barit1
13th Aug 2010, 01:19
Ocampo:
Ever since HSD developed (late 1930s) the Hydromatic design, counterweights has not been used on these props. Whereas earlier props balanced oil pressure against centrifugal counterweights in the hub, the Hydromatic used a double-acting hydraulic cylinder (see the illustration above) to position the blade angle. This permits both feathering and reversing functions to be employed.
I cannot say for sure about the Dash 8 prop, but I believe it's similar to the HSD prop on some Saab 340 models; it definitely has a flyweight governor in the PCU mounted coaxially with the prop shaft, on the aft side of the gearbox. A control shaft extends between the PCU and a servo valve inside the prop hub.
14RF-9 prop description (http://sites.google.com/site/sf340com/propeller)
ejet3
21st Aug 2010, 02:22
Hi guys,
I don't know anything about the dash :{ However i found this which i think answers your question
SmartCockpit - Airline training guides, Aviation, Operations, Safety (http://www.smartcockpit.com/pdf/plane/bombardier/DASH-8-200-300/systems/0016/)
page 18/35 :ok:
BrasiliaCaptain
23rd Aug 2010, 21:34
I'll try to add my rapidly-devaluing two cents to this discussion. First, a disclaimer. I used to teach systems ground school on the ATR, and my boss ensured that we went into great detail on the propeller system. That being said, most of the modern, non-electronically-controlled Hamilton Standard propeller systems operate the same way; however, the control of individual system components among airframes is often quite different. For example, one airplane might have a switch for the pilot to manually activate the electric feather pump, while in another airframe, its operation might be primarily automatic or tied to the pilot doing something else, like pulling a fire handle. Another big difference is nomenclature.
That being said, let's go over a few basics of constant speed propellers in general. You might remember from your student pilot days that there are three ways we can potentially make a propeller spin faster. The first way is to apply more engine power. The second is to decrease the blade angle of the propeller. The third is to increase the IAS of the airplane. In a Cessna 172 with its fixed pitch propeller, you were limited to power and airspeed. Increase either, and your RPM indication will increase. I know the airspeed and power relationships seem moot at this point since we're talking about constant speed propellers, but they become important when we talk about a very important safety feature of the propeller.
With a normally operating constant speed propeller, we have mechanisms which maintain a selected RPM for us by controlling the blade angle of the propeller. In a typical light airplane, the blade angle is controlled by changing oil pressure, which opposes a nitrogen charge, flyweights on the blades, or spring force. In the good old days when engine horsepower and airspeeds (and their associated airloads) were getting to be rather large, using springs and flyweights became impractical. Hamilton Standard developed the Hydromatic propeller for the early DC-series airplanes. The unique feature of these propellers was that they used engine oil pressure to increase, decrease, and maintain blade angle. Your Dash 8 propeller uses this feature.
The hub of the propeller features a pitch change mechanism (PCM) that actually does the work of changing the blade angle. It features a two-way piston pressurized by oil pressure on each side. To change the blade angle, the PCM drains oil from one side of the piston and adds oil to the other. The piston moves, and when it does, it drags an off-center trunnion mounted at the base of each propeller blade. Another device, called the propeller control unit (PCU) is the brain that tells the PCM where to set the blade angle. The PCU is mounted on the reduction gear box (RGB). More on Mr. PCU later.
The PCM and the PCU communicate with each other via the oil transfer tube, a long, hollow tube that passes from the PCU all the way through the engine power shaft out to the hub of the propeller. This tube is turning along with the propeller. To command a change in blade angle, the PCU turns the oil transfer tube. Inside the PCM, the oil transfer tube has an acme screw fixed it. This basically means that when the tube is rotated within the hub, it will travel backwards (or forwards) depending on the direction it is turned. When it moves back and forth, it opens and closes holes within the PCM, venting oil from one chamber, and adding it to the other just like I mentioned above. Then when the blade angle gets to its commanded value, all of the holes are sealed, and the blade angle stays where it is.
A bit more about the oil transfer tube. Another function besides telling the blade angle what to do is to transfer pressurized oil to the PCM, hence the name. This pressure is required to change the blade angle. The oil for this operation is engine oil. However, the pressures required to change the blades are higher than the normal engine oil pressure.
Attached to the RGB is the propeller's oil pump which pumps oil whenever the propeller is turning. It is the normal source for the oil pressure fed to the oil transfer tube. Some of its pressurized oil also goes to the PCU so it can do its thing. The pump gets its oil from a small reservoir which engine oil pressure keeps full when the engine is operating. The propeller's oil pump sucks oil from this reservoir with a standpipe which doesn't go all the way to the bottom of the reservoir. The rest of the oil is accesible by another pump called the electric feather pump. This pump uses electricity to work. It can access all of the oil in the reservoir. It serves as a back-up to get the propeller feathered in case the main pump isn't pumping. Remember, in this airplane, you have no flyweights or springs to drive the propeller towards feather, just oil pressure. The electric feather pump means you can get some oil pressure to the hub in case the main pump fails or the engine flames out and no longer fills the reservoir.
Now let's talk about what happens if for whatever reason, you can no longer get oil pressure out to the hub. If this happens, there is nothing to hold the blade angle fixed (that we know of yet). Remember that a propeller blade that can freely rotate within the hub will go towards flat pitch since it seeks the path of least resistance. This is not good, since it could lead to a prop overspeed, excessive drag, or some combination thereof, none of which are satisfactory. Remember how the PCM has a piston that is controlled by oil pressure that changes the blade angle? And our friend the oil transfer tube and its acme screw? If all oil pressure is lost to the hub, the blades will go towards a flatter pitch, but only by about one degree. The flattened blades drag the piston and the acme screw until it hits a bulkhead within the PCM and the blade angle decreases no more. This condition is called pitch lock. This is not easy to understand without the use of schematics, but the main thing to remember is that if you lose oil pressure to the hub, it pitch locks. The next question I'd ask my students is "can you feather it?" The answer is, it depends on why you lost oil pressure to the hub. If it's because of a bad main pump, or a flamed-out engine, we have another pump, don't we? Yes we do, our friend the electric feather pump. If there are no plumbing problems, we can probably get oil from the reservoir to the hub with the electric feather pump. So what kind of propeller do we have once it's pitch locked? We now have a fixed-pitch propeller that you MIGHT be able to feather. So if a propeller pitch locks at Vr, what will we notice after takeoff? How will we keep the RPM within limits? Do you remember the two ways to control RPM in a Cessna 172? You will notice RPM increasing as your airspeed does, and you can keep the RPM within limits by reducing power on the affected engine, decreasing your airspeed, or both.
We have discussed how the system changes blade angle, where we get the oil pressure to do it, and where the oil comes from, and how we can pressurize it. So now we ask, what happens inside that PCU thing? As a refresher, it's the brains of this operation; it tells the PCM where to put the blade angle. Some of its operation will be familiar to complex light airplane people, while some of it definitely won't.
We talked about the oil pump on the RGB that pressurizes the oil so the propeller can use it. The PCU also uses some of this oil and divides it into two. One is called supply pressure. This is the same pressure we get from the RGB oil pump. The other oil is called metered pressure. More on these oil pressures in a bit.
Inside the PCU is another piston separate from the one in the hub called the servo piston. Remember we said that the PCU tells the PCM where to set the blade angle by twisting the oil transfer tube? The servo piston is what twists the tube. When the servo piston moves back and forth, a ball screw mechanism twists the oil transfer tube and the blade angle changes. The servo piston moves using, you got it, oil pressure. One side is pressurized with supply oil pressure (straight from the RGB oil pump), which doesn't change when the engine is operating normally. On the other side, is metered pressure. Increasing metered pressure flattens the blades, reducing blade angle. Metered pressure is what the PCU modulates to change blade angle, while the supply pressure remains fairly constant. So what happens to the blade angle if the metered pressure were 'dumped' and it went to zero? That's right, the blade angle goes towards feather. In fact, when you move your condition lever to feather, it opens a valve which dumps metered pressure.
Now the question is, "what controls the metered pressure to change the blade angle?" We will start simple on that one. Remember the flyweight governor in your Seminole/Duchess? Well inside the PCU is one of those. When the power levers are in the flight range, this constant speed governor regulates propeller RPM by changing the metered oil pressure. When you move the condition levers to change RPM, you adjust spring tension on the flyweights, and the governor changes the metered pressure. If it increases metered pressure, the blades go flatter, and RPM increases. If it decreases metered pressure, blades take a bigger bite of air, and the RPM decreases.
As a safety feature, this system also has an overspeed governor. The overspeed governor also uses flyweights. However, they are set to govern propeller RPM at some RPM greater than the maximum setting for the overspeed governor. Connecting the pressures produced by each of the governors is something called the least selector valve, which sends one of the metered pressures to the servo piston (guess which one? The lower pressure). Remember, the higher the metered pressure, the higher the RPM. So if the constant speed governor 'jumps the couch' and starts commanding a super-high RPM, its metered pressure will exceed that produced by the overspeed governor, so the least selector valve selects the lower metered pressure produced by the overspeed governor. So now you still have a constant speed propeller, but you no longer get to select the speed with the condition lever. Now if for some reason the overspeed condition cannot be controlled by the overspeed governor (let's say you had a pitch lock for example), another feature kicks in. Above a certain RPM, the overspeed governor will roll the ENGINE RPM back as long as the propeller RPM is above this value. This is sometimes called fuel topping. It works by venting pressure from the P3 bleed line to the engine HMU, 'tricking' it into reducing fuel flow, and hopefully keeping the overspeed from getting worse. If you have this condition, engine power will probably cycle back and forth; power decreases as the RPM increases above the threshold and then increases again as the propeller RPM drops below the threshold. I have been told that is is quite noticeable.
Another thing that makes turboprops different from their piston twin counterparts is engine operation during taxi. In a turboprop like your Dash 8, you typically taxi the airplane with the power levers below a gate or detent. You can't bring the power levers below this position in flight or bad things happen. Before we talk about good or bad, let's talk about what happens with the power levers around this position. When you go below this minimum flight position, two valves come into play. The reverse valve sends supply pressure directly to the metered pressure side of the servo piston, allowing something called the beta valve to control blade angle. When you are below flight idle, you are directly setting the blade angle with the POWER levers. This actually works really well during taxi, but in flight it would be catastrophic. The reason is that to directly control blade angle with the power levers,you are sending supply pressure to the metered pressure side of the servo piston (the valve that does this is the reverse valve, which does this when the power levers are below flight idle; the beta valve sets the exact blade angle). The blade angles you can select are too low for safe operation in flight, and you bypass your low pitch (more on those soon) and your overspeed protections. If you go below flight idle in flight, you will probably ruin the engines and lose control of the airplane.
Another function of the beta valve is low pitch protection for the propellers during flight. Low blade angles in flight can cause large amounts of drag and propeller overspeeds. Think about it this way. In your Seminole, you could maintain 2400 RPM all day until you reduced the manifold pressure below a certain range. At the point, the blade angle could not get any flatter because they just don't go any lower. The RPM would now decrease as the power and airspeed reduced. Well in our big, fancy turboprop, that 'stop' is adjustable; each power lever angle has a minimum blade angle. If the beta valve, which knows your power lever angle, sees the blade angle get too low, it dumps some metered pressure, maintaining that minimum blade angle. I think in the Dash 8 airplanes this is called 'flight disc' or something to that effect. If you're not too busy, you'll notice that while in the flare, when the power levers come back to idle, the RPM no longer stays at the set RPM. The constant speed governor is sending more metered pressure to tell it to get flatter, but the beta valve 'overrules' it and dumps some metered pressure.
There is another low pitch protection that is electrical. There is a microswitch in the PCU that detects if the blade angle is too low for safe flight. It does two things. First, it turns on a light in the cockpit. Depending on the type of airplane, it might say "LO PITCH", "BETA", or something like that. We want to see this light when we are on the landing roll after the power levers are below flight idle. The second function of that microswitch is low pitch protection. When the power levers are above flight idle, this protection is active. If it detects the blade angle getting below a certain threshold, the microswitch opens a valve, dumping metered pressure, driving the blades towards the feather direction, returning the blades to a safe angle for flight. This is usually cyclical if it happens; the system coarsens the blades, and when they return to a safe angle, the valve closes, allowing metered pressure to build, and the process repeats unless the underlying problem is rectified.
I know this is a lot of stuff. Use all of it, some of it, or none of it. If you feel like it, paste into Microsoft Word and substitute the terms I used with the ones DeHavilland uses. If you are really motivated to learn it, I'd try to find some clear system schematics (good luck with that, but they're out there, try FSI's publications perhaps) of the PCU and the propeller hub and try to use my humble contribution here to help you out. But be careful using simulators to play with these failure modes. They often lack fidelity in simulating many of these failures.
The airline I flew for had two propeller-related accidents (not in the ATR), so we had a lot of corporate knowledge about the workings of what is clearly a mechanical engineering achievement. We didn't run a pilot butcher shop and quiz guys too much on propeller stuff during our oral or written examinations, but we did want them to be able to deal with propeller problems correctly.
I'll summarize some of the key points about failure modes.
*Know your propeller RPM limitations. Often, a maximum propeller RPM in flight is linked to a particular torque. Knowing this limitation can help you manage a pitch lock or propeller overspeed.
*Pitch lock: Caused by loss of oil pressure to the hub. You have a fixed-pitch, POSSIBLY featherable propeller. Keep RPM within limits using IAS and power.
*You are operating on the overspeed governor: Propeller RPM is fixed at a value above the maximum normal RPM. If you move the power lever, RPM stays constant, unlike with a pitch lock, when RPM will vary with power setting and IAS. Know the RPM value that the overspeed governor should maintain.
*Low pitch in flight: The beta valve isn't providing low pitch protection, so it's being done electrically. Most likely at low power settings and airspeeds. Indications will probably cycle on and off as the protection activates and deactivates.
I hope this helps.
AerocatS2A
24th Aug 2010, 03:23
Great stuff BrazililaCaptain! I'll just point out a small error that may confuse those who read it. In relation to the PCU you say,
One side is pressurized with metered oil pressure, which doesn't change when the engine is operating normally. On the other side, is metered pressure.
Presumably the bolded bit should be supply pressure (the Dash 8 manuals refer to it as "constant pressure".)
The beta control in the flight range of the power lever arc is referred to by deHavilland as "flight beta".
The Dash 8 manuals were never very clear on how any of this worked, partly because it is vastly simplified compared to what you have written and also because it doesn't adequately explain the difference between the oil pressure that is directly controlling the blade position and the metered vs constant oil pressure that is inside the PCU.
Anyway, I understand it a lot more now than I did before thanks to your post.
Cardinal
25th Aug 2010, 19:03
Brilliant coverage BrasiliaCapt, I've sat before some of the finest E120 instructors and that was a flawless, beautiful, concise disertation on the Hamilton Sunstrand.
For the record, Ocampo, the E120's PW118/Ham14RF-9 combo is nearly identical to the PW120 installation on the Dash, most significant difference is the horsepower.