Do not confuse this with the balance point of your model!
Definition
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.
Mission 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.
Happy flying!
Related posts:
Understanding Model Airplane Downthrust
Model Airplane Pitch Trim Tests
Tip: Is your Model Airplane Tail Heavy?
Tip: Is your Model Airplane Nose Heavy?
Model Airplane Balance Checking
How To Flight Test A New RC Model Airplane
Introduction to Electrics for Gas Pilots
Flying Wing CG Location
Carlos’ Power Rule Explained
Understanding Helicopters Video Series
UAV Pilot Training
Figuring Out Unknown Motor Specs
http://t.co/IBy6PA3C Model Airplane Propeller Downthrust Angle: Do not confuse this with the balance point of your model! #rc
I have about 120 flights on my SIG 4-Star 40. This airplane is my “transition” airplane from a primary trainer. After I became comfortable with my 4-Star I noticed two things. First, when I pulled back on the throttle the nose would pitch up. Second, when flying straight and level at 3/4 throttle, hands off, the elevator was slightly up. All that bothered me, since I got my start in this hobby with rudder-only aircraft that had neither elevator nor throttle control. Aircraft trim was something that I learned early on. I decided to experiment.
First, I tinkered with the engine down thrust angle. After several iterations I found that 2 degrees upthrust solved the pitch-up problem. I can stabilize in level flight, then walk the throttle from full to idle and back again without noticing the airplane pitch up or down. I used Brodak (TM) thrust wedges to set that adjustment.
Next, I sanded out a couple of tapered balsa wood shims to put under the trailing edge of the wing (this is a low wing airplane) to change the angle between the wing and the horizontal stabilizer. Something between 1/32″ and 1/16″ was sufficient to allow me to trim the elevator flat while maintaining hands-off horizontal flight. This change of decalage did not require me to readjust the engine thrust angle.
Before making these changes my takeoffs were sloppy. My landings were terrible, 3 out of 4 would bounce all over the place and more often than not nose over, kill the engine and break the prop. (Note: aircraft C/G is right in the middle of the design range.) After making these changes I can ease it off smoothly at the beginning of a flight and grease it on at the end of a flignt 75% of the time. Maybe the improvement is because I am becoming a better pilot, but I really think the trim changes made the airplane more stable and responsive. Neutral engine down thrust and a flat elevator are, for me, a winning combination.
The 4-Star 40 is an excellent design. It can fly beautifully upside down, do all the basic maneuvers, including normal spins, and just bore holes in the sky without a lot of effort on my part. Just for the record, I power this plane with an OS 46FX that I got second hand. The engine has maybe 200 flights on it (maybe 30-35 hours total run time). I experimented with different props, from 11×4 to 11×7 and have settled on an 11×5 as optimum for my 4 Star 40. Fuel is 10% nitro and either castro or synthetic lube, no apparent difference. Flying fields are about 50 ft above MSL and 450 ft above MSL. Air temps anywhere from 40 deg F to 98 deg F.
What is really amazing is how few of us actually go through the trouble to trim a model airplane properly. Too often nowadays, we try and fix every quirk via transmitter programming. Yes, I am guilty of doing that, too. Thank you for sharing a very interesting story.
One correction to my recent post. When I shimmed the wing on my 4 Star 40 I had the airplane upside down in a cradle. That is why my post said “. . .under the trailing edge of the wing . . . .” More correctly, the post should have been something like “the shim is between the trailing edge of the wing and the fuselage.” The change I made increased the angle of attack of the wing relative to the engine thrust line which allowed me to trim the elevator flat relative to the horizontal stabilizer.
That is what I understood. Thanks for making sure it was clear to everybody.