Constant Speed Propellers | PPGS

what are constant speed propellers and how do they work thanks to polo for suggesting this video topic [Music] chances are you're probably familiar with a fixed pitch propeller most training aircraft have a fixed bit propeller until you get to your commercial rating and you need to get a high performance endorsement fixed pitch propellers are either a climb propeller or a cruise propeller but they don't really have great efficiency over the entire operating range think bmx bike and a cruising bike on the bmx bike you can pedal really really fast and go slow or you can have a hard time starting out in the one gear cruise bike that's a climb versus a cruise prop in between a fixed pitch and a constant speed propeller is an adjustable pitch propeller they're not very common anymore and they can only be adjusted on the ground to a certain pitch and that also limits its efficiency the beauty of a constant speed repeller is that it can adjust to different flight conditions and the way it does that is it uses oil as a sort of hydraulic fluid to move propeller blades and adjusts for each phase of flight that makes it really efficient kind of like shifting gears on a mountain bike from the outside a constant speed propeller looks like a regular propeller but if we strip away the spinner and the engine calling we can see a key component that makes constant speed propeller work and that's the governor i do want to apologize for the animation in advance i did it myself and some things may be slightly off so don't be too hard on me now looking at the blades we'll see that they're connected to some sort of arms and the whole thing is connected to a piston that sits inside the spinner oil flows in and out of the pressurized oil cavity up in the front and that changes the blade angles pretty simple right the real magic behind the operation is the propeller governor when we set the rpm what we're actually doing is changing the pressure on the speeder spring this spring right here speeder spring adds or reduces pressure on the flyweights and these flyweights are spinning when the engine is running connected to the flyweights there is a pilot valve that allows oil to either flow into or out of the propeller hub i know that sounds a little bit confusing so let's run through a scenario let's say you're cruising along and you move your propeller lever forward to increase rpm things happen pretty instantly in real life but here's what goes on behind the scenes when you increase rpm with a propeller lever that compresses the speeder spring and that speeder spring pushes on the flyweights which forces them to fold inward they're not spinning fast enough to overcome the pressure of the speeder spring and so they push the pilot valve down the openings in the pilot valve now allow the oil from the propeller cylinder to flow into the pilot valve and down back into the engine as the oil flows out of the propeller hub the angle of the blades changes to a lower pitch or a higher rpm and the centrifugal force allows the flyways to return back to their natural position which pulls the pilot valve back up and stops the oil flow and now we're back to what's called an on speed condition the opposite happens when we decrease the rpm we release the pressure on the speeder spring and that allows the flyweights to flop out or fly out due to centrifugal force this pulls on the pilot valve and allows oil to flow into the prop hub which changes the angle to a higher pitch as the flyweights and speeder spring find their balance our flyweights return back to their equilibrium point their normal position and we stop the flow of oil setting a new rpm so why is it called a constant speed aren't we just setting different rpms yes we are but the beauty of this system is that it will maintain that engine rpm say you pitch up in a fixed switch propeller the engine would normally slow down as you're getting slower this is called an under speed condition the propeller is spinning slower than what was set our flyweights don't have a lot of centrifugal force and so they fold in because of the speed of spring pressure they'll push the pilot valve down that releases oil from the propeller hub and decreases the pitch the flyweights will keep decreasing the pitch until our propeller hits a low pitch stop or the lowest possible angle and then it'll behave like a fixed pitch prop so that is an under speed condition where the propeller is spinning slower than you set and the oil is draining from the propeller hub to decrease that pitch angle and get the propeller back on speed in an overspeed condition the opposite happens the flyweights pull the pilot valve up and send more oil to the propeller hub and the pitch angle keeps increasing until we hit the high pitch stop when everything is in balance and no oil is flowing in or out of the propeller hub that's called an on speed condition if you were to lose oil pressure because the engine was shut down or if you were just losing oil all the oil would drain out of the propeller hub and decrease your pitch angle i would go to a high rpm and a low angle and your throttle would control rpm if the engine was still running that is but you do need to know that if you were to go full throttle without the propeller having oil in it you would exceed engine rpm because nothing would be governing the max rpm one thing to mention is that if you were to fly a multi-engine airplane the propeller would actually go to a feathering position or a high angle if you were to lose oil pressure and that's so you could reduce drag by feathering the propeller or pointing it more or less in the direction of the wind the other part you need to know about when it comes to constant speed propellers are the controls on a fixed pitch propeller the tachometer shows rpm and it's controlled by the throttle in the constant speed setup you have two controls you have the throttle and the propeller control the throttle controls power output and the propeller controls engine rpm which in turn controls propeller rpm your indicators are now a manifold pressure gauge and that shows the pressure inside your intake manifold you control that with the throttle and a tachometer for rpm that's controlled with a propeller control as you increase the throttle you have more air and fuel flowing into the engine and your manifold pressure will increase if the engine isn't running the pressure is roughly equal to ambient air pressure and that would be a really good indicator if your engine was to fail your manifold pressure would show whatever the pressure is at that altitude when you start the engine your manifold pressure will be low 10 11 12 inches or something like that that's because the engine is sucking air and fuel in and it's producing a lower than outside air pressure some important things to remember when you change power settings make sure you do it in the right order when you add power go right to left mixture if you need it prop then throttle if you add more throttle before increasing rpm you're putting undue stress on the engine it's like trying to ride up a hill in a super high gear on a mountain bike and you don't want that for your engine reducing power you reduce the throttle then the rpm and on the mixture if you need to lean it as always follow your poh for throttle and rpm combinations that you need to follow typically your throttle is going to be lower than the prop so for example you'll have 21 inches of manifold pressure and 2 300 rpm on the prop or something like that so usually the throttle is always lower than the prop and that about does it for constant speed props thank you once again for a video suggestion if you have any video suggestions leave them in the comments below i'm always running out of ideas so anything you can suggest would be helpful once again have fun fly safe and always keep learning see you next time you

Channel: Private Pilot Ground School

Views: 8,098

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Id: ZUPr4IMxQpk

Length: 7min 48sec (468 seconds)

Published: Mon Oct 11 2021