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JohnF
Joined: 13 May 2010
Posts: 124
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 Posted: Thu Jun 16, 2011 7:37 am Post subject: RPM Variations - follow up |
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Wow ! 7th day in a row of great weather...calm wind, cavu visibilty 60+ and dead smooth air
Used auto pilot at density altitude 10,500-ft and rpm set at 5250...varied only up and down occasionlly 10-ft...went to manual control and saw the rpm vary as before, so I am convinced pilot input is the cause of engine speed variations as they coincide with Dynon D-180 indications of "up or down.
Anyway it was fun !
[quote][b]
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Fri Jun 17, 2011 3:29 am Post subject: Re: RPM Variations - follow up |
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Too much "precision" in instruments can cause unnecessary concern. The ASI function in my EIS varies in increments of 24 rpm which tames the readout to meaningful precision level.
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Fri Jun 17, 2011 4:24 am Post subject: Re: RPM Variations - follow up |
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Hi John,
Out of curiosity what is your max WOT flat and level rpm and what is your cruising rpm? Let me know and this may have something to do with a 100 rpm swing and maybe not. How big an up or down? (i.e. 50' pm or 200'pm)
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Tucson, Az.
Rotax Instructor & Rotax IRC
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JohnF
Joined: 13 May 2010
Posts: 124
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| Posted: Fri Jun 17, 2011 4:51 am Post subject: RPM Variations - follow up |
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Roger,
I don't have the precise WOT number at hand, but pretty sure it is 5,650 in
level flight from density altitude 8,500 to 10,500 - seems about the same at
either alt. The Up/down variation seems to be 100-ft or less. The operation
yesterday with the auto pilot holding the plane within 10-ft (as shown on
the Dynon D-180) kept the engine rpm really solid...the most I saw was
10-rpm variations....the Dynon only gives rpms in +/- 10 units, and if rpm
is 'on the cusp' it can 'jump' up and down as the electronics sees which
number is closest.
My cruising rpm is 5,250 - at least that's what I shoot for, and am seeing
4.4 gph at that rpm.
Overall I am happy with the engine's operation...this rpm variation is just
something that I wanted to resolve. In my mind, the pitch changes that I
somehow cause when flying manualy change the load on the engine and results
in some rpm change.
One other thing I see is that when I first start the engine and its oil temp
is in the 60's that the rpm will slowly increase without throttle movement.
After the engine is warmed up that doesn't seem to happen. By the time I get
to the end of the runway and finished my run up and the oil is in the low
120's that 'creep' in rpm is gone.
John
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rickofudall
Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA
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| Posted: Fri Jun 17, 2011 8:55 am Post subject: RPM Variations - follow up |
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John, As oil warms it's viscosity becomes less (gets thinner), it takes less power to pump and RPM goes up. We don't see this in car engines so much anymore because an ECU can compensate and hold a steady RPM as the engine warms.
Rick Girard
On Fri, Jun 17, 2011 at 7:47 AM, John Fasching <n29cx(at)ridgeviewtel.us (n29cx(at)ridgeviewtel.us)> wrote:
[quote] --> RotaxEngines-List message posted by: "John Fasching" <n29cx(at)ridgeviewtel.us (n29cx(at)ridgeviewtel.us)>
Roger,
I don't have the precise WOT number at hand, but pretty sure it is 5,650 in level flight from density altitude 8,500 to 10,500 - seems about the same at either alt. The Up/down variation seems to be 100-ft or less. The operation yesterday with the auto pilot holding the plane within 10-ft (as shown on the Dynon D-180) kept the engine rpm really solid...the most I saw was 10-rpm variations....the Dynon only gives rpms in +/- 10 units, and if rpm is 'on the cusp' it can 'jump' up and down as the electronics sees which number is closest.
My cruising rpm is 5,250 - at least that's what I shoot for, and am seeing 4.4 gph at that rpm.
Overall I am happy with the engine's operation...this rpm variation is just something that I wanted to resolve. In my mind, the pitch changes that I somehow cause when flying manualy change the load on the engine and results in some rpm change.
One other thing I see is that when I first start the engine and its oil temp is in the 60's that the rpm will slowly increase without throttle movement. After the engine is warmed up that doesn't seem to happen. By the time I get to the end of the runway and finished my run up and the oil is in the low 120's that 'creep' in rpm is gone.
John
---
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gpabruce(at)gmail.com
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| Posted: Fri Jun 17, 2011 11:49 am Post subject: RPM Variations - follow up |
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This is true. I am a Control System Engineer and this is a common issue in all control systems and their I/O. It also is an issue on the outputs which could be a reading or something like a valve or speed control such as a governor. Typically there are what is termed "dampening" built into every instrument to dampen out fast oscillations. This does not effect "precision" but it adds a "time" component. If yours is digital, look in the menu options . . . if it has it and it should. In older instrumentation they would install small orifices in the line to dampen out pulses where liquid and air medians were used . . . today, the sensors are right on the primary process such as speed or flow or pressure or temperature . . etc. They see everything like a magnifying glass and they even pick up noise. You don't need that, you need an average of the input over time. The dampener gives you that "time" adjustment. "Precision" is one term and "sensitivity", "dampening", "sampling rate", "stability"are others. Look for "sensitively" and/or "dampening". The "precision" is more about how many decimal points to the right you wish to see, slightly different than the other two terms . . . why I'm saying this is you may just over look the attribute if you look only for "precision". In digital electronics there can be computer code that controls this from a screen or that can be a and adjustable "pot" or potentiometer. Today you can almost count on computer code . . . again it depends on the programmer and his knowledge of terminology . . .there are no absolutes . . . usually "trial and error" sorry to say. "Sampling rate" is yet another term that can also affect it your reading "stability". Hope that sheds some light on it better.
Bruce
On Fri, Jun 17, 2011 at 4:29 AM, Thom Riddle <riddletr(at)gmail.com (riddletr(at)gmail.com)> wrote:
[quote]--> RotaxEngines-List message posted by: "Thom Riddle" <riddletr(at)gmail.com (riddletr(at)gmail.com)>
Too much "precision" in instruments can cause unnecessary concern. The ASI function in my EIS varies in increments of 24 rpm which tames the readout to meaningful precision level.
--------
Thom Riddle
Buffalo, NY (9G0)
Kolb Slingshot SS-021
Jabiru 2200A #1574
Tennessee Prop 64x32
Truth is what stands the test of experience.
- Albert Einstein
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