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Old Feb 14, 2013, 08:54 AM
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Canada, ON, Owen Sound
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Hi Robin: The Turnigy 180D is a micro that has a total travel of 180 degrees, not 90 like a std servo. But it is still a std servo, not designed for retracts.
As you noted, that much trel requires a connector that pivots to allow a straight push on a rod.
My Mosquito will have only two bombs. By moving a frame truss thats centred, I will be able to use a single rod about a 1/4" above the bombay roof. the wire will have two hooks, one for each bomb. I have decided that I don't require the 180 degrees in the TGY180D, so will now use an HXT900 that will easily allow 1/2 to 5/8" of travel without going too far off centre. Thats my story and I'm sticking to it, hehe. Doug B
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Old Feb 14, 2013, 11:18 AM
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Baltimore MD USA
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Doug I'll be watching this w/ much interest because I too was thinking of having the bomb bays open and having bombs but wasn't sure about having them drop? I also will be having rockets on my plane (of course they won't fire) there weren't many that had rockets and they will be removable w/ magnets and guide pins to attach.
But the mossies could be armed w/ 4- 303's, 4-Hispano cannons, 2-bombs and 8 rockets quite a bit of fire power for the day.
It will be a big help to me seeing how you go about making this bomb release and how you open the doors.

Bruce.
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Old Feb 14, 2013, 03:35 PM
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Hi Bruce: I have just now completed the hookup for a staggered bomb drop of two bombs.
I already had working, shortened bombay doors. The doors where shortened from the front because I wanted the 20mm cannons for hard targets -hehe.
I will try to get some pics up of the mechanism for the doors, it works great so far on a HXT 900 servo. The bombs are set up with the same Hxt900 servo, and wire lengths allow a staggered release thru the Aux2 switch. The actual door to roof opening is pretty tight at 1 5/8", so the doors actually help a little to keep the balsa bombs in place while the doors are closed. I worked it 5-6 times and all went well.
Rockets, damn, i forgot about rockets!! 4 on each wing would look so cool! Thanks for looking in, its good to know that there are others interested in our projects. Doug B
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Old Feb 15, 2013, 10:36 PM
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Well, I worked on the LG all day today and all I managed to do was burn up another big servo!!! BMS660DMG+HS. One is still fine, but the other has burnt the motor and potentiometer out. Obviously not going to do the job!
Upon removing the servo from its mounting plate, also found the servo case fractured. Hard brittle plastic, won't be buying anymore BMS aka Bluebird servos - ever. Have now ordered nylon cased MI High torque servos with a magnetic pickup instead of potentiometers.
Also found my bombs had moved, denying the bombay doors from closing 100%. I need another attachment method, so no pics!
Have you ever had one of these days? I've had 2-3 lately! hehe All part of the game of Rc airplanes, right (at least I keep telling myself that). Doug B
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Old Feb 16, 2013, 02:06 AM
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Quote:
Originally Posted by Doug Bartley View Post
Have you ever had one of these days? I've had 2-3 lately! hehe All part of the game of Rc airplanes, right (at least I keep telling myself that). Doug B
Yes - all too frequently Doug. Don't worry, you're not the only one.

I take mine as a timely reminder that I too am human and prone to make mistakes...
And to try and keep my brain in gear next time!

Puts the cost up though doesn't it?
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Old Feb 16, 2013, 03:53 AM
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Baltimore MD USA
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Doug if your doing the landing gear the way Brian has them drawn it doesn't seem
to me like there's enough leverage on that horn I'm having the same trouble.
I bought HITEC HS-5645MG servos w/ the HPP-21 PC programmer this will slow down the action giving the gear a more scale look, the servos at 4.8V have 143 oz of torque. There not burning up but either my wheels are to heavy or the horn has to be made longer or I will have to make a pivot arm to create more force. This is one of the reasons I took a break from the Mossie to think about how to go about this.
I'll be watching what you come with and if I have any other ideas I pass them on
to you.

Bruce
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Old Feb 16, 2013, 04:18 AM
JB
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I folow this thread with great interest
Years ago I bought the drawings, cowls, cockpit, wheels etc. but never got round to building the model. This thread may provide the final push

On my Brian Taylor Mosquito drawing are shown springs assisting the raise of the U/C, quote:" adjust spring so that equal effort is needed to raise and lower U/C".
Shouldn't this allow for enough leverage on that horn?
JB
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Old Feb 16, 2013, 05:51 AM
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JB has the solution I would expect! I try and arrange a spring in the right sense so that it balances the weight of the gear but gives least resistance to the locking down of the undercart.
Not always possible depending on the position of main airframe components, but worth trying for.
Wind resistance will always assist in the early retraction phase.
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Old Feb 16, 2013, 08:27 AM
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Hi fellas: The problem is not raising or lowering the gear. the situation has the servo arm parallel to the lifting rod when the wheels are up. when the gear is down, the servo arm is approx 90 degrees to the rod. as such it relies on some tension from the servo to lock the arms past centre.
The trick is getting just enough pressure to assure a lock without putting any strain on the servo.
Thats where I just messed up, I had set the servo travel at 100% for the down positionl it provided total lock. What I didn't think about was how much load was actually on the motor/potentiometer to hold the position. It did not appear to be any issue when I used a small 500mah/15c/2S battery to test. The problem showed within 1 minute when I hooked it up to the planes Bec battery, an 1800/20c/2S.
Testing this morning showed the travel adjust could be set at 88%on the big battery and shows no heat in the servo. Obviously the little 500mah was not supplying the full power under the 100% load. Live and learn I suppose.
I just ordered some new servos again, this time I'll try MI High Torque metal Gear Prod Id 019000001 (HK) rated at 27kg on 6v and 31kg on 7.4v (I'll be using 6v). This servo is different in that it does not use a potentiometer, instead a Magnetic Rotary Encoder. With higher torque combined with slower speed, I hope to apply enough hold on the arm without loading the system. The extra gearing to get the slower speeds should help take some load off the electric parts. I'll defintely use the planes intended battery and pay attention to lock up loads too!!!!
When BT designed this, the biggest servo he could get was likely 60oz, unless he used giant scale stuff, springs would defintely have been needed. There's no mention of what was used, but it was designed with a single servo in the wing centre with 1/16th" rods. I have no idea how this ever worked!!
If this doesn't work I'll be gutting the system, and using a full 180 retract servo, to get full lock on the arms both ways. The only retract servo currently available strong enough I can find is the JR791.
I'll certainly not use BMS (Bluebird) servos again with their brittle plastic cases, only servos with nylon based construction. fwiw Doug B
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Old Feb 16, 2013, 10:58 AM
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FWIW it sounds to me like he could have been working both gear sets off a single torque bar running through the entire wing centre section. The Mosquito lends itself to that. A metal pinion on the bar (about 8swg), centre fuselage, and a worm drive on a speed 300 would just leave you fiddling with the end switching for 180 deg. rotation of the bar.

You couldn't use that system with detachable wings of course (well, someone like me, without tooling couldn't easily do it), but it's also one good way of using old servos with burned-out amplifiers (of which I seem to have a small stock - see earlier post!).
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Old Feb 16, 2013, 03:06 PM
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Hi Robin: I pretty sure everything will be fine if I carefully watch the travel adjustments more carefully. After this mornings discussions, I retested. leaving the good servo, properly adjusted, powered up for 15 minutes, checking it for heat every 1-2 minutes. The servo was cold at the end.
My stupid mistake, putting the brain in gear before making changes would help -hehe.
I've come up with another plan for the bombs, complete restructure and different holddown. Not much progress yet, trying to use the little bit of brain matter I was given. Doug B
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Old Feb 17, 2013, 01:29 PM
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While I'm stifled again with the LG, I have successfully completed a new bomb drop system that actually works.
The biggest issue was the confined space, putting a 1 1/2" diameter bomb into an opening of 1 5/8". The pictures will show the bombs hanging from dowels recessed 1/8" into the tail, and supported on the front by 1/16th wire into the noses. The wire inserts into the nose approx 3/8". A plastic insert in the nose helps the wires slide easily.
On the bench, the wires move forward, letting the tail fall from the dowels. Because Aux2 switch is the only available one to use, I eliminated 90% of the travel one way, so pos1 doesn't drop the bombs, pos2 does!!
I also replaced the HXT900 servo for the bombay doors with a TGY190D, slightly stronger, but with more travel available for fine tuning. The Hxt wouldn't stop humming when the doors where closed. The pics show the linkage from the door servo to the twin arms that link to the doors. Someone had mentioned they wanted to see the hookup.
I might mention the crossbar for the dual arms is made from 3/16" dowel, it would be better from 1/4" as this does bend slightly at full travel down. I may put in another support at centre for now. Doug B
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Old Feb 17, 2013, 02:58 PM
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Hi Doug, I think you may find that the airflow, once the doors are open, keeps the bombs where they are as the wire goes forward so the nose would drop first. If so the tail of the bombs might bind on their pins. Might be worth a test with a fan simulating the airflow?

It all looks good though.

I have every sympathy with you on the undercarriage woes. I spent more time on the undercarriage of my Stirling than the rest of the model... and now I'm just waiting for some glue to dry before installing and testing my Caribou mock-up... why do I have to choose such tricky subjects? At least the Stirling uses a 'dumb' geared motor so it stops and off-loads once the end switch is operated, but the Caribou will be my first experience of the RCLander electronic retracts. At least if they're not man enough to overcome my lousy engineering I could use ordinary servos... this is such a period of uncertainty!
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Old Feb 17, 2013, 04:22 PM
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Hi Robin: The rear dowels only go in the bomb about 1/8", I'm hoping the air will hold them there until the forward pins are out. Each bomb has a 9mm 110gr bullet buried in its nose to get some weight forward.
Thanks for the encouragment on the LG. This time is was my own dumb error. Using a powerful servo allows far more pressure, but at a cost in servo draw.
Have you got a build blog on the Caribou? I'd be interested in seeing what you built for sure!!
I have an 84" Buffalo, its only been flown twice to date, LG is mechanical. Watch out for the big CG change when you put the gear up, with the LG swinging forward, mine changes a lot!!
I'm going to some scouting for heavy duty mecancial retracts or air retracts that could be converted to servo activated. With the new servos available now it should n't be hard to make them for 15 lb planes. Doug B
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Old Feb 17, 2013, 05:52 PM
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Canada, ON, Toronto
Joined Mar 2010
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Pretty nice work on the drop mechanism! You could put a flexed piece of plastic betwwen the bomb and the bottom of the bay to force the bomb out once the pin is pulled.

Also if the rear hole that fits over the dowel is shaped like a rocket exhaust then there is no way for the dowel to hang up in the hole.

Good to see I am not the only one getting screwed over by retracts.......
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