David Davis Posted May 21, 2020 Share Posted May 21, 2020 I normally use Hitec servos but one day several years ago my supplier was out of stock of the Hitec servos which I wanted and they asked me whether I would accept JR servos instead. I agreed to their suggestion and fitted them to the elevator and the rudder of the WOT 4 XL. They proved to be quite satisfactory prior to 2018 when I crashed the model through trying to land it too slowly and stalling in from about 12 feet (3 metres). Having repaired the model I naturally turned on the radio to check that everything was working. On turning on the receiver the JR elevator servo vibrated for a few seconds before centring itself. I did not think that this was normal so I replaced it with a new metal geared Hitec servo. A few days ago I switched on the radio and this time it was the JR rudder servo which twitched before centring itself. It was replaced by a new Hitec 311 servo. Yesterday afternoon was quite windy in central France so I didn't get to the flying field before 16.30. I put the model onto my Great Mate Stand and screwed the wing on finger tight. My intention was to get the engine, an old but perfectly sound Laser 150 V, running on both cylinders, not necessarily to fly. Having fueled up and having fitted a glow stick to heat one glow plug and cables to heat the other, I gave it a twirl on the starter motor. It started on one cylinder. I removed that pathetic glow stick from one cylinder and removed the igniter from the other, connecting it up to the non-firing cylinder. Soon I had the engine running on two cylinders! I hadn't intended to fly but now that I had my engine running properly, I thought I'd give it a go. After all the WOT 4 is a pretty heavy aircraft with a 25cc four stroke in the nose and I'm quite an experienced pilot so between us we should be able to handle a wind. I took off and got to height, I was enjoying myself! I did a few circuits then remembered that the wing bolts were only finger tight. I decided to land. I used the rudder to turn the model onto finals when I suddenly had no control and it crashed into the ground from about twenty feet. A post mortem was held by my French colleagues including a retired marine engineer and a retired aviation technician. It was found that the out put arm of the servo had shifted so that when the rudder stick on the transmitter was in the cental position, the output arm and the rudder were way over to the right. I had taken out the JR servo and replaced it with a Hitec 311 but I had not changed the output arm, this allowed the arm to slip on the splines and jam the rudder over. Now what does Homer Simpson say? I suppose that the socket on JR arms must be a bit bigger than Hitec arms. They probably have a different number of splines too. I'm told that Futaba servos have 24 splines while Hitec have only 20. Anybody know how many splines there are on a Hitec servo? Damage is largely limited to the fuselage bottom and the undercarriage. She'll fly again. Edited By David Davis on 21/05/2020 06:18:28 Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted May 21, 2020 Share Posted May 21, 2020 Look Here Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted May 21, 2020 Share Posted May 21, 2020 Sorry to hear about the damage to your WOT4 and you have unfortunately experienced the differences in manufacturers arm specifications. Could be a tricky one to detect when swapping parts over and I was told (and can see the logic) of not forcing servo arms back and forth with your finger/thumb. The rational is that the servo motor is massively geared down and resists being forced to rotate so the gears can be damaged. I do wince when people get their model out and waggle the control surface forcing the servo back on forth. Food for though as I collect all my spare servo arms in one big container! I think in future I'll keep them separate and marked up from which make/model servo they are spares for. Hope the rebuild does not take too long and thanks for the information. Quote Link to comment Share on other sites More sharing options...
Jon H Posted May 21, 2020 Share Posted May 21, 2020 Most unfortunate DD but easily done in the heat of battle. I have individual boxes that snap shut for all of my futaba and hitec arms to not only help me find the right one, but prevent me using the wrong one. Quote Link to comment Share on other sites More sharing options...
FlyinBrian Posted May 21, 2020 Share Posted May 21, 2020 I have had a similar situation swopping Hitec servo for a futaba, the futaba arm fits the hitec quite well but will slip uder load. Overall Futaba's 25 tooth spline is IMHO the best idea as by rotating the arm 90 deg on the spline you can finely adjust the centre position with minimum use of sub trims. Having said that I use mainly Hitec purely because I like and trust them. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted May 21, 2020 Share Posted May 21, 2020 Argh. There's good arguments for sticking to your chosen standard... Hitec definitely missed a trick with the 24/25 tooth thing. Although, now we have subtrim and whatnot it is not something that affects the geometry any more. DD, not sure if you know, but Ripmax website calls for an 8kg servo for that rudder. A Hitec 645 would be about right I think? A 311 is only a 3kg output. Quote Link to comment Share on other sites More sharing options...
Geoff Copping Posted May 21, 2020 Share Posted May 21, 2020 Posted by Chris Walby on 21/05/2020 06:45:12: Sorry to hear about the damage to your WOT4 and you have unfortunately experienced the differences in manufacturers arm specifications. Food for though as I collect all my spare servo arms in one big container! I think in future I'll keep them separate and marked up from which make/model servo they are spares for. Hope the rebuild does not take too long and thanks for the information. A timely posting, having recently got back into RC planes, from the various new models I've acquired I decided to put all the spare arms in a coffee jar. I too will now make a point of keeping them labelled. Thanks for that. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted May 21, 2020 Share Posted May 21, 2020 ...and look out for the old chestnut of swapping Hitec and Futaba servos, for example - they may rotate in the opposite direction! Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted May 21, 2020 Share Posted May 21, 2020 hard lines there David...don't know if I'm right or wrong but I thought that the servo arms were only for the brand they came with, so I've always stuck to the futaba servo/futaba horn etc.....never tried to fit another brand?...are they interchangeable or not?.. ken anderson...ne..1.servo dept. Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted May 21, 2020 Share Posted May 21, 2020 Most seem to use the Futaba standard these days Ken. Other than JR/Spektrum and Hitec of course. Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted May 21, 2020 Share Posted May 21, 2020 Posted by Geoff Copping on 21/05/2020 09:57:11: I too will now make a point of keeping them labelled. Thanks for that. A tip for anyone who keeps servo arms, or any other its for that matter, in those sets of clear plastic drawers. Rather than just writing the name of the item on the label, If you cut out the manufacturer's name, e.g. Futaba, JR, Hitec, from the backing card or box, and use that as the label on the drawer with the respective servo arms in, it makes it a lot easier to spot; particularly if like mine, the drawers keep finding their way back into different drawer compartments. For any other bits, I draw a picture of the part on the label, or if I have enough of them, stick one on the front of the drawer. Quote Link to comment Share on other sites More sharing options...
David Davis Posted May 22, 2020 Author Share Posted May 22, 2020 Thank you for all of your advice and sympathy gentlemen and Nigel, I will order a Hitec 645 for it. I have subsequently found out that on their standard size servos, Futaba servos have 25 splines, Hitec have 24 and JR 23. Quote Link to comment Share on other sites More sharing options...
Paul Marsh Posted May 22, 2020 Share Posted May 22, 2020 That's what I do, never mixed up before, then again, I have so much stuff, organize everything, props, fuel tanks, pilots, engines, servos, rx, etc, are all got their own space, labeled, cataloged and stored on a spreadsheet for stock taking purposes, so I know I've only got so many 2mm metal clevis left and order more. Quote Link to comment Share on other sites More sharing options...
Jonathan M Posted May 22, 2020 Share Posted May 22, 2020 Excellent organisation Paul. I think everyone has explined themselves very well indeed! When I'm finished installing a set of servos in a model, I empty all the unused bits into a spare little plastic bag with a self-sealing closure (senior moment, forget what they're called! but as in bottom-left of photo above) and write on the outside exactly where they're from. As an avid-collector of random little bags, shoe-boxes, plastic food-packaging, tins, jam-jars and the suchlike, all the little plastic bags are usually then found in the same place... just have to remember what place that is! PS - David, glad the WOT4XL will live to fly another day! Quote Link to comment Share on other sites More sharing options...
Nigel R Posted May 22, 2020 Share Posted May 22, 2020 The 2L ice cream tubs make excellent and free (once you've eaten the ice cream) workshop organisers Not quite as neat as Paul's parts drawers but I have a tight budget (and a young family who seem to eat quite a lot of ice cream). Quote Link to comment Share on other sites More sharing options...
Paul Marsh Posted May 22, 2020 Share Posted May 22, 2020 Best to get one of these: Label the box: Quote Link to comment Share on other sites More sharing options...
David Davis Posted May 31, 2020 Author Share Posted May 31, 2020 Posted by Nigel R on 21/05/2020 09:17:24: Argh. There's good arguments for sticking to your chosen standard... Hitec definitely missed a trick with the 24/25 tooth thing. Although, now we have subtrim and whatnot it is not something that affects the geometry any more. DD, not sure if you know, but Ripmax website calls for an 8kg servo for that rudder. A Hitec 645 would be about right I think? A 311 is only a 3kg output. Going on your advice Nigel I bought a Hitec HS 645 MG servo and installed it in the model to drive the rudder. Transmitter on, receiver on and... the rudder juddered for several seconds before coming to a central position. I then recalled my club president saying, we have presidents in France rather than chairmen, that if you installed an analogue servo next to a digital servo, the digital servo would have this effect on the other servo. Sure enought the elevator servo was a Hitec D645 MW digital servo! Fortunately I had several unemployed Hitec HS 5485HB servos new and in their boxes. These are also digital servos so I installed one and the problem went away! Ok it's not metal geared but the gears are Karbonite, it's a 6.4kg servo and I'm no CPLR so it'll probably be alright. I seem to be building up a collection of analogue metal geared standard servos. Must build something to to fit them in! Quote Link to comment Share on other sites More sharing options...
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