Do not confuse this with the balance point of your model!
Downthrust is when the motor on the nose of a model airplane is mounted angled slightly down. The idea is for the propeller to pull the model forward and down a little bit. This is done to balance out the tendency of many model airplanes to climb when more power is applied.
Ducted fans may run into the same issue, but there may not be much you can do about it. Pusher propellers work in reverse and may need upthrust instead.
Testing the Downthrust
Intuitively, it is very easy to understand the need for downthrust on a motor mount. Just try it!
Fly your model airplane straight and level. You want the lowest throttle setting that will fly the model at a comfortable cruise speed. Wait a few seconds for the model to stabilize. Then push the throttle stick all the way forward. Don’t yank it there, but move it smoothly over a span of a couple of seconds. Watch carefully what happens to the model.
Did it dive to the ground? Imagine you are coming in for a landing and you have to do an emergency go around. You give it full power and it dives for the runway. That is really bad. You have too much downthrust. Do not fly the model again until the problem is fixed.
Did the model start climbing when you applied more power? For most model airplanes, this is not the ideal behavior. In a sport model or trainer a modest climb is not too bad. That is how I have most of my own model airplanes set up. But a steep climb so you are at risk of stalling the model is definitely a no no.
Finally, did the model stay level, and just smoothly accelerated? Wow. A model that does that is a rare sight. You have my permission to jump up and give your spotter a high five. Yes, land the model first.
CG and Downthrust
Airplanes are only stable in pitch if there is a downforce on their tails when flying level. When the elevator stick is in neutral, the elevator will be raised a little bit. There is a very important consequence from this. Model airplanes always pitch up when they fly faster.
This is easy to understand. When the airplane speeds up, there is more air flowing past the elevator. It now works better, so it presses down harder. Voila! The nose goes up.
Find the right balance point for your model before you try and adjust the downthrust. Use whatever test you like, such as the dive test. The balance point is a lot more critical. You also definitely do not want to be adjusting both of these at the same time.
This is also the reason why you should not use a programmable mix from the throttle to the elevator. Do yourself a favor and keep these two forces as separate as possible.
Given the complex relationship between the balance point and downthrust, you may be wondering if it is possible to perfectly adjust the amount of downthrust. In other words, is it possible to perfectly trim any model so that moving the throttle stick never causes a pitch change?
I have to say no, for a simple reason. The elevator responds to changes in the airspeed of the airplane. The downthrust is reacting to changes in the propeller’s RPM. If you throttle up or down too quickly, the thrust from the propeller will be out of balance with the airplane’s pitch trim. This is because the RPM can change more quickly than the airplane can change speed.
Also, while doing aerobatic maneuvers the airspeed of the model is going to vary. Again, the two forces will be out of balance with each other.
Note that I am assuming that the propeller blast is not hitting the elevator directly. This can happen even if the motor and the horizontal stabilizer are lined up vertically. You see, the downthrust can cause the prop blast to reach the tail too high to be of help.
If the prop blast hits the horizontal stabilizer directly, you may have an easier time trimming the model.
Having said all of that, the goal should not be find the perfect trim for your model, but to find the best trim that works for you.
Which brings me back to something I have said before. The best way for you to trim your model is by you flying it. Wait for typical weather conditions and do the maneuvers you typically do.