172 with constant speed prop advice

[BeRad]

Hi. I'm resuming my flight training next week after a long lay-off. The plane I'll be flying is a '69 172k with a 180 hp engine and a constant speed prop.

I've read up on using a controllable prop in the Airplane Flying Handbook, the Handbook of Aeronautical Knowledge, and other places. However, I haven't found much 'real world' information, especially specific to the 172. (I would think this plane might be similar to flying a 182, but I'm only guessing.)

I would love some advice on this subject regarding power and prop settings, and any other relevant areas. (For instance power settings in the traffic pattern and on run-up etc...) I know my instructor will explain all this to me but I would like to be as prepared as possible to make things easier, and so I can concentrate more on flying the airplane and over-all situational awareness.

Thanks!

 

[Areeda]

We had one of these in our club for a while. They are pretty nice but I'm not sure the CS prop is worth the money.

Check the (meager) STC information for appropriate limits on RPM/MP combinations and work within them. In general you need to pay very little attention to the prop. It stays high RPM for take off, climb and landing. In cruise leave the throttle full open (at all but fairly low altitudes) and bring the prop back to quiet things down.

Joe

 

[Henning]

A constant speed prop is operated the same regardless of what airplane it's in. Use the number sets published in the AFM/ POH as modified by the STC. I prefer to stay on the Low RPM*High MP side of the envelope.

 

[skidoo]

snip ... I prefer to stay on the Low RPM*High MP side of the envelope.

Just curious as to why this is your preference? Which Trade-offs are you aiming at?

 

[Henning]

Just curious as to why this is your preference? Which Trade-offs are you aiming at?

Aiming for fuel efficiency, not trading off anything for any given allowable power rating.

 

[BeRad]

Thanks, this is just the conversation I was hoping for. I've been wondering if having a controllable prop would change the way I would use the throttle, (besides using manifold pressure instead of rpm for setting power). I've found a couple other good sources as well and I think I'm understanding the general procedures better now. From the various materials I'd read before I had expected it to be far more complicated than what it seems to be.

Use the number sets published in the AFM/ POH as modified by the STC.

Check the (meager) STC information for appropriate limits on RPM/MP combinations and work within them.

This is partly what was confusing me. I had stopped by the airport and copied the added pages from the POH and found nothing about rpm or manifold pressure. I'm assuming now that I probably missed a page.

 

[poadeleted20]

Unless the STC developer also produced engine power and performance tables for this installation, you may have a problem. Unlike the more common O-360 conversion for 172's with a FP prop (for which the 172Q manual provides accurate performance and power setting information), I don't think Cessna ever put out a manual covering a 180HP O-360 with CS prop on the 172.

What you would need would be the Lycoming O-360 Operator's Manual, available direct from Lycoming for about $20. In it, you will find the power charts for this engine which will allow you to determine what MP/RPM settings will give you what power output the associated fuel flows for those power settings. Armed with that data, you can then determine experimentally what speed you'll get at various altitudes with those power settings.

It's also possible that the power charts from a Cessna 172RG Cutlass (180 HP, CS prop) might give you the power setting information for this combination if the engine you're using is the same as the Cutlass (O-360-F1A6, I believe). Of course, the performance data for those power settings would not be at all correct.

 

[Areeda]

A constant speed prop is operated the same regardless of what airplane it's in. Use the number sets published in the AFM/ POH as modified by the STC. I prefer to stay on the Low RPM*High MP side of the envelope.

Just curious as to why this is your preference? Which Trade-offs are you aiming at?

Aiming for fuel efficiency, not trading off anything for any given allowable power rating.

OK off to the theoretical and esoteric side of the discussion, way to much practical information so far

I also like the Low RPM/High MP side of the envelope, but I have not seen significant differences in fuel efficiency.

The most noticeable change is in engine/prop noise. Lower RPM lower noise.

Next I believe these engines seal their rings better with the modestly higher internal cylinder pressures (ICP) that this combination delivers.

Fuel burn seems fairly constant for a given horsepower produced and specific leaning procedure. I doubt these engines can be run LOP so if we go 75° rich of peak or lean to roughness and back to smooth + a bit we seem to get to the same gph at 65 or 75% power regardless of the RPM/MP we use to set the power.

I do understand lower RPM means lower internal frictional losses I've just never seen it on a fuel flow meter including the R1820's.

Joe

PS I hate disagreeing with Henning, he will probably prove me wrong but at least I'll learn something.

 

[poadeleted20]

You will have some engine management issues that you wouldn't have with a constant speed prop. One major difference is that during climbout, you will have all of the available engine horsepower available to you, whereas with a constant speed prop (unless maybe it's pitched specifically for climbing) you won't.

You sure about this? I think you've got some confusion here about fixed vs constant-speed props. BTW, no FP props (even when pitched to favor climb performance) produce full power at normal climb or even "cruise climb" speeds. If they did, the engine would overspeed way too easily.

As a result, leaning becomes much more critical. Cessna doesn't even bother to put EGT gauges on some models of the 172 equipped with the O 360 since they feel that you can't fry the engine no matter how you lean it.

The prop you put on the engine doesn't make any difference in the effect of leaning on CHT. Cessna may have had a reason not to put EGT's on the 172Q (the only C-172 they built with an O-360 engine), but I don't think it was because "you can't fry the engine no matter how you lean it."

Same thing with the IO 360 used on late model 172 SP's. Engine management really isn't a big deal.

Again, while it's not hard to manage the mixture on the IO-360 in the C-172S in cruise or in climb, it's still possible to "fry the engine" if you mismanage things.

The same engine used in a Diamond DA 40 is equipped with a constant speed prop, and on hot days, you have to fight with power and mixture settings to keep CHT's to a reasonable level, in my experience with those.

The DA40 has the IO-360-M1A. The C-172S has the IO-360-L2A. Very similar, but not the same engine, and the CS prop on the Diamond vs the FP prop on the Cessna changes things.

 

[Tom-D]

A couple pieces of mis info in this thread,

the STC is required to give the engine power setting in the re-write of the flight manual for the aircraft. the added pages to the POH must contain that info. take another look for this in the POH. If you can't find it there see the STC for the numbers and the proper alteration to the flight manual supplement.


All props that are approved for installation on any aircraft/ engine combo must allow the engine to reach rated horsepower. even if they are considered a climb or cruise props. This is why we only see a 1 or 2 inch difference in climb or cruise props for the same installation.

 

[poadeleted20]

the STC is required to give the engine power setting in the re-write of the flight manual for the aircraft.

Not so. If the STC developer can certify that the performance is at least equal to the original configuration, they don't have to do that. Best example of that is the AFMS for the Penn Yann 180HP O-360 STC for C-172's originally equipped with the O-320 engine -- no power settings, no performance data, nothing.

All props that are approved for installation on any aircraft/ engine combo must allow the engine to reach rated horsepower. even if they are considered a climb or cruise props.

No such requirement in the FAR's for FP-propped installations, and with an FP prop, you won't see it in any stable flight condition. The only way you'll ever see 100% power in a FP-propped plane is very briefly in flight at sea level with full throttle before RPM redline is exceeded and the throttle must be pulled back.

This is why we only see a 1 or 2 inch difference in climb or cruise props for the same installation.

The reason we see that (actually, it's often larger than that -- 4 inches difference for the AG-5B Tiger) is other certification requirements such as minimum climb capability (affecting maximum pitch) and overspeeding during normal climb (affecting minimum pitch).

 

[RotorAndWing]

So, what are we arguing about?

You must be new here...........

 

[poadeleted20]

>You sure about this? I think you've got some confusion here about fixed vs constant-speed props. BTW, no FP props (even when pitched to favor climb performance) produce full power at normal climb or even "cruise climb" speeds. If they did, the engine would overspeed way too easily.

Isn't that what I said?

No, it isn't.

 

[tonycondon]

does it have a tow hook?

 

[Tom-D]

Not so. If the STC developer can certify that the performance is at least equal to the original configuration, they don't have to do that. Best example of that is the AFMS for the Penn Yann 180HP O-360 STC for C-172's originally equipped with the O-320 engine -- no power settings, no performance data, nothing.

I don't know how old that STC is, but you will not get it done that way today.

No such requirement in the FAR's for FP-propped installations, and with an FP prop, you won't see it in any stable flight condition. The only way you'll ever see 100% power in a FP-propped plane is very briefly in flight at sea level with full throttle before RPM redline is exceeded and the throttle must be pulled back.

It's not a FAR requirement, it is a certification requirement. try getting approval to install a prop that will not allow the engine to produce the rated horse power.

The reason we see that (actually, it's often larger than that -- 4 inches difference for the AG-5B Tiger) is other certification requirements such as minimum climb capability (affecting maximum pitch) and over speeding during normal climb (affecting minimum pitch).

At each and every annual we IAs are required to run each engine and see that it adheres to its type certificate, or properly altered condition. It's properly altered condition must be documented, or the aircraft is not airworthy. So if it doesn't make the numbers, show me the paper.

 

[poadeleted20]

No such requirement in the FAR's for FP-propped installations, and with an FP prop, you won't see it in any stable flight condition. The only way you'll ever see 100% power in a FP-propped plane is very briefly in flight at sea level with full throttle before RPM redline is exceeded and the throttle must be pulled back.

It's not a FAR requirement, it is a certification requirement.

I've looked through FAR Part 23 and can't find any requirement that a FP prop be pitched to allow the engine to develop 100% of rated power, and FAR Part 23 is the certification requirement. Is there something I missed?

At each and every annual we IAs are required to run each engine and see that it adheres to its type certificate, or properly altered condition. It's properly altered condition must be documented, or the aircraft is not airworthy. So if it doesn't make the numbers, show me the paper.

Read the TCDS for any FP prop airplane and you'll see the RPM limits for the static run you IA's are required to do on an annual are way below full rated power. For example, my Tiger's engine is rated at 180HP at 2700 RPM at sea level (i.e., 30 inches MP). The TCDS sets the static RPM limits between 2175 and 2325. In that range, at full throttle at sea level, the engine is developing about 83% power. If it developed full rated power on a static power run, it would be turning 2700 RPM, and would be unairworthy.

I would point out that in most cases, failing a static run on the high side is due to an excessively worn or illegally modified prop. OTOH, failing on the low side is usually due to the engine not making the power it should, although it may also be due to an illegal prop (too much pitch).

 

[RotorAndWing]

CFR 23 maybe the certification requirement depending upon the airframe. In the example of the older Cessna 172 the CAR's apply, not CFR 23.

 

[Tom-D]

I've looked through FAR Part 23 and can't find any requirement that a FP prop be pitched to allow the engine to develop 100% of rated power, and FAR Part 23 is the certification requirement. Is there something I missed?[/COLOR][/COLOR]


Does FAR 23 cover STCs?

Read the TCDS for any FP prop airplane and you'll see the RPM limits for the static run you IA's are required to do on an annual are way below full rated power. For example, my Tiger's engine is rated at 180HP at 2700 RPM at sea level (i.e., 30 inches MP). The TCDS sets the static RPM limits between 2175 and 2325. In that range, at full throttle at sea level, the engine is developing about 83% power. If it developed full rated power on a static power run, it would be turning 2700 RPM, and would be unairworthy.

True, and if you don't see those numbers you better have the paper to modify the aircraft,

I would point out that in most cases, failing a static run on the high side is due to an excessively worn or illegally modified prop. OTOH, failing on the low side is usually due to the engine not making the power it should, although it may also be due to an illegal prop (too much pitch).

I guess your cause to debate was my use of the term rated power, Rated power is given on the engine type certificate, the numbers used at annual time are the installation or aircraft type certificate. They will be different.

In general, the prop pitch that will give the lowest RPM allowed during a static run, is a cruise prop, and the prop which gives the highest RPM allowed is a climb prop.

When I do a out of annual run up and the numbers are not the onesallowed I do some trouble shooting, the first Thing I check is the prop make and model, then the engine.

some of my customers will install a borer seaplane prop when they go to floats, and forget to remove it when they go back to wheels, so we must do the paper to make it legal.

 

[poadeleted20]

I guess your cause to debate was my use of the term rated power, Rated power is given on the engine type certificate, the numbers used at annual time are the installation or aircraft type certificate. They will be different.

Exactly my point. Glad we're on the same page.

In general, the prop pitch that will give the lowest RPM allowed during a static run, is a cruise prop, and the prop which gives the highest RPM allowed is a climb prop.

Typically, there is a set range for each type of prop for that aircraft type, and there is usually some overlap. For example, the stock AA-5A Cheetah can be equipped with either a 57-pitch climb prop or a 59-pitch cruise prop. The static ranges for the two props in the TCDS are 2250-2375 and 2250-2348, respectively. As you can see, while the top end is slightly lower for the cruise prop, the lowest allowable RPM is the same for both. For the AG-5B Tiger, the ranges for the three stock props are:

61" 2100-2275
63" 2050-2225
65" 2000-2175

Again, there is overlap, although the ranges are lower top and bottom for the higher pitches. Of course, this can vary from type to type, so consult your TCDS or other documentation. And it's a darn good idea to know the ranges for your aircraft/engine/prop combination, as low static RPM when powering up for takeoff is probably the biggest warning sign of serious engine issues (unless it's just your old mechanical tach reading low, but that's easy to check).