Cruise Prop?

[Dayton Murdock]

Hi All
Lately I have been trying to improve the efficiency of my -4 at cruise settings. I have change the horizontal incidence 1/8?shim to a ??shim under the spar, clean up some gaps, waxed the Red paint.
At 8500? I run 2500 RPM?s 7.0GPH 135IAS about 155kts-160kts across the ground. 65degs F, 10600 density alt. What I need to know is the manifold pressure for an O320 should under these conditions? I thinking of changing my prop pitch for better cruise performances.

[frankh]

running your O320 lean of Peak Dayton?...I mean now that you found the mixture control and all...

[pierre smith]

....than you're doing.

This engine only makes 75% at WOT or 2700 revs at 7500 DA. At 2500, I suspect you're making 55-60% power. At 7500' you probably only get 22 1/2" MAP anyway at WOT.

Regards,

[Dayton Murdock]

Hi Peirre

If I run WOT a 8500' it will over rev to 2850rpm 165 IAS ish 170-175kts across the ground.to get 2500 rpm it is at 1/2 throttle and 50deg lean of peak. What i am after is 150kts and 6.0 -5.5 gph fuel burn.
will my -4 be faster at sea level?

[Radomir]

If you can hit 2,850 RPM at 8,500 ft, your prop is badly mis-pitched.. I'd personally be looking for a different prop

[BruceMe]

Originally Posted by Dayton Murdock

Hi Peirre

If I run WOT a 8500' it will over rev to 2850rpm 165 IAS ish 170-175kts across the ground.to get 2500 rpm it is at 1/2 throttle and 50deg lean of peak. What i am after is 150kts and 6.0 -5.5 gph fuel burn.
will my -4 be faster at sea level?

8k DA is the sweet spot. Lower or Higher is slower or more gas, take your pick. You might get 6gph 150KTAS. I get roughly that on my fastback -4. I think your prop is pitched perfect for your goal. It should rev-out, or you're pitched for speed.

[rvbuilder2002]

Originally Posted by Radomir

If you can hit 2,850 RPM at 8,500 ft, your prop is badly mis-pitched..

Maybe... and maybe not.

As we all know, a fixed pitch prop on an RV is a compromise . How well a particular prop is matched to the airplane depends on what your primary desire for performance is. A lot of people think that if the prop doesn't act like it did on the C-172 they used to fly, then it is mismatched. The prop on most certificated fixed pitch airplanes is pitched so that in most all conditions, a pilot can not over speed the engine. Not a bad thing for low time pilots, but it is a serious impact on performance. A related issue is that these airplanes are expected to spend most of there flying at lower altitudes (6000" and below). Proping an RV this way will produce a poor performing airplane, but there are a lot of them out there flying this way (we all see the prop sleeves on RV's at fly-ins with a cruise speed of 175 MPH listed)

The prop on this RV-4 is probably a perfect match if the desire was for higher altitudes max speed cross country cruising. If flown at 12,500 at full throttle it will probably turn up to between 2650 and 2700 at 7.0 to 7.5 GPH (properly leaned) with a TAS of about 170+ kts. This is probably somewhere between 55% and 60% power. A secondary benefit of a fixed pitch prop like this, which at first seems under pitched, is you also get better takeoff and climb performance at lower altitudes (under 5000 ft). This makes it a climb prop and cruise prop. in the same prop.
If the desire was for better performance at lower altitudes, then more pitch is probably needed.
A very good resource for information on fixed pitch props used to be a section that was included in the construction manual related to propeller selection (section 15 I think). I don't know if it is still included in the current issue construction manual or not, but it would be worth getting a hold of by anyone dealing with fixed pitch propeller choices.

[Tony Spicer]

Originally Posted by Radomir

If you can hit 2,850 RPM at 8,500 ft, your prop is badly mis-pitched.. I'd personally be looking for a different prop

You think? Have a look at the first two posts here:

http://www.vansairforce.com/communit...ad.php?t=28210

Ellipse is Paul Lipps. More on him here:

http://www.biplaneaerobatics.com/elippse

Tony

[Kyle Boatright]

Originally Posted by rvbuilder2002

Maybe... and maybe not.

As we all know, a fixed pitch prop on an RV is a compromise . How well a particular prop is matched to the airplane depends on what your primary desire for performance is. A lot of people think that if the prop doesn't act like it did on the C-172 they used to fly, then it is mismatched. The prop on most certificated fixed pitch airplanes is pitched so that in most all conditions, a pilot can not over speed the engine. Not a bad thing for low time pilots, but it is a serious impact on performance. A related issue is that these airplanes are expected to spend most of there flying at lower altitudes (6000" and below). Proping an RV this way will produce a poor performing airplane, but there are a lot of them out there flying this way (we all see the prop sleeves on RV's at fly-ins with a cruise speed of 175 MPH listed)

The prop on this RV-4 is probably a perfect match if the desire was for higher altitudes max speed cross country cruising. If flown at 12,500 at full throttle it will probably turn up to between 2650 and 2700 at 7.0 to 7.5 GPH (properly leaned) with a TAS of about 170+ kts. This is probably somewhere between 55% and 60% power. A secondary benefit of a fixed pitch prop like this, which at first seems under pitched, is you also get better takeoff and climb performance at lower altitudes (under 5000 ft). This makes it a climb prop and cruise prop. in the same prop.
If the desire was for better performance at lower altitudes, then more pitch is probably needed.
A very good resource for information on fixed pitch props used to be a section that was included in the construction manual related to propeller selection (section 15 I think). I don't know if it is still included in the current issue construction manual or not, but it would be worth getting a hold of by anyone dealing with fixed pitch propeller choices.

Scott,

How are/were the "company" FP aircraft set-up? Most of the FP guys I know run 2350-2500 rpm in cruise. I'm sure there are folks out there turning 2700 rpm, but I don't know anyone who runs the engine that fast in cruise flight.

Thanks,

[rvbuilder2002]

Originally Posted by Kyle Boatright

Scott,

How are/were the "company" FP aircraft set-up? Most of the FP guys I know run 2350-2500 rpm in cruise. I'm sure there are folks out there turning 2700 rpm, but I don't know anyone who runs the engine that fast in cruise flight.

Thanks,

Van's only has one airplane with a fixed pitch propeller...The RV-7 used by Mike Seager for transition training. It is pitched rather light to have good climb and high altitude cruise but since it is a Sensenich O320 metal it is limited to 2600 RPM.

I admit that the majority of fixed pitch RV pilots run at these RPM's. I think it is because they are afraid to run faster. Generally it is believed that higher RPM's cause a higher rate of wear (and it probably does) but I believe it is a very small percentage difference. RV pilots probably cause more unnecessary wear doing cold weather starts without proper preheat, etc. than they would running at higher RPM's.

My first RV-6A that I began flying in !993 had an O-320 160 HP with a fixed three blade wood prop from Performance propellers. I ran it at 2650-2700 for 650 hours and it was still running great when I sold it. To this day it is the smoothest running RV I have ever flown, particularly at these high RPM's. Judging by smoothness it was happiest turning fast. I routinely flew cross country truing 165 Kts along side of 180 HP RV's that would be turning slower RPM's and would use more fuel than me on any given crossed country leg.