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prop rpm

R

Robbie S

PATRON
Lonoke Ar.
quick ?.just finished an 18 with Dakota big wings and slots.31 tires .balanced 0360.mccaulley 82 42.im seeing 2300 or so on climb out with 2700 flat and level wide open indicating about 125mph.112 at 2500.does these numbers sound like its pitched ok.im used to 160 hp cubs with flat borers so don't know.it also has zero thrust mod.i had another 180 cub with stock wing and I think it was quite a bit faster flat out.i know wing is slower and has differ purpose but not that much.take off run feels ok for a 1210 lb plane.thanks
 
Yes, the prop RPM sounds about right, for that prop and engine. The speeds seem about 5 to 10 MPH fast for 31"Bushwheels. Sounds more like what I would expect to see with 8.5 tires

Bill
 
skybolt64 said:
quick ?.just finished an 18 with Dakota big wings and slots.31 tires .balanced 0360.mccaulley 82 42.im seeing 2300 or so on climb out with 2700 flat and level wide open indicating about 125mph.112 at 2500.does these numbers sound like its pitched ok.im used to 160 hp cubs with flat borers so don't know.it also has zero thrust mod.i had another 180 cub with stock wing and I think it was quite a bit faster flat out.i know wing is slower and has differ purpose but not that much.take off run feels ok for a 1210 lb plane.thanks
Click to expand...

interesting. we weren't happy with nearly the same numbers. we're trying to work out a similar issue with an -18 with a stock wing and VG's and 160hp. had a mccauley 82/44 and static rpm was 2380, 2450 at liftoff. swapped to a 82/42 and barley gained any rpm. then swapped to a 82/41 and still didn't get the rpms we'd like, maybe 30 rpm increase. we'd turn 2700 in cruise at about 100KIAS. all at 2000lbs gross weight. our main goal is to get out of the hole on takeoff, so we're looking for a bit more static to help us in that regard. exhaust baffles are nearly new and no obvious signs of blockage. we're going down the road of re-jetting the carb to see if we can pour some gas on the problem and increase our rpm. i'll return with our results.
 
Just for reference
I see about 2350 static with 150hp and 82/43 borer
about 2500 on climb out at 60 mph and 2700 flat wide open
Just for reference
 
125 MPH @ 2700 and 112 MPH @ 2500 gives you an effective prop pitch of 47-48". With an 82-42, that math doesn't work.
My 160HP Cub with an 82-43, 31s, etc. would go 120, but it was at 3000 RPM. That math does work.
I don't have an O-360 Cub, but the math doesn't lie.
 
skybolt64 said:
quick ?.just finished an 18 with Dakota big wings and slots.31 tires .balanced 0360.mccaulley 82 42.im seeing 2300 or so on climb out with 2700 flat and level wide open indicating about 125mph.112 at 2500.does these numbers sound like its pitched ok.im used to 160 hp cubs with flat borers so don't know.it also has zero thrust mod.i had another 180 cub with stock wing and I think it was quite a bit faster flat out.i know wing is slower and has differ purpose but not that much.take off run feels ok for a 1210 lb plane.thanks
Click to expand...

I was flying the same engine / prop today and seeing 105 - 107 mph at 2350 / 2400 (since 2005). I'd leave it alone.

Crash
 
thanks everyone..im going to solid up my numbers on next calm day with the gps both directions..airspeed indicator may be a little off,,but it did just come back from overhaul with the tach and everything else.by the way,the prop I am using is the big,thick fat legal one that goes with the cubcrafter 180stc,not the boerer pencil tip prop.once I get just a little time on her and the neighbors stop stopping by to look..i have a new catto in the shop to go on a friends backcountry cub when done....so guess what..LOL.and I guess I do keep forgetting about the 31 inchers under there.you know,25 years ago my first rebuild I put 8.50s on it and they looked like monsters here in Arkansas.now the 31s look normal beside the 35s on the backcountry.I know I don't need them here in Ar.,,but they look so cool.LOL
 
Rule of thumb that I've seen is
RPM x Pitch / 1056 = MPH
FWIW I bought a new Sensenich prop for my C150/150 a few years ago, it was factory-stamped 56 but when the prop shop measured it it was darn near 57.
 
I'd install a M/P gauge at least for test flights. It might help sort out any apparent inconsistencies.
 
hotrod180 said:
Rule of thumb that I've seen is
RPM x Pitch / 1056 = MPH
FWIW I bought a new Sensenich prop for my C150/150 a few years ago, it was factory-stamped 56 but when the prop shop measured it it was darn near 57.
Click to expand...

Carey Gray said:
Do the numbers backward. 125MPH X 1056/2700=48.8" of pitch.
Click to expand...
That is an interesting rule of thumb. How do you take into consideration different diameters and chord of the blades? Also how about the drag coefficient of the airplane? The same prop on a 150/150 will give different results on a 150/PA-18, Bush wheels, floats etc. A new full sized prop will give different results than a same dimension prop which has been "repaired" down to minimum dimensions. With these thoughts, when comparing different props on the same airplane the rule of thumb is a good starting point.
 
That formula is a rule of thumb, not a hard & fast written in stone. It calculates how far the a given pitch prop screws itself (and the airplane) forward in the air for a given rpm.
pitch x rpm = forward travel in inches per minute x 60 = inches per hour, /12 = feet per hour, /5280 = mph.
The times 60 divided by 12 divided by 5280 thing works out to divided by 1056.
 
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mydoghasa170 said:
interesting. we weren't happy with nearly the same numbers. we're trying to work out a similar issue with an -18 with a stock wing and VG's and 160hp. had a mccauley 82/44 and static rpm was 2380, 2450 at liftoff. swapped to a 82/42 and barley gained any rpm. then swapped to a 82/41 and still didn't get the rpms we'd like, maybe 30 rpm increase. we'd turn 2700 in cruise at about 100KIAS. all at 2000lbs gross weight. our main goal is to get out of the hole on takeoff, so we're looking for a bit more static to help us in that regard. exhaust baffles are nearly new and no obvious signs of blockage. we're going down the road of re-jetting the carb to see if we can pour some gas on the problem and increase our rpm. i'll return with our results.
Click to expand...

so a little post modification info for those interested. We initially chased a lean condition idea but after our mechanic checked out the jet, turns out we were a bit rich and bogging down on takeoff. Our guy did a little port, polish, and helped the air with a little annular action through the Venturi (essentially increasing the air volume) and compensated for the rich condition. We gained back significant RPMs. Took some washout out of the wings and now it's a properly performing cub.
 
skybolt64 said:
thanks everyone..im going to solid up my numbers on next calm day with the gps both directions..airspeed indicator may be a little off,,but it did just come back from overhaul with the tach and everything else.by the way,the prop I am using is the big,thick fat legal one that goes with the cubcrafter 180stc,not the boerer pencil tip prop.once I get just a little time on her and the neighbors stop stopping by to look..i have a new catto in the shop to go on a friends backcountry cub when done....so guess what..LOL.and I guess I do keep forgetting about the 31 inchers under there.you know,25 years ago my first rebuild I put 8.50s on it and they looked like monsters here in Arkansas.now the 31s look normal beside the 35s on the backcountry.I know I don't need them here in Ar.,,but they look so cool.LOL
Click to expand...

Have you used a digital tach checker to verify that tach is accurate? If not, you need to. I had my tach overhauled, and it came back 50 rpm off. I figured that sucked, but mechanics said its legal....just placard the error on the panel. But, I traded that overhauled tach for a new one.....which it turned out was 100 rpm off at max.....again, legal, just placard. I ordered a digital tach and never looked back.

without knowing if you have tach error, you've no way of knowing if what you're doing is legal or smart.

MTV
 
Just a small amount of carb,or intake or induction tube ice will reduce rpm. if you pull carb heat and get a reduction you most likely have some ice. its a rare day I don't get some kind of ice.I can feel it in the 160hp my 52 pitch sench,can turn up 3000rpm level flight on a good no ice day.the 180s HP cubs its smoother with ice,my be the big prop.
 
Most pilots I fly with don't under stand ice, we tow signs big sign s we need 100percent I have only 3000 hrs in cubs and 18,000 total all in pistson.I can't spell but I do know carb,induction Ice .Did high performace check out in piper 235,the pilot did not know what the carb heat gage was for nor did her try the carb heat ,well I pulled the carb heat and it lost 400 rpm,he could not beleive it.That said every time you pit up a big banner you are at 100 percent and you can tell you are not getten it when you only have2200 rpm. so 15 to 20 min later you find a carb heat setting that will keep ice out. there for, when someone tells what RPM they are getting if low most likly they had ice at that time.
 
You should get a reduction in RPM any time you pull carb heat because you are running hotter less dense air. It should drop more than normal with carb ice and than slowly increase as ice clears.
DENNY
 
If you are running over rich you will still get a drop, engine might just quit. I had loose float bowl screws cause the over rich problem I would loose 300 rpm when I pulled carb heat. I have flown in 100 degree heat and it would still drop RPM when I pulled carb heat, because it still raised the temp on the air going to the carb. In both situations you are going to change the air fuel mixture and you should see a drop in RPM. Now if temps where -40 carb heat might help atomize fuel and give you more power but at -30 I still got a RPM drop. Just to be clear when I say RPM drop I mean 75-100 RPM. If I drop more than that I suspect carb ice.
DENNY
 
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My standard "gauge" is if it'll WOT/run out to at least 2700, you're about right. Both my props will make about 2750 and I personally feel thats ideal.
 
as far as best pitch goes. I would think one would want to see max RPM in a Vx climb. That would be when you would need it most.
just my 2cents.
 
I'd love to de-pitch a couple inches for purposes of takeoff/climb, but then my groundspeed would have to be measured on a calendar. If only there were some way to magically control the prop from inside the cockpit........
 
I just repitched a 74/48 sensenich to 52. At 48 on a 150 hp engine it would spool up to 2700 climbing at 70 mph....climbed like crazy.
Static was 2570, a bit over TCDS. and WOT level was approaching 3 grand, it would reach 112 mph with 26 goodyears though.
52" on that prop works out right at the top limit for the type certificate. and WOT 2750 and 115 mph on a timed average.
 
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