Yorkman Posted July 21, 2008 Share Posted July 21, 2008 Went back to try the York again today. Since the last abortive flight ended in a spiral dive into the corn, I have :1/ Bought a Spektrum Dx6i2/ Moved the c/g forward3/ Put smaller props on (and checked motors/esc/battery after full power run-all ok)So,range check carried out-satis.Bit blustery, but perfectly flyable, all controls move as requested, off we go.With the smaller props on, ground roll is as it used to be-protracted-but lifts off and climbs away in her usual fashion.There follows a couple of minutes of -erratic!-flying as I try to get the damn thing trimmed into some semblance of straight and level-quite a lot of nose down needed, as well as fighting a constant determination to turn right...Then as we (I've just about caught up with the plane by now) complete a slow orbit to the left back towards what would be base leg, she suddenly pitches sharply up to the vertical...and flops over the top of a 'loop' (not something the full size ever did, i bet!)...Mindful of the fact that last week I 'drove' the thing into the ground on full power trying to recover, I chop the throttles-and she pulls out of the dive without a hint of a problem, I open the throttles, drop the flaps, and stick her straight back down on the strip..and try to calm my nerves!Now what on earth happened?The only thing I can think of is servo runaway-but...can that happen? She has a hitec mini servo on the tailplane for elevator control, on a long extension lead...Look forward to hearing people's thoughts.Martin Quote Link to comment Share on other sites More sharing options...
Piper Cub Posted July 21, 2008 Share Posted July 21, 2008 Motor thrust lines not set properly? If you're having to hols in down elevator, that sounds like the motors dont have enough downward thrust. Always wanting to turn either suggests both props are turning the same direction or insufficient left thrust.Have you tried a trial glide and see what happens ie get someone to throw it nice and level with the motors not running to see if it still wants to turn left or pitch up? If it does this with the motors not running, it sounds like you got some serious warps in the airframe. Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 21, 2008 Author Share Posted July 21, 2008 Hand launch not really on the cards....a bit hard to get hold of!Going to try again tomorrow and if I get sufficient height without anything unexpected happening, will cut the power and see how she glides.My concern is the undemanded pitch-up... Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 21, 2008 Author Share Posted July 21, 2008 Oh-and have moved the battery forward another inch.. Quote Link to comment Share on other sites More sharing options...
Piper Cub Posted July 21, 2008 Share Posted July 21, 2008 Where's the Cof G? Should be around a third of the way behind the leading edge. A C of G too far forward can cause probs too. Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 21, 2008 Author Share Posted July 21, 2008 c of g is a little forward of where it always used to be-and she flew for 2 or 3 years without problemshttp://www.modelflying.co.uk/forum/forummessages/mps/dt/4/UTN/4018/V/1/SP/that's the start of this story Quote Link to comment Share on other sites More sharing options...
winchweight Posted July 21, 2008 Share Posted July 21, 2008 Cof G sounds very suspect. Never heard of a quality servo running away. Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 21, 2008 Author Share Posted July 21, 2008 just had a quick measure- C/G is at 4 inches on a root chord of 11.5 inches.But, as I said-she flew balanced here many times no problems-though a lot heavier Quote Link to comment Share on other sites More sharing options...
Lee Bentley Posted July 21, 2008 Share Posted July 21, 2008 From what I have read so far, CG aside, your main query seems to be the sudden undemanded full up elevator. Incorrect CG wouldn't cause that. I have seen something similar at our club and it turned out to be quite simple. I'm not sure about the Dx6i but it happened with a Futaba 35mHz rig where the pilot had 'FAILSAFE' enabled, on checking the settings he had up elevator set on loss of signal. Guess what, he lost signal and the aircraft suddenly pitched up! Signal was regained quite quickly and was able to land safely,We all then retired to the club house for a drop of 'water' to calm our nerves.... Lee Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 21, 2008 Author Share Posted July 21, 2008 well, that was my first thought, as it was the first flight with a new radio...but 'failsafe' on the spektrum (supposedly) leaves all flying controls in their previous positions and closes the throttles-neither of which happened.and the thing was barely, what 50 yards away, in completely open ground, not somewhere you'd expect interference...I'm puzzled. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 21, 2008 Share Posted July 21, 2008 Failsafe does NOT operate on any channel in the DX6i ( or DX7 ) other than the throttle. It does rather sound like an oversensitive elevator ( rearward COG ) but if you are happy that it is not, then try exchanging the elevator servo, ensure that the receiver is being fed with sufficient power ( BEC or UBEC or seperate battery ? )Crank your tr aerial at 90 degrees to the case top - maybe you flew into the "doughnut of death" ?Your right yaw is possibly inbalance between the 2 motors / props , and you could always cheat it out by having rudder trim...possible mixed proportionally to throttle as master.PS I take it your receiver is QC enabled....did you check for flashy LEDS after landing?? Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 21, 2008 Author Share Posted July 21, 2008 Hi Timboyes i checked the qc and that works, didn't look at the LEDs, I have to confess.the brownout scenario just doesn't fit as all the 4 motors were still turning at unchanged power setting as it shot upwards..the yaw problem is fairly easy to check-as soon as i get some altitude! throttle back and see if it goes away...actually....just remembering....this model has a history of elevator servo failures...though never in the air..think it's on its third...I'd forgotten all about that..was a few years back..recommend a good mini servo? Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 21, 2008 Share Posted July 21, 2008 Loads to choose from these days...but HS85MG is my preffered weapon of choice for minis.Just another thought...you say a long extension is fitted to this servo - interference should not really be an issue on your 2.4G but how about voltage drop ? Is it heavy duty srvo cable- and are you using 6V or 5V BEC Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 22, 2008 Author Share Posted July 22, 2008 cancelled todays test for further investigation..think will replace servo for peace of mind, UBEC should arrive today (certainly will before next flight) and will set it up on 6v, and try again! Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 22, 2008 Share Posted July 22, 2008 Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 22, 2008 Author Share Posted July 22, 2008 Hi Jetsomeelevator servo as no1 suspect has been terminated...replaced by a newer,faster,better model....well, the 'mg' version as recommended by Timbolinkage is about 2" long bit of ally tube-no slop there!onwards and (hopefully) upwards! Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 22, 2008 Author Share Posted July 22, 2008 the flaps are fine.they operate off a single central servo so can be ruled out of the spiral dive question.but I take on board what you say about forces/linkages etc. Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 22, 2008 Author Share Posted July 22, 2008 no mate, decreasedgone from 2 nicad packs to one lipo, but, good point about stall/spin...don't think that's what happened, as it actually needs quite a bit of abuse to make it stall, and despite not being very much in control at the time, it was in a semblance of straight and level flight, before suddenly spiralling down.. Quote Link to comment Share on other sites More sharing options...
Former Member Posted July 22, 2008 Share Posted July 22, 2008 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 22, 2008 Share Posted July 22, 2008 Most of these points have been covered in the post earlier Jetsome Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 23, 2008 Author Share Posted July 23, 2008 Yep, did that. 5 minutes flat out against the garden shed. Battery barely warm, ESCs slightly above ambient, motors hot but not cooking.Rx voltage will not be a factor from now on as 6v UBEC fitted.Next test pencilled in for next Monday. Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 23, 2008 Author Share Posted July 23, 2008 Thanks pal. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 23, 2008 Share Posted July 23, 2008 Yep...we all have our digits crossed ! Quote Link to comment Share on other sites More sharing options...
Alistair Taylor Posted July 23, 2008 Share Posted July 23, 2008 Good luck with the tests YorkmanYou said that this model had been through several elevator servos already, and that you needed a fair bit of down trim on the elevator to keep the nose down under power. Might this be putting sufficient strain on the elevator servo in flight to cause premature failure and (which might explain the involuntry loops) occasional blowback of the elevator, not to mention increased current draw in flight? Looks to me like the elevator is sitting right in the fast (and turbulent) prop wash from the inner motors, which probably wouldn't help the situation.If this is the case, sorting the motor thrustlines would help, as would stiffening the elevator linkages and hinges (is it in two parts?), sealing the hinge line, and (as I think you've just done) fitting a more robust servo.Hope this helps (and makes vague sense) AlistairT Quote Link to comment Share on other sites More sharing options...
Yorkman Posted July 23, 2008 Author Share Posted July 23, 2008 Thanks for your thoughts Alistair.Yes, it's puzzled me that the elevator servos seem to have such a short life-I've got a 1/4 scale ASW 17 built in 1985 still on all its original gear! The elevators are quite big, but, therefore, don't actually move very far and the linkage is short and direct (the servo is mounted on the tailplane), and large control inputs (hopefully) aren't required-it's not like it's aerobatic! However, I've put a metal gear solid Hitec in there, upped the Rx voltage to 6v, the proof will be in the flying Quote Link to comment Share on other sites More sharing options...
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