Lynn,
I've often wondered how the air flow is affected by joined tubes, with a slip tube outer. My concern is tripping the boundary layer and causing turbulence enough to restrict flow. Is there any really good way of achieving a nice consistent flow with such a join.
I thought of rounding off the inner facing edges, but then wondered if that minor diameter increase would disturb the continuity of velocity.
Probably all a trade off.
George ( down under)

     Fellows,
    >
    >  I have been thinking up this crazy idea of trying to make intake     > runner tubes out of carbon fiber. My question is regarding
    length of     > the velocity stack. Does the length make a difference? Could a     > velocity stack be more or less the entire length of the tube?     > Meaning, if I have a 12" intake tube, could the entire length of
    the     > tube be a gradual taper to the diameter of the block opening?
    Would     > that mess with the speed the air is traveling in the tube?     > Aerodynamics is not something I have a very good handle on, and am     > hoping someone out there in Fly Rotary land can shed some light.
    >
    > Thoughts???
    >
    > BTW,  Any reasons why carbon fiber should not be used for intake     > tubes? May make the velocity tubes a mute point.
    >
    > Thanks for any thoughts.
    >
    > Ben Schneider
     
    It would take the rest of your life to read all there is to read
    about intake (tuned) lengths. So here is my view on it.
         The pipe organ idea is in play, but in my opinion is gifted too
    much interest. In actual operation just about anything works prety
    close to just fine. I just reviewed some formula Super Vee stuff
    and note that a number of intakes were tried. Super Vee was a
    class where a 1600 CC VW (Rabbit) engines were used in race cars.
    The stock parts had to be used but could be modified by machining,
    The intake runners could be up to 32MM, and the mechanical fuel
    injection (probably from the diesel I think) could be used.
         HP well above 180 was the result, from VW parts. A straight 32MM
    tube with a bellmouth. Not very exotic.
         The tuned length of any system has many harmonic peaks. They look
    like the primary frequency but are removed from it by a factor of
    4. So you have harmonic peaks at multipals of 4. So, 4,8,16 and so
    on. So there are secondarys that show up to either side of the
    primary, and some of those are reenforced by combinations of the
    secondaries, so the 16th harmonic can be quite clear and easy to
    find on a scope.
         The outcome is that it would be foolish to build an intake with
    one primary frequency in mind. Say you want to cruise at 6,000
    RPM, and built tubes to peak at that frequency. If you cannot keep
    the engine at that RPM exactly, the effect is lost. Then you find
    the engine in a null between peaks and performance is now less
    than that of the thrown together mess on that plane in the next
    hanger.          On takoff you would want to have say 6,500 or 7,000 available.
    Where would you tune for that?
         So I drop back to what works in most situations. In general long
    columns of air operate at lower frequencies. Short columns at
    higher frequencies. Large diameters at lower and small diameters
    at higher, and so on.
         The ideal shape for intake runners is tapered. Yep, thats it. not
    often seen due to complexity of construction, but if you want to
    talk ideal.................
         The horn shape at the end of the tube is to prevent a vena
    contracta that reduces the effective diameter of the tube. A short
    rounded edge that turns back 180 degrees is the very best and
    works well in confined spaces like a plenum. For carbs, not so
    good. At harmonics of, and just off peak frequencies, you will get
    a ball of fuel standing in space just outside of the horn. In
    those cases, a longer tapered horn works better to cover up a
    number of frequencies and make the ball less obvious.
         The tuned length thing works less well in bent systems, and since
    the nearly best length will involve runners over the top of the
    engine, there is at least a 180 turn involved, and for side port
    engines, two turns involved. So, to start off we are in tuning
    trouble with a bent system that tends to null our best math
    picture of ideal.
         Round runners are ideal for efficiency in moving air through the
    smallest cross section. However, in a turn
    the "D" shape takes over from the round, in that the mass of the
    airflow tends to move along the outer wall, and at some speeds may
    be seen moving along the inside wall in the wrong direction, and
    also may
    slow and upset the flow along the outer wall. In most cases the
    "D" shape may have slightly less cross section than the original
    round runner. Also there may be improvements should a slightly
    course or rough surface be left along the flat of the "D" so as to
    keep flow attached and reduce the higher velocity along the
    outside. Uniform velocity is better than a number of velocities or
    even reversed flow.
         The taper............
         Large tube diameters have lower velocity and small higher for any
    fixed depression. If you imagine the rotor as a blade and the
    intake flow as a sausage being pushed through the blade, then you
    are one sick puppy.............
         However, it works for me.
         So would high velocity at the port face, (where the rotor cuts off
    each chunk) be better than low velocity?
         The higher velocity for any unit of time means more sausage (or
    fuel air mixture) entering the chamber.
         Yes high velocity is better. But drag in the runner develops at
    the square of velocity, so if the runner is the same diameter for
    all of its length, drag will be high, the boundary layer will be
    thick and easy to upset. Suppose then that the taper was only in
    the last several inches of the runner. Or if you want the ideal,
    the taper starts at the horn and runs the length of the runner
    tightening to the port size at the manifold face. Less drag
    overal. Thinner more stable boundary layer, and the highest
    possible velocity at the port face.
         Another thought..........
         The primary and secondary ports feed just one rotor each set. The
    primary ports are those in the center iron, and are smaller than
    the secondary ports in the end irons. The engine operates for most
    of its life on the primaries, as little HP is called for in normal
    driving. The primaries are small, so as to maintain high velocity,
    and crisp throttle response.          I would join the two intake ports and run a single runner for each
    rotor housing. Less volume consumed.
    Lighter intake system. Smaller plenum. Probably no loss of
    performance. The molded part then could be tapered, have the "D"
    shape, the rough inside turn, a length of straight tube to mate to
    a slip tube to "tune" runner lengths.          Lynn E. Hanover