C145 allowable rpm limits

[DavidA]

In my 1951 C170A pilot operation handbook, it lists cruise data for the landplane with a upper limit of cruise, I guess that means "continuous". RPM of 2500, But on the seaplane, it lists a upper limit of 2700.
Does that mean I could be using 2700 for continuous cruise and thereby, getting a little more speed, albiet with increased fuel consumption?
In a related question, with a McCauley 1A170 prop, what pitch is considered "climb" and what pitch is considered a "cruise" prop?

[Jr.CubBuilder]

I've been pondering the same kind of thing. I've got a climb prop on mine (7651) and it's my understanding that the 7653 pitch was the cruise prop, but I've read of several planes with 7656 pitch props on them. From the sounds of it the 7656 prop will give a cruise of 125mph to 130mph, but I bet the climb performance is not very good.

My plane was on floats before (wheels now) and when I bought it a 7648 was installed. According to the logs it was pitched back to a 45, but after I had it checked out at a prop shop it turned out to be a 47. That prop gave me a top cruise at 2450rpm of about 85mph, and an awesome angle of climb. After I had it pitched up to 51 I started getting about 100-105mph at the same rpm. Now I can't really think of any reason why I couldn't spin the motor another 100rpm on cruise to get a little more speed unless it started getting to hot, but I haven't done any experimentation yet.

[zero.one.victor]

Cruise rpm: I cruise mine anywhere from 2200 on up to redline, depends on how fast I want to go & how much fuel I'm willing to burn. Mine cruises real nice at 2500-2550 or so, good for around 120 IAS this time of year turning a DM76-51 prop. I average about 8 gph,start to stop.
My Univair catalog's McCauley section lists the DM7653 as the standard prop, and the DM7651 as a climb prop, I assume a DM7655 would be considered a cruise prop. The MDM7655 is listed as standard, with MDM7656 as a cruise & a MDM7653 as climb. It lists the DM7647 & MDM7649 as standard seaplane props.

Eric

[Jr.CubBuilder]

Does anybody know what the difference is between the DM and MDM props? I'm guessing the MDMs have a fatter airfoil?

[GAHorn]

MDM's have a thinner/lighter (more efficient) airfoil. Example: at station 24, the DM prop has a min/max thickness of .49-.54" while the MDM is .415-.465.
(The MDM blade is the same as the more modern EM 6-bolt prop.)

[DavidA]

to gahorn: I have found your comments and replies very enlightening on this site, and was hoping you could answer the initial question on this thread, namely, can I use 2700 rpm as a continuous rpm for cruise? It appears my 53 inch pitch is ok for climb but I am not getting near the advertised cruise indicated speed, so it must be a climb prop and not a cruise prop. any comments?
thanks

[GAHorn]

Max continuous allowable power on the C145/O300 engine is 2700 rpm. (This information is found in the engine Type Certificate Data Sheet.)
The reason Cessna charts might indicate less than that rpm is because the standard prop was originally a DM7653 and that prop/engine combination typically made achievable only the power settings depicted. Notice that this engine only produces max rated hp (145) at 2700 rpm. Therefore if you're not turning 2700 ...then you're not getting 145 hp.
The DM 7651 was offered by Cessna as the factory climb prop. The cruise prop offered was the DM 7655.
The cruise prop will not necessarily make an airplane cruise any "faster". Think of it this way: Does an auto cruise faster in 1st, 2nd, or 3rd gear? Answer: It depends. Are we carrying a really heavy load at high mountainous altitudes? Or are we drag racing at sea level? Or are we merely cruising along at moderate highway speeds in the plains? And at what throttle setting are we doing any of the above? Wide open? Or 3/4's throttle? Or half?
Generally, if you fly at altitudes that make full open throttle practical, then a cruise prop will likely do well for you.....PROVIDED that you aren't making that flight after departing a hot, high, 1200 foot strip!
If all you care about is getting best fuel economy while cruising along at or above 7500 MSL, then perhaps a cruise prop might make you happy. (Especially if your present climb prop makes you feel like you're running along in an auto stuck in second gear! .... i.e., full throttle and not much forward speed.) But if you frequently try to launch out of short strips, or even reasonable ones while hot, high and/or heavy....that cruise prop will show you lot's of bird nests up close in the tree tops at the departure end! I know. I've got one, and Jamie and I have loaded N146YS up with full fuel, lots of baggage/camping gear and seen obtacles up close and spent lots of time climbing in the desert trying to reach a cool altitude while pretty heavy. There have been times I've wished I had a finer pitch. (The real fix is more horsepower ....but that comes at a price I'm not willing to put into my 170 ...either in terms of dollars OR non-originality. It's a personal thing. Many of the truly great old classic airplanes in history were underpowered. (Ask any Boeing 727-200 driver. Q: Is it a slug when hot/heavy. A: Yep. Q: Wouldn't you rather fly something else? A: No #@%&* way!) It's almost part of the charm, not to mention the ego boost, to be able to deal with it gracefully, safely, and tactfully.)
But the 7655 has been a generally good compromise for me because I just plan on it's reduced take-off/climb performance and stay light when operating on less than 2,000' Length or above 4,000 Elev. fields. (Last SUMMER (hot) we were especially heavy and eventually saw 13,500' as we climbed up over Jim Bridger's old stomping grounds east of Salt Lake on our trip out west to the convention. The ol' girl does a good job as long as you give 'er room and time. What's the other side of the coin?
Well, when we're at reasonable weights, say 2100 lbs or less, and just going someplace, she'll clip right along at or better than book while averaging about 7.8 gph. (She can show her white nav-light to a certain green machine up in Dallas without too much trouble.)
No, she isn't the performer my Cessna 206 was. But she also doesn't cost me what the 206 did to operate. Fuel costs per mile was about the same, but those annuals, and cylinders, and accessories, and....well,...you know what I mean.
Besides.......even a ho-hum pilot can operate a plane if you give him enough excess horsepower ....and 206's have the small wheel at the wrong end and have crummy-looking tailfeathers to boot!
In any case, my own opinion is that Cessna knew what they were doing when they standardized the 7653 prop. It's a good all-around choice, and anything else compromises the all-around performance for improvement in only one small area.

[auxtank]

What George says regarding the TCDS is true: Type Certificate Data Sheet NO. E-253––covering the Continental C145-2, -2H, -2HP; and 0-300-A, -B, -C, -D, -E engines––specifies maximum continuous allowable power as 2700 rpm (producing 145 hp)

Incidentally, for anyone interested, a pdf file of the engine TCDS is available at:

http://www.airweb.faa.gov/Regulatory_an ... enFrameSet (Search for C145 or E-253)

The TCDS also specifies takeoff hp and rpm to be the same as the maximum continuous allowable power (i.e., 145 hp and 2700 rpm). However––and I find this point quite interesting––the TCDS specifies a five minute period for the takeoff setting: “Takeoff hp., 5 min., r.p.m., full throttle at sea level pressure altitude.”

I am aware that many POHs designate time limitations for maximum power operations. The IO 520-F in the Cessna 206’s I sometimes fly are only rated at 300 BHP and 2850 RPM for 5 minutes at takeoff (maximum continuous power is 285 BHP at 2700 RPM). Perhaps the CAA, or later the FAA, requested Continental to specify a 5 minute takeoff power setting for the C145/0-300, and that is the reason why the TCDS includes a takeoff power setting (identical to the continuous allowable setting) with what appears to be a 5 minute limitation. Does anyone out there know if takeoff power settings are required to be designated in the TCDS as a separate and time limited specification even when they are the same as maximum continuous power settings.

As far as a C145/0-300’s ability to turn 2700 rpm continuously goes, I have no doubt that Continental tested these engines by running them extensively at full power. But, I believe the original certification for the C145 was for a TBO considerably less than the 1600 hours the engines now enjoy, and I would be surprised to learn that the manufacturer ran a C145 or 0-300 in an aircraft under real-world flying conditions at 2700 rpm for 1600 hours.

Dave, you observed that Cessna recommends and upper cruise limit of 2500 rpm in the landplane version of the C170A and a limit of 2700 rpm in the seaplane version of the same model. You asked: “Does that mean I could be using 2700 for continuous cruise and thereby, getting a little more speed, albeit with increased fuel consumption?” I will give you my answer to that question in a minute. But, I think your question begs another, and I will try to answer it first. That question is: What difference(s) does Cessna see between the landplane and the seaplane version of the C170A that would account for an upper limit on cruise rpm of 2500 for the former and 2700 for the later.

Cessna may have considered that the flatter prop of the seaplane model would permit a modest increase of 8% in continuous rpm. As George pointed out, this recommendation does not run counter to the TCDS. That said, my own opinion is that Cessna’s specification of a higher rpm limit for cruising in the seaplane reflects a triumph of marketing concerns over maintenance concerns. In other words, I believe that Cessna, recognizing that speed is an important factor influencing aviation consumers’ choices, decided to disregard its own well-founded recommendations for cruise power settings in the landplane for the purpose of increasing the book-number for the seaplane’s cruise speed. In fact, the cruise-rpm limit of 2500 on the landplane may be a nod to the same desire to claim a somewhat higher-than-otherwise cruise speed.

Some of my reasons for suggesting that marketing concerns may play a role in Cessna’s engine operating recommendations are contained in the FAA approved Continental publication Form X-30015, Operator’s Manual Aircraft Engine 0-300 and “C” Series. In the Specifications section, found on page 1, Continental suggests a cruising rpm of 2450. Significantly, on page 4, under the heading Cruising, Continental cautions: “Do not exceed recommended cruising R.P.M. or manifold pressure for long periods of time. Excessive speeds and loads hasten wear and increase operating costs.” (Emphasis added). The manual contains another caution against operating at 2700 rpm when not absolutely necessary: in the takeoff and climb section (again, page 4) pilots are told to “[m]aintain rated R.P.M. only until immediate obstacles are cleared; then reduce to climb power setting.” Additionally, the Foreword to the pamphlet advises operators that “[c]areful observance of these suggested procedure will help the engines to serve faithfully.”

For those that are not convinced that it could be a questionable practice to exceed the engine manufacturer’s recommended cruising rpm of 2450 for what Continental calls “extended periods of time,” I offer the notice given on the cover of the engine Operator’s Manual: “In order to properly use this engine, the user must comply with all instructions contained herein. Failure to so comply will be deemed misuse, relieving the engine manufacturer of any responsibility.” That sounds like a legal disclaimer to me. Clearly, Cessna’s suggestion to cruise the seaplane version of the C170A at up to 2700 rpm does not conform to Continental’s own recommendations for operation of the C145/0-300.

So, Dave, in answer to your question about using 2700 rpm for continuous cruise, I would suggest following the engine manufacturer’s recommendations contained in the Operator’s Manual; i.e., don't do it. I personally don’t feel the added wear and tear and increased fuel consumption is worth the small increase in speed. Of course that is coming from someone whose ragwing is pulled around by an 80/41 prop with 8.50 mains hanging on C180 gear…you can see where my priorities lie. When it comes to engine operation, my inclination is to resolve discrepancies between the airframe manufacturer’s suggested engine operating procedures and those of the engine manufacturer in favor of the engine maker’s approved procedures.

I might add that, while I don’t recommend cruising or operating continuously at 2700 rpm, I don’t typically pull the throttle back as soon as obstacles are cleared. The reasons are that, even with an 80/41 prop, I don’t see redline with full power on climb out and I want the extra cooling at climb power and climb speed that the excessively rich mixture at full throttle provides.

Although I see the wisdom in Continental’s suggestion not to cruise above 2450 rpm for extended periods of time, I will say that IMHO cruising at 2700 rpm (if your plane will make that at cruising altitude) might not be the worse thing you can do to your engine. As many here have said in the past, regular flights and oil changes are probably the best thing you can do for your engine. In other words, 0 rpm for extended periods of time might be the worse thing you can do to your engine.

For anyone interested in receiving a pdf file of the 0-300 Operators Manual, if you send me a PM requesting the file along with your email address, I will send you a copy.

Gordon Sandy
1948 Ragwing

[GAHorn]

While I haven't read the requirements for TCDS recently, I believe those requirements do indeed require the mfr to list both maximum continuous power AND max take-off power limited for 5 minutes. Since the max continuous is 2700...then the max continuous-5 minute limit cannot be any less, considering that 2700 is also the max rpm under all conditions.
Therefore, I believe it may be misleading to consider 2700 a 5 minute limit. Think instead of the 5 minutes being a limit of the take-off event only.
As for T.B.O. on this engine.... It is common to the industry that a newly introduced engine have a recommended time of service before overhaul. Key word: recommended. In other words, the recommendation has little to do with the capability of the engine. When the C145/O300 engine was first introduced, the mfr estimated that it's ability to produce rated power reliably was approximately 600 hours, and therefore that recommendation was published. However, actual experience in the field (after numerous tear-downs demonstrated acceptable engine wear) later indicated that the engine was capable of a much-extended TBO period, and the mfr then submitted and obtained approval for an increased TBO. (Presently 1800 hours.) Virtually ALL new aircraft engines first enter the marketplace with a conservative TBO which is later increased. (Rolls Royce Viper turbojets first entered the market with a 400 hour TBO, later increased to 1200 hrs, still later increased to 2400 then 3600, then 4400, and now....are potentially never overhauled, as they are an "on condition" engine. Some engines have over 17,000 hours on them!)
Does the 2,000 hr TBO of a competitor's engine mean that competing design is somehow much superior to the C145/O300? No. It only demonstrates that the competitor continued to evaluate their engine and was able to convince the FAA and the public that their engine could possibly reach that figure, but that Continental had seen the decline of new airframe applications for their C145/O300 and therefore decided to quit spending money on research and FAA approvals for an engine not expected to gain significant further sales due to any recommended increase. Engine reliability had little to do with it. (There's little incentive for an engine mfr to continue to extend the TBO of an engine that new airframe mfrs are not likely to install. Cessna had purchased 5,000 Lycoming 150 hp engines for their Cardinal in 1967....which turned out to be a piteously underpowered airplane. So the Cardinal got more horsepower with a different engine.....and Cessna continued to mfr the 172/Skyhawk and used all the Lyc 150 hp engines they had in-stock up by sticking them in Skyhawks. It was the end of the O300, and TCM had no reason to further extend the TBO of the engine.
Operating the C145/O300 at less than 2700 rpm continuously will likely help it last longer. Operating it at 1000 rpm will likely make it last even longer. But how many miles will it cover? At some point the graph-curves of miles travelled vs the time spent travelling will cross and it is detrimental/excessively expensive to operate at reduced power. The C145/O300 engine is not working very hard at 2700 rpm actually, and other than fuel costs, suffers little additional wear at 2700 vs 2450, IMHO. That said, if you find yourself operating at 2700 continuously then you you are severely under-propped. A coarser/higher pitched prop will serve you better in most cases. Then you may still operate at full throttle (more efficient manifold pressure) and still get the benefit of higher forward speeds at lower fuel consumption and less piston-travel per mile. (potentially less overhaul costs when that time arrives.)

[auxtank]

George,

Thank you for catching and correcting my mistake concerning TBO (it's 1800 hours not 1600). Also, your point about the relationship between manifold pressure, prop pitch, and efficiency is an important one.

When the C145/O300 engine was first introduced, the mfr estimated that it's ability to produce rated power reliably was approximately 600 hours, and therefore that recommendation was published. However, actual experience in the field (after numerous tear-downs demonstrated acceptable engine wear) later indicated that the engine was capable of a much-extended TBO period, and the mfr then submitted and obtained approval for an increased TBO.

Your description of the process for determining the manufacturer’s recommended TBO agrees with what I have heard in the past, and I can see a considerable market-force incentive for the engine manufacturer to publish conservative cruise rpm limits and other related operating cautions (such as those cited in my earlier post): Responsible engine operation of the early engines leads to extended TBO approval, which leads to more sales.

Thanks again, George, for correcting and expanding on my earlier post.

Gordon Sandy
1948 Ragwing

[spiro]

regarding differences in max recommended cruise rpms for land v. seaplane. The charts in the book show %hp to be only a function of rpm. But that's only true for a fixed pitch prop. With a constant speed prop the charts will show hp as a function of rpm and MP. So I believe that if you change (fixed pitch) props then you need a different chart of rpm v. hp.

I've got 7653 and 8042 props for my 170. Cruising at 2400rpm w/ the 7653 definitely works the engine a lot harder than the 8042 at the same rpm. As a practical matter I cruise the 7653 at about 2350 and the 8042 at about 2450 and get about the same gph, which means roughly equivalent power produced.

So it wouldn't surprise me that 2500rpm would be the recommended max cruise for a landplane prop and 2700 for a flatter seaplane prop.

paul

[N1478D]

. . .
Well, when we're at reasonable weights, say 2100 lbs or less, and just going someplace, she'll clip right along at or better than book while averaging about 7.8 gph. (She can show her white nav-light to a certain green machine up in Dallas without too much trouble.)
. . .

George, that's surprising that you bring up me seeing your white nav-light! Every time it's been me standing on the ground after landing and waiting for a long time, then, there it is, your white nav-light on a S-L-O-W downwind!

Better put Petit Jean on your calendar, it's one not to miss. There's enough fireflies up there to help on your night landing!

[Metal Master]

The following may not answer anyone’s questions but might inform others that are reading this. It is my cent & half worth
Engine RPM vs. Prop Pitch.

In my seminar discussion on propellers and constant speed propellers I use the comparison to an automobile transmission and gearing. Like this:

When starting a car with a stick transmission particularly when climbing a hill with a heavy load (compared to an airplane taking off at full gross or a float plane). The car is started in first gear. This allows the engine to produce the maximum rpm at the maximum torque curve if the gearing is selected properly. This is analogous with an airplane with a flat pitch propeller or a constant speed propeller selected for maximum RPM.
Once the car is started up the hill load is decreased on the car and we are moving at a steady rate or accelerated to the maximum RPM, the car is shifted to second gear. In the airplane there is a narrower RPM band of operation and we reach that point much more quickly and or we reach a steady state of steep climb clearing a 50 ft obstacle and maintain the higher RPM until we decide to change to cruise climb. After we reduce the rate of climb with a fixed pitch prop the load is reduced on the wings the rate of climb decreases and the load on the prop is decreased or kept the same and or speed increases depending on whether we decrease throttle or maintain maximum RPM. Remember lift, weight, drag, thrust must stay in balance with one another respectively or one must exceed the other to climb or descend.

When we are traveling down the free way in the car we can shift into the highest gear. Load has been reduced, RPM has been reduced, and manifold pressure in a carbureted car goes up because the throttle is closed further but also goes down because the engine is running at lower RPM. In the constant speed propeller equipped airplane somewhat the same condition exists, Pitch the airplane for your cruise speed, the propeller is moved to a steeper pitch (Higher gear) Adjust the throttle for desired manifold setting, RPM prop control for appropriate RPM for the desired power setting and set Mixture for desired % economy power combination. The constant speed propeller maintains the set RPM whether we climb or descend if no other propeller adjustments are made. With a fixed pitch propeller we can’t change pitch in flight. The fixed pitch is selected to give the desired performance characteristics usually selected between climb & cruise. The flatter the pitch the better the climb the higher the RPM at given throttle & fuel mixture setting and the highest HP available. The steeper the pitch of the prop the climb is reduced because at the same throttle fuel mixture setting the RPM is reduced thus making less horse power. When the airplane is then in level cruise flight the propeller is unloaded and RPM increases which allows us to make a higher horsepower in cruise flight and the airplane is moving through the air faster. But in the latter case climb performance suffers because the engine cannot make as much RPM when the prop is heavily loaded in the climb. Such as trying to start a car up hill in third gear it will not go up the hill and you will burn out the clutch trying to make the engine reach the higher RPM. Or you will stall the engine keeping the clutch engaged. In this way the propeller in a way is stalling in the air (it may not actually be stalled but remember it is an airfoil) and it may not be operating at its optimum lifting capability for the situation.
We can reach a point where the throttle is wide open and the fuel mixture is set at the leanest setting and the engine cannot achieve maximum horse power because the RPM can not be increased because the pitch is so steep.
Horsepower cannot be directly indicated. The two most common instruments for determining power output are the (RPM) Tachometer and the (MAP, MP) Manifold pressure gage. Neither of which actually measure horsepower output. Most fixed pitch equipped airplanes do not have a manifold pressure gauge. We depend on the manufactures of engine, airframe & propellers to give us charts to tell us what engine RPM and or Manifold pressure to Give us the best Climb and Cruise performance with a given propeller and what horse power at what RPM. Then we can fly the airplane and determine if what they tell us is true.
I have always thought that any given propeller shop may adjust the prop pitch depending on wear of the propeller in chord and length to a large degree accurately or inaccurately depending on the skill and experience of the technician performing the work. The actual RPM that the prop will turn depends greatly on the Density altitude, condition of the engine more than that of the numbers stamped into the propeller, A worn out engine will not make as much power as a fresh engine with the same propeller. The same is true of magnetos slipping or being timed improperly or worn out plugs or misfiring plug wires.
A good example of what a manufacturer will give you is the Lycoming O-235 L2C engine. The data plate on the engine itself gives three different Horse Power ratings compared to RPM. This information is not in the operator’s manual for the airplane because the airframe manufacturer in the case of the Cessna 152 selected the RPM and horsepower by the propeller installed. And thus the tachometer has specific RPM limits dictated in the Type certificate of the airplane. An STC is available that allows a higher HP out put with a Sensenich propeller at one of the Higher RPMs stated on the Engines data plate.

This is why the RPM gage is not just there to tell you that the engine is still turning and why it is important to know its accuracy or inaccuracy. You can determine base line data for what the best climb rate RPM and cruise rate RPM you should get with a given propeller by studying the manufactures given specifications. If you know what your engine actually does with another given propeller and if you know how far it is off from the expected performance figures. Because I have observed that no two props with the same model and pitch fly the same and once they have been re-profiled and or overhauled the spread becomes even greater. You will only know what actually happens by flying the airplane. I have seen flatter pitch propellers installed by owners wishing to have a higher climb rate only to discover after installation the they are exceeding the maximum RPM limit of the engine at full throttle in the climb let alone at cruise. The effects of this are that the engine is now operating out of its optimum torque band. The prop needs to be re-pitched to put it back at the proper RPM torque curve. The question of whether they are gaining the performance they desired, is to fly the airplane against standard day settings at known weights and see if the performance is that which is desired. Or by the seat of the pants feel “ I am off the ground better and shorter” than I was before while staying at the recommended RPM limits.

I hope I did not confuse the issue,

Jim

[GAHorn]

The following may not answer anyone’s questions but might inform others that are reading this. It is my cent & half worth
Engine RPM vs. Prop Pitch.

In my seminar discussion on propellers and constant speed propellers I use the comparison to an automobile transmission and gearing. Like this:

When starting a car with a stick transmission particularly when climbing a hill with a heavy load (compared to an airplane taking off at full gross or a float plane). The car is started in first gear. This allows the engine to produce the maximum rpm at the maximum torque curve if the gearing is selected properly. This is analogous with an airplane with a flat pitch propeller or a constant speed propeller selected for maximum RPM.
Once the car is started up the hill load is decreased on the car and we are moving at a steady rate or accelerated to the maximum RPM, the car is shifted to second gear. In the airplane there is a narrower RPM band of operation and we reach that point much more quickly and or we reach a steady state of steep climb clearing a 50 ft obstacle and maintain the higher RPM until we decide to change to cruise climb. After we reduce the rate of climb with a fixed pitch prop the load is reduced on the wings the rate of climb decreases and the load on the prop is decreased or kept the same and or speed increases depending on whether we decrease throttle or maintain maximum RPM. Remember lift, weight, drag, thrust must stay in balance with one another respectively or one must exceed the other to climb or descend.

When we are traveling down the free way in the car we can shift into the highest gear. Load has been reduced, RPM has been reduced, and manifold pressure in a carbureted car goes up because the throttle is closed further but also goes down because the engine is running at lower RPM. In the constant speed propeller equipped airplane somewhat the same condition exists, Pitch the airplane for your cruise speed, the propeller is moved to a steeper pitch (Higher gear) Adjust the throttle for desired manifold setting, RPM prop control for appropriate RPM for the desired power setting and set Mixture for desired % economy power combination. The constant speed propeller maintains the set RPM whether we climb or descend if no other propeller adjustments are made. With a fixed pitch propeller we can’t change pitch in flight. The fixed pitch is selected to give the desired performance characteristics usually selected between climb & cruise. The flatter the pitch the better the climb the higher the RPM at given throttle & fuel mixture setting and the highest HP available. The steeper the pitch of the prop the climb is reduced because at the same throttle fuel mixture setting the RPM is reduced thus making less horse power. When the airplane is then in level cruise flight the propeller is unloaded and RPM increases which allows us to make a higher horsepower in cruise flight and the airplane is moving through the air faster. But in the latter case climb performance suffers because the engine cannot make as much RPM when the prop is heavily loaded in the climb. Such as trying to start a car up hill in third gear it will not go up the hill and you will burn out the clutch trying to make the engine reach the higher RPM. Or you will stall the engine keeping the clutch engaged. In this way the propeller in a way is stalling in the air (it may not actually be stalled but remember it is an airfoil) and it may not be operating at its optimum lifting capability for the situation.
We can reach a point where the throttle is wide open and the fuel mixture is set at the leanest setting and the engine cannot achieve maximum horse power because the RPM can not be increased because the pitch is so steep.
Horsepower cannot be directly indicated. The two most common instruments for determining power output are the (RPM) Tachometer and the (MAP, MP) Manifold pressure gage. Neither of which actually measure horsepower output. Most fixed pitch equipped airplanes do not have a manifold pressure gauge. We depend on the manufactures of engine, airframe & propellers to give us charts to tell us what engine RPM and or Manifold pressure to Give us the best Climb and Cruise performance with a given propeller and what horse power at what RPM. Then we can fly the airplane and determine if what they tell us is true.
I have always thought that any given propeller shop may adjust the prop pitch depending on wear of the propeller in chord and length to a large degree accurately or inaccurately depending on the skill and experience of the technician performing the work. The actual RPM that the prop will turn depends greatly on the Density altitude, condition of the engine more than that of the numbers stamped into the propeller, A worn out engine will not make as much power as a fresh engine with the same propeller. The same is true of magnetos slipping or being timed improperly or worn out plugs or misfiring plug wires.
A good example of what a manufacturer will give you is the Lycoming O-235 L2C engine. The data plate on the engine itself gives three different Horse Power ratings compared to RPM. This information is not in the operator’s manual for the airplane because the airframe manufacturer in the case of the Cessna 152 selected the RPM and horsepower by the propeller installed. And thus the tachometer has specific RPM limits dictated in the Type certificate of the airplane. An STC is available that allows a higher HP out put with a Sensenich propeller at one of the Higher RPMs stated on the Engines data plate.

This is why the RPM gage is not just there to tell you that the engine is still turning and why it is important to know its accuracy or inaccuracy. You can determine base line data for what the best climb rate RPM and cruise rate RPM you should get with a given propeller by studying the manufactures given specifications. If you know what your engine actually does with another given propeller and if you know how far it is off from the expected performance figures. Because I have observed that no two props with the same model and pitch fly the same and once they have been re-profiled and or overhauled the spread becomes even greater. You will only know what actually happens by flying the airplane. I have seen flatter pitch propellers installed by owners wishing to have a higher climb rate only to discover after installation the they are exceeding the maximum RPM limit of the engine at full throttle in the climb let alone at cruise. The effects of this are that the engine is now operating out of its optimum torque band. The prop needs to be re-pitched to put it back at the proper RPM torque curve. The question of whether they are gaining the performance they desired, is to fly the airplane against standard day settings at known weights and see if the performance is that which is desired. Or by the seat of the pants feel “ I am off the ground better and shorter” than I was before while staying at the recommended RPM limits.

I hope I did not confuse the issue,

Jim

Thanks, Jim. I think you have reduced my risk of future complaints.

[N1478D]

Diesel engines have their horsepower at a much lower RPM than gasoline engines. Diesel trucks can start a big load with an engine at idle, while a gasoline engine has to rev up to generate the horsepower to move the load. Wonder if the new diesel aviation engines would make for better climbers?