Why a Uniflow? How, and why, does it work? Should I use pressure, or not?
Nearly every 3 to 6 months these questions, and others regarding using Uniflow tanks, are posted on the Internet CL forums. This may be one of the least understood subjects in Controline “wet” powered aircraft.
It’s really not that difficult to understand, you just have to step back and realize what’s happening inside the tank. We can talk about CG forces slinging the fuel around, yes it’s true, but the Uniflow has only a small role in that scenario. Uniflows are hard at work, even when the plane is sitting still, at least until the inside end of the uniflow tube becomes uncovered.
Let’s start at the beginning. It was back in the 18th. Century when the principle was discovered.
Simply put, it was found that a column of liquid in a vessel, would drain at varying rates when the surface of the liquid was open to, or vented directly to the atmosphere. When the vessel is sealed, and a vent tube is placed extending to some point below the surface in the column of liquid,,the drain rate remains constant, until the vent is uncovered. At that point, the drain rate will vary as before, when the surface was open to the atmosphere.
When I was a child I first saw a simple demonstration of this principle while washing the family dishes. Have you noticed that if you turn a water glass on it’s side under water, then turn it bottom side up, you can raise the bottom of the glass (now, the top) completely free of the surface, until you raise it high enough that the rim breaks free of the water surface? At that instant all the water in the glass immediately drops out, and back into the dishpan, because it has vented to the atmosphere.
This demonstrates the underlining principle that makes a uniflow work.
If we had a modified glass, A glass with a pair of tubes, one coming through the bottom of the glass, (Uniflow) and the other about 1 inch above the lip. (when held upside down) This will be the drain. The Uniflow can be extended to a point, at least 1 inch below (above) the rim, we could put the Uniflow to work by filling the glass as before, and blocking off the tube going through the bottom (the Uniflow) with a finger tip before tilting the glass upside down, and raising the glass above the surface of the water.
Bring it up high enough so the drain is free of the surface, so you can see the speed of the flow.
With the Uniflow vent blocked off by your finger, The column of water doesn’t materially change because of the vacum that is established inside of the glass. It cannot vent. The column of water stays up inside the glass until you take your finger off the Uniflow (vent).
The speed of the draining, is dependent on the height of the Uniflow, above the drain. The speed will remain constant until the submersed Uniflow end breaks free of the column of water within the vessel.
Uniflow, and the stunt tank
Basically, there are only two types of Uniflow stunt tanks, but lot’s of ideas on how they should be constructed.
Metal, or Hard tanks are not usually set up as a clunk tank. A clunk tank is designed to attempt to have the fuel pickup follow, and stay in the fuel during the manuievers.
Plastic or Soft tanks are less understood by many. They are almost always set up as a clunk tank.
The biggest misunderstanding seems to be about where the Uniflow should terminate inside the tank.
In all cases, we want the Uniflow to terminate ahead of the pickup. Typically, in a hard tank, this means about 3/8” to 1/2” ahead, (towards the front) of the pickup. Get the Uniflow too far to the rear, and you’ll almost certainly have problems.
Tests by one of our top pilots Paul Walker, show that the Uniflow will work well when placed at the front, middle, or, traditional location. He noted that he preferred a location near the middle.
The plastic (soft) tank is often setup with the Uniflow connected to the pick up line. The reasoning seems to be that the builder wants to maintain the traditional location of the Uniflow. While this seems logical, I believe it is in error. Why? Because the end of the Uniflow is moving with the clunk, and the relative, or apparent height of the Uniflow is moving around. This upsets the symmetry of the engine run. It makes the engine see the tank as moving above, and below the NVA. Alternately going slightly rich or lean with the movement.
Study the illustration at the top of this article, and you’ll notice that the Uniflow end is not coupled to the pickup. It is instead, directed to the approximate center of the tank.This sets the symmetry of the engine run, or, makes it possible to rotate the Uniflow up or down, and thereby adjust the apparent height of the tank relative to the NVA.
What about the pickup?
Hard tanks, usually use the end of the tube soldered about 1/4” to 3/8” from the back of the tank.
Soft tanks usually use a clunk. Some use the ones that come with the tank. I prefer
to use a sintered bronze pickup. I believe there’s an advantage beyond not needing
an in-
Muffler pressure or not?
I prefer to use muffler pressure to the Uniflow line. I feel that I get a much more consistent style of run. Others preferred to forgo pressure, but, I am left wondering if they might have their Uniflows coupled to the pick up.
Uniflow tank
Construction
Using the plastic RC tank in CL Stunt.
By
John Miller
Plastic RC tank setup for uniflow
When you set up a tank as shown, you will have a stunt friendly uniflow tank. The apparent tank height can be adjusted by rotating the Uniflow up or down as needed.
The actual tank does not move, just the end of the Uniflow, inside the tank.