Winter build project: Wolfgang Matt’s ATLAS

Both John's and Chris's builds are coming along well. Mine has crawled to a halt while I recover from last weekends foam-fest...and this week I've also been printing and laminating membership cards in anticipation of everyone renewing their subscriptions tomorrow night.

Not sure why you had to bend the top plank, Chris. But that's the beauty of building from a 40 year-old plan. You never know how accurately drawn it was in the first place and then we all add in our own little errors along the way.

The top plank went on flat Mike, no bending, I took the top of F5 down a little to ensure the angled sides were flat and of a consistent height.

I did bend the angled sides though, unlike someone, who took the easy and a lot quicker... and possibly more sensible route of cutting them

Some sand corners whilst others cut corners.

Trying to be too clever, when I drew out the motor mount I worked out the thrust angles and shifted the holes to compensate.

However I've just discovered my mistake this evening, on the plans from former 3 down to the nose the whole fuselage section has been angled to that of the down thrust angle.... This means no compensation for the down thrust was needed and I've basically mounted my motor 1.5mm above '0' line.

Needless to say I remained calm, I didn't say anything too outrageous, instead I went to the fridge and pulled out a bottle of Wolf Rock Red IPA (one I've not tried before).

Can anyone hazard a guess as to how badly this mistake affect my Atlas in flight? I guess there are bound to be mistakes in a first build, but I'm kicking myself for not seeing it in time.

Can you not just glue in some dowels to plug the original holes and then re-drill in the correct position?

I agree with Chris 73 - You could also glue some thin ply washers across the repair to create a clean area to re drill.

If your rework is criticised - say its a reinforcing modification !

Hi Chris, If you are worried about it do as Chris H suggests, it's an easy job. However, I would be very surprised if being 1.5mm of the original vertical position would make any difference to flight characteristics, as long as the downthrust is correct. When you install the nosering and balsa cowl, just line it up with the revised position.

If Chris does fill the holes and move the motor down, his will be 1.5mm lower than mine because I made the same mistake. Mine is not going to change since I've already built up the nose and I'm convinced that:

1. It will make no difference
2. I will probably build in other small errors that might cancel out this error
3. Who says Wolfgang Matt got it right all those years ago? Did he experiment with the thrust line until it was perfect or did he make a TLAR model and when it flew well he left it alone?
4. The vertical weight distribution of the electric setup is going to be different from the 40-year-old IC setup and who knows whether this will affect the desired thrust line. I have read that Prettner recommends a different dihedral angle (more) for the electric Curare due to the vertical CG being higher than the IC version with a pipe below the wing

Thanks everyone, I'm leaning towards leaving well enough alone, ideally having spotted my mistake I'd like to correct it, but due to where I am in the construction, I'm worried I could easily make things worse. Filling the holes will require punching out the captive nuts that were snugged up with epoxy and I can't get at them from the back anymore, to chip it away, so it would be a case of punching them out and hoping the integrity of everything else holds. The 1.5mm discrepancy would mean drilling up 2.5mm of the plug and 1.5 of the ply, the drill will naturally want to follow the easiest path, especially as it will be done free hand, so the holes might not end up where I want them. I could knock up a guide block to try and minimise the risk, but I'm still hampered by working at the end of a tunnel! and won't easily see what's going on.

And I'm reassured by Jon and Mike's considered opinion that it's unlikely to detract from the flight performance.

I think I'll just put this down to experience and something to really think through for the Aurora or Beetle, i.e. the next build

Thanks again.

Chris

I was going to say at least yours would be unique and then we find out that Mike has made the same mistake

If you have gone that far with the construction, I would definately leave it as is, as Mike has said (or at least I think he did, don't know what TLAR means ) I bet Wolfgang didn't experiment with the centre of thrust in 1mm increments!!