From: Idris Francis, Petersfield Hampshire, UNITED KINGDOM

Transcription

1 Problems with E Flite electric retract under-carts, and JR style connectors. as posted at following earlier discussions. From: Idris Francis, Petersfield Hampshire, UNITED KINGDOM idrisfrancis@fightbackwithfacts.com Thanks for your most helpful post, confirming as it does - and more - my findings. What follows is intended to be my final assessment of these units: As before, the fundamental design error is the use of a lead-screw, notoriously inefficient at transmitting power because of high frictional losses inherent in rubbing the two threaded surfaces one on the other. Unless of course a ball-bearing lead screw were used, though unlikely to be available in this size or suitable for the forces involved if it were. Nor does the small size of the motor help efficiency. Having now removed the drive motor and electronics (using a cutting disc as I cannot find a Hex key to remove the screws) I can confirm that what appears to be just a motor driving the in-line lead screw does in fact have a tiny 3 stage gearbox built in to the end. I suspect that in those small sizes with a small number of teeth efficiency is not too good, It is at least arguable that were the overall gear ratio and the thread pitch both twice as great the inefficiency would be significantly less and available pull significantly greater. In my view the speed could be halved to improve force, and still be fast enough (though as doubling the ratio would also double the force, speed would not fall by that same factor of 2) This might go some way to reducing the several problems which arise from such low efficiency: 1/ The system struggles to overcome friction in the lead screw and (as you mention) in the locking pin etc. My own test on a complete unit, done by holding a finger lightly against the wheel when it is being raised, shows that available pull before stalling fluctuates a great deal - a graph would be a saw-tooth. This is because of the less than perfect fits and surface finishes moving one on another. In my case the margin for error - the difference between worst running current and electronic trip - was so small as to be unusable, the slightest touch would stop the wheel moving. 2/ 5 cells instead of 4 made marginal improvements but nothing like enough to make it reliable. 3/ One time in 6 or so, one leg or the other would refuse to unlock and extend. 4/ Unwanted cycling of the controls, presumably because of the electronic trip being activated, occurred quite frequently. At times merely the metal to metal contact of a screw touched against the mounting frame would cause one or other unit to cycle. 5/ On 4 cells the maximum current when the legs continued to move, for 2 units, was close to 1 Amp. When stalled 2 Amps with a buzzing sound until the trip activated after 2 seconds or so. On 5 cells it was 2.7 Amps. 6/ Because such currents would cause terminal voltage of normal Rx batteries to drop to dangerous levels - loss of decoding, delayed recovery, and slow throttle fail-safe activating - I would not dream of operating them from the Rx battery even if I could make them work reliably - unless perhaps in the smallest size - and in my view and those of others I have read, using the Rx battery is just asking for trouble. Having removed the motor and cut off its lead screw I could operate the unit by hand. It immediately became obvious that the transverse locking pin was marginally tight in it slot, on one side. This caused it to tilt, making things worse. It was then also clear that the sharp corners in the frames between the locking slots and the transition slots were too tight. There is no need for the corner to be sharp, so careful work with a needle file freed the whole

2 thing up. Before doing so, even without the under-cart leg fitted, pulling the linkage free from the locks took significant force, afterwards no force to speak of. It also became clear that the diameter of the transverse pins being so small, the slightest tilt of a pin could give rise to significant friction at its "shoulders" where it passes through the side frames. In one unit I found insufficient transverse clearance between the frames for the transverse pin and the alloy block to move smoothly if the securing screws were properly tight and had to fit shims to obtain clearance. That the fit of these parts is so critical helps explain why some have problems and some do not - all depending on the clearances (which of course have to be minimal because of the 30 to 1 magnification seen at the wheel) It may also be that it is with the larger units, longer legs and heavier wheels that these problems are most severe. Having removed the drive system and freed up the mechanism I decided that marginal operation and unwanted cycling were simply not acceptable, and the obvious solution would be a more efficient motor with conventional gears. Fortunately I had precisely such a mechanism immediately to hand - a retract servo! Initial tests showed that even with a 7" leg and 4 ounce wheel, a bog-standard 60 degree Futaba ball race servo would handle the forces involved in one unit. A 17 kg cm 160 degree retract servo would of course do so with torque to spare. Because the aircraft in question is a TopFlite Thunderbolt 63" one, already built, it would have been very difficult to retro-fit the long linkages that would have to pass between the wheel wells and the upper skin to reach the centre servo bay, and bench tests showed that such long linkages need to be very stiff to prevent bowing that would prejudice secure locking at one end. And of course tricky to set up because the locking pins are not visible when installed in the wing. Plan B was therefore to cut off the unwanted ends of the frames and bolt directly to it each unit the two retract servos I had to hand, so that the output arm of the servo was on the centre line of the retract unit. A single nut and bolt will suffice as long as the far end of the servo is then supported in the wing.) By a stroke of luck I had a steel rod with a threaded end that matches the transverse pin and installed it as a push rod about 2" in length between the locking pin and the servo arm. These being proportional servos it was easy to match the travel to that of the locking positions before installing in the wing. I had to cut away enough wing skin to allow the servo to fit, but not that much and not difficult to rework. I could have used the frame and servo to reinforce the structure but on this wing it does not seem to be necessary. I now have - at considerable overall expense in parts, time and frustration - under-carts which work every time, do not jam up, which lock precisely in place at either end and which have considerably more servo power available than they need - so much so that the servo does not noticeably slow down when lifting the leg. This incidentally confirms a point I made earlier, that the efficiency of the standard unit is of the order of single figure percentage points. Because the servo arrangement is so much more efficient it will not result in excessive drain from the Rx battery. As for future retract models - at least one of the small cheap units, working on much the same principle as the mechanics of the E Flites are entirely satisfactory in models up to say 56" span. And that same unit can have grub screws fitted to adjust minutely the play when up and down. I cannot envisage paying E Flite prices for units that I then have to convert to servo operation after fine-tuning the fit of the parts - I did so on this one occasion because it was the easiest way out of where I had found myself. I have taken a few photographs which I will add here shortly (dated 8th August).

3 If on the other hand E Flite decide to offer just the mechanisms without the drive system - as Robart do as an alternative to their pneumatic retracts - and at a significantly lower price, then they would be an attractive option. Especially if the frames were arranged to accept servos bolted on and quality control of the moving parts were improved. All of this and previous postings are of course only my opinion, others are free to disagree, and I have no commercial reasons whatever for making these observations. continued Connector Problems. When installing these E Flite electric retracts in a wing, and subsequently re-installing them with Spektrum 7000 retract servos fitted I was puzzled when the leads fell out of the supplied Y lead, several times, when I simply turned the wing over, despite my not having tugged them, as far as I was aware. On closer inspection I found that there was so little grip between the individual connectors within the plugs and sockets that they would almost fall apart under their own weight and that of the cables - and would certainly do so under even modest G forces in flight. In summary: 1/ All connectors involved that were loose were new, as fitted to the retracts and servos and all JR style with brown, orange and white leads. 2/ Given that "pattern" connectors are made by manufacturers other than the original manufacturers of the different types, it is not possible for me to know who manufactured these particular plugs and sockets. 3/ Cross-checking these plugs (each of which contains 3 individual formed sockets) and sockets (each of which contains 3 individual plugs) confirmed that in terms of inadequate mating forces, or grip: i/ The new plugs were a loose fit in all the cable sockets I tried, old and new (I cannot now remember checking them on the solid square pins in Rxs) This includes the Y lead plugs checked in the sockets at the other ends of the same Y lead. ii/ All other plugs I had to hand (mostly Futaba but a few old JR) were acceptably tight in all sockets including both old and new JR types. iii/ 4 new Futaba style i.e. ("pattern") extension leads I bought last week, are so tight that it is quite difficult to grip the plastic parts well enough to join or separate them. On the basis that the force needed to mate or separate the connectors is directly related (through mu, the coefficient of friction) to the contact pressure between the contacts, I considered these plugs unreliable and unsafe, even if physically tied together, and I replaced the entire lead on each servo and the entire Y lead (the E Flite Y lead in any case having inconvenient lengths) Naturally I complained about these loose-fitting plugs, that seem to be to be downright dangerous, to my everhelpful model shop, who then copied to me Horizon Hobby's response, that a/ They must have thrown away the cables under complaint with packaging of another item returned at the same time and therefore could not test them. b/ They have tested current stock and been unable to find any such problem (though forces needed to mate or separate were not quoted and so this seems to be more of a subjective assessment than a measurement). c/ All plugs of any manufacture would be expected to make proper and firm contact with the solid steel pins in Rxs - I now know that these are all made to an international specification and are therefore very consistent in size and shape.

4 d/ However the individual pins (that as I now know, have to be formed from thin sheet to allow crimping to the leads, rather than being solid) can vary from manufacturer. e/ For this reason, plugs should only ever be connected to sockets made by the same manufacturer, and not "mixed". f/ I should solve the problem by using plastic clips or heat-shrink tube to prevent the plugs and sockets coming apart. g/ No indication was given that any further action would be taken regarding my report of these problems, though I was offered a replacement Y lead - which I refused. h/ It is the responsibility of the modeller to ensure that proper contact is maintained. My view of the above response is that: h/ I fail to see how the individual sockets within plugs, that provide consistent reliable contact on solid Rx pins would fail to make the same on formed pins designed to the same nominal size and shape. j/ In all my R/C years I have never before been told by anyone that different makes of nominally compatible plugs and sockets should not be mixed, and indeed the supplying dealer assured me, before Horizon responded, that they can be. k/ Many cables on the market are clearly "pattern" parts, which according to the above theory should not be used! l/ Once out of their packets these pattern parts are indistinguishable from those they mimic - so how could anyone know which is which? m/ No buyer of a servo knows who manufactured the plug on its lead, whether the originators of the design or a copier. n/ For all these reasons, I refuse to accept that different brands of plugs and sockets should not be mixed, and will continue to do so, as long as careful checks confirm adequate grip and reliable contact. o/ I would not dream of launching a model that relied on physical clips or tape to hold together connectors that had virtually no grip of their own - to do so would in my opinion would amount to negligence, with all that it would imply in the event of a crash. p/ If I am mistaken and it really is the case that different brands of similar connectors should not be mixed, it seems clear to me that this is an unacceptably dangerous situation that should never have been allowed to develop, and requires widespread publicity (a) about the nature of the problem, through the national agencies and magazines and (b) detailed advice on how to check for reliable and acceptable contact. For these reasons I am in touch with the BMFA and others to try to sort this out as a matter of urgency I confirm that I have no commercial or other interests in these matters other than the safety of model flying Idris Francis 08 Aug 2010 idrisfrancis@fightbackwithfacts.com

5 . Sorry if anyone thinks this long-winded, but I tried to make it complete in itself and as helpful as possible. I will put the same on the few other web sites where I have seen this subject raised.