It took me years to unravel this mystery.
Yesterday I wrote about how to fly your model and trim it with the right amount of downthrust. How can you estimate the amount of downthrust you need before your maiden flight? Why do you need downthrust in the first place? I got an email a while back asking these questions. Let me try and answer them.
In a Nutshell
The propeller pulls on the nose of the airplane. If the airplane pitches up, that means there is something out of balance with the load it is pulling on. Right? But what could that be?
Center of Mass
Years ago I used to think that the center of mass of the model airplane is what determined whether downthrust was needed or not. The center of mass is the three dimensional balance point of the model airplane.
Measuring it is not hard. Hang the model from three different points in the airframe. I would do a wingtip, the prop shaft, and the tail. Drop a string from the point where the model is hanging. Where the three strings intersect, that is the 3D balance point of the model. This technique can be used to find the 3D balance point of any object.
Once you know the center of mass, angle the motor mount so that the thrust line goes through it. I used the method once on a problematic model airplane that I was designing. I was having trouble trimming it all out, so I decided to go the extra mile. It worked beautifully.
Center of Lift
I have heard that it is really the center of lift that determines the amount of downthrust needed. This makes a certain amount of sense. Lift is the other major force generated by the airplane. It is what holds the airplane up and it acts at a right angle to the thrust line.
Center of Drag
Applying everything that I know about airplane design, the above two answers do not look quite right. The drag is the main force that the thrust is working against. All diagrams of the four forces acting on an airplane tell you that.
In the typical airplane, specially at high speed, the drag force will be much bigger than any turning moment caused by the center of mass or center of lift.
But why did the center of mass work so well for me years ago? Think about it. With the exception of the motor and battery, most of the airframe of a model airplane has about the same density. Skin friction drag is a big portion of the drag on an airplane. This is just a factor of the surface area. In other words, the center of drag is probably not too far from the center of mass.
Let me tie this into my discussion from yesterday. The downforce on the elevator is substantial and will alter the effects of the center of drag. It also makes it harder to do a simple calculation to figure out how much downthrust you need.
I can think of two relatively simple tests that will help you eyeball the right amount of motor downthrust before the maiden flight. Do not expect high accuracy from either method.
First, you can use the center of mass as a reasonable facsimile. The results might be more accurate if you leave off the motor and battery when you string up the model.
For an even easier and less accurate method, use the center of lift. You see, the wing is responsible for a large portion of the drag on a flying airplane. It has a large induced drag at low speeds. The wing also has a large surface area that creates a lot of skin friction drag at high speeds.
This is what you can do. Hold the airplane by your fingertips about a third of the way out from the wing root. Pull forward on the prop shaft. Depending on how much the model pitches up, that is a rough idea of how much downthrust you need.
This test ignores the drag from the fuselage and tail, which depending on the model can be substantial. But it is hard to beat for simplicity.