daft turbine questions from a piston pilot
11-10-2009, 09:48 PM
#1
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daft turbine questions from a piston pilot
Pardon the silly question, but apart from the increased weight and complexity, why aren’t jet engines designed to be feathered as a props are? I’d imagine the N1 blades at the front create a lot of drag just windmilling, so why isn’t there a way to reduce drag? Or are jets typically so powerful that engine out climb performance is still healthy?
On an equally silly note, do jets and turboprops have counter-rotating engines? I ask because I remember being taught about the descending blade producing more thrust in PPL school. Since turboprop and turbojet blades are airfoils, how do those planes escape the need to correct for it the same way a piston pilot has to?
On an equally silly note, do jets and turboprops have counter-rotating engines? I ask because I remember being taught about the descending blade producing more thrust in PPL school. Since turboprop and turbojet blades are airfoils, how do those planes escape the need to correct for it the same way a piston pilot has to?
11-11-2009, 12:45 AM
#2
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Power is the solution to all problems. Aside from that a turbine engine with variable pitch fans is something too unfeasible (if not close to impossible) to build (or make it work right), most do have variable stator vanes, which control air flow to the high pressure compressor. There's no need for a jet to have counter rotating blades and most turboprops do not (I can't think of any that do actually), I'd venture to guess it's due to the increased cost of developing and certifying the same engine twice.
Now talking about "correcting how a piston pilot does", for the most part we do, all twin turbine have a VMC, but we also have a higher speed envelope that we can achieve on one engine, which mitigates most of the problems.
Now talking about "correcting how a piston pilot does", for the most part we do, all twin turbine have a VMC, but we also have a higher speed envelope that we can achieve on one engine, which mitigates most of the problems.
11-11-2009, 05:23 AM
#3
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Jet's don't need counter-rotating as it's pretty much just direct thrust. Also, even if you could "feather" the front blades of a turbo-jet, the whole frontal area is still drag anyways. Even the High-Bypass ratio engines I wouldn't think it's going to make a huge difference if you could stop the rotation of the blades.
There are counter-rotating configurations with some TPE-331 installations. I think the Cheyenne 400LS did, but can't remember. I know I've seen it on something. I can't think of any PT6 equipped bird that does this though.
There are counter-rotating configurations with some TPE-331 installations. I think the Cheyenne 400LS did, but can't remember. I know I've seen it on something. I can't think of any PT6 equipped bird that does this though.
11-11-2009, 05:58 AM
#4
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Turbojets don't make as much drag as a prop when windmilling. They are designed to be efficient at very high rpms, which means they don't do much energy transfer at windmill speed...I think a lot of the air just flows through with minimal drag. Also frontal area is much less when compared to power output...so the good engine's power can easily overcome the frontal area drag.
11-11-2009, 06:11 AM
#5
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I cant answer your question about turbofan engines, but my guess is the added drag is negligiable compared to a prop. As far as counter-rotating turbo props, never heard of it, because it does affect efficiancy, how much I really dont know, obviously there are critical engine issues you dont really see in small piston twins that counter-rotate, but like anything else its not a real issue with some training. You'll for sure have to go back to using those rudders on take off though...lol
in my best southern drawwwwwl (You may have met P-factor but you aint met his daddy)
in my best southern drawwwwwl (You may have met P-factor but you aint met his daddy)
Last edited by TPROP4ever; 11-11-2009 at 06:12 AM. Reason: my spelling sux
11-11-2009, 08:58 AM
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The bear is an example of Contra-rotating propellors - twin propellors on a common axis with an opposing spin.
Counter rotating propellors are multi engine aircraft where the left and right engines turn opposite.
I believe all of the Piper twins (Navajo, Semenole etc) were counter rotating. The Cessnas aren't.
Counter rotating propellors are multi engine aircraft where the left and right engines turn opposite.
I believe all of the Piper twins (Navajo, Semenole etc) were counter rotating. The Cessnas aren't.
11-11-2009, 09:10 AM
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The bear is an example of Contra-rotating propellors - twin propellors on a common axis with an opposing spin.
Counter rotating propellors are multi engine aircraft where the left and right engines turn opposite.
I believe all of the Piper twins (Navajo, Semenole etc) were counter rotating. The Cessnas aren't.
Counter rotating propellors are multi engine aircraft where the left and right engines turn opposite.
I believe all of the Piper twins (Navajo, Semenole etc) were counter rotating. The Cessnas aren't.
11-11-2009, 09:12 AM
#10
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