Ultra high flow fuel systems
I would like to give you some update on the latest research about fuel systems for the new generation engines.
We are slowly going further beyond the concept of high flow systems to ultra high flow system.
The reason for this is that our engines are getting bigger, with the current LTMA-1 regulations allowing up to 75 lb static thrust and LTMA-2 up to 90 lb static thrust, capped to the aircraft wet weight plus 5 lbs.
Although people are not allowed to use more than this thrust, we do provide precise thrust-to-RPM graphs that allow the modeller to down-rate the engine to comply with regulations and supply a proof for it.
These engines, even de-rated, can use very high peak flow. This is because the latest ECU and fuel pumps are fast and over sized, which means that they are able to supply very high peak flow when the acceleration condition dictates it.
In this case ( large de-rated engine on fast acceleration ), maximum cavitation vacuum could be reached on high flow systems and create a momentary condition for a single burst of kerosene bubbles that could generate a flame out condition.
I also found out that for the latest generation product ( JB-220, B-300F ) that do not normally require de-rating, fuel restrictions could occur over time in bends when Tygons harden or with slime/ dust accumulation ( from the vent line ).
I recently serviced a flying target that had all new Tygon lines, cleaned up air traps but was showing poor results on the vacuum gauge. The engine was still performing OK, thanks to our professional Ultra High Flow system but was getting on the edge of cavitation point. After scratching my head for a long time, I inspected the inside of the fuel vent fitting. I found out that remains of fuel + oil had evaporated, leaving a greasy deposit that had trapped lots of thin sand dust, almost clogging the vent! this was extremely difficult to detect, as hidden in the aluminum fitting and we only got along with it because of the flow capacity of the system downstream the clunk and dual vent system ( redundancy ).
So, because of all of these points I have decided to bring to the market redundant/ Ultra High Flow fuel systems.
Redundancy comes with the new Ultra High Flow fuel stopper system that I introduced recently here.
That system is unique in many ways ( CAD, CAM, 3D print process... ), but most importantly in that it allows the modeller to use dual vent AND dual clunk systems with dual UAT!
That can be used on twin engine or single engine models!
Of course, no ECU system is capable to drive dual pump systems as of now, so a Y fitting has to be placed just before the pump.
This product is labelled UHF because it can be used to setup a redundant parallel system ( like the one I am designing on the Mibo A-10 ) or it can use 1/4" ID Ultra large Tygon tube ( 6.4 mm ID ). 1/4" lines are now the norm for us on 400 and 500 N engines and I have setup a few B-300F with these lines as well with complete satisfaction.
The other new product is a Ultra High Flow fuel clunk that is being released to the market now.
This one is labelled UHF because it is designed with an ID of 5.0 mm ( 0.3 mm higher than the 3/16" Tygon line ).
Also I have recently seen some people being victims of the communicating vessels law.
On the latest air trap designs, capiliary drag is very law, allowing the fluid to travel between the air trap and the tank. This is especially true with the larger air traps I designed a few years ago that feature large plated paper filters.
Plated paper is great in that it offers very little resistance to fluid travel, thus reducing risks of cavitation. However it also allows the fluid to escape from the air trap and the filter, thus allowing air to enter.
This would happen if the level of the pickup line is placed somehow att he level of lower that the top of the fuel filter unit when the tank is empty.
This can happen in case of rotation of cylinder type air traps, or if the model is place on its side for storage or transport.
In the case shown above, air has entered the filter section and will enter the pump at some point. Even if the air is purged, micro bubbles can stick to the membrane and enter the pump. It the case above, after purging the air, a thorough high flow test while tapping on the air trap will be required to remove these bubbles.