Constant speed means, if you set for example 3000 RPM, the propeller tries to maintain this RPM by changing its angle. Thats what is meant by “constant”.
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Yes, read my further posts as I come to grip with understanding just how ignorant I’ve been about how much effort is required to move a propeller through the air.
I had, honestly, not taken into account the load placed on the engine when the propeller is set so coarse thst the engine’s RPM is going to be low even though the throttle is wide open.
I think I’ve been confused by turboprops having a propeller RPM gauge and trying to make sense of that as applied to a piston engine with a constant speed prop.
I know and, put my hand up, that I’ve been confused and conflating things here, which is why I’m here asking for clarification — to learn.
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Propeller tips don’t go hypersonic. Hypersonic means > Mach 5.
Sorry, still trying to keep up with all the posts.
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Ok, if that is the case, then on the Staggerwing, for instance, you don’t really have a way to measure what RPM you want the propeller spinning at, like you do in say a turboprop where you’ve got an actual propeller RPM gauge, yes?
The tachometer is for the engine and you’re not really able to know what RPM the propeller is spinning at.
So, why then, on a turboprop is knowing propeller RPM important and not on a piston engine?
Well it is the same, there is no difference in the working of the propeller system itself. The engine itself runs at 20.000 to 30.000 RPM usually the propeller maybe 2000 RPM or less by a reduction gearbox. On most pistons engine versus propeller RPM is 1:1.
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What does it matter? If the ratio is staying the same it makes no difference if it is displaying engine or propeller RPM.
You always bring up entertaining discussions!
Love them 
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It only matters in my understanding of mechanical systems.
You, obviously, don’t know me personally, but if we were friends in real life you would see my shop and all the mechanically focused aspects of my world and you’d know why it matters.
So, yes, if it technically doesn’t matter then why are we adjusting propeller speed to a given RPM in a turboprop for cruise or climb, etc.?
You always adjust propeller RPM no matter if its a piston or turboprop. In case of a fixed shaft turboprop or any piston engine it doesn’t matter if the RPM indicator is showing propeller or engine RPM though. There might be a reduction gearbox installed between the engine and propeller but 100% RPM is still max and 0% is still min. In revolutions per minute the figure on the gauge is different, thats all.
You can have constant speed props with any type of engine.
They are just more often found on more complex aircraft, which nowadays are more likely turbine powered.
The constant speed prop just allows you to operate the engine in its peak performance RPM more often.
It’s like shifting gears in a truck.
Yes. I understand that, definitely.
Can we take a step back?
On the piston engine, for example we have the engine’s load (manifold pressure) and the engine’s speed (RPM). We use those values to determine how to set various states for climb, cruise, high speed, etc.
Ok.
How does that differ for the turboprop? I know that temperature is very important, and RPM runs at such high values that we look at percentages of RPM via N-values rather than specific RPM values. The question is why, then, do we also care about the RPM of the propeller? If we don’t really need to know that value on the piston engine, why do we make adjustments on the turbo prop using that propeller RPM value? (Along with torque, N-values, etc.)
If you are going up hill on the truck, and the engine works best at 2800 rpm, you can shift gears to try keep it there.
Too high a gear, and the engine bogs down and won’t maintain the RPM.
The same in the airplane, climbing with too coarse a pitch, the engine bogs down.
In a turbine, without the airflow through the engine, you will overheat.
In a flat stretch in the truck you can roll right along at 2800 RPM in high gear.
In the plane flying straight and level you can coarsen the pitch and do likewise.
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It’s hard to convey what I’m trying to get at.
My opening post came about because of this confusion between piston engines with constant speed props and turboprops.
They both have tachometers, however one (piston) is showing engine speed and one (turboprop) is showing propeller speed. This confused me and continues to do so.
I fully understand the concept and application of gearing and why we want to reduce or raise a given “gear” via propeller pitch to alter engine speed for economy, lifespan, etc.
What I just still don’t get is why in the turboprop don’t we make our adjustments based on torque and N-values alone? Why do we even measure propeller RPM as a separate value and even utilize that value as a part of our adjustments for various performance characteristics? Why don’t we care about that value in a piston engine?
The values are not that high for the propeller, roughly the same as for a piston, thats not the reason for the N1, Ng or whatever its called on the TBM. The prop RPM is in the 2000 to 3000 RPM range. The engine is running at a very high speed 20.000 to 30.000 RPM. You are right, why does it matter on a turboprop? It depends on the type of turboprop engine, on a fixed shaft engine it doesn’t matter and you often have only one RPM gauge. On a free turbine engine like the PT6 on the TBM the engine itself (compressor section) is not physically connected to the propeller (power section), so the ratio is not the same. The engine in essence can run freely from the propeller on a free turbine engine.
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Both show propeller speed (= engine speed) free turbine turboprop engines show both engine AND propeller speed. I want to add that except for engine start and faults you don’t use the Ng / N1. You set power using torque (instead of MAP), unless limited on turbine temperature (ITT). So torque and ITT are primary.
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On a free turbine you can start the engine while holding the propeller as they are not interconnected. I fly the ATR which has a propeller brake on the right engine. We can start the right engine in “hotel-mode” during which the engine runs and can be used as an APU while the propeller doesn’t turn. Just to illustrate the free turbine principle. The ATR engine has the compressor section split in two, we have a NL (low), NH(high) and NP (propeller) RPM gauge. But as I said we don’t use NH or NL other than engine start or faults.
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I read so much of what you share freely throughout this forum and massively appreciate how much you know and share with us all — for free.
Thank you.
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I have been a power plants classroom instructor for ATPL students, its way more complex when going into beta mode etc.
But I’m gonna sleep now 
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Cavitation is a generally something to worry about for water propellers (“screws” in Navy-speak). Turn a prop fast enough and the low pressure behind the blade is enough to allow to literally carve a hole in the water because water - like basically all liquids - is incompressible in any real meaningful sense. When pressure is reduced enough it can’t expand so instead it flashes to vapor. You can increase the pressure all you like, but that gallon-jug of water is still going to be a gallon-jug of water 1,000’ below the surface. Unlike, say, a gallon-jug full of air. That’s because air - like all gases - is compressible.
There are a bunch of reasons you don’t want your (air) propellers to have blade tip speeds going supersonic, but they boil down to the very fact that air is a compressible fluid. “Supersonic” means “faster than sound,” and sound - as we know - is made up of compression wave travelling through a medium. So if an object moves through the air, it will both compress air slightly in front of itself as it pushes the air out of the way, and slightly reduce the pressure behind itself as the displaced air tries to rush in behind it.
When an object exceeds the speed of sound in a fluid, it creates what is caused a “shockwave” and a discontinuity in the pressure-field of the fluid surrounding the object. That shockwave has REALLY high pressure air immediately in front of it, and since fluids generally heat up in direct proportion to the increase in pressure, that shockwave gets really, really hot. As in, potentially hot enough to melt steel or set stuff on fire. Great care is spent designing supersonic objects to avoid “shock impingement” and prevent BadThings™ like that from happening. Even more complicated (and potentially worse) things can happen when multiple shockwaves hit each other at odd angles.
Now image a 3-, 4- or 5-bladed propeller spinning around with tips all going supersonic speed, each one carving its own flattened-cone shaped shockwave, and the whole plane moving a few hundred knots through the air … You end up with an incredibly complicated system of interacting shockwaves, tons of vibration ranging from ultrasonic down to probably a few hundred hertz depending on the resonant frequencies of the entire airframe and powerplant, and a LOT of very, very hot spots all over the skin of the entire aircraft due to shock impingement effects.
So … while experiments have been done with supersonic propellers, there’s a reason why they don’t exist in practice.
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Tips hat at this thread and smiles!
Great question and replies from everyone above!
Well done
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