This one might cause a little controversy, but who cares. I never could fly worth a flip anyway.
I call this the rule of 3/16. It has worked on every airplane I have ever owned, but there are those who say, “yes, but”. Nevertheless, if you use this technique to determine the optimum place for your c.g., you won’t be too far off.
As I have said several times before, set your c.g. at the factory specification. Use any good technique for measuring the c.g., or read last month’s crap trap. Do it the same way every time!!!!
Measure your elevator rates for high rate and low rate. Set them at 7/32 and 5/32”, respectively. Note that this is not the 20% I have been harping on. If you use 100% for high and 80% for low, you will actually have “a little above 3/16 and a little below 3/16” for elevator throw. Set Ailerons and rudder about where you usually do. Adjust them to your liking.
Fly the airplane, trim it, and fly the course. Fly on both the high rate and low rate. One of the benefits of setting the throws at 80% is that you get to try two throws with every test flight.
One of four things will happen: You will like the settings. You will not have enough throw. You will have too much throw. You will crash. If you crash, skip to page 22 for special instructions.
If you have too much throw (your airplane is too sensitive or is jumpy in level flight), then move your c.g. forward. Removing ¼ oz. from the tail will move the c.g. 1/16”. Twice that much is required in the nose, so move your battery pack forward if possible. 1/16” c.g. movement is about the least amount you can detect, so don’t bother with putting a dime on the nose.
If you don’t have enough throw (your airplane won’t turn quickly at full stick), then move the c.g. back 1/16” and try again.
Do this repeatedly until you are flying at low rate with between 5/32” and 3/16” throw.
Here are some other things you need to keep in mind:
Final note: It usually makes good sense to put your servos as far back as possible. It gives you more flexibility in adjusting the c.g.
- Exponential really confuses things if you are not banging the sticks. The airplane starts into the turn slowly, then accelerates. This means that it is turning fastest during the last part of the turn—you don’t really want this. I don’t like it—you might. I won’t argue with you either way.
- Jumpiness can be caused by a bad servo, loose linkages, wind, a broken wing, or just a crappy airplane. If you are making a good smooth turn at One, using full stick and 3/16” throw, then you are probably not too tailheavy.
- Being too noseheavy is not all that bad. It is sort of like flying an overweight airplane—you can still win with one, but you might could do a little bit better. Just a little.
- A noseheavy airplane will have more tendency to dip its nose in the turn, and conversely, a tailheavy airplane will try to roll out of the turn. (see previous crap traps) Wing weight might need to be adjusted as you change the c.g.
- As you move your c.g. and change your throws, continue to measure them. Sometimes you get them out of whack without knowing it. You can always start over.
- Inertia forces are greater than aerodynamic forces during take-off. This means that having a tailheavy airplane with a small amount of throw might be hard to get off the ground, then go straight up or snap as the aero forces take over from the inertia forces. Be careful taking off tail heavy airplanes.
- Some airplanes just won’t turn as good as others. If you are getting snapping at One, wiggling coming out of Three or One, or just unpredictability of any type, then back off the throws and fly it a little more smoothly. You will go just as fast, or faster.
- Keep good data. It will help next time.
Food for thought: I laugh a lot when I hear, “I like a lot of throw so I can get out of trouble”. How do you think you get into trouble?
Dub
