DA50 help

[49dimes]

I am going to mess with the motor in the morning when I get home and I'm sitting here thinking about it! Am I right in the thinking that a larger prop with more load would cause a rich condition, slower turning not using the fuel as quickly? Or is it the opposite that the engine would go lean not enough fuel to turn the larger mass? I'm not sure why this has me so perplexed...

The engine is going to work harder with a bigger prop. So working harder means more fuel .

Also know your engine works harder in warm humid conditions so again more fuel needed.

Just the opposite in cool dry weather.

 

[Bartman]

Dimes!

I think it's just the opposite! In warm humid conditions, the moisture in the air leaves less room for oxygen so the mixture will go rich. Our engines make less power on hot humid days, there isn't much we can do about it except lean out the mixture a bit so it will do as much as it can with the oxygen it's getting.

On cold days, when the air is dry and dense, more oxygen is getting into the engine so it will go lean unless you give it more fuel by opening up the needle valve just a bit.

All planes go through this, less engine power and less lift from the wings on hot/humid days. More power, more lift, so much better performance on dry cold days.

That bigger prop shouldn't change the mixture. If you try both props on the same day, I'd expect the mixture to be the same. The engine can only make so much power and so it will spool up to the highest RPMs it has power to make and that'll be it. I'd guess you could put the bigger prop on and reset the mixture for the new top end RPM limit but the change in the carb setting will be very small.

I may be wrong, it's happened before.

 

[Alky6]

Hi Bartman: you are technically correct. However, I believe @49dimes is referring to another phenomenon that occurs, and usually much more pronounced in an alcohol engine that has a much wider range of stoichometric ratio. Essentially, the fuel is acting as a cooling process. A little more rich adds some cooling and is less apt to pre-ignite or detonate. Higher load and higher outside temps mean higher temps inside - potentially leading to detonation. When I was racing full scale boats (back in the day) I had a much more difficult time jetting the two-stroke engine when it was hot and humid out (and hotter water that was used for cooling the engine) and was much more prone to sticking a piston than in cooler drier weather. In some cases I did need to richen up. Usually that went hand in hand with a bigger prop too.

But, yes, to maintain the correct stoichometric ratio for peak power you need to richen on cold dry days and lean on hot humid days. We always hoped for a cold day on days to set speed records.

 

[49dimes]

Hey Bart. Yes you are correct about air density and tight grouped air molecules in cool dry weather and how it affects tuning. I was explaining from how hard the engine works or is made to work (loading/resistance). The bigger prop is higher load (higher resistance) and makes the engine work harder and run hotter. The bigger prop is not going to change the fuel air ratio as Paul explained but richening the needles will help the engine run cooler and get some power back. That's why its best to start over on the tune when changing different size props. "Tune for loading" if you will. Tuning for atmospheric conditions and changes is always a factor but again I explained it from a loading standpoint for the sake of the symptoms explained by the author. Go back to square 1 and re tune for the prop so you can run the engine as efficiently as possible . The physics are the same in regards to "resistance" or loading. Does not matter if its fluid flow energy, electron flow energy (and in our case) expansion flow energy. The more resistance encountered the more energy expended to an economical point.

 

[49dimes]

Hi Bartman: you are technically correct. However, I believe @49dimes is referring to another phenomenon that occurs, and usually much more pronounced in an alcohol engine that has a much wider range of stoichometric ratio. Essentially, the fuel is acting as a cooling process. A little more rich adds some cooling and is less apt to pre-ignite or detonate. Higher load and higher outside temps mean higher temps inside - potentially leading to detonation. When I was racing full scale boats (back in the day) I had a much more difficult time jetting the two-stroke engine when it was hot and humid out (and hotter water that was used for cooling the engine) and was much more prone to sticking a piston than in cooler drier weather. In some cases I did need to richen up. Usually that went hand in hand with a bigger prop too.

But, yes, to maintain the correct stoichometric ratio for peak power you need to richen on cold dry days and lean on hot humid days. We always hoped for a cold day on days to set speed records.

I got treated (after flying) to a blown 455 alcoholic Chevy step-side. About 1200HP today .

 

[Bartman]

Hi Bartman: you are technically correct. However, I believe @49dimes is referring to another phenomenon that occurs, and usually much more pronounced in an alcohol engine that has a much wider range of stoichometric ratio. Essentially, the fuel is acting as a cooling process. A little more rich adds some cooling and is less apt to pre-ignite or detonate. Higher load and higher outside temps mean higher temps inside - potentially leading to detonation. When I was racing full scale boats (back in the day) I had a much more difficult time jetting the two-stroke engine when it was hot and humid out (and hotter water that was used for cooling the engine) and was much more prone to sticking a piston than in cooler drier weather. In some cases I did need to richen up. Usually that went hand in hand with a bigger prop too.

But, yes, to maintain the correct stoichometric ratio for peak power you need to richen on cold dry days and lean on hot humid days. We always hoped for a cold day on days to set speed records.

Interesting

 

[Bartman]

....................The bigger prop is not going to change the fuel air ratio as Paul explained but richening the needles will help the engine run cooler and get some power back. That's why its best to start over on the tune when changing different size props. "Tune for loading" if you will. Tuning for atmospheric conditions and changes is always a factor but again I explained it from a loading standpoint for the sake of the symptoms explained by the author. .................

I can see how that would factor in, X gets the square.

 

[Alky6]

I got treated (after flying) to a blown 455 alcoholic Chevy step-side. About 1200HP today .

View attachment 102507 View attachment 102508 View attachment 102509

Whoa! Badaasss!!!!!

 

[49dimes]

The engine is going to work harder with a bigger prop. So working harder means more fuel .

Also know your engine works harder in warm humid conditions so again more fuel needed.

Just the opposite in cool dry weather.

@Travis Haney Let me clear up some things......Simply put.....Engine working harder more fuel consumed. Engine working less hard less fuel consumed. My use of adjective's was poor. Sorry for that. Hope you got it running with just resetting the needles. I see a lot of guys "chase" needle settings down here because atmospheric conditions have been changing from day to day (it seems) down here and IMO they try and tune for the most power and are really running their engines to lean. I take some heat myself on my own philosophy of tuning for a slightly rich condition (not overly rich) and run a 32-1 mix for the sake of good engine lubrication, performance and cooling. Hope you got it going with out having to take the carb off.

 

[stangflyer]

Engine tuning... Hmm!! How many of us have crossed that bridge too many times? LOL... It can be an art all its own. It's unfortunate that these carbs on our model engines don't come with "self sensing and adjusting" needles. But until everything goes to EFI, we're all just going to have to adjust them manually. Too many times I see fellas either "under" prop their engines to get the quickest and highest rev out of their engines. Which is actually hard on an engine. Just as equally hard on them is "over" propping them. Working an engine too hard will wear that puppy out quick. And in that equation of either under or over propping engines, comes in the factor of needle adjustments. Then we factor in humidity, temperature and overall density of the air. Oh and then lets factor in oil to gas mixture. It can be a scientists worst nightmare.