The fuel clunk/ pickup line is one of the most important component of the fuel system together with the air trap and is very often mis/ installed or undersized/ wrongly chosen. The clunk is what will enable you to pick the fuel from the tank in the best conditions and minimize air sucking during aerobatics. Here is a video of a tank showing what happens to the fuel and the clunk in flight on a semi-rigid plunger system:
In this 30 second sequence, the clunk is out in the air for half of the time! A properly chosen and installed clunk is essential to avoid emptying your air trap before the end of the flight!
The HF/ UHF fuel clunk was created and tested over the course of one year. It is one of our first product completely in-house designed and produced. It was designed to complement our super flexible Viton tubes and offer the best pickup system available on the market.
The device was CAD designed and is CNC machined to 4/100 mm on our new 4 axis CNC lathe.
The idea behind this new creation is to improve the concept of high flow clunks further by introducing some specific features aimed at jet models.
I worked a long time with the vacuum gauge at high pump flows to find the right shape, density and port to allow a high flow in all positions and super high suction at the taper to extract the last drop of fuel from your tank. The weight of the clunk is also optimized to allow our Viton lines to let the clunk move everywhere WITHOUT PINCHING the tube.
The following features were finalized after a long research:
ID: 5 mm/ 6.6 mm/ 8 mm
This is very important. The internal diameter of the clunk is slightly bigger that the diameter of the 3/16"/ 1/4" / 5/16"Tygon tubing ( 4.8 mm/ 6.4 mm/ 7.9 mm ). This forces the tube to expand further before the fuel enters it and reduces entry turbulence, thus risks of cavitation. This is the largest ID available on the market.
Barb OD:: 8.92 mm/ 10.6 mm/ 11 mm
The large barb makes a very good clamping effect for added safety. I found out that only one barb makes the best sealing against fluid leak. However there is less grabbing effect. Thus the following feature. This is the largest barb on the market.
Safety wire zone length: 12 mm
The cylindrical section after the barb is large and enables multiple safety wiring. This extra length also makes it much easier for the safety wire operator and allows the use of thicker wire. This is the longest safety wire zone available on a clunk.
Body max diameter: 15 mm/ 22 mm/ 23 mm
This makes the 3/16" ID clunk compatible with the standard fuel stoppers available on the market ( Dubro, BVM ). It is a good size to pass the clunk through the stopper hole, even if not perfectly clean line it is often the case on kevlar tanks. The 1/4" and 5/16" ID clunk is compatible with our UHF fuel stopper.
Taper: 30 degree
I found this to be the optimal angle to grab fuel in most tanks corners. It also keeps the tip of the clunk heavy enough so that it always faces down.
Taper edge: 0.3 mm
This is an important feature as a thin edge reduces pickup turbulence and together with the proper taper angle, will allow you to suck the last drop of fuel from your tank. The edge is continuous with no notch that tend to suck air bubble. The minimum edge thickness is a function of the material strength used and machining technique. By using high density brass and ceramic insert cutters, we can achieve this ultra-thin wall while keeping strength at the tip.
This is the thinnest edge available on the market.
The clunk can be chosen cross drilled for an all flexible plunger or with a round tip for a semi-flexible plunger line. Please refer to our fuel installation article for an optimal choice.
The HF clunk is machined from high density 360 brass alloy. This is very durable, does not degrade in kerosene/ diesel/ alcohol and offers excellent cutting and finish with ceramic tools. Tensile strength is 17 000 PSI. Our alloy is ASTM B16 certified and quality traced.
Weight 20 grs/ 0.7 oz ( HF ) - 40 grs/ 1.4 oz ( UHF ) - 60 grs/ 2.1 oz ( XHF ):
Allows for an optimal inertia that will bend our Viton fuel line just enough to drive the clunk everywhere without pinching the tube.
All these features allowed me to create a very effective product that fits our UHF fuel stopper very well.