For years I have stressed the point that when you start flying giant scale models, you should do it in a scale-like manner. To rely on the wing’s lift as full-scale pilots do, rather than on the propeller’s thrust like many sport RC models are flown. The larger airplanes in many ways, fly better than their smaller counterparts but at the same time, they require serious attention to detail. This is especially true for your first really big model. If you are about to fly a model that IMAA legal (or close to it), let’s look at some things you need to check before a successful first test flight.
Formula for Success
By all means find an experienced friend who can help you check the new model out and, even make that first flight for you. Remember, a successful first flight doesn’t start at the flying field; it starts on your workbench. I give you my word, that if you build a sound and straight model, your chances of a good first flight will be 100 percent better. Instead of going over every single thing that could become an issue, here’s a pre-flight checklist of questions to ask.
I call it my “10-top reasons not to fly!” That is to say, if you can’t answer yes to all ten questions, then you should reconsider your decision to fly, at least for the time being.
1. Are all your controls installed properly with hinges that don’t bind and with 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, smooth operating hinges. Pull on the control surfaces and make sure everything is tightly secured. Make sure you use more hinges as well. If you normally use three hinges for a given control surface, then use five in your bigger models.
2. Are your servos and pushrods properly installed? Tight control surfaces aren’t much good if you have excessive play and slop in your control system. I like to install my servos in plywood plates that are secured with screws to large hardwood rails. I also add ply strips under the servo screws. This provides more material for the screws to thread into. Depending on the type of airplane you’re flying, you can use several different kinds of pushrods. For lightly loaded, fabric covered models, I still use hardwood dowels and wire pushrod ends. Flexible pushrods with plastic outer sleeves should be braced every six to eight inches with scrap balsa so it can’t flex under load. For the heavier models, larger heavy-duty composite pushrods and/or flexible pushrods should be used.
3. Are your servos, pushrods and control horns the proper size? Servo torque or output strength must be up to the task . To minimize bowing and flexing, the pushrods and wires should be large enough to handle all expected flight loads. The control horns must also be strong enough to transfer the control system’s power to the control surface. Always use 4-40 size pushrod wires and clevises and some sort of keeper on the clevis. This can be a spring retainer, 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 Mfg.’s swivel control horns and clevises because they use small screws and locknuts to join them together.
4. Is your battery pack big enough and properly charged? With bigger control surfaces and stronger servos on board, you need larger capacity batteries. Start thinking 4-digits when it comes to battery size. My minimum for even moderately large models is 2000mAh. If you are using a large amount of servos (one for each control surface), or if your model will be experiencing high flight loads, (doing aerobatics), then go with even higher capacity battery packs as they are all now readily available. Some radio systems are able to use High Voltage packs (6V), and there are servos specially made for the increased voltage. It is also a good idea to use larger heavy-duty radio on/off switches.
5. Is your engine running properly? To fly safely, your engine needs to run reliably and idle low enough so you can land easily. Throttle response should be as smooth and the carb must be adjusted properly. For a 2-stroke gas engine with a high and a low needle, a slight burble in the midrange is acceptable. Your prop also should be the correct size for the engine and it should definitely by balanced to minimize vibration. The engine should be mounted securely and for gasoline engines, a kill-switch should be used to for safety. A very convenient set up is 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, then make sure its battery pack is fully charged and operating properly. How about the glow plug or spark plug? Are they new? They should be.
6. Does your model balance properly? Is the center of gravity within the acceptable range shown on the plan? Next to battery failures, improper CG location is the next most common reason for losing a model. Try to place as much weight forward of the CG as possible while building the model. 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 wing tip is not a lot heavier than the other. This will help minimize aileron trim changes.
7. For gasoline engines, is your fuel fresh and properly mixed with 2-stroke oil? If this is the first flight of the flying season you should not use the fuel that been sitting in the container all winter. Properly dispose of it and buy new gasoline. Make sure you use a filter when filling the container and use a filter when you fill your model’s tank.
It doesn’t take much debris to clog up your fuel line. Also make sure to use gasoline safe fuel line and tank hardware.
8. Did you range check your radio? This is a very basic thing that is often overlooked in the excitement of a first flight. Make sure you can get at least 75 feet with the TX antenna collapsed. If you get servo jitter try relocating the RX antenna wire to a different position. I usually install the RX antenna externally to keep it away from long servo leads in the tail of the model. Also check the radio range with the engine running.
9. What’s the wind like for the test flight? Try to pick a calm day for your first flight. You’ll be nervous enough without having to worry about gusty of cross-wind conditions. Make it easy on yourself and wait for the best conditions.
10. Finally, are all the controls working properly and moving in the correct direction? With a computer radio, make sure you are on the correct model memory for the model you want to fly. Often, reversed controls can happen just by having the wrong model settings dialed up.
If any of these questions cannot be answered satisfactorily then please consider not flying. Go back to the workshop and fix anything that in question. If you build your model properly from the beginning and test run your engine at home then most of the time, the check list will serve only to reinsure your confidence in your model and your craftsmanship. Confidence in yourself and your model goes a long way in making the hobby both safe and fun.