The blue knob

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evapilotaz

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Can some explain to me what the blue knob is in high performance airplanes? Is that for varible pitch propellers. What does it exactly do and how do you use it? Sorry I don't have high performance rating so I'm un educated.
 
it's not specific to HP, it controls the pitch of the prop/RPM. I'm sure someone will come along and tell me why I'm wrong, but that's basically it.
 
MAKG1 said:
It's an airplane transmission.

You take off in low gear and can back it off in cruise.

CS props are not limited to HP airplanes. They were even in some 172s.
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That.
 
Shawn said:
Holy crap...do you have any idea how much easier the concept of a CS prop would be to explain if every description just started out with that simple statement...
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It's how I've been explaining it to CAP cadets on their first orientation flights. It's not EXACTLY correct, but it's close, and it does serve a very similar purpose. Even the 12 year old kids have some idea what a transmission is for.
 
evapilotaz said:
Sounds good. Thanks for the replies. Does the manifold pressure have anything to do with it?
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Check the POH for approved MP/RPM combinations.
 
It's also a volume control for the noise from the prop tips. Also comes in handy for lowering CHTs sometimes.
 
Re: Manifold Pressure... Think of it as a measurement of the throttle position. More specifically the air pressure inside the intake system between the throttle and the pistons. It can be used to calculate how much "power" (lay term, not scientific) that a running engine is making. Thats the oversimplified, unscientific and not entirely factually correct version.

The lower the MP, the less air (and fuel) the running engine has coming into it. The higher the MP, the more air (and fuel) that is going into the running engine. If you have a turbocharged car/truck that has a boost gauge.... thats a manifold pressure gauge.

In planes with fixed pitch props, the throttle position is linked to the RPM and there is no need for a separate manifold pressure gauge. If you had one in a fixed pitch plane it would go up and down in lockstep with the RPM gauge. In planes with adjustable or constant speed props, you can adjust the throttle (and the manifold pressure) independently of how you adjust the propellor.

I dont know your background or how much you know about reciprocating engines, but back in the early 2000's AvWeb had a series of columnists. One of them, John Deakin, had an EXCELLENT series of articles/blogs on engine operation. Including a 6 part series on turbocharged engines. He was also a fan of Lean Of Peak operations. Attached are two links, to his articles on the blue knob (prop RPM control) and the Black knob (Throttle/Manifold Pressure control)

http://www.avweb.com/news/pelican/Pelicans-Perch-15-Manifold-Pressure-Sucks-182081-1.html

http://www.avweb.com/news/pelican/Pelicans-Perch-16-Those-Marvelous-Props-182082-1.html

Additionally here is the link to his index of articles. I encourage anyone who operates piston planes to read these from time to time.

http://www.avweb.com/news/pelican/182146-1.html
 
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FastEddieB said:
See the Pilot's Handbook of Aeronautical Knowledge, p. 7-6.

Check back in if you're still unclear after reading that.
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:rolleyes:


That thing reads poorly.

Best book is "from the ground up"

The gas peddle and the gear shifter analogy works quite well too.

Don't think I've ever told a student to just read the AIM, just not the best way to convey the information, almost as bad as those old gleim books
 
Do some suggested reading and then grab a complex aircraft, a CFI and get your complex endorsement. Then you will understand.
 
James331 said:
:rolleyes:


That thing reads poorly.
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It's not great. But a good place to start and I like to point inquisitive students to sources of information, and the PHAK is an essential source.

Don't think I've ever told a student to just read the AIM, just not the best way to convey the information, almost as bad as those old gleim books
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Not sure where the AIM comes in here. I'd bet there's a good video on YouTube on constant speed props. I'd look for one and post a link, but better that the OP get in the habit of searching out informative content to answer his questions.
 
FastEddieB said:
It's not great. But a good place to start and I like to point inquisitive students to sources of information, and the PHAK is an essential source.



Not sure where the AIM comes in here. I'd bet there's a good video on YouTube on constant speed props. I'd look for one and post a link, but better that the OP get in the habit of searching out informative content to answer his questions.
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*PHAK

My bad
 
Not all blue knobs go to constant speed props. I flew behind a variable pitch, ungoverned prop for 15 years. My wife never even realized what she was missing until after we upgraded to constant speed.
 
SkyDog58 said:
Do some suggested reading and then grab a complex aircraft, a CFI and get your complex endorsement. Then you will understand.
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I know of at least one instance where that's not even remotely true. Me.

I have a mooney with a CS prop and a few hundred hours in it. Did tons of reading, asked on here and others forums, no one could recommend anything more than "it's a transmission, check your poh for valid combinations" stuff. I even remember reading an avweb article that said something about "pumping losses" making an engine less efficient at anything other than WOT. Confused? oh, yeah. I can't think of a reason, can't find a reason, not to leave it full forward.

So, my operational procedure is... leave it max rpm. According to the POH, it gives the highest cruise speed, it needs to be full forward for takeoff, and I didn't buy this airplane to go slow. About the only reason I can see to adjust it is to save gas, and the price of fuel doesn't matter that much to me. I did play with it a bit before getting the ANR headset, but losing 15kts wasn't worth it.

What would be helpful, to me, would be if someone could give reasons for different settings (not the specific settings, I'll look in the POH), for different purposes (takeoff, climb, cruise, etc) and why. All I've been able to find is "save gas", and I don't much care about that. Anything else?
 
As a point of information, there's a thread going on on the Cirrus Owner's site where multiple owners report that for any given fuel flow, they fly a bit faster at an rpm less than maximum.

I recall a test in my SR22 where I found the same thing.
 
Interesting. How much is a bit? I'm definitely going to play around next time. I just went from one valid combination to another without pausing in between looking for a local maximum.
 
Mistake Not... said:
What would be helpful, to me, would be if someone could give reasons for different settings (not the specific settings, I'll look in the POH), for different purposes (takeoff, climb, cruise, etc) and why. All I've been able to find is "save gas", and I don't much care about that. Anything else?
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It's also a volume control for the noise from the prop tips. Also comes in handy for lowering CHTs sometimes.

Take off with all knobs forward (if using sea level mixture), pull back the blue knob a little so as not to bother the neighbors in the climb, pull it back a bit more if you want a comfy cruise, leave it back until base or final to make the neighbors happy again, then full forward again prior to landing.

Also, some emergency checklists say to haul back on the blue knob to get best glide in some planes.
 
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Mistake Not... said:
Interesting. How much is a bit? I'm definitely going to play around next time. I just went from one valid combination to another without pausing in between looking for a local maximum.
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Note that @FastEddieB said "for a given fuel flow". You already mentioned that you weren't interested in saving gas. But if you were, set the prop to full, adjust the throttle to the speed you want, then note the fuel flow. After that, set full throttle and then pull back on the prop until you get the same fuel flow. You should see a higher speed.
 
Mistake Not... said:
I know of at least one instance where that's not even remotely true. Me.

I have a mooney with a CS prop and a few hundred hours in it. Did tons of reading, asked on here and others forums, no one could recommend anything more than "it's a transmission, check your poh for valid combinations" stuff. I even remember reading an avweb article that said something about "pumping losses" making an engine less efficient at anything other than WOT. Confused? oh, yeah. I can't think of a reason, can't find a reason, not to leave it full forward.

So, my operational procedure is... leave it max rpm. According to the POH, it gives the highest cruise speed, it needs to be full forward for takeoff, and I didn't buy this airplane to go slow. About the only reason I can see to adjust it is to save gas, and the price of fuel doesn't matter that much to me. I did play with it a bit before getting the ANR headset, but losing 15kts wasn't worth it.

What would be helpful, to me, would be if someone could give reasons for different settings (not the specific settings, I'll look in the POH), for different purposes (takeoff, climb, cruise, etc) and why. All I've been able to find is "save gas", and I don't much care about that. Anything else?
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Not sure what plane you have, but full forward for me is limited, max continuous is basically youre saying a climb prop give you a faster cruise than a cruise prop.
 
First, I'm going to echo the comments above... especially given the circumstances you describe. Even with the experience you have in your aircraft it's always worth taking A CFI on-board periodically, especially while you explore new features or procedures that you're not as familiar with.

btw, you don't mention which Mooney you have, I just pulled a POH online for an M20J, which has the recommended Op Procedure for a cruise climb of 26/2600. There are prescriptions in the POH, not just options.

In terms of reasons, there are a few.

1. Wear/Engine Life - At lower revs the forces, friction and therefore wear on the engine reduces. In the last Mooney I flew, the green arc went all the way to 2700, but in my current plane I need to be in the range 2100 - 2500 for green arc operations.
2. Noise - Even if you're wearing an ANR headset, those on the ground are not. There's quite a difference to the noise footprint.
3. Vibration - ANR headsets can help reduce the noise, but vibration is also a big cause of fatigue, lower RPMs reduce the vibration
4. Speed control - I've not flown a Mooney in a while, but in my current plane I'd find it impossible to fly at the speed needed for turbulent air penetration, remain in the green arc for the decent and enter the pattern without using the full range of the MP and RPM green arcs.

I'm sure there are others, but those are ones that spring to mind for me at the moment.
 
James331 said:
Not sure what plane you have, but full forward for me is limited, max continuous is basically youre saying a climb prop give you a faster cruise than a cruise prop.
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I'm limited to max throttle (2 min, iirc). After I reach pattern altitude, I throttle back to 24" (75%), and stay there. Which is something else I found confusing. I know I read an avweb article (I think talking about LOP operations) where it recommended max throttle until leveling off from cruise. Which would be against my POH. So..

I'd like to understand what you mean "climb prop" vs "cruise prop". I get it for fixed pitch props. But my POH clearly (or maybe not :) seems to indicate fastest cruise is at 2700rpm and 24". Anything less is, well, less.

If I have a mental model of this at all, the way I think about it is "Max rpm gives max thrust, but it's very inefficient. You can develop less thrust but more efficiently at lower rpm."
 
CMongoose said:
First, I'm going to echo the comments above... especially given the circumstances you describe. Even with the experience you have in your aircraft it's always worth taking A CFI on-board periodically, especially while you explore new features or procedures that you're not as familiar with.
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Getting my IR now. He thinks it's dumb to burn gas I don't have to, but it simplifies operation and doesn't appear to be hurting anything. For example, 90kts level is '22" gear down, flaps half". 500fpm, 90kts is same thing, 17".

btw, you don't mention which Mooney you have, I just pulled a POH online for an M20J, which has the recommended Op Procedure for a cruise climb of 26/2600. There are prescriptions in the POH, not just options.
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1981 'J'. I'll check mine and post it (I have it scanned, but not with me). "recommended", though, isn't a prohibition. If it makes any difference, I'm one of the few with the IO390 upgrade. But the manual for that says all POH numbers still apply, so probably doesn't make a difference.


2. Noise - Even if you're wearing an ANR headset, those on the ground are not. There's quite a difference to the noise footprint.

Good point. So, lowering rpms near an airport, for example, would be a good thing.

3. Vibration - ANR headsets can help reduce the noise, but vibration is also a big cause of fatigue, lower RPMs reduce the vibration

My instructor was motivated to get his prop balanced after flying my airplane. I won't take credit for it, but whoever had it before me had a REALLY good prop balance done.

4. Speed control - I've not flown a Mooney in a while, but in my current plane I'd find it impossible to fly at the speed needed for turbulent air penetration, remain in the green arc for the decent and enter the pattern without using the full range of the MP and RPM green arcs.

Are you worried about "shock" cooling? Because (again, teh interwebz are full of contradictions) from what I've read "shock cooling" isn't something I have to worry about unless I'm trying to beat jumpers to the ground. Those guys do apparently run through cylinders like they're disposable. I just close the cowl flaps (if open, close in cruise anyway), throttle back and push over until I get the airspeed I want. In the summer, I stay high until way late and then pop the speed brakes.

I'm sure there are others, but those are ones that spring to mind for me at the moment.
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thanks. I'm genuinely interested in learning to fly this airplane better. I do not in any way claim what I do is optimal, just doesn't seem to be hurting anything and gives me what I want.
 
Mistake Not... said:
If I have a mental model of this at all, the way I think about it is "Max rpm gives max thrust, but it's very inefficient. You can develop less thrust but more efficiently at lower rpm."
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Horsepower=RPM x Torque x constant. Pull back on the throttle and you reduce HP. Pull back on the prop and you also reduce HP.
 
asicer said:
Horsepower=RPM x Torque x constant. Pull back on the throttle and you reduce HP. Pull back on the prop and you also reduce HP.
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Ok. But isn't thrust (force pulling airplane forward) a function of RPM only? You can keep available horsepower constant, but drop into a lower gear, your top speed is reduced? (To make a probably incorrect analogy.)
 
Mistake Not... said:
Getting my IR now. He thinks it's dumb to burn gas I don't have to, but it simplifies operation and doesn't appear to be hurting anything. For example, 90kts level is '22" gear down, flaps half". 500fpm, 90kts is same thing, 17".

1981 'J'. I'll check mine and post it (I have it scanned, but not with me). "recommended", though, isn't a prohibition. If it makes any difference, I'm one of the few with the IO390 upgrade. But the manual for that says all POH numbers still apply, so probably doesn't make a difference

Are you worried about "shock" cooling? Because (again, teh interwebz are full of contradictions) from what I've read "shock cooling" isn't something I have to worry about unless I'm trying to beat jumpers to the ground. Those guys do apparently run through cylinders like they're disposable. I just close the cowl flaps (if open, close in cruise anyway), throttle back and push over until I get the airspeed I want. In the summer, I stay high until way late and then pop the speed brakes.

thanks. I'm genuinely interested in learning to fly this airplane better. I do not in any way claim what I do is optimal, just doesn't seem to be hurting anything and gives me what I want.
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Come join us at MooneySpace. There's a lot of information available there specific to our planes. You won't be the only '81 J, and I think there are others with the IO390.

I fly approaches at 90 knots, 16-17", Takeoff Flaps, Gear Up. Lowering the gear starts my descent, beginning 1-1/2 dots high. In a VFR descent, I don't reduce the throttle, I just push for 500 fpm and walk the throttle back and mixture forward to maintain cruise MP & EGT. But my C-model doesn't run LOP very well those guys are WOT from departure to pattern entry.

Congratulations on having a good plane, and working to increase your knowledge and ability. See you around!
 
Mistake Not... said:
I'm limited to max throttle (2 min, iirc). After I reach pattern altitude, I throttle back to 24" (75%), and stay there. Which is something else I found confusing. I know I read an avweb article (I think talking about LOP operations) where it recommended max throttle until leveling off from cruise. Which would be against my POH. So..

I'd like to understand what you mean "climb prop" vs "cruise prop". I get it for fixed pitch props. But my POH clearly (or maybe not :) seems to indicate fastest cruise is at 2700rpm and 24". Anything less is, well, less.

If I have a mental model of this at all, the way I think about it is "Max rpm gives max thrust, but it's very inefficient. You can develop less thrust but more efficiently at lower rpm."
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A CS prop and a fixed setting might as well be a fixed pitch.

In climb having a high rpm and taking a smaller bite of air makes sense, but in cruise when the engine doesn't need to put as much work into climbing, let the prop take a bigger bite of air and spend the climb energy going forward. If that makes sense.

My R1 can do nearly 100mph in first gear, but it dos well over that in 6th and its a lot happier doing it.
 
Leaving a CS prop set for the same RPM for both takeoff and cruise is not the same as leaving it at the same pitch setting. The pitch will vary to maintain that RPM as other conditions change.
 
SkyDog58 said:
Leaving a CS prop set for the same RPM for both takeoff and cruise is not the same as leaving it at the same pitch setting. The pitch will vary to maintain that RPM as other conditions change.
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This is true, but shy of something like a fully automatic prop on something like a PC12, there is a reason that knob turns.
 
FastEddieB said:
As a point of information, there's a thread going on on the Cirrus Owner's site where multiple owners report that for any given fuel flow, they fly a bit faster at an rpm less than maximum.

I recall a test in my SR22 where I found the same thing.
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Engine power within its powerband is roughly MP*RPM, so you will get more out of higher RPM. At least with decent mixture settings.

MP is how hard you push on the piston. RPM is how often you push on the piston. You can get more out of it by pushing more often or pushing harder. Your engine will like it better if you do the former.

There are several loss terms. With a big bore SLOW engine, pumping losses are not going to limit you. The engine will breathe just fine, as 2500 or 2700 RPM (or whatever) is a pretty easy target. I'd expect valve train losses to dominate, 'cause the pushrods are enormous. But that shouldn't be a factor unless you overspeed the engine.

Your cirrus engines may have advanced cams, which will rob some performance at high RPM in order to enhance it lower and suppress detonation. That's a design choice, not a fundamental. Or, the prop may be optimized lower.
 
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