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by Bryan Jones
Have you ever been presented with a deal too good to be true? Sometimesthey are good deals, other times... well. One thing we have in our benefitliving in the Houston area is a very large group of RC airplane flyers.There are several outlets for buying and trading model airplanes and theirrelated accessories. Regardless of where you go to find the used equipmentyou desire, there are a few tips I have learned you may want to consider.
Airframes These are the easiest items to inspect. The first andeasiest items to check is the covering or paint. Having a well-appliedand thoroughly sealed covering or coating is important in keeping oil andother materials from the underlying wood or fiberglass. Water or oil soakedstructures will eventually weaken and fail. Look in the engine compartmentfor the sealing I have mentioned. Exposed wood is easy to spot. Anotherarea critical to an airplane's structural integrity is the wing saddleand attachment structure. Look here for cracks or evidence of previousrepairs. Generally, any joint having been repaired will be weaker thanoriginally constructed. If the joint shows sign of repair, this indicatesdesign or crash damage. Assume it is crash damage and inspect the tailfeathers and other exposed inner surfaces in the fuselage.
Wings are a little more of a mystery than the fuselage. Without breakingthe wing, place it over your knee and apply bending pressure. Listen forcracking noises (Stop then!). Look for splinters falling out any openings.Check control surface tightness and proper operation. Look for wing tipdamage. Wing tip damage comes in two forms: first, the underside scrapescaused from ground loops and hard landings. Second the crunching effecton the end of the wing tip caused by cartwheels. Cartwheels will trasha model quicker than almost anything.
Engines Purchasing a used engine is not quite as easy as purchasingan empty airframe. The first item of concern is external damage. Look fordirt, particularly that packed in between the forward cooling fins or aroundthe carburetor. This is a pretty good indicator of a crash. Don't forgetlooking for the broken cooling fins and bent needle valves. Once you havechecked the eninge externally, look at the cylinder head. Assure all headbolts are present. Check the crankshaft. Look for buggered threads.
One thing I strongly recommend is checking the shaft for runout witha dial indicator or similar instrument. I wouldn't accept any more than0.002" TIR (total indicated runout) on .60 and smaller engines; 0.003"TIR on all others. Bear in mind, this measurement should be weighed inrelation to the rest of the engine and these runout measurements are prettyhigh.
Look into the exhaust port on the cylinder. If the muffler is attached,remove it. Slowly turn over the engine while feeling the condition of thebearings and the piston/cylinder liner fit. Look down the port at the pistonand the liner. Look for gouging and excessive scraping or scratches. Feelthe engine as it is turned over. Notice any grinding or gritty feel inthe bearings. Try and find out if the engine has ball bearings or sleevebearings on the shaft. A ball bearing engine (with good bearings) is morevaluable.
Hang onto that dial indicator we used earlier and set it up to checkshaft looseness. When you get the indicator set up, pull the shaft in theopposite direction than it is being pulled when you set up the indicator.On enginges 60 or smaller, 0.001" to 0.002" is reasonable. Larger enginescan withstand 0.003" to 0.005" looseness.
Finally, check the thrust on the shaft. While holding the engine inone hand, push and pull the shaft while turning it. Note any noises orunusual feels such as metal on metal rubbing or gritty feel. This is notparticularly a problem in the inactive or reverse thrust direction, butmay be a real problem indicator in the active or normal thrust direction.
I have purposely skipped the four-cycle engines for a couple of reasons.First, this subject deserved more space than available and second, I wouldhave to research the issue more before writing.
Radio Gear This is a more challenging area than the previous two. Bearin mind the consequences of a complete radio failure... not pretty. Keepthis in mind when you are about to make that killer deal. I have a feweasy items to look for when buying used radio gear. These items typicallydo not indicate the actual condition of the internals but are a very representativeindicator.
First, the general external appearance of the transmitter, receiver,and servos are important. Look for dirt, glue, or fuel residue. None aregood. Even more important, check the switch harness from one end to anotherif you must use a used item. I don't recommend it. I only use switchesI have purchased new. One failed switch or switch lead and the game isover.
The external condition of the transmitter is a good indicator of howthe entire system was treated by its previous owner. Check the bottom andback of the transmitter case for excessive scratches. This indicates theamount of use the system has had. Less scratches, less use, good, good.Check the feel of the gimbals. Smooth and tight. Check the trim switchesand auxiliary switches. Extend the antenna, checking for bends or damage.Turn on the transmitter and check the output/power needle response. Obviouslythe batteries may be dead or undercharged.
Look at the reciever antenna. Is it in good shape? A kinked or stressedantenna indicates rough use and possible damage. Look for cracks in thecase. Check for narrow band certification. Check for bent pins in the opensockets.
The servos are the least important items, but don't forget, it onlytakes one well-placed servo failure to wreck your plane. First, check theoutward appearance. The leads are important as well. Look to see if thewires are damaged where they are attached to the plug. Look for plug damage.CAREFULLY check the gear train by rotating the servo head. If you stripthe servo, you may have to buy a wrecked servo. Don't do this step if youdon't feel sure of what you are doing. If you do, feel and listen for brokengear teeth.
Flight battery pack -- be very careful. I wouldn't recommend using aflight pack if you don't have a cycler/charger to verify the capacity andhealth of the battery. Don't forget to look at the lead. It's just as importantas the battery switch.
Finally, connect the components of the system and operate with the transmitter.Check each channel individually, check dual rates, chek programmability(if applicable), check servo response (noise, chatter, dragging, speed,etc.). If possible, perform a range check -- collapsed antenna at 200 feetminimum fully operational.
These are just a few items to keep in mind when purchasing used equipment.Even if everything checked out as described here, there is a possibilitythat the equipment was near breaking down or someone was trying to sellaway a hidden problem.
[from The Flightline, Pearland TX, Bryan Jones, editor.]
