The first flight of a new RC model is always an exciting event. Whether you’ve spent ten minutes or ten months getting your aircraft ready for this moment, you’re bound to feel anxious – and probably a little nervous. The best cure for a case of the pre-flight jitters is having confidence in the mechanical, electronic, and aerodynamic soundness of your model. In this article, I will explain the baseline inspection steps that I execute before flying any new model.
The steps shown here are primarily intended for multi-rotors and airplanes. Helicopters are a special case. The concerns are the same, but the techniques for addressing them are quite different. Even excluding helicopters, there are far too many variables among the different types of multi-rotors and airplanes to strive for a one-size-fits-all strategy. Rather, consider these steps as cornerstones of an individualized inspection routine that you can create for your specific model. It is often helpful to create a checklist to guide you through the process.
Don’t Harsh My Vibe, Man
Vibration is bad news on RC models. It causes premature fatigue of mechanical and electronic components, distorts camera images and wastes onboard energy. For models with piston engines, some degree of vibration is unavoidable. You just have to isolate the fragile components the best that you can and live with it. On electric-powered models, however, there is no excuse to not find and eradicate all sources of vibration.
The most common cause of vibration is an unbalanced propeller. Always assume that any new propeller is off-kilter. Most of the propellers I have ever used required some degree of tweaking to make them balanced. The good news is that balancing a propeller is usually a simple process.
The key to eliminating vibration in electric powered multi-rotors and airplanes is to balance the propeller(s)…even brand new ones. This magnetic balancer works well with most RC propellers.
The balancing device that I use in my shop is the Top Flite Power Point magnetic balancer. It suspends the prop magnetically, with the axis of rotation perfectly horizontal. This set-up allows you to detect very small balance deviations. This is especially helpful when working with small propellers.
To use the Top Flite unit, the prop must have a 1/8″-diameter (3.12mm) or larger thru-hole so that it will fit on the suspended shaft of the balancer. The vast majority of RC props fall within that category. One notable exception is the self-tightening style of propeller used on the DJI Inspire and Phantom models. There are alternate methods and products than can be used to balance those types of props.
With the Top Flite device, tapered sliders hold the prop firmly to the shaft and ensure that everything is centered. The shaft is then placed between the magnets. If the prop is out of balance, the heavier blade will rotate downward. The breeze from a ceiling fan or even a heavy breath can cause the prop to windmill in the fixture, so be sure to control your environment. I usually take several measurements (reseating the prop on the shaft each time) to ensure that I’m getting consistent results.
You can either lighten the heavy blade (by sanding away material), or add weight to the light blade. I typically add small pieces of Scotch tape to the backside of the light blade until it balances…making sure the entire piece of tape is firmly adhered. If a prop is molded in black plastic, I’ll use electrical tape for balancing just because the colors blend better. Since electrical tape is heavier than Scotch tape, it is also good for fixing props that are way out of balance.
I also keep a fingertip prop balancer in my field bag. It is handy to have if I ever need to rebalance a scuffed prop at the field (props with any significant damage should be replaced). Although not as precise at the magnetic type, the fingertip balancer works well enough in most situations.
Fingertip balancers such as this are less precise than magnetic balancers, but they are handy for adjustments at the field.
A well-balanced propeller will run vibration-free at all throttle settings. If you still have vibration issues, make sure that there is no slop between the prop and the adapter that attaches it to the output shaft of the motor (or gearbox). Many aftermarket propellers come with a selection of spacers to fit different shaft sizes, so choose the proper one for your application. You may also need to verify that the motor shaft is not bent. These shafts are usually easy and inexpensive to replace.
There are rare times when a propeller is off-balance because a manufacturing defect caused the hole to be nonconcentric with the propeller’s hub. In those cases, you’re probably better off just getting another prop. Similarly, you should always use a reamer (tapered or step) if you have to enlarge the hole in a prop. Unless you have the tools and skills of a machinist, enlarging the hole with a standard drill bit will make a mess of things.
Always use a reamer when enlarging a prop hole. Standard drill bits do not work well for this application.
All of this assumes that you are working with a 2-blade prop. Although much less common, 3-blade and 4-blade props are also off-the-shelf items that should be balanced. The techniques for addressing those types of props are a bit more involved. I plan to do a follow-up article on that exact topic, so stay tuned.
One of the factors that most influences the flight characteristics of an aircraft is its center of gravity (CG). The CG is the point at which the aircraft balances on all axes, with the fore/aft balance often being the most critical. The distance between the CG and the center of lift (CL- the focal point of an aircraft’s lifting forces) can have a profound impact on a model’s flight stability…especially with fixed wing aircraft.
The distance between the center of gravity and the center of lift can have a profound impact on a model’s flight stability…especially with fixed wing aircraft.
With multi-rotors, the CL is variable and determined by the relative speeds of the motors. In fact, changing the CL by increasing or decreasing the RPM of individual motors is the primary control method for multi-rotors. The job of coordinating the balance between the CG and CL is performed by the flight controller (FC).
Due to the active oversight executed by the FC, attaining a precise CG is not critical for most multi-rotors. If one side of the machine is heavier than another, the FC will compensate by feeding a little extra juice to the motors on the heavy side. That being said, you are likely to attain your best efficiency and performance after fine tuning the CG.
With aerial-photography rigs where you typically fly in a level attitude, you may find it best to locate the CG right at the collective center point of the rotors. Racing quads spend much of their flight time pitched forward, so you may find better results by shifting the CG towards the nose. Experiment and analyze your results.
With fixed wing airplanes, the CG is crucial. Sometimes shifting the CG just a few millimeters can make the difference between a stable model and an unflyable one. That margin is usually much larger, but it is something that you must always pay attention to. Whether an airplane is prebuilt, assembled from a kit, or scratchbuilt from plans, the directions should specify a CG location.
When measuring CG, you should make sure that the airplane is ready for flight with everything installed. Leaving off a seemingly innocuous item such as a spinner or tailwheel could drastically skew the results. Also be sure to account for any weight shifts that may occur in flight (emptying fuel tank, retracting landing gear, bomb drop, etc). You’ll want to measure the CG at both extremes.
The blue sphere shown here is the head of one of the sewing pins that have been inserted in the wing of this foam model. The pin heads allow me to quickly verify the model’s CG with my fingertips without marring the delicate finish.
Measuring the CG can be as simple as balancing the model on two fingertips. Although imprecise, this method often works well enough for common mid-size airplanes. If you try this technique with a foam aircraft, you may find that your fingernails leave indentations in the surface of the wing. In these cases, I’ll glue a sewing pin with a spherical head into each wing panel at the desired CG location. The pin heads then become my finger rests. This prevents damage to the foam and also adds the precision necessary for smaller models. Just be sure that you do not stab the pin into any wiring or structure within the wing.
A more accurate way to measure an aircraft’s CG is to use a tool such as the Great Planes CG Machine. It can handle airplanes in a wide variety of sizes and it measures the CG location in millimeter increments. If you have a model that is awkward to hold or is particularly sensitive to the CG placement, using a tool like the CG Machine is really the only sane way to go. It works for multi-rotors too.
This CG Machine offers the most precise and reliable way to measure the CG of RC airplane models.
If you must make adjustments to get the proper CG for your model, it is best to start by relocating heavy components such as the battery. If that isn’t enough, you’ll have to consider adding ballast to the nose or tail. Sometimes you can make the weight increase useful with things like a larger battery. Other times, adding dead weight is your only option. Hobby shops sell self-adhesive leads weights for this purpose. Whatever the case, you should never fly a model until you are sure that the CG is within limits
The CG Machine can also be used to measure the CG location of various multi-rotors. My Strider 250 racing quad is shown.
Another significant factor that will affect the performance of your model is the amount of power that the motor produces. Regardless of the type of aircraft, more power will allow it to fly faster, carry more payload, or climb more quickly – perhaps even some combination of those things.
Another significant factor that will affect the performance of your model is the amount of power that the motor produces.
With piston engines you wouldn’t normally measure or calculate the actual horsepower of the motor. The expected power level of a given engine is usually known. So it is common to verify that things are in the ballpark by measuring the RPM (using an optical tachometer) with a specified propeller and fuel.
Electric motors are a different matter entirely. Their power output is determined by numerous variables. Explaining the details of how these elements mingle to determine the motor’s power will have to be another follow-up. With a ready-to-fly airplane or multi-rotor that includes (or specifies) the motor, propeller, and battery, the work is already done for you. Yet, it is still important to know how to evaluate the power system to ensure that everything is working within the design parameters.
The tools I use for evaluating my power systems are the Astro Flight Super Watt Meter and Great Planes Power Match. These two units are very similar but the Power Match can store readings and also has additional capabilities for balancing LiPo battery packs. There are several other brands of meters that do the same thing. I particularly like these units because all of the pertinent data is displayed on the screen at once.
Being able to measure the performance parameters of an electric-powered model is vital for ensuring that all of the components are operating properly. These measurement tools also provide valuable data when you make component upgrades or changes.
It is very important to have your model secured in one place when evaluating the power system. I anchor my multi-rotors with 10-pound steel ingots placed over the skids. I usually have an assistant hold my airplane models in place. You’ll produce a good bit of wind, so I recommend doing these tests outside.
The meter must be plugged in between the aircraft’s battery and the Electronic Speed Control. Once the system is armed, you advance the throttle to full power. These types of static tests impart the highest loading that the power system will experience. So, you should run the motor only long enough to gather your readings and then back off.
The meter displays the real-time amperage drawn by the system, the power produced (in watts), the voltage of the battery, and the cumulative amp-hours consumed. It’s all useful info. Knowing the maximum amp draw, you can verify that it does not exceed the limits of any component in the system. The wattage reading allows you to ensure that the motor is not producing more power than it is rated for. A low voltage reading may indicate a bad cell on the battery (LiPo batteries should drop no lower than 3.5-3.7 volts/cell under load). Some modelers extrapolate the amp-hour value to predict the model’s flight time, but I typically do not.
A power meter should be considered required equipment for pilots who fly electric-powered models.
I execute this test before for the maiden flight of all but my smallest airplane models. There have been numerous times that I uncovered a problem that could have caused a crash if left unnoticed. I do not typically run the tests for ready-to-fly multi-rotors unless I need to troubleshoot a problem that was sensed by other means.
A power meter should be considered required equipment for pilots who fly electric-powered models. As you become more experienced, you will find these meters to be indispensable tools when tweaking a power system for the best performance. Sometimes a simple propeller change is all you need to liven up a lackluster model. Sometimes it takes more. Using a power meter to establish before and after measurements will uncover the precise effects of any change that you make.
It’s All About Control
The final preflight element that I want to talk about doesn’t really concern multi-rotors. It’s all about measuring the amount of throw on each of an airplane’s control surfaces. Like CG, control movements are typically called out in manuals and plans. More throw will make the airplane more responsive, while less throw will make the model more docile. The suggested throws should be considered a starting point.
As mentioned in my overview of computer transmitters, you will likely want to configure your model with dual or triple rates so that you can adjust the responsiveness of your model in flight. You will simply set a different level of control throw for each position of the rate switch. While you’re in there, review the section about exponential as well. It is too often overlooked.
Specifications for control throws are typically provided as units of distance. For instance, “+/- 15mm” would indicate that at full deflection, the trailing edge of the control surface should move 15 millimeters from its neutral position (measured at the widest point of the control surface). Throw specifications are sometimes provided in angular degrees, but it’s uncommon.
The distances that an airplane’s control surfaces move will largely determine the responsiveness of the model in flight. This clip-on meter makes obtaining those measurements super easy.
The easiest way to measure control surface throw is to use a deflection meter. I use the Great Planes AccuThrow. You just set it in place over the subject control surface with the arced ruler near the trailing edge. Then you move the control stick to full deflection and read the measurement. Easy.
Before putting that new model in the air for the first time, follow the steps shown here to make sure that it’s as ready as it can be. I think you will find that it is time well spent. You’ll begin your flight with fewer variables and more confidence.
Terry spent 15 years as an engineer at the Johnson Space Center. He is now a freelance writer living in Lubbock, Texas. Visit his website at TerryDunn.org and follow Terry on Twitter: @weirdflight