building precision effects
 

hi guys,

I am building my jet from scratch and have made all parts from cnc (formers then wood planking...).

I was wondering what effect would have small errors of symmetry when building parts.
ie: - two half stab not identical
- both wings not excalty identical (even if visually same).
- small differnces between airframe right and left side

and so on....

jet fly quite fast and definetly the most accurate it is the better it is but how small mistakes influence flight...
I trust that some manufacturers manage to make perfect symmetry but I also beleive that many models look perfect from the eyes point of view but in reallity they are not...

any comment / advise / explantation?

RE: building precision effects
 

My guess is if the errors are so small you cant find them by looking at it, you wont notice any bad flight characteristics in the air. Thats what trim tabs are for. ;)

There's lots of planes out there with parts way out of alignment and they still fly fine.

RE: building precision effects
 

one of the concern was if you setup you plane at 50km/h to fly straight, then what happens at 200km/h or more...

I am sure that I can trim the plane and make it fly straight (even if this could reduce speed due to drag), however, with increase of speed, this might be an issue...(I don t really know if the answer is yes or no for RC jet...)

RE: building precision effects
 

If you're worried about roll trim, I doubt you will detect any trim changes from slow speed to high speed. I know first hand, because one ARF (jet) I built (ahem, assembled!) was off in wing incidence, and had visually noticeable aileron trim required. For pitch trim, if you develop trim change issues here, it is most likely from CG or thrustline issues.

Just build the model as if it were going to be a pattern or aerobatic model, check the wing incidence angles at a few wing stations, etc.

RE: building precision effects
 

Just how much of an error are we discussing here? If it's only a millimeter here and there then you won't notice a thing. But if we're taling a lot more than it'll likely have some minor impacts. But if it's more than a couple of millimeters here and there you really need to work on your craft skills or CAD skills.

One place small differences will hurt you is the critical areas like the wing leading edges. But a template or other jigs to control the shaping can be used to help out.

As for setup angles of the wing to tail surfaces you'll want to be within a degree of square and within a few millimeters of symetry in span and chord. But if the elevator hinge line is out by 2 or 3 degrees it won't affect the flying much. Same if a fin is tilted slightly to the side. But if that same fin isn't alinged to the centerline to within a degree then you'll have some unwanted effects. Same with the wing to stabilizer angles.