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Constant speed prop use simplified...

pierre smith

pierre smith

Well Known Member
Mornin' all,

Since I've seen quite few questions from the newer guys, I though I'd try and explain just how easy a C/S prop is to use.

A few basics: Most C/S props have internal low pitch stops which limit the blades' travel with power off.

The governor has a high RPM stop (it's adjustable to give redline revs on takeoff), that limits the travel of the control arm, which is usually cable driven from the cockpit.

On takeoff, the prop control, mixture and throttle are all forward and the governor allows the prop to go to redline and as you accelerate, the blades automatically increase in pitch to prevent overspeeding and still maintain redline RPM's, courtesy of the governor's action.

After you're comfortably clear of the runway and climbing, a power reduction can be made, first by reducing the manifold pressure (retarding the throttle) and then simply screwing the prop control out (or backing it up on a quadrant), usually to 2500 RPM or so. What happens is that the governor increases the blades' pitch, trying to satisfy the new RPM you ask for.

You can leave the prop control alone for the rest of the flight and when a power reduction is needed, just use the throttle as usual.

As you enter the pattern and throttle back, the governor will flatten the blade pitch in order to maintain the 2500 RPM that you dialed in, resulting in quite a handy bit of braking. Continue reducing throttle as necessary and when you notice the RPM come down you will know that the blades have reached their internal stops and can go no flatter...the reason for the revs coming down.

At this point if you are on final, you have two options...leave the prop control alone or push it forward. In the event of a go-around, having it forward will allow full RPM's. If you leave it where it was, the engine will only go to the last RPM setting, in this case, 2500 RPM, which, BTW, is more than enough for a go-around in these airplanes...heck, my F/P Catto only turns 2400 climbing at 140 MPH!!

There are other handy things you can do with a C/S prop and that is to ease it forward to 2600 or so on nearing the pattern, to initially slow you somewhat (the blades went flatter), or leave it alone.

That's basically it.
 
Great write-up, Pierre! The only difference I use (and teach) is that with the carburated engines, it is OK to just pull the prop back to 2500 and leave the throttle full forward during initial climb after takeoff. The enrichment valve in the carb at full power actually helps to cool the engine, and will result in lower CHT's.
Vic
 
I'm with you guys! C/S Prop needs to be no more complicated than Pierre describes. I also pull the RPM's back a little after I've reached a safe altitude, while leaving the throttle full in (I don't believe in the "Oversquare Bogeyman" when it comes to small Lycomings...). I generally don't push the prop forward in the pattern until I know that the power is low enough so that it won't surge and make noise for the neighborhood below.....

Paul
 
Ironflight said:
I'm with you guys! C/S Prop needs to be no more complicated than Pierre describes. I also pull the RPM's back a little after I've reached a safe altitude, while leaving the throttle full in (I don't believe in the "Oversquare Bogeyman" when it comes to small Lycomings...). I generally don't push the prop forward in the pattern until I know that the power is low enough so that it won't surge and make noise for the neighborhood below.....

Paul
Click to expand...

Yep, I quit worying about oversquared operation when I flew a FP airplane with a MP gauge. You can't "lug" an engine any worse than a FP on takeoff or climb...
 
Nobody in the posts above has stated WHY they bother to decrease RPM to 2500 after reaching a safe altitude. Is it simply for noise reduction? Assuming that you are still climbing, if you want full cooling effect, it would seem that higher RPM and a faster turning fuel pump would be of benefit. Anything wrong with continuing at full throttle/max RPM until reaching cruise altitude?

erich
 
The mark of a professional. On behalf of all of us who live near airports, (and want them to stay open) thank you and well done. ;-)
DM


Ironflight said:
I generally don't push the prop forward in the pattern until I know that the power is low enough so that it won't surge and make noise for the neighborhood below.....

Paul
Click to expand...
 
One thing I teach my complex students is to leave the throttle alone after takeoff until reaching a safe altitude. A lot of engine failures occur after the pilot has changed something...why give it the opportunity at 500 ft AGL? Back the prop off to about 2500 RPM slowly, and leave the throttle where it is. Manifold pressure right after takeoff is usually somewhere in the 26" range during takeoff (depends on field elevation and pressure, of course), but because pressure decreases 1" for every thousand feet or so, the manifold pressure will drop to the rule-of-thumb "inches less than RPM" by the time you reach pattern altitude. (Or a little higher depending on the MP-RPM spread.)

Another note, most engine manufacturer's do allow higher manifold pressure settings than RPM. I'm looking at a 182Q manual here at my desk (don't ask). At a pressure altitude of 2000 ft, Cessna allows 23 inches of manifold pressure at 2100 RPM for 68% BHP. Obviously, check with your manufacturer, I'm just pointing out that the rule-of-thumb is a good rule-of-thumb, there are other allowable settings.

Two thumbs up for the set-it-and-forget-it C/S prop setting (Unless looking for a lower fuel burn). If you choose to go full prop during your before landing checklist, wait until you see a RPM decrease with power decrease before increasing your prop setting. I don't like pilots shoving the prop forward quickly in the pattern, causing a quick jump in RPM and drag so close to the ground. Everything should be done smoothly.

Apparently I didn't read everyone's post before hitting reply. You guys already covered these points. Sorry for the redundancy.
 
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Yeah, I was thinking of IO-520's

Ironflight said:
I'm with you guys! C/S Prop needs to be no more complicated than Pierre describes. I also pull the RPM's back a little after I've reached a safe altitude, while leaving the throttle full in .......Paul
Click to expand...

...we had in the Cessna Agwagons, which had a 5 minute full power limitation, but not our little Lycs. Good point.

Thanks,
 
erich weaver said:
Nobody in the posts above has stated WHY they bother to decrease RPM to 2500 after reaching a safe altitude. Is it simply for noise reduction? Assuming that you are still climbing, if you want full cooling effect, it would seem that higher RPM and a faster turning fuel pump would be of benefit. Anything wrong with continuing at full throttle/max RPM until reaching cruise altitude?

erich
Click to expand...

Noise, engine life have been mentioned.

Also, many engines have a "takeoff" rating and a "max. continuous" rating, requiring a reduced power setting for anything over a couple of minutes.

I usually fine tune the rpms in cruise to find the best combination of smoothest rpm (least vibration) and lowest cabin noise.

CDE
 
Flyfalcons said:
Noise reduction and engine longevity. Max RPM, espcially on a two blade prop, is quite loud.
Click to expand...

Noise yes... but it doesn't hurt most engines to stay at max RPM. Most Lycs will go to TBO at 2750 RPM the whole way. In fact, most of the helicopter engines turn a lot higher than that without any structural modifications.
 
We hear "magic" numbers like 2500, or 2100, or whatever...but shouldn't the RPM be whatever value lets the prop run "on point" for the altitude you're at? I know next to nothing about aerodynamics and prop theory, but recall reading or being told that the big advantage of a CS prop is that it lets the prop run at maximum efficiency for whatever conditions it is in. Surely 2500 (or 2100 or whatever) is not that setting (otherwise, why not just get a FP prop?)?

Granted, you'd be hard pressed to know the *precise* RPM setting at any time that gives you that, unless you had some sort of table or something...

one ref I found (no doubt there are jillions out there):

http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm
 
Flying Scotsman said:
We hear "magic" numbers like 2500, or 2100, or whatever...but shouldn't the RPM be whatever value lets the prop run "on point" for the altitude you're at? I know next to nothing about aerodynamics and prop theory, but recall reading or being told that the big advantage of a CS prop is that it lets the prop run at maximum efficiency for whatever conditions it is in. Surely 2500 (or 2100 or whatever) is not that setting (otherwise, why not just get a FP prop?)...
Click to expand...


Prop RPM depends on the mission (for me). If I'm just out turning 100LL into noise, I'll pull the prop back to 2400 or so; If it's acro, all knobs are as far forward as they can go. But for cruise, then yes, you need to play with it to determine what is the best combination of speed, economy, noise, etc, for YOUR airplane.

A "rule of thumb" is just a starting point, not the end all.
 
Toobuilder said:
Prop RPM depends on the mission (for me). If I'm just out turning 100LL into noise, I'll pull the prop back to 2400 or so; If it's acro, all knobs are as far forward as they can go. But for cruise, then yes, you need to play with it to determine what is the best combination of speed, economy, noise, etc, for YOUR airplane.

A "rule of thumb" is just a starting point, not the end all.
Click to expand...

Clearly, but the question still stands :)

I guess one should be able to generate a chart which plots maximum efficiency as a function of airspeed, altitude and temperature, no? Anybody ever do such a thing for an RV? Someone must have, and if so, was it generated analytically or empirically? Getting the flight test data might be prohibitively expensive...
 
Flying Scotsman said:
Clearly, but the question still stands :)
Click to expand...

Well, not really...

Flying Scotsman said:
I guess one should be able to generate a chart which plots maximum efficiency as a function of airspeed, altitude and temperature, no? Anybody ever do such a thing for an RV? Someone must have, and if so, was it generated analytically or empirically? Getting the flight test data might be prohibitively expensive...
Click to expand...


Who is to say that your RV-x would respond exactly the same as the "test subject" RV-x? If you're really trying to find optimum, I think you need to fly YOUR airplane and find out what that is. Besides, what better way is there to spend a few hours cross country on autopilot than playing with power levers and recording data? :)
 
My RV was a three speed!

I know the whole idea of this thread is how simple a C/S prop can be, and when I first started flying my RV6A, I did it just like Pierre described. But somewhere along the line I ran across a chart for the O-320/O-360 series that gave the specific recommended power settings for three typical cruise conditions. That was handy, because then I could set a specific power setting and predict accurately what performance I'd get.

Here's the chart I developed for my POH (I had an O-320):

MPH KTS RPM MAP Fuel Flow Pwr %
192 167 2400 23" 8.4 GPH 75%
180 156 2200 23" 7.4 GPH 65%
174 151 2200 21" 6.3 GPH 55%

So instead of a one-speed transmission, at least I had a three-speed! In practice, I think you could probably simplify it to 24 squared, 22 squared, and 21 squared and it would work out about the same.

You probably get better fuel performance with the prop trimmed for mission, but I never actually tested it. I also felt like I was treating my engine better by not always spinning it at 2500.

On the other hand, my Harley cruises at 4000 RPM and it's lasted for 35 years!
 
Toobuilder said:
Who is to say that your RV-x would respond exactly the same as the "test subject" RV-x? If you're really trying to find optimum, I think you need to fly YOUR airplane and find out what that is. Besides, what better way is there to spend a few hours cross country on autopilot than playing with power levers and recording data? :)
Click to expand...

Oh, believe me, I *dream* of that day!

I was just wondering if anybody else had done it for their plane, out of curiousity and as a starting point (and to find out, I guess, what kind of test flight profiles you'd need to do in order to really, fully characterize the performance of your system).
 
Hard to parse prop efficiency and engine efficiency

What we want is powertrain efficiency. In general, WOT improves engine efficiency, as that minimizes pumping losses, while lower RPM improves engine efficiency by minimizing fricitional losses. Propeller efficiency is generally improved at lower RPM, due to minimizing the Mach number out at the tip. Thus, depending upon your altitude, desired % of power, and tolerance for "oversquare" operation, WOT and lower RPM are your best bet. All of these factors pale in comparison to what's going on with the mixture knob.
 
Considering all the differences in replies here, no one should ever have to worry about using a CS prop "wrong", because apparently there isn't any. I think the only absolute is to make sure the knob is full forward on takeoff and final approach... But I suspect someone will chime in and even disagree with that. :)
 
Pierre, the blue knob moves? ;) :D

Just kiddin', and actually, nice rules of thumb to start with, and it does become second nature, as you said.

But it would be interesting to see if there is a gouge for most effecient RPM. Guessing with all the variables of prop type, altitude, MP, speed, etc., it may not fit into an easy box...though it would be interesting to see.

And I would agree that the red knob will have as big (or bigger) impact in terms of nmpg/smpg. I did some X-C testing (as Toolbuilder mentions) at 21, 23, 25, and 2700 RPM (9,500', WOT, 100 ROP, IO-540) and found that my FF increased about 2 gph for each step up of 200 RPM, nmpg went down about 1 nmpg for each step, and range decreased about 10% for each step. Leaning to peak (OK, cause I was up high) gave back those 2 gph, and increased range markedly (no surprise there!). Of course max range is far slower (L/D max, or close, depending on winds, right?) but this was an interesting exercise to find rules of thumb for cruising in my RV...to get an idea of how fast and how far I'd go at each RPM, and how much it'd cost me in range and fuel burn to go from slower to faster. (YMMV, of course).

Maybe one good reason to ease back on the revs when cruising is to keep CHTs in an engine-friendly range, though I'd have to say that during those tests, the CHTs stayed pretty good, backing up the premise that the engine can run at 2700 all its life. At 21-2300, I saw 330-360, at 2500, I saw 340-370, and at 2700, I saw 370-390 (with 1 at 400...watching that one). I've read that 380 is a sweet spot (not gospel, just some reading).

So Pierre, here we go complicating your nice "CS made easy" post...sorry! :)

Cheers,
Bob
 
You guys have been busy.....

....while I was sleeping:)

I can summarize: Everything forward for takeoff.

Pull throttle back a little for cruise....that's all....don't touch the blue knob.

Regards,
 
Fortunately....

....at 2700 RPM, our prop tips aren't supersonic like the IO-520's at 2850 with their long props...man it is ear-splitting on takeoff...kinda like an unmuffled rotary engine at Road Atlanta races.

Blue skies and tailwinds...
 
Two Questions, please

1. Is there any way to set a CS prop so that the pitch will not change? In other words, if you set it for the highest pitch (lowest rev's) will it hit a limit and stay there? I'm only asking because of some testing that might be done that will work only with a constant pitch (not constant speed). I understand that you might not want to fly that way under normal conditions.

2. Isn't there, for any given prop, an advance ratio or advance angle for which it was designed to give best efficiency? Therefore wouldn't the best speed for the prop vary with IAS or TAS? While best engine efficiency will be at WOT, won't there be a best setting that includes the prop efficiency in its calculation?
 
hevansrv7a said:
1. Is there any way to set a CS prop so that the pitch will not change? In other words, if you set it for the highest pitch (lowest rev's) will it hit a limit and stay there? I'm only asking because of some testing that might be done that will work only with a constant pitch (not constant speed). I understand that you might not want to fly that way under normal conditions.
Click to expand...

As the airspeed of the aircraft increases, the prop will advance to a course pitch in an attempt to keep the RPM constant until the piston in the hub "bottoms out". At this point, the blades can not move any further, but they will back off the stop again as soon as the speed is reduced. In theory, you could machine the spacer that the piston "bottoms out" against to provide the maximum blade angle you want, then ask the governor to provide a hydraulic "stop" on the other side to keep it from backing off that pitch by pulling the prop control all the way out. In this case, you'd have effectively a fixed pitch prop at this one single blade angle.


hevansrv7a said:
2. Isn't there, for any given prop, an advance ratio or advance angle for which it was designed to give best efficiency? Therefore wouldn't the best speed for the prop vary with IAS or TAS? While best engine efficiency will be at WOT, won't there be a best setting that includes the prop efficiency in its calculation? [/FONT]
Click to expand...

Not sure exactly what you are asking, but yes, most engines are run WOT in cruise, and the prop is adjusted to suit. Since the engine can be adjusted to provide the "efficiency" you want (best power, best economy, etc), the propeller is only used to create thrust. More thrust is more efficiency, so you are really looking for the best speed for the power, and that is your most efficient propeller RPM. Making the propeller spin faster does not always make the airplane go faster.
 
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Yup

hevansrv7a said:

2. Isn't there, for any given prop, an advance ratio or advance angle for which it was designed to give best efficiency? Therefore wouldn't the best speed for the prop vary with IAS or TAS? While best engine efficiency will be at WOT, won't there be a best setting that includes the prop efficiency in its calculation?
Click to expand...

If I remember my aerodynamics classes correctly (long time ago and I have advancing CRS), the answer is yes there is a particular advance (meaning essentially an average prop angle of attack) that is appropriate for each speed. Without hitting the books again, I couldn't say whether it is at max L/d or just max L or somewhere inbetween.

This would give a constant speed prop operator the chance to tune for that advance, if the owner decided to do enough research and testing to find the sweet spots and chart them.

Since I rent, I just use Piere's methods on the antique spammers that I fly at the moment. Too much stuff hanging in the breeze to bother looking for efficiency.
 
Perhaps we need to split this into two seperate threads - "Constant speed prop use simplified" and "Constant speed prop use made as complicated as humanly possible" :D
 
Training

So I was taught a little differently ... was it wrong?... After take off I reduce to 25sq and maintain that through climb. Once at altitude I reduce to 2350 sq. If I need to climb it's prop in to 2500 followed by the throttle and climb etc. On decent I don't touch the 2350 prop until very late in final for noise management.

Re-educate me!

DWS



Ironflight said:
I'm with you guys! C/S Prop needs to be no more complicated than Pierre describes. I also pull the RPM's back a little after I've reached a safe altitude, while leaving the throttle full in (I don't believe in the "Oversquare Bogeyman" when it comes to small Lycomings...). I generally don't push the prop forward in the pattern until I know that the power is low enough so that it won't surge and make noise for the neighborhood below.....

Paul
Click to expand...
 
dwschaefer said:
So I was taught a little differently ... was it wrong?... After take off I reduce to 25sq and maintain that through climb. Once at altitude I reduce to 2350 sq. If I need to climb it's prop in to 2500 followed by the throttle and climb etc. On decent I don't touch the 2350 prop until very late in final for noise management.

Re-educate me!

DWS
Click to expand...

No re-education required - looks fine to me. The thing is...."square" mens nothing. It's an accidental relationship brought about by the units used. if you were reading RPM in Radians per Second, and manifold pressure in psi, no one would think of them as square. the numbers are fairly arbitrary - the point of Pierre's post is that the operation is simple if you want it to be!

Paul
 
dwschaefer said:
So I was taught a little differently ... was it wrong?... After take off I reduce to 25sq and maintain that through climb. Once at altitude I reduce to 2350 sq. If I need to climb it's prop in to 2500 followed by the throttle and climb etc. On decent I don't touch the 2350 prop until very late in final for noise management.

Re-educate me!

DWS
Click to expand...


By "sq", are you saying that you pull the MP back to 25 inches as well ("25 squared" = 2500 RPM/25 inches)?
 
Ironflight said:
No re-education required - looks fine to me. The thing is...."square" mens nothing. It's an accidental relationship brought about by the units used. if you were reading RPM in Radians per Second, and manifold pressure in psi, no one would think of them as square. the numbers are fairly arbitrary - the point of Pierre's post is that the operation is simple if you want it to be!

Paul
Click to expand...

Paul is dead-on here...if you stop and think about it, the 25" MP is measured in *inches* of *mercury*. What the...? Why on earth should that have anything to do with the proper stoichiometric ratio and spark advance timing to get proper burn and such on an engine? It just so happens that the two measurements (25" and 25 hundred RPM) *sound* close together (they're not even that, really...25" and 2500 RPM are different by two orders of magnitude).

So how did this old wive's tale about running "square" or avoiding "over-square" come about?

Again...shouldn't this be more analytical in nature? There should be a chart that says the max efficiency for speed, or climb is at X" and Y RPM at Z Density Altitude and S speed, etc.?

I suspect this is what goes into the algorithms and control loops for a FADEC, no? Along with other things, such as pitch/ROC/etc.?

I confess to always setting "25-squared" at 1000' AGL upon departure, but now I'm rethinking this seriously...just keep everything where it is but ensure that RPM is below redline, and let 'er climb, then reconfigure at cruise altitude? (Until the -7A is done, this is for Archers and Arrows, though :( ).

But since I expanded this thread to max efficiency and whatnot in the first place, I wanted to commend Paul on pointing out what should be obvious...the units chosen are affecting the operational decisions (or in other terms, the "flight rules"), and that shouldn't be the case.
 
What we want is powertrain efficiency. In general, WOT improves engine efficiency, as that minimizes pumping losses, while lower RPM improves engine efficiency by minimizing fricitional losses. Propeller efficiency is generally improved at lower RPM, due to minimizing the Mach number out at the tip. Thus, depending upon your altitude, desired % of power, and tolerance for "oversquare" operation, WOT and lower RPM are your best bet. All of these factors pale in comparison to what's going on with the mixture knob
Click to expand...
.


I fly a 10. Shortly after take off, I lower the RPM to 2200 and fly WOT until I reach the altitude I want. It is nice and quiet and I feel that it is easier on the engine than at full throttle. I get a lot of flak when I mention this to others, but no one has indicated that it is hard on the engine.

Gary Specketer
 
RPM Restriction

I don't think it's been mentioned yet but don't forget about the Hartzell recomendation to reduce RPM to 2600 or less "as soon as practical after take-off" or some such wording. This is only if you have electronic ignition or high compression pistons on an O-360. The BA blade removes a 2050-2250 avoid slot but the high RPM restriction remains. This was due to vibration studies done by Hartzell.

As others have stated, I also reduce MP and RPM at about pattern altitude after take-off to 25... umm, to some settings that are easy to remember for climb. I'll cruise at 2300-2400 and leave it there until just before landing. I once forgot to run the blue lever up before landing and had to go around. As I crammed all three levers up to full the resulting surge and noise must have sounded like a real amateur at the controls.

Paul Danclovic
Jamestown NC
RV-8A N181SB
 
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carb enrichment feature and related pubs

The enrichment valve in the carb at full power actually helps to cool the engine, and will result in lower CHT's.
Vic

Somehow, what Vic said above got buried in this thread, but is absolutely correct, especially here in South FL during the hot season. To prove it, try one takeoff where, at a safe altitude, you pull back to 25" then 2500 rpm, then one where you simply pull back to 2500 rpm. If method B does not result in lower CHTs after prop RPM reduction, than A, there is something wrong.

One other note is that fuel flow, and percent power, is a function of RPM and MAP. I reduce the RPM in cruise to lower fuel flow and go faster. But when I stop going faster, I stop reducing. My O-360/carb/CS RV-6 also has a 2050-2250 RPM range to avoid, but about 2300 RPM, WOT, at 8k', seems about the best, re min fuel flow and best TAS.

If you're really into this stuff, the best two pubs I''ve read on it are:

EI pub "The Pilot's Manual...":

http://buy-ei.com/Information/Pilots Manual.pdf

And Lyc pub "Key Reprints":

http://www.lycoming.textron.com/support/tips-advice/key-reprints/index.html
 
wv4i said:
but about 2300 RPM, WOT, at 8k', seems about the best, re min fuel flow and best TAS.
Click to expand...

I have played around with just about every permutation of cruise power settings and have also found that 2300/25" works the best for cruise.
 
Seems to me Dan C used to play around a lot with C/S prop settings, LOP stuff and got pretty good economy with his big engine......perhaps he'll pontificate on the matter. Presently I'm flying an Arrow (200 hp) and in instrument mode for training purposes I'll generally run 2400/21-22". Almost never "square" . But again, that's an Arrow.
 
Extra fuel at fuel throttle

Per Lycoming's "Key Operations" publication, extra fuel at full throttle also occurs in fuel injected engines.

"Part throttle takeoffs should be avoided. The fuel injector metering jet is a two-hole unit, which is interconnected with the throttle. The secondary jet is fully opened only at full-throttle conditions. The richer fuel flow supplements engine cooling and deters engine damaging detonation" (page 27)

Bill Swatling
RV-7 fuselage
 
Another CS prop operation question ...

I found this old thread of Pierre's on the operation of a CS prop and, having read through the thread, it seems like there are very few hard and fast rules of correct operation.

I have gotten into the habit of pulling back the RPMs and throttle to around 2500/25" upon reaching a safe altitude, then to 2350 at my cruise altitude. Because I seldom climb very high and because I'm a really cheap guy, I find that I'll pull the throttle back to 21-22" to keep fuel burn low. When I'm just smashing bugs and enjoying flight, I see no reason to go any faster than 150 knots.

My question, however, has to do with "balls-to-the-wall" lever/ball positions. It's my understanding that top speed is obtained with everything forward. I asked a few race guys and they all admitted that they race with everything forward.

If that's the case, then why have constant speed props at all? The forward position is used for take-off and climb. When I pull back the blue lever I can actually feel the plane surge forward. By keeping the prop lever forward in level flight, how is that any different from having a FP climb prop? The RV-1, for example, climbed like a homesick angel but topped out at around 120 knots.

I know that the explanation will be either ridiculously simple (and I'll feel like an idiot for asking) or engineeringly complicated (and I won't understand the explanation). Regardless, I'd like to hear an explanation for how a CS prop gives top speed at high RPM but a FP prop doesn't.

Oh, and as an aside, I did some experimenting with cruise settings and speeds the other day and did NOT get my fastest speeds with all levers forward, whatever that means.

Any takers?
 
I'll try to give a simple explanation of how a C/S prop differs from a F/P climb.
Even a F/P climb prop will not allow full power during take-off and climb.
A C/S prop will allow rated rpm during take-off, but then it will increase pitch during cruise to keep the engine from going over red line.

So even though the lever remains full forward, the pitch does not remain constant.
 
Not forward.

Hi Don, FWIW, I found that my -10 goes faster with the prop pulled back to around 2630 RPM, not 2700 RPM's.

Alan Barrett told me at Sun 'n Fun several years ago, that I could run my IO-540 at WOT for it's entire life and I wouldn't hurt the engine, maybe lower the overall TBO.

He added that with that being said, however, the airplane may be faster with a little more coarse pitch, or "bite", so I tried it and whaddya know! The Dynon TAS reading went from 212 TAS at 1,000' MSL to 214 TAS. The next year, I tried some more (less RPM) and went 217 MPH TAS.

All this is with the blended airfoil Hartzell on my -10. You may or may not get the same results in a -7 or Lancair..or a different prop....just try it.

The racers may not be telling all either:)
 
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props

The standard Hartzell prop does seem to work best at 2600 r/m or slightly more.
When you get into the 250 m/h range it is difficult to get a fixed pitch prop that will still give optimum top speed performance and get you off the ground. It can be done but the takeoff performance will almost always suffer.
The British racing seaplanes were examples of the extremes with fixed pitch props. The final aircraft in this series had 2350 horsepower. The aircraft on display has a fixed pitch prop. They were able to make this work only because it was a seaplane. I have seen discussions of absurdly long takeoff runs that would not have been possible in that era(late 20's early 30's) on runways.
Dave Anders and Tracy Saylor(RV6) had problems getting past 225-235 m/h until they modified their Hartzell props by twisting the blades and in Daves case reducing the diameter. The reliability when doing these mods becomes a big question mark. Daves RV4 at the peak of its development was doing around 260 statute.
 
Pierre wisely provided this as a "general rule". Like all such rules there are exceptions, for better or worse. One of the exceptions not addressed (at least these old eyes havent spotted it) is the technique of proper leaning. Forget this in some aircraft at certain airports at your own peril. Other exceptions like running full rpm all of the time may take much longer to manifest their problems. Glad to see everyone thinking beyond that.
Great discussion. Thanks Pierre.
 
Pitch stops?

Good timing to see this thread again.

With a std CS prop loss of oil will drive the blade to fine pitch. How do you know when to adjust the blade stops? There has to be some point because if the gov has to do a lot of work to keep the RPM limit, then loss of oil or gov failure in level flight will result in immediate overspeed.

I ask, because on my ground run I did not quite hit full power and was able to drive the RPM to 2860 for a brief second before I caught it. I guess the good news is the engine is making 210+ hp, but I am thinking I need to adjust the fine limit back some.

I surmise that the optimum condition would be for the prop stop limits to be such that on the ground, motionless, you could not get more than say 2700 RPM. With forward motion that RPM will increase (like a fixed pitch prop, since it is at this point) to say with failure you should see no more than 2750 or 2800 in level flight.

Any thoughts?
 
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Screw the pitch stop in until the max RPM is 2700 when starting the take-off roll. It usually takes a 1-2 secs for the gov to catch up and control the RPM, so noting the RPM with a rapid throttle increase during TO is the best way I've found to adjust it. Keep turning the stop in until you see that "overspeed" come down to around 2700 or 2750. Most digital tachs and EFIS have peak RPM readings so you can use those to catch the RPM.

Or... you can back off your gov screw letting the RPM rise to say 2800, then screw in the pitch stop till you see the max RPM at 2750, then go back and re-adjust the gov to 2700.

If your gov is already set method one elimates having to adj the gov twice.

Does any of this make sense :confused:
 
JRS got it. The one "variable" we cannot change is the blade twist. The prop is probably optimized for cruise airspeed and something like 2500 RPM. If the efficiency drops, when you go to 2700 RPM, more than the engine power goes up, then you'll be faster at the lower RPM. CS props are better than fixed pitch cruise props on takeoff, but they're still a ways off from ideal. the inner part of the blade is probably stalled at the start of the roll, even on CS props.

As to the other question: if you lose oil pressure on a CS prop, you'll probably find you have to pull the throttle way back, and slow to 80 knots or so, to keep the prop and engine from over speeding. Of course you'll want to look for a place to land since the engine is likely to fail from oil starvation in the very near future.
 
Hartzell BA prop

I also noticed that the Hartzell BA prop on the RV-8 in my avatar was a couple knots faster when the blue lever was pulled back a wee bit to between 2600-2650 RPM than it was full forward at 2700 when at wide open throttle.
 
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