Squaring it up

[Tony Patman]

How important is it to get the alignment of the wings, tail and fin exactly right? I guess any anomaly will introduce some sort of bias, resulting in a tendency of the model to bank, but isn't compensating for this what trim is for? I guess that the more accurate the alignment, the less trim will be needed, right? But what degree of tolerance in alignment would be practical? 1mm too high or low at the wing-tips? 5mm?

[Alistair Taylor]

Hiya Tony

with my building skills you'd expect me to know loads about this....ahem.

In a nutshell - you can build it as far out of true as you want to, and trim it back for straight and level flight. Free flight designs often do just this - with tailplane and wing "mis"-aligned in order to get a constant turn in one direction.

BUT

- your plane may prefer to turn one way over the other, and in extreme examples you could run out of control throw when trying to correct a turn/stop a roll
- as you increase speed, the effectiveness of trim increases, so your plane will only fly straight and level at one speed
- control surface deflections add drag, so your plane may aquire the flying characteristics of the Catalina flying boat, which was said to take off, cruise, stall and land all at the same speed
- trim that works one way up, may not work inverted. e.g. down trim to compensate for rearward CG

Hope this helps

I did once see a rubber-powered free-flight biplane that had a seriously warped wing, after being left in a cold damp garage overnight. It flew better rolling circuits at chest height than some aerobatic pilots I've seen....

AlistairT ;)

[Grant Darnley]

The degree of tolerance will depend on the particular aircraft, a pylon racer doing 200mph will amplify a 1mm warp to levels a trainer could only imagine.

How much you can personally tolerate depends on what you want to do with your aircraft, the rules are different for every type. My profile has the port wing leading the starboard by almost 1/2"(i was on painkillers that night, dumbest idea) but because it flies slowly and probably spends more time in the hover than horizontal I don't notice it. My pattern ship was built as accurately as possible, none of the critical distances are out by more than a millimetre and this shows in the tiny trim changes to fly straight.

[Alistair Taylor]

You might find the following instructive, with regards to the building tolerances allowed for full size high performance fighter jets;

http://www.domain-b.com/aero/eurofighte_typhoon.htm

AlistairT

[001]

What matters is the accuracy that your flying style demands. I had a friend who was absolutely exact in everything he did, building, covering and flying. When I build a plane, whether it is from a plan, kit, or ARTF I think, is it up to *****'s standard. It never is, but I try.
For a sports model a few mm. of misalignment is not too drastic, unless it all adds up in the wrong direction. For a trainer, try to start off keeping to the designers dimensions/angles.
What seems to cause a big adverse effect is a (designed)rudder/fin hinge line that is far from the vertical, because it always adds unwanted pitch in the (rudder) turn.

[Tony Patman]

The challenge is being able to judge the tolerances and know where and how much to adjust the wing seating, for example. Therefore my question was about how fastidious I need to be. From your very helpful answers (thanks), the answer seems to be "as precise as you can be bothered, but don't worry too much if it isn't perfect."
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