Timeline for Why, with a constant speed propeller, does moving the propeller control forward increase drag?
Current License: CC BY-SA 4.0
20 events
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| Jul 26, 2021 at 18:47 | history | edited | Michael Hall | CC BY-SA 4.0 |
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| Jul 26, 2021 at 0:55 | comment | added | Kenn Sebesta | @PeterKämpf, Michael Hall, I revised my answer and worked out the math. The engine braking for an O-360 is an astounding 13hp! The prop spinning faster isn't what's causing the deceleration on its own, it's the negative AoA (which M. Hall explains well) which is being driven by the piston engine braking. Without the braking, the AoA would likely be scarcely negative. I'm not sure the magnitude is the same for a turbine, though, since turbines and diesels share the common trait of not having a throttle body. | |
| Jul 26, 2021 at 0:07 | comment | added | Kenn Sebesta | I think I see your point now, and I did not make this clear in my answer because I took it for granted that the reader would understand why the prop was spinning faster. Thus what was important to understand for me was where the energy was going (because E = $F*d$, so without $E$ we have no $F$). This answer still doesn't really address the fact that the engine losses are where the energy is going, and I think that's a crucial point to understand. | |
| Jul 25, 2021 at 23:42 | history | edited | Michael Hall | CC BY-SA 4.0 |
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| Jul 25, 2021 at 23:34 | comment | added | Michael Hall | Thanks for the feedback from both of you! I added a paragraph that amplifies these points somewhat, please let me know if you have any suggestions on wording. | |
| Jul 25, 2021 at 23:33 | history | edited | Michael Hall | CC BY-SA 4.0 |
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| Jul 25, 2021 at 21:26 | comment | added | Peter Kämpf | @KennSebesta I just upvoted Michael's answer. And the explanation of the prop extracting energy from the airflow is spot on. Basically, the prop blades have now a negative AoA and drive the engine (compressing air all the time). Just like a car in small gear going downhill. | |
| Jul 25, 2021 at 18:42 | comment | added | Michael Hall | @Kenn Sebesta: I respect Peter's IQ and scholarly acumen, but I am a simple unfrozen caveman pilot and prefer common language explanations based on relatable observation over complex equations. ;) However, your remark about "pumping air" leads me to believe there might be a better answer than simple drag. That the prop is extracting energy from the airflow rather than adding energy to create thrust. I'm just not sure where to go with that idea.... | |
| Jul 25, 2021 at 18:08 | comment | added | Kenn Sebesta | I suspect it's completely resolvable through first principles. Maybe @PeterKämpf could weigh in? | |
| Jul 25, 2021 at 18:07 | comment | added | Kenn Sebesta | I have no experience in a turboprop, but it sounds like you have an equivalently sharp deceleration event. Are the turbine blades and prop blades set up such that they have a very similar slip ratio? If not, then it might be the case that the turbine blades are also pumping a fair amount of air. It's an interesting question! | |
| Jul 25, 2021 at 18:04 | comment | added | Kenn Sebesta | We agree that the AoA is negative, but the disagreement is whether a free-spinning prop has a negative AoA to the point of producing much thrust. My reasoning is that it will only be negative to the extent that energy is required to turn the prop until the prop no longer slips, i.e. the advance ratio goes to 1. The energy required for this is a function of parasitic drag and shaft torque. The parasitic drag is likely very small so the majority of the energy is spent spinning the shaft. | |
| Jul 25, 2021 at 18:03 | comment | added | Michael Hall | And might the answer be different between a turboprop, (the basis of my answer) and a piston engine? (the basis of your answer) Because a turboprop is closer to a free spinning blade, while a piston engine has the resistance of compression, which puts it much closer to the example of downshifting a car or motorcycle. | |
| Jul 25, 2021 at 17:50 | comment | added | Michael Hall | @Kenn Sebesta, I am open to suggestions to improve this answer, but a prop under power will have a positive AOA as it "bites" the air and pulls the aircraft forward, while a free spinning blade will have a negative AOA from the force of the airflow making it turn. Excellent point about the fact you can't ignore the effect of the engine turning because the prop really isn't spinning free, but my gut tells me that the primary contribution to deceleration is the increased drag from the flat pitch. | |
| Jul 25, 2021 at 17:45 | comment | added | Kenn Sebesta | I have my doubts that this explanation is correct. The angle of attack for a freely spinning propeller blade is going to be very well low. True, additional propeller parasitic drag is created by virtue of the prop spinning faster, but I suspect these losses are negligible in face of the pumping losses from engine braking at several thousand RPM. | |
| Jul 25, 2021 at 17:43 | history | edited | Michael Hall | CC BY-SA 4.0 |
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| Jul 25, 2021 at 17:15 | history | edited | Michael Hall | CC BY-SA 4.0 |
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| Jul 25, 2021 at 17:01 | history | edited | Michael Hall | CC BY-SA 4.0 |
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| Jul 25, 2021 at 16:52 | history | edited | Michael Hall | CC BY-SA 4.0 |
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| Jul 25, 2021 at 16:46 | history | edited | Michael Hall | CC BY-SA 4.0 |
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| Jul 25, 2021 at 16:41 | history | answered | Michael Hall | CC BY-SA 4.0 |