Hi, I am new to the the Giant scene and I could use some advice.

I unexpectedly won a 89 inch Yak in Ebay. It is similar to the Hanger 9 Yak but without the foam core build. Balsa/Ply, with turtledeck sheeted wings. I am trying to get it in the air as cheaply as possible without too much compromise.

A bit about me, so you get an idea of what I want to do with the build. I am UK based, and I have been flying for a couple of years now. I have 6 aerobatic models, built 2 from balsa kits, one is repaired balsa from anothers crash, and two are kit foamies. I fly what you could describe as slow speed 3D with some XA, mainly tumbles and pop tops, IMAC and pattern thrown in. I hoping to be able to do some of that, although I suspect proper full on XA 3D will rip this to bits. I am a member of the BMFA, and my local flying club. We have a 400ft grass runway that we fly giant scale and turbines from every week so flying it will not be a problem.

I got the airframe and motor very cheap, 180£ or about 200$ , and so I am trying to build a budget giant around it. I got really lucky, as despite the price the airframe is clean, straight and unflown.

I already have the below components that came with the airframe, and my stock so these are the fixed parameters I am trying to work around.

Two sets of 2x 6 Cell 5000amp batteries in a series harness giving 12s
120amp HV Aerostar ESC OPTO
180KV Overlander Thumper 8085 6000W motor.

It came with a 22x6 master airscrew prop, this appears to be quite a conservative prop based on some guesses, so I plan to use this to start and move up gradually unless you guys say its a bad idea =-)

I am trying to design a Servo and Radio system around this. I am an FrSky user. I will hang battery and temp telemetry off the system, but don't discuss it below as I have a good understanding of it already. The Hanger 9 yak requires 12KG servos, so I am starting with that as a guide.

I will say right off the bat, I would LOVE a proper 7.2 Volt HV servo system, with branded brushless Servos, However the 12KG requirements put this out of budget for me ( which I am setting around 25£ or 30$ a servo ), so I am trying to design something workable and cheaper. I already fly 3d successfully ( ish ) on 0.12 response servos, so I don't view 0.01 as an absolute requirement.

Redundant Li-FE setup :

2x Li-fe batteries 6.6 Volts
FrSky RB20 redundancy bus ( unregulated hence the LI-FE )
1x FrSky RX8R pro receiver ( I have seen issues flying around others ignition systems so the interference protection seems like a good idea )
XM+ redundant receiver in tail

4x 13kg 0.12sec Power HD Servos, one per aileron, two for the split elevator.
1x 20kg Power HD Servo for the rudder ( push pull cable )

I like these servos as - they meet the KG requirement, they run from an unregulated 6.6 V, they are acceptably fast, they have an alloy case.

KISS solution alternative to the above - no redundancy, but less failure modes.

1x FrSky RX8R pro receiver
1x 6.6 volt li-fe battery
Servos powered direct from battery via custom loom

High Voltage (HV) Savox/Hitec/Futaba build

I am honestly struggling to design a viable, branded HV servo solution. The problem I have is that budget ( 25£ ) HV Savox and Hitec servos top out at 8KG. The surfaces on the Yak are pretty massive, and I worry 8KG is not enough. There is a massive jump in price the 12KG rated HV Savox/Hitec servos are about 80£ each....so it would look like the above builds, but with the batteries and servos swapped to :

Li-Po ( 7.4 V )
Savox SV-0220MG 8KG 0.13sec

Low Voltage (LV) Savox/Hitec build.

I can have a regulated ( 6v ) output from a BEC. 4-Max in the UK do a nice 20amp BEC that has switchable voltage output. The problem as I see it is that the BEC is an added point of failure, so I need two, which means added weight and complexity, and I have to use a redundancy bus. However "branded" LV servos then fall into my budget, although they are slow. (0.18ms)

2x 20amp BEC
1xRB20 redundancy bus
Savox SC-0251MG 16KG 0.18sec

It is probably going to catch fire, but what the hell its the cheapest build.

1x 2 cell Li-Po
1x 20 amp bec
1x X8R FrSky receiver
5x Tower Pro 996R servos. 10KG 2.0secs

Apart from the airframe, the motor and the brand of radio gear ( FrSky ) I am open to any suggestions.

A few thoughts


1 - Rather than guess at the torque requirements, do some calcs. there is a spreadsheet floating around that will give you numbers, I have attached it.

12Kg*cm might be light indeed if that Yak has ailerons as big as the latest EF EXP type planes... but on the other hand 10 years ago that's all we had and they worked fine in planes that size (with smaller surfaces). Do the math and check.

2- 0.12 ~0.15 s/60 will be enough for traditional low and slow 3D. If you are into the high energy XA style you might feel like you are chasing the aircraft a bit. Under .10 should feel fast for anything, never quite understood why some airplane guys obsess about servos faster than I use on my gyro driven heli tail rotors...

3 - If you are happy using RX batteries stick with that and KEEP IT SIMPLE. Nothing is more reliable than a pair of batteries. Dump the smart redundancy bus also and just plug them both direct parallel into the reciever - pilots (me included) have done this for decades, its simple its cheap and it works.

4 - If you do use a BEC, do not run 2 in parallel. A number of BEC makerrs specifically forbid that. Remember that some failure modes of BEC (like leaking unregulated battery voltage to the regulated side) are going to fry the system regardless of if there was another in parallel....

5- One budget option is to consider lightly used name brand servo's of a higher spec

6 - If budget is really this tight, consider if you really want to fly this size of airframe or might be better going a bit smaller for now. 60cc equivalent electrics get expensive FAST if you fly a lot. Short flight times will have you yearning to invest in a lot of large expensive batteries and possibly high powered field chargers, generators, etc.

Thanks for taking a look, answers in line below.

Originally Posted by jharkin

A few thoughts


1 - Rather than guess at the torque requirements, do some calcs. there is a spreadsheet floating around that will give you numbers, I have attached it.

Thanks, running them now.

12Kg*cm might be light indeed if that Yak has ailerons as big as the latest EF EXP type planes... but on the other hand 10 years ago that's all we had and they worked fine in planes that size (with smaller surfaces). Do the math and check.

2- 0.12 ~0.15 s/60 will be enough for traditional low and slow 3D. If you are into the high energy XA style you might feel like you are chasing the aircraft a bit. Under .10 should feel fast for anything, never quite understood why some airplane guys obsess about servos faster than I use on my gyro driven heli tail rotors...

That is what I am hoping ! I don't think I will be doing much High XA with this.

3 - If you are happy using RX batteries stick with that and KEEP IT SIMPLE. Nothing is more reliable than a pair of batteries. Dump the smart redundancy bus also and just plug them both direct parallel into the reciever - pilots (me included) have done this for decades, its simple its cheap and it works.

I am happy with receiver packs. However, I don't think the FrSky receiver will allow that - its rated to 3 amps, and don't I think that is going to cut it. Frskys solution is the RB10/20, rather than a high amp receiver like futaba or JR. I think with one FrSky receiver I need to have the Servos with power running directly from battery and signal only from Receiver by pulling the ground and live and running it straight to the Lif-Fe pack. I have never actually done this but I understand it works in theory =-)


4 - If you do use a BEC, do not run 2 in parallel. A number of BEC makerrs specifically forbid that. Remember that some failure modes of BEC (like leaking unregulated battery voltage to the regulated side) are going to fry the system regardless of if there was another in parallel....

OK that could be a problem. The RB20 does't regulate voltage.

5- One budget option is to consider lightly used name brand servo's of a higher spec

Servos of the requisite spec are a bit thin on the ground, but I am open to the idea.

6 - If budget is really this tight, consider if you really want to fly this size of airframe or might be better going a bit smaller for now. 60cc equivalent electrics get expensive FAST if you fly a lot. Short flight times will have you yearning to invest in a lot of large expensive batteries and possibly high powered field chargers, generators, etc.

I have this figured out already. I fly a 1.6 meter Aerobatic model that takes 5000mah 6 Cell batteries. So I already have 4 batteries, a 10 amp twin charger, and 2 car batteries for field charging. I can charge next to the car in the pits, and I can recharge both in 30mins and get about 6 pairs in before the car batteries run out, enough for a days flying.

A pic of the airframe and some of my other aircraft I flew today.

Originally Posted by BobHope

I am happy with receiver packs. However, I don't think the FrSky receiver will allow that - its rated to 3 amps, and don't I think that is going to cut it. Frskys solution is the RB10/20, rather than a high amp receiver like futaba or JR. I think with one FrSky receiver I need to have the Servos with power running directly from battery and signal only from Receiver by pulling the ground and live and running it straight to the Lif-Fe pack. I have never actually done this but I understand it works in theory =-)

Unless there is something very unusual about it, the receiver should not have any fix amp limit - internally all the + and - pins on the servo ports in a typical receiver are simply ganged together on a power bus.


The "3 amp limit" commonly floated around for RC comes from the reccomended 3 amp continuous current rating that JST published for the 0.1" (2.5mm) pins in our servo plugs. Note particularly that this is a continuous rating, and going over it in short bursts doesnt mean instant failure.

If you run 2 batteries with 2 servo plugs into open ports on the receiver that is at least 6 amps of continuous rating, which should be enough for 12kg servos. Even if you go over a bit in short burst it wont cause issues as long as the averages are reasonable. Ive pushed 5 amps continuous though a single plug for 15 minutes while charging - it gets warm and its not recommended but it doesn't melt.

Originally Posted by BobHope

OK that could be a problem. The RB20 does't regulate voltage.

My point was that many BEC makers dont want multiple BECs run in parallel as there can be compatibility issues - some dont have diodes on the output and will backfeed and cause damage to the one turned on last, etc. And sometimes they can fail in a way that having 2 of them wont save you.

Originally Posted by BobHope

I have this figured out already. I fly a 1.6 meter Aerobatic model that takes 5000mah 6 Cell batteries. So I already have 4 batteries, a 10 amp twin charger, and 2 car batteries for field charging. I can charge next to the car in the pits, and I can recharge both in 30mins and get about 6 pairs in before the car batteries run out, enough for a days flying. .

If you get serious about flying big electirics I would skip the car batteries and buy true deep cycle batteries to run your chargers. Examples of deep cycle are marine batteries, golf cart batteries and solar storage batteries. Car starting batteries are designed for high current bursts and shallow discharge and you will probably kill them in very short time using them for deep discharge.

Thanks for that spreadsheet, it is awesome.

The powerHD servos have 180 Oz-in and they look viable for aileron and elevator.

The rudder is a suprise though, looks like I need a bigger servo than 326 Oz ( 20KG ) as it requires 401. However I note the rudder is counterbalanced, do I take this into account in any way in the spreadsheet ?

Ok thank you this is very helpful, so essentially it would look like the below diagram , try to ignore my battery pole labelling .