Why it's important:
You are almost ready to fly, but in order to fly well, you need a proper center of gravity. In my experience, 3D printed planes have a nasty habit of being tail heavy. While neither nose or tail heavy scenario is good, tail heavy is the most dangerous as it will create an unrecoverable stall. If the plane is extremely tail heavy it simply will not fly at all. The first time we flew the first Pitts biplane prototype it did a very tight back flip immediately off the runway landing upside down on it's top wing. Amazingly it did no damage, but clearly an adjustment was in order. In the case of the Fokker DR1, the center of mass (in airplane speak the center of gravity) of the computer model worked out almost perfectly for the real world. There are going to be variables for each model, however. These variables could be filament, covering weight, infill density, etc. Use the point position below as a safe starting reference.
What it should be:
If you have chosen to cover your plane with the iron on silk method, your covering weight is going to be negligible unless you choose to use lead paint and apply it with a broom. Assemble the model prior to covering as a quick balance check. The center of gravity should be 59.58 mm from the forward firewall. Before you get the rulers out, know that this actually coincides with the back face of former F-7.
How to correct an out of balance airplane:
If it is minor: Battery adjustment can be effective as it is arguably the heaviest object on the plane. simply shift the battery forward or backwards on the battery tray to tweak your balance.
If It's nose heavy: If you have done a fantastic job with ultra light weigh PLA, you may actually find your model to be nose heavy. In that case, adding weight to the tail is the obvious solution. This can be done with small weights at the extremity of the tail, or simply by re printing the rudder or elevator with a heavier density infill.
If it's tail heavy: If you find that your model is tail heavy you have 9 hollow jugs in your dummy motor that can double as a convenient spot to conceal small weights in just the right place.
First of all, ensure that your controls all have freedom of movement and equal deflection in their ranges. Also ensure that your transmitter has the trims centered. As tempting as it may be to see what this model can do, your first flight should be fairly short. Once the plane lifts off, notice if you are applying forward or back stick pressure to maintain a level flight attitude. If this condition cannot be adjusted with fairly minor elevator trim, then a weight adjustment is in order. Make the needed adjustment to the model and re center your trims then make the flight again. Hopefully at this point everything will be in an acceptable range. In that case you are good to go.
Conclusion and a request:
The design of this model has been a labor of love on my part. hundreds of hours have gone in to the design, building, testing, and documentation of this model. I really appreciate your patronage in purchasing this model and welcome any suggestions or feedback you would like to share that can improve or make the building process better. Additionally, I want to see what you have created! send us a pic and we will post those images to our website with a photo credit. The model files are yours to use for your personal use. You may reprint as needed, but before you share your files with others please consider the time spent on our end for the creation of it. We love creating these models but can only continue to create them with the generous support of fellow modelers like yourself. Thank you for that consideration and happy flying!