Prop Pitch

[donchandler]

  how much, if at all, does the temperature throught the day, and humidity changes, effect prop pitch?

[Paul Walker]

What "effect" are you referencing?

[Howard Rush]

You may be thinking of a different parameter.  Pitch is a geometric measurement one makes on a prop.  It is something measureable, but it is only vaguely related to anything aerodynamic.

I've found with an electric motor running the same speed with an APC prop, the airplane will go a tad slower when the air density decreases (or maybe when the temperature increases).  Both thrust from the prop and drag on the airplane oughta be proportional to density, but I think something else is at play.  Humidity has an extremely small effect. 

[Steve Helmick]

"I've found with an electric motor running the same speed with an APC prop, the airplane will go a tad slower when the air density decreases (or maybe when the temperature increases).  Both thrust from the prop and drag on the airplane oughta be proportional to density, but I think something else is at play."

Could it be increased flexibility of the plastic prop as temp rises? I see Paul is using a CF 3-blade...does the change to a CF/epoxy prop prevent the airspeed drop? I'd think timing accuracy would be a significant problem...time for 10 level laps? Too boring!    Steve

[Randy Cuberly]

Here in Tucson Arizona where we regularly fly in temperature variations between 68 degrees and 105 + in the summer time during one morning flying session, I have noted several things (possibly) at least partially related to prop pitch and temperature.
First as, everyone knows, as the temperature begins to climb in the early morning it is necessary to lean the mixture of the engine or add nitro to maintain a given RPM...no surprise there.  However, I've found that to maintain the same lap times it is typically also necessary to increase the engine RPM by as much as 250 to 300 RPM at the extremes of those temperatures.  My guess is that this is due to the decrease in air density and it's effect on thrust from the prop.  This means of course that the air temperature effect would be a greater factor in speed than the decrease in drag on the airplane and lines.
  
It may also be partially (small amount) due to the general loss in power and change in torque curve of the engine.
I have also noted that this effect seems to be more drastic with two blade props than with three blade props, but seems to be unrelated to whether the engine is piped or muffled  Some or all of this may of course be due to the difference in efficiency of the individual props but the difference is significant and very noticible.  (all props in question are carbon)

I would mention that here in Tucson the humidity is typically very low and doesn't change significantly from those low temperatures to the high extremes.  Typical numbers are less than 10% relative humidity, often as low as 3-4%.

There are a few times later in the summer (late July and August when the humidity goes significantly higher but I haven't done any testing during those times.  I suspect that the differences if any are small due to changes in humidity.
During that time the nitro flows freely!!  25% to 30% isn't unusual.
With that much nitro it's usually necessary to open the needle even at the higher temps.

Randy Cuberly

[Howard Rush]

OK, I did an experiment.  Yesterday was cool; today was warm.  After my last trim change yesterday, lap time was 5.32 seconds, temp. was 67F, density / sea-level-standard-day density was .986.  Today, lap time was 5.5 seconds, temp. was 84F, density / sea-level-standard-day density was .949.  Another parameter of interest is Reynolds number / sea-level-standard-day Reynolds number.  That was .98 yesterday and .92 today.  I neglected to measure RPM to see if something funny is happening with the speed regulation, so I don't know if the difference is due to the prop or the electrics.  Could be the prop is going soft with the heat and flattening out or something, or it could be that it's operating in a range where characteristics change fast with Reynolds number, but that sounds farfetched, don't you think? 

I used Natural Weather Service data and this nice calculator: http://wahiduddin.net/calc/calc_hp_dp.htm for all but the Reynolds number item.   

[BillLee]

...
I used Natural Weather Service data and this nice calculator: http://wahiduddin.net/calc/calc_hp_dp.htm for all but the Reynolds number item.   

Great page! Thanks for the link, Howard.

[Douglas Ames]

You mean - propeller efficiency vs. density altitude.

[Brett Buck]

  I neglected to measure RPM to see if something funny is happening with the speed regulation, so I don't know if the difference is due to the prop or the electrics.  Could be the prop is going soft with the heat and flattening out or something, or it could be that it's operating in a range where characteristics change fast with Reynolds number, but that sounds farfetched, don't you think? 

    No, actually I was interested in somebody doing this experiment because I had gotten the same sort of effect. I am pretty sure my carbon/epoxy props are not flattening out. But I was pretty certain that I needed significantly more in-flight RPM to get the same lap times as the air got hotter.

    Brett

[Howard Rush]

You mean - propeller efficiency vs. density altitude.

That's not what I mean.  Density altitude is a number that gets calculated because it's more intuitive for pilots than straight density.  You can't calculate anything from it.  I think the ratio of density to SLSD is more useful: minimum loop radius is inversely proportional to it, for example.  The Web site give you both and derives everything.  It's pretty cool.

[bob whitney]

i have an electric scorpio that i flew at Brodaks last year   here in FLA at O sea level i was running 10,000 RPM, and getting 5.2 laps . at Brodaks at 2500 FT above sea level and worm weather  my lap times dropped to 5.9 sec had to up the RPM to 10,3000 to get back to 5.2 sec, i use my own 12 / 6 APC copy and it dos not flex this year the weather was terrable  rain and very thick air and i did not need to up the RPM to get 5.2 laps

[donchandler]

  Thanks guys-exactly what I was looking for. Here in Red Bluff, CA. early morning temps go from high 60's to triple digits by afternoon. Morning flights are fairly trouble free on low nitro. By afternoon this time of year I have increased nitro content and tweaked the neeedle for more rpm but its seems like I am dragging something behind the airplane. OS 46's and 40's mainly (Tom Dixon mods) with a ST .51 that actually seems to perform well no matter what the temp.

Thanks Again, I'll pay more attention to rpm at takeoff.