Buying Used

BUYING USED EQUIPMENT - by Bryan Jones [from The Flightline, Pearland TX, Bryan Jones, editor.]

Have you ever been presented with a deal too good to be true? Sometimes they are good deals, other times... well. One thing we have in our benefit living in the Houston area is a very large group of RC airplane flyers. There are several outlets for buying and trading model airplanes and their related accessories. Regardless of where you go to find the used equipment you desire, there are a few tips I have learned you may want to consider.

Airframes- These are the easiest items to inspect. The first and easiest items to check is the covering or paint. Having a well-applied and thoroughly sealed covering or coating is important in keeping oil and other materials from the underlying wood or fiberglass. Water or oil soaked structures will eventually weaken and fail. Look in the engine compartment for the sealing I have mentioned. Exposed wood is easy to spot. Another area critical to an airplane's structural integrity is the wing saddle and attachment structure. Look here for cracks or evidence of previous repairs. Generally, any joint having been repaired will be weaker than originally constructed. If the joint shows sign of repair, this indicates design or crash damage. Assume it is crash damage and inspect the tail feathers and other exposed inner surfaces in the fuselage.

Wings are a little more of a mystery than the fuselage. Without breaking the wing, place it over your knee and apply bending pressure. Listen for cracking noises (Stop then!). Look for splinters falling out any openings. Check control surface tightness and proper operation. Look for wing tip damage. Wing tip damage comes in two forms: first, the underside scrapes caused from ground loops and hard landings. Second the crunching effect on the end of the wing tip caused by cartwheels. Cartwheels will trash a model quicker than almost anything.

Engines- Purchasing a used engine is not quite as easy as purchasing an empty airframe. The first item of concern is external damage. Look for dirt, particularly that packed in between the forward cooling fins or around the carburetor. This is a pretty good indicator of a crash. Don't forget looking for the broken cooling fins and bent needle valves. Once you have checked the engine externally, look at the cylinder head. Assure all head bolts are present. Check the crankshaft. Look for buggered threads.

One thing I strongly recommend is checking the shaft for run out with a dial indicator or similar instrument. I wouldn't accept any more than 0.002" TIR (total indicated run out) on .60 and smaller engines; 0.003" TIR on all others. Bear in mind, this measurement should be weighed in relation to the rest of the engine and these run out measurements are pretty high.

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 the bearings and the piston/cylinder liner fit. Look down the port at the piston and the liner. Look for gouging and excessive scraping or scratches. Feel the engine as it is turned over. Notice any grinding or gritty feel in the bearings. Try and find out if the engine has ball bearings or sleeve bearings on the shaft. A ball bearing engine (with good bearings) is more valuable.

Hang onto that dial indicator we used earlier and set it up to check shaft looseness. When you get the indicator set up, pull the shaft in the opposite direction than it is being pulled when you set up the indicator. On engines 60 or smaller, 0.001" to 0.002" is reasonable. Larger engines can withstand 0.003" to 0.005" looseness. Finally, check the thrust on the shaft. While holding the engine in one hand, push and pull the shaft while turning it. Note any noises or unusual feels such as metal on metal rubbing or gritty feel. This is not particularly a problem in the inactive or reverse thrust direction, but may 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 would have to research the issue more before writing.

Radio Gear- This is a more challenging area than the previous two. Bear in mind the consequences of a complete radio failure... not pretty. Keep this in mind when you are about to make that killer deal. I have a few easy items to look for when buying used radio gear. These items typically do not indicate the actual condition of the internals but are a very representative indicator.

First, the general external appearance of the transmitter, receiver, and servos are important. Look for dirt, glue, or fuel residue. None are good. Even more important, check the switch harness from one end to another if you must use a used item. I don't recommend it. I only use switches I have purchased new. One failed switch or switch lead and the game is over.

The external condition of the transmitter is a good indicator of how the entire system was treated by its previous owner. Check the bottom and back of the transmitter case for excessive scratches. This indicates the amount of use the system has had. Less scratches, less use, good, good. Check the feel of the gimbals. Smooth and tight. Check the trim switches and auxiliary switches. Extend the antenna, checking for bends or damage. Turn on the transmitter and check the output/power needle response. Obviously the batteries may be dead or undercharged.

Look at the receiver antenna. Is it in good shape? A kinked or stressed antenna indicates rough use and possible damage. Look for cracks in the case. Check for narrow band certification. Check for bent pins in the open sockets.

The servos are the least important items, but don't forget, it only takes one well-placed servo failure to wreck your plane. First, check the outward appearance. The leads are important as well. Look to see if the wires 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 strip the servo, you may have to buy a wrecked servo. Don't do this step if you don't feel sure of what you are doing. If you do, feel and listen for broken gear teeth.

Flight battery pack -- be very careful. I wouldn't recommend using a flight pack if you don't have a cycler/charger to verify the capacity and health of the battery. Don't forget to look at the lead. It's just as important as the battery switch.

Finally, connect the components of the system and operate with the transmitter. Check each channel individually, check dual rates, check programmability (if applicable), check servo response (noise, chatter, dragging, speed, etc.). If possible, perform a range check -- collapsed antenna at 200 feet minimum 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 possibility that the equipment was near breaking down or someone was trying to sell away a hidden problem.