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Thread: IO-520-D Engine Management on Takeoff and Climb

  1. #41

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    The 88" prop was designed for the IO-550 which turns at 2700rpm. The 86" prop was designed for the IO-520 which turns at 2850rpm. The extra 2" is mainly just noise.

  2. #42
    skywagon8a's Avatar
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    Over the years I've used four different props and three different engine configurations in my 185. Starting with the original 88" 2-blade, and a 80" 3-blade both with the stock IO-520, a 80" wide chord 3-blade with a converted stock engine to turbo/intercooled 310 hp. And now a IO-550 with the 82" Hartzell Blended Airfoil prop which is shown on Ponk's http://www.pponk.com/HTML%20PAGES/Ha...pellerPDF.html site. This current combination gives me absolutely the best takeoff/climb/cruise performance of all and is the quietest smoothest operating combination that I have found. The STC for the engine and prop is owned by Texas Skyways http://txskyways.com/. A friend had the 88" 3-blade on his 185 and it was an extremely noisy dog, both on wheels and floats.
    N1PA

  3. #43

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    My 185 with a 520 and 86" 2 blade has reached tbo with never having a cylinder off yet[milleniums], it is used for SES training and hauling very heavy loads on floats 3525#gross plus. While training light i use 2700 rpm from the start and go to 25 -2500 on climb out. Loaded heavy - full rpm til on step then 2700-2750 rpm, on climb out full MP and 2500 rpm til 500 ft then to 25MP and 2500rpm, it has worked well for me for 14 years. I have flown many 185's with lots of prop and engine combos, I think the 88" 401 may be a bit long for the 520 and making more noise, i may go to a 84" 401 for mine for my next weed wacker or a MT.

  4. #44
    Iflylower's Avatar
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    What a great topic.

    A few misconceptions MANY pilots have....

    Manifold Pressure: It is technically a pressure guage. BUT, it makes people think of pressure in the case, when it's almost always a vacuum. The engine is always able to run and handle full atmospheric pressure. The air being drawn into the Filter/carb/throttle body/intake whatever is always from the intake stroke of the engine (with minor ram rise "dynamic loading" from moving through the air.) You can't "overboost" a cylinder with standard atmosphere (no turbo/supercharger). Never. The engine is always most efficient at WOT (wide open throttle), when one is not limiting or starving the engine of air. There are other factors of course, but that's what makes 8-9,000 feet with WOT such a great altitude for most normally aspirated planes. Anytime the throttle is at less than WOT, you are limiting the air that the cylinder is "drawing in," So, at idle - 12ish MP, the engine is getting very little air, but not fully cut off - the engine wouldn't run.

    Fuel Enrichment at full throttle: A lot of people think it's for engine cooling, and that is a secondary benefit. But, the fuel enrichment at WOT is another detonation avoidance tool. It's designed to slow down the combustion event on the power stroke at high power settings. That's really it. In cruise and even take off, you can use the mixture to set exactly what YOU want (best power), but being slightly on the safe side is better.

    Oversquare: Again, the idea that you overpressure the cylinder with prop back more than manifold is wrong. THE PROBLEM is and always has been detonation. Most of our engines are tuned for full Take off power. Otherwise, we'd have variable advanced timing, fadec etc. Setting our timing for full power takeoff means we make a compromise at other settings, especially towards lower power settings. I personally like and have used leaving full throttle and bringing the prop back first. 2700 RPM to 2500 RPM is an 8% reduction in Revs. 2850 RPM to 2,500 is a 12% reduction in Revs. At sea level with full MP available, bringing the RPM's way back creates a situation where the engine is running slower (cycles/revs) but getting full power from fuel/air mix. The combustion event has the possibility of completing earlier in the power stroke. Think of it this way. At 1,400 RPM the stroke takes twice as long, but the combustion takes the same amount of time. You in effect are burning your power earlier in the stroke and you're PEAK PRESSURE POINT or PPP, happens at a point in the stroke where you don't get all the BANG for your buck. So you're running the risk of losing HP because you didn't hit your optimal power point in the stroke and you run out of combustion early, which leads to detonation.

    I have a cool chart image from GAMI that I'll try and insert.

    USE at your own risk.

  5. #45
    Iflylower's Avatar
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    Quote Originally Posted by Speedo View Post
    My employer wants the engine to be pulling us into the pattern, rather than "pushing" the prop. This comes from their experience, and since they have been at it for a long, long time, I suspect they have found something that works for them.

    On a related thought, if you arrive at TPA 6 minutes out that is roughly equivalent to 10 miles out - about 5 miles early if you go by the AIM.

    Another cool subject....

    The idea of the engine pulling on descent vs being pushed (windmilling) is important, but more a throw back to radial engines. The idea of windmilling, or the air driving the Revs on an engine is called "Reciprocating Load." It's a no, no or at least something to limit on radial engines with the master rod, connecting rods geometry. Let's face it. All these planes/engines are designed to pull us through the air, not get pushed back. There are the load bearings on cranks and normal forces to consider as well. Radial guys and really most of us should try and keep at least a small amount of power on till pulling power to land.

    It doesn't take much, but having a small amount of power over idle and using drag to slow the plane with approach flaps and gear or leveling to get a slower speed will all be a help. This is not a player for cubs, barely a player for Cessna's and really are for larger recip aircraft.


    On the descents with a faster, bigger engined recip, I don't mind mixing the 1" manifold reduction at a time, with the slightly pulling decents, and the keep bringing mixtures richer at a quarter inch at a time. Throwing a full rich mixture after lots of leaning is another way to cool and engine too fast from the inside.



    Last note: I always operate how the owner wants me to protect their equipment (as long as it isn't illegal or dangerous.)

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