I don’t remember there was a low value for ground run, I was check my POH, I had the impression it’s mainly for getting warm oil in the spinner and checking the cable but surely low values do indicate that max pitch stop is exceeded…now I am tempted to try stall turns with throttle & propeller well back to see if engine keeps running north of 1500rpm (if I am wrong and it goes bellow 500rpm, mags will quit but in aerobatics we wear parachutes and have plenty of air under)
Ibra wrote:
was a low value for ground run
What it says more or less is that a drop of more than 400 RPM should be avoided to reduce wear. So I put the 1100, which is 1500 minus 400.
I am still eager to learn what happens in high coarse high speed descent if you cut off throttle.
From Mooney M201 POH, nothing about max RPM drop during prop re-cycles
In the other hand, I am aware of this for descent, I think it’s mix of harmonics engine-propeller friction and vibrations plus the extra drag from high angle of attack of the prop, it’s easy to notice 
UdoR wrote:
I am still eager to learn what happens in high coarse high speed descent if you cut off throttle.
‘Hot Prop STC’ of the PA24? I am sure it’s a nice way to remove ice from non-de-iced propellers, it will inflate and deflate like boots !
If you abuse it, you can get oil from the spinner on windshields & wings
It’s one shot though
I still don’t get what this is about. A variable pitch prop is nothing else (effects like stalled blades ignored) than a transmission with a variable transmission ratio. Or in the car analogy a car with an infinite number of gears. If you try to stop at a red light without using the brakes and with the highest gear in (idle of course) you will have a hard time to stop. Try the same in first gear and you get enough deceleration to be able to stop in front of the line. Almost, that is, because below a certain speed the engine will take over again and drive the car instead of the momentum of the car driving the engine. In an airplane it’s exactly the same. Put it into low pitch and get a high descent rate. Put it into high pitch and get a much better glide ratio. You can use that as a plan B in a messed up Ziellandeübung. It works amazingly well.
A stalled prop can only improve the glide ratio even more because a stalled blade cannot bear any substantial aerodynamic load, just like a stalled wing, and thus the airflow cannot exert any force upon the airframe.
Peter wrote:
Max propeller drag is when the engine is absorbing the max amount of energy from the propeller (which is being driven by the airflow).
This is an incorrect statement 
Its an example of correlation versus causation
Generally the absorption of power through the engine ensures that the CSU moves the blade to a finer pitch which results in a greater angle of attack on the blade while wind-milling than would otherwise occur increasing drag. That is the correlation. This isn’t necessarily the case for fixed pitch, where the engine just reduces the RPM, in this case if you removed the engine the blade would potentially spin faster creating even more drag!
Ibra wrote:
From Mooney M201 POH, nothing about max RPM drop during prop re-cycles
I know there is such a restriction for PA28s and PA32s with VP prop. (But I don’t recall how much.)
It’s exactly the same as a car going downhill with no power being applied.
If you put it into a low gear which gives a high rpm, you get a lot of engine braking – the amount of potential energy from the descent that’s being used to turn the engine. Same in the aeroplane – blue lever forward, high rpm, fine pitch on the propeller so maximum blade area exposed to the airflow. Maximum drag from both frontal area of prop and effort required to turn the air pump.
In a higher gear you get a lower rpm and less engine braking – less energy is taken to turn the engine because it’s going round fewer times per minute. In the aeroplane the blue lever is back, lower rpm, coarser pitch on the prop. Again less energy is going into turning the engine, and the coarser pitch gives less frontal drag – but my bet would be it’s the lower rpm making the lion’s share of the difference.
In an engine failure scenario, assuming the governor reverts to fine pitch in the absence of oil pressure, that’s like a car that locks itself into first gear when the engine fails. If you then wanted to extend how far you would roll in the car, you would of course depress the clutch so that the roadwheels no longer had to turn the engine round at some high rpm. In a piston-engined aeroplane we’d don’t generally have a device to disconnect the engine from the propellor. Twins feather, which achieves the equivalent effect albeit it’s more akin to lifting the wheels off the surface of the road and preventing them turning at all, rather than letting them freewheel.
Continuing the gedankenexperiment, I hypothesise that if our piston aero engines featured a clutch we could depress to disconnect the engine from the prop, the drag we would see from a propellor so-disconnected and windmilling without turning the engine would be close enough to that seen from a feathered prop that the difference wouldn’t be of practical significance.
Thoughts?
Peter wrote:
It will also happen at a closed throttle because the engine, with no combustion, is an air pump, and at a closed throttle it has its suction inlet blocked.
I’m not sure that’s correct?
With the throttle closed against the stop, a small amount of air and fuel still enters the cylinders and the plugs are still firing so combustion continues. Otherwise on the ground the engine would stop when you closed the throttle completely? Of course, it will only be idle-level combustion so it may indeed be that the prop is being turned principally via airflow, but there must be enough power being generated to turn the engine at whatever speed it turns at (on the ground) with the throttle fully closed.
We could quantify this. If we know that a fully closed throttle (on a warm engine) on the ground gives an idle speed of e.g. 600rpm, and we descend with a closed throttle and the prop turning at 2,200rpm, then the air-pump effect is accounting for 1,600rpm of the 2,200rpm we’re seeing, with combustion accounting for the rest.
The above explains why a windmilling prop on an engine that has actually failed produces noticeably more drag that one which is still running but with the throttle pulled right back. The prop on the failed engine consumes the potential energy necessary to turn the engine at X rpm, whereas in the practice scenario it only consumes that necessary to turn it at X-600 rpm. Logically, in a real failure you would therefore see a lower rpm for a given airspeed (assuming prop either against the fine stop or fixed-pitch) than you do in the practice scenario.
Closing the throttle does not normally close the inlet completely, only ~99%. Or depending on the carburettor design it may close it, but air continues to flow via air bleed/bypass circuits that enter the throat via a route other than the throttle butterfly. Otherwise it would stop, like it does if you put your hand over the carburettor intake on a car engine.
Graham wrote:
Continuing the gedankenexperiment, I hypothesise that if our piston aero engines featured a clutch we could depress to disconnect the engine from the prop, the drag we would see from a propellor so-disconnected and windmilling without turning the engine would be close enough to that seen from a feathered prop that the difference wouldn’t be of practical significance.
No , depending on the speed of the aircraft and the blade angle, the drag would be potentially very very high. At very low forward speed and with a very course prop your statement may be true but at higher speeds and normal blade angles no. There is some NACA research on this topic from the 40s. i..e graphs that shows the speed and blade angle ranges. At high speed in a dive with fine pitch blade the prop would spin very fast, enough to get the tips supersonic. The CSU of course varies all this, within its governing range.
A free spinning turboprop, will produce enormous drag with the blade set fine.
Ted wrote:
No , depending on the speed of the aircraft and the blade angle, the drag would be potentially very very high. At very low forward speed and very course prop your statement may be true. There is some NACA research on this topic from the 40s. At high speed in a dive with fine pitch blade the prop would spin very fast, enough to get the tips supersonic. The CSU of course varies all this, within its governing range.
There we go then. But the drag would surely be less – a lot less – than it would be if the prop were turning the engine?
Depends what we mean by high speed, of course. In an engine failure scenario in an SEP were slowing to something much less than normal cruise speed and certainly not in a high-speed dive.
