It helps to visualize the forces on the fuel in the tank.
1. The "normal" condition - level horizontal flight at the basic 'altitude', 5 to 6 feet above ground, 'sees' the combination of "centrifugal" force and gravity. CF is around 3g horizontally outward. Gravity is (duh) 1g, vertically downward. The angle resulting force (resultant) is 3 units out and 1 unit downward. The combined force is the diagonal of a rectangle of those proportions: square root of (CF^2+G^2) = (3^2+1^2) = square root of 10, 3.16g. The number is approximate, but useful...
2. In inverted level flight, the same numbers apply but the "down" direction is inverted with the tank.
3. When maneuvering g are present, their action is added to the standard conditions. Corners are transient and of short duration. Round loops, both ways, last longer. All such turns generate CF of their shape, which we can consider full or partial circles. A long time ago, I arrived at around 10g for round loops and perhaps twice or more that as a reasonable guess for 'square' corners.
These forces are aimed like radii of the figure. Going through the bottom of a low round loop, that approx 10g adds to the basic 1g - both forces aim vertically down.
At 45° line angle, the vertical effect of maneuver g is as Sin45° downward. Lift is also tilted 45°, so to hold altitude. Consider level flight at 45°: Total lift at that height must equal level lift plus Sin45° to maintain altitude. Total wing lift, there, is more than level flight's 1g
Lift is generated perpendicular to the wingspan. Maneuvering g loads also act perpendicular to the wingspan so are also tilted. Above 45° these tilts also apply. We pass through top dead center with maneuver load, if present, acting against wing lift, and the 'tilt' of the g load on fuel is vertically upward CF reduced by that 1g.
Cornering loads could be considered 'flat,' tangent to the flight hemisphere, because they are small compared to the hemisphere. E.g., in a corner up from level flight (away from the ground, not from the viewpoint of an imaginary pilot) the diagonal values of g load and lift increase load until the model is vertical, then slightly reduce it. The g required overwhelms any need to worry about lesser loads present.
At 45° similar brief transients occur. Over the top, the very brief duration pretty much allows us to go through on momentum and regain more standard conditions. This applies for wingovers and the top leg of the hourglass.
What has all this to do with tank venting? (That's where we started, right?)
First, think an open top 'fuel tank' with the fuel pickup rising vertically in it to the fuel surface. Some magical relationship exists between that level and the engine's fuel draw situation - most engines differ; aligning the fuel surface with some structural part of the engine may or may not give our desired 'run.' Once the 'magic relationship' location is found, raising the fuel surface (and pickup) will 'pour' fuel downhill to the 'system:' lowering the fuel surface (and pickup) below that point will require the engine to draw fuel 'uphill.'
Fuel surface high? Richens the run. Fuel surface low? Leans the run. Easy to understand, right?
The fuel surface determines the effect. With a uniflow tank setup, the end of the VENT TUBE inside the tank acts as the relevant fuel surface. Think of the varying g conditions. For CLPA we want a consistent run "both ways'" which means for all g, attitude and height conditions, inside and outside load conditions. Yes, g loads will still have effect, but if they vary consistently "both ways," we can use it. Well, it HAS worked for 70 years plus...
Before I fly a new model, I check for a reasonably "correct" tank height. (My wig-wag test, many times explained here and on other sites.)
Basically:
I hold the model wings vertical, outboard tip down. Start engine and set a 'just rich' run. (relevant fuel 'surface' effects easier to observe.) With fuselage level, roll the wings to about 45° 'pilot up' and watch for an RPM or sound change -note any such. Roll it about 45° 'pilot down' and check again. If there is a noticeable setting or RPM change, stop the engine (safest: nose vertically down, outboard tip up to uncover fuel pickup.) Adjust tank height: lower it from the richer position, or raise it from the leaner one. Run the same process again, until there's minimum change between the rolled positions.
It's not perfect or final, but should allow a safe first few flights while you fine tune the height to get the best tank height for that model. Warps and other structural details may (often do) require detail tweaking. At least, it should survive for you to do the tweaks!
Topic: Fuel tank question (Read 2043 times)
