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  #31  
Old 11-11-2011, 04:10 PM
Finley Atherton Finley Atherton is offline
 
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Quote:
Originally Posted by gereed75 View Post
Fin, What do the charts say relative to RPM and efficiency??
As an example lets take a FF of 7.5 g/h and Best Economy mixture settings for an I0-360 180 hp:

2,700 rpm gives 88 hp
2,000 rpm gives 108 hp

Another example. At Best Power mixture settings and a FF of 8.5 g/h:

2,700 rpm gives 82 hp
2,200 rpm gives 99 hp.

Fin
9A
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  #32  
Old 11-11-2011, 04:12 PM
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DanH DanH is online now
 
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Fin's observation is a bit more clear if you consider the necessary manifold pressure setting.

With constant RPM....

Lean the mixture to 50 LOP. Fuel flow decreases. To return fuel flow to 8.5 you increase MP. Same fuel flow and more manifold pressure means more HP.

Richen the mixture to 150 ROP, best power. Fuel flow rises. To reduce fuel flow back to 8.5 you must pull the throttle. 8.5 and less MP means less HP.

Exactly what change in manifold pressure would be required I do not know.

With constant RPM and constant MP LOP means less power and 150 ROP is more power, as you expect.
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  #33  
Old 11-11-2011, 04:51 PM
Finley Atherton Finley Atherton is offline
 
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Originally Posted by DanH View Post
Fin's observation is a bit more clear if you consider the necessary manifold pressure setting.
Dan,
Thanks for the assist. You are clear and concise as always.

Quote:
Originally Posted by RV10inOz View Post
Just for the record, best economy is usually Round 20-40lop and for a combination of economy and a bit more speed from 10-20lop. Best power is more like 75ROP
Not trying to disagree with you (us Aussies have to stick together) but its difficult to know the exact figures as Lycoming puts out some contradictory information.

In the Operators Manual they say don't go leaner than 150 ROP at Best Power mixture settings. In their Lycomings Operations flyer they say lean to 100 ROP for Best Power cruise. Also in the Operations flyer is a representative diagram that shows max power for cruise at about 140 ROP and their Best Power band for cruise from about 100 ROP to about 170 ROP. So from this diagram at least, 75 ROP is outside the max power band.

The representative graph in the flyer gives the best Specific Fuel Consumption at about 20 LOP with almost no difference apparent between Peak and 20 LOP but I agree speed should be a bit higher nearer Peak (assuming throttle setting is the same).

Fin
9A

Last edited by Finley Atherton : 11-11-2011 at 05:23 PM. Reason: Clarification
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  #34  
Old 11-11-2011, 05:09 PM
fehdxl fehdxl is offline
 
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Quote:
Originally Posted by steve91t View Post
...on a long cross country, the ETA would be screwed up. And it's not. It's dead on.
Can you expand on what you mean by this?

Thx,

-Jim
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  #35  
Old 11-11-2011, 06:51 PM
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RV10inOz RV10inOz is offline
 
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Ah Finley, you have started the path to discovery and true enlightenment!

This is going to sound cocky, but I am sure those who do know will agree. The Lycoming publications are often WRONG and as you have pointed out contradict each other

There is better info out there
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  #36  
Old 11-11-2011, 08:53 PM
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DanH DanH is online now
 
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Quote:
Originally Posted by RV10inOz View Post
This is going to sound cocky, but I am sure those who do know will agree. The Lycoming publications are often WRONG and as you have pointed out contradict each other
There is better info out there
Lycoming dyno chart....full throttle, 28.5 MAP and 2700 RPM. Mixture is pulled full range from bog rich to LOP.

Max power is a range from 200 ROP to 100 ROP, just as Fin found in the Lycoming publications.

Show us the better info, please.

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  #37  
Old 11-11-2011, 10:27 PM
steve91t steve91t is offline
 
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Wow, lots of great information. Thanks for all of the responses.

I will say this quickly, there is no way that my dad is setting the mixture so incorrectly that he's loosing 15 kts. Not possible. He is setting it correctly, I promise.


fehdxl: What I mean is that if the GPS says we'll be somewhere in 1 hour, and we are, then that's correct. We've also noted that the GPS and the TAS match. If they didn't, the EFIS would be telling us that we have a kickin tail wind ALL THE TIME.

It's not like I'm looking for a few knots here. According to some, we are missing 15+ kts.


If my dad flies 155 miles in 1 hour, he was doing 155 miles per hour. Now, let's say he's INDICATING 155 kts true and he flies 155 miles. If he gets there early, then his indications are reading slow.

Something just isn't adding up. And I honestly do not think it's the static system or the way he's leaning the engine.

I will say this though. We have not flown the box pattern like you guys have said to completely rule out the static system, but like I said, there's no way it's 15 kts off and we didn't know it. Also, leaning an engine so incorrectly that you loose 15 kts is not something that you wouldn't notice. I can tell you that isn't the issue.



It's got to be the prop then, right? That's the only thing that I've read on here that actually says will slow you down. But by how much remands a mystery. Some say not much at all, some say it's substantial.

Anyway, thanks again for all of the help. I know I've learned something.

Maybe we could switch to some prop talk since I think that's the problem.

Steve
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  #38  
Old 11-12-2011, 12:58 AM
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RV10inOz RV10inOz is offline
 
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Dan, please read what I wrote...... I have been through this time and time again, as was pointed out previously they often contradict themselves from one publication to the next. And on many topics.

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.

Lets look closely at what you and I are debating here. Remember Peak EGT is where everything should be referenced from. It is the point we can measure in flight and work away from in either direction.

Now first look at BSFC, and see where it begins to flat line. Using the lower EGT trace as it is easier to see (they should be all relative) then go up the the EGTs and you will see that the point at which you hit the EGT is at 10degF LOP and it continues to 40F LOP.

Now look at the HP line, from left to right coming from LOP to ROP, the HP max's out at 25F ROP and a smidgen more by 75/80 ROP. Moving further to the right the power does nothing through to 200ROP and beyond and it actually starts falling.

Gee look at that.....the graph just proves my earlier statement.
Quote:
Just for the record, best economy is usually Round 20-40lop and for a combination of economy and a bit more speed from 10-20lop. Best power is more like 75ROP
Now lets remember that the chart you posted is a detonation test. Cylinders running really hot, and everthing full bore to test what they were looking for. I wonder what that does to the "actual numbers" as you get into a detonation test does that mean best power is achieved for longer going ROP due to the high CHT required for the test? In other words as the fuel is increased to richer and richer, did the HP measured stay higher for longer rather than taper off?

Here is a generic TCM chart, note the peak power at half way from 50 to 100 ROP, and scale depending it seems power drops away again more rapidly than the Lycoming chart.


And here is another one from Pratt & Whitney, same thing oddly enough.


These graphs are not new, in fact they all have been accepted fact for 50+ years.

Now my concern is that your detonation graph is possibly misleading for the typical real world application with CHT's in the mid 300's and not trying for detonation. I am not for one minute saying that Lycoming cheated, or fiddled the data. The only explanation off the top of my head for the very flat power curve at very ROP settings is that combustion was very strong in such a high temp test. Maybe it is just a scaling thing, but my eyes are hurting reading these graphs on my laptop.

So any better data out there you asked? No more so than that curve with a caveat that the flat HP line ROP could be affected by the nature of the test.

As for other sources, if you can find them in an old 2nd hand bookstore; The Aircraft Engine and its Operation by Pratt & Whitney, Basic Theory of Operation - Turbo Compound Engine by Curtis Wright.

Last edited by RV10inOz : 11-12-2011 at 02:05 AM.
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  #39  
Old 11-12-2011, 02:37 AM
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RV10inOz RV10inOz is offline
 
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Some more reading which I had to go re-discover, this is lifted from an Avweb article so all attributions to them.

Quote:
Heads up, crucial concept coming! Assume that we increase MP to increase power output to something substantially above 65% power. Now suppose we lean the mixture a bit, until the actual power drops to exactly 65% power again?

Here's exactly what that looks like:

What's Going On Here?


This is a real picture of a Cessna 414, previously trimmed up very carefully for straight and level flight at equal MP, RPM, and mixture settings. After that, the throttle is increased, and the mixture decreased, to produce what you see here, with no tendency to yaw/roll/turn. This situation is actually very sensitive to very small differences in power.

Remember,both engines are producing exactly the same power!

The left MP is 3" higher than the right, but the left fuel flow is 3.2 GPH lower! Note further, the EGT is only 10? F higher, but the CHT (as shown by the missing bars on the Graphic Engine Monitor display) are 1 to 3 bars lower, with each bar representing 25? F.

Ponder this: cooler CHTs, less fuel, same power. Sounds like magic, doesn't it? Why, if we could run that MP up high enough, and pull the mixture back far enough, we might invent perpetual motion! Unfortunately, this is another aviation case where a little is good, but "more" isn't. There are other forces at work.


Steve back to your speed tests,
Quote:
I will say this quickly, there is no way that my dad is setting the mixture so incorrectly that he's loosing 15 kts. Not possible. He is setting it correctly, I promise.
Are you so sure? have you sat there and done the same? I can show you a 20 knot difference in my RV10.

More important is do the GPS box and compare the GPS calculated TAS, to the one shown on your EFIS or from manual calculation. If they are not within 2 knots, it might be time to play with your static port. I have gone from 9 knots too slow to 6 too high until I got sub 2 knots, just by small adjustments around the static port.
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  #40  
Old 11-12-2011, 07:14 AM
David-aviator David-aviator is offline
 
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Quote:
Originally Posted by steve91t View Post
Hey guys, I'm asking this question for my dad who owns a very nice RV7 with 180 hp and a 3 bladed CS prop. I can't remember who makes the prop. Anyway, it has an amazing climb but cruise at 2550 or so is right at 155 kts true at 8.5 gph.

How is it that I'm seeing guys with less power cruising at 165 kts? I know the 3 bladed prop will have a better climb than a 2 bladed, but the cruise is going to be a little slower. How much slower? I think it weighs around 1100 full IFR, glass panel.

I just wanted to see how some of you guys are getting faster cruise speeds. I know Smokey's RVX can cruise at 160 KTS easily. Of course that's a light, slippery little plane.

http://www.youtube.com/watch?v=9-x-2UTXuGE
Vans reports 55% cruise with the 7A to be 177 mph. 155kts = 178mph. That figures, the airplane is a 7.

My spread sheet HP calculator says 8.5 gph = 55.56% power with 180 HP being 100%. (BSFC at .51)

I do not detect a problem with the performance of this airplane.

If your dad wants to go faster, tell him to increase power. Push it up to 75%, he will be going 198 mph or 172 KTAS.

If he decides to junk that MT prop anyhow, I will gladly go dumpster diving to retrieve it.
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