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11-12-2011, 07:17 AM
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Join Date: Jan 2005
Location: Ottawa, Canada
Posts: 2,357
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8.5 gph at just rich of peak EGT is likely somewhere around 63% power, which is pretty close to the reported 65% power. This calculation is based on an old Lycoming document that describes how to calculate power from fuel flow data.
As near as I can tell from Van's claimed performance, you are about 9 kt slower than would be expected, based on his 75% claims, corrected to 63% power, which gives 164 kt.
I changed from an older Hartzell two-bladed prop (7666 blades, as sold by Vans before the blended airfoil Hartzells came along) to a three bladed MT prop. I lost about 5 kt cruise speed with that prop change. So, if your airspeed indications were perfect, your engine was producing the expected power, and there was no excess drag on the aircraft, I'd expect your dad's aircraft to produce somewhere around 160 kt at that power setting with that prop.
The aircraft could easily have 5 kt of error in the static system and/or the EFIS ASI. My EFIS ASI had up to 2 kt of instrument error. And my static source position error was up to 2 kt also. The fuel flow indicator or tachometer might not be perfectly accurate either.
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11-12-2011, 07:43 AM
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Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
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Quote:
Originally Posted by Kevin Horton
8.5 gph at just rich of peak EGT is likely somewhere around 63% power, which is pretty close to the reported 65% power. This calculation is based on an old Lycoming document that describes how to calculate power from fuel flow data.
As near as I can tell from Van's claimed performance, you are about 9 kt slower than would be expected, based on his 75% claims, corrected to 63% power, which gives 164 kt.
I changed from an older Hartzell two-bladed prop (7666 blades, as sold by Vans before the blended airfoil Hartzells came along) to a three bladed MT prop. I lost about 5 kt cruise speed with that prop change. So, if your airspeed indications were perfect, your engine was producing the expected power, and there was no excess drag on the aircraft, I'd expect your dad's aircraft to produce somewhere around 160 kt at that power setting with that prop.
The aircraft could easily have 5 kt of error in the static system and/or the EFIS ASI. My EFIS ASI had up to 2 kt of instrument error. And my static source position error was up to 2 kt also. The fuel flow indicator or tachometer might not be perfectly accurate either.
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If 8.5 gph = 63% power, then at 100% burn will be just 13.49 gph.
That simply is not true for the 0360 at 180 HP, it burns more fuel than 13.49 gph at 100%.
I calculate a burn of 11.5 at 75%, and 15.3 gph at 100%. In flight performance confirms these figures to be reasonably accurate. I have seen 199 mph true (172kts) with WOT at 8500, burn right at 12, which is consistent with the prop turning about 2780, slightly more than 75% power.
63% power at 8.5 gph is a reasonable expectation with an 0320.
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11-12-2011, 08:13 AM
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Join Date: Jan 2005
Location: Ottawa, Canada
Posts: 2,357
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Quote:
Originally Posted by David-aviator
If 8.5 gph = 63% power, then at 100% burn will be just 13.49 gph.
That simply is not true for the 0360 at 180 HP, it burns more fuel than 13.49 gph at 100%.
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This is a bit of an apples and oranges comparison. The 63% power at 8.5 gph is only valid if the fuel flow is just a tiny bit rich of peak EGT, as reported by the original poster. You wouldn't want to run nearly this lean at 100% power, and even if you tried the mixture would be leaner than best power mixture so the engine wouldn't produce 180 hp.
Quote:
Originally Posted by David-aviator
I calculate a burn of 11.5 at 75%, and 15.3 gph at 100%. In flight performance confirms these figures to be reasonably accurate. I have seen 199 mph true (172kts) with WOT at 8500, burn right at 12, which is consistent with the prop turning about 2780, slightly more than 75% power.
63% power at 8.5 gph is a reasonable expectation with an 0320.
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The Lycoming data I referenced earlier suggests 75% power at 2700 rpm and mixture set for best power would give a fuel flow between 11.2 and 12.1 gph. 100% power would need a fuel flow between 14.1 and 15.4 gph. This is pretty consistent with your numbers.
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11-12-2011, 08:32 AM
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Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
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Kevin,
Thanks for the quick reply, we appear to be on the same page.
So does Steve's father have a problem or not? It is common knowledge the MT prop is not quite as efficient on the top end, but I believe the deficit is just 2 or 3 knots, not 15.
I'd like to see what this airplane will do at 8500, WOT and 2600 rpm and leaned to 100 ROP. I'm betting it will come in at 198-200 mph true.
My experience with the MT 7 and the Subby H6 was IAS slowed up a couple knots when running at 2700 vrs 2600 rpm.
Like Steve said, it has great performance on take off and climb.
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11-12-2011, 10:16 AM
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Join Date: Nov 2010
Location: Huntersville, NC
Posts: 138
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I'll have him try everything recommend here.
On the specific flight that I was on, where I was flying, we were between 8 and 9000, WOT, leaned rich of peak, 2450 RPM. That was giving us 65% power according to the EFIS. Cruise speed was 155 kts true, 162 kts over the ground.
We did two flights that day. We flew from Roanoke, VA to Knoxville, TN and back. On the way there, it took us roughly an hour and a half. On the way back, just a couple of hours later, it took us an hour and 15 minutes. I flew both legs, and I can't remember exactly how much we leaned it. I do know we were WOT from the time we took off until our decent.
I took a quick video during one of those flights, but I can't make out much more than the MP. It looks like it's 22.4. This is at 8000ft.
http://youtu.be/H6pYg1ekrU4
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11-12-2011, 04:27 PM
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Join Date: Jan 2005
Location: Ottawa, Canada
Posts: 2,357
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Quote:
Originally Posted by David-aviator
So does Steve's father have a problem or not?
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Well, it sounds like he isn't completely happy with the aircraft, so in his mind he has a problem. Certainly something doesn't quite add up with the reported performance. It could be one or many of the following (in no particular order): - airframe with excessive drag,
- engine that is low on power,
- prop with poor efficiency,
- airspeed system error,
- OAT indicatotr instrument error,
- static source position error,
- tachometer indication error, and/or
- fuel flow system indication error.
Quote:
Originally Posted by David-aviator
I'd like to see what this airplane will do at 8500, WOT and 2600 rpm and leaned to 100 ROP. I'm betting it will come in at 198-200 mph true.
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I agree that it would be useful to see the results from that test.
It would also be useful to see the results from a four-sided box pattern to check airspeed system error at the IAS that is normally seen in cruise. Fly four legs at roughly 90 degrees to each other, at the same altitude (within 50 ft of the target altitude). Adjust power as required to obtain exactly the target IAS on each leg. Wait a few seconds to allow the GPS to stablize, then record GPS ground speed and GPS track on each leg. Also report the altitude, altimeter setting and indicated OAT. It can be tough to get a good set of data using this test (a really, really good set of data has a standard deviation of 0.1 kt, as reported by the spreadsheet I created for Kitplanes - anything over 0.5 kt is probably not worth looking at). So, do this test three or four times in one flight, so hopefully at least one of the data sets will be a winner.
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11-12-2011, 05:49 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by RV10inOz
The "cherry picked" piece of information you have just posted is most likely perfectly representing the test conducted. So I am not sure what your point is. I am not suggusting this chart is fudged, and it does look exactly as I would expect. And this chart is at odds with what Lycoming often print. Sometimes they print great stuff, other times it astounds me that they still print it.
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Ok, let's check the leading Lycoming competitor's clone.

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RV-8 SS
Barrett IO-390
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11-12-2011, 09:35 PM
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Join Date: May 2007
Location: Rio Rancho, NM
Posts: 136
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Need more power!
My experience is with a 6A but when we first started flying our prop (wood-Sterba) was over pitched and max RPM was unacceptable. After it was repitched and balanced we were suddenly making the power we needed. The engine was unchanged but the power, and also speed, increased to what was published by vans. We were able to cruise at 165 knots at 7500msl at aprox 2650 rpm. We later upgraded to the fixed metal prop vans sells and picked up another 3 knots tas. Our fuel burn was between 9.5 and 10 gph with a carburetor.
I think the rpm needs to get increased, increasing fuel burn, and the tas will be in the ball park of what is expected.
Last edited by C. Brenden : 11-12-2011 at 09:41 PM.
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11-12-2011, 10:09 PM
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Join Date: Nov 2010
Location: Huntersville, NC
Posts: 138
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I've got a quick question. Is MP effected by how the engine is leaned? Is an engine running at full rich at 8000ft WOT going to have the same MP as an engine leaned at 8000 ft WOT at the same RPM?
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11-13-2011, 05:51 AM
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Join Date: Jan 2005
Location: Ottawa, Canada
Posts: 2,357
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Quote:
Originally Posted by steve91t
I've got a quick question. Is MP effected by how the engine is leaned? Is an engine running at full rich at 8000ft WOT going to have the same MP as an engine leaned at 8000 ft WOT at the same RPM?
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For all practical purposes the answer is yes.
If you had a very sensitive, high resolution MP gauge you might be able to see a tiny change in MP after you leaned, as the airspeed would be slightly different due to the change in power and there would be a bit more or less ram pressure in the inlet. But I really doubt you would be able to see that effect with typical MP indication systems.
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