I've always eyeballed it in the past. Is there any way to do this more precisely. I hate eyeballing stuff.

Thanks in advance.

Steve

G'day Mate,
I use a lazer incidence meter, it can be used on engines, wings, tails, whatever.

Levels, angle meters, straight edges and protractors along with commercial or homemade incidence meters or sticks.

You'll need to block up the model and set it along a set of reference lines that you establish. Then useing straightedges that are similarly packed up to level and an angle meter (a little right angle deal with a weighted angle finding needle) or a protractor you can set the angles of the firewall or mount.

ANother way is to use a full scale drawing of your nose area to mark lines on the inside surfaces of the sides to align your firewall to when building. For downthrust it's just an angle of the line. For the right thrust you place the line on the right side slightly to the rear by an amount you determine from drawing the angle on the top view. Or you can figure out the amount using trigonometry. With the lines you can build the model and have the thrust line come out pretty much exactly where you want it to be. Of course this also means that you need to make your sides exactily alike and build it up over a centerline so that the sides are properley aligned so the firewall lines come out where they are supposed to be.

But that is the way you should be building anyhow. Never leave accuracy up to good luck. The accuracy of the last piece you glue on is only as good as the foundation of accuracy that starts with the proper alignment of the first two parts joined by the first glue joint.

EDIT- It just occured to me from reading your post again that this may not be a building method question. Are we talking about how to set the angles or what angles to choose in the first place?

If it's what angles to use then that depends on what the model is being designed to do. Aerobatic models that are set to be neutral or close to neutrally stable need no downthrust and perhaps only a 1/2 to 1 degree of right thrust. On the other hand a high wing trainer with the more forward CG and higher location of the center of drag will need as much as 4 to 6 degrees of downthrust and often 2 to 3 degrees of right thrust. Sporty models that are inbetween these two need angles that are in between depending on the design. Low wing models generally needing less downthrust than high wing models.

I suspect that the methods used to find the recommended angles don't need to be any more accurate than the original quess work that went into finding the "correct" angles to begin with. Bolting a straight edge to the crank snout will give a nice expanded reference to check the engines' allignment with. Fine adjustments can always be made at the field after you see how the model perfroms.

These are very good and I appreciate all of your inputs. I am remounting a 3D aircraft (Harrier 3d). It is definitely fully semetrical. It always looked like it had some down thrust from the factory but I'm not sure that was intended.

Is there any good builders manual that any of you know of that's really comprehensive.

I think I will start with the trig method and by bolting a straight edge on centerline just to see where I am.

An aircraft has a lot of angles to it. How is neutral defined or identified.

On a 3D model it would be as simple as the airfoil's chord line. Make it all 0'ed to that and rely on a touch of trim for the small amount of elevator up trim you may need. If you trim the CG locaton back far enough that the model has no tendency to want to lift it's nose then the elevator trim needed won't even be visible. For side thrust you may want anywhere from 0-1 degree of right thrust. But you can't go wrong in this case with good ol' zero.

I found the trig method a bit hard because it is hard to measure that accurately with a ruler or a tape. I leveled the aircraft with a level on the canopy base which seems to run exactly straight with the fues. I could then easily see with a small level on the backside of the motor (horizontally mounted) that I had a slight amount of down thrust. The funny is that it appears that the firewall also has some down thrust built into it. This may be intended or may be a flaw. Using the line I established the tail also had a slight amount of negative incidence which I found to be surprising.

So, I need to take my measurements again leveling off of the cord line. However, I don't have any tool to determine what the cord line is.

G'day Rino,
Slight negitive tailplane incidence is normal, because the tail plane (horizontal stabilizer) flys level, & this will make the wing fly at a slightly positive angle of attack, & should make up trim unessesary & down thrust in the firewall is also normal, it allows the plane to fly faster without climbing excessively.
Everything in flying is a compromise.

If what you have here is a 3D plane, the only angle needed anywhere is right thrust on the engine, everything else should be lined up zero-zero. Set the wing up level by establishing an even distance from the bench top to center of the leading edge and center of trailing edge, it isn't rocket science. The plane doesn't care if the canopy is level, just get the tail surfaces measured like I described doing the wing, and check the firewall with a carpenters' square. It doesn't hurt to check allignments in the forward and in the plan view while you're at it. This plane should behave the same upright or inverted, so it should be set up the same way as much as possible.

I'm learning a lot and your comments are all being taken in. No to find an incidence meter I like.

thanks

r

For your zero reference I would use either the stab or the wing chords' center line.....which ever is the most difficult to change. The wing and stab should be checked at several points along the span, all you need is a carpenters' square off a perfectly flat table, but the incidence meter tool is nice to have.