[s_tones]

So as not to hijack the other thread....

Anybody care to summarize the operational management of the cs prop in the different phases of flight for a simple minded fp guy?

I'm particularly interested in how you stay out of trouble in terms of over speeding the prop and what is the most efficient way to run the cs prop in flight.

Steve

 

[Andy Hill]

Steve...

Here follows a few thoughts / suggestions They are not comprehensive, some will probably disagree with some/all of them... They are based on a UK education, so even more room for disagreement

1. Training Nothing will substitute for some proper "training" by an Instructor. In the UK now you need a "differences training course" signed off by a qualified instructor for a VP/CS Prop. It might only be an hour or 2, but worthwhile...
2. Setup You need to ensure both the "static RPM" of the engine / prop combination are set [see Prop Manual] to a little below 2700 RPM (typically), but also the governor. In practice you might find the governor is factory set OK, but the prop will need doing. When you "smartly" set full power for takeoff, the governor will not act quickly enough to prevent exceeding 2700 RPM for a period... So you set the "fine pitch stop" to, say, 2650 RPM to limit the initial RPM to that... and then the governor maintains it...
3. Overspeeding You will only tend to overspeed for Fine Pitch stop not set (see above), governor not set (see above), or lack of oil pressure (non-aerobatic prop) e.g. under negative 'g'. Suggest set 2400RPM for aerobatics, and smartly retard throttle if you hear engine speeding up... lack of oil for any period of time is hardly doing the engine any good either! Apart from these, the prop/engine is basically impossible to overspeed, one of it's advantages for aerobatics etc. (i.e. "carefree handling").
4. Handling At "Max RPM" / "Fine Pitch" = Blue Lever fully forward, you enable the engine to produce full power (e.g. 180HP), albeit at a cost in noise / lack of fuel efficiency etc. Since you usually want full power for takeoff and landing (in case of a Go Around), then you will check/select the Blue Lever all the way forward in your pre-takeoff and downwind checks. In the cruise, when you might not want full power, and do want fuel efficiency, you will select a lower RPM e.g. 2400 RPM/2200 RPM.
5. Over-boosting Selecting too much manifold pressure / too little RPM can "over boost" the engine, with potential damage (apparently!). Various rules of thumb are out there, but 1st 2 digits of RPM + 4.5 for Max MP is one... e.g. 2200RPM, don't use a lot more than 26" MP - if you want to go faster, use more RPM as well... As a consequence, when changing power settings, if increasing set RPM first, then MP, and reducing power, reduce MP first, then RPM...
6. Pre-Take off With, say, 1800 RPM set, cycle the prop lever 3 times (1st flight of day, maybe only once later flights) to reduce RPM by (some say only 50RPM on 1st cycle) 200-300RPM. Checks the system, and (allegedly) circulates the warm engine oil into the prop.

As I say, I am sure your question and my reply will offer lots of room for debate

Andy & Ellie Hill
RV-8 G-HILZ

 

[N131RV]

How I do it.

So as not to hijack the other thread....

Anybody care to summarize the operational management of the cs prop in the different phases of flight for a simple minded fp guy?

I'm particularly interested in how you stay out of trouble in terms of over speeding the prop and what is the most efficient way to run the cs prop in flight.

Steve

It depends on the airplane and engine of course:

For me, it's very similar to the way I fly a 210 with C/S Prop.

For takeoff, full fine (max rpm). I usually leave it there until I've got the gear and flaps up, then reduce power FIRST (always with C/S) then reduce RPM to cruise (2350 or so).

Cruise, at 75% power or less, "oversquare" gets maximum efficiency (but not always). Square means MP in inches and rpm/100 and MP match, like 25"/2500.
So oversquare means rpm/100 is less than MP in inches IE: 23"/2200 rpm ( a common cruise number).

I typically cruise at 5000' MSL and above: WOT, lean of peak at between 2250 and 2350 rpm with whatever MP I can get.

Landing, that C/S is a nice fat brake. Enter the pattern, drop power to around 15" or so (varies from plane to plane) and push the prop full in (fine pitch).

Over speeding is not normally a problem as the prop will adjust to stay at the current RPM setting. You can over speed your prop if you lose oil pressure.

Just how I do it, YMMV.

JP

 

[KirkGrovesRV8]

So as not to hijack the other thread....

Anybody care to summarize the operational management of the cs prop in the different phases of flight for a simple minded fp guy?

I'm particularly interested in how you stay out of trouble in terms of over speeding the prop and what is the most efficient way to run the cs prop in flight.

Steve

Hi Steve,
You don't get more simple minded than me
I take off max rpm and reduce rpm to below 2600 at around 500ft (hartzell blended airfoil prop) usally 2550 or so. I leave the throttle wide open as I climb, as I reach my altitude I level off reduce MP to around 21" and then reduce rpm to 2350rpm. And then basically leave it alone until you are abeam the numbers on downwind and slowed down. Then as a final check boost pump on and prop and mixture full forward, then land

 

[Webb]

Simply put

When you want to increase power - Prop first then throttle
When you want to decrease power - Throttle first and then prop

If you can remember this, the rest comes easy.

Image the prop is the standard transmission in a car. You do not want to step on the accelerator at slow speed in high gear (coarse pitch). You want to have it in a lower gear (fine pitch) when you want to increase the power. Shift to the lower gear (prop to fine pitch) and then add power. When you want to slow down or reduce power, you take your foot off the gas and let the engine slow down, not shift it into a lower gear (fine pitch) that increases the rpm. Simple right?

Here's a great example....On take off, the prop is all the way forward and full power is applied. When at a safe altitude, the power and rpm needs to be lowered. Let's assume you want to finish climb out at 25 squared. The old pro pulls the throttle back to 24 inches of MP and then backs the prop back to 2500 rpm. Amazingly, the MP increases 1 inch and the engine is now set at 25 squared. If you pulled the prop back first, you would have lots of power on an slower running engine which is harmful.

Hope this makes sense now.

 

[Bryan Wood]

But when landing...

(Agree with everything previously posted.) However, when you are setting up to land you will now be flying a fixed pitch prop for all practical purposes. After stuffing the prop control forward for landing and with the power back your prop will be in fine pitch and you will then need to transition from the manifold pressure guage to the tach. This is likely the only time during flight where your tach will act like you are used to seeing it in a fixed pitch plane by changing in relation to throttle movements. The manifold pressure guage will either not be reading at all, or only showing minimal pressure.

 

[erich weaver]

When you want to increase power - Prop first then throttle
When you want to decrease power - Throttle first and then prop

If you can remember this, the rest comes easy.

Image the prop is the standard transmission in a car. You do not want to step on the accelerator at slow speed in high gear (coarse pitch). You want to have it in a lower gear (fine pitch) when you want to increase the power. Shift to the lower gear (prop to fine pitch) and then add power. When you want to slow down or reduce power, you take your foot off the gas and let the engine slow down, not shift it into a lower gear (fine pitch) that increases the rpm. Simple right?

This is the standard operating procedure that is written in all the instructional books, and I have no problem with it. However, under normal operating conditions and using normally aspirated engines like ours, you would be hard pressed to come anywhere near engine damage by simply adjusting the RPM and MP in the "wrong" order.

Furthermore, RPM is typically only adjusted two or three times once the flight has started: maybe a slight decrease in RPM after take-off to lessen noise, again at cruise altitude to "go into high gear" and save fuel, and once when preparing to land (back to high RPM). That last change isnt really a change at all, since at that point, the low MP setting prevents any increase in RPM from occurring - you are simply preparing for a possible go-around. These three standard RPM changes are not necessarily accomanied by an associated adjustment in MP. Many folks reduce MP upon reaching cruise altitude for fuel savings, but you dont have to, and there are definte advantages to operating oversquare (MP > RPM/100), if you are low enough to be able to achieve that condition. Better yet, push that throttle all the way in and go lean of peak to get your fuel savings - whole 'nother topic there.

Anyway, my point is we really dont need to get too hung up on whether to push the throttle or the prop knob first - it simply isnt going to matter under normal circumstances. That said, if you happen to be throttled way back, cruising at 2000 feet at 2200 RPM and you decide you have to catch your buddy that just went screaming by, yes, you are well advised to increase RPM first, then MP, so that you avoid encountering a condition where MP is very high, and RPM is very low.

erich

 

[hecilopter]

I make it easy

Take off - prop forward, full throttle
500 ft - prop back to 2500, climb to cruise altitude
at Altitude - prop back to 2300 for cruise & lean

--Notice I never touch the throttle from full for takeoff, in climb & cruise

Decent to land - pull throttle back to 23" and pull back gradually to maintain 23", richen as required
Pattern - reduce throttle to 100 mph
Downwind - Now push prop all the way forward & fly with throttle

--Notice I never touch the prop in decent

None of the above conditions even come close to hurting the engine.
Makes it dirt simple.

 

[L.Adamson]

Pattern - reduce throttle to 100 mph
Downwind - Now push prop all the way forward & fly with throttle

--Notice I never touch the prop in decent

None of the above conditions even come close to hurting the engine.
Makes it dirt simple.

Yep....................if you're somewhere over that 100 mph & shove the blue knob in; you'll feel it in the stomach & you'll most likely remember not to do it again!

Other than that, I'm typical full throttle too. But at my altitudes it's usually 22" mp at best.

L.Adamson --- RV6A

 

[frankh]

somebody care to

Care to elaborate on where the "reduce throttle before RPM" rule comes from?

I'm hving a hard time beliveing reducung RPM with full throttle is harmful but I'd like to know what evidence is out there on engine damage one way or the other.

Frank

 

[Finley Atherton]

I routinely use the % HP display on the engine monitor for adjusting the prop during climb. Typically, after take off I climb with full throttle and bring the prop back to show 75% power. As the altitude increases, I increase the rpm to maintain about 75% power. At about 7,500ft both throttle and prop are full fwd with the HP decreasing below 75% as the climb continues.
My understanding is that full throttle helps cooling due to an excess of fuel and this should help keep CHTs lower in the climb.

Fin
9A

 

[hecilopter]

Probably the over square days

Care to elaborate on where the "reduce throttle before RPM" rule comes from?

I'm hving a hard time beliveing reducung RPM with full throttle is harmful but I'd like to know what evidence is out there on engine damage one way or the other.

Frank

I'm going to guess somebody made that rule so by reducing MP before rpm will never let you get over square. Opposite true on the increasing side (rpm before MP).

 

[frankh]

Not so sure

I routinely use the % HP display on the engine monitor for adjusting the prop during climb. Typically, after take off I climb with full throttle and bring the prop back to show 75% power. As the altitude increases, I increase the rpm to maintain about 75% power. At about 7,500ft both throttle and prop are full fwd with the HP decreasing below 75% as the climb continues.
My understanding is that full throttle helps cooling due to an excess of fuel and this should help keep CHTs lower in the climb.

Fin
9A

About that Fin,

The throttle kinda only indirectly adjusts fuel flow..The mixture knob is really what controls fuel, whereas the throttle controls the flowrate of air allowed into the cylinders....Slowing the flowrate will reduce the fuel but this is an indirect function if you see what I mean.

yOu want full throttle in the climb only if you need the power..Indeed your setting yours to 75% which is just fine, but adjusting the mixture is what will contol the fuel flow.

If you want to run ROP keep the CHT's down then keeping full rich will do this...But the same thing can be done at less power (and a lot less fuel burn) by running LOP if you are fuel injected. Typically LOP ops are used at 65% or less although the need to do that maybe a myth too but it works so why not.

Frank

 

[mannanj]

Care to elaborate on where the "reduce throttle before RPM" rule comes from?

I'm hving a hard time beliveing reducung RPM with full throttle is harmful but I'd like to know what evidence is out there on engine damage one way or the other.

Frank

Frank:

Do some research on radial engines before and during WWII. That era is where the "square" rule come into being too. Seems that the radial engine with the "master" rod could be damaged by disregarding the throttle-rpm rules as stated above.

With modern (circa 1920) horizontally opposed engines, the old rules are good practice, but I doubt that you could hurt a Lycoming unless you totally disregard the common practice for an extended period of time.

 

[airguy]

It's primarily for boosted engines, non-boosted engines will be difficult to damage by operating oversquare (within reason).

 

[Finley Atherton]

About that Fin,

The throttle kinda only indirectly adjusts fuel flow..
Frank

Frank,
For a carburetor at least, I was under the impression that at full throttle an additional jet/passage?? is opened (independent of the mixture setting) which delivers extra fuel to help keep CHTs lower during full throttle take off. If this is correct, then bringing the revs back while maintaining full throttle in the climb may help CHTs.
Any carb experts out there able to verify or refute this?

Fin 9A
0-320 carb

 

[Norman CYYJ]

Every fix pitch plane runs over square on take off and climb. No damage done.

 

[Scott Hersha]

As stated earlier, some of the power application/reduction rules came from the radial recip days when most of these engines were supercharged and you could 'overboost' the engine - BMEP limits exceeded. Some of the reasons to be careful with how you reduced power started with geared engines, like the cessna 421, where pushing the prop lever forward without slowing first - fine pitch - caused the gearing to be loaded from the wrong side, ie. the prop driving the engine instead of the other way around. This lead to early engine problems in the 421 and other geared engines. In a normally aspirated direct drive engine, like the Lycoming O-360 you can't really hurt it with the prop lever. That being said, it's not a good idea to cycle the prop on run-up while breaking in a new or newly overhauled engine, other than once initially to make sure the prop governor is working. This is according to ECi. On my RV-8 with a Whirlwind prop, my max continous RPM limit is 2600. So on takeoff at full forward fine pitch, I get about 2700 (2650-2700) and at about 500' or abstacle clearance altitude, I pull the prop back to 2600 or below - usually about 2550 - and leave the throttle wide open. The wide open throttle opens up the power enrichment jets (I'm told) allowing extra fuel for cooling in the climb. WOT (wide open throttle) in a carbureted engine like mine is also theoretically more efficient in cruise because the butterfly valvle is pretty much out of the way allowing more efficient airflow through the carb and into each intake tube. Back in the traffic pattern for landing you shouldn't have to push the prop forward and be shoved forward in your seatbelt unless you need it to slow you down. This I'm told won't hurt the engine, but it's not that comfortable and could just be a result of poor planning. On base or final, slowed to 100K or below, pushing the prop full forward will have no noticeable effect, but will prepare you for a go-around. If you're flying formation, set 2550 RPM and leave it alone until after you land.

Scott

 

[flyeyes]

(snip)However, when you are setting up to land you will now be flying a fixed pitch prop for all practical purposes. (snip)

Hi Brian.

I hate to disagree with you but I think I do here.

IMHO one of the most important differences between a FP and CS prop is in the landing approach and it's very important to understand.

With the CS prop retarding the throttle doesn't just reduce thrust, it simultaneously increases drag. Very small throttle movements have a disproportionate affect on the glide ratio. This is especially obvious in an RV with the short wings.

When landing a FP RV, airspeed control is critically important, and unforgiving if you have a short runway. An idle speed set 100 RPM too high can result in a couple of hundred feet or more of float/excess rollout. A failure to get the throttle hard against the aft stop might force a go-around.

A CS RV is a different animal. You have to be aware of the potential for a very high sink rate/loss of energy if you're slow and suddenly retard the throttle, but it also makes you very flexible. In many ways (the instantly available drag) it is like a piston twin. The RV has the advantage over a twin of much less inertia so you can quickly arrest the sink with a little power if necessary.

Most of my flying time is behind CS props, and it has made me a little lazy. My RV time is 95% CS, but I find the landing and formation flying workloads to be much higher with a FP prop. Short or spot landings are relatively trivial tasks in my RV-8.

 

[L.Adamson]

Hi Brian.

I hate to disagree with you but I think I do here.

I kind of figured that he just meant the fact, that you no longer use the prop control. He did mention that it was in fine pitch.

But I like your excellent explanation of what happens in the landing process regarding C/S & F/P. It's very informative for those that are not aware.

L.Adamson --- RV6A/ Hartzell CS

 

[Bryan Wood]

Hi Brian.

I hate to disagree with you but I think I do here.

IMHO one of the most important differences between a FP and CS prop is in the landing approach and it's very important to understand.

With the CS prop retarding the throttle doesn't just reduce thrust, it simultaneously increases drag. Very small throttle movements have a disproportionate affect on the glide ratio. This is especially obvious in an RV with the short wings.

When landing a FP RV, airspeed control is critically important, and unforgiving if you have a short runway. An idle speed set 100 RPM too high can result in a couple of hundred feet or more of float/excess rollout. A failure to get the throttle hard against the aft stop might force a go-around.

A CS RV is a different animal. You have to be aware of the potential for a very high sink rate/loss of energy if you're slow and suddenly retard the throttle, but it also makes you very flexible. In many ways (the instantly available drag) it is like a piston twin. The RV has the advantage over a twin of much less inertia so you can quickly arrest the sink with a little power if necessary.

Most of my flying time is behind CS props, and it has made me a little lazy. My RV time is 95% CS, but I find the landing and formation flying workloads to be much higher with a FP prop. Short or spot landings are relatively trivial tasks in my RV-8.

With 100 or so rpm affecting the approach so dramatically very fine power adjustments can be made with a visual clue as to what is being changed. If you use the manifold pressure guage in this scenario by the time it gives a useful indication at such low power settings the approach will be all over the place. I think we are actually implying the same thing here, just in different ways. The point of my post was simply to show a way to measure and give useful feedback to the pilot of what you are talking about. I took that part for granted. Thank you for the additional clarification.

 

[roee]

Hi Brian.

I hate to disagree with you but I think I do here.

IMHO one of the most important differences between a FP and CS prop is in the landing approach and it's very important to understand.

With the CS prop retarding the throttle doesn't just reduce thrust, it simultaneously increases drag.

James, I think you need to examine or clarify that statement. Please correct me if I'm wrong, but here's my understanding:

Once you are set up for landing with a CS prop, you're throttled back sufficiently that your RPM will be low enough that the prop is effectively off the governor. In other words, at such a low RPM due to retarded throttle, the prop will be at max fine pitch no matter what you do with the blue knob. Hence, at that point, it is effectively a fixed pitch prop.

Now, the differences that you describe in the amount of drag and reduction of glide ratio do exist. But they are simply attributed to the fact that a CS prop at fine pitch is more finely pitched than your average FP prop. In other words, if you happened to have a FP prop that was pitched as a pretty drastic climb prop (same pitch as the CS in fine pitch), you would experience the same amount of drag and reduced glide ratio as you described with the CS prop.

So in effect, I think Bryan's original statement is true. You are at that point effectively flying a FP prop, since the governor is out of the picture. But it's an extremely fine pitched FP prop, hence the high drag that James describes.

If there's an error in my statements, please somebody point it out. This is a great discussion, but I'd like to demystify this issue once and for all.

... When landing a FP RV, airspeed control is critically important, and unforgiving if you have a short runway. An idle speed set 100 RPM too high can result in a couple of hundred feet or more of float/excess rollout. A failure to get the throttle hard against the aft stop might force a go-around.

A CS RV is a different animal. You have to be aware of the potential for a very high sink rate/loss of energy if you're slow and suddenly retard the throttle, but it also makes you very flexible. In many ways (the instantly available drag) it is like a piston twin. The RV has the advantage over a twin of much less inertia so you can quickly arrest the sink with a little power if necessary. ...

That's good practical advice.

 

[Bryan Wood]

About that Fin,

The throttle kinda only indirectly adjusts fuel flow..The mixture knob is really what controls fuel, whereas the throttle controls the flowrate of air allowed into the cylinders....Slowing the flowrate will reduce the fuel but this is an indirect function if you see what I mean.

yOu want full throttle in the climb only if you need the power..Indeed your setting yours to 75% which is just fine, but adjusting the mixture is what will contol the fuel flow.

If you want to run ROP keep the CHT's down then keeping full rich will do this...But the same thing can be done at less power (and a lot less fuel burn) by running LOP if you are fuel injected. Typically LOP ops are used at 65% or less although the need to do that maybe a myth too but it works so why not.

Frank

Frank,

I think Finley is right on this. At full throttle with the mixture control full rich the mixture is richer than at partial throttle and the engine temps will be lower as a result. If memory serves me right it is the "Enrichening Valve" that causes this to happen.

 

[flyeyes]

James, I think you need to examine or clarify that statement. (snip) throttled back sufficiently that your RPM will be low enough that the prop is effectively off the governor. In other words, at such a low RPM due to retarded throttle, the prop will be at max fine pitch no matter what you do with the blue knob. Hence, at that point, it is effectively a fixed pitch prop.

(snip)

You're right of course that if RPMs are below the governor's range, the prop will be against the fine pitch stop. The difference between that and a typical FP airplane is that the fine pitch stop results in such high drag that it would be unsuitable as a FP prop, even a climb prop.

Remember that we typically have the pitch stop set to allow full RPM on the takeoff roll (although somewhat short of full RPM static). This means that a FP prop pitched that flat would be RPM limited and unable to produce full power in flight, even at Vx.

In practice, I think most people carry just enough MP pressure to keep the prop off the stops and only pull all the way to idle late in the flare. I frequently practice power-off approaches but I'm very careful because the very steep descent necessary is unusual and I'm vulnerable to descending into someone flying a more normal flatter (and faster) approach.

I typically don't fully advance the prop until short final in the RV. In the limited amount of FP RV flying I have done, the props are so coarse because of the speed range that even "climb" props leave the airplane very slippery.

IMHO a CS prop completely changes the "personality" of an RV.

Of course, other airplanes are different. I'm fortunate enough to have access to a Lockwood AirCam which is maybe the wierdest airplane I've ever flown. It is a twin with fixed-pitch props. Because of the airplane's extremely narrow speed range and high drag, CS props would be a useless extravagance. Even with an engine shut down, the airplane's drag overwhelms the drag of the dead engine/prop and it's just a matter of adding a little throttle to the good engine and putting in a little rudder.

 

[Garage Guy]

CS management with some Hartzell props

Another issue for some of us running Hartzell CS props is that -- depending on prop, engine, and ignition system combination -- there may be ranges of RPM and throttle settings that Hartzell thinks you should avoid:

http://www.vansaircraft.com/pdf/Hartzell_c2yr.pdf
http://www.vansaircraft.com/pdf/Hartzell_c2yk.pdf

Not really hard to do, but it can add a little bit of complexity to the blue knob management list.

--Paul

 

[s_tones]

Whooooa

Thanks you guys.
I think I get the jist of it although it is still a bit cornfusing.
I think I'll have to actually do it to make sense of it but the information above is a great start.

Steve

 

[flyeyes]

Thanks you guys.
I think I get the jist of it although it is still a bit cornfusing.
I think I'll have to actually do it to make sense of it but the information above is a great start.

Steve

It seems to me that we have pontificated a lot without really answering your original question.

I'll tell you how I manage my prop on a typical cross-country:

I start the engine , warm up and taxi with the prop control fully forward.

During the run up, after checking the mags, I "cycle" the prop a few times. This means that with the engine at 1800 RPM or so and the stick fully back I will bring the prop control smoothly back until I see/feel the RPM begin to drop. You can watch the tach but you are really just looking for a drop which you can hear and feel. The airplane will feel as if it wants to surge forward as the prop takes a "bigger bite" of air. Your oil pressure will also drop a little. As soon as I feel the drop, I quickly but smoothly move the prop control back forward. You don't want to hold it back, just see it begin to move. I do this 2-3 times on the first flight of the day to circulate warm oil through the prop.

For takeoff (near sea level) the prop and mixture are fully forward. I advance the throttle smoothly to the stop and rotate, looking for Vy until clear of obstacles, then pitching down a little for a cruise climb. Everything stays forward on a normal takeoff until I'm above all obstacles. I note the WOT/full rich/sea level EGT. The intial prop RPM is about 2600 but quickly builds to 2700 as the airplane begins to move.

Others will differ here, but my first power reduction after takeoff is a slight reduction in the prop RPM to 2500-2600 RPM. The throttle stays fully forward, and the manifold pressure reduces itself automatically because of the climb. The prop RPM reduction dramatically lowers the noise produced by the prop tips.

The EGTs will begin to fall as the airplane climbs, and I will pull the mixture back in small increments, keeping the EGT at or below the takeoff EGT (I have my airplane setup so the WOT EGT is about 200 degrees rich of peak)

I normally cruise between 9 and 12 thousand feet, and leave the prop and mixture alone until leveling off and allowing the airplane to accelerate as much as it can. The throttle will stay wide open, and I usually set the prop between 2400 and 2600 depending on the noise/speed tradeoff I'm looking for. I then lean the airplane and typically don't touch the prop again until short final. The throttle will stay wide open until I begin my descent.

The prop goes fully forward (slowly) on short final at about 85 knots and maybe 15" MP.

 

[s_tones]

Thanks James...one more question

It seems that folks commonly run WOT throughout the flight until preparing to land. This seems at odds with the way I fly a fixed. I only run WOT on takeoff and generally fly at 65% power just to be nice to Mr. Engine.

Are you effectively reducing engine power by manipulating the prop pitch in cruise? Does the manifold pressure correlate directly to power output?

Excuse my ignorance

Steve

 

[Bryan Wood]

It seems that folks commonly run WOT throughout the flight until preparing to land. This seems at odds with the way I fly a fixed. I only run WOT on takeoff and generally fly at 65% power just to be nice to Mr. Engine.

Are you effectively reducing engine power by manipulating the prop pitch in cruise? Does the manifold pressure correlate directly to power output?

Excuse my ignorance

Steve

Yes. A simple rule of thumb, though debated by some with slide rule and pocket protecters is the following.

Manifold pressure added to the RPM/1000 can be used to closely approximate power.

Example:

23" + 2300 rpm would be 23+23

For my engine after checking the Lyc graphs if they add up to 43 then engine is running at 55% power, if they equal 46 it's close to 65%, and 49 is 75%. My understanding is that for an 0-360 you would use 42,45, and 48 instead. Any combination of mp and rpm will get you close.

I originally read this in an issue of the RVator and was a tip shared by Van. Some of the folks with in panel electronics that give percent power could likely tell you how far off this trick is, but it will usually get you so close that it really doesn't matter if it is off a little. You'll probably find that the Lyc charts are not totally useful with your RV because with the airbox setup that we use you will get approx. 1.5" of manifold press. boost in flight. None of the charts that I've seen allow you to make the power that we are seeing and you have to kind of guess anyway.

 

[flyeyes]

It seems that folks commonly run WOT throughout the flight until preparing to land. This seems at odds with the way I fly a fixed. I only run WOT on takeoff and generally fly at 65% power just to be nice to Mr. Engine.

I generally cruise at 65% or less as well. With a FP prop, you can't reduce RPM except by reducing throttle or slowing down (e.g. climbing). This limits your options and reduces your efficiency. You can optimize a FP prop for one flight regime (typically 8000' cruise on an RV) but is it going to be inferior at other times to the CS. Of course, if you're Jon Johansen and spend 99+% of your time in long-range cruise, you can gain the advantage of less weight, cost and maintenance with the FP prop ;-).

The engine is most efficient at WOT, because this minimizes pumping losses. If the throttle is partially closed, air has to make it around the throttle plate and this hurts efficiency.

[QUOTE)
Are you effectively reducing engine power by manipulating the prop pitch in cruise? [/QUOTE]

Exactly. At my normal cruise altitudes I can't make more than 60-70% anyway, even at WOT and 2700 RPM

Does the manifold pressure correlate directly to power output?

Yes, but so does RPM and (within limits) fuel flow. You can adjust power with any of the three levers. Maximum power output on a Lycoming is always at WOT (around 30" MP at sea level), Max RPM (usually 2700 depending on the model) and Best Power mixture (usually about 80 degrees rich of peak EGT. Lycoming publishes power output tables for specific engine models that you can use to determine output for a given condition.

If you're operating lean of peak, power conveniently correlates exactly with fuel flow, so you can use the mixture as your primary power control.

Excuse my ignorance

Ha! That's what this board is for, ideally ;-). Spend some time with Deakin's articles on engine management (available if you root around AvWeb) and you'll know more in a few hours than many pilots learn in their career.

I can scratch the surface pretty well, but there are posters here who really understand how engines work.

Steve

 

[erich weaver]

I agree, consult the Deakin articles for a thorough education. His phhilosophy is a little different on one point however - he would say "throw out the power tables, who cares what power setting you are at?" Instead, determine what your flight mission is (go fast, be fuel efficient, etc) and set your MP, RPM and fuel flow accordingly.

One other note: power is a complicated thing, and depends on MP, RPM, fuel flow (mixture), and to a lesser extent temperature. When flying cruise, rich of peak, you are generally fixing your RPM and mixture, and adjusting your power with MP (throttle). When flying lean of peak, you generally are fixing RPM and MP (keep at full throttle) and adjusting power with fuel flow (mixture). The standard power tables will not give you correct info when running LOP. Instead power can be estimated by multiplying fuel flow (gph) x 14.9. This applies to normally aspirated lycomings and lycoming clones.

Check out this thread for additional discussion of LOP operations

http://www.vansairforce.com/community/showthread.php?t=16293&highlight=erich

erich

 

[s_tones]

As always, thanks. I will follow up on those other sources.

Steve