Variable Pitch Propellors Options

[c150student]

Right, I know I should probably know this already, but I may as well stop trying to be 'proud' and actually try and further my understanding.

Now obviously, for my aircraft technical written exam, I had to know a certain amount about variable vs fixed pitch propellors. Having only flown a c150, i.e. fixed pitch, I dont actually know how to control a variable pitch aircraft's propellor.

I know the basics (at least, I think I do). The pitch of the propellor is changed rather than the throttle (which is only changed slightly) and the different pitches allow for different amounts of thrust, and also cruise at a higher altitude.

Now, firstly, are variable pitch props the same as constant speed props? (Im thinking the are, but best to get this clear from the start )

Secondly, how do you actually control a variable pitch prop? When do you set the throttle, what, exactly, does the 'blue lever' do? And what is manifold pressure, and how is that important to the pilot (if I recall correctly, from a short flight in the rear seat of a Seneca I had over a year back, manifold pressure is more important than rpm).

Thanks for this. I know I ought to know it, but before you start going 'what an awful pilot, hes so dangerous etc', bear in mind that I havent actually been trained on complex types yet. Its just always been a grey area of mine.

[BMeister]

Hey there 150,

Thanks for your advise in my forum post, I'm a little familiar with some of these things, because I've been flying a DiamondStar a little..

so If I could I'd like to give a little of my understand about these,


The throttle controls the power output which is registerd on the manifold pressure gauge and the properller control regulates engine RPM. Which alters the pitch of the prop to take a bigger bite of air.


A controllable-pitch propeller (constant-speed) permits the pilot to select the blade angle that will result in the most efficient performance given the flight conditions. A low blade angle and a decreased pitch reduces the propeller drag and allows more engine RPM (power) for takeoffs. After airspeed is attained during cruising flight, the propeller blade is changed to a higher angle to increase pitch. The blade takes a larger bite of air at a lower RPM and consequently increases the efficiency of the flight. This process is similar to shifting gears in an automobile from low to high gear.

Im trying hehe. probably doesn't answer your question and you knew this already but I tired

PS. I also found this a little helpful for me too: http://www.avweb.com/news/maint/185020-1.html

[Piltdown Man]

Or...

you use the BLUE knob to control the RPM. Push it in = more RPM. More RPM = lower gear (if such an analogy is valid for aviation), The BLACK knob (throttle) controls power. Push it in = more power. Now, here's the clever bit. As the power increases, the constant speed unit (CSU) determines that it can take a bigger slice of the air (to keep the RPM constant) and does so. So, rather weirdly, it will appear that the throttle controls the pitch. But in effect, it does. And as a pilot, all you have to is set the amount of power you want and the RPM takes care itself.

But what I have described above is a constant speed prop, which is the most common form of variable pitch props. The pure "Variable pitch" prop is on where you adjust the pitch, generally between coarse and fine and you effectively have two choice of two fixed pitch props on one aircraft, varaible at the whim of the pilot. The last option is the adjustable prop where you set a blade angle on the ground appropriate for the flight you are about to undertake.

PM

[c150student]

Thanks for the help you guys.

I think Im understanding more of the theory, but could we just go back to a 'real-life' situation?

Say I want to take off, and make an approach afterwards. What would I need to do with the throttle/rpm gauge and the prop lever/manifold pressure gauge? Obviously, all aircraft are different, but in general...
Its all nice and simple when you have one lever which directly affects just one gauge. Kind of like a car with 1 gear

Thanks again

[The Airbuser]

Say I want to take off, and make an approach afterwards. What would I need to do with the throttle/rpm gauge and the prop lever/manifold pressure gauge? Obviously, all aircraft are different, but in general...

Uhmm, about the manifold gauge. Manifold press is the same as atmospheric press when the engine is a "normally aspirated" engine, that is, no superchargers nor turbochargers (as the C150 and the PA28). Manifold press is different from atmospheric press when the engine has turbocharger, or supercharger. (The PA-34 for instance)

Back to practical...

At takeoff you want max power, so, you put max rpm (with the blue lever), and max allowable power (with the black lever). At climb, you reduce power (black) AND rpm (blue), at cruise, you reduce even more power and more rpm. At approach, the checklist usually commands to put max rpm (blue lever), so in the case of a go-around you will have full power from the engine when you put go-around power (black). Usually on piston engines GA power is the same as takeoff power.

[c150student]

Thanks, I think Im getting it now (sorry if Im being slow)

So to get more power, I would push forward the throttle and rpm lever (unless Im at a high altitude, in which case wouldnt I leave the blue lever back a little, so that the prop can get a bigger 'bite' of the air?)

At a given throttle setting, would adjusting the prop rpm affect the fuel consumption?

[Piltdown Man]

Near enough, but... For take off you'll generally find that it is everything forward. For climb, you may well back off the Power and RPM (the aircraft manual will tell you which order to move the knobs - some engines don't like having more inches of manifold pressure than they do hundreds of RPM) and for cruise, you set up an RPM/inches/TGT from the book (it is interesting, especially in the UK to see how many people haven't read the book on power settings) to get a resulting IAS/TAS/Fuel flow etc. To add power, like climb power from a cruise setting, you'll generally push (ease is a better word) the knobs forward in the following order - Red, Blue and then Black (Mixture up, pitch up, power up). But, as ever, read the book on the engine and prop - generally doesn't mean always!

PM

For what it is worth, RPM x Torque = Power. So as you change RPM, you change the power output of the engine and thus the fuel flow. From memory, flying a C206 in Tasmania a few years ago, you could change your fuel flow by nearly 2 gph (from worst to best), without changing IAS/TAS just by altering Manifold Pressure, RPM and EGT. I used to set more Inches than RPM and peak EGT - I didn't invent this, I got the settings from Mr. Cessna's book.

[The Airbuser]

So to get more power, I would push forward the throttle and rpm lever (unless Im at a high altitude, in which case wouldnt I leave the blue lever back a little, so that the prop can get a bigger 'bite' of the air?)

At a given throttle setting, would adjusting the prop rpm affect the fuel consumption?

Uhm...the cruise power settings are on the AOM, so normally you wouldn't have to "invent" yourself the settings. Usually, all possible cruise settings (normal cruise power, High performance cruise, economy cruise) are taken from the book and will give you all the proper settings (manifold press, RPM and mixture).

When the blue lever (RPM) is full forward, the prop has more drag, because is moving a bigger mass of air. So, in high altitude, you would normally have less RPM than for example climb or takeoff power.

And, since the prop angle (pitch) creates drag, if you have less pitch (less RPM) then you would have less fuel consumption than if you had more pitch (more RPM).

Eduardo

[Kilrah]

And, since the prop angle (pitch) creates drag, if you have less pitch (less RPM) then you would have less fuel consumption than if you had more pitch (more RPM).

Isn't it the opposite? When you increase pitch the RPM will decrease for a given power setting...

At a given throttle setting, would adjusting the prop rpm affect the fuel consumption?

The throttle setting will define the amount of air/fuel mixture that is sucked in by the engine at each cycle. So as you increase RPM more cycles take place in a given amount of time, so more fuel is used overall and power output is increased.

[c150student]

Thanks guys

Im understanding it more now, but I think to fully appreciate it, I'll have to book myself a flight in a complex aircraft at some point!

[The Airbuser]

Isn't it the opposite? When you increase pitch the RPM will decrease for a given power setting...

You know what? I got it all mixed up about the pitch/RPM relation...

I'll have to read something

[rjb4000]

Uhmm, about the manifold gauge. Manifold press is the same as atmospheric press when the engine is a "normally aspirated" engine, that is, no superchargers nor turbochargers (as the C150 and the PA28). Manifold press is different from atmospheric press when the engine has turbocharger, or supercharger. (The PA-34 for instance)

Manifold pressure is only the same as the atmospheric pressure when the engine is off. As soon as the engine is started, a vacuum is created in the intake manifold that restricts the amount of power that the engine generates. When the throttle is increased, more air fills the vacuum and the pressure increases (rise in manifold pressure).
Having a supercharger or turbocharger has nothing to do with this (aside from providing more air to the intake).

Think of the propeller control as the gearshift on your bike. As you start off you want to be in a higher gear (increased propeller RPM) and as you fly down a hill you want to be in a low gear to elicit more power from your pedaling (low propeller pitch). The problem is that when you try to start off up-hill in the lowest gear you'll kill your legs trying to get the bike started. An airplane engine will react the same way, and so it's important to not stress the engine by requesting more power in a low gear (low RPM).