As the plans to my GP J3 Piper Cub do not show a datum line how do I mark one as so I can set the correct wing,stab & engine incidence? Your help in this matter is well appreciated. Thanx

NGH:

Just eyeball one and draw it in. Sound strange? The actual datum line is not important, so long as the wing, stab, and engine are angled correctly one to the other. If you see that you will have to move any part too far using your first line, erase it and draw another.

Bill.

I like to draw a line parallel to the horizontal stab under the assumption that its incidence is zero. From there just adjust the wing incidence according to it.

Since the GP kits key together the alignment tends to be pretty close. I would suggest that you go ahead and mount the wing. Then, using an incidence meter block the plane up so that the wing is at the proper incidence (probably about 2 degrees). Then mount the stab and check it's incidence. Make whatever adjustments are necessary.

I would build it over the plans like they say, mark the firewall on fuse side from the plans, and mount the wing, then mount the tail last and eyeball it so it is level with the wing side to side. The kit is built in a way that all the thinking is done for you. Mount everything where it is on the plans and you don't have to worry about that stuff. You can double check it, but thousands of those kits have been built just by putting it together, and they all fly. Don't over think it.

NGH:

Flyboy has the best suggestion of all. Just put it together and fly.

Bill.

Thanx everybody. Bill I was in the right direction as I thought it was an imaginery line just to
pull the stab, wing & engine incidence together with one another. On mounting the wing an setting the incidence from there I have heard that it is better to mount the stab an set everthing else from the stab. Any truth to that? Anyway thanx for confirming that the datum line is an
imaginery reference point now on to building.

NGH:

If you are worried about it, the stab will probably be a couple degrees negative relative to the wing. And the engine either neutral or a slight bit down also.

Go here:

http://www.rcuniverse.com/forum/Wher...1063414/tm.htm

Post #5 explains the interaction of incidences and thrust line in fair detail. Should take care of your questions. If not tell me what's not yet clear.

Bill.

I believe that in an ARF the incidence is pretty well set by the manufacturer, and in sport/sport scale it is less important than it is for competition aerobatic flying. I agree wholeheartedly that the only things that matter are the alignment of the wing/stab/engine. The fuselage simply acts as a means to keep them flying in close formation

In my GP J3 ARF, there is no slop that would allow any deviation from what GP has determined is the optimum wing/stab incidence and the engine thrust line is fixed by the firewall and careful installation of the engine into the engine mounts. Engine mounting is relatively straightforward, so I have concentrated on making sure the wing incidence (washout, or lack thereof) is identical for both left and right wings, making sure the engine is mounted straight in the mounts. and ensuring that the flying surfaces are square in all three axes (lateral, longitudinal and vertical) in accordance with the GP instructions

HTH,

Jim

Hello NGH:

To Flyboy I repectfully disagree. I am now building a GP 40 size cub kit. The plans call for +2.5 degree on the stab and wing with the engine at 0 degrees. What I would and am going to do is use my Robart incidence meter. I will put the fuse in a stand and check the stab incidence. I will juggle the fuse untill the meter reads 0 with the wing installed but the hold down bolts undrilled. I will check carefully the centering and incidence. If the wing and stab are 0 then I will drill the holes and I'm done.

I will agree that checking the motor is a PIA. I will not check it. Here Flyboy is right, I won't mess with it unless flying it shows signs of balloning or diving under power. This is just my preference, Flyboy is right that if you build'em straight there is no need to check.

I'm just a persnickety perfectionist that has to check and double check and check the darn thing again. This from a guy who checks the drain plug on cars he changes oil on at least 3 times before returning it to the customer! Good luck on your Cub.

Mark Shuman