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BEB's Dawn Flyer Build


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I've just checked. The plan doesn't say one way or the other. If you look at the pics in the mag Lyndsay's model (the darker coloured one) has them on the bottom of the wing - as you'd expect. But you're right Lindsay's mate Geoff, who made the lighter coloured one, has indeed put them on the top!

I have no idea why he did that! In fact it would require a bit of extra work because, as usual, the rear spar is at the bottom of the aerofoil - so at the top of the section at this point there is nothing to fasten to! So it would require an extra fitting to give you something to glue the rear servo hatch mounting rail to? Very odd, I expect Geoff had his reasons - maybe Lindsay can enlighten us!

BEB

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Tonight has been about fitting the wing to the fuselage. I lifted the joined wing from the board - all's well - and came to fit it to the fuselage and like everyone else found it was too wide. But in my case only by a very tiny amount - in fact it was so close I could have just pushed it home - but that wouldn't be so smart!

So, why is my wing a closer fit that other people's - many of whom are citing a 3mm or so discrepancy? Well I think the answer is that I actually chamfered back the ends of the tail members - to the angle of F5A and then fixed the former in place. This means my F5A is probably 2-3mm further back. Now strictly speaking this was an error - the ends of the tail longerons should obviously be square - but it turned out to be a fortuitous error as it resulted in F5A being just enough further back that wing very nearly fits! (Its an ill wind and all that!) So all I had to do was re=square off the ends of the tail longerons and I was in business...

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And from the rear,...

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Very nice! Now the next job is, as VA has suggested, a bit of a tricky one. We have to fit the front locating dowel and make the wing fixing screw holes - in pretty precisely the right places. First up let's get the wing in the right place. I used my favourite length of string and measured off the distance from a marked point on the exact centre of the nose to one wing tip, then try this length against the distance from the same point on the nose to the other wing tip. Obviously these distances should be the same. Jiggle the wing until that is the case. Then sight over the joined pair of centre wing ribs and check that they are exactly over the centre of the wing fixing plate hole. Work around these two conditions (equal lengths and over the hole) until satisfied. Once the wing is where you want it, tape it there to stop it moving,...

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You can see my datum mark on the nose in that picture. Now carefully lift the assembled model up, get in from the opening that will be the battery hatch and, through the dowel locating hole in the former, put a pencil mark on the leading edge of the wing - this marks where the dowel has to go,..

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(Its tricky holding a taped together model in one hand a camera in the other - but I'm sure you get the idea.

Now we need to locate the rear wing bolt hole. Squint down from above, no paralax, and mark the position on the two joined centre ribs which is exactly over the centre of the wing fixing plate hole - job for the Mark 1 eyeball! here is the point marked on the ribs as the wing sites in position,...

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Now take the wing off, and glue some scrap balsa either side of that point, like this,..

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This is just like the balsa blocks we put inside the wing at the front under the sheeting, it there to give us some "body" to drill into. The wing bolt will be M5, so we drill a hole 4.8mm - which is a nice clearance for an M5 bolt,...

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And to prove my point - here's the bolt in position,...

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Sorry, that's a bit blurred, but you get the idea.

'cont' in part two....

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OK now onto the dowel at the front, we have marked the point on the leading edge and so now we can drill it - 6mm for a 6mm dowel. Fitting the dowel, 5 min epoxy, we need to be sure its nice and straight which is easier if we use a long piece of dowel and then cut it off to length later,...

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And here it is nicely trimmed off,..

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This has been test fitted to the fuselage - and its spot on, so good news there.

Now we turn our attention to the wing mounting plate and we discover a small problem. The wing bolt will be M5 as I've said, but the ply plate that the wing fixing plate is made from is way too thin (3mm) to accommodate the prongs of a M5 captive nut sad So I'll have to glue a backing plate to the mount to make it thicker. I quickly made up a backing plate from a 3mm ply off cut from the blank the wing bracer had come from and drilled a 6mm hole in that,...

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This is glued to the underneath the existing wing mounting plate using 5 min epoxy again and the holes lined up with the off cut of the wing dowel,...

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Now obviously you want to whip that piece of dowel out once the plates are clamped together and before the epoxy sets. But a tip here - 5 min epoxy takes forever when you are waiting for it, but when doing a job like this its amazing just how quickly it can go off - especially if you are distracted at the critical moment (knock over your coffee or the phone goes etc!) and you're left with the damn dowel glued in there!!

As an insurance policy - if I don't want something to be glued I just dip it in this,...

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Handy stuff to have in the workshop for jobs like this or glueing in hinges etc - all those times when you don't want something stuck. As it happens the dowel was pulled out straight after the photo was taken - but better safe than sorry!

I will leave that epoxy to go really hard now before I subject it to the stress of pulling in the captive nut.

Once the nut is in place I will test that the bolt hole is in the right place and that wing fits with both the dowel and the bolt. Assuming all is OK I will then apply the centre sheeting to one side of the wing and use the existing hole as a guide to drill through it, then sheet the other side and again use the existing hole as a drilling guide. But that's all for next time.

BEB

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I am curious to know why when locating the wing BEB used string against a mark on the front fus rather than a mark on the tail end.
To my mind the reduced angle to the tail is an advantage anyway, but the important thing is to get the wing aligned with the fin & tail. Any error at the nose end could be rectified by adjusting motor thrust with washers etc. Any error at tail cannot be rectified easily.

I expect BEB has a perfectly good reason and anyway his workmanship is probobly so good it wouldnt make any difference ( but it might on a beginners model ) So tell me why!
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Hi BEB

I have been following your build with great interist and have a couple of questions.

firstly right back at the start when you fitted the moter you had to quite a lot of trimming and fetteling to get it to clear at the top, would it not have been easier to just move the motor down to cleare the top longeron or is there some reason why this cannot be done.

Secondley being very new to flying i thought that the leading edge on wings had to be curved to help with it pushing through the air i.e less resistance = better efficiency and longer flight times but i notice in your last set of photos that the leading edge is still flat fronted, can you tell me why this is.

Thank you

Mouse.

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Evening All,

Oh questions - great! Please feel free to ask questions I'm keen that the blog should be a dialogue! And its a big part of the point of the mass build. I'll take Mouse's question first;

Could I have dropped the motor slightly and thus avoided having to trim away some of the top longeron in the nose? The short answer is - no. Two reasons, one aerodynamic the other practical. The aerodynamic one goes like this: when the designer draws up the plan there are certain key lines he puts on there - one of these is the engine thrust line. This line is not really negotiable! Its position, with respect to the incidence angle that the main wing and tailplane are set at is very important in ensuring that the aeroplane will be able (with minimum trim) to fly straight and level. We move the thrust line from where Lyndsay put it at our peril!

To get a better handle on this let me ask you a question: suppose my problem had been that things were a bit tight on the righthand side of the motor - do you think it would be OK for me to just move the motor off the centre line to the left? Well obviously not. If I did that when I come to fly the model the thrust would be acting off centre and the model would yaw to the right - a bit like a twin with one engine out, not as severe but the same sort of idea. I would have to put a fair bit of left rudder trim in permanently to counteract this. Not good.

Well the fact is that aerodynamically there is a sort of "vertical centre" as well. Its not in the geometric centre so its not so obvious, but its real none the less. So we can't move the thrust line off this point. Now you might be thinking "would a millimetre or two make that much difference?" Well maybe not - but this is a relatively small model - 1-2mm is a sizeable amount here - and I wouldn't risk it.

The practical reason is simplier, I'd already fixed the motor position on F1 - almost the first thing I did. To drop the motor I'd need to drop the mount - that would mean a new F1 and that former was glued into the model by then! Big job.

Trimming that longeron is not a problem. It is not really a structural part. Its main job is to hold F1 and FC in the correct relative position and give you something to glue the sheeting to. Its the sheeting which is providing 90% of the strength here. I know its only 1/16" thick, but as a stressed skin its very strong. So we can live with losing a little of the longeron.

Now on to the question about the leading edges. They will be curved - but not just yet! Having them flat fronted at this point helps me to align the wing - I have flat face at the front to locate on. Now I have the wing alignment fixed by the dowel and the screw I can finish the job of rounding the leading edges.

Now on to Darran's question. Some models (most actually except out and out aerobatic types) have at least some dihedral - that is the wings slope upwards towards the tips. This tends to make the model more stable in the roll axis. Trainers - which you want to be very stable have lots of this and its really noticable. But for more general models too much dihedral can be a problem. It makes the model slow to roll and makes inverted flying quite tricky. Dihedral right way up gives stability - upsidedown it becomes anhedral (wings sloping down towards the tips) and in the absence of any sweep back that is very unstable. Flying a trainer inverted well is actually quite a balancing act!

This model has a very small amount of dihedral - just a bit. Its hardly noticable but it is there. So the wings are not glued together flat - they have to make a very shallow "V" shape. How much? Well that is set by how "V" shaped the wing bracer is. So what I did was make the wing bracer flat on the bench for the wing I was going to add, and that of course then meant that the other wing had to be "up in the air".

I'll deal with kc's question in the next post - there are word limits!

BEB

 

 

Edited By Biggles' Elder Brother - Moderator on 23/01/2014 00:03:34

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kc asks an interesting question - and yes there was indeed a reason I did it the way I did!

This model is a bit unusual. With most models you build the fuselage all in one piece - from nose to tail. Here we built the front part of the fuselage - the canoe bit - and then we stuck the whole tail assembly onto that. So this model effectively has a join in the middle!

Casting our mind back to when we glued the tail on you can see that I did it by drawing a straight line on the reverse side of the plan, then marking two points on the centreline of the "canoe", one on the nose, one near the back of the canoe. I then positioned the canoe so that these two points where on the line I'd drawn and fixed the canoe in that position. I then put a centre mark on the tail and located the tail so that the mark was also over that same line. That way I can be reasonably sure that the whole fuselage (canoe and tail) are straight.

The consequence of that is that the tail position is actually defined relative to the mark on the front of the nose. That mark is our ultimate datum point - everything so far is defined from that. The tail is where it is because of that mark on the front.

Now, consider an analogy. Suppose you wanted to cut 5 lengths of wood each 2" long, how would you mark that out? Well the best way is to put your rule in place - left side at the end of the wood, then mark off 2", 4", 6", 8" and 10". The not so good way would be to mark 2", then move the start of your rule to that point and mark another 2", then move again and mark 2" and so on. The second method is inferior because you are creating a new "zero point" or datum for every measurement. Your second 2" becomes dependent on how accurate you marked your first 2". And worse, your third 2" depends on how accurate your first and your second 2" were, And so on. Your error accumulates as you go along

The moral of this story is "pick one single datum and stick to it" - then measure everything from that. This way your errors don't accumulate and all your measurements all have the same accuracy.

Well - as I've said the nose mark is my primary datum for the build - everything is measured from there. The tail was put on relative to that point, the wings have been put on relative to that point and ultimately the tailplane will go on relative to that point. Now that wont stop me "triangulating" and checking the wing against tailplane in the more conventional way - but its a check not the primary measurement. If I found a discrepancy there then I'd know to go and look for the source of an error.

As I say this is not important normally with a "one piece" fuselage - but with two part fuselages like this I feel this is better way to maintain accuracy in the build.

BEB

Edited By Biggles' Elder Brother - Moderator on 23/01/2014 09:24:26

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And now - the blog bit - or what BEB did tonight!

Epoxy set I installed the captive nut and tried the wing on the fuselage to check that the dowel and the screw are both in the right places,...

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And BINGO! Spot on. The bolt passes through the wing straight into the captive nut with the wing in position. I then double checked the wing alignment with my string again - in fact you can see the famous string in the picture! All was well.

The next job is to do the centre sheeting. But before doing that its a good idea to connect up these aileron servos and check everything is working OK,...

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I use an old NiCad pack to power things for this. The Rx is 617 - taken out of my PZ Spit which sadly bit the dust a couple weeks ago sad. A 7 channel Rx is a bit overkill for this - but I don't have a 4 channel free. One advantage of the 7 is I can set the ailerons on 2 channels which makes life a bit easier in some respects.

Now we need to prepare the skins. On the Typhoon I experimented with a different way of doing this using CA - following the technique of the great Dave Platt - (blessings be upon his mighty name). This was very successful I thought so I decided to do the same here. The skin is assembled prior to going on the model,..

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For details on the method for preparing this this see my Typhoon blog here.

Usually you'd make this skin in two parts - left and right. But the dihedral is so slight that I felt 1/16" sheet could cope with the small amount of double curvature - so I did these in one piece.

So here is the bottom sheet in place,...

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This was done with CA - working from the inside just like we did with the first piece of the front sheeting.

Now we cut two small holes for the aileron servo wires to exit through. Don't forget if like me you need to use extension leads here use lead locks or some other method of preventing the connectors from separating,...

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I put a dribble of CA on these servo wire holes as it helps to harden them up and make them a bit wear resistant. But do that before you put the wires through - servo leads don't like CA!

Using the wing fixing bolt hole (we drilled in the centre ribs last night) as a guide, we can now very gently drill through from the top and through the bottom sheeting. Remember to put a bit of scrap wood behind the sheeting to stop it splitting or splintering around the exit of the hole.

And finally we fix the top centre sheeting - but of course we have to use aliphatic here and pin everything,...

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I'll let that dry overnight now. Just the cockpit and the tailskid to do now I think.

BEB

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I allow to answer myself as I suppose BEB still enjoys his well deserved sleep. I sit in Greece this week, so I am 2 hours ahead

the wing seats neatly on the wing seating which gives it stability against radial movement. The front of the wing is fixed by the dowel, the single screw holds it down on the wing seating which together gives also axial stability. This is a very stable solution, something I like about this plane - especially when you have a few Seagull models which always use 4 screws in fiddly positions you enjoy closing only one screw for a change

VA

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Measuring wing alignment.
Thanks BEB, I knew you had a good reason!
However, for general models wouldn't you say measuring back to the tail is more likely to be accurate? My instinct ( = experience of all the things i have ever made) is that measuring at a shallow angle is not as accurate as measuring at a much greater angle. In practice I mean, not in geometry theory. Its the accuracy of where the string intersects the point of measurement that changes with angle. ( i.e if the string crosses the line at a very shallow angle its difficult to decide the actual point of intersection)
In the same way using a thin thread is likely to be more accurate than a thick piece of string.
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BEB

i thought it would be for dihedral, If I have got this right when the brace was fixed in the first wing the end of the brace was not flush with the board, so to get it flush to the board for the second wing fit the wing would need lifting from the tip. Thus meaning when glued and mounted there will be a slope to the wings.

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Yes Darran that's correct - when the brace was only in one wing then if you laid that wing flat the half of the brace sticking out was off the board - here's a photo from earlier in the build that shows that,..

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This is the installation of the first half of the brace and you can see that the free end is just off the board - its only slight because as I say this wing has very little dihedral indeed.

kc - yes I agree - as I said above with a more conventional fuselage - all built in one piece - then yes I would tend to use the tail to wing as the longer distance increases my percentage accuracy for the same absolute accuracy - e.g. a 0.5mm error in 500mm is a smaller percentage error than a 0.5mm error on 200mm. I'm not so much in agreement on the string though! I prefer to use stout string as I can be really sure it wont stretch! But each to his own of course.

Re: one bolt versus two bolts - I agree with VA and John. This wing is very solidly grounded with the arrangement of one dowel and one bolt - mainly as folks have said because of the wide seating combined with a more or else flat bottomed wing.

A brief progress report....

I've decided to start on the cockpit. Lyndsay's a little non-specific in this area on the plan - he gives a reasonable amount of detail but there are one or two bits left to the builder's discretion! So I'm feeling my way a bit here. Matters aren't helped by for the life of me I can't see how F2A is supposed to work! It has a hole for the dowel - but the only way you could use that would be to cut away a chunk of the LE around the dowel? I'm not doing that - its what's holding the front of my sheeting in place!

So I've decided to go my own way here a bit. First I put the wing on, having drilled through from the back now for the wing bolt,....

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I've swapped the black wing bolt for a white one - as although the black one was great with a blade on top to turn it, it was too short really. I've put a sheet of cling film between the wing and the fuselage - so I don't accidental glue the wing on!

Next I cut F2A much shorter and sanded it to a very shallow "V" - so it can sit on top of the leading edge, then having marked a centre line on the wing top and the formers and also a position line for F3 I arranged F2A and F3 on a length of 6mm sq. I then glued this 6mm sq peice in place with CA. That gives me this,...

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I can then glue this to the top of the wing, and also glue F5 in onto the wing top at the back as well,...

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And from the front so you can see F5,..

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The glue for fixing these to the top of the wing is aliphatic. These will need a little reinforcement I think so I'll add a little triangle stock tomorrow. Once these are settled I can sand F5 down a little - its a bit high - and then sheet this.

This method of building any hatchway "in place" is always to be recommended if possible - its one way of ensuring you get a really good fit with narrow gaps. Trying to do this by measuring is very difficult!

Anyway - we'll leave that aliphatic to dry now. More tomorrow.

BEB

Edited By Biggles' Elder Brother - Moderator on 23/01/2014 22:36:08

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