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Geoff Smith 112/06/2011 11:17:11
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545 forum posts
88 photos
Hiya all. Come up against a problem with the build of my Webbit. Now everything looks straight(ish) but the No1 eyeball spotted a distinct list the left with the electric motor. It wasn't till then I realised there was a problem. I have offered the wings to the fuselage and surprise, surprise the measurement is spot on from the wing tip to the end of the fuselage the problem appears to be in a former that is not quite straight in the fuselage. I have tacked a piece of balsa to the rear of the fus and on a level (checked) surface I have used a plumb bob and discovered that the fuselage is about 2mm out of true. This may not seem a lot but it sticks out like a sore thumb. The only way out, as I see it is to sand the front cheeks down on one side until it stands straight and attach the motor to the front former, this will help with the CoG as well. Anyone have any better ideas considering the nearness of our trip to Greenacres. Oh and BTW can you lot sort this weather out before the 24th.lol Cheers my friends.
Geoff
 
Very Wonkey Webbit Builder No 20
Biggles' Elder Brother - Moderator12/06/2011 11:32:53
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15748 forum posts
1460 photos
OK Geoff, item number 1 - fear not, I'm sure we can sort this!
 
Now from what you say I imagine that the motor is angled to the left slight when viewed from above - is that correct?
 
If so the first thing to realise is that in an aeroplane wonky fuselages in themselves are not a problem. In our models the fuselage only does two jobs - a place to put the radio gear and its the bit that we fix the wings, motor and tail assembly to. Only the latter of these two is important from a flying performance point of view!
 
What I'm saying is; provided the wings, the motor and the tail are aligned OK then nothing else about the accuracy of the fuse really matters.
 
So, double check. Are the wings and the tailplane/fin all reasonably square? If so the problem is just with the motor mount - everything else is just cosmetics - and as I said previously you wont see that once its in the air.
 
To square up the motor I would suggest the use of a few washers behind one side of the mount. If it points left, then place the washers under the left side of the mounting - thus packing it up. Much easier than sanding the nose square and adding another former!
 
BEB
Lindsay Todd12/06/2011 11:40:57
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1705 forum posts
1707 photos
sound advice BEB, no point in creating work if you don't need too.
Andy Gates12/06/2011 11:43:21
638 forum posts
20 photos
Geoff
 
I do not know the Webbit directly but I will try and help.
 
I am not quite sure exactly which way the twist is from your description (bit early for me on a sunday).
 
I will recap.
 
You are standing your fuselage on the front former on a flat level bench, and dropping a plumb line from the tail post which it is showing out of plumb by 2mm.
 
If that is the case, it may be OK depending where you are viewing it from.
 
If you are viewing it from the underside of the fuselage and the plumb is to the right then that sounds correct. Likewise if you are viewing from the top and the plumb is to the left then that too probably OK. This is to offset the motor to reduce a yaw charecteristic and is found in the vast majority models.
 
If they are opposites to the above, then you have added left thrust instead of right thrust. Easiest solution is to mount the motor with a packing piece under the left of the motpr to correct the alignment.
 
I know what you mean by deadlines, I am also targeting the same weekend for a meeting and my 84" machine is still in the balsa stage.
 
Good luck
Geoff Smith 112/06/2011 11:44:13
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545 forum posts
88 photos
Thanks for this BEB. The wings are definitely spot on as I took a measurement from the wing tip to the rear of the fuselage and it was exactly the same both sides. I have offered the tail assembly to the fuselage and, shaky fingers apart it seems to line up. I haven't attached the tail yet because I am changing the operating system from snakes to pushrods. I felt that the snakes exerted too much pressure and as I am only using 9g servos I wanted to keep it as free as possible. Do I need any side/down thrust on the Webbit? Steve seems to think not but I notice a lot of the lads have put some in. Cheers mate.
Geoff
 
Very Wonky Webbit Builder No 20
 
PS Do I get a prize for being the last to finish? LOL
 
G
 
Geoff Smith 112/06/2011 11:50:07
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545 forum posts
88 photos
Thanks for this Andy. I am viewing the plumb bob from the top of the fuselage and it is to the right so it is the wrong way, something to do with a bloke called Murphy I think. I have found some longer screws so now I need some small washers for the backing. Cheers my friend and good luck with your build, so what are you doing talking to me on here? get on with it man.
Geoff
 
Very Wonky Webbit Builder No 20
Richard Harris12/06/2011 12:00:09
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2116 forum posts
1965 photos
Geoff,
You are worrying too much, it will be fine
 
Now this will sound a bit odd. When I check the thrustline of my motor I use a piece of wood, say 350mm x 20mm x 6mm thick with a hole drilled in the middle. This is then bolted on to the prop driver, I make sure the wing is square measuring from tip to tail.
Measure from the wood to LE each side and adjust the motor to suit until it is parallel.
 
The reason, my eyes are not the best in the world!
 
Rich
Biggles' Elder Brother - Moderator12/06/2011 12:44:16
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15748 forum posts
1460 photos
Regarding a bit of side and down thrust - I would get it square first and then fly it an see. If it does need a bit right or down thrust just add another washer where approriate!
 
I think Richard is dead right - you can worry about these things too much. If you get it more or less square, so its not miles out, a bit of trimming will usually do the rest to be honest. OK, not the right attitude for a pattern flier maybe, but for those of us just out for a bit of fun you'd be amazed what you can get away with and it'll will still fly OK!
 
BEB

Edited By Biggles' Elder Brother - Moderator on 12/06/2011 12:45:23

Geoff Smith 112/06/2011 13:48:27
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545 forum posts
88 photos
Thanks for this gentlemen. Unfortunately I am a perfectionist and a worrier which is not the best of combinations but I cope. What I have done this morning is cut another former to fit in front and up to the original of the (twisted) motor former. I have placed, behind the 'low', side a piece of 1/32 balsa and epoxied the whole lot up. It has straightened it all up with the motor now in line with the centre line of the fuselage, if I need to incorporate any thrust lines it will be easier to get the washers under the mounting. I will have to extend the cheeks a bit so the front former will then fit the new motor position. Cheers my friends
Geoff
 
Not so very Wonky Webbit Builder No 20
Geoff Smith 112/06/2011 13:50:10
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545 forum posts
88 photos
Posted by Richard Harris on 12/06/2011 12:00:09:
Geoff,
You are worrying too much, it will be fine
 
Now this will sound a bit odd. When I check the thrustline of my motor I use a piece of wood, say 350mm x 20mm x 6mm thick with a hole drilled in the middle. This is then bolted on to the prop driver, I make sure the wing is square measuring from tip to tail.
Measure from the wood to LE each side and adjust the motor to suit until it is parallel.
 
The reason, my eyes are not the best in the world!
 
Rich

Thanks for this Richard, another one for the memory bank. Cheers mate.

Geoff

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