Test-flying your big bird

Model Airplane News, May 2001 by Yarrish, Gerry

Sooner or later, you will reach the point at which you want to test-fly a new model yourself and not leave it to a more experienced builder. Let's look at some of the many things you need to check and consider to ensure a successful first test flight of any model, and especially your first giant-scale plane.

FORMULA FOR SUCCESS

By all means, ask an experienced friend to help you check a new model to see whether anything is missing or whether the setup is incorrect. A second pair of eyes is always good. Remember: a successful first flight doesn't start at the flying field, it starts on your building board. I give you my word that if you build a sound, straight model, your chance of having a good first flight will be 100 percent better. Here's a preflight checklist of questions; I call it my "10 top reasons not to fly!" If you can't answer "yes" to all 10, you should not fly-at least, for the time being.

1. Are all your controls installed properly with hinges that don't bind and have only a minimal hinge gap? Big airplanes have larger engines, props and control surfaces than you may be used to. Flutter can be a real problem if you don't use strong, smoothly operating hinges. Pull on the control surfaces and make sure they are all tightly secured. Use more hinges; if you usually use three hinges for a given control surface, use five in your bigger models.

2. Are your servos and pushrods properly installed? Tight control surfaces aren't of much use if you have excessive slop in your control system. I like to install my servos in plywood plates that are secured with screws to strong hardwood rails. I also add doubler strips under the servo screws to give them more material for the screws to bite into. You can use several kinds of pushrods, depending on the type of airplane you fly. You can still use dowels and wire pushrod ends, but make them with larger-- diameter dowels and support them with balsa guides at their midpoints to prevent them from bowing under load. Flexible pushrods that have outer sleeves should also be braced every 6 to 8 inches with scrap balsa so that they cannot flex. For heavier models, use heavy-duty flexible pushrods.

3. Are your servos, pushrods and control horns the proper size for your model? Servo torque (output strength) must be up to the task at hand (see Figure 1). To minimize bowing and flexing, the pushrod wires must be large enough to handle the expected flight loads, and the control horns must be strong enough to transfer the control system's power to the control surface. Always use 4-40 pushrod wires and devises and some sort of keeper on the clevis. This can be a spring retainer, a retainer clip, a small O-ring, or a thin slice of fuel tubing pushed over the clevis to prevent it from accidentally springing open. I prefer to use Robart swivel control horns and devises, as they're joined by a small screw and locknut.

4. Is your battery pack big enough, and is it properly charged? With bigger control surfaces and stronger servos on board, you need batteries with a higher capacity; start thinking four digits for battery size. My minimum even for moderately sized models is 1200mAh. If you use many servos in your model (one for each control surface), or if your model will experience high flight loads (doing a lot of aerobatics), use even higher capacity batteries; 1400, 1500, 1800, and even 2000mAh packs are readily available. For moderate flight loads and current drains, the new nickel-metal-hydride (NiMH) cells offer a higher capacity without a large size and weight increase. It is also a good idea to switch to a larger, heavyduty on/off switch when using larger battery packs.

5. Does your engine run properly? To fly safely, your engine must run reliably and idle low enough to allow you to land easily. Throttle response should be smooth, and the carb must be adjusted properly. For a 2-stroke gas engine with a high needle and a low needle, a slight burble in the midrange is acceptable. Your prop should also be the correct size for the engine, and it should definitely be balanced to minimize vibration. The engine should be mounted securely, and with gasoline engines, use a "kill" switch for safety. It's very convenient to have a second kill switch installed so you can stop the engine with your radio. This can save the day if you find that the idle is too high for a safe landing. If your engine has an electronic ignition system, make sure its battery pack is fully charged and operating properly. How about the glow plug or spark plug? Is it new? It should be.

6. Does your model balance properly? Is the center of gravity (CG) within the range shown on the plan? Second only to battery failure, improper CG location is a common cause of losing a model. While building the model, try to place as much weight forward of the CG as you can. If in doubt, screw some lead weight to the firewall and err in favor of nose heaviness. A nose-heavy condition may not be good, but it is a lot better than being tail heavy. You should also check to make sure that one wingtip is not a lot heavier than the other. This will help to minimize the need for aileron trim changes.