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Thread: prop pitch and detonation?

  1. #1
    bwilson4web's Avatar
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    prop pitch and detonation?

    Hi,

    To solve a problem with prop gearing, I'm planning to use an Ivoprop, in-flight, electrically adjustable prop. But I remember back in the 1980s, constant speed props needed advanced training if nothing else because of the engine oil powered, mechanical coupling to the engine. Additionally, too much pitch and throttle could lead to detonation and certainly at high prop pitch there would be rpm limited power and thrust if a go-around is needed.

    So I understand in 1997 the FAA required complex airplane endorsement for "an airplane that has a retractable landing gear, flaps, and a controllable pitch propeller, . . . " (para 61.31.) I understand this is the "AND" of all three elements are required. So a controllable pitch propeller alone is not enough to require a 'complex airplane endorsement,' right?

    On the practical side, the risks of a variable pitch prop are real. Too much pitch, low altitude and adding full throttle could put a high compression engine into detonation. I've seen this in manual transmission cars when in a high gear and flooring the accelerator triggered the 'ping.' Certainly, I'll want to address this during ground testing but I haven't read much about such testing. Have you come across articles that address prop pitch, altitude and detonation?

    I know 100LL gives us a built-in, detonation advantage but that is more true for the relatively low, 8-to-1, compression engines. However, I'm planning to run a 9-to-1 compression engine which reduces the detonation margins.

    Thanks,
    Bob Wilson

  2. #2
    steveinindy's Avatar
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    So a controllable pitch propeller alone is not enough to require a 'complex airplane endorsement,' right?
    I'm not sure so if I were you'd I'd call the local FSDO and ask. Most of the employees are nice and helpful. However, at least so far as I can see it, additional training is seldom if ever a bad thing so why wouldn't you go get a checkout on a complex aircraft?
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



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    The first post is the first that I have ever heard that too much prop pitch could cause combustion problems. I would double check the source of that info. If that were true then there would be lots of problems with airplanes that have fixed pitch propellers. The maximum pitch of a fixed pitch prop is determined by the acceleration on take off that is desired and the desire to be able to obtain max engine RPM. Some of the Reno racers run really low pitch props that give awful takeoff performance but really shine once at speed (150 mph+). Those guys are not reporting any problems and they really really look at what their engines are doing. If the assertion in the initial post were true, those guys would be blowing up their engines. They are not.

    What cuts into your detonation margin is changing the pistons to increase the compression or adding turbo or supercharging (which adds up to the same thing).

    Best of luck,

    Wes

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    You can save a call to the FSDO by using Google. FAA Order 8710.3E defines a complex airplane for the airplane single-engine land or airplane multiengine land rating as one that has a retractable landing gear, flaps, and controllable propeller.

    The "and" is important. If all you have is a controllable propeller, your airplane does not qualify as complex. You have to read the FAR's with your lawyer hat on. The words mean what they state in this case, no more, no less.

    Best of luck,

    Wes
    N78PS

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    To the question on complex aircraft, the standard has always been that all three components are necessary for the requirement to apply. As far as detonation at high pitch settings is concerned, most engine manufacturers will provide a minimum static RPM at flat pitch and full throttle. The pitch setting in flight needs a relationship to manifold pressure in order to achieve a safe power setting with reguard to detonation. Your post dose not mention what engine you will be using, but aircraft engines will have a power setting chart adjusted for a range of altitudes. A standard axium for power adjustments is to lead with RPM when increasing power, and with manifold pressure when reducing the power setting. This is a good way to avoid the problems associated with high manifold pressure/low RPM operation.

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    I once hung a Beech R-003 manualy controlled prop on a Continental A-65. My concern was exceeding the maximum manifold pressure for the RPM, or overboosting the engine. I had no MAP guage yet. The factory operating manual for the A-65 had a table of max RPM vs MAP. I knew that I could use max RPM at S.L. with no problem. I then did the math to detemine the max MAP that could be attained at various altitudes and applied those MAPs to the minimum RPMs. The max MAP for 2150 RPM, for example was reached at a fairly low altitude. I made a placard.
    Does this make sense. I eventualy sold the prop and hung a metal fixed pitch on it. Flying through rain boogered up the finish on the Beech R-003. And I sold the uninstalled MAP guage.

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    bwilson4web's Avatar
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    Quote Originally Posted by Awhite View Post
    To the question on complex aircraft, the standard has always been that all three components are necessary for the requirement to apply. As far as detonation at high pitch settings is concerned, most engine manufacturers will provide a minimum static RPM at flat pitch and full throttle. The pitch setting in flight needs a relationship to manifold pressure in order to achieve a safe power setting with reguard to detonation.
    Thanks! I'll ask the vender, Recreational Power. I ordered the data package for the engine and though it provides a lot of useful information not found on the web site, there are pieces missing. For example, it does not have the exact center of gravity for the engine mount and any details about recommended mounting such as rubber bushings. Also, I didn't see the redline until I got the data package.
    Quote Originally Posted by Awhite View Post
    . . . Your post dose not mention what engine you will be using, but aircraft engines will have a power setting chart adjusted for a range of altitudes. A standard axium for power adjustments is to lead with RPM when increasing power, and with manifold pressure when reducing the power setting. This is a good way to avoid the problems associated with high manifold pressure/low RPM operation.
    I'm looking at a Hirth 3502, a 60 hp, geared, two cylinder, two-cycle, water cooled, fuel injected and oil injected engine. The engine specifications list the compression ratio as 9.5:1 which is why detonation is important. Fortunately, the throttle body includes an 'equalizer' tube so I'll be able to tap it for one end of a manifold pressure gauge. At 5,000 rpm (redline is 5,300), things happen rapidly so avoiding detonation is pretty important.

    I have been planning to record the engine metrics available from the fuel injection controller but there is no knock sensor. I will consider adding one but this is one of those cases where calibration of the knock sensor is not trivial and may only announce the last cause before the engine fails. My understanding is knock sensors work by detecting the high-frequencies found in detonation but that presumes knowledge of the normal, audio spectral distribution. One reason for having forums like this is to ask questions and hopefully find if not a credible answer, at least the right question to ask the vendor.

    Thanks, Bob Wilson

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    I believe Wes is stating it backwards about "some of the Reno racers run really low pitch props", and that while take off is poor they are good at speed.

    Pitch means the angle of the prop blade to the air, that is if there is lot of angle, then the prop is taking a big bite of air each revolution, in other words it is a high pitch (high angle) prop.

    The high angle or high pitch prop may bog down the engine, and not be good for takeoff or slow speed climb, by not allowing full rpm and full power of the engine.
    This high pitch prop may work well in cruise or full speed runs where full rpm is not needed or easier to develop than it would be in still air on takeoff.

    So high pitch means low rpm, good for cruise and full speed,and great for fuel economy.

    Low pitch means a flatter prop, less angle or bite on the air, and more rpm, good for takeoff and climb, but not so for top speed and real wasteful of fuel economy.

    It's just like riding your bike, you want low gear( low pitch) high rpm to go up a hill, but high gear, low rpm to go down a hill or cruise.

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    Bill is correct that I should have said high pitch. Its a high number on the prop protractor and low RPM's on takeoff.

    9.5 to 1 is not a high compression ration except in relation to standard aircraft engines. Acro guys run 10 and 10.5 to 1. I will also note that the characteristics of two stroke engines are different. Your big question will be how efficient your cooling design is since 2 strokes make more heat. I will hazard a guess that in practical terms cooling will be what determines your detonation margin, not your prop pitch.

    I will observe that the two stroke installations that I see run smaller props than the 4 stroke installations. Two strokes make less torque which is likely why the installed props run narrower chord blades (see Ivo props) than the big paddles that we can put on Lyco's (See Hartzell Claw).

    Everything on an airplane is a trade-off. You rarely can change one variable without also changing another couple.

    Best of luck,

    Wes
    N78PS

  10. #10

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    Quote Originally Posted by bwilson4web View Post
    Hi,

    too much pitch and throttle could lead to detonation
    There are many factors at work, but yes, this is one of them. If parameters are all held the same, and you do nothing but slow the prop, there is a timing effect. If the engine was near detonation at higher rpm, and you slow it, the pistons are now moving a bit slower. The spark is at the same piston position, but the flame front is progressing in the chamber that has a different rate of expansion (slower). So, one could have higher peak pressures, maybe high enough to have detonation be a problem. This phenomenon is described by either the GAMI folks or Mike Busch, I forget which. Look for a Pelican's perch on oversquare operations and it may be in there.
    However, as I said there are many factors at work. Some flyers report trouble high CHT's on climb out. They leave the throttle full in, but drop from 2700 rpm to maybe 2500 and the CHT's come down.... moving them farther from detonation risk. Why? the engine is making less power, so you've go less heat.

    Wes said it well, detonation will likely depend on the cooling margin. I'm doubtful one can ground test and answer this question. There seems to be consensus building around the idea if you can keep your CHT's cool- below say 380F, the likelyhood of detonation is very small. So... paying attention to airflow will payoff.

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