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Use of different octanes in flight

 
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pkent



Joined: 26 Feb 2013
Posts: 10

PostPosted: Fri Jun 14, 2019 15:52    Post subject: Use of different octanes in flight Reply with quote

I have just read the following which surprised me :
"...from 1933 to 1937 airlines used 87 octane only to boost take-off power and 80 octane for cruising. By 1937 87 was used for cruising and 90 for take-off."

Q. Can anyone expand on this dual-fuel use? Thank you.
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sithomas



Joined: 01 Jan 2019
Posts: 10

PostPosted: Tue Jun 18, 2019 08:12    Post subject: Reply with quote

I understand Amelia Earhart also used different octane fuels on her round the world flight. The concept is to get full power for high weight take offs using the higher octane fuel, and then switch to the tanks with lower octane for lower power cruising. The higher octane fuel availability was an issue at some locations, but also the cost delta of the fuels played a part.

I believe that some current light planes with mogas STCs do the same thing - they use avgas for take off and climb, and use mogas for cruise. In this case, an additional driver is the vapour pressure of the avgas can prevent the engine missing due to vapour problems during the climb.
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jschauer



Joined: 19 May 2004
Posts: 93
Location: Justin, Texas

PostPosted: Tue Dec 31, 2019 15:10    Post subject: Reply with quote

You are misunderstanding the octane rating. Avgas grade 80/87 was 80 octane. When used for takeoff an over-rich mixture was produced by the carburetor and the fuel had an equivalent rating of 87 octane. 100/130 had the equivalent octane rating of 130 when the carburetor was in AUTO-RICH. Todays 100LL has the equivalent rating of 130 octane when used in AUTO-RICH.
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