On Sat, 2008-12-20 at 15:11 -0700, John Denker wrote: > On 12/20/2008 10:24 AM, Ron Jensen wrote: > > > Lets look instead at finding out the real reasons why the output > > behavior is not as it should be. > > > > First question, prop_81in2v.xml gives a minimum and maximum propeller > > pitch of 12.0 and 31.8. Any idea if these numbers are right or are they > > just guesses? > > > > Second question, prop_81in2v.xml gives a minimum and maximum rpm of 900 > > and 2400 rpm. Any idea if these numbers are right or are they just > > guesses? > > > Hmmm ... prop_81in2v.xml is not a Hartzell part number or > even close, so I have no idea what RW propeller that file > is supposed to represent. Similarly, there is no documentation > in the file to tell me.
It looks like the c_power and c_thrust tables were taken from propC10v.xml which are taken from data in NACA Report 378. > I have here a RW 1979 Cessna R182 manual that says the propeller > has a low pitch of 15.8 and a high pitch of 29.4. Mr. Cessna is > kind enough to specify that this is measured at the 30" station > (otherwise the numbers would be meaningless). The Type Certificate Data Sheet for the 182S/T which are the only 182s listed for the IO-540-AB1A5 give 17 / 31.8 at the 30" station. That matches what was once in prop_81in2v.xml, I'm pretty sure the 12 is a typo. > The RPM numbers quoted above are probably about right as to the > governor. Full forward on the prop control sets the governor for > 2400. I reckon full back might set the governor for somewhere > near 900 but I'm not sure. I recall it is impressively low, but > I don't recall the exact number. I usually only see this number > during emergency glide conditions, and then I usually have other > things to look at. Of course the governor numbers are not > hard limits; at any prop setting if you pull the throttle back > far enough the propeller will drop out of regulation. That's > because the aforementioned pitch limits *are* hard limits; the > mechanism literally hits the stops. The FGFS model seems to > capture this out-of-regulation behavior OK. > > There is also transient behavior; it is easy to overspeed or > underspeed the prop temporarily. > > ======================== > > Now let me explain why that's not the right starting point. > Two reasons: physics and engineering. > > You don't need to tell me the propeller and engine interact. > I'm pretty sure I knew that already. That's exactly why > they should be tested separately. And yet, you are testing them together and posting your results. > Think about software engineering: We write modules and test > them individually. Yeah, they interact, which is why we > start by testing them separately, so if there is any funny > business we know where to look. We always *end* by testing > everything together, but that's not where we start. > > The same engineering principle applies to hardware. That's > why dynamometers and prony brakes were invented. I guarantee > you Lycoming tests the engines on a test stand before they > go anywhere near a propeller; I've seen the data. (I don't > have a copy; sorry.) Indeed. I use and abuse a test propeller configuration for engine test. > As to the physics: In the steady state, if we know the torque > and the revs, we don't need to know *anything* about the > propeller to ascertain engine performance. It doesn't matter > whether the engine is connected to a dynamometer or to a > propeller or to bunch of pom-poms on a broomstick. You can > formalize this in terms of Sturm-Liouville theory if you want. > The fundamental equations of physics are low-order differential > equations, and they need only a small number of initial > conditions and/or boundary conditions. > > Since it has been "stipulated" that there is misbehavior in > the engine, basic engineering principles suggest debugging > the engine before allowing it to interact with other > subsystems. > > Otherwise there is jeopardy of ending up with two bugs: > in particular, unrealistic prop behavior that compensates > and masks some part of the unrealistic engine behavior. Looking at engIO540AB1A5 I note its configured for 230 horsepower @ 2575 RPM, that makes it underpowered by about 5% as the correct numbers are 230 horsepower @ 2400 rpm. I modified the attached configuration with 250 hp @ 2575. That allows us 230 @ 2400 plus a bit more power when it exceeds redline. You've never told me what RPM you expect the engine to achieve with the prop set full coarse? Thanks, Ron
engIO540AB1A5.xml
Description: XML document
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