BH 50CC Yak 55 build


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Like all good plans, I realized my mistake about 10 minutes after building the battery mount.. The cowl doesn't fit anymore.

While installing the cowl, the cowl needs to drop down to allow the exhaust pipe to fit past the cowl ring into the cutout..

Lucky for me the battery mount is pretty much symmetrical, so I just flipped it over.. Whew.. Got lucky on that one.. Am I OK having the Ignition battery wire running next to the throttle servo and RX batteries, or do I need to separate them?

Tried to fit the cowl again, and remembered that I never cleared a spot for the spark plug. Fixed that really quick, and now had to figure out how to route the sparkplug wire..

I finally decided on passing it through the two pipes, and so I needed a support to prevent metal to metal contact.. Any thoughts on how this worked out?

Also got the power system finalized, and am now trying to finish the radio install.. Anyone know how to program a 9303.. Anyone want to buy a 9303? :)

I would like to have my throttle control on a 3 pos switch, up is off (Opti-kill disabled) , middle is normal, and down is plus 10% on throttle (warmup/starting). I used to know how to do it, just need to figure it out again :)


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Dang, where you at? I just called a minute ago, your cell goes straight to V.M.

I need help. I keep seeing the same words over and over again..

I am We Todd did.
I am Sofa King we Todd did.

So, my rudder is all hosed up now.. I was very careful to setup the rudder on 3" centers, directly in line with the hinge. I then went and got a H9 3" servo arm. Of course, the 3" holes are offset, so now the geometry is all hosed again.. ARGH!


Dave thanks for posting your progress and pics. I'm looking forward to getting mine soon and your posts gives me some insight on how to approach the build.


Circus Staff
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if the cables cross... then it is about right if the rudder horn is 180 degrees on the hinge... but the tillar oughta be 5 to 10 degrees aft offset at the servo rack... that will be pretty dang close...


Professional Moderator
Perfection is unobtainable. Get it close and go fly the piss out of it!

Hmmmm, maybe that's why so many run and hide when I fly??? Naw!


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I just can't wrap my head around the damn tiller!

I ditched the Hangar9 servo arm and went back to the arm I had on the last plane. It was drilled for 3.5" and offset forward. I redrilled at 3", keeping the offset, and it seems much better.. Good and snug at center.. The cables have less tension once I clear the center, but it translates into very little play on the rudder, so I figure it should be good..

BTW- Yes, they are crossover cables..

Spent some time on the radio last night.. Figured out the throttle and OptiKill, and went to work on the Ailerons.. Left aileron came in around 25 deg, changed the arm length on the ail to get +/- 40 deg. Moved over the to the right ail and I am maxing out at 38 deg. Dug around for a while, and it seems the hard point for the control arm is set back further, so given the same linkage length, I wound up on a different spline on the servo, which of course messes with the geometry, and on it goes..

Anyway, I don't have enough length in the linkage to put the servo arm on the correct spline and still be centered, so I left it where it was, and modified the <u>other</u> ailerons linkage to match the throw of 38 deg..

Hmmm, hope that makes sense..

Not sure if I talked about it before, but I also added a Emcotec BMS. It is basically a dual channel current. voltage. and milliamp counter.. What can I say, I like gadgets :)
This thing is pretty cool.. It only measures down to 150mA, so the bare fuse just shows "< .15A", but once I get all the servos onboard it reads 18-20 mA per pack at idle.. If I wiggle the sticks I can get about 1.5A out of it, and applying pressure to a control surface will easily get me over 2A.

Anyway, I this is my first try for A123, and I really like the ability to see all this data..

Maybe lunch time I can try to dial in the Elevators....

I will also post back on what I had to do for the throttle idle up and optikill switch.. I still don't understand how it works, but it does :)


Well-Known Member
Here is a quick shot of the BMS. The display is set to show the present current for both batteries.

The system starts and stops on it's own, so I don't need to pop the canopy and start it or anything like that. Of course, it has no control over the batteries, and it cannot stop the power, it just watches it for me..

I cheated big time on the mount.. I just measured the spacing of the vertical braces where they are slotted through to the top skin, and then cut a piece of plywood to match. Inserted from below and bolt into place. For a little snazz I was going to add a thin sheet of CF, but the piece I had was too small, so I just slapped some monokote over the wood :)

One thing I didn't like about the BMS, is the connectors are straight out the bottom, in this orientation it is very hard to clear the wing tube.. if I go forward I interfere with the floor of the canopy, if I go aft I will block the wing bolts.. I actually had to make two of these plates, the second and final one had the display about 1/4" forward of the first one, just enough to prevent pressure on the connectors.


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Just for fun :)

I just charged both my A123's after tuning the radio and running the engine a little bit..

BMS showed 547mAh and 533 mAh.

Charger shows 535mAh and 537mAh ... Unfortunately, I hadn't marked one and two yet, so I don't know which is which.. Those readings seem really close between the charger and the BMS..


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Sorry Al, I don't know.. We weighed it, but I forgot the weight.. I thought it was a touch under 17lbs ( I honestly don't remember anymore), without elevator extensions, throttle servo and BMS. BMS is about 1oz.

Tried to maiden her today, but I wasn't sure about the CG, so I went back home.

Now I have to leave for two weeks.. Oh well, at least I know she is still in one piece :)


New Member
anyone doing an electric version? im planning on a Hacker A60-18L, Castle ICE 160HV, 12S 5000 40C, APC 24x12E. should be about 3000watts =]

... now i just need the cash


anyone doing an electric version? im planning on a Hacker A60-18L, Castle ICE 160HV, 12S 5000 40C, APC 24x12E. should be about 3000watts =]

... now i just need the cash
I'm considering it.... I have an A60-18L and CC 110HV...


New Member
OK, last round..

I just wanted to see these for myself :)

I taped my spinner and prop in place (well, almost in place).

I guess I should mention that this is one of the very first 50cc Yaks from Bill. As such, it doesn't have the CF mains the mainstream versions have. I gotta say, I kinda like the polished aluminum. I have a set of CF mains on order just in case I change my mind, or don't like the weight.

Also took two shots to try and show the canister exit area, but I don't know how well those turned out.

Of course, last shot is the landing gear bolt pattern..

I think I mentioned before, I need a 3.5" spinner.. Anyone up for a trade?

I will also be needing some turnbuckles, not sure of the length yet. If anyone has some for sale, or wants to trade, lemme know. I will probably be needing 2" - 2.5" lengths.
any video of you flying the Yak . I just picked one up.
Thanks Rob


Well-Known Member
Wow, has it really been two months since I last looked at this thread?

Finally got to fly her on Sat.. Just two quick hops.. Had to add about 20oz to the nose to balance her at the back of the wing tube! Overall weight is about 18.5 if memory serves.. I don't think I can move anymore weight around.. Maybe a Vess ring to help get the weight forward?

Actually, now that I think about it.. Maybe I need to look at either moving the Elevator servos up into the canopy, or find some lighter servos (currently 8611's)..