Graham wrote:
But the drag would surely be less – a lot less – than it would be if the prop were turning the engine?
The engine of course does absorb the power, but the airframe would just as happily absorb the drag force of an even faster rotating propeller. So yes there is a correlation but the engine is not the cause. 
The resistance of the engine does of course affect the how the CSU drives the pitch.
In your example it depends, while the engine forces the blade to any finer angle than the coarse stop, then I think the answer is generally yes, but at slow speeds where CSU can get the blade to the coarse stop then I think the engine actually helps to reduce drag by causing it to spin slower. There is more to it than that because it depends on relationship between speed and blade angle. The key is to get the blade to coarse.
If you take your example and consider an autorotating helicopter, with a failed engine, it is essential that the rotor or horizontal propeller is disconnected by a clutch or by other means to allow the blade to spin fast enough to create enough drag force (aka known as lift ) otherwise it drops like a stone…
This stylised graph gives some insight, but you have to consider how the CSU and the engine interacts, which will change the blade angle in its governing range, so its not the whole story.
There was an attribution of propeller drag (drag from non-zero thrust) to the total aircraft drag (the linear sum of induced drag from wing AoA, zero-lift parasite drag from aerodynamic section give the zero-thrust drag without propeller, then one adds drag from propeller as well as it’ airframe interference), the study shows that the drag in power-on level cruise is 30% higher than the drag in power-off glide at similar 90mph speed
I am not sure how much of that can be extrapolated to find a figure with variable pitch and idle power descent? on idle power one still expect large drag increase from propeller (windmill & engine) compared to a feathered stopping propeller on coarse pitch but it’s not clear how much impact can be attributed to propeller pitch on similar ASI?
https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/AIAA-46372-872.pdf
AIAA_46372_872_pdf
Ted wrote:
There is some NACA research on this topic from the 40s
Everything about the ‘propellers science’ was 99.99% known by WW2, the 0.01% remaining to be discovered is getting debated all around the place these days anytime you ask to buy a new propeller
vic wrote:
I cannot see the logic why there might be CS props that go to fine pitch with dead engine
On single engines, they are designed to go fine pitch after the engine/spinner start to lose oil pressure (there is a return spring in the spinner), the logic is that you can still fly the aircraft with whatever remains as engine power even with low oil pressure as the engine progressively dies, if the engine blows up, I doubt the position of blue matters that much (the same happens during governor failure, it reverts to fine pitch then you have to quickly pull to reduce speed and reduce MP to maintain level flight without max rated RPM over-speed)
On twin engines, they go coarse pitch after the engine/spinner start to lose oil pressure to avoid too much yaw
Aerobatic props fail coarse to prevent overspeed if oil pressure to the prop drops. Otherwise they typically fail to fine pitch as explained, or in the case of an electrically actuated prop they may fail in place.
Also if you have a governor failure but not an engine failure, you probably want it to fail to fine pitch rather than coarse pitch.
Same as if your car with an electronic sequential gearbox failed and you could only have one gear to limp home with, you’d want a low gear rather than a high one.
RobertL18C wrote:
Ok surprising you can get 1000 rpm in flight, you may need to check your low pitch stops!!
On the Bonanza, this is normal. At idle throttle and best glide, typical RPM is 1800 RPM with the prop set to any value except almost at the full stop. Decent rate is 1400 FPM. Reducing the prop makes very little difference because the prop is resting on the prop pitch stop until the prop control is in the governing range. The last half inch of travel of the prop control, one sees the RPM begin to reduce and 1000 RPM to 1200 RPM is achieved at the full aft stop of the control. The descent rate drops to 800 to 1000 FPM, and the aircraft pitch must be increased from about – 3 degrees to almost level to hold the best glide airspeed. Glide is a little over 10 to 1 in this configuration. With the prop control anywhere outside the governing range, the descent is more like 6 to 1. It is a dramatic demonstration and part of the BPPP training. The prop control by itself makes an enormous difference in the glide. If you run out of oil and the prop can’t be controlled, don’t expect the book numbers for glide, We teach to use the wing tip to sight estimate the glide, you can glide to anything it covers and mentally drawing an arc from tip to tip is a no wind glide distance. If you can’t set the prop control to the full aft position, there is a seam about two thirds of the distance out on the wing to the tip that will give a sight picture for the steeper glide. Best glide happens if you have a catastrophic engine failure and the engine seizes and is probably close to 13 to 1. I haven’t tried it, but using the throttle could improve glide as well.
This is a good article by Peter Garrison going through the variables.
https://www.flyingmag.com/gliding-props-and-arithmetic/
I went to Eckalbar’s book on effect of coarse pitch on glide ratio on the Bonanza, and he posits an improvement from 1200 fpm to 700 fpm, and it struck me as not realistic? Garrison’s mechanical analysis looks sound and a glide ratio improvement of around 20% seems more reasonable? Eckalbar doesn’t give an RPM in coarse pitch.
The two SEPs with constant speed that I fly are Lycoming powered (late model 182 and Pitts S2), and at least in the Lycoming coarse pitch brings RPM to around 1700 RPM but will check next time I fly an SEP.
On an MEP the feather lock is designed to be comfortably below coarse pitch windmilling speed, so around 1300 RPM compared to around 1700-1800 windmilling RPM outside the feather range.
I went to Eckalbar’s book on effect of coarse pitch on glide ratio on the Bonanza, and he posits an improvement from 1200 fpm to 700 fpm, and it struck me as not realistic?
On typical 80kts best glides, that sugget moving from 1:11 to 1:6? it does not sound right
RobertL18C wrote:
I went to Eckalbar’s book on effect of coarse pitch on glide ratio on the Bonanza, and he posits an improvement from 1200 fpm to 700 fpm, and it struck me as not realistic? Garrison’s mechanical analysis looks sound and a glide ratio improvement of around 20% seems more reasonable? Eckalbar doesn’t give an RPM in coarse pitch.
It very much depends on the Bonanza. My experience is not much different than Eckalabr’s. It is a dramatic difference. Pilots who have not previously had this demonstrated are amazed and it is an unforgettable experience.
