Paul wrote:
    A fixed pitch prop has only one load for each RPM.
    A variable pitch prop and engine dyno has many loads for the same RPM.
    What ever is going on I could not get it to run well
    on the dyno for what ever reason.

    Unconfuse me Paul,

    Fixed pitch = I got that, But variable pitch has many loads I don't
    understand.

    If I start at a steady state cruise and touch nothing, power and load
    must be equal.

    If I pitch up slightly ( without any throttle position change ) I change
    this equibrilum. Prop load would BEGIN TO increase, but SIMULTEANOUSLY
    as the RPM began to fall, the flyweights in the prop governor react
    allowing oil movement to decrease prop pitch which ( and here's where
    the confusion is ) should keep the load on the engine the same.

    If this senerio is correct, then for any given power setting the load on
    the engine will be as stable as a fixed pitch. The only difference is
    that the pilot gets to pick the power/RPM relationship ( within
    reason---detonation being one of the reasons ) whereas the fixed pitch
    pilot has only one power/RPM relation available at any given RPM and
    must decide between cruise performance,ie. buy a cruise prop, or climb
    performance, ie buy a climb prop.

    None of this will help with your dyno tuning problems, but I'm trying to
    get a handle on controllable pitch props when used with Tracy's engine
    controller and a standard aircraft prop governor. Please
    comment. Thanx, Dave M


    Me too and we are still collecting data. It is also usual
    for the pilot to increase the pitch and load the engine down for cruise.
    A fixed pitch prop loads the engine 100% only at maximum speed.
    The rest of the time there is potential power in the engine that
    is not being used. This has a major effect on the BSFC and the MPG.
    See the last attachment. It is a complicated subject.

    Paul Lamar ...No rotor no motor.


    Dave,

    I think what Paul is getting at is the question of advance ratio,
    which is the key to propeller efficiency.

    Advance ratio is simply the ratio of the airplane's speed through
    the air to the propeller's rotational speed -- mathematically, V / nD.

    With a fixed pitch prop, the propeller will be working at its
    optimum efficiency at only one particular airspeed; at all others it
    will be sub-optimum. With adjustable pitch you can adjust for the
    optimium advance ratio at each particular airspeed, so your prop can
    always work at its peak efficiency.

    Think of advance ratio as analogous to angle-of-attack of the wing.
    Let's say you have a fast plane flying in a formation with a slow
    plane. The fast plane will have to fly at a higher angle of attack
    to keep its airspeed down. Obviously it will not be flying at its
    most efficient speed and will have a lot of induced drag at that
    attitude. Efficiency will suffer.

    It's the same with advance ratio. You want to adjust the prop pitch
    to achieve the optimium advance ratio for that particular speed.
    With a constant speed prop, that means the particular pitch that
    gives the optimum advance ratio will correspond to a particular
    engine rpm.

    That's where the question of fuel flow comes in. If you run your
    engine at a higher rpm than optimimum for that particular airspeed,
    you will be burning more fuel than necessary. That's why that
    magazine test with the rotary came out so bad. The rotary engine had
    enough power to fly the same speed as the Lycoming, but at lower
    prop rpm. But since the test stipulated they fly at the same rpm,
    the more powerful plane was at a disadvantage -- just like the fast
    plane that has to fly at an inefficient high angle of attack to stay
    with the slow plane.

    So let's say you have climbed to your cruise altitude and are
    leveled out and setting up for cruise. You start pulling back on the
    prop to coarsen the pitch and lower the rpm -- in order to find the
    optimimum advance ratio for your cruise airspeed.

    As you do that the engine is loaded down because it has to make the
    same power at a lower rpm. (Your plane is still flying at the same
    speed, so it requires the same amount of thrust horsepower to move
    it through the air). Now you also need to increase your manifold
    pressure if you are going to make the same power -- and you need to
    increase your manifold pressure some more if you are leaning and
    want to maintain the same power).

    So with an ECU that relies on MAP, engine rpm and a VE lookup table,
    this could be tricky. The lower rpm, but higher manifold pressure
    (and possibly leaner mixture) is the tricky part.

    Perhaps an ECU that uses mass air flow might work better in this
    situation. I believe Tracy's ECU uses the MAP, rpm and VE table
    method -- as does megasquirt. For fixed pitch prop this should work
    great, but I'm not so sure for constant-speed. (Just one more reason
    to go with the carbs).


    Regards,

    Gordon.

Megasquirt also uses throttle position and engine temperature.

Paul Lamar

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