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Author Topic: FAQ: Octane Analysis  (Read 49280 times)

Gary LaPook

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Re: Octane Analysis
« Reply #15 on: December 04, 2011, 01:59:54 AM »

First though, since you disagreed, I'll repeat: "higher octane is required for the highest power settings" -
Somehow that seems fundamental enough to "our points" that I'll close with a summary on "why this is so", after what immediately follows:

As to your more immediate "3 points" -

GL-


In my experience, the more conservative reading (the NOTE) is the one the operator is bound to live by (one case of "the Note taketh away").


GL -
"Second, note 5 shows a higher mp necessary, 36.5 inches, to obtain takeoff power using 91/96 compared to only 36.0 inches on page one. Just what do you think the 36.0 inch setting on page 1 is referring to? It is referring to the use of 80/87 to obtain takeoff power. If this were not true then page one would omit a power setting for full power and simply refer to note 5. I pointed out the difference in manifold pressure settings in my prior post, I'm surprised that you didn't pick up on this."

I think it refers to 600 HP at 2250 RPM 36.0" MP at Sea Level for 5 minutes, no disagreement on that.  But it's not the "highest power setting" available.  Not sure I even challenged that per se, but if I slipped into it then "you win".
You suggest that were my foolishness true, "page one would omit a power setting for full power and simply refer to note 5" - really?  Is that a "full power" setting when a higher one is available?  I disagree.  ;D


GL -
"Third, note that the time limit for takeoff power is one minute when using 91/96 (note 5) while the limit when using 80/87 for takeoff shown on page is five minutes. Again, this would be omitted if you couldn't produce takeoff power with 80/87. And the lower time limit when using 91/96 makes sense since you are using a higher mp."


And as promised, to in sum to "higher octane is required for the highest power settings" -
"Just what do you think" the "highest power settings" are?
The "highest power settings" involve the highest combustion pressures - and, again, the data is clear (from the now notorious "Note 5"):
-  Takeoff (one minute), hp, rpm, in.Hg., at:
      Crit Alt (ft.)                     600-2250-36.0-2300
      S/L Press Alt (ft.)            600-2250-36.5-S.L.
Gary, those ARE the "highest power settings" on the TCDS (look at the pressures and relevant altitudes - that's the key), and per poor ol' TCDS E-143 Rev. 4 (1973, mind you...) those settings DO require "higher octane" fuel, namely 91/96 octane. ;)

You have claimed that there is a general rule for interpreting a TCDS that  "the note taketh away" and you rely on this to surmise that note 5 "taketh away" the power setting for takeoff power using  80/87 octane printed on page 1 and instead requires that, to obtain 600 hp takeoff power, you must only use the settings in note 5 including using 91/96 octane fuel. I was going to attach a bunch of other TCDS's to show that there are many notes that do not "taketh away" but I realized that this was not necessary. There are actually 6 notes on TCDS E-143 for Earhart's engines that we have been examining and none of the six "taketh away" anything and that includes note 5. Take a look at them.

I think I see where you went wrong and it is entirely understandable, every pilot that has flown behind a constant speed prop would probably make the same error. You see that the manifold pressure in note 5 is higher than the manifold pressure on page 1 and you said to yourself "aha, higher manifold pressure means more power!" As I said, I think every other pilot would say the same thing.

I'm sure you know this, so I expect to hear a loud noise when you slap yourself on the forehead. Carburettors meter fuel based on volume not on weight but the power output of the engine is not determined by the volume of fuel but by the weight of the fuel that goes through the engine every second. I have attached a 1944 Army Air Force graph showing the density of various grades of aviation fuel. 87 octane is denser than the other grades. At 30° C 87 octane has a density of 5.97 pounds per gallon while 95 octane is only 5.78 pounds per gallon. Although pilots' normal experience shows increasing power with increasing manifold pressure those pilots do not have experience with switching to a grade of AVGAS that has a lower density. The TCDS shows that you have to use higher manifold pressure with 91/96 in order to force a greater volume of fuel through the carburettors in order to supply the same weight of fuel per second so as to produce the 600 hp that could be produced with a lower manifold pressure using the denser 87 octane fuel.

I have also attached the Pratt & Whitney power setting table for this engine. You will note that it takes an additional gallon (volume) per hour using 91/96 octane low density fuel to move the same weight of fuel through the carburettors every hour to produce the same 100% power and the same takeoff power as when using 80/87 octane high density fuel. This confirms that the extra half inch of manifold pressure is needed to compensate for the lower density of the 91/96 fuel and its use does not lead to a higher power output.

(Also, you seem to ignore the "600" at the beginning of the takeoff power settings in note 5, and this "600" means 600 hp, the same as the 600 hp when using the power settings on page 1 for 80/87 octane.)

gl
« Last Edit: December 04, 2011, 05:05:20 AM by Martin X. Moleski, SJ »
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Richard C Cooke

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Re: Octane Analysis
« Reply #16 on: December 04, 2011, 06:45:15 AM »

I'm sure you know this, so I expect to hear a loud noise when you slap yourself on the forehead. Carburettors meter fuel based on volume not on weight but the power output of the engine is not determined by the volume of fuel but by the weight of the fuel that goes through the engine every second. I have attached a 1944 Army Air Force graph showing the density of various grades of aviation fuel. 87 octane is denser than the other grades. At 30° C 87 octane has a density of 5.97 pounds per gallon while 95 octane is only 5.78 pounds per gallon. Although pilots' normal experience shows increasing power with increasing manifold pressure those pilots do not have experience with switching to a grade of AVGAS that has a lower density. The TCDS shows that you have to use higher manifold pressure with 91/96 in order to force a greater volume of fuel through the carburettors in order to supply the same weight of fuel per second so as to produce the 600 hp that could be produced with a lower manifold pressure using the denser 87 octane fuel.
gl
That would be correct if you had no mixture control.  The manifold pressure determines how much air goes in and the mixture control on the carburetor determines how much fuel goes in.  So to get the desired air fuel ratio with the low density fuel, the mixture the control lever would have to be set slightly towards the rich side compared to its position when using the high density fuel.

Richard Cooke
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Richard C Cooke

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Re: Octane Analysis
« Reply #17 on: December 04, 2011, 09:34:17 AM »

Here is a NACA report from March 1938 that shows what they saw as the benefits of various fuel grades.

Interestingly they got their various ratings by blending 100 octane fuel with 18 (yes 18 octane) "fuel" in appropriate proportions.

Richard Cooke
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Gary LaPook

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Re: Octane Analysis
« Reply #18 on: December 04, 2011, 11:52:00 AM »

I'm sure you know this, so I expect to hear a loud noise when you slap yourself on the forehead. Carburettors meter fuel based on volume not on weight but the power output of the engine is not determined by the volume of fuel but by the weight of the fuel that goes through the engine every second. I have attached a 1944 Army Air Force graph showing the density of various grades of aviation fuel. 87 octane is denser than the other grades. At 30° C 87 octane has a density of 5.97 pounds per gallon while 95 octane is only 5.78 pounds per gallon. Although pilots' normal experience shows increasing power with increasing manifold pressure those pilots do not have experience with switching to a grade of AVGAS that has a lower density. The TCDS shows that you have to use higher manifold pressure with 91/96 in order to force a greater volume of fuel through the carburettors in order to supply the same weight of fuel per second so as to produce the 600 hp that could be produced with a lower manifold pressure using the denser 87 octane fuel.
gl
That would be correct if you had no mixture control.  The manifold pressure determines how much air goes in and the mixture control on the carburetor determines how much fuel goes in.  So to get the desired air fuel ratio with the low density fuel, the mixture the control lever would have to be set slightly towards the rich side compared to its position when using the high density fuel.

Richard Cooke
EXCEPT that the mixture control  is set to its maximum position for all takeoffs (unless from a high altitude airport, Lae was not a high altitude airport) so there is no way to make the mixture richer if switching to lower density fuel, it is already maxed out for 80/87 octane fuel. Because of this you cannot increase the mass flow through the carbs to compensate for the lower density fuel by use of the mixture control, that is why you need the higher manifold pressure.

The full rich position on an aircraft carburetor is equivalent to the normal carburetor in a car. The mixture control on an aircraft carburetor "leans" the mixture from that point for flight at altitude, it cannot "enrichen" the fuel air mixture any farther. To help you understand this I have attached an excerpt from the P&W maintenance manual for Earhart's S3H1 engines that describes the mixture control. There is a diagram included of the NA-R9B carburetor but this is NOT the type that was used on the Wasp engine, the manual also covers the Wasp Jr. engine and this is the carburetor for that engine. There is no diagram of the NA-Y9H installed on Earhart's Wasp engines. I have also included an excerpt from the FAA A&P Mechanics Powerplant Handbook that describes the action of the type of mixture control used on Earhart's engines.

gl

« Last Edit: December 04, 2011, 08:18:28 PM by Gary LaPook »
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Gary LaPook

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Re: Octane Analysis
« Reply #19 on: December 04, 2011, 11:54:30 AM »

Here is a NACA report from March 1938 that shows what they saw as the benefits of various fuel grades.

Interestingly they got their various ratings by blending 100 octane fuel with 18 (yes 18 octane) "fuel" in appropriate proportions.

Richard Cooke
Interesting, but the report only deals with the anti-knock properties of aviation fuel, it does not address the issue we are discussing, the difference in power output due to different densities.

gl
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JNev

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Re: Octane Analysis
« Reply #20 on: December 06, 2011, 05:46:42 AM »

Gary, I didn't "get it wrong" - there are "reasons" the higher octane is imposed where the "highest numbers" are involved -

Higher MP among other constants = higher combustion pressures.

The highest combustion pressures (at the highest MP settings) = requirement for 100 octane.

That's the point.

"Mixture" has little to do with it, although that statement alone could easily provoke several pages of argument.  The ratings involve "full (or auto) rich".

"Octane" has little to do with density in the sense you speak of either; it has everything to do with suppression of "knock".  The physics are true, at the nano level - but at the practical level YOU NEED THE OCTANE TO USE THE MAX ATTAINABLE PRESSURES.  Torturing the data doesn't change that.

---

Richard, excellent find - thanks for sharing that. 

I HIGHLY recommend the reading of that NACA report (link in Richard's post above) for those who honestly want to understand the effects of octane on power extraction from the internal combustion engine (Wasp included) - this drives exactly at the point I have struggled make in this string.  Don't take my word for anything, read it and see for yourselves if interested.

LTM -
- Jeff Neville

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« Last Edit: October 27, 2014, 08:27:44 AM by Jeffrey Neville »
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Harry Howe, Jr.

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Re: Octane Analysis
« Reply #21 on: December 06, 2011, 11:21:53 AM »


The issue that we were initially discussing was the question:  IF the 81 gallon tank that contained about 40.5 gallons of 100 octane fuel had been "topped off" with 40.5 gallons of 80/87 octane fuel, THEN would the distance to takeoff have been appreciably lengthened?
No Worries Mates
LTM   Harry (TIGHAR #3244R)
 
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Martin X. Moleski, SJ

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Re: Octane Analysis
« Reply #22 on: December 06, 2011, 03:33:20 PM »

I believe that was before we dove into the weeds and the topic got (sensibly) split off. 

Marty has uncommon good sense on when to do that ...

Thanks for the compliment, but it is undeserved.

Someone sent me a message in the background, suggesting the split.

I do not have much sensitivity to thread drift.  I contribute to it regularly.   :(

Quote
- and seemingly infinite patience -

That is also an illusion.  I am often busy with other things--especially these days at the end of the semester.   ::)
LTM,

           Marty
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Irvine John Donald

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Re: Octane Analysis
« Reply #23 on: December 07, 2011, 06:22:03 AM »

If AE was flying with one tank of 100 octane because she had been told or led to believe she needed it for extra power on takeoff then why would she have that tank topped up with non 100 octane. She believed she had plenty of fuel, with ample reserve, for the trip to Howland.
She did know that Howland's airstrip had just been constructed and has no idea what shape it was really in. So saving any 100 octane for a "boosted" takeoff on a new runway on a small island would have been prudent. She didn't plan on getting lost and needing every tank to be 100% full.  But she did need to plan her takeoff from Howland. If some of the arguments/inferences in this thread are true then why was she carrying two types of octane in the first place? 
Respectfully Submitted;

Irv
 
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Gary LaPook

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Re: Octane Analysis
« Reply #24 on: December 07, 2011, 10:52:04 AM »

If AE was flying with one tank of 100 octane because she had been told or led to believe she needed it for extra power on takeoff then why would she have that tank topped up with non 100 octane. She believed she had plenty of fuel, with ample reserve, for the trip to Howland.
She did know that Howland's airstrip had just been constructed and has no idea what shape it was really in. So saving any 100 octane for a "boosted" takeoff on a new runway on a small island would have been prudent. She didn't plan on getting lost and needing every tank to be 100% full.  But she did need to plan her takeoff from Howland. If some of the arguments/inferences in this thread are true then why was she carrying two types of octane in the first place?
Because she was misinformed. High octane fuel is needed in high compression engines to prevent detonation (knock) that can quickly damage the engine. Earhart's S3H1 engines are low compression engines with a compression ratio of only 6:1, designed to make full takeoff power with the 87 octane fuel available at Lae. One hundred octane fuel only became available, in experimental quantities, in 1934. The S3H1 had been designed to use the existing 87 octane fuel because it takes a lot longer than the one or two years that 100 octane fuel was available, prior to the actual construction of her engines, to design a new aircraft engine. Only one of the 107 other versions of the Wasp engine, many designed during WW2 after 100 octane fuel was freely available, required the use of 100 octane fuel and even that one version only put out 600 hp, just like Earhart's engines. (That one engine type was fuel injected and had many other modifications.) Think about it, there was a lot of pressure and incentive for Pratt & Whitney to increase the power of these engines, ("don't you know, there's a war on") but they were unable to do so even when 100 octane and other even higher octane fuels were available, all the way up to 145 octane.
Ask all of your friends and I bet that they will all say the same thing, that putting high octane fuel in their cars, that have engines that use "regular," will make the car go faster and accelerate faster because the engine will make more power with the "high power," high octane fuel. All of your friends will be wrong. Simply putting high octane fuel in your low compression engine that is not designed to use high octane fuel, that does not require the use of high octane fuel to prevent detonation, will not allow the engine to make more power. Apparently, in 1937, this was also a common misconception but, since the 100 octane fuel was just newly available, perhaps this misconception can be forgiven. The power setting table for these engines proves this, using the higher octane 91/96 fuel produces the exact same power output as when using 80/87 octane, the higher octane does not produce more power and the same is true if using 100 octane fuel.

Another question presents itself, where did Earhart get the 100 octane fuel, did she tanker it all the way around the world?

gl

gl
« Last Edit: December 07, 2011, 09:42:42 PM by Gary LaPook »
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Irvine John Donald

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Re: Octane Analysis
« Reply #25 on: December 07, 2011, 12:48:13 PM »

But even if she is misinformed she is making her plans with the information she has. Not the technically accurate information of today but what she was told then. Why else fly around with a tank of 100 octane?  And if it had just been introduced on the market then maybe some slick sales guys made a statement that later turns out to be false but she didn't know that at the time.  Think 1937.  What info she had is what she made decisions on.
Respectfully Submitted;

Irv
 
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Richard C Cooke

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Re: Octane Analysis
« Reply #26 on: December 07, 2011, 01:35:35 PM »

But even if she is misinformed she is making her plans with the information she has. Not the technically accurate information of today but what she was told then. Why else fly around with a tank of 100 octane?  And if it had just been introduced on the market then maybe some slick sales guys made a statement that later turns out to be false but she didn't know that at the time.  Think 1937.  What info she had is what she made decisions on.
She should have had a pilots operating manual that should have said what to do when you have to take off with low octane fuel.  Most that I have seen give an alternate take off boost and rpm setting.

Richard Cooke
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Irvine John Donald

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Re: Octane Analysis
« Reply #27 on: December 07, 2011, 02:07:11 PM »

I think that has been covered previously in this thread. See Jeff Neville reply #9 for info.  The choice is still AE's as to what octane and power settings to use to take off from Lae with a full load of fuel.
Respectfully Submitted;

Irv
 
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Gary LaPook

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Re: Octane Analysis
« Reply #28 on: December 07, 2011, 07:11:05 PM »

But even if she is misinformed she is making her plans with the information she has. Not the technically accurate information of today but what she was told then. Why else fly around with a tank of 100 octane?  And if it had just been introduced on the market then maybe some slick sales guys made a statement that later turns out to be false but she didn't know that at the time.  Think 1937.  What info she had is what she made decisions on.
She should have had a pilots operating manual that should have said what to do when you have to take off with low octane fuel.  Most that I have seen give an alternate take off boost and rpm setting.

Richard Cooke
-----
Sure, see the attached power setting table from Pratt & Whitney. It says when using normal (not low octane) 81/87 octane fuel, the power settings to get the full 600 hp are 2250 rpm and 35.5 inches of manifold pressure. When using higher octane 91/96 fuel, to get the SAME 600 hp you must use 2250 rpm and a higher 36.0  inches of manifold pressure.
What isn't clear about this?

https://tighar.org/smf/index.php?action=dlattach;topic=528.0;attach=404

gl
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Richard C Cooke

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Re: Octane Analysis
« Reply #29 on: December 07, 2011, 08:35:57 PM »

What isn't clear about this?

https://tighar.org/smf/index.php?action=dlattach;topic=528.0;attach=404

gl
The date it was issued, so we can be sure this was the information available to AE.

RC
« Last Edit: December 07, 2011, 08:37:35 PM by Richard C Cooke »
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