[ccsmith51]

I have my Sonex for sale and once it is gone I plan on purchasing a flying RV-4. There are many variations to choose from, and for the most part I can make a decision on what I want, based on my personal mission requirements.

However, there is one area I'd like some input on. If you had to choose between a 160 HP C/S or 180 HP F/P RV-4, will all else being equal, which would you opt for? My best friend Dan says I should opt for the extra HP and the lower maintenance and OH costs of the FP, and he usually knows what is right for me!!

But with such a major decision, I'd like to hear others thoughts, if anyone is so inclined.

Thanks, Chris

PS: I put this question here because I couldn't figure out how to put it in the RV-4 area...

 

[Sid Lambert]

If it were between a 180 FP and a 160 CS I'd chose the 180 FP. The 180 with a good prop will climb pretty darn well and outrun the 160 on the top end with room to spare. More time is spent in the cruise than in the climb for the RV.

Saying that... There is more to the airplane than the engine prop combo. Choose the best aircraft and you will be happy with either combo.

 

[Scott Hersha]

No question for me. I would pick the 160 with the constant speed. That is the best combination for that airplane. With all things being equal, the 180 with a perfectly pitched prop (for cruise) will only give you about 4 extra knots and lower overall fuel efficiency. Your useful load may be less, it will take a lot more runway to get airborne (although still pretty short - 800' vs. 400'), you won't climb as well, and formation, if so inclined, will be more challenging. Slowing in the pattern will require more planning, which also means short field arrivals will probably be a little less precise. Assuming it's a lycoming, the 160 can be STC'd for auto fuel if you can get it without ethanol. I know you don't need an STC, but lycoming doesn't recommend using auto fuel in any of their engines with more than 8.00:1 compression ratio - at least that's what they used to say..... who knows what they'll say with the for sure future demise of 100LL.

Good luck with your search. Your going to love the RV's, no matter which one you choose.

 

[Chino Tom]

YMMV

This could end up being another CS/FP looooong thread. I would opt for
the 160/CS. The 160/CS would give you the takeoff and climb performance
superior to the 180/FP, but not the cruise. However, the CS would allow you to
optimize cruise/fuel flow and make the best of the smallish 32 gallon fuel tanks
of the -4.

 

[Sam Buchanan]

I have my Sonex for sale and once it is gone I plan on purchasing a flying RV-4. There are many variations to choose from, and for the most part I can make a decision on what I want, based on my personal mission requirements.

However, there is one area I'd like some input on. If you had to choose between a 160 HP C/S or 180 HP F/P RV-4, will all else being equal, which would you opt for? My best friend Dan says I should opt for the extra HP and the lower maintenance and OH costs of the FP, and he usually knows what is right for me!!

But with such a major decision, I'd like to hear others thoughts, if anyone is so inclined.

Thanks, Chris

PS: I put this question here because I couldn't figure out how to put it in the RV-4 area...

This could end up being another CS/FP looooong thread. I would opt for
the 160/CS. The 160/CS would give you the takeoff and climb performance
superior to the 180/FP, but not the cruise. However, the CS would allow you to
optimize cruise/fuel flow and make the best of the smallish 32 gallon fuel tanks
of the -4.

Yep.

The forum has just gone through a pretty thorough thread on a similar topic:

http://www.vansairforce.com/community/showthread.php?t=60970&highlight=constant+speed

Whichever combination you choose, you will have an outstanding airplane.

 

[ccsmith51]

Hi Tom and Sam,

Thanks for the input. I'm sure that whatever -4 I purchase will be a fantastic plane.

Before I posted I followed the thread that Sam mentioned, but it was focused more on the pros / cons of CS vs FP on the same engine. I'm more interested in thoughts on a smaller engine with a CS vs a larger engine with FP.

Thanks, Chris

 

[Bill Wightman]

The RV4 has a lighter feel on the controls than the 6, 7, or 8 does. Its probably the best flying 2 seat RV, in terms of flying quality.

With that in mind, I'd suggest paying close attention to the weight and balance of any 4 under consideration. With pilot and full fuel, it should balance toward the front of the envelope. With about 200 pounds in the back seat, you'll be near the aft CG limit. Also, I'd be wary of empty weights over 1100 pounds, esp considering the GW is only 1500.

Perfect balance, and a 1000 pound empty weight would be what I'd want to buy.

 

[elippse]

With all things being equal, the 180 with a perfectly pitched prop (for cruise) will only give you about 4 extra knots and lower overall fuel efficiency..

Typically, the speed increase will be slightly more than the cube root of the HP ratio, which for (180/160)^1/3 is 4% or over 8 mph for a 200 mph speed. The reason for the "slightly more" is that the additional speed will decrease induced drag and the over-4% higher speed will give about 8% more dynamic pressure, MAP, into the intake. I would have to add my vote for the higher HP and FP prop since the cost of maintenance of the CS prop will offset any higher fuel cost. Consider, if you have to spend $2000 in prop maintenance over some period of time, that will buy at least 400 gallons of av fuel at $5/gal. Since the fuel-flow/time will be proportional to HP, it will be 12.5% higher per time, but due to the increased speed, will be only 8.2% higher per mile. At an SFC of 0.45 at 75% power, the 160 HP will be 9.15 gph and the 180 will be 10.29 gph, 1.14 gph difference, or 349.6 hours for 400 gal. It will actually be slightly more than this since a good FP will be more efficient than a CS, so it will give even slightly more speed. Not a hard decision, really!

 

[L.Adamson]

I would have to add my vote for the higher HP and FP prop since the cost of maintenance of the CS prop will offset any higher fuel cost. Consider, if you have to spend $2000 in prop

I'd like to know, if any RV owners have actually spent $2000 in C/S prop maintenance costs during their RV flight time. I suppose there could be a few..........but not as using it as a standard dollar amount for comparing costs. Unless a C/S owner feels that they absolutely must go by suggested maintenance schedules, then the only real maintenance is a grease job during the yearly condition inspection.

I vote for the C/S. I don't like F/P at all!

L.Adamson --- RV6A Hartzell C/S

 

[Jetj01]

I like Constant Speed

Having owned 3 fixed pitch RV-4s and now my Constant Speed RV-4FB, I vastly prefer the contstant speed/180 hp combo. The thing I like best is it makes arriving at a destination, especially a short field situation, a no brainer. Previously, with a FP prop and 180 hp, getting the plane slowed down, especially over obstacles on a short field, can be interesting.

Another thing I like with the CS prop is the lower noise level in cruise. I routinely slow my prop down to 2350 rpm and the noise level and engine smoothness is much better. While mentioned is made of the cost of a CS prop, I have never put any money into any I have ever owned other than original purcahse...knock on wood. However, I think turning that RPM down using the CS is saving wear and tear on my motor. BTW, I caution anyone about 'putting in grease' during the annuals. If it is working, leave it alone!

Finally, in both formation flying and aerobatics, a CS prop makes both easier. Easier rejoins and speed control and easier nose down aerobatics with RPM control.

Ooops, another: CG is so much easier to control with the CS prop and passengers and baggage. Last week I had a 230 pounder in back and there was no problems with aft CG.

My airplane, fully painted and interior finished weighed in at 1030. So yes, they can be built quite light and I think the cost is worth it. I did enjoy both my wood propped RV-4 (X2) and my Sensenich equipped RV-4, but when it came time to build my own, the previous three purchased, the CS was my choice. no regretts.

Tailwinds,

 

[L.Adamson]

BTW, I caution anyone about 'putting in grease' during the annuals. If it is working, leave it alone!

Check the owners manual for specifics. I don't have mine in front of me, but looking up one on the web,..........it stated 12 months, or 100 hrs (whichever comes first). At most, you'll only be adding enough until grease comes out the fitting on the opposite side (and there is even a limit as too how much can be put in). It's not a complete grease change.

L.Adamson --- RV6A

 

[Jetj01]

That is what the manual says...

L.

Your right, that is what the manual says. So on my Rocket, at the first condition inspection, I and the A&P did exactly that, per the manual. Then it started ever so gently 'weeping' a small spray of grease on the base of the blades and actually making it to the front of the canopy every flight. Like a gently mist of grease. Of course I was upset because the prop was a new, rediculously expensive Hartzel. The A&P thought that maybe we had blown the seals.

The following day I called Hartzel to find out the process to send in the prop for what ever was needed to be done. The technician asked me if I had recently serviced the prop. I said I had and he asked me why...had it been malfunctioning? I replied no and he said that in general, if the prop is working normally and not experiecing any leaks to NOT SERVICE IT ANNUALLY. He also said that in a few more flights, that excess grease would be done weeping and I would be back to normal ops. He was right. And I have left well enough alone since then.

Hey, I'm just a simple homebuilder! Your mileage may vary

 

[L.Adamson]

L.

Your right, that is what the manual says. So on my Rocket, at the first condition inspection, I and the A&P did exactly that, per the manual. Then it started ever so gently 'weeping' a small spray of grease on the base of the blades and actually making it to the front of the canopy every flight. Like a gently mist of grease. Of course I was upset because the prop was a new, rediculously expensive Hartzel. The A&P thought that maybe we had blown the seals.

The following day I called Hartzel to find out the process to send in the prop for what ever was needed to be done. The technician asked me if I had recently serviced the prop. I said I had and he asked me why...had it been malfunctioning? I replied no and he said that in general, if the prop is working normally and not experiecing any leaks to NOT SERVICE IT ANNUALLY. He also said that in a few more flights, that excess grease would be done weeping and I would be back to normal ops. He was right. And I have left well enough alone since then.

Hey, I'm just a simple homebuilder! Your mileage may vary

In my case, the prop had sat brand new & unused for 12 years. The rubber caps that cover the zirc fittings still looked new.

After being run a few hours, there was a light spray on my rightside of the windscreen. We put some new grease in, as the old grease had become a bit seperated. The light spray stopped. I've done two condition inspections since then, and just added enough, to where it comes out the opposite side where the zirc fittings are removed. In my case, it's worked well. The prop had cycled perfectly since day one, and light grease drops don't hit the windscreen. I have observed a small spray of weeping grease that you've described on one blade during this last inspection.

L.Adamson --- RV6A

 

[elippse]

Additional considerations

First off, I must say that I don't favor any particular solution. CS is good, FP is good; fixed gear is good, retract is good; magnetos are good, so are EIs; tail-dragger good, nose dragger good; aluminium good, so is composite! Choose what is best for your application. But to make your decision, disregard those opinions that say "only such and such is good", and arm yourself with good data and information, not opinions.
As I wrote, a 180 HP v a 160 HP will have at least 4% more speed; but a high-efficiency FP wil be 3% to 6% more efficient than a CS, so it will give 1% to 2% more speed, which means that with the HP increase, a 200 mph plane with 160 HP and a CS will see at least 210-212 mph with a high-efficiency FP and 180 HP, not counting reduced induced drag and increased MAP. And on a long trip it will burn about 6% to 7% more fuel. Now as far as ROC, let's look at some data. Assuming both props perform at the same efficiency in a climb, say 82%, the CS will have the advantage of being able to develop rated rpm, whereas the FP will turn about 2350 rpm, but with 20 HP more. So the 160-CS will be at 160 X 0.82= 131.2 thrust HP, and the 180-FP will be at 180 X 0.82 X 2350 / 2700=128.5 thrust HP, a 2% difference! An RV-6A requires about 31.6 thrust HP at best L/D. The 180 will have 96.9 thrust HP for climb, which at 1450 lb would give 2204 fpm, whereas the 160 will have 99.6 thrust HP to give 2266 fpm! Not much difference, huh?
I know there are a lot of CS operators who say that their props cost nothing to operate, but I've known several people who have had to spend several thousand dollars to comply with ADs and prop wear. A CS prop, and associated controller, have many parts that are subject to wear and must be inspected on some time period, whether its 100 HR or 2000 HR or 5 years or whatever. To ignore this or brush it aside is not to give an unbiased comparison. Only by setting up a page with real pros and cons listed for each case can you arrive at a reasonable conclusion. Keep in mind your mission, too. If it's lots of cross-country, an FP might be better; if its aerobatics and lots of take-off and climbs, a CS might be better. Minimize bias and maximize data for your choice.

 

[Toobuilder]

...Keep in mind your mission, too. If it's lots of cross-country, an FP might be better; if its aerobatics and lots of take-off and climbs, a CS might be better. Minimize bias and maximize data for your choice...

I come up with the opposite: If I did lots of short flights, I could "live with" a FP, but dealing with 2750 RPM for hours on end going cross country would be intolerable.

In any case, cross country or touch and goes, I require 2750 RPM on every takeoff and the ability to turn 2350 at max cruise, WOT. There are a variety of fixed pitch props that excel at one or the other condition: but no fixed pitch can do both in the same flight.

In my opinion, the only time a FP makes sense on a ?sport? aircraft is if its speed range is very limited ? such as a Cub, which essentially has just one speed for takeoff, cruise and landing (~ 65 MPH). Something like an RV with a HUGE speed margin simply leaves too much performance ?on the table? for my taste.

So to answer the OP?s question ? Even if the 180/FP outperformed the 160/CS in climb AND cruise, I would have a hard time not seeing 2750 RPM at takeoff, and a REALLY hard time listening to it buzz away at 2750 during cruise. The economy gains and the comfort of the 160/CS would negate by far any slight performance advantage of the bigger engine. Just my .02

 

[L.Adamson]

I come up with the opposite: If I did lots of short flights, I could "live with" a FP, but dealing with 2750 RPM for hours on end going cross country would be intolerable.

In any case, cross country or touch and goes, I require 2750 RPM on every takeoff and the ability to turn 2350 at max cruise, WOT. There are a variety of fixed pitch props that excel at one or the other condition: but no fixed pitch can do both in the same flight.

I thought & do the same thing. I dial back to 2350 on those long cross countries, and am rewarded with the pleasant sound of an easy going engine.................instead of a wound up one!

L.Adamson --- RV6A, Hartzell CS

 

[Sam Buchanan]

I thought & do the same thing. I dial back to 2350 on those long cross countries, and am rewarded with the pleasant sound of an easy going engine.................instead of a wound up one!

L.Adamson --- RV6A, Hartzell CS

I usually cruise at 2350-2400 rpm, too. Even this leisurely engine speed yields 150 kts TAS (~7 gph) with my little O-320 and (gasp!) fixed-pitch prop.

 

[Steve Sampson]

I have an O-320 c/s -4 so perhaps this will help.

To me the extra cost of the c/s prop ( a 2-blade MT) is a bit of an extravagance. I can justify it because I come and go from a 1020' one way strip. The going is not the problem, but without the c/s prop the coming/stopping would be problematic for me. The book says you can do it every day, but with me at the controls I need all the help I can get.

The advantages of the c/s are obviously starting and stopping but also the cruise. I tend to run along at 2000 rpm, 21lph and 22". Its very relaxed. (IT has P-mags by the way which probably make the 21lph possible.)

For me the advantage of the 180fp would not be all out speed, I never run around flat out, but the reduced capital and maint cost. Without my short strip it is the direction I would have gone.

The 180 -4 is really a bit overpowered and you would need to keep an eye on the rough air limit.

The MT is undoubtedly nicer, though if it was a Hartzell you would I think need some weight in the baggage cmptmt to balance it.

 

[Toobuilder]

I usually cruise at 2350-2400 rpm, too. Even this leisurely engine speed yields 150 kts TAS (~7 gph) with my little O-320 and (gasp!) fixed-pitch prop.

I'm guessing you are not WOT. If the throttle blades are anything but wide open, the efficiency of the engine goes right out the window.

 

[Sam Buchanan]

I'm guessing you are not WOT. If the throttle blades are anything but wide open, the efficiency of the engine goes right out the window.

Correct, WOT would be the prop limit of 2600 rpm.

You're saying 150 kts @ 2350 and 7 gph isn't efficient???????

 

[Toobuilder]

Correct, WOT would be the prop limit of 2600 rpm.

You're saying 150 kts @ 2350 and 7 gph isn't efficient???????

Sure it's good, but relatively speaking, not as good as it could be. Any "throttling" of an air pump reduces its efficiency, so if you are restricting the airflow by partially closing the throttle, you are leaving something on the table. You are in effect dragging a 320 cubic inch engine around, but by choking it down with the throttle, only flying with the benefit of 250 cubic inches (or whatever).

Maximum efficiency of an internal combustion engine is realized with the induction system unobstructed.

Charles Lindbergh illustrated this when he used to fly P-38 combat missions as an "advisor"... He would consistently land his airplane following a sortie with plenty of fuel remaining, while the rest of the formation was on fumes. His technique was high MP, low RPM, and LOP EGT. Since he flew the same duration, speed, and equipment as the others, his relative efficiency was higher.

 

[Sam Buchanan]

Sure it's good, but relatively speaking, not as good as it could be. Any "throttling" of an air pump reduces its efficiency, so if you are restricting the airflow by partially closing the throttle, you are leaving something on the table. You are in effect dragging a 320 cubic inch engine around, but by choking it down with the throttle, only flying with the benefit of 250 cubic inches (or whatever).

Maximum efficiency of an internal combustion engine is realized with the induction system unobstructed.

Charles Lindbergh illustrated this when he used to fly P-38 combat missions as an "advisor"... He would consistently land his airplane following a sortie with plenty of fuel remaining, while the rest of the formation was on fumes. His technique was high MP, low RPM, and LOP EGT. Since he flew the same duration, speed, and equipment as the others, his relative efficiency was higher.

All true. However, I haven't seen much interest in routinely flying RV's at full throttle. All the instances I recall are pilots who are much more interested in efficient and satisfactory operation at less than full throttle. The real world puts us in situations where we need to conserve fuel or match our speeds with the other planes in the gaggle.

 

[elippse]

High efficiency with an FP

One of the things I'm testing is the use of carb heat to reduce power at WOT. In initial tests of my new three-blade at 14,880' dalt, the prop is giving me 3020 rpm vs 3000 rpm design. By use of carb heat, I got the rpm down to 2900, could lean more, and FF was 5.4 gph. When you increase induction temperature, power goes down due to reduced inlet charge density, the so-called volumetric efficiency, BUT, the engine efficiency goes up by the sq. rt. of the increased temperature ratio*. So you guys with FP props can drop your rpm but still leave the throttle wide open so that you don't get the HP drop across the partial pressure drop of the throttle plate, and get even better SFC and lower FF by judicious use of carb heat. Now you can have the higher efficiency of the FP and still lower rpm if desired at even better efficiency! You've been taught all along that carb heat is BAD, and you look upon any use of it except to prevent carb ice as a use that must be lived with, but only to prevent carb ice. Sure!, when taking off you want as much power as possible, so if you have carb heat on for landing and must go around or a touch-and-go, turn it off. But it can actually be a good thing!

* For those who want to see how that is possible, look at the little formula in the upper left of your Lycoming's HP-MAP-RPM chart, that shows power is sq.rt.(( 459.7 +Ts)/(459.7 + T)) where T is OAT and TS is 59F. The increased efficiency shows up because volumetric efficiency, charge density, is directly proportional to (459.7 + Ts)/(459.7 + T). the square root shows that the engine efficiency goes up with increased temp! For those who are still not convinced, look it up in C.F. Taylor!

 

[Toobuilder]

All true. However, I haven't seen much interest in routinely flying RV's at full throttle. All the instances I recall are pilots who are much more interested in efficient and satisfactory operation at less than full throttle. The real world puts us in situations where we need to conserve fuel or match our speeds with the other planes in the gaggle.

I'm wondering if you are thinking full throttle = 100% (or maximum, for altitude) output? Maximum engine output is found by combining WOT and max RPM. Reducing one or the other also reduces engine output, but reducing only the RPM is a more efficient use of the engine

Engine efficiency for a given output is maximized with an unthrottled engine. If you must reduce power output, simply pull the prop back.

And yes, I have a great deal of interest in efficient RV (and Hiperbipe) operations... Thats why the throttle is wide open on takeoff and not touched again until I'm headed downhill somewhere near my destination.

 

[Sam Buchanan]

I'm wondering if you are thinking full throttle = 100% (or maximum, for altitude) output? Maximum engine output is found by combining WOT and max RPM. Reducing one or the other also reduces engine output, but reducing only the RPM is a more efficient use of the engine

Engine efficiency for a given output is maximized with an unthrottled engine. If you must reduce power output, simply pull the prop back.

I understand what you are saying.

And yes, I have a great deal of interest in efficient RV (and Hiperbipe) operations... Thats why the throttle is wide open on takeoff and not touched again until I'm headed downhill somewhere near my destination.

In a dozen years of active involvement in the RV community, I don't know if I've ever heard of anyone routinely using the throttle procedure you are describing. Not saying it is never done, but it sure isn't customary.

Guess you will just have to stand alone as the most efficient flyer among us.

 

[L.Adamson]

I understand what you are saying.



In a dozen years of active involvement in the RV community, I don't know if I've ever heard of anyone routinely using the throttle procedure you are describing. Not saying it is never done, but it sure isn't customary.

Guess you will just have to stand alone as the most efficient flyer among us.

I read about it it, tried it, but it doesn't seem to work out for me, as I look at the fuel flow. I end up throttling back, when the intention is fly with other RV's at slower speeds than WOP. My C/S has the prop restriction of 2000-2250, & I prefer not to be under 2000.

L.Adamson --- RV6A

 

[Bill Wightman]

WOT here!

In a dozen years of active involvement in the RV community, I don't know if I've ever heard of anyone routinely using the throttle procedure you are describing. Not saying it is never done, but it sure isn't customary.

Guess you will just have to stand alone as the most efficient flyer among us.

I fly a profile very close to what Toobuilder is describing. Basically, its WOT from takeoff to top of descent. I could understand running at less than WOT if under 8000 MSL or so, but I almost never cruise that low.

You can run WOT at altitudes commensurate with 70% power or so for the life of the engine. That's the way to save fuel, and keep the TAS's up at the same time.

 

[Chino Tom]

I too use Toolbuilders same profile when going somewhere. But I'm always
8,500+. I have tried the carb heat thing also as well as just backing the
throttle back a bit (1/2 inch max) to try and equal out the EGT's. Mostly
it just changes which peaks first. The RPM is adjusted for desired % power
at WOT. I lean until the first cylinder peaks. Any more and the engine begins
to run rough (maybe two cylinders on the lean side) and richen
until smooth (the first peaked cylinder back at peak). I have an old fashioned
carburator.

Of couse when hamburger hunting locally at low altitude the throttle is
not WOT but pulled back to loafing mode (20" or so).

 

[Toobuilder]

In a dozen years of active involvement in the RV community, I don't know if I've ever heard of anyone routinely using the throttle procedure you are describing. Not saying it is never done, but it sure isn't customary...

This proceedure is not RV specific. If you are going cross country, it is simply how C/S aircraft are flown, period. My dad flew our Mooney this way, my flight instructor taught me to fly his Cardinal this way... Everyone I know who has a C/S does this. If there is a blue knob in the airplane and you are going cross country, the throttle is wide open. Cruise speed control is determined by the blue and red knobs.


There are certainly times that I'm throttled, like if I'm just out tooling around turning avgas into noise, but if it is my mission it to get from A to B efficiently, then it's WOT and LOP.

 

[elippse]

Not saying it is never done, but it sure isn't customary.

Guess you will just have to stand alone as the most efficient flyer among us.

Number me in that bunch also, 'cause when I go cross-country at 11,500' or 12,500', that throttle stays all the way in through take-off, climb, and cruise until I descend. If I'm just puttering around locally, then I pull it back to around 2500.

 

[Toobuilder]

I read about it it, tried it, but it doesn't seem to work out for me, as I look at the fuel flow. I end up throttling back, when the intention is fly with other RV's at slower speeds than WOP. My C/S has the prop restriction of 2000-2250, & I prefer not to be under 2000.

L.Adamson --- RV6A

Understand that I'm not talking about formation, aerobatics, sightseeing, ect. I'm talking about using the airplane as a transportation device. You know, take off, go high, fly in a straight line for 3 hours, land far from home... On such a mission you might be in a hurry (or bucking a headwind) and you might select 2700 RPM and slightly ROP for maximum power, or perhaps you might be looking for economy which would put you at 2300 and as LOP as your ignition can support. On such a mission you are only making fine adjustment to power output with the prop/mixture. In any case, you want that air pump up front doing just that - pumping air. How much?

...as much as it can get.

 

[L.Adamson]

Everyone I know who has a C/S does this. If there is a blue knob in the airplane and you are going cross country, the throttle is wide open. Cruise speed control is determined by the blue and red knobs.

And it seems like so many people that I know, don't do it this way. I've tried it several times, but don't see an advantage fuel flow wise. When I'm flying as a lone aircraft, I'll often go WOT & pull the rpms back; but I haven't been successful doing that, when trying to maintain speeds with slower aircraft, such as RV9's . My flights usually range from 7500 - 10500 msl.

L.Adamson --- RV6A Hartzell C/S

 

[L.Adamson]

Understand that I'm not talking about formation, aerobatics, sightseeing, ect. I'm talking about using the airplane as a transportation device. You know, take off, go high, fly in a straight line for 3 hours, land far from home... On such a mission you might be in a hurry (or bucking a headwind) and you might select 2700 RPM and slightly ROP for maximum power, or perhaps you might be looking for economy which would put you at 2300 and as LOP as your ignition can support. On such a mission you are only making fine adjustment to power output with the prop/mixture. In any case, you want that air pump up front doing just that - pumping air. How much?

...as much as it can get.

Okay................I was typing my new reply as you did this one. It about covers it.

L.Adamson