, hate to think what it pulls in the air with those large surface's, jets don't pull as much, but its worth taking into consideration and not poo pooing it as a myth.

Mike

Hi Jason /Carsten.

Yes the Wea does show spikes which get as high as 7-10 amps on my Hawks. Expanding the graphs to max resolution shows the instantaneous currents including very short transient "spikes" but a I said the average current is around 3 amps, the highest load on the Hawk being when the flaps are travelling. These spikes are also shown when the graph resolution, the time base or x axis is condensed so there does not appear to be any "averaging" or "smoothing" by the Wea software. Conformation of this "average" amperage is provided by the amount of recharge which goes back into the batteries, which in broad terms reflects the average current used. On this basis I am more than happy to use Wea Micro servos in larger jets and a Micro 12 Gyro 111 is going into my BVM F4
The Micro is rated at 5 amps continuous but can handle 10 amps with a battery connected to each end of the bus.

The other two jets I have flown with the Wea 12-22, a PST Reaction and Revision, both show lower average currents, just over 2, again verified in broad terms by the recharge so the Reaction was converted to a Micro 10 and is flying very happily with it. I also have two Bobcats equipped with Wea Micros, all of which use Powerbox sensor switches which are rated at, I believe, 5 amps. Zero problems in 100 plus flights.

My own simple experiments have shown that in broad terms, current consumption is a function of torque produced but the more powerful digital servos obviously become more efficient, in relative terms as torque demands rise.

A lot is spoken/written about servo overkill. One must decide what is required. Sure, our servos generally are far more powerful than required to overcome aerodynamic forces (even though BVM got the original Bobcat servo spec. very wrong !!) but that is only one aspect of servo power.(I think wearypanda is dead right) The other aspect is the holding power to provide the stiffest control surface to resist flutter so apart from cost I see no downside to using "overpowerful" servos. Few modellers really understand flutter but I believe powerful digital servos with very strong holding power have been instrumental in minimising flutter problems.

I do not think Weatronics is compatible with Futaba S bus (I am sure bus systems are fantastic, my new house will be controlled by one ) but the Weatronics is in itself a "dual receiver" system so I cant see it working with the S bus receivers.

Finally, accurate control set up is vital to minimising current draw. To this end I ensure that all control surfaces are as free as possible over their entire movement and the Weatronic system allows perfect matching of dual servos on a single control surface (like the AW Hawk stab. ) as it has 32 points on the servo curve, each point having 4096 adjustment steps. As a result the dual servos on my Hawk stabs are perfectly matched producing absolutely minimal current draw and very little servo buzz. It is quite amazing how even the tiniest servo mismatch can significantly increase current draw. The Hangar 9 meter is indispensible here.

and for those deterred from Weatronics by cost, that is not really true. After buying a Tx module the Micro dual receivers are actually quite cheap and remember the larger dual receivers come with built in regulators, dispensing with Powerboxes and similar power management systems and matchboxes and all are available with integrated gyros which is a MUCH cheaper option than separate units.

To summarise the Weatronics system is a fantastic device for evaluating the precise performance of the entire electrical system in a complex model, in flight, ensuring that the battery performance is all that is required by providing real information, no more guess work.

What's wrong with poo pooing myths if you have real data with which to work and that data contradicts the myth !!!

Regards,

David.

Very true and in many cases the mounts that the servos are screwed too can even take 1/2 of the servos rated torque.

You have to remember if not using a battery at the hub all those amps are being fed on one servo lead from the RX

I did write a long reply, but decided in the end that your are just too intranciant for words. It must be wonderful on your planet where the only person that can be right is you.

Mike

I have to concur with David G's findings. I run Weatronics as well and my CARF Hawk never pulls more than 3 amps overall with 7x JR 8711's and even then only for a split second. Most of the flight averages 1.5 amp draw. You just don't see the big amp draws in jets like you do in 3d planes.

Not trying to detract from the Futaba thread, I'd luv to have an 18mz but, it'll never happen because it doesn't have a module and I can't plug my Weatronics gear into it

Mike

????

I also agree with David..

This is a thread about Futaba Sbus and the power requirement of servos.

I use a Wea micro system in my Bandit (8 x 8411) which records the Rx battery voltage every second and logs it. The attached graph of a 12 minute flight shows the voltage varied between 5.8 and 6 volts. I am very comfortable that my system is coping with the power demands of the eight servos. (just a fraction of what the Wea system can do.)



Roger

David,
I had prepared a long post, but I decided to cut it short

The measurements I talk about, has been done with professional equipment, measuring the current spikes directly at the batteries (more accurately, at the connector at the receiver, after 30 cm 2 mm2 wires).
The Powerbox Royal is quicker and will show the higher peaks than Weatronic will, hence my feeling that Wea softens/averages the peaks somewhat (HW filter at the ADC, software average or probably some combination of that)
So rest assured, the peaks are much higher than you see from the Wea software!

Still changes nothing about the fact that you will have a higher voltage drop along one single cable with a number of servos on, than compared with a seperare cable to each servo (as well as some kind of fault tolerance)

Unless you are using sbus servos, there is no point in running sbus at all.  Having to use decoders quickly ballons the cost.  Further, when its all said and done ther is not any advantage. 

If the total amps for each hub is 3A,  keep in mind the base hub will see ALL amps.  this can quickly get over 3A in cumulation.