New generation air traps and avoiding cavitation.
1. Introduction
The air trap market has long been dominated by the BVM UAT and its derivatives. However with the coming of bigger and more powerful engines, this little device that was designed about 15 years ago is reaching its limits.
Quite a few companies have come to the market with new generation air traps, using a completely different approach from the original UAT. These units are much more suited for high flow engines and some of them will protect your fuel system very efficiently from the dangers of cavitation as I will explain below.
Furthermore, the latest technology in fuel bladders completely eliminate the need of using an air trap and increases the reliability of the fuel system by an order of magnitude, as we will explain further down.
2. The concept
The venerable BVM UAT was designed many years ago to protect the best ducted fans and the first JPX jet engines from sucking air bubble that would almost certainly lead to a flameout. The idea behind this device was to be able trap a small air bubble that would otherwise make its way to the engine and create a flameout condition.
2.1. Imperfect fuel pickup
The main reason why air would get into your fuel line is not because of a leak but because the rigid fuel tank concept and the fact that a clunk is not a perfect system. Although it is heavy and mounted on a flexible line to stay in the fuel as much as possible, there will be situations when it will get out of the fuel and suck air! This will happen towards the end of the flight, as the tanks get mostly empty and/ or while performing aerobatics.
Here is a video example illustrating the fact. This video is taken on board an aerobatic plane. During this 30 second sequence, one can see that the fuel clunk is out of the liquid for exactly 50% of the time, pickup up air instead of fuel during this period!
This is the reason why fuel bladder systems are so interested for AUVs and competition models. The bladder, if setup and filled properly, is designed to run without a vent and collapses as the engine drains fuel. The "tank" is always full, without the possibility of introducing air in the system. Additionally, atmospheric pressure pushes on the bladder and contributes to reduce the effect of suction drag ( we will see this later ). As a result, an air trap is not necessary, reducing the weight of the fueling system significantly.
On older, more conventional tank systems, however, an air trap is strongly recommended, adding complexity, increasing risk of failure and adding dead weight of about 2 to 3 lbs ( fuelled air trap ).
On this matter, the BVM UAT was designed for fairly reasonable fuel flows by today’s standards. Just imagine that the biggest engines available on the market nowadays can burn 1 liter of kerosene per minute, whereas the first JPX turbines were burning a quarter of this amount.
So as good as the older system are, they only offer a few seconds of maximum thrust fuel flow buffer. However, for hard core aerobatics flyers and people who like to fly at full thrust for a large portion of the flight and keep high thrust level in the dive, the amount of air that is sucked by the clunk can be up to 15 seconds of full thrust fuel flow!
For this reason I strongly recommend choosing and air trap that offers 30 seconds of full thrust fuel flow buffer capacity.
Baldders are not legal in the US over rupture concerns, back when people were using blood/ plasma bags for their tanks. However our latest generation of professional bladders are extremely strong and fuel resistant. We make a lot of these for military / industrial UAVs and have an impressive reliability and dependability record.
2.2. Cavitation
What happens with high fuel flow is that the pump is required to deliver a lot of pressure to the engine through the 4 mm/ 2.4 mm PU tubing. This means that the suction upstream the pump, ie vacuum levels generated, can be very high. This may lead to a phenomenon that is not very well known to the modelers: fuel cavitation.
Kerosene is very aerophile. It can absorb over 10 % of its volume of air. This air is dissolved into the fuel until the condition for releasing is met: namely depression. This depression does not have to be very high to create a fuel cavitation condition. You can see it by using a plastic syringe filled with fuel and pulling the piston while closing the tip. The kerosene will foam quite quickly. The air dissolved releases.
What happens in our models is that air bubbles are created by both a certain level of vacuum AND turbulence (this is very close to what happens in a can of beer. As soon as you open the container, its pressure drops and thousand of micro bubbles of CO2 start coming to the surface). The reason behind this is that the micro swirl in turbulence creates local point of lower vacuum where the vapor pressure of air dissolved in kerosene equals the local level of vacuum. This is where the liquid "explodes" in a micro bubbles. Thousands of micro swirls create thousands of micro bubbles. Then the high energy stream keeps on swirling passed the creation point and prevents the bubbles from re-dissolving.
Real size passenger aircraft are constantly prone to fuel cavitation. In fact, every time the aircraft is climbing, the fuel is literally “boiling” in the tank above 30,000 feet for a certain amount of time until all the dissolved air is released. This is why commercial aircraft have got their low pressure fuel pumps mounted in the tanks. These pumps are designed to take a mix of air and fuel and re-pressurize it considerably in the downstream lines so that all the air bubbles are forced to dissolved again before the fuel reaches the engines and their high pressure pumps.
We do not have wet pumps in our models. The created air bubbles will not dissolve immediately when the vacuum/ turbulence condition stops because they are made of air, not kerosene. Condition for the bubbles to re-dissolve would be to agglomerate into a bigger bubble with a lower tension surface into a locally calm area of the stream. In any case, re-dissolving would take between one to two minutes. Way too slow to avoid bubble propagation into the engine.
additionally, old generation air traps are small in volume and made of a soft material by design. This combined with what I call a high suction drag ( too much vacuum upstream the pump creating fluid drag) could lead the pump to vacuuming the air trap. You’d see the walls of the air trap slowly bending until they touch the filter bag. At this point, the bag would get its usable surface decreasing rapidly and start to cavitate itself: air bubbles would form downstream the bag as if it was letting the bubbles through. In fact, what happens in this case is that the clogging of the bag creates a restriction that leads to increasing the suction downstream it. When this suction reaches the cavitation point, it looks like the bag is bleeding air bubbles.
This phenomenon can also happen in any point of the fuel system upstream the pump when a restriction occurs.
So having that in mind, some manufacturers have decided to come up with new designs to improve the resistance of the unit to cavitation as a secondary function of the air trap, while improving the primary function ( air buffer ) by increasing the available volume.
One very simple way to avoid cavitation is to use fuel bladder systems. These provide a slight positive feed to the pump due to the fact that atmospheric pressure pushes on the soft layer of the tank. Also the removal of the air trap reduces the length of the tubing and number of fitting upstream the pump, reducing suction drag and cavitation risks. FInally, our professional bladders are designed with high flow in mind and all tubings, stoppers and fittings in the systems have the same internal bore of 4, 5, 6 or 8 mm depending on the bladder capacity.
2.3. Summary
To resume, when legally possible, use a bladder system for your UAV or competition model.
If not, use a modern air trap which:
1. Acts as a buffer for the non perfect clunk ( air trap )
2. Reduces the likeliness of cavitation
3. Acts as a first level of filtration ( raw filtration that will not substitute to the post pump fine filter )
I will analyze here for you the Hanson ( BVM ) , PST and Ultimate Jets solutions.
3. Devices description
3.1. BVM Universal Air Trap
The BVM air trap is the “ancestor” but is still widely used around the globe and sold from our shop as kit bundle on some variants. I remember using this unit as fast as 1995 back with my ducted fans. At this time the concept was revolutionary. It is made of a semi clear semi soft plastic tank and an automotive synthetic fuel filter bag commonly called “Kuss”. The fittings are fastened through the plastic, and the cap is sealed with a few turns of Teflon plumber tape. The optimum orientation is 45 degrees nose up.
3.2. The Hanson SuperTrap
The Hanson super trap is a declination of the venerable BVM UAT that came on the market in 2006.
As such it uses the same soft Nalgene bottle and the Kuss automotive filter bag.
The main difference is that the bag is safety wired onto an aluminium assembly that provides two advantages: to keep the bag open is all circumstances, and to enable the use of a ¼” SMC push fitting, or large barbed fitting. This assembly also houses a micronic filter so that this unit can be used as a filtering device as well.
A Hanson Super trap dismantled. It includes from left to right: a ¼” push on fitting, an aluminium housing, a micronic filter, a filter retainer and the Kuss bag.
I found two problems with this unit: firstly the micronic filter is quite restrictive and not really required considering the fact that the Kuss bag also provides a fairly good level of filtration ( in fact as much as a standard aluminium turbine filter ). Secondly, although it is a very good idea to use a large push on fitting, it appears that this SMC unit is quite restrictive. The inner diameter is only 2 mm, whereas the aluminium housing minimum inner diameter is 2,8 mm.
So this makes this unit optimally suited for 60 to 80N engines as such. The solution to improve the flow of this unit is to:
1) Remove the micronic filter while retaining the Kuss bag by just unfastening the inner filter retainer
2) Redrill the push on fitting to 2,8 mm and clean it thoroughly, or go for the barbed fitting option.
The SMC push on fittings and filter retainer inner diameters are shown here. The top fitting has been redrilled to 2,8 mm and compares to the filter retainer on the right. The bottom fitting is stock and has an inner bore of 2 mm.
The optimum orientation of this unit is 45 degrees nose up. This is to allow an optimum purging of the air during refill.
A last note about the Nalgene bottle cap. It is a screw-on cap that has quite a large and loose drive. To avoid introducing air via the screw, it is wrapped with a few turns of Teflon tape. It is important to check this area every year for tightness and replace the tape if in doubt. This also applies to the UAT.
The teflon tape is being renewed on this air trap. 4 to 5 turns of plumber tape tightly wrap are required.
A leak in this area would introduce air directly downstream of the air trap…
Note that after having tested both the original BVM UAT and modified Hansen air trap, we realized that they actually perform the same. So the ancestor actually holds quite well compared to some younger contendants!
3.3. The PST Air Trap
The PST air trap is a bit more recent and was released on the market in 2008.
The PST air trap is a very clever device that uses materials issued from other fields of activity ( like the UAT ). So don’t be surprised to discover that the body is in fact a baby bottle!
The advantage of this design is that it is inexpensive to produce and the bottle is quite strong and completely transparent. Additionally, the tolerance is such that the sealing of the bottle neck is achieved with 100% success by an o’ring system with no risk of subsequent air leak.
The filtering unit is comprised of a sintered brass pickup wrapped up with a geotextile felt. This assembly is very effective and has the advantage to increasing the capitation surface, thus reducing the suction drag and the possibility of cavitation.
The core of the PST air trap: the filtering felt assembly that is housed within an anodized cylinder. Note the size of the fittings and the large sealing o’ring.
The bottle top is made of an aluminium core that includes the fuel line fittings and a hard connection tube to the bronze pickup. It acts as a cap and the sealing is made by a large o’ring forced along the side of the bottle. It is very effective. The top plastic cap is only used to keep this aluminium assembly in the bottle. It does not provide any sealing.
The filtering felt removed shows the sintered brass pickup and the large brass tube. The restrictive point on this designed is the brass pickup.
The PST air trap comes without any mounting device. However, one can purchase the plywood mount kit. His brings the price up substantially and is quite heavy. The optimum orientation of this unit is nose up.
3.4. Ultimate Jets UFBK air trap
The Ultimate Jets UFBK series have been designed to combine all the advantages of the previous systems while being as light as possible. As such, the units are now fully serviceable kevlar tanks that enclose a fixed brass tube and brass/paper felt filter assembly. They come in 4 different sizes:
1) 160 ml for max 160 N engine class
2) 250 ml for max 250 N engine class
3) 500 ml for max 500 N engine class
4) 1000 ml for max 1000 N professional/ UAV engines.
The brass fittings as well as the inner pickup tube are optimized for high flow with no restriction along the path of the fuel. The units are made of Kevlar fiber and glued with Hysol 9462. They are coming with a CNC cut large aluminum threaded cap and sealing o'ring. All the internal components are user serviceable and available as spares.
The tank itself is a cylinder and is very rigid. The dimension is computed to provide a buffer of 30s of fuel usage at full thrust. This is why we are providing different unit sizes. The size, shape and position of the filter assemblies is matching the different sizes of the tanks. These points are extremely important to allow an optimum volume of buffer liquid around the filter that will give a good flow capability in case of cavitation.
The extra large filter assembly. A practical device made of aluminum brass and paper.
The kevlar tank wall is thin enough to allow seeing the liquid and detecting a cavitation condition.
On the smaller UBFK series, we have added a VaccuTest pocket. This aims at giving you an indication of how strong the suction drag is upstream the unit. If the fuel level reaches the bottom of the pocket when you are doing a full power test for one minute, then there is a danger of cavitation at this point. The restriction upstream is too strong and you need to check your fuel system again.
The pocket designed to show a restriction condition upstream the tank.
The tank is mounted on a set of 3D printed brackets. these screw on any horizontal bracket. For vertical mounting, the unit can be held in place by the gap left below the screw cap.
The UFBK-1000 on its mounting bracket.
The optimal orientation of the 160 and 250 units is horizontal with the vent fitting placed on the upmost position. However, vertical type units can be ordered as well.
4. Devices comparing
3 generations of air traps side by side. From left to right: the Hansen air trap, the PST air trap, the GBR jet medium size air trap.
4.1. High flow and cavitation workbench test
Each device was plugged to a Hausl pump controlled by a Jetcat ECU in an open loop. The fuel was directly taken from my Jersey Modeller jerrican and returned there. I placed a Festo ball valve on the feed line to simulate an increase of the suction drag. I also placed a vacuum gage on the other feed line of the air trap device to be able to measure the feed line vacuum charge generated. I finally plugged a restrictor of my design on the return line downstream the pump. This restrictor is designed to simulate the injectors charge of a 160N thrust engine.
The basic cavitation test setup. The air trap is looped back into the open jerrycan. The fuel pump is controlled by the Jetcat GSU in device test mode.
I then ran the benchmark for each device, doing a series of tests.
1) full power test with no restriction.
This test enables me to check the depression generated in the air trap at the maximum pump output of 6,2V and to verify that no cavitation condition exists at this stage.
2) Full power air retention test.
This test is designed to see how the air trap manages with air in the outer compartment in all flight positions. I do a full power test with the air trap gradually emptied from full and moved it in all the positions. I look at the pump line to see if any cavitation bubble shows up.
It is important to characterize the minimum level available for cavitation protection because when slowly reaching the cavitation point, the tank will gradually empty till reaching the minimum level, at which point the air will be released in the system. So knowing the minimum level will give you an idea of the time frame available with a proper protection level. This is also why a big air trap will give you a better protection.
3) Cavitation test.
I run the system with the ball valve ¾ closed on the feed line and see at what voltage the system starts to cavitate. This is a fast pump increase, contrary to the previous test. I note the depression value and level drop at this point to characterize the absolute cavitation point of the air trap.
A vacuum gage is used to monitor the depression in the air trap. This gives a precise indication of the restriction created by the partly closed ball valve on the feed line.
4.1.1. Hanson Super Trap/ BVM UAT
1) Full power test.
The Hanson air trap/ BVM UAT did not cavitate at 6,2 V with no restriction. The depression generated in the air trap was 5 inches of mercury ( in Hg )
2) Full power air retention test
The Hanson air trap/ BVM UAT did not send cavitation bubble at ¾ and ½ full. However it started to cavitate when put vertically and 1/4 full (which corresponds to a 50 degrees climb in most circumstances ). This is a good result.
The minimum acceptable level in the Hanson air trap before cavitation. This picture shows the air trap after the pump had stopped, with no more vacuum applied.
3) Cavitation test
The Hanson air trap/ BVM UAT started to cavitate with the pump set at 3 V. This corresponded to a restriction equivalent to 10 inHg. At this point the walls of the air trap had started to deplete under the action of the suction.
So when you test your system at full thrust, wait one minute at this power level and check you air trap. If you see the walls depleting on this unit, this is an indication that something is wrong upstream the air trap and that you need to work out your fuel system again.
4.1.2. PST air Trap
1) Full power test
The PST air trap did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 3 inHg. So this unit generates less suction drag than the Hanson air trap.
The PST air trap in the test. Note the fully open ball valve for the full power test.
2) Full power air retention test
The PST air trap started to send cavitation bubbles into the pump at ¾ full in the vertical nose down position. This corresponds to an inverted flight in most cases. This could be a problem since it means that as soon as you deplete the air trap due to high fuel flows and suction drag, you cannot do any aerobatic maneuvers anymore. In other words, this system does not tolerate a high suction drag ( restriction in the system ) on an aerobatic aircraft and 200N class engine or more.
3) Cavitation test
The PST air trap started to cavitate at 3,5 V with a depression of 13 inHg. This is slightly better than the results found with the Hanson air trap/ BVM UAT. The level drop in the air trap at the cavitation point was 20 ml or 2cm from the top / 1 cm at the cylindrical section.
So if you want to setup this system correctly, just do a full power test for 1 minute and see if the level in the air trap drops by less than 1 cm. If not, you need to rework your fuel system upstream the air trap.
4.1.3. UFBK 250
1) Full power test
The medium size CAT did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 2 inHg. So this unit generates less suction drag than the previous ones.
The medium size CAT ready for testing.
2) Full power air retention test
The medium size CAT did not send any air into the system at ¾, ½ and ¼ full. It is an excellent result that gives an optimal protection. The air retention capability is only compromised if the fuel quantity reaches the bottom of the filter. There is no detrimental position for this unit since the volume taken by the filter is perfectly optimized.
3) Cavitation test
The medium CAT just started cavitating with the ball valve closed at ¾, when reaching 6,2V. This is an amazing result and means that this unit can accept twice the flow and restriction compared to the other units before cavitating. I was honestly not expecting such an amazing result. I believe that it is due to the pleated paper filter characteristics.
The equivalent depression was 15 inHg and the fuel level dropped slightly below the test mark, which means that it is suitable for the purpose.
4.1.4. UFBK 500
1) Full power test
The large size CAT did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 2 inHg.
2) Full power air retention test
The medium size CAT did not send any air into the system at ¾, ½ full. It did cavitate a ¼ full in the vertical position nose up and nose down. It is an excellent result that gives an very good protection . This CAT being designed for 120 to 300N, you will probably never end up ¼ full unless you have consumed the last drop of fuel in your tanks, or your running a nearly completely blocked fuel system for a long time.
This result is slightly different from the medium CAT because the filter is slightly shorter for the size and the volume of fuel left at the flats ends of it is slightly larger than on the sides.
3) Cavitation test
The large CAT did not cavitate with the ball valve closed at ¾, when running at full 6,2V! I had to close the valve further to 80% to get it cavitate. This is an equally amazing result and means that this unit can accept probably three times the flow and restriction compared to the other units before cavitating.
The equivalent depression was 17 inHg and the fuel level dropped slightly below the test mark, which means that it is suitable for the purpose.
4.1.5. UFBK 1000
1) Full power test
As expected, the large size CAT did not cavitate at 6,2V with no restriction. The depression generated in the air trap was 1 inHg.
The extra large CAT in the test.
2) Full power air retention test
The extra large size CAT performed as the large one . The length of the filter relative to the length of the CAT is the same. So it did not send any air into the system at ¾, ½ full. However it did cavitate a ¼ full in the vertical nose up and nose down position. It is an excellent result that gives an very good protection. Since the CAT has a volume of 400 ml, you’d have to use 300 ml of the CAT fuel before reaching the cavitation point on low level.
This is the minimum level available for cavitation protection on the extra large CAT.
3) Cavitation test
The extra large CAT did not cavitate with the ball valve closed at ¾, when running at full 6,2V. I had to close the valve further from the large CAT to get it cavitate. This is a great result and means that this unit can accept well over three times the flow and restriction compared to the other units before cavitating.
The equivalent depression was 19 inHg, which is a huge restriction and the fuel level dropped slightly below the test mark, which means that it is suitable for the purpose.
In essence, this air trap is a big buffer tank that can allow very big engines to run on a very restrained fuel system for a long time.
4.2. Devices weights
Hanson Super Trap ( modified )/ BVM UAT: 61 grs
PST air trap: 93 grs
UFBK 160: 43 grs
UFBK 250: 63 grs
UFBK 500: 115 grs
UFBK 1000: 125 grs
4.3. Pros and cons
Hanson Super Trap/ BVM UAT:
Pros: very good filtering capability, translucent, low price
Cons: soft wall, small capacity, average resistance to cavitation, no mounting device included.
PST air trap
Pros: very good filtering capability, hard wall, medium capacity, average resistance to cavitation, completely transparent.
Cons: heavy, low air separation characteristics, no mounting device included, performance to price ratio.
Pros: good filtering capability, hard wall, largest capacity, very high resistance to cavitation, translucent, mounting device included, weight, performance to price ratio, entirely user serviceable.
Cons: Harder to set in a vertical position for the 160 and 250 sizes.
5. Avoiding restriction
Having a perfect air trap for your system is great, however there are a number of modelers using the venerable BVM air trap in their model who've had perfect results for years. So, how do they achieve this?
The answer is avoiding restriction and minimizing suction drag in their system. The fuel system follows the chain rule. It is only as good as the weakest link. So the same attention to details must be used for the whole fuel system and some important guidance must be followed, from the vent line to the suction side of the pump:
. Use large tubing ( 3/16" or 1/4" ID Tygon )
. Use large fittings ( 3/16" or 1/4' ID bras tubes and fittings )
. Avoid any reduction in your ID ( Inner Diameter ) on the fuel path from the clunk, to the fuel stopper down to the air trap.
. Avoid tight turns in your fuel tubing ( avoid to pinch it ) and keep the tube path very clear and neat
We are offering a line of UHF ( Ultra High Flow ) accessories specifically designed to help you on this matter.
. UHF fuel clunk
. UHF fuel stopper
. UHF viton tubing to plumb your clunk and plunger
. UHF Tygon tubing
. UHF fittings and brass tubes
. Specific PYCABS Tybon tube clips that allow for a very clean and easy setup of your system
Although, air has a 30 times lower dynamic viscosity ( 15 cSt vs 0.5 cSt for hydrocarbons ), it is also good practice to keep the vent line and fitting of the same large diameter as he remaining of the line.
All of this is of the utmost importance to keep your system healthy and avoid putting too much load on the suction side of the pump. On that matter, bladder fuel system offer the best possible setup.
6. Fuel pump considerations
Our fuel pumps are geared type, electrical motor driven.
Brushed motors are typically driven by voltage. The voltage value applied to the motor will translate to a certain flow ( non linear relationship ). Well, in theory.
That is because geared pumps have a specific characteristics: they are weak on the suction side and strong on the pressure side ( meshed gear will struggle at grabbing your fuel, but once it is entrapped, will excel in compressing it ).
In practice, if the suction drag on the pump side is too high, the pump will see a cavitation condition occur at the entry of the gear mesh. If this happens, there is a great probability that the cavitation will propagate beyond the gear, to the engine. Also the pump will see and RPM increase as well as a drop in the downstream flow.
All these would most certainly lead to a flameout. However, bear in mind that pump cavitation usually occur at higher vacuum value than air trap cavitation.
7. Servincing considerations
Even if you have created the best system in the World, you will need to keep it that way.
Fuel systems tend to degrade over time. This can be due to several factors:
. Fittings oxidization
. Fuel lines hardening
. Dust accumulation ( filters, vent lines )
. Slime ( kerosene/ diesel algae )
For this reason, I recommend the following:
. Fittings oxidization: use the best quality brass fittings ( our fitting, tubes and clunks use quality traced low oxidation brass alloy ) and inspect them before the beginning of the season. Change when oxidation is visible.
. Fuel lines hardening: Festo lines will slowly darken and harden with time when immersed in diesel and kerosene. when you see that the line becomes brownish and hard, change it. Do not use Festo lines inside the tanks ( clunk plumbing ) but Viton lines that are very flexible and virtually unaffected by hydrocarbons.
. Dust accumulation: Flush the air trap reverse several times at the en of the season to clean it.Inspect your vent lines at the same time and on a regular basis if you operate in dusty environment. Evaporated fuel will leave a greasy deposit in the vent line that will eventually get dust to stick to the line and possibly clog it. Rinse the line with non mixed diesel or kerosene if this occurs, then flush with alcohol to facilitate a dry tube.
Slime: inspect your filter on a regular basis. If you see a gel substance accumulating, clean and mix some kerosene with fuel bugs killer. Leave for a couple of days and flush the system. We have created a biopack system fuel treatment 10 years ago and been using it with great success. I also always fill my air trap with this mix at the end of the season.
8. Conclusion
The choice of a proper air trap and fuel hardware is not always as obvious as it might seem when you include suction drag reduction in the equation.
The first criteria, however, should be size/ fuel buffer capacity.
The second criteria should be resistance to cavitation
Although choosing a modern air trap will help you in reducing suction viscosity, the fuel system follows the chain rule. It is only as good as the weakest link. So keep the same attention to details when setting up the whole system and use appropriate hardware that are designed to work with each other.
Finally, think in terms of durability and servicing. Don't forget the slime problem neither!
All of this will help you in achieving a perfect system that will get you to enjoy trouble free flights for years.
