Constant Speed Prop

[cwyckham]

I am seriously confused about cruise settings for constant speed props. I've attached three performance charts from a C182, a C421, and a Diamond DA20-A1 Katana. I was always under the impression that one chose the rpm to fly at based on whether you wanted speed or range. High rpm would give you high speed, and low rpm would give you better range. That doesn't seem to be the case, though. It seems that the speed and range only depend on the %bhp you choose to run at. But if that's the case, why does Cessna provide so many options?

For some reason, the Cessna charts are broken up in terms of rpm and not %bhp, so I'll have to interpolate in the charts to show what I mean. If we start with the 421 chart (sorry it's such a bad scan) at 15000', 1020 lbs, and 60% power, we see that at both 1950 rpm and 1600 rpm the tas is 218 and the fuel burn is 203 lb/hr. Similarly, for the 182 at 8000', standard temperature, and 60% power at 2100 and 2400 rpm, you get a tas of 142 and fuel burn of 11 gph.

So what gives? Why would Cessna complicate things so much by giving all sorts of options and setting things up in terms of rpm and mp when the only thing that matters is %power? Wouldn't I just choose the %power I want to trade off range and speed and then use the lowest rpm I can to achieve that power without engine damage, depending on my altitude? That will reduce noise and wear and tear on my engine. So why would Cessna think that I want the option of getting the exact same performance at a higher rpm?

In fact, the Diamond Katana is set up exactly like this. You decide on a %power and an altitude and they give you a single rpm and mp. Works for me. I don't have charts for any other aircraft available.

Chris

 

[tonycondon]

with the katana, fixed pitch prop, only one setting of the throttle will give you 75% power. on the 421 and 182RG, many many many combinations of manifold pressure and RPM will give you 75% power, or whatever % power setting you desire. Looks like cessna just pulled out the nice round numbers and gave those, interpolation will give you a good swag at the numbers in between. Most of us cant control the MP within .1 inches or RPM within 10 RPM so it is easier to set the chart to nice round MP and RPM values instead of even % power settings.

 

[jesse]

They show you all your options to get that certain power setting. I like options. Yeah--they could just show you one setting. But there might be some reason or another that you prefer a different RPM to achieve it.

Options are a good thing.

 

[KennyFlys]

It scares me when Jesse makes sense.

 

[cwyckham]

with the katana, fixed pitch prop, only one setting of the throttle will give you 75% power. on the 421 and 182RG, many many many combinations of manifold pressure and RPM will give you 75% power, or whatever % power setting you desire. Looks like cessna just pulled out the nice round numbers and gave those, interpolation will give you a good swag at the numbers in between. Most of us cant control the MP within .1 inches or RPM within 10 RPM so it is easier to set the chart to nice round MP and RPM values instead of even % power settings.

The Katana chart I posted was for a DA20A1 which has a constant speed prop. They give one rpm and the mp for each power setting / altitude combination.

 

[alaskaflyer]

Interesting. As you say, for the Cessnas I fly with constant speed props I set the manifold pressure for the cruise performance I want then reduce the rpm to the lowest allowed on the charts. But that's for level cruise or cruise descents.

One possibility for the differences would be a cruise climb? Certainly a climb - even a slow cruise climb - is more efficient at a higher rpm and so the charts might be useful for setting those prop/power configurations?

 

[cwyckham]

Interesting. As you say, for the Cessnas I fly with constant speed props I set the manifold pressure for the cruise performance I want then reduce the rpm to the lowest allowed on the charts. But that's for level cruise or cruise descents.

I'm not sure I understand what you mean. The cruise performance you want isn't determined by the manifold pressure, it's determined by the %power which is a combination of mp and rpm. Are you basically saying that you choose the power you want then choose the combination on the charts that gives you that power at the lowest rpm? That's what I would do. I just can't figure out why they're giving me all sorts of options at higher rpms for the same power setting, or why they've built the chart in a way that's easiest to use if you choose the rpm first, then decide on the mp setting to get the performance you want. That seems backwards to me.

 

[cwyckham]

They show you all your options to get that certain power setting. I like options. Yeah--they could just show you one setting. But there might be some reason or another that you prefer a different RPM to achieve it.

Options are a good thing.

Options are only a good thing if there's some concrete reason that you'd want to choose a different option from the usual optimum. Otherwise, options are just confusing. What are the reasons for wanting to fly the same power setting (ie. same performance) at a high rpm? (That's not some sort of trick question, I really don't know much about this stuff.)

From what I can gather in my very limited conversations with pilots about this, nobody really has much of an idea about how to choose a cruise setting. Most seem to just have a favorite pair of numbers they always use for no particular reason. I was taught to just set the rpm at the max continuous and then forget about it and treat the aircraft like a fixed pitch propeller aircraft except for takeoff and landing.

The last thing I want is lots of options with no information on how to choose between them.

Chris

 

[alaskaflyer]

I'm not sure I understand what you mean. The cruise performance you want isn't determined by the manifold pressure, it's determined by the %power which is a combination of mp and rpm. Are you basically saying that you choose the power you want then choose the combination on the charts that gives you that power at the lowest rpm? That's what I would do.

Sorry, not very articulate at this time of night.

 

[jesse]

Options are only a good thing if there's some concrete reason that you'd want to choose a different option from the usual optimum. Otherwise, options are just confusing. What are the reasons for wanting to fly the same power setting (ie. same performance) at a high rpm? (That's not some sort of trick question, I really don't know much about this stuff.)

I would rather a POH show me all my options. If it only shows one way to achieve a certain power setting there may come a day where you'd like that power but with a different RPM. Like I said in Flashchat sometimes a certain RPM can be just plain annoying to listen to. I'm sure some of the higher time complex pilots on here might be able to give a scenario to where it can also be useful to know other ways to achieve it.

Most of the time I chose the RPM/Manifold Pressure based on the airspeed and fuel burn. I set the RPM at whatever doesn't annoy me which generally tends to be the lowest one. I do recall instances in the Beech Sierra though where I didn't run the lowest RPM. If I remember there was something that annoyed me. I think it was the way something vibrated. I don't recall--I don't have one of those fancy ANR headsets.

 

[poadeleted20]

You are very nearly correct that speed and range depend only on %bhp. Yes, there may be small differences in engine and prop efficiency at different MP/RPM combinations, but they are pretty small. What you get is the option to vary the mix of engine stress versus engine wear. Using a higher RPM/lower MP setting reduces stress but increases the number of tach hours per flight hour, thus giving you more flight hours to 100-hour, TBO, etc. Lower RPM/higher MP reduces tach hours/flight hour (increasing the book maintenance interval) but increases stress, thus increasing the likelihood of reduced engine life. And there's always the vibration/resonance issues Jesse mentioned above. At the end of the day, it's a trade-off you make based on knowledge of the engine and your personal preferences, and there's no one right answer for all operators and all engines.

 

[Aztec Driver]

You are very nearly correct that speed and range depend only on %bhp. Yes, there may be small differences in engine and prop efficiency at different MP/RPM combinations, but they are pretty small. What you get is the option to vary the mix of engine stress versus engine wear. Using a higher RPM/lower MP setting reduces stress but increases the number of tach hours per flight hour, thus giving you more flight hours to 100-hour, TBO, etc. Lower RPM/higher MP reduces tach hours/flight hour (increasing the book maintenance interval) but increases stress, thus increasing the likelihood of reduced engine life. And there's always the vibration/resonance issues Jesse mentioned above. At the end of the day, it's a trade-off you make based on knowledge of the engine and your personal preferences, and there's no one right answer for all operators and all engines.

In addition to all of that, noise issues and passenger comfort come into play. Lower RPM usually means lower noise for the passengers without headsets.

 

[mikea]

What you all said.

I'll add that that power chart comes from the engine manufacturer, not the airframe manufacturer. Mine is right out of the Lycoming engine owner's manual.

 

[KennyFlys]

On the surface it may not make sense but I agree with the options matter. I think one really needs to simply experiment with different settings in cruise to see the physical effects other than the numbers indicated.

That's something I'll have to try out sometime.

 

[gismo]

What you all said.

I'll add that that power chart comes form the engine manufacturer, not the airframe manufacturer. Mine is right out of the Lycoming engine owner's manual.

That's not always the case (and IME not even typical) Lycoming and Continental publish engine operators guides for all of their engines which usually contain nomigrams for determining engine power and fuel flow over the full range of allowable operation instead of the tabular form found in most airplane POHs. What the engine manuals obviously won't list is the TAS/IAS for any power settings.

And AFaIK the altitude/OAT/MP/RPM combinations listed in an airplane POH are rarely if ever mandatory, just "suggestions" IOW you can use any combination as long as they conform to the limitations in the POH or engine manual. Having several combinations listed allows for more precision when interpolating for settings not listed. BTW most flight planning software allows the entry of this data to control the behavior of the program's range and speed calculator.

 

[midlifeflyer]

What you all said.

I'll add that that power chart comes from the engine manufacturer, not the airframe manufacturer. Mine is right out of the Lycoming engine owner's manual.

I'd guess that they are different. The power chart in an aircraft POH will \show expected airspeed at each setting, something that I expect would not be shown in the "pure" engine one since the same engine could be put in different aircraft with =very= different airspeed results.

 

[cwyckham]

Thanks, guys. Ok, I understand now that there may be times when you want to have options.

I still think that the Cessna tables are built backwards, though. If you're going to build a table, you usually take the things you already know and put them along the top and side, and then put what you want to find inside the table. The thing we know is the performance that we want to achieve. It would make more sense to me to rebuild the Cessna 182 table, for example, so that you have the % power down one side with the consequent TAS and fuel burn next to it. Then you can list three different rpms with the correct mp next to each. But if you group it in terms of %power, then it's far more clear to everyone that it's the power that determines you performance, not some sort of voodoo combination of of rpm and mp. It also makes it much easier to use the table. People keep telling me that it's not so hard to interpolate, but there's no reason we should be interpolating at all.

If I ever end up using a constant speed prop aircraft on a regular basis, I think I'd want to rebuild the table for myself in that way, anyways.

Chris

 

[cwyckham]

Here's a copy of what I meant when I described a more usable table layout. I just took a fragment of the original 182 table I posted before and interpolated and reformated. To me, it seems to be more usable this way. Do you guys agree, or would you prefer the Cessna layout?

Chris

 

[tonycondon]

i like your chris.

 

[CowboyPilot]

In addition to all of that, noise issues and passenger comfort come into play. Lower RPM usually means lower noise for the passengers without headsets.

That's why I added a third blade to the propeller on the redbird.

Regards.

-JD

 

[flyingcheesehead]

Lower RPM usually means lower noise for the passengers without headsets.

And those with headsets as well.

I don't take pax without giving them a headset.

 

[Dwight B. Van Zanen]

That's why I added a third blade to the propeller on the redbird.

Regards.

-JD

I understand the planes used by the Border Patrol have a 6 blade prop and run the exhaust through the plane (and a tuned muffler) to the tail, so they can idle down and be almost silent as they cruise over the border areas with their infrared detectors on.

 

[cwyckham]

I understand the planes used by the Border Patrol have a 6 blade prop and run the exhaust through the plane (and a tuned muffler) to the tail, so they can idle down and be almost silent as they cruise over the border areas with their infrared detectors on.

Cool. Don't they still have to have their lights on at night? And if not, how do they deal with legal night VFR accross the border? Or do they just have to keep a lookout with the IR gear and ATC to avoid the VFR traffic coming through?

Chris

 

[Dwight B. Van Zanen]

I assume they need the same minimum lights to be legal at night, but I would guess they are turned down as much as they can and be legal. Quiet works for them day or night. Some of these planes are the unmanned drone aircraft the military also uses, with the Rotax 914 engine. They have been used along the southern border for a while, but now also in the north. The pilot can be a few thousand miles away, and for them it is like a flight simulator - except that if they impact other aircraft it is real.

 

[Dr. Bill]

I thought they just put women on the "un-manned" drones?

 

[Henning]

I'm not sure I understand what you mean. The cruise performance you want isn't determined by the manifold pressure, it's determined by the %power which is a combination of mp and rpm. Are you basically saying that you choose the power you want then choose the combination on the charts that gives you that power at the lowest rpm? That's what I would do. I just can't figure out why they're giving me all sorts of options at higher rpms for the same power setting, or why they've built the chart in a way that's easiest to use if you choose the rpm first, then decide on the mp setting to get the performance you want. That seems backwards to me.

Chris, Look up the "Lindberg Doctrine" It's the paper Charles Lindberg wrote for the US forces before WWII when he was down helping Chenault and the Flying Tigers. He showed how operating at minimum RPM and maximum manifold pressure for the performance they wanted produced the most efficient results greatly extending the mission range and allowed for more "fighting fuel" along the way. It's what allowed the B-17s and B-24s fly the mission ranges that they did.

 

[tonycondon]

Also, that operating method greatly increased the capabilities of the P38s in the Pacific.

 

[Henning]

Also, that operating method greatly increased the capabilities of the P38s in the Pacific.

It's the power management technique all WWII aviators were taught. I feel really lucky to have flown with quite a few WWII pilots.

 

[cwyckham]

Chris, Look up the "Lindberg Doctrine" It's the paper Charles Lindberg wrote for the US forces before WWII when he was down helping Chenault and the Flying Tigers. He showed how operating at minimum RPM and maximum manifold pressure for the performance they wanted produced the most efficient results greatly extending the mission range and allowed for more "fighting fuel" along the way. It's what allowed the B-17s and B-24s fly the mission ranges that they did.

I had always thought that was true until I started looking at performance charts. It seems obvious that running things slower would be better from a maintenance perspective. If the friction force in the system is constant no matter what speed you run it (not sure if that's true), the running it slower would also result in less work lost due to internal friction.

Perhaps the difference is that I'm looking at a modern and very low power engine compared to Lindhberg's very powerful, and very old engines. I assume Cessna's charts are accurate, so the efficiency savings must be lost to round-off error in these smaller engines.

I think the answer is still to run lower rpms, but due to noise and maintenance not efficiency.

Chris

 

[Henning]

I had always thought that was true until I started looking at performance charts. It seems obvious that running things slower would be better from a maintenance perspective. If the friction force in the system is constant no matter what speed you run it (not sure if that's true), the running it slower would also result in less work lost due to internal friction.

Perhaps the difference is that I'm looking at a modern and very low power engine compared to Lindhberg's very powerful, and very old engines. I assume Cessna's charts are accurate, so the efficiency savings must be lost to round-off error in these smaller engines.

I think the answer is still to run lower rpms, but due to noise and maintenance not efficiency.

Chris

Remember, drag is a cubic component. There's also the kinetic energy of turning the piston around at the end of each stroke, that's cubic as well IIRC. The slower you can run the engine the more efficient. Look at all the big high efficiency engines, they turn 300rpm and less.

 

[cwyckham]

Remember, drag is a cubic component. There's also the kinetic energy of turning the piston around at the end of each stroke, that's cubic as well IIRC. The slower you can run the engine the more efficient. Look at all the big high efficiency engines, they turn 300rpm and less.

Actually, if the engine's internal friction force is independent of speed (which I think it is), then the power loss will vary directly with the rpm (ie. piston speed), not as a cube. It would work quite differently than the power loss due to aerodynamic drag which varies as the cube of velocity. The work done to effect the change in momentum of the pistons would also vary directly with rpm. I think that means that the power loss due to the change in piston momentum would go as the square of the rpm, but don't quote me on that.

That's neither here nor there, though. The fact is that the Cessna charts indicate that the efficiency varies with power output only and not with rpm. If I assume the charts are correct, the only conclusion I can make is that the increase in efficiency gained by running at the same power output but lower %bhp must be small enough to be hidden by round-off error. I don't get why that would be, but that's what the chart tells me.

Chris

 

[poadeleted20]

My wife's been flying "unmanned" aircraft for years. The currently correct definition for UAV is "unoccupied air vehicle."

 

[Dwight B. Van Zanen]

I didn't think this group was all that "politically correct". Perhaps I should just use UAV, and then you can infer whatever you want for the U.

 

[poadeleted20]

I didn't think this group was all that "politically correct".

Let's just say that we have quite a few woman pilots here, and I would much rather share the air with aircraft they're flying than with UAV's.

Perhaps I should just use UAV, and then you can infer whatever you want for the U.

Sounds good to me -- I still call the AIM the "Airman's Information Manual" now and then, but I can't go wrong by calling it the "AIM." Likewise airport traffic areas, terminal control areas, getting cleared "out of the zone" (control zone), etc., etc.

 

[Anthony]

My wife's been flying "unmanned" aircraft for years. The currently correct definition for UAV is "unoccupied air vehicle."

I take "Man" to mean mankind. We have bastardized our language with a bunch of unecessary and longer statements by using congressperson or chairperson, etc. Why there can't be a female chairman is beyond me.

 

[N7702Y]

There are idiosyncratic differences between airplanes of the same make and model flying with the same engines. Why? Beats me. Maybe props slightly out of balance, worn governors, older engine mounts, difference in torque on engine mounts (within the min/max range). There are probably too many variables to track, but the fact is there are differences.

I ran into this first when I learned to fly. I traded off on a pair of 150's, both manufactured the same year, same engines, same props, same set up. One ran more smoothly than the other unless you bumped the cruise RPM up a bit. Never did figure out the difference.

There was a Twin Comanche discussion a while back on best cruise RPM. Some pilots got good results (smooth running engines) with RPMs as low as 2000, others feel more comfortable at 2500, most seemed to like 2450.

Like many other things in aviation, there is no one right way. Best to experiment with the airplane that you are flying and see what works best.

Jay

 

[poadeleted20]

I take "Man" to mean mankind. We have bastardized our language with a bunch of unecessary and longer statements by using congressperson or chairperson, etc. Why there can't be a female chairman is beyond me.

At the university where I worked, we simply had "Chairs." I suppose that suggests the departmental leaders were pieces of furniture, which, in some cases, accurately reflected their leadership ability.

 

[kgruber]

What you all said.

I'll add that that power chart comes from the engine manufacturer, not the airframe manufacturer. Mine is right out of the Lycoming engine owner's manual.

You should use the one provided by the airframe manufacturer.

 

[jdwatson]

I like the way Chris organized the performance charts. I had always presumed that Cessna organized the charts the way they did because one could interpolate between the values in the table for a give altitude and temperature. The way Chris organized the data one cannot count on a linear interpolation between values. Like I said, it was *my* presumption not fact.