The blue knob

For those of you that are AOPA members, there's an "Engine & Propeller" online course that you can view that explains the relationship between blade pitch, MP, RPM, etc.
 
Simplest way someone explain to me: Think of a bicycle. If you have to go up a hill, first you need to have the proper shift set or else your legs (engine) will suffer. You have to set your climbing gear. You are going downhill, change your gears so you can be efficient.

Which rpms/mp is up to POH first and experience second
 
I hear the snap on knobs are better than the Craftsman or Harbor Freight but I don't have one in my plane. But I have one in the Arrow that I fly but I don't know what brand the knob is...I hafta check on that.
 
ktup-flyer said:
so it's a good for nothing piece of junk?
Click to expand...

A well built CVT is an amazing thing. Can keep the engine at its best power production RPM throughout a range of speeds. Very efficient when done right.

Many problems with modern CVTs is that they have to add a bunch of crap to them to make them FEEL like they're shifting, because morons need that feeling when they're driving like they're running a standard slushbox, and that stuff adds unnecessary complexity and more things to break. You shouldn't feel a CVT "shifting" at all.

Early CVTs were also built really wimpy to keep them light and therefore broke too much.

Subaru's modern ones aren't bad at all. They still have to add the stupid "shift point feel" to them though, since their competition is dual-clutch stuff with paddle shifters for those who can't figure out how to freaking use a clutch pedal.

(And yeah, dual clutch manuals are amazing gadgets also, but let's not pretend the average driver needed one if they knew how to freaking drive...)
 
denverpilot said:
Early CVTs were also built really wimpy to keep them light and therefore broke too much.
Click to expand...

First one I remember, with a "Variomatic" transmission:

https://en.m.wikipedia.org/wiki/DAF_Daffodil

8624057_orig.jpg
 
denverpilot said:
A well built CVT is an amazing thing. Can keep the engine at its best power production RPM throughout a range of speeds. Very efficient when done right.

Many problems with modern CVTs is that they have to add a bunch of crap to them to make them FEEL like they're shifting, because morons need that feeling when they're driving like they're running a standard slushbox, and that stuff adds unnecessary complexity and more things to break. You shouldn't feel a CVT "shifting" at all.

Early CVTs were also built really wimpy to keep them light and therefore broke too much.

Subaru's modern ones aren't bad at all. They still have to add the stupid "shift point feel" to them though, since their competition is dual-clutch stuff with paddle shifters for those who can't figure out how to freaking use a clutch pedal.

(And yeah, dual clutch manuals are amazing gadgets also, but let's not pretend the average driver needed one if they knew how to freaking drive...)
Click to expand...
In theory the CVT is best, but I've yet to see one that can withstand high torque numbers. The CVTs on the market now aren't anything to write home about. I would like to match one up with a rotary and see how it does. They seem almost perfect for each other. I'll stick with my 6 speed manual for now.
 
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.
Click to expand...

I trained in a DA-20 A1 (Katana) back in '96. It had a blue knob and I promise you, it wasn't a high performance airplane. So, step #1 is to disassociate the presence of a blue knob from high performance airplanes. Exhibit B would be the Beech Sierra which is often overtaken by continental landmasses due to tectonic drift.

Step #2 is to understand that variable pitch propellers (where the pilot directly controls the pitch of the blades) are exceedingly rare. What you're likely referring to is a constant speed prop where the pilot selects a desired RPM with the blue knob. The prop governor then adjusts the pitch of the prop to maintain that RPM regardless of speed and throttle changes (to the extent possible. At some point, even with nearly flat pitch there isn't enough power to produce the selected RPM, such as during taxi).

Otherwise, I suspect the other posts have covered it.
 
It's actually run by the Turbo Entabulator.


That should make abundantly clear the relationship between throttle, prop and mixture.

Cheers
 
  • Like
Reactions: Ted
denverpilot said:
A well built CVT is an amazing thing. Can keep the engine at its best power production RPM throughout a range of speeds. Very efficient when done right.

Many problems with modern CVTs is that they have to add a bunch of crap to them to make them FEEL like they're shifting, because morons need that feeling when they're driving like they're running a standard slushbox, and that stuff adds unnecessary complexity and more things to break. You shouldn't feel a CVT "shifting" at all.

Early CVTs were also built really wimpy to keep them light and therefore broke too much.

Subaru's modern ones aren't bad at all. They still have to add the stupid "shift point feel" to them though, since their competition is dual-clutch stuff with paddle shifters for those who can't figure out how to freaking use a clutch pedal.

(And yeah, dual clutch manuals are amazing gadgets also, but let's not pretend the average driver needed one if they knew how to freaking drive...)
Click to expand...

One of the best CVT's is the Toyota Synergy Drive used in their hybrids. It uses a planetary gearset. The engine is linked to the final drive via the sun gear, and electric motor driven planet gears are used to split the power between the engine and the electric drive system so that the engine can supply anywhere between zero (freewheeling gears) to 100% where the planet gears are locked and you essentially have a solid shaft connection between the motor and the final drive. Those can withstand high torque, and there are no frictional losses from clutches or bands.

I'll post up my go-to cutaway diagram when I find it.
 
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?
Click to expand...

The faster your engine turns, the faster things wear. And at wide open throttle this would be at higher HP's so that also would affect wear. But if you dont care about fuel prices I'm guessing you might not care about hours before TBO either.

The faster the prop tips turn, the louder they are. So noise both to the cabin and the folks on the ground might be a factor

One other reason might be to pull the prop back a bit (rather than the throttle) to stretch range, which might make the TIME difference worth it in having to let down for a fuel stop. But you can do this with the throttle too.

Personally I'm all about higher throttle MP settings and lower prop speeds for a given HP setting. Makes for a more pleasant ride and a bit easier on the engine. But I DO care about fuel cost and TBO.
 
FastEddieB said:
Just argument by assertion, stating a fact not in evidence.

Support it with some data and maybe we'll get somewhere.
Click to expand...
Yep. There are several effects that work in the opposite direction. Like engine cooling. That works FAR better at low throttle, high RPM, than the other way around. It's not at all obvious that wear is greater at high RPM. There is a whole helluva lot more oil flowing.
 
As a general rule, it's more fuel efficient to use more manifold pressure and less RPM. Lower engine speed decreases friction losses, and greater manifold pressure decreases pumping losses. Your propeller may be more efficient at lower RPM as well, it's certainly quieter in the cabin. What effect this has on the rate of engine wear isn't obvious, but for privately owned aircraft, I have to think that amount of wear during the enroute part of a flight isn't all that significant, most of our engines time out more than they wear out.
 
As a data point, ROTAX engines are rated up to 5,500 rpm continuous and have roughly the same TBO as non-geared engines (2,000 hours).

rotaxtach.jpg


If high rpm by itself "used up" engine longevity, that would not be expected, right?
 
FastEddieB said:
As a data point, ROTAX engines are rated up to 5,500 rpm and have roughly the same TBO as non-geared engines (2,000 hours).

If high rpm by itself "used up" engine longevity, that would not be expected, right?
Click to expand...
Oh....snap. lol :ohsnap:
 
FastEddieB said:
As a data point, ROTAX engines are rated up to 5,500 rpm continuous and have roughly the same TBO as non-geared engines (2,000 hours).

rotaxtach.jpg


If high rpm by itself "used up" engine longevity, that would not be expected, right?
Click to expand...

A more interesting comparison would be measuring mean time before failure of that Rotax operated at "100% power at 5500 rpm", and at some lesser RPM number (but perhaps at a higher MP and therefore same power output?). Apples to apples. The big piston air cooled 6 bangers that put out their peak rated power at 2500 RPM are not the same beasts that the Rotax is, at any output..
 
Sac Arrow said:
Hey, I tried to throw it off track. Failed so far.
Click to expand...

Well it would help if you reported back whether your knob twists or snaps. If the latter, do you ever worry about it prematurely popping off? Also, do you prefer the shaft moving up and down instead of in and out?
 
Mistake Not... said:
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.)
Click to expand...

Props have less slippage the slower they turn. If 75% power gives you 120 knots, it doesn't matter if that 75% is made with 23/2300 or 24/2100, you'll still go 120 knots. But your fuel flow will be lower at 24/2100 and vibration and noise will be lower at 24/2100. Theoretically.
 
asicer said:
Well it would help if you reported back whether your knob twists or snaps. If the latter, do you ever worry about it prematurely popping off? Also, do you prefer the shaft moving up and down instead of in and out?
Click to expand...

None of those things, really (I purported to not touch them, right?) No, I'm more concerned about alienating pilots in the UK and Australia by inflecting "lehver" instead of "leever" in to the reference when discussing the issue.
 
One thing about a Constant Speed prop that a lot of beginning pilots dont recognize is that the prop is automatically going flatter and coarser on its own in response to throttle and other factors (like going into a dive) without the pilot moving the prop knob. Sort of like an automatic transmission.
 
JohnAJohnson said:
Props have less slippage the slower they turn. If 75% power gives you 120 knots, it doesn't matter if that 75% is made with 23/2300 or 24/2100, you'll still go 120 knots. But your fuel flow will be lower at 24/2100 and vibration and noise will be lower at 24/2100. Theoretically.
Click to expand...

I'm not seeing that in my Owners Manual. It seems to vary more with altitude than with MP/RPM settings. But then again, my "knob" isn't blue, and it neither twists nor snaps . . .
 

Attachments

  • image.jpg
    image.jpg
    128.7 KB · Views: 16
  • image.jpg
    image.jpg
    242.2 KB · Views: 16
My final comment about Constant Speed Props is, inspite of being "constant speed" you do get an rpm drop when you lean. Good thing you do, it makes it so you can lean via rpm drop (lean to rpm drop, then richen).
 
denverpilot said:
Subaru's modern ones aren't bad at all. They still have to add the stupid "shift point feel" to them though, since their competition is dual-clutch stuff with paddle shifters for those who can't figure out how to freaking use a clutch pedal.
Click to expand...

The Euro sports car makers only continue to deliver cars with clutch pedals because American purists still want them, even though dual clutch automatics are quicker than the same car with a manual gearbox.
 
RoscoeT said:
The Euro sports car makers only continue to deliver cars with clutch pedals because American purists still want them, even though dual clutch automatics are quicker than the same car with a manual gearbox.
Click to expand...

Wanting a manual trans is no longer about being "faster", it's just more fun to row the gears in a sports car than it is to let the computer do it. It's also less complex so that failures are much less likely, and less costly when they do fail.
 
coloradobluesky said:
My final comment about Constant Speed Props is, inspite of being "constant speed" you do get an rpm drop when you lean. Good thing you do, it makes it so you can lean via rpm drop (lean to rpm drop, then richen).
Click to expand...

Any power or speed change is going to temporarily
MAKG1 said:
It does at run-up power, but only because the prop is at the fine stop and can't make max RPM. It doesn't work while airborne.
Click to expand...

At least not very well. If you abruptly change the mixture, you might get a slight momentary deviation in RPM. Or not, depending on how well the governor works.
 
You must log in or register to reply here.
Back
Top