At what cost speed?

[A64Pilot]

1 minute ago, bluehighwayflyer said:

Again, I agree, although I don’t know what most think.  My need for oxygen is mostly measured by the headache I get at the end of the flight.   Under 10,000 feet I don’t and above 10,000 feet I do. :-)

Yeah, you definitely need O2 then, really big indicator you have sleep apnea is waking up with a headache, due to lack of O2. Headache can be dehydration of course, but if it’s that O2 doesn’t help.

I think but don’t know, but those O2 concentrators are particularly good for lower altitudes if you don't want to mess with bottles.

Personally I don’t like the Cannula’s, I just fly low enough not to need it and leave the O2 in the wall locker. Mask is even worse. Cannula dries up my sinuses and nose.

[Shadrach]

3 hours ago, bluehighwayflyer said:

Correct.  I never flew my J LOP at that high a power setting, which of course would be 75 percent power at 10 GPH LOP, but 160 KTAS at 10 GPH LOP would probably be possible down low and some J owners have probably done it successfully while air racing or breaking in a new engine.  I found 145 KTAS at 8 to 8.5 GPH LOP in the 8 thousand foot range to be the sweet spot for cruising in my J, personally.  This was after a lot of experimentation flying around at Carson’s speed much slower.  If I had an Ovation, though, I would probably be spending a lot of time 10 GPH LOP, although I also agree with you that very few others probably do.  
 

 

 See my post above. That was High power LOP down low and Peak up high. About 1gph difference in FF between 4 and 11K. Round trip  Block speed was 158kts avg FF 9.5gph.

[Shadrach]

1 hour ago, bluehighwayflyer said:

Hi, Ross.  Yes, Sir.  For everything there is a season.  The ability to run LOP is actually what I miss most about my old J model.  Well, that and it’s striking good looks.  

Understood. My not so well made point was that my draggy old stock F model will push near 150Kts high power LOP down low, a properly set up J model might be closer to 160 than we think. Roy Lopresti used to run his J LOP in the cafe races of the 70s and they were flown as low as 500 agl.

[Shadrach]

3 minutes ago, bluehighwayflyer said:

I understood and agree, Ross, and there is a time and a place to fly them that way, as you demonstrated above.  But @A64Pilot is also right that most folks don’t cruise their J’s, or Es or Fs for that matter, at 75% power LOP.  I never cruised my J even at 75% power ROP.  I suspect if I had, though, that it would have made 160 KTAS.  But barely.  @jetdriven’s J and some of the other highly aerodynamically refined J’s around here will probably beat that handily.  

FWIW, I only find my C to be 10 to 12 knots slower than my J was, all else being equal.  Not nearly as big a difference as most people believe.  

Yep, block speeds for most of the 4 cyl fleet is likely within a 15kt range.

[A64Pilot]

FWIW on my pre-buy test flight I got 168 kts out of my J at takeoff power at about 1000 MSL, but burning 19 GPH too, that’s not at all a realistic cruise of course, just a data point.

You can run low altitude at high MP LOP, but you cannot generate high power LOP. TANSTAFL

I know that to be true, but I don’t know how much power LOP cost, but I think it’s pretty significant

[0TreeLemur]

If fuel cost was $100/gal and we had to fly, we'd be tempted to fly at the maximum L/D speed, or glide speed to reduce the total cost of the trip.

If fuel cost was $0.01/gal, we could fly as fast as possible all the time with very little cost penalty.

Carson's speed, which theoretically is 1.316*glide speed, is pretty slow.  That speed minimizes the increase in FF per knot with respect to increases in airspeed.  Its the optimum of the (gallons/h per knot) per knot function.  Carson called it "how to waste fuel most efficiently" while flying faster than glide speed.

As fuel prices near $10/gal, the cost of fuel approaches %50 of the operating cost for a M20C.   That's where minimizing fuel consumption becomes a significant factor in controlling the total flight cost.  Thankfully, most of us, in the USA at least, are not paying near $10/gal.

Here's another way to look at the data, assuming $6.00/gallon 100LL.    With reference to the test numbers in the original post, this figure illustrates how different combinations of engine settings produce the same power, resulting in the same airspeed with significant differences in fuel cost.  This is an important thing to keep in mind.  Some engine settings result in more cost efficient ways to fly at less than max. speed than others.   Is it better to reduce RPM, reduce MP, or some combination of the two? 

[A64Pilot]

In theory it’s best to run RPM at the bottom of the green, min RPM and high MP

This reduces pumping losses and frictional losses from the engine, and the prop is more efficient at low speed due to drag reduction.

I’ve seen posts that x prop is most efficient at 2550 RPM, I don’t believe that’s actually true, what I believe is that it generates the most thrust at 2550 RPM, but from a pure efficiency, not power perspective within limits the slower the better.

However I had NIST calibrated instrumentation one time and while I don’t doubt this low RPM thing is true, the difference for identical airspeed just wasn’t measurable it was in the noise band, just wasn’t significant.

I believe the most efficient point is an altitude where just LOP at low RPM gives an indicated airspeed of L/D max, but that’s way higher than I fly, I have no idea what altitude would give you that. Just don’t forget the fuel you burn getting there though.

Altitude is your friend, less air density equals less drag or higher true airspeed for the same drag is another way of looking at it.

As much aa we hate to, the thing that saves more fuel than anything else is slowing down.

I know many people who will say things like my car gets its best milage at 80 mph or similar, to those I only smile because it simply isn’t true, but you can’t convince them so don’t even try, because there are sure their car is different.

https://www.consumerreports.org/cro/news/2009/09/tested-speed-vs-fuel-economy/index.htm

[0TreeLemur]

17 hours ago, A64Pilot said:

I believe the most efficient point is an altitude where just LOP at low RPM gives an indicated airspeed of L/D max, but that’s way higher than I fly, I have no idea what altitude would give you that. Just don’t forget the fuel you burn getting there though.

Altitude is your friend, less air density equals less drag or higher true airspeed for the same drag is another way of looking at it.

That's a good point.  Up at 12,500 DA, our C indicates about 116 knots in cruise which is near the theoretical Carson's speed, with a TAS of 143 at about 65% power.

[A64Pilot]

On 11/20/2022 at 5:33 PM, Shadrach said:

Yep, block speeds for most of the 4 cyl fleet is likely within a 15kt range.

I think that’s probably true as it’s roughly 10%.

But it may be much less than that at higher altitudes, if HP and thrust are the same I think the higher you go, the less drag so airframe drag becomes less the higher you go, so any aerodynamic advantage a J has may make less difference.

[Utah20Gflyer]

On 11/24/2022 at 1:13 PM, 0TreeLemur said:

That's a good point.  Up at 12,500 DA, our C indicates about 116 knots in cruise which is near the theoretical Carson's speed, with a TAS of 143 at about 65% power.

Yeah, If you are up high and bring back prop RPMs while still running WOT you aren't far off Carson speed.   Given a far enough trip this is how I prefer to operate.   Fuel burn drops quite a bit and you can still make a Cessna jealous.

[Hank]

7 minutes ago, Utah20Gflyer said:

Yeah, If you are up high and bring back prop RPMs while still running WOT you aren't far off Carson speed.   Given a far enough trip this is how I prefer to operate.   Fuel burn drops quite a bit and you can still make a Cessna jealous.

I fly 7-10K, 2500 and beat Cessnas in both flight time and fuel burned for the trip. Even my little C will beat a 172 by 30% less time and 10% less fuel.

[larryb]

And then there is the question of engine life and maintenance cost. In my Encore I cruse at 60% power, 25" 9.5GPH, LOP, and around 150 Kt TAS in the mid teens. Yes that's not as good as others report, but I do have FIKI. I can go 20kt faster at 14 gph ROP. But my fuel cost is a lot higher per mile, and I feel like my engine life would be a lot less as well. How much less? I'd really like to know. 

[A64Pilot]

On 11/25/2022 at 9:18 PM, larryb said:

And then there is the question of engine life and maintenance cost. In my Encore I cruse at 60% power, 25" 9.5GPH, LOP, and around 150 Kt TAS in the mid teens. Yes that's not as good as others report, but I do have FIKI. I can go 20kt faster at 14 gph ROP. But my fuel cost is a lot higher per mile, and I feel like my engine life would be a lot less as well. How much less? I'd really like to know. 

Lycoming has published a couple of times that 65% or below for max engine longevity, but doesn’t say how much more life. I figure an engine is an engine, the harder you run it the faster it wears. I think your cylinders will last longer run at lower output

I think it’s 75% and do all the other things right and you’ll make TBO, if you want to go beyond, fly 65% or lower.

But you don’t want to fly so low as to not keep temps in the green.

I think people tend to report optimistic airspeeds

[Schllc]

On 11/20/2022 at 3:06 PM, A64Pilot said:

Yeah, you definitely need O2 then, really big indicator you have sleep apnea is waking up with a headache, due to lack of O2. Headache can be dehydration of course, but if it’s that O2 doesn’t help.

I think but don’t know, but those O2 concentrators are particularly good for lower altitudes if you don't want to mess with bottles.

Personally I don’t like the Cannula’s, I just fly low enough not to need it and leave the O2 in the wall locker. Mask is even worse. Cannula dries up my sinuses and nose.

Some people have issues with cannula's drying the sinuses, and some do not.  I am like you, mine dry out to the point of nosebleeds the day after with constant flow systems.

However, with the conserver systems like precise flight, and mountain aire, I have zero issues.  In fact, I flew from Florida to Oregon and back over seven days, all flights above 16k, which turned out to be about 26 hours of flying, with the precise flight system.  Not only did i return with a little over half the o2 in the tank.  I didn't have any problems at all with dry sinus.  

1000 thumbs up for conserver systems.

[A64Pilot]

I don’t have a constant flow either, and it doesn’t make my nose bleed, just annoying. As I don’t fly over mountains I just stay lower.

I  don’t have a turbo so it’s no great loss. 

If I were to start using O2 often I’d put some kind of moisture adder like you see on medical O2 systems, they usually have a bottle with water, but I bet a tube with a sponge or wet rag would work.

But my system hasn’t come out of the wall locker in years

[Pinecone]

On 11/25/2022 at 10:18 PM, larryb said:

And then there is the question of engine life and maintenance cost. In my Encore I cruse at 60% power, 25" 9.5GPH, LOP, and around 150 Kt TAS in the mid teens. Yes that's not as good as others report, but I do have FIKI. I can go 20kt faster at 14 gph ROP. But my fuel cost is a lot higher per mile, and I feel like my engine life would be a lot less as well. How much less? I'd really like to know. 

I run my 252/Encore (upgraded) at 65%, 32 inches, 2300.  10.3 GPH at 174 KTAS. So I am going move than 20 knots faster for less than 2 GPH.’

No FIKI though

[Fly Boomer]

3 hours ago, A64Pilot said:

If I were to start using O2 often I’d put some kind of moisture adder like you see on medical O2 systems, they usually have a bottle with water, but I bet a tube with a sponge or wet rag would work.

Wine.

[jetdriven]

That McCauley prop is most efficient from a nmpg Standpoint at 2500 rpm.  Pulling the prop back costs you 3 knots per 100. But at higher altitudes or lowe power settings it’s like 5-10kt below 2350.  2200 rpm is 15 knot slowe than 2500 but the ff isn’t low enough to make it pay. The ff doesn’t drop off as fast. So the trip burn is higher and the trip takes longer. 

[A64Pilot]

18 hours ago, jetdriven said:

That McCauley prop is most efficient from a nmpg Standpoint at 2500 rpm.  Pulling the prop back costs you 3 knots per 100. But at higher altitudes or lowe power settings it’s like 5-10kt below 2350.  2200 rpm is 15 knot slowe than 2500 but the ff isn’t low enough to make it pay. The ff doesn’t drop off as fast. So the trip burn is higher and the trip takes longer. 

The slower speeds in your example come from reduced engine power, watch the fuel flow when you roll the RPM back.

It’s sort of one of those things that works on paper, not so much in the airplane. The theory is you reduce RPM, but increase MP to regain power, but at altitudes that your efficient at, most likely your at full throttle anyway, at 10,000 full throttle if your lucky is 20” MP. 20 squared is poking along.

If we had a power required chart for a Mooney we would see that slowing down saves fuel, until you get to the region of reversed command, which is pretty darned slow.

However if you look at a power required chart you’ll notice while it’s U shaped but the bottom of the U is sort of flat, and both ends are real steep, if you cruise in the flat part you get good fuel economy, it’s when you begin to climb the walls that fuel burn gets steep for each kt of speed added.

I’m not a fan of bottom of the green RPM cruising to save fuel, if you go out and fly it to see, and add MP until you maintain exact speed you’ll see any fuel saving is minimum at best, but again you have to be pretty low to have the MP to add.

But again under normal conditions running low RPM just isn’t much of an option because you lose so much power you end up crawling along.

 

[Pinecone]

Cue smiles from turbo owners.

I will have to play with this.  I can run LOP so HP is directly rated to fuel flow.  And run the same HP, LOP, at different RPM and see what the IAS and TAS do.

I suspect that while the prop may be most efficient at 2500 RPM, the longer dwell time (time for the crank to rotate from ignition to exhaust opening) will over shadow that and make lower RPM better.

[A64Pilot]

Props not necessarily more efficient at 2500 RPM, there is no one “best” RPM, a prop will have an efficiency curve based on HP applied, air density, and airspeed, but from a cruise perspective it’s more efficient at lower RPM, but how much? 

I’ve got a whole bunch of turboprop time and a whole bunch test flying them, what’s neat on a turbo prop is that you can leave engine HP the same and change the prop RPM, torque changes of course but SHP doesn’t, at least on free turbines, Garrets you can’t. So you can set a cruise speed and play with prop RPM, if it had a sweet spot then IAS would peak at that sweet spot RPM, except for noise and vibration I could tell essentially no difference, and that was three, four and five bladed props from Hartzell, Avia and MT. You set a prop RPM for noise and vibration for cruise and max for TO but only because you had torque limits and the higher RPM at the same torque gave more power, for instance 58.7 lbs torque was 750 SHP at 2200 RPM on a -34, but to make 750 at 2000 took 64.5 lbs of torque. 64.5 was max allowable on that engine.

A whole lot of these truths or tricks etc like for instance full throttle is more efficient because less pumping losses with the throttle plate flat you can prove on paper, but can’t measure in the airplane, there is a difference, but it’s small.

Don’t mean to tell you don’t go out and test, heck it’s fun, hurts nothing, you may learn something and we all need an excuse to fly anyway. My opinion is we need to rely less on internet experts and to go out and see for ourselves.

From my testing the big difference is speed, best MPG at L/D max, but who can fly that slow? But the curve is flat on the bottom so that’s where Carson’s speed comes from, a significant speed increase with just a little more fuel, but you can exceed Carson’s speed with just a little more, but it’s diminishing returns, meaning you get to a point where a 5% increase in speed may cost 15% or more increase in fuel or something, thise are made up numbers. I find a 135 kt or so speed being a good compromise for me, now that’s just for flying around where the goal isn’t a location, I’m just wasting fuel, military term OFO, but that’s the majority of my flying now.

Other thing is if I fly Carson’s speed LOP with cowl flaps closed my Cyl head temp isn’t in the green or at the very bottom anyway, so I need to speed up.

To see what difference speed makes, go fly with the same RPM etc, only changing MP and write down fuel flow each five kt increase, you’ll see where the curve gets steep, that graph should match the power required curve pretty close.

My MVP cheated, it calculated and displayed MPG if it had a GPS input, so you could make a change, adjust throttle to recover exact speed let things settle and see f the MPG changed.I imagine that pretty common. Ideally set it on autopilot on heading and altitude hold, allow several minutes at each test point, equilibrium takes awhile. I hand flew all mine an autopilot would have been nice.

[jaylw314]

On 11/27/2022 at 6:03 AM, Schllc said:

Some people have issues with cannula's drying the sinuses, and some do not.  I am like you, mine dry out to the point of nosebleeds the day after with constant flow systems.

However, with the conserver systems like precise flight, and mountain aire, I have zero issues.  In fact, I flew from Florida to Oregon and back over seven days, all flights above 16k, which turned out to be about 26 hours of flying, with the precise flight system.  Not only did i return with a little over half the o2 in the tank.  I didn't have any problems at all with dry sinus.  

1000 thumbs up for conserver systems.

O2 through cannulas can dry the nasal passages if you're blasting more than 2L/min through them.  The conserver cannula/systems reduce that to something more like 250-500 cc/min, so unless you crank up the flow to wastefully high levels, dry sinuses is probably not going to be a major issue.

On the other hand, the mechanical irritation from having two fingers picking your nose for 4 hours can certainly lead to some issues 

[jaylw314]

On 11/27/2022 at 7:19 PM, jetdriven said:

That McCauley prop is most efficient from a nmpg Standpoint at 2500 rpm.  Pulling the prop back costs you 3 knots per 100. But at higher altitudes or lowe power settings it’s like 5-10kt below 2350.  2200 rpm is 15 knot slowe than 2500 but the ff isn’t low enough to make it pay. The ff doesn’t drop off as fast. So the trip burn is higher and the trip takes longer. 

I guess the idea that the prop is most 'efficient' at 2500 rpm needs some more precision as to what definition of 'efficient' that refers to  If you get slower and use less fuel when slower, and faster with more fuel when faster, why is 2500 rpm so special?

At least in the POH range charts I posted, the range increases gradually at any given power setting with slower RPM, there doesn't appear to be a sudden change in ff vs RPM at 2500 

FWIW, I usually cruise at 10-12,000' MSL.  Up there my IAS at WOT/2500 RPM is down to 120-130 KIAS, but eventually I start getting impatient and crank it up to 2600 RPM  There is definitely less noise at 2500 RPM, and I have it dynamically balanced for 2500 (Hartzell 2-blade), but at that altitude the noise is much lower anyway

Edited November 29, 2022 by jaylw314

[A64Pilot]

Props are a sort of strange thing, initially I thought that a constant speed prop would just add pitch as necessary to absorb the power and there wouldn’t be much difference between props.

But when I started flying different props on the same airplane and could compare one with the other I discovered how wrong I was, but you can generalize props into being good fast cruise props or stump pullers that give lots of thrust at low speeds.

Our G-10 powered airplane used a prop from a commuter aircraft and I was told it worked really well at 200 kts plus, unfortunately that was well beyond our VNE.

Some props are real stump pullers, pull hard at low speed but then lose it as speed increases

Bush pilots and people who fly amphibs call these “Borer” props, I have no idea where the name came from but as long as you can run them ground clearance wise they tend to be long thin props, where go fast props tend to be shorter and more “paddle bladed” meaning wide chord props.

Then where the prop pulls from also plays into it, when I first started with the GE airplane the Avia three blade was the hands down winner. Joe Brown who owns Hartzell took that personally and they sent me everything they had that could fit, three and four bladed. Nothing beat the Avia, so they went back and designed a prop from scratch, what made that prop pull so hard I was told was that it was heavily tip loaded meaning it’s blades weren’t washed out as much as normal and the tips produced the majority of the thrust. Worked really well but during the vibe survey we ran into the reactionless mode so we had a rather large avoid range RPM wise.

Honestly I think the Avia was a better prop, but you can’t get it overhauled in the US. The Hartzell is just more fragile a prop than the Avia, the Avia had thick blades and turned rocks into dust with almost no damage, plus the blades literally screwed in, to replace a blade all you did was remove the clamps and unscrew it, screw in the new blade, carefully align the red marks and install the clamps, of course balance it.

Crop dusters often damage props.

[Pinecone]

Interesting.  I recall that the P-47 was not well liked at first.  And especially by the Americans who has flown with the RAF in Spitfires before moving to the USAAF.

Then they put the paddle bladed props on the them and significantly increased the climb and acceleration without reducing top speed.