constant pitch prop`

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makapala
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constant pitch prop`

Post by makapala »

I just did my differences training today for wobbly prop irl in a Reims C172 (I know I know)
Anyway, my question is - during circuits the FI wanted me dialing a bit back on the power FAST - like.. within 200 ft of the ground.. after fly PA28's fixed prop, this seems very counter intuitive.
Does anyone have any tips or tricks on practicing standard patterns/circuits with a wobbly prop?

David

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ratty
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Re: constant pitch prop`

Post by ratty »

makapala wrote: 23 May 2023, 11:00
Does anyone have any tips or tricks on practicing standard patterns/circuits with a wobbly prop?
Everybody, apparently. If you do a web search you'll find all sorts of opinions. I thought this Reddit thread summed it up: https://www.reddit.com/r/flying/comment ... peed_prop/
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AKar
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Re: constant pitch prop`

Post by AKar »

For whatever my perspective is worth, one ought to take each instructor's (no matter how qualified) instructions exactly as that: instructions of one instructor, weighed as appropriate. It didn't take me but five hours of training in gliders to figure out that every instructor has his own ways of teaching and thinking. Sometimes "right", sometimes "wrong" but most often simply subjective. And anyone who doesn't alter one's opinions every now and then is indicative of not learning oneself!

In what comes to the constant speed props in circuit flying and things like that, I can only offer my technical/practical point-of-view: perhaps consider avoiding unnecessarily redlining the engine for extended periods of time. With some installations, especially high-horsepower ones with two-bladed props, noise is really something to consider and cutting just a bit back on prop can bring the tips back subsonic. As you often, but not quite always, need to reduce the throttle a bit to bring back the prop in a healthy manner, this may, in some conditions, require a relatively early power and prop reductions. But if you feel like rushing it, then perhaps you ought not to be doing that.

-Esa

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DHenriques_
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Re: constant pitch prop`

Post by DHenriques_ »

makapala wrote: 23 May 2023, 11:00 I just did my differences training today for wobbly prop irl in a Reims C172 (I know I know)
Anyway, my question is - during circuits the FI wanted me dialing a bit back on the power FAST - like.. within 200 ft of the ground.. after fly PA28's fixed prop, this seems very counter intuitive.
Does anyone have any tips or tricks on practicing standard patterns/circuits with a wobbly prop?

David
Normal procedure flying patterns with constant speed props is to use recommended power settings for the initial takeoff, when applicable reaching Vy reduce power to the recommended climb setting and bring back the prop to match that setting. This should get you through the crosswind and onto the downwind. Once on downwind, bring back the power and prop to recommended cruise temporarily. From there you can reduce power as required to slow the airplane down leaving the prop set at cruise. Fly a stabilized approach from the key position and once through the base turn and onto final, bring the power back momentarily, run the prop up to the recommended RPM for a go around then bring back the power as needed. (Reason for lowering the power before running up the prop is to avoid a propeller surge)
That's about it really.
When in doubt always follow the recommended procedure in your aircraft's flight manual.
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Oracle427
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Re: constant pitch prop`

Post by Oracle427 »

I would be curious to understand the reasons behind the desire to pull the power back so soon.

I agree with Esa about the knowledge, techniques and habits passed along by each instructor is a part of the greater learning experience. Everyone has past training, experiences, opinions and rationalizations for doing things certain ways. Always be inquisitive, keep an open mind and keep learning.

There are always many exceptions to everything I have learned. Everything that follows are my techniques and rationalizations in my own aircraft with a naturally aspirated engine.

I would not be inclined to reduce power 200 feet AGL after takeoff as it is trading away performance at a critical time.
For closed patterns, I will keep maximum performance until starting my crosswind turn when I am in a better position in case of an engine failure. At that point I will pull back the throttle quite a bit so I can maintain an appropriate airspeed in the pattern. For me that is 16" with the gear up.
If climbing up to cruise altitude, I will keep full throttle and rpm right up to my target altitude, then I will reduce MAP and RPM to my cruise settings. The only engine control I adjust is the mixture which I lean out to maintain steady EGTs from pattern altitude to cruise altitude.

Some engines have a 2-5 minute power restriction and power changes will be needed as appropriate as per the POH to avoid exceeding the limits. For example, if the engine I am flying behind requires an RPM reduction I'll take care of that as soon as I've established myself in a cruise climb which will happen in under 2 minutes after takeoff. Mother nature usually takes care of most, if not all, of the throttle reduction provided I climb high enough. If I start at ~28" at sea level at full power, I will only be at approx ~20" when I reach 8000 ft despite my throttle still being wide open. All I need to do is reduce RPM, lean and finish all other configuration for cruise.

Reducing power will cut out the full power circuit on the fuel metering system. (I believe that is what it is called) This circuit feeds extra fuel to help keep CHTs down and prevent detonation at wide open throttle. As a result keeping the throttle all the way in tends to keep the engine somewhat cooler under certain circumstances such as proper cruise climb speeds, cowl flaps open, etc.

Reducing throttle alone on a constant speed prop may not do a whole lot to reduce prop noise unless the prop RPM is also reduced following the throttle reduction. Props turning fast make a lot of noise! If you have the opportunity, note the stark difference in noise levels between two aircraft with similar engines where one is rated at 2800 rpm and the other at 2700 rpm at takeoff.

A couple of things in my brief prior to every flight.

Does your airframe have a time limit for certain power settings? Are you operating at high density altitude? You may already be operating below the restricted power range above certain density altitudes, unless the aircraft is turbo or super charged.
Do you have noise abatement procedures to comply with? Is there a certain flight path to fly? Would reducing power make the aircraft climb out more slowly and therefore possibly make more noise in the noise abatement zone?
Are there obstacles to clear?
Where am I going to go if my engine fails prior to reaching crosswind turn, what about if I reach crosswind?
Flight Simmer since 1983. PP ASEL IR Tailwheel
N28021 1979 Super Viking 17-30A

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DHenriques_
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Re: constant pitch prop`

Post by DHenriques_ »

Oracle427 wrote: 23 May 2023, 14:07 I would be curious to understand the reasons behind the desire to pull the power back so soon.
Generally speaking the reduction in power after takeoff isn't usually an issue. Naturally at Vy you wait until you have maneuvering altitude before the reduction.
The reduction offsets in that the power reduction is offset by an increase in prop efficiency as the pitch change is made.
Naturally cooling is monitored carefully all the time. Any cooling issue presented by the minor decrease in power should easily be controlled if even needed by the cowl flaps.
All mentioned here is also subject to runway length, terrain clearance, and post takeoff consideration for obstructions such as property, housing, etc.
Bottom line is that it's not written in stone. You fly your aircraft per the suggested manual settings and use common sense where and if needed.
DH

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AKar
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Re: constant pitch prop`

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Oracle427 wrote: 23 May 2023, 14:07 Reducing power will cut out the full power circuit on the fuel metering system. (I believe that is what it is called) This circuit feeds extra fuel to help keep CHTs down and prevent detonation at wide open throttle. As a result keeping the throttle all the way in tends to keep the engine somewhat cooler under certain circumstances such as proper cruise climb speeds, cowl flaps open, etc.
This is a very good point, and something that is not always considered when babying the engine. Pulling back the last inch or so from full throttle may not come with intended results at all. And the undesired results may not be immediately obvious enough for the pilot to ever notice or attribute correctly, unless into things, like, analyzing JPI graphs. And should issues ever develop, I doubt this way of operating the engine is too often recognized as contributory factor.
Oracle427 wrote: 23 May 2023, 14:07 All I need to do is reduce RPM, lean and finish all other configuration for cruise.
Speaking of manuals, some are written to include... unnecessary climb power reduction, which is sometimes taken as an obligatory operating procedure. Some engines are rated to operate at their maximum power indefinitely, and sometimes even an RPM reduction is allowed at sea level full throttle, at least implicitly by the manual's graphs. A timely example would be the O-540-A in the Comanche 250. Obviously not saying that doing either of these would always or necessarily be a smart thing to do. But I do think it is worth considering, if the engine is properly monitored and known to have no overheating tendencies at rated power climbs, to operate it precisely in the manner you describe instead of unnecessarily throttling back soon after takeoff only to work the throttle back in after a few minutes of climb.
Oracle427 wrote: 23 May 2023, 14:07 Reducing throttle alone on a constant speed prop may not do a whole lot to reduce prop noise unless the prop RPM is also reduced following the throttle reduction. Props turning fast make a lot of noise! If you have the opportunity, note the stark difference in noise levels between two aircraft with similar engines where one is rated at 2800 rpm and the other at 2700 rpm at takeoff.
I was first to say that reducing at 200 ft sounds stupid, but caught my tongue as, in fact, I know just one context in where something like this is/was sometimes done, though perhaps not quite that aggressively. Floats were very much present in the GA flying I was involved with back in the days. And sometimes during water takeoffs something like this was indeed done, precisely to counter the noise issues associated with props at or near maximum RPM. The main villains were airplanes such as Cessna 185, in particular when equipped with the original two-bladed propeller.

-Esa

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DHenriques_
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Re: constant pitch prop`

Post by DHenriques_ »

AKar wrote: 24 May 2023, 02:45
Oracle427 wrote: 23 May 2023, 14:07 Reducing power will cut out the full power circuit on the fuel metering system. (I believe that is what it is called) This circuit feeds extra fuel to help keep CHTs down and prevent detonation at wide open throttle. As a result keeping the throttle all the way in tends to keep the engine somewhat cooler under certain circumstances such as proper cruise climb speeds, cowl flaps open, etc.
This is a very good point, and something that is not always considered when babying the engine. Pulling back the last inch or so from full throttle may not come with intended results at all. And the undesired results may not be immediately obvious enough for the pilot to ever notice or attribute correctly, unless into things, like, analyzing JPI graphs. And should issues ever develop, I doubt this way of operating the engine is too often recognized as contributory factor.
Oracle427 wrote: 23 May 2023, 14:07 All I need to do is reduce RPM, lean and finish all other configuration for cruise.
Speaking of manuals, some are written to include... unnecessary climb power reduction, which is sometimes taken as an obligatory operating procedure. Some engines are rated to operate at their maximum power indefinitely, and sometimes even an RPM reduction is allowed at sea level full throttle, at least implicitly by the manual's graphs. A timely example would be the O-540-A in the Comanche 250. Obviously not saying that doing either of these would always or necessarily be a smart thing to do. But I do think it is worth considering, if the engine is properly monitored and known to have no overheating tendencies at rated power climbs, to operate it precisely in the manner you describe instead of unnecessarily throttling back soon after takeoff only to work the throttle back in after a few minutes of climb.
Oracle427 wrote: 23 May 2023, 14:07 Reducing throttle alone on a constant speed prop may not do a whole lot to reduce prop noise unless the prop RPM is also reduced following the throttle reduction. Props turning fast make a lot of noise! If you have the opportunity, note the stark difference in noise levels between two aircraft with similar engines where one is rated at 2800 rpm and the other at 2700 rpm at takeoff.
I was first to say that reducing at 200 ft sounds stupid, but caught my tongue as, in fact, I know just one context in where something like this is/was sometimes done, though perhaps not quite that aggressively. Floats were very much present in the GA flying I was involved with back in the days. And sometimes during water takeoffs something like this was indeed done, precisely to counter the noise issues associated with props at or near maximum RPM. The main villains were airplanes such as Cessna 185, in particular when equipped with the original two-bladed propeller.

-Esa
Perhaps this quote from the FAA "Airplane Flying Handbook" (12-7) referring to this exact question will help clear the air on this issue . I will add that in all the years I was active as a flight instructor teaching in complex aircraft I never once had an engine issue related to power reduction after takeoff.
Others experience may vary, but I never had a problem.

Here's the quote from the manual ;

" As the airspeed increases after lift-off, the load on the engine is lightened because of the small blade angle. The governor senses this and increases the blade angle slightly. Again, the higher blade angle, with the higher speed, keeps the blade AOA with respect to the relative wind small and efficient.
For climb after takeoff, the power output of the engine is reduced to climb power by decreasing the manifold pressure and increasing the blade angle to lower engine rpm. At the higher (climb) airspeed and the higher blade angle, the propeller is handling a greater mass of air per second at a lower slipstream velocity. This reduction in power is offset by the increase in propeller efficiency. The blade AOA is again kept small by the increase in the blade angle with an increase in airspeed.

Again..........I hope this added information proves helpful.
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AKar
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Re: constant pitch prop`

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DHenriquesA2A wrote: 24 May 2023, 08:26 I will add that in all the years I was active as a flight instructor teaching in complex aircraft I never once had an engine issue related to power reduction after takeoff.
Others experience may vary, but I never had a problem.
This discussion over possible engine issues a power reduction after takeoff refers specifically into 'mini-reductions'. Say, instead of staying all-in, the throttle is reduced only enough to see a drop of an inch or so in MP, to maximize the insult for the sake of this example. In some engine models, this does cause a drop from the takeoff fuel flow disproportionately large to any drop in power (in fact, the power may increase from the effective leaning as much as to fully compensate for this slight throttling).

I first encountered this specific "case study" when reduced power takeoffs were discussed and if they should be attempted in common GA pistons. (Obviously, the conclusion was that, for heavens sake, no. For reasons including, but not limited to, this specific technicality.)

-Esa

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Re: constant pitch prop`

Post by DHenriques_ »

In a normally aspirated engine, you’re gonna lose an inch of manifold pressure at 1000 feet anyway. Not sure where you want to go with this but I’ll let you take it.

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Re: constant pitch prop`

Post by MkIV Hvd »

DHenriquesA2A wrote: 24 May 2023, 11:43 In a normally aspirated engine, you’re gonna lose an inch of manifold pressure at 1000 feet anyway. Not sure where you want to go with this but I’ll let you take it.
That was the sound of a mic dropping... :lol: :mrgreen:

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AKar
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Re: constant pitch prop`

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DHenriquesA2A wrote: 24 May 2023, 11:43 In a normally aspirated engine, you’re gonna lose an inch of manifold pressure at 1000 feet anyway. Not sure where you want to go with this but I’ll let you take it.
Yes, which kinda makes the small reductions in power pointless regardless in those things specifically, of all the ideas out there. But perhaps enough said on such a minute point. That all comes down to reducing all the way back into 25'' or whatever in the world the book happens to recommend, instead of reducing just a bit down low there.

-Esa

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Re: constant pitch prop`

Post by DHenriques_ »

AKar wrote: 24 May 2023, 12:38
DHenriquesA2A wrote: 24 May 2023, 11:43 In a normally aspirated engine, you’re gonna lose an inch of manifold pressure at 1000 feet anyway. Not sure where you want to go with this but I’ll let you take it.
Yes, which kinda makes the small reductions in power pointless regardless in those things specifically, of all the ideas out there. But perhaps enough said on such a minute point. That all comes down to reducing all the way back into 25'' or whatever in the world the book happens to recommend, instead of reducing just a bit down low there.

-Esa
If it helps to clear the air I am NOT a fan of making minute power reductions after takeoff. You either fly it as the engine manufacturer calls for or you takeoff using recommended settings and if and when you reduce power you do that to the recommended settings as well.
How's that?
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makapala
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Re: constant pitch prop`

Post by makapala »

thanks everyone. I flew with a different FI from a different school this week and said 'don't f**** touch any of that until safe maneuvering altitude" hah

As for the POH, the 1968 Reims 172 checklist is at best vague, at worst dangerous. Everyone I've flown that plane with has found massive gaps in the information. AAAANNYWAY, it's powerful for a 172 and has its quirks... and we're all still learning as it's new to the airfield.

I reduced to 25/2600 at 1000ft then brought it back to 24/2500 for the cruise.

Oh.. and on our pre-flight checks yesterday we found a bird nest in the engine. Wagtail flew out when we opened the oil inspection flap.



mak

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AKar
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Re: constant pitch prop`

Post by AKar »

makapala wrote: 03 Jun 2023, 09:13 As for the POH, the 1968 Reims 172 checklist is at best vague, at worst dangerous. Everyone I've flown that plane with has found massive gaps in the information. AAAANNYWAY, it's powerful for a 172 and has its quirks... and we're all still learning as it's new to the airfield.
I assume this is the 'Reims Rocket' we are talking about here? I've never flown one nor even in one but have been otherwise somewhat involved with at least one example. I understand it is, well, as rocket as 172s ever go.

-Esa

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