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Author Topic: FAQ: Octane Analysis  (Read 62195 times)

Gary LaPook

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FAQ: Octane Analysis
« on: November 26, 2011, 12:45:47 PM »


Let's see, 2 tanks with 2 gallons more equals 4 gallons.  Assuming 40 gallons per hour fuel management, that represents 1/10th hour or 6 minutes which, at 133 mph represents about 13 miles.  Could be the difference between getting wet and landing on a reef.
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Yep, that may be true but nobody would take the wings apart to add just four gallons to the existing 1147 gallons because nobody would ever expect that the pilot would make a flight, cutting it that close on fuel.

gl
« Last Edit: July 17, 2012, 10:00:03 PM by J. Nevill »
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Harry Howe, Jr.

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Re: Octane Analysis
« Reply #1 on: November 26, 2011, 01:33:25 PM »


Let's see, 2 tanks with 2 gallons more equals 4 gallons.  Assuming 40 gallons per hour fuel management, that represents 1/10th hour or 6 minutes which, at 133 mph represents about 13 miles.  Could be the difference between getting wet and landing on a reef.
--------------------------
Yep, that may be true but nobody would take the wings apart to add just four gallons to the existing 1147 gallons because nobody would ever expect that the pilot would make a flight, cutting it that close on fuel.

gl

Let's see, 2 tanks with 2 gallons more equals 4 gallons.  Assuming 40 gallons per hour fuel management, that represents 1/10th hour or 6 minutes which, at 133 mph represents about 13 miles.  Could be the difference between getting wet and landing on a reef.
--------------------------
Yep, that may be true but nobody would take the wings apart to add just four gallons to the existing 1147 gallons because nobody would ever expect that the pilot would make a flight, cutting it that close on fuel.

gl

Gary
Yep, but that pilot took off with an 81 gallon tank only half full.  The 40.5 additional gallons might have given an additional 1 hour's endurance (133 miles)  Oh well, can't  mix octanes.
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Harry Howe, Jr.

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Re: Octane Analysis
« Reply #2 on: November 26, 2011, 04:01:36 PM »


Jeff
I don't think AE was much concerned with range.  The Lockheed estimate with 1200 gallons was 4100 to 4500 miles.  Doing a simple ratio with 1100/1200 reduces that to 3700  to 4100 or so, still far above the Lae- Howland distance of 2600 or so.

So, the 40.5 gallons left behind wasn't too important, but three things that a pilot has no need for are 1. Altitude above the plane, 2. Runway behind the plane, and gas left in the fill truck.
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Harry Howe, Jr.

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Re: Octane Analysis
« Reply #3 on: November 28, 2011, 11:32:05 AM »


Of course octanes can be mixed, and I'll never understand why the 81 gallon tank wasn't filled with 80 octane before takeoff .  That extra 40.5 gallons could have meant an additional 1 hour's flight should it have become necessary.  As I said before, gas in a truck at an airfield behind you is of no use to a pilot looking for a tiny sliver of an ialand in a large ocean.
The number of flat-out stupid errors made on that leg were just astounding.
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Mona Kendrick

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Re: Octane Analysis
« Reply #4 on: November 28, 2011, 10:16:01 PM »

   I spent a little while this evening looking for Lae's runway length and can't seem to find any sources -- but I do seem to recall once reading that it was about 3000 ft.  In any case, it was barely long enough for such a heavily loaded takeoff as NR16020's; the takeoff run used virtually every bit of runway, as described by witnesses.  Diluting the 100 octane by adding 40.5 gallons 80 octane would have reduced the power for takeoff and added 246 lb. of weight (40.5 gal. X 6 lb./gal. = 246), thus increasing the length of takeoff roll needed to get off the ground.  In short, insufficient runway is probably why she didn't top off the 81 gal. tank with 80 octane.

LTM,
Mona
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Gary LaPook

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Re: Octane Analysis
« Reply #5 on: November 29, 2011, 10:11:37 AM »


Of course octanes can be mixed, and I'll never understand why the 81 gallon tank wasn't filled with 80 octane before takeoff .  That extra 40.5 gallons could have meant an additional 1 hour's flight should it have become necessary.  As I said before, gas in a truck at an airfield behind you is of no use to a pilot looking for a tiny sliver of an ialand in a large ocean.
The number of flat-out stupid errors made on that leg were just astounding.

In terms of 'range' and best habit, you are spot on - the fuel in the truck, like the runway behind you on an intersection take-off and the altitude above you can't help you once committed to the wilderness of the air.

But, in terms of take-off performance / power requirements, you can't mix octanes - the 100 would be needed for max power at take-off.  I don't think 100 was available at Lae to top off that particular tank; had it been, the additional fuel that would have been left after take-off would have padded the range capability that much more since 100 octane can also be used at any other time (it's just overkill at lower power settings).

Mona Kendrick's point below yours is a good analysis of the fix AE would have been in had she diluted her existing 100 octane supply for the sake of topped-off tanks.  Her point, and yours earlier that plenty of range was actually already afforded by the 1050 +/- gallons aboard at take-off bear out AE's reasoning.  Given that sufficient range was provided for as AE and FN likely saw it, it probably would have been a relative no-brainer to go with a partial load of 100 octane aboard.

LTM -
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The engines could be run on either 80 octane or 100 octane fuel. How much horsepower did they make with the 80 octane and how much horsepower did they make with the 100 octane?

gl
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Harry Howe, Jr.

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Re: Octane Analysis
« Reply #6 on: November 29, 2011, 11:29:12 AM »


I agree with the points above.  Now, I'll pick some nits.
Assume that they took off with about 1110 gallons, which at 6 lbs per gallon represents a fuel load of 6660 lbs, then the additional 243 pounds (40.5 x 6=243) would represent an increase of about 3.65%.  The plane, passengers, baggage, etc weighed something (Probably as much as the fuel) so the additional 40.5 gallons probably represented less than 2% more.  If they were cutting the weight/takeoff roll that close that an additional 2% would have mattered then they picked the wrong runway.

 Gas left behind on a truck at the airfield is of no use.
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Gary LaPook

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Re: Octane Analysis
« Reply #7 on: November 29, 2011, 12:46:37 PM »

Now you've heard about all I know on AvGas - just enough to be dangerous...
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I have attached TCDC 590, can you point out where is requires the use of 100 octane fuel to get the 550 hp takeoff power?

gl
« Last Edit: November 29, 2011, 04:02:25 PM by Martin X. Moleski, SJ »
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Gary LaPook

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Re: Octane Analysis
« Reply #8 on: November 29, 2011, 03:57:46 PM »

Gary, in this case, the aircraft TCDS does not specify minimum octanes.  You can find that information in the Wasp S3H1 engine TCDS E-143.  Minimum octane there is shown as 80/87, and if you follow the tables and relevant note 5 it gets you to a 91/96 octane requirement for certain higher power settings at sea level (600 HP for 5 minutes). 
LTM -
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Take a close look at note 5 of TCDS E-143, which I have attached. You will see that there is NO increase in power when higher octane fuel is used. The engine was designed and certified to put out the full takeoff power rating of 600 hp using 80/87 fuel. Note, that in order to get the same 600 hp using 91/96 octane fuel you must increase the manifold pressure a half an inch compared to setting takeoff power using 80/87. By the end of WW2 there were four grades of avgas, 80/87; 91/96; 100/130 and 115/145 so "91/96" does not mean 100 octane in the TCDS.

I have represented Pratt & Whitney and UT including Sikorsky for many years and if they could have gotten more horsepower out of the engines by simply increasing the octane required they would have, since they could have then have sold the engines for more money. Prior to WW2 100 octane fuel was very new. I watched a show about the Doolittle raid and learned that after Doolittle left active duty in 1929 he worked for Shell Oil. Doolittle believed that more powerful engines were needed for modern military planes. At his instigation Shell Oil developed 100 octane fuel so that these more powerful engines could be developed. Shell delivered the first load of 100 octane fuel to the Army Air Corps for test purposes in 1934. In 1937 100 octane was very rare but when the TCDS was last amended, 100 octane was ubiquitous so, if 100 octane would have produced more takeoff power, why didn't P&W amend the TCDS to provide for the use of 100 octane? If you look at all the TCDS for P&W round engines you will find NONE that allow for a higher power output when higher octane fuel is used. There are 107 variants of the Wasp engine, including the military R1340 versions. Of all these variants only one specified the use of 100 octane fuel, the R1340-37, which is a geared engine. There are extensive differences between it and Earhart's engines including a strengthened head and fuel injection, a higher compression ratio and greater supercharger ratio. Note, even with these extensive improvements and the use of 100 octane fuel, the engine still only produced 600 hp. The main improvement from all these changes was raising the critical altitude all the way up to 11,000 feet.

gl
« Last Edit: November 29, 2011, 11:24:03 PM by Gary LaPook »
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John Ousterhout

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Re: Octane Analysis
« Reply #9 on: November 29, 2011, 08:05:56 PM »

http://tighar.org/Projects/Earhart/Archives/Research/Bulletins/27_LaeGallery/27_LaeGallery.html gives the Lae runway length as 3000 feet.
I corrected my earlier post shortly after re-reading it, but Gary beat me to it. Sorry for any confusion.
Cheers,
JohnO
 
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Harry Howe, Jr.

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Re: Octane Analysis
« Reply #10 on: November 29, 2011, 09:43:41 PM »


Mona
Sorry, My Bad, Apologies     Reply #33 was about Howland,  Lae had 1 runway  3000 feet.  Looks to be oriented NW to SE  150 degrees  runway numbered 15 and33.

The additional 243 lbs could possibly have affected the takeoff roll and since she had 1110.5 gallons and plenty of range (endurance) for the proposed leg it was probably prudent to go without the 40.5 gallons.  I might have considered gassing up and leaving Fred behind.  MHO
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Gary LaPook

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Re: Octane Analysis
« Reply #11 on: November 30, 2011, 03:15:04 PM »

Thanks Marty.  As soon as I can I'll brush up on minimizing my clutter - sorry.

For now -

Gary LaPook - "Take a close look at note 5 of TCDS E-143, which I have attached. You will see that there is NO increase in power when higher octane fuel is used. The engine was designed and certified to put out the full takeoff power rating of 600 hp using 80/87 fuel. Note, that in order to get the same 600 hp using 91/96 octane fuel you must increase the manifold pressure a half an inch compared to setting takeoff power using 80/87. By the end of WW2 there were four grades of avgas, 80/87; 91/96; 100/130 and 115/145 so "91/96" does not mean 100 octane in the TCDS."

And again, "increasing octane does not increase power" anyway - it merely allows one to safely apply power settings that create the highest combustion chamber pressures - to obtain the highest power output.  How?  By resistance to detonation and preignition that is more prevalent in the use of less-buffered / more volatile fuels.

Consider Note 5 again - it says (verbatim) -

(a) Ratings are based upon the best mixture strength and 450F cylinder head temperature.
(b) With the use of grade 91/96 fuel, the following ratings may be used:

Maximum continuous, hp, rpm, in.Hg., at:
- Critical altitude (ft.)                                   550-2200-33.0-4500
- Sea level pressure altitude (ft.)                  550-2200-34.0-S.L.
Takeoff (one minute), hp, rpm, in.Hg., at:
- Critical altitude (ft.)                                   600-2250-36.0-2300
- Sea level pressure altitude (ft.)                  600-2250-36.5-S.L.
[/u]
LTM -

However, the main data on page 1 states, verbatim:

Maximum continuous, hp, rpm, in.Hg., at:
- Critical altitude (ft.)                                   550-2200-32.5-5000
- Sea level pressure altitude (ft.)                    550-2200-34.0-S.L.
Takeoff (five minutes), hp, rpm, in.Hg., at:
- Critical altitude (ft.)                                   600-2250-35.5-3000
- Sea level pressure altitude (ft.)                    600-2250-36.0-SL (See note 5.)

There are three ways to show that your interpretation is in error. First you claim that the 91/96 octane is only needed for 600 hp takeoff power but note 5 also shows power settings for continuous power. If you were correct, then by your logic, 91/96 was also necessary for the continuous rating of 550 hp.

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.

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.

gl
« Last Edit: December 01, 2011, 03:17:27 AM by Gary LaPook »
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Gary LaPook

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Re: Octane Analysis
« Reply #12 on: November 30, 2011, 04:35:20 PM »

Gary, you are ignoring Note 5's full import - and the greater meaning of this discussion - 

The whole point is: the higher octane is required for the highest power settings - period.

My whole point is that higher octane was NOT required for the highest power settings.
Quote

 - and after 35 years of dealing with authorities on this stuff it is clear what the answer would be
I've spent over 20 years dealing with these things too, working with IAs, A&Ps and factory engineers at P&W so I think I am qualified to interpret this TCDS too and I don't see any ambiguity at all.
Quote

So, I guess we disagree, no loss, but I suggest keeping your 91/96 octane handy...  :D
- Jeff

I am attaching the P&W power setting table for the S3H1 engine that shows settings for the 600 hp takeoff power for both 80/87 octane and for 91/96 octane and these setting are the same settings as the settings in the TCDS, the 80/87 settings from page 1 and the 91/96 settings from note 5. This power setting table confirms my interpretation of the TCDS and refutes yours.

You didn't respond to my three points either but if you don't want to, then I will drop this conversation too. We have stated our arguments and I have posted my source documents so readers can decide for themselves which point is more convincing. :D

gl
« Last Edit: November 30, 2011, 05:31:15 PM by Gary LaPook »
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John Ousterhout

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Re: Octane Analysis
« Reply #13 on: November 30, 2011, 07:19:26 PM »

I take gary's point to be that the half-full 100-octane tank could have been safely topped up with 87 octane, without endangering the takeoff from Lae or Howland.  The connection with NOT landing on Nikumororo is a little harder to see, and I trust he will clarify shortly.
I actually enjoyed the technical treatise, even if slightly off topic.
Cheers,
JohnO
 
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Gary LaPook

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Re: Octane Analysis
« Reply #14 on: December 01, 2011, 03:15:15 AM »

I take gary's point to be that the half-full 100-octane tank could have been safely topped up with 87 octane, without endangering the takeoff from Lae or Howland. 
That is my point and the extra hour of flight endurance could have allowed Earhart to find Howland, or some other island or, if she landed on Nikumororo then the extra fuel would have provided many hours of extra radio transmitting time, take your pick.
Quote
The connection with NOT landing on Nikumororo is a little harder to see, and I trust he will clarify shortly.
I actually enjoyed the technical treatise, even if slightly off topic.
Not connected with the landing on Nikumororo, an example of "thread drift."
gl
« Last Edit: December 01, 2011, 03:22:03 AM by Gary LaPook »
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