I guess I'll have to jump in on this one. I did quite a bit of research
    and experimentation on turbos. I have real experience with running them
    up to 18000 ft. My plane has a total of 150 hrs of turbo time and 50
    hrs N/A time. I really should update my website with a discussion of
    turbo selection so I don't have to keep re-typing this...

    I agree that the TO4B series is probably the best choice for size, cost,
    and availability. Paul is right that the stock turbo will produce
    sufficient power, but at that power level it is over speeding and will
    not last long. That cannot be fixed by putting on a larger waste gate
    alone. Turbo speed(in a given turbo) is simply a function of mass flow
    ( i.e. desired horse power) and pressure ratio. That is regulated by
    the throttle (i.e. how much h.p. you use), not the waste gate. A bigger
    waste gate will only reduce the back pressure created for a given
    position on the compressor map. But given an altitude and power
    output, changing the waste gate will not affect turbo speed, surge,
    stall, over speed, or thermal efficiency.

    The waste gate only determines what portion of the exhaust energy is
    flowed through the turbine. So the idea is to feed the turbine only the
    energy that is needed to create the desired boost. Anything more just
    increases the back pressure and reduces the efficiency of the system
    (different than compressor thermal efficiency).

    In my plane, only the stock turbine housing will fit. It is amazingly
    compact. I originally ran the stock series 5 turbo. It was good for 70
    flight hours, of which only about 15 were high altitude/ moderate boost
    hours. It died. My replacement (current) turbo is the TO4B 60-1
    compressor attached to the stock turbine housing that has been reamed to
    accept a TO4 p-trim turbine. After 80 flight hours, including at least
    40 hrs of high duty, it shows no sings of being abused. This turbo is
    capable of making at least 40psi of boost (MAP well above 80in. at sea
    level).

    I need a turbo that size for proper efficiency and compressor speed at
    altitude, not high boost. Since I will never use that much boost I
    opened up the waste-gate. The waste gate has been ported and the flapper
    removed. In this configuration, full throttle at S/L will give me a MAP
    just under 50", which is perfect.... I am only using the exhaust needed
    to obtain the desired boost.

    This translated to just over 30" MAP at 16k feet altitude and WOT.
    Again, perfect. Minimum loss of efficiency from back pressure, minimum
    loss from throttle body pumping. Aircraft Speed approaching Vne.
    Compressor speed well into the green according to the compressor map.

    It sounds like the end of the story, but I still plan some changes to
    the turbo when I get around to it. I plan to put the flapper back in,
    and aggressively clip the turbine blades. The waste-gate has another
    important function in my set up. Besides bypassing unneeded exhaust, it
    helps to regulate and smooth out the natural variations in boost. These
    variations only really become evident when running WOT, particularly
    when running lean of peak. With the flapper removed (waste gate passage
    fully open all the time) I have found that I can either aggressively
    lean the mixture, or operate at WOT, but not both at the same time. If
    I try to do so, the natural variations in engine RPM have an amplified
    effect on boost and engine RPM. i.e. a slight decrease in RPM leads to
    decreased exhaust energy which leads to decreased boost which further
    decreased RPM. It all happens rapidly and seems like someone is playing
    trombone with the throttle. At less than WOT, the throttle valve tends
    to dampen out these variations. The flapper valve on the wast gate
    (connected to the stock automatic actuator) did the same thing when it
    was connected. I am not sure why running rich also fixes the problem,
    but it does.

    So here is the IMHO Dave Leonard simple guide to selecting a turbo for
    the aircraft 13B: A compressor in the size neighborhood of the TO4B
    60-1 (since it is oversized and underused the exact size does not really
    matter as long as it is in the neighborhood - check the maps, many are
    in this size range). Then, the turbine should be as BIG as you can
    fit... (A/R in excess of 1.0). Have some sort of automatic regulation
    on the waste gate, then make the wast gate big enough and/or the turbine
    clipping aggressive enough so that full WOT will give you what you
    decide is the maximum MAP you want to be able to achieve (for the time
    being I recommend less than 50" MAP at sea level).

    The amount you need to clip you turbine blades will depend on the
    turbine and compressor that you finally choose. A larger turbine and
    waste gate is probably preferable to a more aggressive clip.
    Alternately, a huge aircraft style wast gate with a normalization is
    probably appropriate if you prefer.

    BTW, keep in mind that if you go to an automotive guy to get the turbo
    built they will try to talk you out of it. Very few of them understand
    what you need, they seem to be too in tune with what the car guys
    want/need.



    And finally, regarding mass vs. volume for turbine sizing... My first
    issue with the above discussion it that you discounted mass flow as the
    preferred way to size the turbo. I initially (years ago) agreed with you
    that volume flow seemed more approiate, and it will work (and some turbo
    maps are in volume). But the density of the air does matter. The
    reason is that we need some sort of standard reference pressure to use
    when deciding how much volume we need in order to achieve the desired
    h.p. from the engine. Since h.p. is mass dependent, I have found that
    is far easier to use mass flow on the turbine maps. This then will
    correlate directly to the proper higher pressure ratio for use on the
    compressor map.

    Another way to look at it: since the pressures are different in
    different parts of the intake system, then volume flow is going to be
    different depending on whether you are looking at compressor inlet,
    outlet, throttle body etc... but the mass flow is the same throughout
    the system. If you play with it long enough, you will see that mass is
    the better measure.

    David Leonard

    Turbo Rotary RV-6 N4VY
    My websites at:
    http://members.aol.com/_ht_a/rotaryroster/index.html

    http://members.aol.com/_ht_a/vp4skydoc/index.html
    http://leonardiniraq.blogspot.com


    Thanks for your input on this Dave. So far you have got more
    out of the largely stock 2nd gen turbo than anybody else.

    I agree with you on mass flow. So many air molecules
    combined with so many fuel molecules equal HP.

    You state:
    "A bigger waste gate will only reduce the back pressure created
    for a given position on the compressor map."

    This is not clear. I am not sure I agree. If no other changes
    are made a larger opening in the waste gate WILL change the position
    on the compressor map because less turbo HP is going into the
    compressor. The limit case is no exhaust gas going to the
    turbine at all. The position on the compressor map will be at the
    very bottom in that case.

    I think not everybody wants 50 inches of Hg boost at sea
    level or flat rating to 16,000 feet.
    Suppose a larger (or different type) waste gate alone gave 29
    inches of HG at sea level (no boost) and the waste gate was
    gradually closed to get more inches of Hg boost at altitude.
    In other words flat rating but maybe not to 16,000 feet as you have
    done. Better yet a red line RPM was established for the turbine
    and the waste gate controlled to keep the turbine within
    the red line RPM. Would that not be cheaper than increasing
    the size of the compressor to slow the turbine?

    I think that would be a good starting point for most people.

    The HP of a given turbine is a function of the blade speed to
    exhaust gas velocity ratio. Dynamic pressure if you will.
    Dynamic pressure is both a function of velocity and air
    density. As the blade goes faster than about 50% of gas speed
    HP starts to drop as turbine torque is a linear inverse function
    of turbine RPM. Chart 2. Therefore I agree that using a larger
    compressor (A to B) reduces turbine RPM and increases the HP put
    into the compressor IF the RPM is beyond the 50% gas velocity point.
    Chart 4. Note the similarities with a combustion engine
    and a propeller. The HP curve can be thought of as
    potential HP. Only when the HP curve intersects the load
    curve have you extracted any appreciable HP from the turbine.
    Another good analogy is dyno load verses prop load.

    One might get the impression this is a very complex subject.
    One would be right :)

    Paul Lamar


    Thanks for jumping in Dave.

    I have nothing against the maps that use mass flow. If you want a
    turbo because you want to increase your horsepower you have to think
    in terms of mass. The Garrett website gives good info on this. It's
    a simple matter to figure out how much mass flow you need based on
    your air-fuel ratio and BSFC. You can then look at the compressor
    maps and choose a compressor that will give you the mass flow you need.

    But what do you do if you don't want to increase your horsepower,
    but want to carry your existing horsepower to a higher altitude?
    Then you have to think in terms of volume, because you're going to
    need more volume in the thinner air -- not more mass because you're
    not after more power. (Same mass, more volume) And those compressor
    maps are no help at all for us.

    Still, it's no problem if you do the arithmetic. Then you can use
    the mass flow maps just fine. The mass and volume relation is fixed
    in the ideal gas law. That's why I posted the formulas. I have
    reposted them here in a simple 4-step form. I will also put them
    into a spreadsheet and that will make the calculations easier.

    Thanks for describing your setup. With 50 inches of manifold
    pressure it makes considerably more power than a normally aspirated
    engine at sea level -- a ground-boosted aircraft engine, in other
    words. Nothing wrong with that -- it's always nice to have extra power.

    What I'm interested in though is a turbo normalized setup that
    is dead simple from a flying point of view. A good example of this
    is the Mooney 231 or Turbo Arrow, neither of which has any kind of
    waste gate adjustment whatsoever -- either automatic or
    pilot-controlled. Same thing on the Seneca turbocharged twin.

    There is just a small pipe upstream of the turbo that bypasses some
    of the flow. That's it. It's basically a fixed waste gate, but it can
    be adjusted on the ground. (See attached picture). The turbine and
    compressor are sized to achieve the desired boost and turbine speed
    as you climb. This can be done just as easily on the rotary if we
    choose the right size turbine and compressor.

    Another simple setup is the turbo Skylane and Saratoga which do have
    a waste gate that is linked to the throttle. You fly the plane just
    like a normally aspirated bird. You push the throttle in for takeoff
    and pull it out a bit for cruise.But you do have to monitor manifold
    pressure more closely in climb and descent. A lot of STC'd
    turbo normalizing systems use this setup too.

    Notice that the waste gate is closed at takeoff. Why doesn't the
    turbo over speed or the manifold pressure over boost? Because the
    turbine is big enough that the sea level exhaust energy can't spin
    it up. The exhaust energy is a function of exhaust speed across the
    turbine, which is a function of pressure drop across the turbine,
    which, in turn, is a function of the pressure delta between the
    turbine outlet (which is ambient) and manifold pressure. This
    pressure delta increases as you climb, provided you can maintain sea
    level manifold pressure. So the exhaust speeds up, and the turbo
    spools up the compressor, adding just the right amount of boost you
    need to keep manifold pressure up -- provided you have sized the
    compressor and turbine just right.

    We can use this natural balance, if you will, to our advantage in
    designing a turbo system that needs no pilot control whatsoever. I
    like the simplicity of this approach because I find that simpler
    systems make for more enjoyable flying. That's why you see the move
    toward single-lever control -- something turbine pilots will tell
    you is a very good thing.


    Regards,

    Gordon.

Gordon,
Good information!
I agree on the bypass system and advocated that sort of system a little
while ago, however my suggestion was a bypass to the turbo with  a gate to
restrict either the exhaust or the bypass. My reasoning is that you can't
overspeed the turbo if you restrict the incoming exhaust to the turbo.
I also believe that by  remotely locating the turbo you will reduce the
speed and heat of the exhaust.

I still believe this is the best way to set-up an appropriately sized turbo.
It allows for variation and fine tuning as well. Once set up it doesn't need
to be varied in flight.
It appears from what you have stated, others think this is the right way of
doing things as well.
George ( down under)



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