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confused about my prop

tmbg

tmbg

Well Known Member
Hi folks,

I have an RV6 that I bought to fly while I finish up my 7, and it's got an O-320-nothing 150hp engine with a sterba 68x72 wood prop.

I turn 2500rpm static at 2000' DA, I can easily run over redline in cruise, and max cruise at 8000' DA at 2700rpm is around 146KIAS.

I've seen other posts on here where people said they have 150hp O320s with similar props and they turn far less static. I even saw someone with the same sterba 68x72 on a 150hp, and turned 2250rpm.

The only conclusion I can come to is that either my engine is producing considerably more than rated power, or my prop isn't the marked pitch.

I'm trying to figure out what sort of prop I should be looking for to make this airplane faster, and also it's tail heavy; I can't get it in aerobatic limits as is, and a metal prop or even a CS prop would go a long way toward helping the CG.
 
There's another thing you haven't mentioned. Have you checked you tach? If it is a mechanical analog tach, those things are notorious for being inaccurate.
It's not at all unusual for them to be 200 rpm off.
 
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yeah, that's been on my mind as well. I have a friend bringing an optical tach out tonight so I can rule that possibility out!
 
Perhaps. Does anyone have a procedure for measuring/estimating the pitch of a prop?
 
I have the same prop on my -4 with a 0320/160. 2500 rpm on take off and I think my numbers are pretty much the same as yours. IMO, this setup is a good and smooth climber but lacking for more speed in cruise.
 
146 KIAS at 8000' DA equates to about 190mph, which is consistent with Van's performance spec for a 150HP RV-4.

I had a similar set-up on my RV-4 with a Pacesetter 200 which would easily overspeed at low altitude. I now have a Catto that is pitched much coarser but won't overspeed at low altitude and has a comfortable WOT cruise RPM at high altitude, but the trade-off is a lower static and climb RPM with resulting reduced performance in those phases.

A FP prop will always be a compromise, so it's mainly a matter of finding the sweet spot for your kind of flying. A metal prop is more efficient, but the Sensenich will be subject to a 2600 RPM limit. Their ground adjustable would allow you to play around and find your best compromise, but they don't come cheap.

a 72" pitch from one manufacturer won't neccesarily equate to a 72" pitch from someone else. Nominally it's the Tan of the blade angle at the tip multiplied by the circumference (2*PI*D), but there seems to be some variation in practical measurement and the airfoil section can influence the effective pitch relative to nominal.
 
You really need a manifold pressure gauge to know if you are loading the motor.
RPM alone is just not enough to let you know what is going on.
 
I put an optical tach on it last night, and confirmed that my tach is correct. Optical tach showed 2520rpm static, mechanical was right at 2500
 
length?

have you actually measured the length?
most common mod to a prop is to shorten it.
a previous owner may have chopped off 2" because it wasn't revving enough on a different engine?...or...heaven forbid, someone took a chunk out of the end when they hit something, and this was the 'repair'.

perhaps Mr. Sterba could shed some light on this? or for $600, try a different pitch? ;)
 
I haven't actually measured the length yet, but I did have it off the airplane and sitting next to an AD 68x78 prop a couple weeks ago, and they were the same length, so I'll be surprised if it's shortened.

I will measure the length, a friend has a blade protractor so I'm hoping to be able to measure the pitch, and I'm going to try to install a manifold pressure gauge to try to make sense of this!
 
Don't be too quick to accept or condemn by measuring pitch unless you have another prop from the same manufacturer to compare.
Every prop manufacturer measures pitch a little differently.
 
Hello Ian,

What is your density altitude, weight and best ROC that you see when your climbing out at with the 2500 RPM?

2700 RPM at 8000 ft DA is pretty much spot on for 75% power and at 190 mph TAS is bang on for Vans numbers.

Tim
 
Mel said:
Don't be too quick to accept or condemn by measuring pitch unless you have another prop from the same manufacturer to compare.
Every prop manufacturer measures pitch a little differently.
Click to expand...

I'm more looking for gross deviations in pitch... posts like this make me think there has to be something drastically off somewhere:

Daver
a couple of data points
I know the following examples are not RV4s but they might give you some additional insight.

My -9A 150 HP 68x72 Sterba prop gets about 2200 static and about 2450 in cruise but I don't have the wheel pants on yet. I'm thinking I'll pick up some cruise RPM with those installed.
Click to expand...

a 300rpm difference in static rpm with ostensibly the same engine and prop... obviously density altitude is a factor there, but I can't imagine it being THAT much of a factor!
 
tmbg said:
a 300rpm difference in static rpm with ostensibly the same engine and prop
Click to expand...

You could get a difference in static if the airplanes were headed differently into the wind, or the surfaces winds were stronger or lighter or the engines were leaned differently.
 
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