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Matt Dralle
Site Admin
Joined: 08 Nov 2005
Posts: 25203
Location: Livermore CA USA
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 Posted: Thu Mar 31, 2011 9:01 am Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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Dear Listers,
With the IO-390 and Hartzell 72" CS on my RV-8, a combination of 2150 RPM and 25-26 MP gives me nice 200MPH True and about 10.5-10.8 GPH. These number are down low, maybe 3000ft. Higher altitude gives lower GPHs. At 2150, the cockpit is pretty quiet too.
Increasing the RPM and Decreasing the MP doesn't really change the GPH that much, but starts dropping the speed.
In all configurations, I'm running LOP at about 1360 on the hottest cylinder. Oil temps are about 178 F. Max MP at low altitude is about 28-29.
Are there any issues running in a low-RPM, high-MP configuration as described above?
Thanks,
-
Matt Dralle
RV-8 #82880 N998RV
http://www.mattsrv8.com - Matt's RV-8 Construction Log
http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel
Status: 160+ Hours TTSN - Paint job is all that's left...
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Matt Dralle
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recapen(at)earthlink.net
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| Posted: Thu Mar 31, 2011 10:08 am Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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Matt,
I don't have an IO390 but the manual for my IO360B shows that combination as outside of the suggested area of continuous operation - yes it was a quick look and I didn't purely run the numbers.
I would guess that combination might be detonation-prone.
YMMV - see if you can get the appropriate chart for the IO390. They are a bugger to figure out too - but there are some guidelines.
Ralph
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ssenegal(at)sanbrunocable
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| Posted: Thu Mar 31, 2011 10:58 am Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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Matt,
I'm curious as to what model 72" Hartzell prop you're running?
Steve
(650) 346 6967
Sent from my iPhone
On Mar 31, 2011, at 6:21 AM, Matt Dralle <dralle(at)matronics.com> wrote:
| Quote: |
Dear Listers,
With the IO-390 and Hartzell 72" CS on my RV-8, a combination of 2150 RPM and 25-26 MP gives me nice 200MPH True and about 10.5-10.8 GPH. These number are down low, maybe 3000ft. Higher altitude gives lower GPHs. At 2150, the cockpit is pretty quiet too.
Increasing the RPM and Decreasing the MP doesn't really change the GPH that much, but starts dropping the speed.
In all configurations, I'm running LOP at about 1360 on the hottest cylinder. Oil temps are about 178 F. Max MP at low altitude is about 28-29.
Are there any issues running in a low-RPM, high-MP configuration as described above?
Thanks,
-
Matt Dralle
RV-8 #82880 N998RV
http://www.mattsrv8.com - Matt's RV-8 Construction Log
http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel
Status: 160+ Hours TTSN - Paint job is all that's left...
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BGray(at)glasair.org
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| Posted: Thu Mar 31, 2011 11:00 am Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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Ralph,
I looked at your attached chart and didn't see any warning about
operation limits. These type charts are just used to predict what
various RPM/MP/Altitude combinations will result in a given airspeed. As
long as he stays in his engine/prop allowed range it's fine. Those
numbers can be found on the props TC certificate or by a call to the
props manufacturer. Low RPM, high MP has been used for years, every
turbo-charged engine does it.
Bruce
WWW.Glasair.org
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recapen(at)earthlink.net
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| Posted: Thu Mar 31, 2011 11:52 am Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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There's a place just to the left of the dividing line towards the bottom that says "LIMITING MAN. PRESS FOR CONTINUOUS OPERATION". It points to both the left and right portions of the chart.
That is what I was referring to - and that is what I try to go by. I would think it is even more critical if you're turbocharged.
Or am I really misinterpreting the charts.....maybe Bart (Lalonde) or Rhonda (Barrett) or Mahlon (Russell) will chime in here and keep us honest.
I'll take an education....
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Kellym
Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ
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| Posted: Thu Mar 31, 2011 2:28 pm Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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There is a difference between the early IO-360s that did not have
dynamic counterweights on the crank and the later versions used in the
Cardinal RG and Mooney 201 that have the dynamic counterweights. I am
pretty sure the IO-390 does have the dynamic counterweights. Point being
that the solid crank models have harmonic ranges between the Hartzell
prop and the engine that restrict operations, usually in the 2100-2350
range. The later models do not have those restrictions but do have
cautions against low MP and low rpm. So it is the prop and crank
combination that matters.
On 3/31/2011 12:46 PM, Ralph E. Capen wrote:
[quote]
There's a place just to the left of the dividing line towards the bottom that says "LIMITING MAN. PRESS FOR CONTINUOUS OPERATION". It points to both the left and right portions of the chart.
That is what I was referring to - and that is what I try to go by. I would think it is even more critical if you're turbocharged.
Or am I really misinterpreting the charts.....maybe Bart (Lalonde) or Rhonda (Barrett) or Mahlon (Russell) will chime in here and keep us honest.
I'll take an education....
--
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Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
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khorton01(at)rogers.com
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| Posted: Thu Mar 31, 2011 3:43 pm Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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That RPM/MP combo might very well be detonation prone at 50 to 100 deg ROP. But, I'm guessing (big emphasis on the word guessing) that it should be fine as long as you are well LOP, and all CHTs are not too hot.
I'm no engine expert, so this advice is worth what you paid for it.
Kevin Horton
On 2011-03-31, at 13:07 , Ralph E. Capen wrote:
[quote] Matt,
I don't have an IO390 but the manual for my IO360B shows that combination as outside of the suggested area of continuous operation - yes it was a quick look and I didn't purely run the numbers.
I would guess that combination might be detonation-prone.
YMMV - see if you can get the appropriate chart for the IO390. They are a bugger to figure out too - but there are some guidelines.
Ralph
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Doug Gray
Joined: 10 Jan 2006
Posts: 112
Location: Sydney, Australia
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| Posted: Thu Mar 31, 2011 4:26 pm Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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On Thu, 2011-03-31 at 09:21 -0700, Matt Dralle wrote:
| Quote: | a combination of 2150 RPM and 25-26 MP
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The Lycoming engine plots say to me that operation at full power, full throttle, full rich (but with zero RAM air boosting) down to 2400 RPM is OK. But the limit line is clearly saying that if the RPM is set below 2400 RPM the maximum power has to be dialed back.
This is not unreasonable since the engines are rated at max RPM. Power is the product of RPM and Torque. To retain the same HP at reduced RPM requires increasing the Torque output - increasing the peak forces withing the engine. Both the max RPM and max Torque define the operating limits of the engine from a mechanical point of view. (You need to hit the cylinders harder to get the same HP at lower RPM)
The max RPM and the lines marked 'Limiting man pressure for continuous operation.' is how these limits are conveyed to the operator. What the limiting factor for the torque is really not generally known - perhaps detonation is a factor - peak cylinder pressures or perhaps it is something else. Talk to the design authority for this engine - if you can find one.
For the few Lycoming engines I have looked at the max power is limited to somewhere between 65 and70% at 2150 RPM.
My question is for the propeller manufacturers, when they test for suitability on the respective engines do they take this secondary limitation into consideration? If you are operating your engine outside this range you may also be operating the propeller outside its tested and approved range as well. (Bigger hitting cylinders will augment their tendency to resonance.)
Just my thoughts on the matter - worth just exactly what you paid.
Doug
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Matt Dralle
Site Admin
Joined: 08 Nov 2005
Posts: 25203
Location: Livermore CA USA
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| Posted: Thu Mar 31, 2011 7:59 pm Post subject: Low RPM/High MP vs. High RPM/Low MP... |
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Hum, interesting. In that graph, albeit for the IO-360, the "Limiting MP For Continuous Ops" at 26 MP is 2100 RPM. 27 MP is 2200. Assuming the IO-390 is similar, 2150 at 26.5 ought to be good to go. BTW, I flew tonight and at 26/2150 I was seeing about 10.2-10.5 GPH (at) 2500ft. Better than I recalled.
Where can I get a sweet chart like that for my IO-390? I looked quickly though the manuals that came with the engine, and I didn't see one.
Thanks for the feedback everyone!
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Matt Dralle
RV-8 #82880 N998RV
http://www.mattsrv8.com - Matt's RV-8 Construction Log
http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel
Status: 160+ Hours TTSN - Paint job is all that's left...
At 10:07 AM 3/31/2011 Thursday, you wrote:
[quote]Matt,
I don't have an IO390 but the manual for my IO360B shows that combination as outside of the suggested area of continuous operation - yes it was a quick look and I didn't purely run the numbers.
I would guess that combination might be detonation-prone.
YMMV - see if you can get the appropriate chart for the IO390. They are a bugger to figure out too - but there are some guidelines.
Ralph
--
| | - The Matronics RV-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV-List |
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Matt Dralle
Matronics Email List Administrator |
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