[Courtesy of Andre Baird, abaird "at" iafrica.com, August 2000]
There has been some considerable discussion about turbine safety and fire prevention on the list. This prompted a few ideas of my own I would like to share. My background in R/C flying goes way back to when proportional radios just became available. I sometimes wonder how many folks today even know what the terms Galloping Ghost or Reed R/C means.
I am hinting that some of the newcomers should not be so quick to judge the ideas of experienced modelers. They have seen it all. To mention just one of them, if memory serves me right, I remember how many years ago BV came to Cape Town to test a new fan/model system and then flew on to Johannesburg (which has one of the highest density altitudes) to be sure his product performed and worked safely.
Back to safety issues.
Radio Failsafe idle or kill? I cannot believe this is an issue. Any PCM R/C system that goes into hold (or PPM system with glitch's for that matter) is experiencing a serious safety problem and should not be operated as if it is a minor event/inconvenience. The failsafe should be set for engine kill and upon getting back control, be immediately landed safely. The R/C system should be checked as well as the installation.
If the ECU is the problem have the MNF correct it. If the problem persist change the R/C system and/or ECU. If follows that if the failsafe were to be RFI (radio frequency interference) induced which may or may not be emanating from the ECU, shutting down the turbine and the electric fuel pump may very well have the RX regain signal integrity due to the lower RFI profile presented to the RX front end.
Fuel systems: I have included a drawing at http://www.bairdtech.com/bmt/fuelsystem.jpg, of my own flavour of fuel plumbing system which works very well. I say this because I have never experienced a flameout or wet start with this system over a period of 2 years since using it.
Fire safety starts on the GROUND and in the PIT area. The most important safety feature of this system is the shutoff/bypass valve. While it can be operated manually, having it servo operated by the user or even better automatically by the ECU adds considerably to the safe operation. At a recent jet fly it was noticeable that turbines without this feature regularly used their fire extinguishers at start up.
The other really tidy feature is the return line to the refueling container which allows you to get all the air out the tanks without spilling fuel all over. Also you can walk away from the process and not pump fuel all over the pit area. The anti-spill tank serves to prevent the vent from spilling fuel as well, especially when traveling in the car or trailer. The tank hold drops of fuel which may siphon past the valve at low pressures over time. It empties automatically when the engine runs because the check valve only lets air/fuel in. It also leaves you car/workshop free of Kerosene smells.
You may notice there is no header tank. I repeat the reasons why I don't believe in header tanks, unless they are fitted with a bushwhacker type felt filter, in which case the felt filter may as well be in the main tank. (If you have to, consider the main tank a big header tank.) My first reason is that all the flameouts I have seen recently, had header tank systems. Second reason is the header tank traps small, sometimes harmless bubbles and releases them as larger bubbles or worse, a single large bubble to the pump when the aircraft experiences slow changes in direction of G forces, often in what appears to be almost level flight, whereupon the air goes for the middle of the tank. The last reason is that in multiple tank systems it gets to be under considerable negative pressure at high fuel demand sometimes flattening itself causing unpredictable behaviour when fuel demand changes. Simple proven physics. Debating this is a bit like debating if the earth is round or flat.
A simple way to prove the worth of the felt filter is to fit it in a transparent tank and then empty with an electric pump using transparent tubing. You will notice how even when the filter is only fractionally submerged in fuel it still does not draw bubbles. I am told the large scale aerobatics pilots do the same for those large gas engines.
By the way, I use brass tubing of 3mm internal dia between the stopper and filter, joined by fuel tubing at the ends. This considerably eases fuel throughput and prevents kinks or "dropback" of the clunk. It is even more important for the secondary tanks which if restricted enough could flatten the main tank.
This is why I recommend TWO separate inlets to the main tank and NOT a "T" piece for this very reason. It also makes for fewer joints. When considering the size of fuel tubing inner diameter please remember that the total resistance is the sum of all the sections of fuel passage calculated by multiplying the length of any given diameter with that diameter, i.e. you can maybe get away with short small diameter sections but longer sections needs to be opened up. It may seem complicated when installing but you will be rewarded with an easy to use and safe system.