Transition from 100LL to UL 91/94 Avgas for our Mooneys

[Shadrach]

2 hours ago, tony said:

It’s really both.  Remeber the STC for mogas?  My F wasn't included because of vaporization issues experienced with the fuel system during testing.  

The vapor pressure qualities of UL91 are nearly identical to 100LL. Mogas definitely has performance issues in some aircraft fuel systems but 91UL would not have those issues. Detonation margin would be the only real concern with 91UL in an IO360 and that could be mitigated by retarding the timing as Rich stated.

 

[Hank]

1 hour ago, Shadrach said:

The vapor pressure qualities of UL91 are nearly identical to 100LL. Mogas definitely has performance issues in some aircraft fuel systems but 91UL would not have those issues. Detonation margin would be the only real concern with 91UL in an IO360 and that could be mitigated by retarding the timing as Rich stated.

With resulting power loss. Who builds the new performance tables, takeoff distances, etc?

[tony]

1 hour ago, Shadrach said:

The vapor pressure qualities of UL91 are nearly identical to 100LL. Mogas definitely has performance issues in some aircraft fuel systems but 91UL would not have those issues. Detonation margin would be the only real concern with 91UL in an IO360 and that could be mitigated by retarding the timing as Rich stated.

 

So I not 100 percent sure, my memory isn't as good as it used to be, but I believe you need to show compliance, by test for the aircraft on a 120 degree day.    

Edited June 7 by tony

[Pinecone]

IIRC, you don't need a 120 degree day, but you heat the fuel to 120 degrees, put it in the plane and do a full power, full rich climb to max certified altitude

[McMooney]

just a thought, could you just not use 100% power instead of changing timings ?

[jetdriven]

5 hours ago, McMooney said:

just a thought, could you just not use 100% power instead of changing timings ?

How do you do that? Put a stop on the throttle? Then you can’t get 100% of available power at altitude.  You could set a limitation to restrict MP to 25” Or something but it still doesn’t solve the vapor pressure issues.  

[Paul Thomas]

On 6/7/2024 at 6:56 AM, tony said:

Its really both.  Remeber the STC for mogas?  My F wasn't included because of vaporization issues experienced with the fuel system during testing.  

Do any M20's have an STC for mogas?

[kortopates]

Do any M20's have an STC for mogas?

No


Sent from my iPhone using Tapatalk

[A64Pilot]

I’ve done hot fuel tests, but only with turbines.

For the P&W -67AG we had to do it with 100LL as it’s an alternate fuel for Pratt.

The correction factors used are conservative, so the hotter the day the easier it is to pass the test.

There are multiple issues with fuel cert, some things maybe aren’t so obvious like prop vibe surveys for instance, same with changing timing, things you may never have thought of become problems.

As far as Auto fuel STC’s, I would think that now days they would be very difficult to obtain, the reason is Auto fuel isn’t standard anymore, there are Different fuels for Summer and Winter, California has their own fuel for example and I think there are even different fuels for major cities all based on pollution, where back in the day it was just one fuel.

This may be of interest

https://www.faa.gov/sites/faa.gov/files/PAFI_Fuel_Development_Best Practices.pdf

[Shadrach]

On 6/8/2024 at 1:08 PM, jetdriven said:

How do you do that? Put a stop on the throttle? Then you can’t get 100% of available power at altitude.  You could set a limitation to restrict MP to 25” Or something but it still doesn’t solve the vapor pressure issues.  

Should not be vapor pressure issues with UL91. Converting from SAE to Metric shows them to be nearly identical in terms of volatility. It’s reasonable to conclude that the average IO360 application, timed to 20° BDTC, has sufficient detonation margins to tolerate UL91 under normal operations. Whether it meets FAA detonation margins in said application is anyone’s guess. 

100LL vapor pressure 5.5-7.0 psia or 39.2-48.2 kPa

91UL vapor pressure  38-49 kPa or 5.51-7.1psia

 

[A64Pilot]

On 6/10/2024 at 9:21 AM, Shadrach said:

Whether it meets FAA detonation margins in said application is anyone’s guess. 

I think that’s it, but FAA merely sets the margins, the engine manufacturer sets some of the conditions, so I suspect that Lycoming knowing they had 100 Octane to work with set some of the conditions quite high, like 500F cylinder heads and probably high oil temps etc.

My suspicion is if more realistic numbers were used that it’s very likely that it would do fine on 91 Octane but the engines and airframes would have to be recertified to get there.

Way back in the 70’s the University of Tennessee which apparently has or had a very advanced Aviation dept. conducted a long term test on I believe a Cessna 421, they ran one engine on 100 Octane and the other on what was then called Gasahol and had no problems. I don’t know what if any modifications or restricted operational limits there were if any, only know one of the Test Pilots / DER’s that wrote and flew the test plan.

I know a few people that run IO-540’s on Premium Auto fuel, but takeoff and climb on 100LL as an example.

I’ve read the majority of Aircraft engines would do fine on much lower Octane, only a few need it, but to ensure safety of all aircraft / engine combinations in every conceivable operation we therefore have to have 100 Octane. 

[Pinecone]

The numbers I heard is that 80% of the aircraft could run on lower octane.  But the 20% burn close to 80% of the fuel used.

[A64Pilot]

13 hours ago, Pinecone said:

The numbers I heard is that 80% of the aircraft could run on lower octane.  But the 20% burn close to 80% of the fuel used.

I’ve heard that too, but think maybe that was an old number back when a great many high power twins were used to fly bank checks and or other Commercial activities.

Nowdays twins are pretty uncommon, at least where I fly, been replaced by Caravans etc. and bank checks haven’t been flown for probably a long time.

But I think even many of the high power twins could burn lower Octane with either limitations or modifications, some I think just probability couldn’t along the lines of the limitations would reduce useful load so much the aircraft just isn’t usable.

It could be done of course likely as simple as a modern detonation sensor and ADI, but I don’t think the money / desire is there. I think in most cases the 25” MP and 400F cyl head temp is likely very conservative for most as it’s blanket numbers and well there are differences in engine models, the trouble engines are well known.

My guess is it’s likely new aircraft manufacturers will if required lead the way, I don’t see for example Cirrus and others closing their doors when LL goes away, and as they will of course be Certified STC houses will follow their model as it’s a whole lot easier to get an STC that pretty much duplicates a Certified product.

But who knows, we have been kicking the ball down the roads for decades, I almost didn’t buy my Maule in 04 due to concern of the impending demise of LL, and almost 20 years later I bought a Mooney with a fuel injected high compression motor that requires it.

I think technically LL is illegal, a law was passed in the 70’s that outlawed ALL leaded fuel and didn’t exempt Aviation, of course they never meant aircraft fuel, but I wonder if the exemption was ever put into law?

My concern has always been some politician will jump on it as a way to gain votes and poof it’s gone.

I think it’s solvable, but it’s going to take $$$, thinking it won’t isn’t I think realistic. Question is how much $$$?

[Pinecone]

Except there is already a 100 octane unleaded fuel that is approved for all piston airplanes.

Modern detonation sensors don't work in aircraft.  It has been tried before. Too much noise in the system.

 

[Shadrach]

36 minutes ago, Pinecone said:

Except there is already a 100 octane unleaded fuel that is approved for all piston airplanes.

Modern detonation sensors don't work in aircraft.  It has been tried before. Too much noise in the system.

 

I was hopeful that George Braly would be able to bring the PRISM ignition system to market. It uses pressure transducers to measure internal cylinder pressure and adjusts ignition timing as needed. Unfortunately there’s been little movement toward certification over the last few years.

[Pinecone]

After G100UL gets up and running you might see that return to the forefront.