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11 hours ago, jonhop said:

I fly strictly by the POH, which requires 100° ROP for best power and 25° ROP for best economy at cruise MP settings. I've observed 130+kts average at 7-8GPH while flying at 25° ROP at 22 to 23" MP at 9500 and 8500 when I escape the San Diego area to the east and north on long cross countries...

Be careful flying 25º ROP at higher power settings. In the 8,500-9,500 ft range you specify that's fine but down lower, that will put you right where you don't want to be.

https://www.jpinstruments.com/wp-content/uploads/2013/05/Mike-Bush-Red-BoxRed-Fin.pdf

http://www.sysdyn.com/xyz/LOPops.pdf

http://csobeech.com/peak-engine.html

http://www.csobeech.com/lean-of-peak.html

And there are many more . . .

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14 hours ago, KLRDMD said:

Be careful flying 25º ROP at higher power settings. In the 8,500-9,500 ft range you specify that's fine but down lower, that will put you right where you don't want to be.

https://www.jpinstruments.com/wp-content/uploads/2013/05/Mike-Bush-Red-BoxRed-Fin.pdf

http://www.sysdyn.com/xyz/LOPops.pdf

http://csobeech.com/peak-engine.html

http://www.csobeech.com/lean-of-peak.html

And there are many more . . .

Roger and good reads... Strictly following the POH chart for power settings until I can install an engine monitor in the future...

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My '81 231 is 919# with a Merlyn and intercooler.  I generally run LOP at 11.0 GPH which is about 70% and get great speeds.  It won't run LOP about about 12k though without developing too much heat, so I run 13.3 ROP up higher into the flight levels.  

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My '81 231 is 919# with a Merlyn and intercooler.  I generally run LOP at 11.0 GPH which is about 70% and get great speeds.  It won't run LOP about about 12k though without developing too much heat, so I run 13.3 ROP up higher into the flight levels.  

Can you explain why LOP generates too much heat? The heat is a result of burning fuel, so less fuel, burnt more efficiently should generate less heat?
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20 hours ago, teejayevans said:


Can you explain why LOP generates too much heat? The heat is a result of burning fuel, so less fuel, burnt more efficiently should generate less heat?

I think the problem is lack of air.  Fewer molecules of air flowing over the engine to keep it cool.  Of course, another option would be to put the cowl flaps in the trailing position.  Then again, he may be talking about TIT not CHT.  I'll let him answer as to whether or not he is able to go further LOP to keep things cool.

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21 hours ago, teejayevans said:


Can you explain why LOP generates too much heat? The heat is a result of burning fuel, so less fuel, burnt more efficiently should generate less heat?

It has to do with the altitude.  The heat capacity of the air is dependent on the mass of the air. At any given  airspeed, the same volume of air flows over the engine.  At higher altitude there is simply less mass of air to transfer the heat that is generated.  If you run ROP, you are using excess fuel to cool the engine. 

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22 hours ago, clh said:

If you run ROP, you are using excess fuel to cool the engine. 

I tend to disagree with this just a bit.  I'm no John Deakin, but I don't think that's it.  I don't think it's the extra fuel carrying extra heat away from the engine.  I think it has to do with timing.  I'm guessing jlunseth (the person that runs ROP at higher altitudes) is probably able to run further below peak EGT on the rich side than the lean side.  From what I've read, maximum CHT happens at about 50 ROP.  No matter which side of peak you run, LOP or ROP, you are creating a less than perfect mixture so the flame front slows down.  The further from peak you get, the slower the flame front, up until the engine dies.  The slower flame front (and likewise higher RPM) gives the piston more time to descend in the cylinder which means there is more room above the piston.  That results in a lower peak pressure which results in a lower EGT.  I think the cooling is mostly due to that lower peak pressure than it is to excess fuel.

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1 hour ago, Bob - S50 said:

I tend to disagree with this just a bit.  I'm no John Deakin, but I don't think that's it.  I don't think it's the extra fuel carrying extra heat away from the engine.  I think it has to do with timing.  I'm guessing jlunseth (the person that runs ROP at higher altitudes) is probably able to run further below peak EGT on the rich side than the lean side.  From what I've read, maximum CHT happens at about 50 ROP.  No matter which side of peak you run, LOP or ROP, you are creating a less than perfect mixture so the flame front slows down.  The further from peak you get, the slower the flame front, up until the engine dies.  The slower flame front (and likewise higher RPM) gives the piston more time to descend in the cylinder which means there is more room above the piston.  That results in a lower peak pressure which results in a lower EGT.  I think the cooling is mostly due to that lower peak pressure than it is to excess fuel.

I was trying to avoid getting into the reaction kinetics, since the observed effect is the same.  Too rich or Too lean will result in reaction limitation due to either fuel or oxygen depletion. Any excess of air or fuel will act as an inert and just adsorb heat instead of generating heat.
 

 

 

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  • 2 weeks later...
On ‎10‎/‎8‎/‎2016 at 1:26 PM, teejayevans said:


Can you explain why LOP generates too much heat? The heat is a result of burning fuel, so less fuel, burnt more efficiently should generate less heat?

I can't fully explain it because I have not dug into it.  But I can give you some reasons.  One, EGT will be higher by about 100 degrees.  This is so because, very roughly speaking, a LOP setting would be -30 dF lean of peak and a ROP setting would be about 125 dF rich of peak.  Peak is the same in both cases, so the LOP EGT is the hotter one.  That means that the pipes are going to run hotter.  Two - and this is something about which there has been some discussion on this Board - there is, let's call it, a secondary combustion event that occurs in the tubes, so the TIT is hotter than the EGT, which is already hotter than if you were ROP.  That means the gases going into the turbo are hotter and that affects the running temp of the turbo.  Three, in my turbocharged engine to get to a good LOP setting I must run at a higher MAP than if I were running ROP.  I typically run at 34".  The reason for that is that there are two ways to go LOP, or to go further LOP if you are already at a LOP setting.  One way is to reduce fuel flow with the red knob, leaving MAP constant.  The other is to increase MAP, leaving fuel flow constant.  More air, same fuel, means a leaner mixture.  The upshot of it is that I am running around 5 or 6" higher MAP at my favorite LOP setting, and so the turbo, already getting a hotter gas stream, is running harder as well.  This is aggravated by going higher, say into the flight levels, where the turbo needs to work harder yet and the air is thinner so cools poorly.   As I said, I have not dug into it but those are the factors I can think of.  There may be others.  The factors I mentioned would directly affect the turbine primarily, but that and the collector will warm the engine compartment, at least that is what I think is happening, and so the CHT's are also higher.  I can markedly affect CHT's in this situation by opening the cowl flaps one notch, but where's the fun in that?  It slows you down. 

My experience is partly driven by what I consider to be acceptable LOP.  Many people just tune the fuel flow down and run 65% or lower, which is fine but greatly affects airspeed.  I want to get there, so I run LOP somewhere in the 70-75% range.  Just dialing the fuel flow back will cut the temps if they start to get too hot at higher altitudes, but so will ROP, so instead of going down in speed, I go ROP up high. 

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  • 3 years later...
On 9/11/2016 at 11:38 AM, Bob - S50 said:

There you go, forcing me to think again...  How much faster by moving 200 lb from the front seat to the back seat?

Short answer:

At higher speeds (75% power) about 0.25 knots.

At lower speeds (45% power) about 1 knot.

That makes sense since the weight distribution will have the biggest effect when induced drag is a bigger player.  As speed increases and parasite drag becomes the main factor, the distribution has less impact. So fly fast!

Long answer:

Using my J POH...

Assume 15 feet from CG to tail center of lift and also assuming an average weight of 2400 lbs.

Moving the CG 0.1' (1.2") will change the pitching moment by 240 #'.

Divide that by 15' to determine the change in tail lift required to offset the moment and you get 240/15 = 16 lbs for each 1.2" change in CG location at 2400#.

Looking at my particular airplane, moving 200 lbs from the front to the rear seat moves the CG about 3.6" (or close anyway, a little poetic license).

That means a change of 48 lbs change in tail lift with corresponding 48 lb change in required wing lift.  Call it the equivalent of 50 lbs less weight.

According to my POH, a 440 pound difference in weight makes (approximately) a 2 knot difference in speed at 75% power, about a 4 knot difference at 65% power and about an 8 knot difference at 45% power.

Since the CG shift resulted in about 1/9 th that amount, I can probably assume it have an a proportional effect on speed.

Now, back to football.

Bob

Bob,

You are the man. I’m not smart enough to know if what anything you are saying is accurate but it sure sounds legit! 

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On 9/9/2016 at 4:33 PM, jrwilson said:

12.4 - 12.7 gph at 28" mp.  84 231 w intercooler.  170 + kts  at 10k, increasing tas with altitude

I'd love to know how you get that. I have a 1980 231 with a merlyn wastegate and intercooler. 28" and 12.5 - 13 GPH and I generally won't see higher than 150 kts TAS at 9-10k feet. 

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21 hours ago, Willie said:

Bob,

You are the man. I’m not smart enough to know if what anything you are saying is accurate but it sure sounds legit! 


Translation...
 

The more you load stuff in the back...the faster you will go... until...

Until you fall out the back of the WnB envelope... then you won’t be flying very fast at all... or for very long...  (roughly speaking)

The tail works harder with a forward Cg... more work, more drag...

Hang out here a while... you can learn a lot of flying technology...

PP thoughts only, not a CFI...

Best regards,

-a-

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On 9/15/2020 at 7:33 PM, khedrei said:

I'd love to know how you get that. I have a 1980 231 with a merlyn wastegate and intercooler. 28" and 12.5 - 13 GPH and I generally won't see higher than 150 kts TAS at 9-10k feet. 

yup...my experience as well...1980 231 with Merlyn and Intercooler...running LOP at 10 gph....higher running ROP

 

Edited by 231LV
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I'd love to know how you get that. I have a 1980 231 with a merlyn wastegate and intercooler. 28" and 12.5 - 13 GPH and I generally won't see higher than 150 kts TAS at 9-10k feet. 

Are you confusing IAS with TAS?

 

Speed depends on:

OAT

Humidity (higher humidity, lower power)

Weight

Instrument error

Pilot error

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23 hours ago, khedrei said:

I'd love to know how you get that. I have a 1980 231 with a merlyn wastegate and intercooler. 28" and 12.5 - 13 GPH and I generally won't see higher than 150 kts TAS at 9-10k feet. 

That seems really slow...like F speeds. What RPM?  I’m using 2500.  Maybe check rigging?

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I use 2500 RPM. I run 90 ROP. 

I'm definitely not confusing IAS and TAS. Im an instructor and know the difference. I also have an OAT on my G500Txi which gives me the readout. I have found that the OAT for the TXi reads about 2 degrees C higher than the one on the window. I will do a 4 way GPS test next time I'm up.

I dont see rigging being an issue. It went for an annual at a MSC a couple years back and they said some of the limits were off slightly and adjusted them. The trim was out of whack after the GCF 500 was installed but the avionics guy had the same MSC come re-rig it. 

I dont have a one piece belly but thats not 20 knots. 

Is it possible my prop isn't the right model for the engine? It was new in 2013 before I bought it.

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Two MSCs in Canada... Left one and right one... West / East...

We know the right one...  :)

28”, 12+ gph, 2500rpm...

With those numbers (FF & RPM) my O typically sees 160kts... (ground speed/gps)  But I don’t see 28” of MP at 10+k of altitude...

@15gph my O sees 170kts...

So...

does your MP, RPM, and FF match book numbers? 
 

Lots of numbers to keep organized... I may have dropped one... :)

Best regards,

-a-

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It won't let me load the picture from the poh. It doesn't have 2500 rpm listed but it shows 2400 rpm and 32.8" MP at 10k feet at 75% power and std temp to give 168ish knots TAS.

 

Thats being said i have the intercooler and merlin wastegate and I was told to use the 252 numbers so I use 28-29 MP which is supposed to be equivalent to 33" on the stock 231.  I was thinking that maybe my intercooler isn't working as its supposed to but even if I use 28 " on these tables I should be going faster than 150kts. 

The only things I can think of are the hot prop but its a factory option so the POH takes that into account and it isn't worth 18kts...

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On 9/15/2020 at 9:33 PM, khedrei said:

I'd love to know how you get that. I have a 1980 231 with a merlyn wastegate and intercooler. 28" and 12.5 - 13 GPH and I generally won't see higher than 150 kts TAS at 9-10k feet. 

It does seem a bit slow.  I too have a merlyn and intercooler, with the original prop I'm 160-165kts at that alt and power settings.  When I'm LOP @ 9.0gph I see around 150kts at 9-10K.   I follow @jrwilsonon Instagram and he occasionally posts photos of his Aspen, he does have a fast one:).   For some reason our Mooneys seem to vary 10kts from airplane to airplane. 

Hopefully you can figure out what is slowing you down, I'd be interested to know what you find.

Cheers,

Dan

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The fuel being burned is a good direct indication of how fast you will be moving...

When deep ROP a couple gph get used for cooling and don’t add to the translation.... Over the ground...

If you want to go fast... there is no substitute for burning fuel...

Typical fuel burn of an M20C 10gph and 150mph... or so... from old memories.

Boosting the MP, but keeping the FF artificially low via mixture... is interesting... but not a way to go fast...

Sure there are cook book reasons to include prop design, TIT, and intercooler outlet temps...

But don’t lose sight of the fact it takes fuel to generate speed... no matter how efficient a TC’d Mooney actually is...

The actual efficiency of the TC’d Mooney really shines in the FLs... it isn’t all that good at NA altitudes...

Best regards,

-a-

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