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Prop pitch aerodynamics (drag)

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No absolute truth here. A still prop moves at a certain angle of attack with a certain drag coefficient at a certain speed. A mindmilling prop moves at a lower angle of attack and a lower drag coefficient at a higher speed. With drag being proportional to drag coefficient times speed, it's totally up to the details of the configuration which produces more or less drag. I've seen WW2 aircraft manuals which explicitly recommend the pilot to make sure the prop is windmilling for maximum gliding performance and manuals which explicitly recommend the opposite.

 

As to in game modelling - last time I checked, prop drag was properly modelled. I tested mostly with a Yak-1S69 and Fw190A-3 back then and found very different gliding performances depending on the prop pitch condition under which I shut the engine off.

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No absolute truth here. A still prop moves at a certain angle of attack with a certain drag coefficient at a certain speed. A mindmilling prop moves at a lower angle of attack and a lower drag coefficient at a higher speed. With drag being proportional to drag coefficient times speed, it's totally up to the details of the configuration which produces more or less drag. I've seen WW2 aircraft manuals which explicitly recommend the pilot to make sure the prop is windmilling for maximum gliding performance and manuals which explicitly recommend the opposite.

 

Would be very interested to see which manuals recommended to make sure the prop was windmilling and which settings was used to get better gliding performance. Obviously feathering the prop would be the way to go if you could do that but which WW2 aircraft manuals recommended setting up a windmilling prop?

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No absolute truth here. A still prop moves at a certain angle of attack with a certain drag coefficient at a certain speed. A mindmilling prop moves at a lower angle of attack and a lower drag coefficient at a higher speed. With drag being proportional to drag coefficient times speed, it's totally up to the details of the configuration which produces more or less drag. I've seen WW2 aircraft manuals which explicitly recommend the pilot to make sure the prop is windmilling for maximum gliding performance and manuals which explicitly recommend the opposite.

 

As to in game modelling - last time I checked, prop drag was properly modelled. I tested mostly with a Yak-1S69 and Fw190A-3 back then and found very different gliding performances depending on the prop pitch condition under which I shut the engine off.

It is all about finding out the configuration of least drag. And that depends how far you can feather the prop. With todays torboprops, you can go as far as the prop not turning at all in the slipstream. Same as when you feather the prop in a B-17. In these, you will stop the rotation by feathering.

 

In single engine prop planes, you often can only go as far as the fully coarse setting. In this setting, the prop will want to turn, but it will turn slow enough to not produce catastrophic damage (hopefully, you will find out about that later). After all like that it has a trim for high speed, and in this situation you‘re gliding at 120% of stall speed to be able to get somewhere other than staight down. This means the prop will go at 1000 rpm or less. It may well be that then the cranking power extracts less energy then than the drag of the prop bades still „set in the wind“.

 

One has to keep in mind, you usually do this when something is seroiusly wrong with the engine. It is up to the intrepid pilot to again prove the right stuff, acting acordingly, and being able to go up and fly again the next day. :)

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Would be very interested to see which manuals recommended to make sure the prop was windmilling and which settings was used to get better gliding performance. Obviously feathering the prop would be the way to go if you could do that but which WW2 aircraft manuals recommended setting up a windmilling prop?

You need to be able to feather the prop for such a recommendation. I haven’t seen any single engine plane of the era that has this feature. All you can do is set the prop all the way to coarse and let it do what it does. In case of being run out of fuel, this is actually a good idea as this maintains pressure in the system, oil as well as pneumatic. Having oil pressure, you can also use then the prop as airbrake to adjust your final approach as busdriver mentioned in another thread. Having pressure in the system ensures that the pitch governor is working.

 

You may also be able to deploy flaps and lower the gear like that. If you make a dead stick landing like that, say in a Tempest, in front of the cheering crowd assembling to witness the spectacle, you can be sure to end up financially broke, as you will have to cover for a lot of free rounds for everyone.

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Just as an aside to all this, flat plate drag from the propeller is why a prop aircraft can never break the speed of sound.

 

Fascinating discussion Gents.

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I could do neither for the first one because of the annoying format and pop ups (made for phones?) but have just finished the second, which is a neat little piece of experimental work. Main point is that it is looking at genuinely free wheeling props - ie does not take into account any additional friction in the engine Z raised and was measured in the NACA report:

 

"The main goal of this investigation was simply to determine whether a stationary or a windmilling propeller has more drag. The answer is complicatedly simple: it depends. It is clear that it depends on the pitch and length of the propeller, and it is probably independent of the wind velocity. A crossover point was discovered where the drag forces for the windmilling and stationary states were the same. This crossover point is also dependent on the pitch, the length, and probably independent of the wind velocity."

 

Basically he seems to be saying what I had intuited - that the rotational speed vs forwards speed argument does not hold for a free wheeling fixed pitch prop : as long as it can adjust it's rotational speed proportionally to the airflow, it will.  

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You need to be able to feather the prop for such a recommendation. I haven’t seen any single engine plane of the era that has this feature. All you can do is set the prop all the way to coarse and let it do what it does. In case of being run out of fuel, this is actually a good idea as this maintains pressure in the system, oil as well as pneumatic. Having oil pressure, you can also use then the prop as airbrake to adjust your final approach as busdriver mentioned in another thread. Having pressure in the system ensures that the pitch governor is working.

 

Exactly: If you can feather then as far as the data that has been presented so far that is what you do. However, I'm still curious which flight manuals JtD was refering to: "WW2 aircraft manuals which explicitly recommend the pilot to make sure the prop is windmilling for maximum gliding performance and manuals which explicitly recommend the opposite."

 

What were the limitations/conditions that mandated that you set up a windmilling prop? Probably you could not feather and had to select a certain blade angle: fine or coarse and probably it depends on the range you can set? Would be interesting to see examples/references to both types of manuals. Anyway one thing is for sure like you pointed out before: A windmilling prop driving an engine that is dead needs the power from somewhere and in the end that will produce an increase in drag. No such thing as a free lunch! ;)

 

In addition, I'm guessing a stopped engine has a higher starting friction than one spinning (at reasonably low rpm of course) with no ignition and maybe you could get the prop to stop and stay stopped if you set a certain pitch angle and then temporarily slowed down lower than speed for best glide ratio to stop the prop windmilling and then speeded up again to speed for best glide ratio with a stopped prop. In addition to the engine friction part, if you did manage to stop the engine by going slow, you could probably ensure that the prop blade stays stalled so even when speeding up the prop would have a hard time staring the engine turning again since it is at a stall aoa.

 

Again, would be nice to see the manuals JtD was refering to and what are the condition when it is recommended to set up a windmilling prop.

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Well manual i'm going to use is that video i posted above showing proccedure (A-20 pilot training vid) when engine is dead (at 15:00);

-Feather the propeller in cruise speed, A-20 is fast plane even so prop will stop spinning.

-When inbound for landing going into shallow dive unfeathering the propeller which will start spinning (second engine tgrottling down with high pitch) so both props will provide enough drag for stable landing.

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Do you guys notice how easily have places like Il-2s can prop hang - it just look so unnatural?

 

What ?

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Do you guys notice how easily have places like Il-2s can prop hang - it just look so unnatural?

 

Not really on topic?  Except that it contains the word "prop".   :)

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Great find, thanks!

 

Mr. Garrison doesn‘t seem to have operated R/C aircraft. When putting the finger on the carburator, blocking air intake, he would have noticed that the prop requires a little more force to crank. The engine, more than anything else, is an air pump. If you restrict airflow, you will have to „pump“ more. Hence it cranks easiest with the carb fully opened. It comes from when the cyliner is trying to suck in the fuel air mixture. If you pull down the piston with a vacuum in the chamber, you have to pull more than with ambient pressure. This is more prononced at higher cranking rpm. Also you car cranks easier when you fully depress the gas pedal while starting it. You notice that especially in cold or bad battery conditions. Hence the better glide with throttle wide open. (Just keep mixture shut!)

 

I will check the manuals regarding glide instructions. Can‘t remember when was the last time I looked that up specifically...

Edited by ZachariasX

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Again, would be nice to see the manuals JtD was refering to and what are the condition when it is recommended to set up a windmilling prop.

 

Pretty hard for me to comply with this right now, as I've lost two hard drives in a row and my library is now considerably smaller. Also not on this computer. At any rate, I'm not sure that changing prop pitch was an option on whatever planes it were, certainly feathering was not possible. Maybe the only variable the pilot had was air speed, and the main point was to keep the oil pump going. However, I recall it was drag. I'd look it up, but can't right now. I'll clarify when I can.

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Pretty hard for me to comply with this right now, as I've lost two hard drives in a row and my library is now considerably smaller. Also not on this computer. At any rate, I'm not sure that changing prop pitch was an option on whatever planes it were, certainly feathering was not possible. Maybe the only variable the pilot had was air speed, and the main point was to keep the oil pump going. However, I recall it was drag. I'd look it up, but can't right now. I'll clarify when I can.

 

OK, sorry to hear about the data loss and no worries about the references: I was just curious about the emergency procedures and the background to the different recommendations but if you ever find them then it would be great to see them.

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Do you guys notice how easily have places like Il-2s can prop hang - it just look so unnatural?

 

What ?

 

Not really on topic?  Except that it contains the word "prop".    :)

 

True: but there are some nice videos around: When it comes to high alfa and prop hanging Monostripezebra is the man: Check out this IL-2 3 min in and the I-16 landing!  :music: 

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True: but there are some nice videos around: When it comes to high alfa and prop hanging Monostripezebra is the man: Check out this IL-2 3 min in and the I-16 landing!  :music:

 

Yes I know -  was going to mention the striped one myself. :)  It is actually an interesting enough topic, IMHO, but surely deserves a separate thread if Tomcat really wants to get into it.

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Definitely: Enough fun and back to business! Found an interesting passage in a translated Finnish Me-109G6 manual: In case of an emergency it seems like you should go to manual setting and set to "glide" and if the engine stops set it to the 6 o'clock position. Anyone know what these settings correspond to in terms of blade angle?

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Yes I know -  was going to mention the striped one myself. :)  It is actually an interesting enough topic, IMHO, but surely deserves a separate thread if Tomcat really wants to get into it.

Yes there is topic about it, sorry.

I would like to make some attention, because for me this prop hang and very hard to stall or to easy to catch lift again after stall by most planes at high AOA at extensive Gs do not feel as it should.

Btw I'm very curiuse how it would be in jet.

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Definitely: Enough fun and back to business! Found an interesting passage in a translated Finnish Me-109G6 manual: In case of an emergency it seems like you should go to manual setting and set to "glide" and if the engine stops set it to the 6 o'clock position. Anyone know what these settings correspond to in terms of blade angle?

 

Not in actual degrees: only fine is 12.00 and coarse is 6.  These might not be the absolute limits - but I think this is saying set as coarse as possible? 

 

edit - although in game I can only get a range from 8.30 to 12.30 for F-4 and G-4.  Different calibration for G6?

Edited by unreasonable

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Not in actual degrees: only fine is 12.00 and coarse is 6.  These might not be the absolute limits - but I think this is saying set as coarse as possible? 

 

I would guess so as well. I also found another reference for the Hawker Typhoon which says you can lengthen the glide considerably by moving the prop control fully back which again would be fully coarse I guess.

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Not in actual degrees: only fine is 12.00 and coarse is 6.  These might not be the absolute limits - but I think this is saying set as coarse as possible? 

 

edit - although in game I can only get a range from 8.30 to 12.30 for F-4 and G-4.  Different calibration for G6?

AFAIR (not having my library at hand now), these 12 (fully fine) and 6 (fully coarse) are indeed hard limits. In „automatic“ mode, the Kommandogerät will not use all the travel but remain between the limits you posted. Most notable is 12:30 position instead of 12 for take off. Calibration should be the same as this dial for prop indicator is something universal among German planes.

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109-G2 2000m 100fuel,engine off, trimm for glide

1)manual prop set to 12 o'clock

0B8XpBj.jpg

2)same just more steady at speed (third test)

Z4cqUbE.jpg

 

manual prop set to 9 o'clock

 

eKielhP.jpg

 

manual prop set to 8:30 o'clock

 

dHkXCOi.jpg

Edited by 307_Tomcat
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Great find, thanks!

Mr. Garrison doesn‘t seem to have operated R/C aircraft. When putting the finger on the carburator, blocking air intake, he would have noticed that the prop requires a little more force to crank. The engine, more than anything else, is an air pump. If you restrict airflow, you will have to „pump“ more. Hence it cranks easiest with the carb fully opened. It comes from when the cyliner is trying to suck in the fuel air mixture. If you pull down the piston with a vacuum in the chamber, you have to pull more than with ambient pressure. This is more prononced at higher cranking rpm. Also you car cranks easier when you fully depress the gas pedal while starting it. You notice that especially in cold or bad battery conditions. Hence the better glide with throttle wide open. (Just keep mixture shut!)

I will check the manuals regarding glide instructions. Can‘t remember when was the last time I looked that up specifically...

Garrison is an engineer who designed and built two personal aircraft. He’s bee writing that column for three decades and had been a wonderful aerodynamics and safety resource for pilots who don’t have access to continuing professional resources.

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AFAIR (not having my library at hand now), these 12 (fully fine) and 6 (fully coarse) are indeed hard limits. In „automatic“ mode, the Kommandogerät will not use all the travel but remain between the limits you posted. Most notable is 12:30 position instead of 12 for take off. Calibration should be the same as this dial for prop indicator is something universal among German planes.

 

Actually those were the limits I could reach in game changing the prop pitch manually with the Kg off, either on the runway or flying circuits. 

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Garrison is an engineer who designed and built two personal aircraft. He’s bee writing that column for three decades and had been a wonderful aerodynamics and safety resource for pilots who don’t have access to continuing professional resources.

I like the column, grateful for you guys pointing it out. Maybe he did consider the effect, but in his case for an obvous (to him) reason didn‘t bring it up. Well, he specifically asked for comments to his article, and I didn‘t go through those.

Actually those were the limits I could reach in game changing the prop pitch manually with the Kg off, either on the runway or flying circuits.

 

You had a nicely serviced aircraft then. :)

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109-G2 2000m 100fuel,engine off, trimm for glide

1)manual prop set to 12 o'clock

0B8XpBj.jpg

2)same just more steady at speed (third test)

Z4cqUbE.jpg

 

manual prop set to 9 o'clock

 

eKielhP.jpg

 

manual prop set to 8:30 o'clock

 

dHkXCOi.jpg

 

Thanks for doing the test. Looks like another indication of the high level of detail in this sim. The results look as expected: The more you feather the prop the longer you glide with a locked prop (I'm assuming it did not rotate?). Did you do any similar test with manual prop and engine idling?

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Thanks for doing the test. Looks like another indication of the high level of detail in this sim. The results look as expected: The more you feather the prop the longer you glide with a locked prop (I'm assuming it did not rotate?). Did you do any similar test with manual prop and engine idling?

Unfortunately prop was windmilling. Edited by 307_Tomcat

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OK, I see. I notice you start off at 400 km/h and then gradually reduce speed to 220 km/h or thereabouts. How about going from 400 to close to stall to stop the prop and then increasing to 220 km/h? Does that work or does the prop start spinning again when you get to 220 km/h?

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OK, I see now that the charts you posted are TAS not IAS. 195-200 Km/h sounds a bit low though, at least compared to the Finnish G6 recommendation which IIRC was 220 Km/h but then the G6 is heavier than the G2 and seeing it's a flight manual they may want to make sure you are a bit on the high side in case of an emergency.

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OK, I see now that the charts you posted are TAS not IAS. 195-200 Km/h sounds a bit low though, at least compared to the Finnish G6 recommendation which IIRC was 220 Km/h but then the G6 is heavier than the G2 and seeing it's a flight manual they may want to make sure you are a bit on the high side in case of an emergency.

I can't stop prop spining.

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Ok, well then I guess it will be difficult to compare to the NACA 464 report results then since we can't see the difference between a locked and a prop cranking the engine.Thanks for trying anyway!

 

I just had an idea though: What about climbing to say 3000 m, go full power hanging on the prop until the engine seizes? that should stop the prop spinning! ;)

 

One could then glide down to 2000 m at 400 km/h IAS and level off and do a recording replicating the earlier flight profiles, only this time with a locked prop.

 

I guess these types of trials blowing up engines will not go down to well with the crew chief but anything for science right? :lol:

Edited by Holtzauge

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Ok, well then I guess it will be difficult to compare to the NACA 464 report results then since we can't see the difference between a locked and a prop cranking the engine.Thanks for trying anyway!

 

I just had an idea though: What about climbing to say 3000 m, go full power hanging on the prop until the engine seizes? that should stop the prop spinning! ;)

 

One could then glide down to 2000 m at 400 km/h IAS and level off and do a recording replicating the earlier flight profiles, only this time with a locked prop.

 

I guess these types of trials blowing up engines will not go down to well with the crew chief but anything for science right? :lol:

Yes, I was thinking the same - that only damaged engine could stop prop from spinning. Edited by 307_Tomcat

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BTW: My trial version of Tacview has expired: Did you use the pro version for your trials or is standard enough?

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I cannot either - in a G-4. I wonder if it is doable though in a twin engine plane? I would try myself but I am too drunk.

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<snip> I would try myself but I am too drunk.

 

Well then you are a bit ahead of me at CET +1: I'm planning to open a bottle of Champagne (the real stuff!) in about an hour. I don't want to waste that stuff when the bell strikes, then any Champagneskoye will do!!

 

Happy new year! :drink2:

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Well it has just popped into 2018 here, I am just finishing my bottle of wine before starting a dry January, the GF is off with her friends at a disco, with a bit of luck she will meet someone rich enough to take her off my hands, giving me more flying time.

 

Happy New Year indeed!  :lol:

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Well it has just popped into 2018 here, I am just finishing my bottle of wine before starting a dry January, the GF is off with her friends at a disco, with a bit of luck she will meet someone rich enough to take her off my hands, giving me more flying time.

 

Happy New Year indeed!  :lol:

 

 

Only a dedicated IL2er would make such a wish! :) Have a Happy New Year, with or without your GF!

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