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CG and safe

Old 04-15-2021, 06:45 PM
  #1  
jangodog
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Default CG and safe

Hi. Just got the Marlin. All setup but when in safe mode the elevator goes up pretty high. How do i excatly know where to put the AR636 rx in the cockpit? I know CG is 8-9cm from edge but is it very importent to place excatly under CG or near the CG? I post som pics.
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Old 04-15-2021, 09:26 PM
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solentlife
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OK ... you've hit on a topic that can create some interesting views and opinions.

For my opinion - I stick with what Hobby Eagle say ... mount aligned properly with level and fore-aft of model. It is more important to get the alignment correct and level correct way round than to force a mount on CoG.

OK why do you need to be relatively close to CoG ? Its down to moments and the gyros reactions. As you move further away from CoG - the amount of movement in arc of model increases as a factor of the distance from CoG ... it can in some cases be quite excessive and violent ...
Having the gyro at or near CoG means that arc of movement still has same degrees - but is shorter in actual traversed distance ... so the 'shock' is much less.
Unlike elevators etc.that need distance from CoG as a moment of beam force - the Gyro only needs tiniest movement of model and it reacts ...

I suggest to you .... mount in best place possible to have LEVEL ... and correctly orientated FORE - AFT ... and if you can - try to keep relatively close to COG. There is no need to be at CoG.
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Old 04-15-2021, 11:37 PM
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Well think iím pretty close to cg only have to tip the rx upwards in order to level the elevator when in safe mode. No problem in normal/safe off mode. Or maybe a little adjustment in the rx with Spektrum programmer. Less up elevator should do the trick.What would you recommend as adhesive for gluing the AR636 in place? Welcro or dobbelsided tape? Thankís anyway iíll get back if i canít fix it.
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Old 04-16-2021, 12:21 PM
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solentlife
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1. Never add pitch to the Stabilised RX to correct attitude ... it should lay flat along the models natural level datum line.
2. NEVER use Velcro to mount a stabiliser ... its fine with a plain Rx ... but Velcro allows too much movement even if only slight - that upsets the Gyro's operation. Use good quality double sided foam tape. Most Stabilisers actually get sold with tape patches for this purpose ... usually 3M brand !

Most Stabilisers have a calibration routine on first setup where you block up the model fully level and as if flying straight ... set gyro calibration to register this as its datum straight and level. Then all you need do when flying .. is power up model .. let it stand on ground for few sec's while gyros initialise (most stabs give a twitch of surfaces to show when completed / along with LED...) ... then you're ready to fly.
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Old 04-16-2021, 02:17 PM
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quorneng
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jangodog
If you are using 'safe' it is vital that the gyro is calibrated to tell it what 'plane level' actually is. This is the attitude the gyro will try to return the plane to when 'safe' is active or the panic button is pressed. It has nothing to do with the position of the control surfaces so trim them to neutral with 'safe' off and then go through the calibration routine.
Unless you complete calibration successfully do not fly with 'safe' activated as you will most likely crash.

A gyro system without 'safe' does not need to be calibrated as it only 'resists' sudden movements but will not return the plane to a pre set attitude. In other words it make the plane more docile but you can still crash it easily enough.
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Old 04-16-2021, 04:52 PM
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jangodog
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I know it's vital that the ar636 is calibrated from start and it was in the first part of the programming with the programmer cabel. But i find it a little strange that the ar636 has to tilt a little upwards in order to level the elevator in safemode. I just struck me last night that i follwed the servo led instruction from the Eflite Viper profile i'm using in the Arrows Marlin. From the Marlin instruction the leds to the rx is very different from the Viper profile. The throttle is no longer channel one in the Marlin. But when i use the Viper profile i guess it's the connection from Viper instructions that's counts? Can i calibrate the ar636 in the plane or should i take it out and do over again in the Spekrum programmer?
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Old 04-16-2021, 11:29 PM
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quorneng
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I can't help with direct Spektrum assistance (I only use Spektrum compatible receivers) but as an observation if you are not confident enough to fly without having full 'safe' available are you sure you should be flying an EDF?
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Old 04-17-2021, 09:14 AM
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jangodog
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Originally Posted by quorneng View Post
I can't help with direct Spektrum assistance (I only use Spektrum compatible receivers) but as an observation if you are not confident enough to fly without having full 'safe' available are you sure you should be flying an EDF?
Iíve got that question more then once and yes i enjoy flying both regular and edf with safe. Use it for take off and landings.
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Old 04-17-2021, 09:45 PM
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ALL gyro stabilisers use level / straight as a datum reference - regardless of whether they have 'Panic' mode or not. The difference being whether you have to calibrate or not. Basic non 'panic' units just insist on being mounted level and fore/ aft. The level function is fixed.

Gyro stabs regardless of brand should always be calibrated after mounting ...
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