Here ya go.
Mostly race set up but most does apply to your motor.
Carl
AMA Class 1 Mouse Racing objective: It is the purpose of mouse race to fly up to 3 models in direct competition in 50 lap preliminary heat races leading up to a 100 lap feature (final) race. A minimum of 1 refueling pit stop is required in the heat(s) and 2 pit stops in the final. The winners are those with the best scores (times) in the feature race.
The engine requirement is any reed valve engine with an integral tank. This means 99% of racers use some form of a Cox Black Widow .049. It has always been a favorite of mine because no machining equipment is required to be competitive. There is no restriction on aircraft type other than it must be able to take off from the ground (ROG) with a fixed landing gear. It's cheap to fly but oddly enough, cubic dollars have little to do with how you place in competition. The deceptively simple looking Cox .049 engine has been known to stump even the most experienced modelers, causing great frustration.
The following article represents much of what I have learned in my 35+ years of competition in this event. It is my hope that those wanting to fly with high performance Cox reed valve engines will find it useful. This is a much updated version of my earlier article.
1.0 Engine
1.1 Crankshaft/Crankcase Assembly
Problem: Cox .049's (when pushed really hard) are prone to breaking the crankshaft. Usually the crank pin parts company from the crank throw web.
Solutions: The use of the Cox “race car” crankcase assembly reduces this problem as the crank throw web is noticeably thicker on these variants, but they are noticeably slower so that's not so good. Davis Diesel cranks can work BUT you really must bench run them first! REASON: Due to tolerance mis-matching many DDD cranks run very slow and give very poor starting and running characteristics. It's a real gamble. The safest bet is to use Cox “Killer Bee” cranks, for greatest longevity and speed, but with one provision.
You must use a 5-40 prop stud screwed all the way into the crankshaft. You will then need a Cox (or equivalent) spinner to hold the prop on. IF you use the standard 1" Cox
prop screw, the crank will shear off at the spines later on down the road. The fitting of a prop stud seems to have cured the crank shearing problem.
Using a modified crankcase with a bronze sleeve bearing in the crankcase is a gamble. Unless the clearance honing is perfect, it can be noticeably slower than stock. My experience has shown that Cox's hard anodizing makes for a very good bearing surface, and so the stock Killer Bee or Venom set up is more than adequate. It’s a good idea to lay some 400 wet/dry sandpaper over a piece of glass, and with the addition of some oil sand the back of the crankcase. This will remove any burrs that might otherwise prevent a perfect seal with the fuel tank. It's also useful to use a 2-56 TPI bottoming tap on the crankcase holes as extra threads in that area help.
It is very useful to disassemble the crankcase assembly. Thoroughly clean everything, and then polish up the crankshaft with 600/fine sandpaper to remove any nicks, scratches or baked on oil that may be present. For re-assembly, use a 5-40 socket head cap screw and an old prop to draw the crank squarely into the drive plate. When re-assembled clean and dry, give it a spin. It should be really free with no binding whatsoever. On the best examples, the crank throw will even rock back to the bottom. The best engines have less than .015" end play on the drive plate.
1.2 Integral Fuel Tanks
Problem: The stock fuel tanks can be slow and sometimes short on range. They may also have trouble holding a consistent needle valve setting (due to leakage).
Solutions: Use one of the larger 8cc stunt tanks for the greatest range. These are commonly found on Golden Bee, Super Bee, Black Widow and Venom engines. Since the stock needle valve w/ spring arrangement is prone to leaking, modify the needle valve assembly as follows. Remove needle valve, discard the spring, install a #4 flat washer, and then add a piece of medium silicone fuel tubing. Inspect the tip of your needle valve to make sure it's not bent. Re-install the needle valve and you now have one cheap, but air tight needle valve assembly.
With the tank and tank back together, check that the venturi size is .082" I.D. If not, drill out the tank and tank back inlet venturi to .082”, (this is what the record holding engine used). Drilling out the venturi larger than .082", sometimes produces an rpm gain, but often your engine won't run as steady and your range will be less. To me, it's just not worth it. Next, sand the metal tank back flat over glass (again with the 400 paper), as some tank backs are warped a bit and do not sit flat when bolted to the firewall. If you wish, you can use a Dremel tool to grind away the screen holder from the venturi area of the tank back. It looks racy, but I doubt that it makes any difference.
A 5-40 TPI prop stud (screwed ALL the way in) & spinner is used to prevent the Killer Bee/Venom type crankshafts from shearing at the splines.
All the parts required to convert the stock needle valve assembly.
The fuel pickup absolutely must be located at the outboard corner of the tank. The normal neoprene tubing arrangement is prone to moving out of place and giving an unstable engine run. I bend a piece of 3/32" O.D. soft aluminum tubing and make the pick-up one solid piece. I file a chamfer at the bottom of the pick-up tube so that it fits perfectly into the back plate. Attach it to the tank back with a short piece of tight fitting silicone tubing. The net effect will be that the pick-up stays perfectly positioned. You will notice greater range and stable running from your engine, with a properly positioned fuel pick-up tube.
The next step is to prevent the integral tank from leaking. This is a must if you wish to hold a consistent needle setting, and have the engine shutdown properly. In some cases it may be necessary to wrap a piece of 1/2A Dacron line around the entire peripheral groove of the tank to help seal it. Hold the Dacron thread in place with saliva or oil for final assembly. It may help to lap the metal tank and tank back joint instead of using thread. The use of a thread gasket is not necessary if you’re using one of the newer nylon tank backs.
*NOTE: The metal tank backs are much more durable and will often survive a crash without breaking. The newer nylon back plate is much more fragile, and will often be damaged the very first time you crash. Metal back plate fitted engines finish more races whenever flying incidents/accidents are involved. In order to finish first...you must first FINISH!
High-Power REED VALVES: The original Cox copper/beryllium reeds tend to 'float' at ~17,000 rpm and so they are not optimal for top performance. In a reed valve engine, top performance is only attainable by using the clear Mylar reed. I find the cross shape better in tanks that use a 'G-Clip reed retainer wire and the rectangular ones better in the tanks using the nylon retaining cap. This last production change allows reed engines to be on par with “Tee Dee’s” in performance! My test bench results indicate that 24,000+ rpm is achievable for steady-state running with such reeds.
What about 'other ' reed materials and shapes? Cox engine designer Larry Renger prefers the Cox stainless steel reed over the Mylar reed. My main concern with the steel reed is that it wears the anodizing right off the mouth of the venturi tube, although the rpm seems to be the same. I've tried other reeds made of thinner steel, floppy disc material, etc. and so far haven't found anything better. One Australian made metal reed was indeed 300 rpm faster, but it broke away after only a few minutes of running. Teflon reeds may or may not work as well. I've not found them to be any faster, and sometimes worse. The final reed sealing check is done by attaching a piece of tubing to the tank venturi and sucking on it. A proper fitting reed will hold the pressure and not leak.
Final Assembly: Clean the 2-56 tank screws with thinner. Final assembly is done using blue Loctite thread-locker. Every time you tear down the engine, replace both the paper
The replacement pick up tube is bent from 3/32" O.D. soft aluminium tubing & chamfered at the bottom to fix exactly. Silicone tubing attaches it to the tank back.
After final assembly, the tank screw hole areas are filled with RTV silicone sealant for leak prevention.
tank gasket and the venturi o-ring. This may seem like a waste, but $2.00 worth of new gaskets now is $100.00 worth of reliability in the racing circle! Once assembled, apply an RTV type silicone sealant over the tank screw head area. The tank screw heads are a major source of leakage and this really seems to help. Removing the sealant is very easy if you need to remove the screws later on.
*NOTE: Some Cox engines have been supplied with screws that are up to .115" longer than normal. Often these engines will have the tank leak and come loose during running! Bottom taping the crankcase screw holes can help here OR use the proper length screws in the first place. It took me an awful lot of wasted time, lost races, and loose leaky tanks before I found out what exactly was going on here. See Photo to note the difference. *NOTE: If you really hate paying a paltry few bucks for new gaskets, they can be home made. Cut the tank gasket from thick vellum paper and from a piece of silicone tubing, thinly slice the venturi o-ring...
1.3 Piston/Cylinder Assembly:
Problem: Not enough power!
Solution: Any type of piston/cylinder assembly made by Cox can produce good results. However, a superior “fit” will overcome nearly any porting deficiency. This means that a well fitted “Babe Bee” piston/cylinder is better than a bad “Tee Dee” piston/cylinder. I recommend a flying test for all piston/cylinder assemblies in your possession, as certain assemblies will occasionally defy all rules of performance. Having said this, I find that good fitting “Tee Dee” #4 piston/cylinder assemblies to be excellent. Many of the very fast engines use these. The Cox Venom cylinder porting seems to be consistently fastest. With just a little Dremel work, you can make your TD cylinders identical. Personally, I rarely grind in my own cylinders as I'm afraid of doing more harm than good...
Aftermarket pistons: Some enterprising individuals have offered for sale standard Cox pistons that have been lightened by grinding the piston skirt on the inside or by cutting the skirt off at the bottom causing increased Sub Piston Induction (SPI). Actually these mods can be useful, if and only if the fit to the cylinder is good. If the fit is wrong then all is for naught. So of course, a test run is always in order when going with this piston type. A note on SPI numbers: Measured with a feeler gauge, SPI should be in the range of .012" - .025". I've yet to determine exactly how much SPI is too much.
Ball-Socket Resetting: It is worth noting that new Cox pistons often come with the connecting rod ball-socket joint set too loose. Therefore it's a good idea to re-set the fit with a Cox factory tool to .002” slop, or less. Using the wrong piston holding fixture can result in a mushroomed head piston (totally ruined), so I don’t use one. Clean the socket area with acetone or brake cleaner first to remove the oil. Then lay the piston on a heavy piece of ½” plate glass or flat thick piece of steel (something quite dense like a vice). With the tool in
This example is over .100" longer than Standard
Genuine Cox Venom porting.
place, tap with a hammer, rotate a bit, tap again and continue, checking often until all excess play is removed. It is necessary to check this joint after every contest. At normally low rpm’s this isn’t critical. However, at racing rpm’s (20,000+) a loose ball-socket joint reduces piston life to a few minutes.
Piston Fit: Optimum piston to cylinder fit needs to be checked with parts being absolutely clean and dry. To check the fit, slide the piston up the cylinder bore (with no finger prints!) until it sticks. Ideally, it should stick flush with the glow plug land or even slightly higher. Now with a slight tap of the finger, the piston should fall right out of the cylinder. If the fit is slightly looser this may be OK, unless starting consistency deteriorates. In this case, the piston is worn too far for that particular cylinder. Keep it to try in other slightly smaller cylinders in your collection later on.
Please note that some new Tee Dee piston/cylinder assemblies are fit just a bit too tight. Those of you with an excellent feel can go ahead and lap the piston to fit. However, Dale Kirn’s easier way to do this is by just running in the engine. Start by cutting down a 5x3 prop until it turns up to 24,000 rpm rich. Run up to 2 dozen tanks of fuel through the engine, but for no more than 2 minutes at a time, to allow for cooling in between runs. It is better to use a Tee Dee crankcase for this purpose, and transfer the piston/cylinder assembly to the reed valve crankcase when it is run in. The reason being that at 24,000+ rpm some reed valve crankcases will wear out quickly, notably the non anodized cast R/C Bee type.
Lastly, it is very important to keep carbon varnish off the piston and especially the cylinder walls. #000 steel wool or medium grade Scotchbrite wrapped around a small dowel wet with solvent easily removes all the carbon. This procedure takes only a minute, but really should be done before every contest to ensure peak performance. *NOTE: the Davis devarnishing brush can also be used but be careful not to get carried away. Bob Davis recommends only a few strokes as his brush actually hones the cylinder. Used vigorously and too much can cause 'over-honing' making the piston fit too loose. Do be aware of this possibility.
Cylinder orientation: This is something very mistakenly taken for granted. I don't know why exactly, but I've had certain engines lose massive rpm or just 'run funny' by mounting the cylinder sideways. That's expressly why my engine cylinders are all mounted upright. Mounting the cylinder sideways also exposes the needle valve to damage in case of a flip over. I will allow that the odd racer seems to be able to get it to work OK for them. All I will say is try a back to back flight test and note how stable your engine runs before going this route.
1.4 Glow Plug
I use only Cox Tee Dee high compression glow heads for maximum reliability and stable running on a new (or newly re-built) engine. My engines also start out with 4 new head gaskets. A good rule of thumb is one head gasket for every 10% of nitro. (hence 35% nitro = 4 shims).
After removing a new Cox plug from its package, carefully sand the seal band on a plate of glass with 400 paper and oil. This will ensure the plug seal area is flat, and will seal properly when tightened. Now examine the plug element, and with a T-pin, make sure the coil is centered. Finally with a T-pin, very gently pry at the element where it is welded to the plug. It should be a firm weld. If it breaks loose (which won't happen often) you'll unfortunately need a new plug!
After awhile, you may notice that your glow head keeps coming loose in flight. This is due to the copper head gaskets becoming old and hardened due to the constant heat cycling.
Since the Cox engines use an annealed copper gasket, replacing the head gaskets with new ones will solve the problem.
*NOTE: I've heard that annealing the gaskets with a torch can work, but have no experience doing that. I find removing gaskets from a cylinder is easier when carefully using a very thin surgical blade or a Davis brush. A micrometer or calipers is really useful here as the odd head shim is thinner than the standard (.005") giving a less than desired head clearance. Head gaskets can also 'fuse' together and the calipers can easily show this.
A Cox replacement head manufactured by Doug Galbreath that uses the Nelson HD plug can offer better performance at less than half the cost (per plug). These are certainly worth using once you have a stable running engine set up. On the odd engine they run slower than normal, which is why you need to use the TD plug as your performance reference. *NOTE: the Galbreath/Nelson head often gives harsher running until your engine warms up, so go easy on the needle adjustment until the engine gets hot.
There are other glow head alternatives in the form of the Norvel 'Speed Plug' and the Merlin plugs. I do not have enough experience to know if they work well or not. They are certainly worth exploring though.
RPM Performance numbers: Ah...it's pretty easy to banter about high performance rpm numbers, but really they aren't meaningful unless one compares apples to apples and under the exact same conditions of prop, fuel, and weather. And YES, I can get some very impressive rpm numbers with 70% nitro, high compression and a large venturi size at sea level. BUT, such an engine will rarely finish a mouse race...so, let's get real. To that extent using Sig 35% nitro fuel, an APC 4.75D X 4P prop and Galbreath head, (with my 'local Leduc weather'): 19,000 rpm is my minimum standard. 20,000 is quite a "good engine" and 21,000+ is superior. Increasing nitro content can easily give a boost of 1000+ rpm over these figures, BUT the engine/glow plug reliability goes down proportionally.
1.5 Miscellaneous Problems
Problem: Engine comes loose in flight, prop falls off and/or glow plug comes loose in flight.
Solutions: 1. Use the right length tank screws with silicone sealant on top. 2. Check tighten the glow plug, prop spinner, and mounting bolts before every race. 3. If field disassembly can’t be avoided, use Loc-Quik super primer T accelerator, with blue Loctite thread locker upon re-assembly. The tank back can be sealed on the field using a single piece of sticky Fascal tape over the entire backplate with only the venturi area cut away.
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Topic: RPM'S OKAY? First rookie rebuild (Read 2507 times)