David Tayler Posted April 10, 2017 Share Posted April 10, 2017 A friend of mine has a Tiger moth with a petrol engine. He recently replaced both aileron servos with Hitec HS81s as a previous cheap 9g servo had failled. With the engine off all works fine.With the engine running the servos jitter and when it is reved up the Left servo moves to full up. The rudder and elevator work normally, as before. He swapped a wing with two cheap servos and a y lead and this works fine on the ground. He has a separate battery for the ignition,and futaba gear, Are the hiTec servos particularly sensitive to interference and is this likely to be coupled on the power or signal lines. Any ideas re filters chokes or other cures would be gratefully received .Thanks David Quote Link to comment Share on other sites More sharing options...
Jon H Posted April 10, 2017 Share Posted April 10, 2017 without knowing the specs of the model in question its hard to be sure but any tiger moth big enough to require a petrol engine would need far more substantial servos than HS81's. Servos of this size are not suitable for large models irrespective of their torque rating Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted April 10, 2017 Share Posted April 10, 2017 What size model is this? Although the Hitecs are reasonably powerful, 9g servos seem a little on the small size for what sounds like it may be a largish model...with longish servo leads? You could try passing a few turns of the servo leads through ferrite rings close to the receiver to decouple any ignition sourced spikes - or try connecting the suspect servos directly to the receiver to see if it's picking up interference in the extensions. Edit - Great minds? Sorry Jon, posts crossed... Edited By Martin Harris on 10/04/2017 18:18:55 Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 10, 2017 Share Posted April 10, 2017 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted April 10, 2017 Share Posted April 10, 2017 That's a point - a loose plug cap can cause all sorts of interference problems. Some types appear to be in place and the engine can run but they need to go on further. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted April 10, 2017 Share Posted April 10, 2017 I have suffered exactly the same problem with HS81s....cost me a beautiful Pitts Special when the ailerons decided they would rather jitter around the neutral position than initiate the left turn I was telling them to do..... It seems to come & go & I believe this is a known type of failure with HS81s. It isn't an "interference" issue. Personally I won't use them ever again.... Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 10, 2017 Share Posted April 10, 2017 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted April 10, 2017 Share Posted April 10, 2017 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Jon H Posted April 10, 2017 Share Posted April 10, 2017 A standard servo in the 3.5-6kg range would be plenty Quote Link to comment Share on other sites More sharing options...
MattyB Posted April 11, 2017 Share Posted April 11, 2017 Putting aside the fact that no micro plastic geared servo is the correct choice for a petrol powered Tiger Moth, two additional points for the OP: As per other posters above, the "jumping" behaviour you describe is most likely ignition related. Check the connection at the ignition unit and the particularly at the plug cap to ensure RF emissions from the ignition are minimised. HS-81s are plain bearing servos. That means they start off accurate enough, but in a relatively short amount of time get sloppy and start to double centre. This is an effect I have seen on all the 81s I have used in slope soarers, so I suspect the effect will be exacerbated by the vibration of an IC engine, especially a petrol single cylinder. Quote Link to comment Share on other sites More sharing options...
cymaz Posted April 11, 2017 Share Posted April 11, 2017 Got to agree with the rest, HS 81's are too small for the job. I have HS 645 mg in any of my petrol models. Best of luck. Quote Link to comment Share on other sites More sharing options...
ChrisB Posted April 11, 2017 Share Posted April 11, 2017 David, You refer to 9g servos, they are tiny, park flyer size servos. How big is the model? If its powered by a petrol engine the chances are its at least 15cc, as most petrols are over 15cc with the odd exception. HS 81 at 4.8v is 2.6kg/cm, that is on the low side. I'd be using 5KG as a minimum on the ailerons, but is does depend on the size of engine and model. Hitec advise that certain servos should not be used with petrol engines due to vibration issues. Although this sounds like interference issues. Quote Link to comment Share on other sites More sharing options...
David Tayler Posted April 11, 2017 Author Share Posted April 11, 2017 Thank you all for very useful comments. Derek is reviewing the HT lead,which he admits was a bit iffy, and I am making some inductors on ferrite beads to go in the aeleron leads.The size of the servos is under review although the plane has flown well for a few years on the 9g servos,I guess that my friend Derek flies it gently in a scale manner.I will post the outcome to the forum when we have sorted it. Many Thanks,David Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted April 11, 2017 Share Posted April 11, 2017 Quick one David, ring inductors are very convenient But if you only have ferrite rods, wrap about six turns around the rod Then tidy up and secure with large shrink wrap Quote Link to comment Share on other sites More sharing options...
MattyB Posted April 11, 2017 Share Posted April 11, 2017 Posted by David Tayler on 11/04/2017 08:59:44: Thank you all for very useful comments. Derek is reviewing the HT lead,which he admits was a bit iffy, and I am making some inductors on ferrite beads to go in the aeleron leads.The size of the servos is under review although the plane has flown well for a few years on the 9g servos,I guess that my friend Derek flies it gently in a scale manner.I will post the outcome to the forum when we have sorted it. Many Thanks,David What size and weight is the model, and what engine does it use? This will help us to recommend the right servos, at the moment everyone is having to guess because we don't know these parameters. Quote Link to comment Share on other sites More sharing options...
onetenor Posted April 11, 2017 Share Posted April 11, 2017 I think any plane over 60" span needs 4/5 Kg servos on ailerons and possibly flaps too where fitted .An excess of force is better than not enough. Especially on long spans. Good luck Quote Link to comment Share on other sites More sharing options...
David Tayler Posted April 18, 2017 Author Share Posted April 18, 2017 Thank you all for your advice.I made two"filters" by wrapping the servo wires round ferrite rings,about six turns, and the jitter and odd servo behaviour has disappeared. It would seen that the cause was interference from the CD unit being picked up on the wing wiring,aggrevated ,maybe, by poor noise immunity on the Hitec servo.Derek is reviewing the rating of the servos in the light of the comments posted on this thread. Thanks.David Quote Link to comment Share on other sites More sharing options...
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