Choosing pitch for high altitude operation.

I have found this site to be very instructive. I am not an rv owner but when I grow up someday I will have one. I currently fly a rans s-16 with an io-320. I have been struggling with low rpm since the day I purchased it. I have gone through all the "engine" stuff to make sure all that is ok (see post in traditional engines). I understand the effects of elevation. Using the standard 3% loss per 1000 feet of elevation puts my 160 hp engine at 127 hp at my base altitude.

Here's the quick and dirty. The plane came with a sterba 68-74 prop. The engine at sea level would only get 2500 rpm. I flew it home to Wyoming where my airport elevation is 7200 feet. The best rpm I could get was about 2450 straight and level full throttle at my elevation. I ordered a new prop from Ed sterba that is a 68-70. I tried that prop and could only get 2500 rpm full throttle s&l. I contacted ed and he offered to re pitch for free. I sent him the 68-74 and he re pitched it to a 68-64. I was expecting to gain a lot of rpm with the 64 pitch. I tried that prop and gained zero rpm increase. I fly at 10,000 feet a lot and need the rpm.

So here are my questions:

Is the sterba prop my problem? (Design, pitch, airfoil)
How does elevation affect rpm?
Should I be pitching for a 125 hp engine?

I can buy a Catto prop off a o-290 for a good price, just wondering if that the way I should lean?

1) what is the RANS supposed to make in knots? 2) What RPM does the 290 make with this Catto?

Rans posts their numbers in mph at sea level. So with an io-320 it's cruise is 172 mph, vne is 185. My average cruise is 140 mph at 2500 rpm (which is my max rpm at this point) That is ground speed off a gps.

The catto is a 68-67 rated for max 2750 rpm. It was made for a 135 hp 0-290.

With the 320 engine, the ground adjustable carbon fiber props from Sensenich or Whirlwind might be worth exploring. With these props you can custom-tune the blade pitch to your airframe and engine.
I love my Sensenich ground adjustable prop on my RV-6. It's definitely not cheap, though, but I've found a pitch setting that works extremely well.

In my opinion 10" of pitch change should have yielded some difference in performance, Static RPM, WOT, something. When making that big of a change the propeller looses quite a bit of surface area, in turn degrading performance. Give me a call, I'm happy to discuss this. 713.417.2519

TAS?

I couldn't help but notice that you posted GPS groundspeed. What is your True Air speed? Or did you fly a GPS triangle and the 140 is your TAS?

Vic

vic syracuse said:

I couldn't help but notice that you posted GPS groundspeed. What is your True Air speed? Or did you fly a GPS triangle and the 140 is your TAS?

Vic

Click to expand...

Hey Vic, I think I met you and your wife at osh last year. Where you hanging out over in the bonanzas to osh tent? I'll be there again this year.

Anyway, my airspeed gauge is in mph so I had to do some converting from knots then back to mph in my e6b. I typically see around 125 mph IAS. This works out to be about 144 mph TAS. I am honestly not real good at watching my airspeed gauge because my gps gs tells me when I'm actually going to get there.

I had an interesting problem that might relate;
I have a Bellanca Cruisemaster, which has a Continental O-470 and constant speed prop. The tachometer wouldn't read full RPM when the prop control was full forward for take off. The Viking forum members advised that aircraft tachometers aren't very accurate, and I should try an electronic one that you set on the dash and it counts the prop blade pass frequency.
So I bought one, and sure enough, the engine governor is set spot on, but the tachometer is 'way off.
So you might want to verify your engine speed before you sink any more effort into this. Borrow one for a flight, but if you buy one, you will use it on every plane you own or fly in.
Here is a link:
http://www.aircraftspruce.com/catalog/topages/trutach.php

SHIPCHIEF said:

I had an interesting problem that might relate;
I have a Bellanca Cruisemaster, which has a Continental O-470 and constant speed prop. The tachometer wouldn't read full RPM when the prop control was full forward for take off. The Viking forum members advised that aircraft tachometers aren't very accurate, and I should try an electronic one that you set on the dash and it counts the prop blade pass frequency.
So I bought one, and sure enough, the engine governor is set spot on, but the tachometer is 'way off.
So you might want to verify your engine speed before you sink any more effort into this. Borrow one for a flight, but if you buy one, you will use it on every plane you own or fly in.
Here is a link:
http://www.aircraftspruce.com/catalog/topages/trutach.php

Click to expand...

I should have probably mentioned that I did check this already with a digital tach. My tach is remarkably accurate. It was only off by 10 rpm. So that's already been checked.

Tonight I put my el cheapo warp drive propellor pitch tool on the tip of the propellor that was re pitched from 68-74 to 68-64 and it dialed in right at 74 degrees at the tips. I'm not 100% sure I did it correct but it seems to me that the re-pitch may have not actually been done correctly. Propman Frank from performance propellers USA offered to check my prop pitch for free. I'm probably going to send it to him and see what's really going on with my pitch.