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Author Topic: Uniflow tank - open vent tube position  (Read 1336 times)

Online Dennis Toth

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Uniflow tank - open vent tube position
« on: March 02, 2018, 04:01:38 PM »
Guys,
I wanted to get some input on located the uniflow open air vent. I have experimented with muffler pressure to the uniflow vent and get a perfect first half followed by a rich 2 cycle second half of the flight. I have heard other have had this with some engine/tank arrangements.

The typical next step is to go open uniflow which can resolve the issue. My question is where to put the vent to minimize upwind/downwind engine variations. In the past we would put this vent on the inboard side of the fuse pointing directly into the airstream. Sometimes we would use a small orifice in the vent end to try and minimize the pressure swing. I have tried putting it behind the engine out of the airstream but in high winds it didn't seem to prevent the swing. Any new ideas?

Best,    DennisT

Offline Istvan Travnik

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Re: Uniflow tank - open vent tube position
« Reply #1 on: March 02, 2018, 04:35:41 PM »
Dear Dennis,
I found that the best is to keep inside end of the Uniflow tube  close to the fuel line's inside end, as close, as possible. Let us say, at 30% of the length of the tank, measured from the rear end. Right at the position of the baffle.
A little bit offtopic: I am convinced that the horizontal positioned engine is far superior,  because the position of the spraybar's hole / nozzle of carburettor is approx 1/2 or 1" outwards  to the conventional. The result is that when the model accelerates (downwind) the air/fuel mix will be rich, when decelerates (upwind, or overhead manoeuvres) will be lean.

Online Dennis Toth

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Re: Uniflow tank - open vent tube position
« Reply #2 on: March 03, 2018, 07:11:27 AM »
Istvan,
Thanks for the post, what I was referring to was not the inside tank location but the other end open vent side location. I was wondering if facing into the free air stream on the inboard side of the fuse is still the preferred location and is some type of restrictor worth using?

Would like to know some details about the baffle arrangement and tank shape you are using.

Best,    Dennis

Online Lauri Malila

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Re: Uniflow tank - open vent tube position
« Reply #3 on: March 03, 2018, 01:40:05 PM »
Dennis,

As long as there is only air going inside the tube, it does not really matter where it ends.
But it’s a good practise to have end of tube on left side above max. fuel level of tank when model sits on ground. If it’s below, it may start siphoning when tank is full and if engine stops too early during flight.
Make it point forward, on a windy day you can add an angled piece to it, sometimes it helps.

Lauri

Offline Steve Helmick

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Re: Uniflow tank - open vent tube position
« Reply #4 on: March 03, 2018, 02:10:24 PM »
Mine are setup so I can use standard uniflow vent (inboard side, above top of the fuel tank while sitting on the wheels), OR muffler pressure, OR hook up to an auxiliary hose that goes to the cavity area in the backplate (no slipstream). I always hook up the auxiliary hose just prior to start...if you fuel up in the pits (like you are supposed to), fuel could well siphon out.

Brett Buck posted about using this scheme if it was really windy, but I just use it all the time. If nothing else, it makes it quicker to set the launch rpm, in case your NV is on the inboard side also, and your fingers/hand blocks propwash to the uniflow inlet. Have you noticed that? I got tired of it.  H^^ Steve
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Istvan Travnik

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Re: Uniflow tank - open vent tube position
« Reply #5 on: March 03, 2018, 06:01:45 PM »
Dear Dennis,
I counted out, how high can be the dynamic pressure at the inlet of the uniflow tube. Dynamic pressure is: 1/2 specific weight of the air, multiplicated by the speed up to the square. (0.62 kg / m^3 * 27^2 m^2 / sec^2 = 452 (kg m / sec^2) / m^2 , (in other words: N / m^2)  or Pascal (Pa). We need to know that the environmental normal athmosferic pressure is round 100.000 Pa.  452 Pa is almost nothing, less than 1/2 % of it.
When you move the tank 1 centimeter towards to the center of circle, the difference of  static pressure of the tank, looking from the aspect of spraybar hole, difference will be 0.01 m * 320 m / sec^2 * 700 kg / m^3 = 2240 Pa. Five times more than dynamic pressure making by the airflow!  When you put some "shade" ( fender, deflector, obstacle etc.) in front of the inlet of Uniflow tube, eliminating more or less the dynamic airpressure, that equals as moving 1/5 cm = 2 millimeters the tank into outer direction. I am convinced the upwind or downwind does not make nothing, since the inlet is in direct blowing of the prop. I think there is no reason to do nothing.
Some plus: When the model flies round loop at 9 meters radius, 0.8 millimeters of vertical moving of the tank will be equal the full dynamic airpressure...
Excuse me for using metric units, do not think me unpolite, but it is a great advantage to use metric units in physical equations to check the correctness / logics of the equation. E.g. when you know, that the result must be a pressure value, Pa, or N/m^2, you must get finally (kgm / s^2 ) / m^2 , or simply kg / m s^2.


You asked my tank inside arrangement, that is more than simple: Baffle is at 70% longitudinal position of the tank, no holes in it, but both sides are 4 millimeters shorter than the full width of the tank. Fuel line is beginning 4 millimeters aft, uniflow tube ends 4 millimeters before of the baffle.   



 
« Last Edit: March 08, 2018, 05:33:44 PM by Istvan Travnik »

Online Dennis Toth

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Re: Uniflow tank - open vent tube position
« Reply #6 on: March 17, 2018, 07:29:49 PM »
OK, got to fly today and set the uniflow tank vent line with a 1/16" orifice pointed into the airstream. The Fox liked it! With this setup I was able to set it rich and get through the OTS pattern. Did some prop adjustments from the MAS 10x6 down to the TF 9x6 and 9x6 1/2. With the smaller diameter it reduced the load and allowed solid runs, one rich 4 cycle and one just breaking to a 2 cycle. All flights held the setting. I now have a large enough tank to play with different nitro levels to get a little bit quicker lap time.

Best,   DennisT


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