Thread: Discussion Xplorer Mk. II
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Old Oct 16, 2012, 09:28 AM
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Originally Posted by satinet View Post
Joe my comment really was incorrect sorry. small = not powerful in that context. I agree that you can use very small/low torque servos even in a big model if you are careful in a number of ways (linkage geometry, flying speed etc).

How much servo torque you need on a surface is a combination of a number of factors. The obvious ones like surface deflection, speed of the airflow, size of the control surface etc. Linkage setup is important so that the servo given a chance. IMHO it is important to make the servo travel down more than up to minimise servo arm length (some discussion on the pike perfection thread).

An f3j model has a large flap and is fairly fast. If you calculate how much servo torque you need to dive a model like that with the brakes out (big flap, big throw and lots of airflown) it's a lot if the linkage setup is not sympathetic (even a powerful servo wouldn't be up to the job). I think the best thing to do on flaps is make the linakge so that it is "locked out" when full flap is deployed. Therefore the servo isn't really doing any work when the flaps are fully out.

Although, like I say personally I wouldn't be doing any sustained dives with my flaps fully deployed even with an f3b model. Let's face it scrubbing off height/speed is the easy bit (I am something of an expert in landing before 10 minutes are up! ).

This a good couple of diagrams I found ages ago on the xploder thread. Sorry I can't find them again to thank the author.

note at full Crow deployment (green lines) the servo horn is pointing directly down the line of the pushrod. This equals servo not having to work hard. And the gears are less vulnerable if the flaps hit the ground.

I'd rather have the greater margin for error with bigger more powerful servos, but I think the same good practice applies regardless - just more so with tiddlers.
This is a great discussion on linkage setup and I totally agree - all my servo flap setup are locked out at full deflection and I think as consequence I have far less slop than I see on many models with full deflection putting stress on the servo itself.

The thing to remember about the 809 is that until the 3.5 center panel of the X or X2 attains a speed of +80 MPH the servo is pleanty powerful to attain high degrees of deflection from a clean position.

Once the deflection starts the speed drops fast and the servo is well within its margin of performance from a speed / surface area and torque requirment perspective. I deployed full flaps as described numerous times and held them all the way to the ground without the described heat and servo failure. Did I make a habbit of doing that ? no its abusive at best but I needed to see oif they would fail...

I also paid very close attention to both geometry and linkage clearance. Even a tiny amount of binding or recentering will cause huge heat and end the life of these servos which is why I posted about abusive usage scenarios not being within the envelope of typical applications for it. Heat is the real enemy here.

Also just a few mm of surface horn adjustment makes a massive difference. I see a lot of guys buring the brass horn too deep, modifying the horn to do so. I did that on my Pike perfect and then realized my servos were sloppy and ill centering at the end of the 2008 WC. I had to replace them all. My bad. lessons learned -
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