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Flying a fixed pitch woody

K

Karbonkid

Member
This may be a day 1 question but I have an rv4 that I have been using as a cross country machine lately. It is an o360 with wood prop 68x72 IIRC.
I have been crusing ~8000 2400 rpm and getting around 155mph, I lean 100 to 200 ROP.
Would anyone cruise differently? Any reason not to be doing 2700?
Any difference in technique with different altitudes?
 
Our son has an old 1989 built short gear RV-4 with the early non swivel tailwheel.
It has the old style wheel pants. it makes about 182 MPH @ 2600 RPM.
It also vibrates the side skins at the back seat @ 2700 RPM, which is annoying.
Generally, we think it's slow for a 160 HP RV-4, but I don't think we would consider flying cross country @ 155 for a minute. Anopther 27 MPH makes a big difference in cross country travel. Taking the Cessna 172 becomes downright painful.
Unless your Miles Per Gallon goes way out of your comfort zone, why not enjoy the speed?
As for leaning, We lean out for engine power and smoothness.
Some one called that 'Rental Power'.
4 cylinder Lycomings with carburetor are well documented to have poor fuel distribution, so any leaning results will be variable. I like to think I'm doing it to prevent the 'too-rich' cylinder from being damaged, even as I worry about the 'too-lean' one. At 8000' I don't think the engine can generate enough power to be hurt by reasonable leaning.
 
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Forty-Eight is great

Karbonkid said:
This may be a day 1 question but I have an rv4 that I have been using as a cross country machine lately. It is an o360 with wood prop 68x72 IIRC.
I have been crusing ~8000 2400 rpm and getting around 155mph, I lean 100 to 200 ROP.
Would anyone cruise differently? Any reason not to be doing 2700?
Any difference in technique with different altitudes?
Click to expand...

KK,
I always use the 75% power number of 48 to figure RPM for cruise at various altitudes. 48 is the combination of the first 2 digits of RPM and MP added together to equal 48. Above 10K you may have 2700 RPM but 19" MP. No worries, its still 75% power.

Leaning with a carb is right out of the Lycoming manual. After climb allow the engine to cool a bit, then slowly lean mixture until a slight RPM drop, then richen until RPM restored and engine smoothness. You can crosscheck EGT, but it still works, after 67 years...LOP isn't recommended with a carb due to fuel atomization through the sump.

Also, I have my Catto Props pitched accordingly to allow rated RPM(2700) at 11,500'. 

V/R
Smokey

PS: there's absolutely nothing wrong with going slow at lower altitudes, 155mph/131knots below 5K is efficient. Above 5K I have found through a lot of traveling to use 65%-70% power. The RV simply performs by the book best @8500".
 
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Karbonkid said:
This may be a day 1 question but I have an rv4 that I have been using as a cross country machine lately. It is an o360 with wood prop 68x72 IIRC.
I have been crusing ~8000 2400 rpm and getting around 155mph, I lean 100 to 200 ROP.
Would anyone cruise differently? Any reason not to be doing 2700?
Any difference in technique with different altitudes?
Click to expand...

Is 155mph indicated or true airspeed? Big difference at 8000'.
 
Smokey nailed it. Just to elaborate on the math, a rough WAG at power is

RPM (100's) + MP (inches) = 48 is approx 75% power
45 is approx 65% power
42 is approx 55% power

You can interpolate as required. Correlates well with the Lycoming power curve for a typical O-320 installation.

Properly pitched, you prop should "wind up" with altitude. Ours is tweaked for 9K' to produce 2700 at WOT. Max altitude/RPM is a matter of taste, but most folks try to correlate with max/design cruise power (65% to 75%). For our plane, this works out to optimum cruise at 9 or 9.5K eastbound and 10 or 10.5 westbound (winds aloft permitting).

Peak EGT will correspond to the change in engine RPM Smokey mentioned.

A "no wind" economy cruise in the -4 works out to about 150 knots...it's worth the effort to dial in your prop to suit your flying lifestyle.

Cheers,

Vac
 
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Karbonkid said:
155 indicated

Next trip will be from lauderdale to New Orleans, I'll try some more rpm.
Click to expand...

Your RV-4 may not be the fastest but you are in the ballpark for cruising @ 2400 rpm. 155mph indicated @ 8000' is about 150kts TAS which is ok for 2400 rpm. You will probably get max performance if the engine is propped so it tops out at 2700 rpm @ 8000'.
 
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My practice....

All: interesting thread. Many years ago, Sport Aviation printed an article that was titled something like "Horses That eat, but don't Run". Worthwhile reading. The point was that Marvel best fuel distribution is at open butterfly. Partial butterfly distorts distribution maximally. So, the thought was that Marvel should be run wide open and use altitude to throttle. That has been my practice since. Course, I am happy to sit at 12.5 or 13.5 (carefully minding the O2 regs, maybe......). My present antique Pacesetter 200 runs out to 2650 at 8,500, and settles on 172-173 knots....using an antique O-320 narrow deck with 8.5 pistons....MTCents J
 
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