RC Airplane World flight school
- lesson #4; weight and balance
Following on from lesson 3, this rc flight school page will teach you how to check the weight and balance of your rc airplane.
Getting the weight and balance correct before you fly your plane is so important, especially the balance.
Every airplane, whether model or real, has a Center of Gravity ('CG') that is determined in the plane's design stage. This CG is crucial to the airplane's flight characteristics, and getting the plane correctly balanced around the CG is an absolute must before you fly.
Checking the all-up weight (AUW) of the airplane isn't as crucial as the balance, although it's still very important if you've built from a kit. If you've bought an RTF, there shouldn't be any issues with the airplane's weight.
To weigh the plane, pop it on some accurate scales with the motor battery pack. Cross-check this weight with the manufacturer's recommended/stated weight to make sure there is no major difference.
If your plane is seriously overweight to what it should be, you need to do something about it because the wing loading is going to be too high for your airplane to fly correctly.
Realistically, your only options are to use a smaller/lighter battery pack or smaller (but still compatible) servos and receiver, and try and push the weight down that way.
Checking the balance (CG)
This is the important one, and it's easy enough to do.
With the motor battery pack in position, as per the instructions, mark the stated Center of Gravity. This CG position should be in the manual, and will read something like "75mm (3inches) back from leading edge", for example.
This means that you measure the stated distance back from the leading edge of each wing and mark the spot with a pen on the wing undersides. You should mark the CG close to the fuselage, rather than further out down the wing. One or two inches out from the fuselage sides is fine.
Once marked, place the tip of each index finger on each spot, and gently lift the plane...
Above, lifing the plane on its CG to check balance
The picture above gives you an idea of what should happen; either the plane will hang more or less horizontally or it will be nose down or tail down.
If it hangs level, then the CG is good and you don't need to do anything. If it hangs slightly nose down, this isn't a problem either.
If it hangs very nose down, or at all tail down, then you need to adjust the balance to get it to hang level.
Your first option is to try moving the motor battery pack forwards or backwards, depending on which way your airplane is hanging. This is the best thing to do to adjust the CG, because you're not adding dead weight to the plane in the form of ballest.
If you can't shift the pack because it's too tight a fit in its compartment, then you need to add ballast to either the nose or tail end of the plane. If this is the case, add the weight as far out as you can, because this will have the greatest effect with the least amount.
So, if you need to add weight to the nose because the plane is tail heavy, then look at putting modeling clay inside the spinner, for example. Or, if you can't do that, tape something to the underside of the nose, as far forward as you can.
If you need to add weight to the tail end of the airplane, because it's nose heavy, then try and add it to the very rear of the fuselage, where both sides come together so to speak. If your plane is foam, you could screw an appropriate size screw into the foam, for example.
Whatever weight you add to the nose or tail, make sure it's very secure! Having the weight fly off during a flight will upset the CG, and this is going to adversely effect the plane's flight characteristics.
It's important to understand that a slightly nose heavy plane will still fly fine. A tail heavy plane, however, is bad news; the plane will be at best very difficult to control, and at worst completely uncontrollable. If your airplane seems tail heavy when it's balancing on your fingertips, do something about it!
Roll balancing your RC airplane
This is often overlooked and isn't as critical as getting the CG correct, but a plane that has one side heavier than the other will want to roll and turn to the heavier side, making your life a bit harder.
To check your plane's roll balance, simply loop some thread around the propeller shaft, and then some around the rear of the fuselage as close to the tail as you can get it. Lift the plane up and let it hang freely - see if it wants to roll to one side or another. If it does, then you need to add some weight to the wingtip of the lighter (higher) side.
Add only enough weight to make the plane hang level, when viewed from the front. Taping the weights to the wingtip is an easy method, although you might want to take the trouble to set them into the wingtip and cover over them to hide them from view.
For foam wings, pushing a small gauge screw, nail or panel pin into the foam is a great way of adding any necessary weight, and is easily hidden with white paint or marker pen.
Above, use 2 lengths of string around each end of the plane to check the roll balance
Take your time with the balancing of your rc airplane, and you'll be rewarded with easier flying and less trimming.
Next up: Lesson 5 - Finding where to fly your rc airplane.
Or skip to the lesson appropriate to your current situation...
- Lesson 1: Buying your rc airplane
- Lesson 2: Know your rc airplane
- Lesson 3: Preparing your plane
- Lesson 5: Where to fly your plane
- Lesson 6: Pre-flight checks
- Lesson 7: Take off / hand launch
- Lesson 8: Flying your rc airplane
- Lesson 9: Landing your rc airplane
Got a question about your rc airplane weight and balance? Join the forum!
