I have a question. Imagine you set RPM to very low, say 1600 to 1800 RPM and reduce power to idle for descent. Then the air flow would be moving the engine and drag should increase significantly. Now of course no one wants to shock-cool an engine, so you would not set this, say, for initial descent. But what if the engine has already cooled down during prolonged descent to non-critical values, say 300°F. Would this be an option to increase sink rate? Or could this harm anything?
@UdoR in an SEP the governor can’t go so low in an ungeared engine in flight, and yes not good for the engine.
The prop aerodynamic drag is high at high RPM then go low to some efficient RPM then very high at very low RPM, the CSU will try to maintain the same efficiency irrespective of RPM & ASI by changing propeller beta angle of attack untill the governor hits the stop (the point where reducing throttle MP implies an RPM drop)
It’s sort of propller drag curve vs rotation speed (or equivalent propeller beta angle of attack and forward speed), it has high drag regimes on very high RPM & very low RPM (unless you feather)
I am not sure if “stalling propeller” under high blades angle of attack is a good or useful option to increase drag and RoD? compared to stalling the wings with a feathered propeller 
It’s not good for the engine as you will be under-gear with high mechanical loads without the right amount of thermal loads and oil temperature & pressure
I think it’s better to go after propeller drag with very high RPM (against low angle of attack or negative beta) at slow ASI values
Max propeller drag is when the engine is absorbing the max amount of energy from the propeller (which is being driven by the airflow). So this will happen at high prop rpm. 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.
This is why, on an engine failure, one is supposed to pull the rpm lever back asap, to minimise prop drag. I had a demo of this in my FAA CPL checkride, and it made quite a big difference to the rate of descent; I’d say 20-30%. Opening the throttle wide helps (reason above) but is not possible to demo in training because the plane will then fly away
With the engine cooled down, say below 300F CHT, shock cooling should not be an issue. It won’t do any harm, but you will get a better result with a high prop rpm.
There probably are some engines on which this mode is not allowed.
These threads below are worth a read also
RobertL18C wrote:
in an SEP the governor can’t go so low in an ungeared engine in flight, and yes not good for the engine.
First not true, I can select anything down to 1000 if I set so. Often descend on 1800 RPM. And second is the question, not the answer.
Yes I do understand that fine pitch gives the brake effect of air brakes, but could it be even more with coarse pitch where the air flow stalls at the blades? I once did that and the sound of the air flow changes significantly while at the same time sink rate increased also to much more than with fine pitch.
Hard to explain that effect.
AIUI, the predominant theory is that most of the prop braking effect is not via the blade drag but via the absorption of energy through the prop driving the “big air pump” behind it; the “windmilling” drag.
But there could be something else…
Ok surprising you can get 1000 rpm in flight, you may need to check your low pitch stops!!
any ful kno that your glide ratio improves considerably in coarse pitch, so no, going to full coarse does not produce more braking (sic).
Why does going to extreme coarse pitch beyond certified stop pitch limits? Not good for the crankshaft and counterweights, most RPM indicators have a green range, it is there for a purpose.
Peter wrote:
AIUI, the predominant theory is that most of the prop braking effect is not via the blade drag but via the absorption of energy through the prop driving the “big air pump” behind it; the “windmilling” drag.
Not only that, but the aerodynamic drag itself will be less with the prop stopped. Compare a helicopter descending in autorotation with one with a stopped rotor…
UdoR wrote:
but could it be even more with coarse pitch where the air flow stalls at the blades? I once did that and the sound of the air flow changes significantly while at the same time sink rate increased also to much more than with fine pitch.
Coarse pitch does increase drag on propeller blades but reduces drag on airframe but you need to exceed typical pitch stop of CSU governor to see that?
CSU is designed to maintain propeller AoA at some efficient value irrespective of air-frame ASI, let’s ignore air-frame forward movement, if you set air-frame ASI = 0, there is a specific propeller AoA (or pitch) and rotation speed (RPM) where propeller drag is minimal…otherwise, the propeller drag is maximal when it “stalls” on coarse pitch and when it “dives” on fine pitch but somewhere in the middle you get less drag, it’s no different than this but there will be a cutoff on the backside of the curve for the propeller given by the back-stop on coarse pitch inside the governor
There is extra drag from engine air pump effect as well…
RobertL18C wrote:
Ok surprising you can get 1000 rpm in flight, you may need to check your low pitch stops!!
I agree, I am puzzled you can reach 1000rpm in flight on CSU propeller with airframe cruising above 70kts (you can assume high 100% propeller efficiency and easily convert ASI to RPM if you know your propeller diameter while it’s windmilling)
Ibra wrote:
I agree, I am puzzled you can reach 1000rpm in flight on CSU propeller with airframe cruising above 70kts
OK you got me, and anyway, this is a gedankenexperiment. I never tried it in cruise.
1970 POH says pitch control test on runup 1500 RPM and reduce RPM to not less than 1100 RPM. This is where I got the idea from that even lower RPM than 1600 could be possible in flight, but this of course was neglecting force of air flow accelerating the prop.
