Fuel Tank….
I use a DuBro 50 oz tank and I plumb it so it sits upside down in the airplane. That reduces the area that the fuel collects in which keeps the clunk submerged with less fuel. I run three lines to the tank, a pickup line, a vent, and a fill. The fill goes to the bottom of the tank so that you can fill and drain the tank from that line. After filling, you must plug this line. I usually use a fuel dot arrangement. On the pickup line, I run a small length (4”) of brass tubing in between the stopper and the clunk to prevent the line from doubling over itself and getting stuck in the front of the tank. I use the large clunk that comes with the tank, and I don’t drill it out. Fuel tubing is Prather large or equivalent, which is about a 3/32” id. Don’t use the larger ID fuel tubing, it can cause feed troubles. I had a problem with an engine cutting out in the turns and traced it to the larger fuel tubing and the routing of the fuel lines. I had a small section of 5/32” ID fuel tubing that ran vertical, and the G loading was enough to back the fuel up because of the extra mass in the column of fluid inside the larger fuel tubing. The G loads would overcome the pressure and stop the fuel flow for a second or two. I also wrap the outside of the fuel tank with fiberglass reinforced packing tape, usually just one layer. I don’t have any idea if this is necessary, or if it’s doing anything important. Somebody told me to do it, and I’ve been doing it since I went to a pressure system, and haven’t had the time to experiment leaving it off. My brain tells me that it’s probably not necessary, but you never know!
Header Tanks…don’t use them anymore. I did for a while and they caused problems for me. The height of the header tank is critical. When your main tank runs out of fuel, and the line between your main tank and your header tank goes dry, you will still keep running on the header, but the mixture goes rich. Of course the main tank never just runs out, the level drops down to nearly empty, you censored a little air here and there, and the mixture is fluctuating between good and rich. Because of this, you have to run a large enough main tank to last the race…so you have a weight penalty by running the additional tank and fuel for the header. I’ve never had much of a problem with the mixture changing from the beginning of the flight to the end with just a main tank, so I trashed the header tank setup. If anything, with just a main tank, your mixture should go slightly rich over the duration of the fuel burn…probably not a bad thing, and I haven’t had a noticeable change.
I place the tank as high in the fuse as I can get it and I make sure that it is angled with the “tail” of the tank downward a fair amount. That will keep the fuel pooled where the clunk is and let you use more of the tank before censored ing air. It also helps keep the clunk submerged in dives. I try to run the vent line up into the chin of the tank, which is the top of the tank in my setup, so that you’ll get a very full tank before it overflows.
Fuel line routing.
As I mentioned before, try to avoid routing the fuel line with any vertical runs. If the fuel line must change elevation, do it gradually over a few inches. You’ll also want to support the fuel line if it has to travel a fair distance so that G forces will not cause it to sag or kink…that can create a vertical or near-vertical run which will give you problems. I use small zip ties on every fuel line to fuel tubing or nipple joint to prevent a line from popping off. The system is under pressure, so they will want to work their way off. If they do come off, you could have a fire and will almost certainly lose your airplane.
On the pressure line from the crankcase to the check valves, I like to use a hard wall tubing like tygon, or something similar to what normally goes between the crankcase and pump nipple on a warlbro-type carb. I don’t know if it’s necessary, but it’s worked well for me. In the old carb days, the pump on the carb worked better with this line and the thinking was that the tubing wasn’t ballooning like a silicone tubing, and the pulses would then be nice and sharp. Again, not sure if it’s doing anything, I just tend to keep doing what works.
Check Valves
I use the YS 0405 valves. I’ve tried the other large-body YS valves, but they take a lot of pressure to “open”…especially when you have two inline. I use two valves inline because my tests have shown that you get more pressure than with just one valve. I don’t know why. The 0405 valves are membrane only on the inside, and do not have a ball or spring. Make sure that the two halves of the valve are threaded together tightly. They sometimes are loose right out of the bag.
Pressure Dump Valve.
I recently started using the B&B specialties smoke valve as a pressure dump valve. I saw this first used by the Powells, and it works well. Craig Greening ordered some silicone o-rings for them, and I replaced the stock rubber ones with silicone so they will stand up to the fuel/nitro better.
Fuel Filter
I use a large Dubro fuel filter in the fuel line before it gets to the mixture valve. You’ll want to trap any junk there so it can’t clog the mixture valve and alter the needle setting.
Mixture Valve
I use the Jet Model Products valve because the pitch of the adjustment threads are coarser than the BVM valve, which means for a given amount of rotation there is a bigger change in the mixture. This allows you to shut the fuel off with the mixture valve at low throttle and still have enough fuel flow at full throttle. I set the linkage up with ball links on both the valve and the servo arm. On the valve ball link, I cut a notch in the nylon part to allow a little extra travel when it hits the metal arm on the valve at the extreme end of the travel. I also like to use a short section of nyrod between the servo and the valve to allow for some flex in case the servo is overdriven because of too much travel. I set my tx mix up so that with low throttle and with the mix knob at 12 o’clock, the valve is as far closed as the linkage will allow. I then loosen the jam nut on the top of the valve and close the needle with an allen wrench until it just barely seats. Then reset the jam nut to keep it from moving and verify the valve is closing all the way by placing some fuel tubing on one end of the valve and censored ing through it. If it is closed, it will hold suction. I set the top end of the tx mix so that at full throttle and with the mix knob at 12 o’clock, I get ¼ turn of rotation on the valve. You probably won’t need a full ¼ turn, but it’s a good place to start. I use a multi-point mix that allows for other points on the throttle curve to have their own mix values, but I usually end up with a pretty linear mix. I do like to set the mix up so that when you pull the throttle down 4 or 5 clicks below full throttle, the mixture valve stays in the same place as full open. If you do this, the top of the mix curve will have a little plateau. This lets you back the throttle off after the race and richen the motor a little. It also is a good climb-out setting to keep the engine cool on takeoff, and allows you to richen the motor in an emergency if you run out of adjustment on the mixture knob.
Operation
Starting
When I start, I use half-throttle and leave it there. When it fires I leave it at half-throttle. When I’m waiting to takeoff, half-throttle. When my launcher is pushing the plane out, half-throttle. Takeoff, half-throttle….I’ve found that at about half throttle I’ll get about 7500rpms and the best cooling on the ground, so that’s where I like to leave it. Your mileage may vary, but that’s what works for me. I’ve also found that when I color a plug, it’s almost always from getting hot on the ground, and not in the air. Some people like to clear the engine out just before your launcher releases the airplane for takeoff. I have had plenty of problems with this, so I don’t do it. The high throttle clear out period raises the pressure in the fuel tank, and when you lower the throttle back down to take off, it loads up and sometimes drops a cylinder. So it’s half-throttle all the time for me.
When I start, I position the throttle at half, close the pressure dump so it is in the run position, and heat the glow plugs before turning the engine. With the starter engaged, the pressure will build slowly and the engine will slowly start to run as fuel starts to get delivered. Once it catches it will usually stay lit. Every once in a great while it will start on only one cylinder, or one cylinder will drop. I’ve never had much luck in getting it to re-light with the engine running, so I close the mixture knob and dump the pressure, but leave the throttle open. That will starve the engine of fuel and it will quit and not be flooded. I then reset the mixture knob and pressure dump and start it again.
On your initial flights with the setup, make sure you can make the engine rich on the ground with the mixture knob. Then just launch and fly it. I’ve never had ANY luck trying to set the mixture on the ground. The temps are just nowhere close to where they will be in the air, and the conditions inside the cowl for induction temp and pressure are not the same either. Once in the air, use your mixture knob to adjust the engine from a rich setting towards lean. For racing the course, I’ve always had the best results by leaving it one click on the rich side of full lean., but sometimes one click rich is too rich. You’ll have to experiment to see what works best for your engine build and cooling setup. Once you’ve got the mixture set, land and make a note of where the mixture valve is positioned at high throttle. Then use the tx mix to position the valve there with the mixture knob at the 12 o’clock position. Now you’ve got equal adjustment on both the rich and lean side. When I race, I do a test flight on Thursday to set the mixture, and unless weather conditions change drastically, or I change nitro %, I don’t have to touch my mixture knob the rest of the weekend.
Shut-down
After the race I pull the throttle down a few clicks, which puts me in the rich range of my mixture curve, and wait for my turn to land. I like to get the engine shut down before it starts to surge at the end of the tank. When it’s time to shut-down, I lower the throttle to the full-low position and dump the pressure. The engine will keep running for quite a while even with the air door/slider completely shut. If it wants to keep running for longer than 10 seconds or so, I’ll open the throttle a little bit and it will quit.
That’s all I’ve got. Let me know if any of this isn’t clear or if you’ve got any questions.
KMac
