Why?

[jwsimmons]

This is a question for all you real brainiacs out there. Why does the performance of propellers change with elevation? What is the science behind it?

 

[jwsimmons]

Re: Why?

Also, how much of the change in performance is usually propeller and how much is carb?

 

[Ron G]

Re: Why?

I think most of it is carb and not prop.its the motor that has to be leaned or richen to give the power at different elevations,a prop is in the water and water dosnt change.but what do i know i dont have to mess with it here.

 

[Texasmark]

Re: Why?

I'm a low altitude guy, but lets take a whack at it. I know from verbiage I have read and my experiences, that air density effects combustion effectiveness. Like I have a 404 cu in tractor rated at 100 drawbar hp. But the same engine with a turbocharger on it cranks out 125 drawbar hp. Difference is the amount of air (weight) crammed into the cylinder before it lights off. Course on this tractor, they increased the fuel delivery also to get to the 125 hp. Didn't get all of that from air.

So, thin air, like at altitude, means less weight and less combustion translating into less hp. So since your prop puts this hp to work, you have to downsize the prop due to less whoopie twisting it.

Like my '05 pickup runs like a scalded cat at my 800 ft altitude. But this summer I went to NM and Ariz. and in the mountains it was a wimp......and the computer is supposed to compensate for that.

My 2c,

Mark

 

[Silvertip]

Re: Why?

The performance of the propeller doesn't change with altitude. However a propeller change may be necessary to compensate for the loss of engine performance at high altitude. Ever go up a steep mountain grade in your car or truck and find the transmission shifting down to maintain speed? In a boat you have only one gear. Therefore the prop determines how much performance you can get from a given amount of power at high altitude. To "shift gears" so to speak, you would change the prop to one with less pitch to compensate for the loss of power.

 

[Silvertip]

Re: Why?

Dang -- you beat me to it again!

 

[WillyBWright]

Re: Why?

Pretty much on target. Fewer oxygen molecules in a given volume of air means less power produced. Less power means a lower pitch to compensate and get the RPMs up. Well, compensate is a bad word. The motor won't/can't put out the same Hp at higher elevations as at sea level. So you use a lower pitch to get the recommended RPMs and thus get the most Hp available, even though it isn't going to be the full Hp written in the specs.

That's also why the pick-up struggles more at higher altitudes. The EFI compensates to give you the most efficient burn, but there's still fewer oxygen molecules to provide combustion. Motors with blowers do better because they cram whatever air is needed and bleed-off the excess. At higher altitude, they bleed-off less and cram more than at sea level.

 

[jwsimmons]

Re: Why?

I originally thought that performance was due to air pressure changes at different elevations and then I read a post that said most of performance change was at the prop and not the carb. I beleive you when you say it's O2 differences (makes tons of sense). Thanks for the info guys. You explained it well for me.