Constant Speed Propellers

[Contacttower]

I often here the words 'constant speed' and 'variable pitch' used interchangably when describing a type of prop arrangement, but I always thought the difference was that with the constant speed prop you select a RPM setting and the CSU adjusts the pitch in order to maintain that engine speed (regardless of Airspeed or man. pressure) and that with a simpler variable pitch prop you simply adjust the pitch directly and have to change it when your airspeed increases in order to maintain a given RPM. Have I completely made that up or is there no difference between the two?

how does the PA28-200R system work?

[Piltdown Man]

I think your understanding is spot on!

PM.

[flyboyike]

The Piper Arrow is constant speed.

[411A]

Spot on, F/Sgt, and going just slightly further, if the prop is on a twin, and the engine oil pressure drops to zero, the propellor will slowly feather, whereas on the single with the constant speed prop, the blades will go toward fine pitch... (I think).

Been awhile since I've flown the latter.

[Dan Winterland]

A CS prop is a VP prop, but a bit more advanced in the operation you describe. Technically, a VP prop will give you a pitch change and that is it, you have to adjust the engine speed yourself. Not many are used now, I can only think of a couple of motor glider which do.

Pre WW2, the Spitfire Mk1 was fitted with a fixed prop which was a compromise, it gave reasonable speed at height but the take off run was huge. the along came the Mk1a, with a two speed VP prop, fine pitch for take off, coarse for cruise The next developement was the Mk2 with a CS prop.

[transonic dragon]

There is such a thing as ground-adjustable VP props, primarily used these days on some ultralights, and the occasional reproduction/vintage aircraft. They are not adjustable from the cockpit.

It means that you can easily adjust the pitch for a coarser pitch if you intend to do a long cross-country flight, or a fine pitch for circuits or short field, etc.

Having said that, I don't think I've ever actually seen one, and I don't know if they are that popular.

[Loose rivets]

On a wonderful clear night many moons ago, I found myself cruising at c5,000' over Chelmsford in a Dove. The aircraft had been batting round Stansted on IR work for much of that afternoon and had used more oil than expected.

Without any warning at all, the right prop went fine and spun up viciously. It was just by luck that I had recently been doing a lot of runaway-prop training in big piston aircraft and before I knew it, I had closed the throttles and pointed at the stars. I remember pressing the feathering button several times before the vibration subsided enough to see the rpm falling below 5,000 rpm. ( I think cruise was about 1,800.) And this was on an engine with no oil left!

Back at base, the engineers were puzzled how the feathering button bracket had become so bent. 30 degrees or so. Two of them could not bend it straight with their hands.

It's funny how much adrenalin one can get into one thumb.

[Contacttower]

Thanks to all for the replies, its interesting to note that what I've been told by instructors is that on a lot of piston singles the prop will fail to fine and consquently increase RPM almost uncontrollably when there is a loss of pressure in CSU, but some aircraft like the Bulldog (old RAF basic trainer) actually have weights in the unit to make it fail to coarse.

[barit1]

The counterweight type prop has basically three forces acting to change blade pitch:

1) Centrifugal force on the blade itself, tending to drive the blade to flat or fine pitch (think of centrifugal force trying to bring all the molecules in the blade into a single plane of rotation). This is of course true for any variable pitch blade.

2) Counterweights in the hub, arranged to rotate the blade to coarse pitch, with a rotating moment somewhat greater than the above action.

3) Hydraulic force in the actuating cylinder, used to provide either pilot or governor control toward fine pitch when desired.

Frank Caldwell earned the Collier Trophy in 1933 for developing the above propeller system. :D

If feathering capability is needed, then additional force must be used, either springs or another hydraulic system.

[The Bartender]

transonic dragon wrote:
Having said that, I don't think I've ever actually seen one, and I don't know if they are that popular.

Ground adjustable props are quite common on ultralight/microlight aircrafts.
Some likely reasons are:
- Unlike on certified aircrafts, there are a lot of engines and props to choose from to ultralight/microlight aircrafts. Because of this, there are not much reliable data available on the performance of prop/engine/aircraft-combinations.
The safe thing for the wallet is therefore to go for a ground-adjustable prop, so you can find your own diameter/pitch sweetspot for your flying.
In addition, the price is usually not to far from a fixed pitch prop, so it's not really a hard decision to make for most. Buying a fixed pitch prop, and find it lacking in performance where you want it, will cost more...

- It gives you the possibillity of adjusting the pitch for STOL or cruise, depending on your needs, without buying a lot more expensive inflight VP og CS-prop.

[rmac]

Some of the Zlins which I fly in Slovakia have the old Walter engines with automatic props the pitch of which is controlled by a spinner at the front which changes the pitch of the prop in accordance with AoA. This system was apparently developed for Messerschmidts in WW2.

Thats about all I know, I normally fly them and someone else fixes them, but they work perfectly. Any engineers care to elaborate on my basic understanding ?

[Sleeve Wing]

Quote :controlled by a spinner at the front which changes the pitch of the prop in accordance with AoA: quote.
Don't think so, rmac.
Think you'll find the pitch changes as the speed increases/decreases.
One of the problems, apart from "don't hand-spin the spinner on the ground !", is that , with a strong tailwind on the ground, the prop will tend to coarsen off.
This means allowing a couple of minutes lined up into wind to enable full power to be available on the takeoff run.
Rgds, Sleeve :ok:

[Volume]

Some of the Zlins which I fly in Slovakia have the old Walter engines with automatic props the pitch of which is controlled by a spinner at the front which changes the pitch of the prop in accordance with AoA. This system was apparently developed for Messerschmidts in WW2.


To be exact, it was developed for the Me108, which is a pre-war four seater aircraft.
You can still buy such propeller from Avia (http://www.aviapropeller.com), a detailed technical description of the system can be found here (http://www.aviapropeller.com/pdf/manuals/053-8912.7.pdf)

[Milt]

Engine RPM Control

Many of us have been campaigning for years to do away with the old descriptor "Pitch Control" and replace it with "Engine RPM Control".

Then along comes FADEC which hopefully optimises engine/prop performance.

[The Bartender]

Here's an example of a prop that automagically set the pitch according to the airspeed over the tip of the spinner:

V-Prop (http://www.silence-aircraft.co.uk/silence%20twister%20propeller.htm)

...and why don't i have one on my microlight? Because i can buy 8 FP wood-props for the price of a V-Prop...:ooh: ...or 5 ground adjustable props...:hmm:

[rmac]

Thanks for the clarification Sleeve, but wouldn't an increased AoA have the function of decreasing the speed of the airflow over the spinner, and therefore have the same effect as you describe ?

Rob

[WHBM]

I've wondered about this too (few sessions in a PA-28-200R).

If the blue lever was really a RPM lever it would be calibrated with the RPM. But it isn't. When you have the lever fully forward for takeoff you can adjust the RPM very much with the throttle just like fixed prop aircraft. And when doing your pre-takeoff checks you exercise the prop pitch lever over its full range, the RPM doesn't vary that much.

So what exactly does the prop pitch lever do ? I am aware that mechanically it moves a gyro governor on the prop but that doesn't seem to be the whole story. A given setting seems to maintain a certain RPM only in conjunction with a given throttle setting.

I seem to recall that the Piper POH just assumed you knew how they worked rather than explaining from first principles.

[barit1]

With a true prop governor, it will maintain true RPM within a fairly wide range of manifold pressure (probably beyond the range of what's good for the engine). It was always a golden rule, when increasing power, to increase RPM first then set MAP with throttle. For this reason, on approach, set high RPM (low or fine pitch) to be prepared for a GA.

But - to better accommodate the training situation, I think some a/c have the prop governor biased by the throttle. When advancing power the RPM moves up to approximately match the ideal RPM/MAP combination.

I don't know if the PA28-200R is one of these, but I wouldn't be too surprised.

[SNS3Guppy]

All constant speed propellers are variable pitch. Not all variable pitch propellers are constant speed.

Variable pitch propellers may be adjustable in flight, or may only be adjustable on the ground. Variabe pitch only means that the pitch of the propeller blades may be changed. it may be automatic, or manual. Some propeller systems and engine systems run at one speed all the time, and vary the pitch to maintain that single speed when the power setting is changed; RPM stays constant but torque changes. Others allow the pilot to change RPM and torque. Some don't allow the pilot to make the changes, some only have very basic changes available.

Manifold pressure doesn't equate to RPM. RPM in a constant speed propeller installation in most piston engines is a function of oil pressure as metered by a propeller governor. The cockpit RPM control does nothing more than adjust the pressure against a spring in the propeller governor; it may be mechanical or electrical. Spring tension opposes a force imposed by counterweights inside the propeller governor, these counterweights and the spring pressure determine how the pilot valve is mechanically moved and how far, and in turn the degree of movement of the pilot valve determines the amount of oil allowed into or out of the propeller assembly, and consequently the pitch of the propeller. The pilot has no direct control over the propeller itself; only the spring tension in the governor. The spring is called the speeder spring.

The other control the pilot has is the feather valve, which may be the same valve as the one controlled by the speeder spring, or in some cases another valve...dumping oil pressure from the propeller assembly and allowing the propeller to feather itself under gas or spring pressure, or sometimes both.

The old wives wisdom of always touching the propeller before moving manifold pressure is a ridiculous idea which is taught in ignorance...it has no valid reason for being taught, and is a false concept. Set your propeller where you want it. Move the manifold pressure as you need it. You do not need to retard manifold pressure before moving the propeller RPM, nor do you need to increase propeller RPM before increasing your throttle. So long as your power settings (manifold pressure/RPM combination) are not outside published settings or limits, you do not need to move one before the other.

[BEagle]

One of the most curious systems was the Ratier prop fitted to the (ahh) de Havilland DH-88 Comet. A 2-speed prop which was set to fine on the ground by pumping up a football bladder type thing with a bicycle pump. As the speed increased, airflow on the spinner moved a deflation valve and the prop pitch moved to the coarse setting.....:rolleyes:

A couple of problemos with that:

1. Both props didn't always coarsen at the same time.
2. Go-arounds would have been rather exciting.......:hmm:

[barit1]

The old wives wisdom of always touching the propeller before moving manifold pressure is a ridiculous idea which is taught in ignorance...it has no valid reason for being taught, and is a false concept.

I'm sure there are engines (probably smaller ones) that will tolerate max throttle at low RPM at sea level - but not the big radials, especially those with older carburetors. Ask the oldtimer engine guys what they think of pilots that abuse the hardware this way. :rolleyes:

[SNS3Guppy]

I'm sure there are engines (probably smaller ones) that will tolerate max throttle at low RPM at sea level - but not the big radials, especially those with older carburetors. Ask the oldtimer engine guys what they think of pilots that abuse the hardware this way.


Why don't you ask me...I'm one of those that spent years flying large radial engines...and here's news for you...none of them came with, or come with newer carburetors.

The carburetor has nothing to do with the manifold pressure setting, nor the RPM, save for carb air temperatures (icing in particular). I'm not sure what you think you're saying here, but you're saying it wrong.

I proposed no abuse of the engine. I quite specifically stated that as long as the engine isn't operated outside the published parameters for that particular engine, and is kept within the power tables published for it, then it doesn't matter a whit whether you move propeller RPM nor manifold pressure (throttle) first or last. It never has made a difference.

What you have (and you appear to be a product of) is bad information being passed from inexperienced and uneducated instructor to student, who then becomes an instructor and passes it along because that's what he or she was taught...a heritage of inexperience and ignorance. The concept that the propeller must always be pushed up in advance of the throttle is an idiotic and ignorant concept which has no grounding in reality nor basis of fact.

If an engine may be run at 2500 RPM at 32 inches, and it's being operated at 28 inches at 2500 RPM, then you may open the throttle without moving the RPM at all. Will you see an RPM increase? No. Will you hurt the engine? No. Do you need to move the RPM first? No. Is there any earthly need to do so, other than ignorant tradition? NO.

Further, if that engine may be operated at 32" at 2400 RPM, and you're presently operating at 28" and 2500 RPM, then you may retard the RPM all by it's little lonesome, and not bother with touching the throttle. Is there any need to retard the throttle first? No. Will it hurt the engine? No. Is there any earthly reason to move the throttle first, other than ignorant tradition? NO.

I said nothing of carburetors, "max throttle," nor sea level, nor the small vs. large engines...you've read into my comments something that was never there, then argued an arguement which is both non-existant, and wrong. But never mind that. What you do need is an education on your powerplant, because clearly you don't understand it.

You do understand what manifold pressure is, correct? You understand why your manifold pressure drops when you close the throttle? You understand that your engine is an air pump, and that closing the throttle sucks the manifold pressure down? You understand that by opening the throttle you're doing nothing more than removing a restriction from the air induction, and allowing the manifold pressure to rise every bit like blocking a vacum cleaner hose and slowly removing your hand from the opening to the hose? You understand that your manifold pressure rises all by itself to barometric when the engine is shut down...and miraculously doesn't hurt a thing...even though nothing is done to increase the speed of the propeller?

Do you understand why manifold pressure drops when RPM is increased? For a given throttle setting, increasing RPM increases engine speed, resulting in more suction from the air pump...the engine, which draws down the manifold pressure slightly. Do you understand why manifold pressure increases when RPM is decreased? As engine speed is decreased by decreasing propeller RPM, the engine produces less suction, and manifold pressure rises. Do you suppose that some how you're increasing the pressure in the engine or harming the carburetor by increasing this manifold pressure? If so, you suppose incorrectly. Do you suppose the engine is somehow being abused when the manifold pressure rises? You suppose incorrectly.

Nothing I described was in any way abusive to an engine. If you believe so, point out exactly what it is, and why, and we can educate you as to how this is not so. Aviation is full of myths. This is one of them. It's been passed along to you by those who didn't understand the concept, but just taught what they'd been taught...much like ground effect being a cushion of air beneath the wing.

What part of the concept do you not understand?

[411A]

Gotta agree with SNS3Guppy, and I too have operated big radials....from the R4360 on the Stratocruiser, to the R2800 on the DC-6B, and even the 1649 Constellation, with it's complicated turbocompound CurtisWright engines.

Big radials need care and attention to detail in their operation, and positively do NOT like rapid/abrupt throttle movements, but otherwise are quite rugged.
In addition, some of the geared engines on GA airplanes have quite specific instructions in their operating procedures regarding rapid/abrupt throttle movement, and two types, the TCM GTSIO series as well as the Lycoming IGSO series...have service bulletins advising against advancing the propellor control to high RPM just prior to touchdown.

[WHBM]

Manifold pressure :

Old "high-performance" sports cars in the 1930s had a manifold pressure gauge. I suspect few knew what part it should play in handling the throttle/gears, but they then disappeared in more recent times. I wonder why.

Actually my current car still has a manifold pressure probe, it is just not displayed but used by the automatic gearbox and all its electronic wizardry to determine when to change gear. If you think about it, the concept is somewhat similar to a constant-speed prop, except in five steps rather than truly constant.

And before you ask, yes I CAN do a manual gearbox on a car :)

[barit1]

Brian Abraham in the "Ring Chatter" thread refers us to some historical lessons. (http://www.beaufortrestoration.com.au/Pages/RestorationChild/Stories/Story_PrsnlEngineer.html)

[lugs4744]

i know this an old thread but i'm currently studying this topic and stumbled across it.My understanding of the subjest is when increasing power you advance the rpm lever first then the throttle for the following reasons, the advancing of the RPM lever will reduce manifold pressure, then opening the throttle will increase it again this keeps the manifold pressure down, if you advance the throttle first you are increasing the manifold pressure straight away and this could exceed the critical pressure of the mixture leading to detonation. Likewise when decreasing power it is advisable to close the throttle first(decreasing manifold pressure) then retard rpm (increasing manifold pressure) otherwise you are again running the risk of increasing manifold pressure to much in the first instance resulting in possible detonation.If i am way off the mark please help me out as I am pretty confident i have this understood(maybe misguided confidence i admit)but some posts on this thread point to this as poor information.