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Guest CaptRolo

Engines and Propellers

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Fixed ,Constant Speed ,Variable Pitch. For Educational purpose and Aircraft Knowledge;I did some research and found this info .********************************************************************** Like a wing, a propeller blade has a thick leading edge and a thin trailing edge. The blade back is the curved portion and is like the top of a wing. The blade face is comparatively flat and corresponds to the underside of a wing. The blade shank is thick for strength and fits into a hub which is attached to the crankshaft directly or indirectly. The outer end of the blade is called the tip. Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation. When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller. Propellers may be classified as to whether the blade pitch is fixed or variable. The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise. There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power. The controllable-pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means. In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed. Click here to see examples of early aviation propellers. The constant-speed propellers have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is windmilling, a situation that can damage the engine and increase drag on the aircraft. Most controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes. CaptRolo

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I am fairly certain that Jon Point is trying to get some kind of "reverse propellor pitch" working on his Saab 340. I would really like to test fly that plane for him, but he seems to have vanished from the face of the Earth ! I have been trying to contact him for the past two months, without success :-(Chris Low.

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>> The constant-speed propellers have a full-feathering>capability. > Most controllable-pitch and constant-speed propellers also>are capable of being reversed. A bit more info..As a general rule, on "single engine" aircraft with constant speed props; the prop won't go to full feather or reverse. Still work good for braking action in the pattern though!L.Adamson

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Hello.Yes the braking effect is vey true, and also the effect, both single and a twin.engine aircraft. The propeller airdesign is very true, but not very accurate. The airdesign and effictness is different from varios aircraft design, the Microsoft corporation designers have never seen this things.Yes, True, but they Do not learn anything from those things.Redo the behavior, and doing more active weather in the next release of simulators, It could bee a true winner if they could take the brain out of their head and think objective,.Not only Dollars for the ms.Just my two cents.Have A good FS 2006(FX!X)I hope we could learn a little more than justa game in the shop.Take care.

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More on constant speed propsThis article is excerpted from Rod Machado's Private Pilot Handbook,Airplanes are operated in a similar manner during cruise flight . There is no need to develop maximum horsepower during cruise flight. Our concern is to obtain a reasonably fast airspeed while keeping the fuel consumption low. After all, we could operate our airplane in cruise flight at full throttle-but why? The larger drag associated with higher speeds would consume enormous amounts of fuel and not allow us to move all that much faster anyway (remember, total drag increases dramatically at higher airspeeds). Therefore, cruise flight is a tradeoff between high airspeed and low fuel consumption.With the proper combination of manifold pressure and engine RPM, you can obtain a reasonably fast airspeed for a given rate of fuel consumption. In cruise flight we select the desired manifold pressure with the throttle, and engine RPM with the propeller control. Now the propeller produces a specific amount of lift (thrust) for a given (lower) fuel consumption.Why Constant Speed Propellers?Controllable pitch propellers on general aviation airplanes are of the constant speed variety. Once the RPM is established, changes in manifold pressure (by moving the throttle) won't affect engine speed .In other words, opening or closing the throttle (or changing the airplane's attitude) doesn't vary the engine's RPM. This is why these controllable propellers are given the name constant-speed propellers. (Of course, if you pull the throttle all the way back, there's simply no power available to keep the propeller spinning. The engine's RPM has no choice but to drop.)The reason constant speed propellers are put on an airplane is to reduce a pilot's workload. Instead of having to readjust the RPM with every change in power, you simply set the RPM and it stays where it's put--just like your home thermostat keeps the temperature constant.What is the value of having a propeller that maintains a preset (constant) speed? It provides you with one less item to readjust while managing power. Let's suppose your Pilot's Operating Handbook suggests the most efficient use of engine power during climb occurs at 25 inches of manifold pressure and 2,500 RPM (pilots refer to this as 25 squared, which proves how weak some of them are in math). As you climb, the manifold pressure decreases approximately one inch per thousand feet (because the outside air pressure decreases one inch for every thousand feet altitude gain). Since you have a constant speed propeller, the RPM automatically stays set at 2,500, despite variations in manifold pressure (or throttle positions). All you need to do is keep adding throttle to maintain the desired manifold pressure during the climb; the RPM needs no adjusting.How to Make Power ChangesWith the ability to vary propeller pitch you need to understand a few very important principles about power management. It's relatively easy to overstress an engine if the throttle and propeller controls aren't used in the proper order during power changes. For instance, suppose your manifold pressure and RPM are set at 23 inches and 2,300 RPM .You want to increase the manifold pressure and RPM to 25 inches and 2,500 RPM. If you increase the manifold pressure to 25 inches first, it will increase the combustible mixture flowing to the cylinders. This would normally spin the propeller faster. Yet this doesn't happen, since the propeller takes a bigger bite of air to absorb the increase in power. Cylinder stress increases as the propeller keeps the RPM from increasing (i.e., the expanding gases push harder, yet are unable to move the pistons faster). Given enough cylinder stress, you could damage the engine. When you want to increase both the manifold pressure and RPM, change the RPM first, then increase the manifold pressure. In other words, move the propeller control forward first, the throttle next.Follow the same philosophy when decreasing manifold pressure and RPM. Pull the throttle back first, followed by the propeller control. Another way of thinking about this is to keep the propeller control lever physically ahead of the throttle during all manifold pressure and RPM changes. A memory aid for this is to keep the prop on top (or always in front of the throttle).Propeller Tips and IdeasBe aware that the propeller governor starts working only when the engine is operating above a specific RPM and not below. In other words, moving the throttle will change the RPM until the propeller reaches its minimum governing RPM. This is why the magneto check we discussed earlier is performed below this minimum governing RPM. Remember, we're interested in seeing how much of an RPM drop occurs on each mag and well as between the mags. Magneto checks done at higher RPMs wouldn't show any mag drops on the tachometer since the propeller would vary its pitch to maintain a specific RPM.On complex, high performance airplanes (those with retractable landing gear and constant speed propellers) we use a verbal checklist while on final approach to land. It's the acronym GUMP. It stands for Gas (fuel pump on), Undercarriage (gear down), Mixture (full in) and Prop (propeller control full forward). Why is the propeller control put in the full-forward (low pitch--high RPM) position just before landing? We do so in the unlikely event there's a need to go-around. A go around is an aborted landing; you apply full power, climb out, and go around for another attempt at landing. In this situation, it's important that the engine develop full power--just like on takeoff. That's why the propeller control is moved to the full-forward position--exactly where it is during takeoff. CaptRolo

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