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Post by Pij on May 28, 2008 18:19:02 GMT 10
I cut 15mm strips from the lead flashing I bought. The lead was about 2mm thick - it is called 30kg flashing, I think. Maybe 30kg/m 2? I weighed out 79g of lead, the same weight I took out of the Duck during stripping. It turned out to be too little - no surprise, considering the extra TE taping and the fin modification. Quickly taping on extra weight, while watching the balance at the 205mm mark, I got it to balance and took it out for test chucks. Later I weighed it at 554g, 30g higher than it was before the reno. If I had left the fins standard, I would have been pretty close to the original weight, I think. As usual, the first chucks were pretty awful, but after I got the trim sorted, it was OK. I'm looking forward to some real flights. Regarding the change of servos: The Hitec servos fitted exactly in the spaces prepared for the previous JP (like eSky) ones. BUT, the first thing noticed was that I had to reverse the servo settings for both aileron and elevator. I fitted proper servo covers this time instead of my razor-covers. It took me a while to get the throws adjusted, but as far as I can measure I have got them exactly as per the instructions, and exactly equal up and down (with 7% differential to achieve that, cf. the 10% I had dialled in previously). I have no exponential set, but am considering using up to 30% on the elevator and none on the ailerons. This was all done using the shortest servo arms supplied ie the 4-arm thing with 3 arms cut off with toenail clippers. I'll try to remember to get some photos of the finished reno. It has temporary ballast taping, but it will do for the first flights. The LED seemed very bright as the sun went down, but when the Duck was inverted on the ground, there was less than 180 o field of view of the light. The LMA went off at irregular intervals. The battery could be low.
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Post by felix on May 28, 2008 19:48:01 GMT 10
hey that looks great mate,well done.did you use that servo stop idea? btw looked at your spar,well designed and you have alot more patience than me lol.
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Post by Pij on May 28, 2008 20:14:03 GMT 10
I was going to use your servo-stop idea inside the servo covers, but I found that when the glider is switched on without the receiver on, the servos go into a "park" position, one forward and one back. The "stops" would have obstructed the servo trying to park forwards, and would then probably contribute to gear stripping. So I had to omit the servo stops, unfortunately I can give you more details on that spar if you are interested. Just ask a Qn in that thread. I'd love to see your composite spar too. Looking forward to the predicted winds over the next few days. Could put those extra 30g to good use
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Post by felix on May 28, 2008 20:42:20 GMT 10
the spar i'll be using is a little more solid than that one.using a material and resin sandwich for mine,not much of a pic seeing as though it looks like a black stick lol.i have it taped to 2 blocks at the mo in the workshop for testing.basically evertime i walk past it i stand on it,kick it,hit it with something etc lol.so far it is holding up well but may be a little brittle as the sharp edges show signs of chipping.sanding the edges round will solve that though i hope. also looking forward to these weekend winds,be finally able to do something other than stagger around in the sky
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Post by chamm37 on May 28, 2008 22:50:39 GMT 10
you going to be at SC felix i am going to be down there on saturday i have rebuilt my old bee and have done some modifications to it as well test on saturday.... i have made the sweep back a bit more and i have put in some more spars and put a centre fin on the back instead of the 2 side fins ;D We will see how it goes thus weekend
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Post by Pij on May 29, 2008 7:31:21 GMT 10
That sounds like a renovation Cham Do you have a picture for us?
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Post by lucky on May 29, 2008 11:52:41 GMT 10
The LMA went off at irregular intervals. The battery could be low. Mine do this too, I investigated and found it usually happens when a servo is at it maximum travel and under a little mechanical load, the battery gets a spike in current draw below 4.2 volts temporarily and the alarm blips for a sec. Its nothing to worry about. I noticed this first flying down Shorncliffe with the lads, it only beeped when another CW approached it, weird!! But it was just that when another wing is near you in combat you are likely to put in full stick lol to either hit it or miss it!
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Post by Pij on May 29, 2008 15:29:58 GMT 10
No mine goes off in a regular sense, continuous alarm not just a blip. Occasionally moving the sticks will shut it up, but more often it continues until I plug in the "Off" plug then turn it back on again.
I did notice this while installing it, too.
The metal-detector effect of the LMA is a weird one. Has anyone else noticed a dramatic change of pitch when a tiny bit of metal, like a staple, is moved nearer or further away?
I've been out for a morning of flying in strong wind. I still don't really have an "After" photo to finish off the reno, but I'll take one. I do have some action shots from today's flying, however, which I'll post in the Flying Reports.
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Post by Pij on May 29, 2008 18:53:19 GMT 10
I just took the "After" pics and weighed the Duck after today's ballasting. It weighs in at 569g now, a big increase on its pre-reno weight, but it needed it today. The photos show the new servo covers and the silver tape covering the temporary nose-ballast on top and the not-quite-fitted weights in the ballast box underneath. You can just see the ends of the 2nd of 3 bamboo skewers in the fins, about 2 flutes from the back.
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Post by GotrekGurnisson on May 29, 2008 19:45:05 GMT 10
I also want to do something about the tear-zone near the inner end of the elevon. It has only shown up as a tear in the covering, but I can see that there is a stress on everything at that point. That's exactly where my Duck cracked the trailing edge ply and then from there developed into a 10-15cm long tear about 5cm or so in from and parallel to the trailing edge. It wasn't fully stripped back during the repair. I'm doing a full rebuild at the moment and I realised that the trailing edge ply was only put on with 3M77 when Mark and I originally built it. It had started to lift completely off. I've used Vise glue to reattach it now, so that should add some additional strength, although I think I might have been better replacing it outright with a carbon ribbon spar.
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Post by Pij on May 29, 2008 19:57:41 GMT 10
I just doubled up on the filament taping there (which adds strength but won't totally stop the flexing) then sanded the tape down hard and rubbed flexible CA into an area a bit smaller than a 20c piece in that spot. I'm hoping to strongly resist flexing in that spot without making it totally rigid. I fear that making it totally rigid will result in either a) the rigid structure breaking or b) just transferring the flexing to the next place along the weakest-link hierarchy.
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Post by felix on May 29, 2008 20:03:19 GMT 10
gotrek that area for some reason has a mass shear stress riser effect for some reason (still trying to work out why???) and i really doubt even carbon will survive for any lenth of time.if it did i reckon the loads would only end up surfacing elsewhere.i have a few untested solutions i believe may work. 1.maybe lay some ply though the wing that intersects the very corner of that joint.it would have to be in a 45 degree angle though and would be best if it went forward and met the spar.that'll distribute the load evenly over more area and any "peak" loads along the spar 2.an easier option would be to radius that turn as much as possible.can be done by curving the duck tail and making the end of the elevons to suit if cosmetics is a problem. 3.other way could be is to stop drill the corner (a similar method is used on plexiglass or fibreglass to stops cracks going further).say at about 10mm along the crack drill a 1/4 inch hole through the foam. 4.drill a quarter inch hole over the actual corner so that the corner void actually forms a pie slice in the circle cut.that would also spread the loads over more of the material (not sure that'd be enough though).the tape or maybe thin card would clean up the hole. 5.cut the tail off and jump on it! ;D
hope that makes sense to anyone bar me lol....could maybe post some pics to make clearer if that helps
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Post by Pij on May 29, 2008 20:22:29 GMT 10
gotrek that area for some reason has a mass shear stress riser effect for some reason (still trying to work out why???) and i really doubt even carbon will survive for any lenth of time.if it did i reckon the loads would only end up surfacing elsewhere.i have a few untested solutions i believe may work. 1.maybe lay some ply though the wing that intersects the very corner of that joint.it would have to be in a 45 degree angle though and would be best if it went forward and met the spar.that'll distribute the load evenly over more area and any "peak" loads along the spar 2.an easier option would be to radius that turn as much as possible.can be done by curving the duck tail and making the end of the elevons to suit if cosmetics is a problem. 3.other way could be is to stop drill the corner (a similar method is used on plexiglass or fibreglass to stops cracks going further).say at about 10mm along the crack drill a 1/4 inch hole through the foam. 4.drill a quarter inch hole over the actual corner so that the corner void actually forms a pie slice in the circle cut.that would also spread the loads over more of the material (not sure that'd be enough though).the tape or maybe thin card would clean up the hole. 5.cut the tail off and jump on it! ;D hope that makes sense to anyone bar me lol....could maybe post some pics to make clearer if that helps I understand sheer stress, but not "riser". I think that in high speed flight the sheer force on that point could well be equal to the strength of the servo (the ability of the servo to resist the tendency of the airflow to push deflected elevon to the undeflected position). The air would still be forcing the elevon's TE down but the servo prevents that happening, so the force is relayed to the foam immediately ahead of the hingeline. It is greatest at the inner end of the elevon because further along the elevon that force is relieved by twisting of the elevon itself. I don't think option 2 would work, because the fixed TE piece is inflexible balsa and the stressed area is very flexible foam. The force would just tear the joint between the two.
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Post by felix on May 29, 2008 20:42:12 GMT 10
stress riser is a vague term used to describe poor design features that allow a part of any component to exceed it's strength capability (goes for wings,suspension,engines components,roll cages,bridges ,whatever)
that said just done a little thinking.....what i think is happenin is this: when the wing is at a positive angle of attack the trailing edge is trying to twist upwards.during a turn one elevon is applying an opposite downward pressure.where those two opposing forces meet applies a shear force to the foam.that explains why the crack forms from back to front right at that junction.also explains why no duck tail equals no tear!!! by radiusing that tail (option2) the shear force would be distributed at a more even rate along more of the foam surface maybe?
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Post by Pij on May 29, 2008 20:48:35 GMT 10
I think it would be spread more evenly along more material, if the ducktail was made of the same stuff as the main body. But I feel that because the ducktail is made of relatively rigid balsa, and the stressed section is made of flexible foam, it will just result in separation. To radius that junction, the balsa ducktail would have to be cut at a curve, wouldn't it?
Unless you meant to radius the junction by adding a radiused section of foam. That could work, I think. We might be imagining 2 different arrangements here.
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