Prop pitch aerodynamics (drag)

[Ribbon]

What i'm really missing in il2 (but i know it will come some day ) is modeled prop pitch drag.

When flying single engine AC's (with no autopitch) i follow routine and set max prop pitch on landing same on takeoff so i never give it much attention.

It should affect speed on different engine settings and situations (when certain speed and inertion is reached max prop pitch will slow down/or limit AC speed, noticable in a dive but i don't expect it being developed till those details).

 

I'm not familiar with ww2 AC's, did they with dead engine and due losing oil pressure go in feather position by them self?

Did they used oil press for prop feathering?

 

Where prop pitch effects comes as crucial is in multiengine AC's where many times i'm RTB with one engine damaged/dead and i noticed there is no difference when feathering/unfeathering prop.

When losing one engine procedure is to feather it's prop to induce less drag and to have less jaw deflection and when on landing when throttle is decreased on second engine to unfeathering it to induce prop spinning again (in a steep dive) and induce as much drag and gain more stability and sincronization with second engine.

The most immersive moments for me are emergency situations so i hope we will have this some day.

 

Happy holidays pilots!

Edited December 28, 2017 by EAF_Ribbon

[Finkeren]

Feathering the prop absolutely decreases drag on a dead engine.

[Matt]

It's all already moddelled. Just compare the decceleration rate in a power off glide with coarse and fine pitch (or high and low RPM if you can't set pitch directly). Coarse pitch will have less drag than fine pitch.

[216th_Jordan]

What Finkeren and Matt said.

 

If, for example, you lost an engine in the pe-2 and don't put rpm to 0% you'll not make it back at low alt, if you do however its possible (with some skill).

[busdriver]

What i'm really missing in il2 (but i know it will come some day ) is modeled prop pitch drag.

When flying single engine AC's (with no autopitch) i follow routine and set max prop pitch on landing same on takeoff so i never give it much attention.

It should affect speed on different engine settings and situations (when certain speed and inertion is reached max prop pitch will slow down/or limit AC speed, noticable in a dive but i don't expect it being developed till those details).

 

I'm not familiar with ww2 AC's, did they with dead engine and due losing oil pressure go in feather position by them self?

Did they used oil press for prop feathering?

 

Where prop pitch effects comes as crucial is in multiengine AC's where many times i'm RTB with one engine damaged/dead and i noticed there is no difference when feathering/unfeathering prop.

When losing one engine procedure is to feather it's prop to induce less drag and to have less jaw deflection and when on landing when throttle is decreased on second engine to unfeathering it to induce prop spinning again (in a steep dive) and induce as much drag and gain more stability and sincronization with second engine.

The most immersive moments for me are emergency situations so i hope we will have this some day.

 

Happy holidays pilots!

 

As others posted, prop drag is modeled. You can prove it to yourself. Take your favorite twin engine airplane to cruise altitude. Starting with the cruise power set equally, select [1] so that your throttle and prop control are ONLY operating one engine. Reduce the throttle to idle [edit...bugger me...and reduce the RPM]. Now one engine is at cruise and one is at idle. Notice the amount of yaw? Now feather the engine you reduced power on. Notice the decrease in yaw?

 

Try that setup again but TRY to slow down to 200 km/h in level flight. Perhaps you've seen Requiem's excellent video ? I notice major difference between a feathered prop and one turning at idle. This was in the 110 below 280 km/h or so IIRC, but it's been a few weeks since I've played a 110 mission.

 

AFAIK there are no self feathering props, I've honestly never heard of such a thing (but you could surprise me). Nor do single engine props feather.

Edited December 29, 2017 by busdriver

[Ribbon]

Sorry guys but i did multiple tests in pe2;

Best way to test it is with both engines off (gliding) and than change prop pitch on one engine.

It may affect speed (not much) but stability affected by drag not at all.

I mean vertical and longitudinal stability unaffected while lateral axis is affected cos of speed loss.

Prop blades have big surface and produce big drag when unfeathered on unpowered engine.

[Ribbon]

As others posted, prop drag is modeled. You can prove it to yourself. Take your favorite twin engine airplane to cruise altitude. Starting with the cruise power set equally, select [1] so that your throttle and prop control are ONLY operating one engine. Reduce the throttle to idle leaving the RPM alone. Now one engine is at cruise and one is at idle. Notice the amount of yaw? Now feather the engine you reduced power on. Notice the decrease in yaw?

 

Try that setup again but TRY to slow down to 200 km/h in level flight. Perhaps you've seen Requiem's excellent video ? I notice major difference between a feathered prop and one turning at idle. This was in the 110 below 280 km/h or so IIRC, but it's been a few weeks since I've played a 110 mission.

 

AFAIK there are no self feathering props, I've honestly never heard of such a thing (but you could surprise me). Nor do single engine props feather.

mmm NO.....modern aircrafts have autofeather option, aside that prop pitch is driven by engine oil press and when engine is off and don't produce any pressure plus wind causing drag on blades it can feather itself, it all depends of engine and propeller type.

[busdriver]

Well you got me WRT to modern airplanes with autofeather...makes sense. The twin that I ride in regularly, a Beech Baron does not. The WWII multi-engine pilot notes or flight manuals I've checked do NOT.

 

You and I have different criteria it seems in whether BoX  models prop drag. IMO your test (engines off) is a poor choice. But I'm not going to try to change your mind. 

 

Cheers!

[Matt]

 

 

Best way to test it is with both engines off (gliding) and than change prop pitch on one engine.

 

That won't work, because the propellers keep the last pitch setting when you switch the engines off. So changing the pitch after switching the engines off does absolutely nothing.

 

Try setting one engine to coarse pitch and the other to fine pitch and then switch both engines off. 

[Ribbon]

Well you got me WRT to modern airplanes with autofeather...makes sense. The twin that I ride in regularly, a Beech Baron does not. The WWII multi-engine pilot notes or flight manuals I've checked do NOT.

 

You and I have different criteria it seems in whether BoX  models prop drag. IMO your test (engines off) is a poor choice. But I'm not going to try to change your mind. 

 

Cheers!

why it is poor choice? it perfectly shows is  prop drag modeled (one should have drag other one don't)  and removes other suspicions, i also did it with both engines on, low throttle but one prop pitch 100% and another prop pitch 0% and result is no roll/jaw affected.

Ok you're pilot and i'm just an AME so you have more trusted judgement when it comes to piloting experience but i thought prop drag would affect stability more.

I'll ask pilots on my job and let them try it in il2, they fly water bombers which can be compared with ww2 bombers in some aspects (slow, big wings and propellers).

[Ribbon]

 

That won't work, because the propellers keep the last pitch setting when you switch the engines off. So changing the pitch after switching the engines off does absolutely nothing.

 

Try setting one engine to coarse pitch and the other to fine pitch and then switch both engines off. 

 

lol i know that and i did it before i shut down engines and i also did it with both engines on as i described above, no change in jaw/roll! and i have perfectly centered and precise joystick.

why don't you guys go test it? devs tied it with rpm and throttle but when you test it like i did; both engines in same conditions (low throttle) but different prop pitch you'll see prop don't induce drag, at least noticable drag compared to other prop.

[busdriver]

why it is poor choice? it perfectly shows is  prop drag modeled (one should have drag other one don't)  and removes other suspicions, i also did it with both engines on, low throttle but one prop pitch 100% and another prop pitch 0% and result is no roll/jaw affected.

Ok you're pilot and i'm just an AME so you have more trusted judgement when it comes to piloting experience but i thought prop drag would affect stability more.

I'll ask pilots on my job and let them try it in il2, they fly water bombers which can be compared with ww2 bombers in some aspects (slow, big wings and propellers).

 

Your gliding test (both engines off) is thinking outside the box I suppose. And it's a unique aspect not commonly found in the game (an undamaged airplane gliding engine out). You post that you tried the test I offered and you got zero yaw or roll, even when slowing down? Hmmm I don't know what to tell you, I have to step on the rudder (and some aileron into the higher thrust engine) to counter the yawing. Again I experience this in the 110.

[Ribbon]

Your gliding test (both engines off) is thinking outside the box I suppose. And it's a unique aspect not commonly found in the game (an undamaged airplane gliding engine out). You post that you tried the test I offered and you got zero yaw or roll, even when slowing down? Hmmm I don't know what to tell you, I have to step on the rudder (and some aileron into the higher thrust engine) to counter the yawing. Again I experience this in the 110.

i'll go test it in 110

[busdriver]

I think you and I must have a different notion of prop drag. For me, it refers to a condition where one prop is spinning at a different rpm than another. The worst case being one prop at high rpm and the other at idle rpm. If I match rpm, or approximately so then I don't expect much yawing or rolling. That is why reducing power on the good engine may be required to regain control of the airplane if you are below what Americans call V_mca. In any case the solution or remedy to the yaw problem requires getting the slower prop stopped and feathered, as this produces less drag than a slower rpm prop.

 

If I read your test correctly, you think having both props stopped but one feathered and the other at fine pitch should produce a pronounced amount of yaw if your concept of prop drag is properly modeled. I don't expect that result, and in any case a stopped prop is simply parasite (or form) drag at that point, with the fine pitch prop having slightly more drag.

[Ribbon]

I think you and I must have a different notion of prop drag. For me, it refers to a condition where one prop is spinning at a different rpm than another. The worst case being one prop at high rpm and the other at idle rpm. If I match rpm, or approximately so then I don't expect much yawing or rolling. That is why reducing power on the good engine may be required to regain control of the airplane if you are below what Americans call V_mca. In any case the solution or remedy to the yaw problem requires getting the slower prop stopped and feathered, as this produces less drag than a slower rpm prop.

 

If I read your test correctly, you think having both props stopped but one feathered and the other at fine pitch should produce a pronounced amount of yaw if your concept of prop drag is properly modeled. I don't expect that result, and in any case a stopped prop is simply parasite (or form) drag at that point, with the fine pitch prop having slightly more drag.

Yeah i was thinking about pure parasite drag that stopped unfeathered prop surface produce not about rpm/np propulsion if engines are unmatched.

So you think more drag on one wing will not affect plane stability?

And i wouldn't call that parasite drag slightly, those blades when unfeathered produce significant amount of drag.

Edited December 29, 2017 by EAF_Ribbon

[Ribbon]

Stopped unfeathered prop will spin but its rpm will not give any propulsion, it becomes pure parasite drag and it is spinning cos of drag.

I won't persuade you more or maybe we just don't understand each other so i'll just leave you this pic to think about it (if you think parasite drag won't affect jaw.

[1PL-Husar-1Esk]

I will try to measure if fine prop pitch will act as air break in dive.

[216th_Jordan]

I will try to measure if fine prop pitch will act as air break in dive.

Without throttle it certainly does.

[unreasonable]

Stopped unfeathered prop will spin but its rpm will not give any propulsion, it becomes pure parasite drag and it is spinning cos of drag.

I won't persuade you more or maybe we just don't understand each other so i'll just leave you this pic to think about it (if you think parasite drag won't affect jaw.Airbus_A400M_EC-404_ILA_2012_11_(cropped2).jpg

 

I agree that must be right, but in the act of spinning, the parasite drag is reduced. That is, if a stopped unfeathered prop was jammed in a fixed position, ie not allowed to spin, it's parasitic drag would be much higher than if it were allowed to spin. I would think in this case it would certainly cause noticeable yawing.

Question is how much less drag do you get when the prop is allowed to spin.

[Ribbon]

I agree that must be right, but in the act of spinning, the parasite drag is reduced. That is, if a stopped unfeathered prop was jammed in a fixed position, ie not allowed to spin, it's parasitic drag would be much higher than if it were allowed to spin. I would think in this case it would certainly cause noticeable yawing.

Question is how much less drag do you get when the prop is allowed to spin.

Yes spinning reduce parasite drag, but not to the point that it doesn't affect jaw at all. Not a slight change in jaw or roll!

[Holtzauge]

I have heard that if the engine is dead you get more drag if the propeller is turning than if it is fixed (assuming you have a fixed pitch prop or can’t feather that is) and NACA report 464 seems to support that: Granted, I did not read the complete thing but the last pages have some conclusions and the attached table where at 100 mph the tested prop had more drag when turning a dead engine and even when idling. The difference was smaller than I expected but as always, I guess it depends on the configurations tested and the result for a Bearcat and Cessna would probably differ.

 

[ZachariasX]

You never want a prop to windmill. It produces most drag then. Why? spinning the prop requires energy, energy that is taken from the planes forward momentum. As soon as the prop comes to a halt, this SIGNIFCANT bleed of energy is stopped and all that is remaining as drag is the exposed frontal section of the prop. This is far less of a brake than making the prop turn and further bleeding energy.

 

Cranking the engine takes a lot of power. Crank one by hand and you see how much that is. Now think of how much that takes to crank a prop at 4000 rpm. In terms of power, in a todays car, you can expect about 25% of the power being lost before it reaches the wheels. In an airplane there is less mechanics in between, but I would be surprised if the internal loss would be less than 10%. So say, just as example, in a Beechcraft Baron, one engine (guessing now) produces 300 hp on the prop at 3000 rpm. In my example this makes 30 hp internal loss (and the engine effectively produce 330 hp). Now, if one engine had problems and you get a runaway prop, say to 4000 rpm, this produces 40 hp of power then. In addition to the drag of the prop blades, you get a 40 hp engine pulling that engine backwards. As soon as you stop the prop (at same prop pitch), you lose this 40 hp pull „in the wrong direction“.

 

In German, there is the term „Schleppleistung“ (anyone knows the proper English term?) that is used to measure the power output of a car on a test bench and that is referring to this internal loss of power. What you do is you measure the power transferred from the spinning wheels to the test bench. After you have that, you cut the power of the engine and see how much power was that was required to keep the wheels spinning at the rpm producing given rated power. This difference is your „true“ power, the one that should be listed in the spec sheet of the car. So if you have bought a 400 hp car, it might well have to have an engine delivering 500 hp for you having 400 hp. And don‘t put your car on a test bench for such. You‘ll be disappointed. Horses do escape.

 

Now this is the Euro-diesel-gate to do it. In my American car, I have „rated bhp“ which were solely good in determining the taxes for the car, but that have nothing to do with the engine, and certainly nothing at all with the car at all.

[unreasonable]

I have heard that if the engine is dead you get more drag if the propeller is turning than if it is fixed (assuming you have a fixed pitch prop or can’t feather that is) and NACA report 464 seems to support that: Granted, I did not read the complete thing but the last pages have some conclusions and the attached table where at 100 mph the tested prop had more drag when turning a dead engine and even when idling. The difference was smaller than I expected but as always, I guess it depends on the configurations tested and the result for a Bearcat and Cessna would probably differ.

 

The free-wheeling prop has less drag than either dead engine or idling in that 17 degrees scenario at any significant speed according to that table, so I do not think it does necessarily support what you have heard.   The text says that whether the drag is higher in the free wheeling vs locked case is dependent on the blade angle: which I am trying to get my head around as I would have thought that a moving prop would have a smaller effective surface due to the angle of the airflow hitting it increasing.

 

All very complex as usual!

 

Edit as Z has just posted - in these NACA tests, and the scenario I was thinking about, "freewheeling" means that the prop is disconnected to the engine, (like a toy prop on a stick), which may not be relevant to the OP's case.

Edited December 29, 2017 by unreasonable

[ZachariasX]

I have heard that if the engine is dead you get more drag if the propeller is turning than if it is fixed (assuming you have a fixed pitch prop or can’t feather that is) and NACA report 464 seems to support that: Granted, I did not read the complete thing but the last pages have some conclusions and the attached table where at 100 mph the tested prop had more drag when turning a dead engine and even when idling. The difference was smaller than I expected but as always, I guess it depends on the configurations tested and the result for a Bearcat and Cessna would probably differ.

100 mph is still rather slow. But you can see how the drag od a windmilling prop increases with speed. In fast aircraft as we have them in the sim, this is the speed where we fall out of the sky. If they measured the same at 200 mph, things would have looked much worse.

 

Also in this setting it is not mentioned how much power it takes to turn the propeller. If it is just on an axis on ball bearings, the parasitic drag mentioned above is not present as it takes no power to crank the prop.

 

As it takes a lot of force to crank a prop in a real aircraft, basically all you have to do with small fixed prop aircraft, is fly slow enough and the prop comes to a stop. The pressure point of the cylinder will keep it stopped if you fly slow.

The free-wheeling prop has less drag than either dead engine or idling in that 17 degrees scenario at any significant speed according to that table, so I do not think it does necessarily support what you have heard.   The text says that whether the drag is higher in the free wheeling vs locked case is dependent on the blade angle: which I am trying to get my head around as I would have thought that a moving prop would have a smaller effective surface due to the angle of the airflow hitting it increasing.

 

All very complex as usual!

 

Edit as Z has just posted - in these NACA tests, and the scenario I was thinking about, "freewheeling" means that the prop is disconnected to the engine, (like a toy prop on a stick), which may not be relevant to the OP's case.

Yes, exactly.

[HagarTheHorrible]

Eh ???????

 

A spinning prop has a higher drag ratio than a fixed feathered prop ?  What is difficult or complex about that, it's perfectly logical, it has no more, or less drag it is simply an indication of how "feathered" the prop is compared to the airflow passing over the blades.

 

If a prop spins without engine power then it has to get it's energy from somewhere, airflow,  and that comes at the cost of drag.  A perfectly feathered unbraked prop , always perpendicular to the airflow, won't spin, any deviation, creating sufficient energy,  from that will cause it to spin.  Obviously it requires a certain amount of energy to spin the prop and engine but all that is doing is demonstrating a visual clue as to how much drag is acting on the blades that you might not otherwise be aware of, it is however no more, or less, draggy.

 

Which is one possible explanation

[unreasonable]

Hagar, I think you have missed the point.  

 

The issue - at least in my mind - is whether a fixed unfeathered prop has more or less drag than a rotating unfeathered prop. The answer is maybe: it depends on what is rotating.  If the prop is genuinely freewheeling - ie disconnected to the engine so that the prop is not driving the crankshaft - it can sometimes have less drag than when the prop is stationary, or when it is rotating on idle or with a dead engine. That is what the table shows. 

[Aap]

What I have heard, a fixed unfeathered prop causes drag according to the area of blades, but a rotating unfeathered prop causes drag that is roughly equal to as if you had a solid disc there.

[ZachariasX]

Ok, let‘s put it this way: a propeller goes through the air as a screw through wood. Be it a sort of wing or not. The pitch of your propeller determines directly and acurate almost to the single km/h your airspeed. So you know your airspeed and you know your rpm, then you can very precisely get the pitch angle of your propeller.

 

This means that a propeller that is set to 3000 rpm at 200 km/h will want to rotate at 6000 rpm if you go at 400 km/h. If you set it to twice the pitch (here: not angle, but it‘s „flank lead“) it will want to go again at 3000 rpm.

 

If we assume the propeller having blades that are two domensional sheets, then this propeller will not have drag in a 90*, feathered angle (it has no frontal cross section then in relation to the airflow then).

 

Now let us assume the aircraft is moving at 200 km/h (example above). If I again set the prop to that same pitch, it will just rotate either 3000 rpm or 6000 rpm, as long as we assume it can rotate freely. Once it reached the respective rpm, it will not produce drag at all anymore, as the airflow „sees“ the prop directly from the front.

 

This means, if the prop could rotate aboslutely without any resistance, it is not relevant whether it is feathered ir not, as in both cases, the air is looking at the prop directly from the front. It will go faster or slower until this state is reached.

 

In the real world, as said, it takes considerable force to crank a prop, and this force is pulling back on your engine nacelle.

 

Why you must increase pitch is that you want to let the prop spin as slow as possible as the slower it turns, the less power is extracted form your aircraft as it takes force to crank the engine. On the practical side: If your prop is set to 3000 rpm at 200 km/h and you‘re going 4000 km/h the propeller will want to turn at 6000 rpm while extracting the cranking power required to do so. This is a pretty exciting state of things.

 

The example Holtz posted reflects the propellers desire to reach the matching rpm in relation to its pitch and flight speed, in which it has minimal frontal cross section (and minimal aerodynamic drag). If you block the prop and drag it through the air, it cannot follow its instinct to minimize its cross section in relation to the air and you will have more drag, if there would be no cranking power further deduced.

 

Cranking a 12 cylinder 30L engine requires substancial power.

[HagarTheHorrible]

Presumably a feathered and BRAKED prop will only be more draggy than a feathered UNBREAKED prop when the blades reach the critical energy state that overcomes the inertia latent in the engine and gearing mechanism and then the blades start to increasingly turn into a brake.

 

Despite the potential downside of an increase in drag from a breaked propeller presumably it might still be a greater benefit if a windmilling prop might cause further damage to the engine, fuel pumps ect. Being able to halt a prop from spinning, or at least having an option, might be a better than having no option at all even if a locked propeller might come with it's own gremlins.

 

I assume that the drag of a spinning, unpowered (feathered or not) is directly proportional to the thrust that a powered propeller spinning at the same speed would produce.

Edited December 29, 2017 by HagarTheHorrible

[Aap]

The free-wheeling prop has less drag than either dead engine or idling in that 17 degrees scenario at any significant speed according to that table, so I do not think it does necessarily support what you have heard.

As you see in the table, the difference between free-wheeling and fixed gets smaller when the speed increased. At some point it will catch up and pass it. Basically, when the props forward velocity becomes bigger than rotational velocity, angle of attack will become negative and create reverse thrust, so actively slowing down the plane.

[HagarTheHorrible]

As you see in the table, the difference between free-wheeling and fixed gets smaller when the speed increased. At some point it will catch up and pass it. Basically, when the props forward velocity becomes bigger than rotational velocity, angle of attack will become negative and create reverse thrust, so actively slowing down the plane.

???

 

You don't get anything for free, (energy)

 

Forward velocity will always be greater than rotational velocity, if for no other reason than to just to overcome the engine inertia. If the feathered prop was feathered past 90 deg then it would simply spin ( if it was possible) the engine in the reverse direction, it wouldn't create reverse thrust, it would simply have the same drag ratio to spinning in the other direction. It would only create reverse thrust if the engine was the prime mover (energy source) and not the air flow.

Edited December 29, 2017 by HagarTheHorrible

[Requiem]

...

 

In German, there is the term „Schleppleistung“ (anyone knows the proper English term?) that is used to measure the power output of a car on a test bench and that is referring to this internal loss of power. What you do is you measure the power transferred from the spinning wheels to the test bench. After you have that, you cut the power of the engine and see how much power was that was required to keep the wheels spinning at the rpm producing given rated power. This difference is your „true“ power, the one that should be listed in the spec sheet of the car. So if you have bought a 400 hp car, it might well have to have an engine delivering 500 hp for you having 400 hp. And don‘t put your car on a test bench for such. You‘ll be disappointed. Horses do escape

I don't speak German but I believe the terms you're looking for (with the airplane engines in this sim currently) are Indicated Horsepower and Brake Horsepower?  Indicated Horsepower is a theoretical maximum output of an engine without considering any power losses due to friction in engine operation, but when these losses are taken into account (aka Friction horsepower) you are left with Brake Horsepower, the actual horsepower available for use when it reaches the propeller.

[Aap]

???

 

You don't get anything for free, (energy)

 

Forward velocity will always be greater than rotational velocity, if for no other reason than to just to overcome the engine inertia. If the feathered prop was feathered past 90 deg then it would simply spin ( if it was possible) the engine in the reverse direction, it wouldn't create reverse thrust, it would simply have the same drag ratio to spinning in the other direction. It would only create reverse thrust if the engine was the prime mover (energy source) and not the air flow.

 

???

 

You don't need to get anything for free. You can trade kinetic energy to potential energy etc.

When forward velocity vector is greater than rotational velocity vector and you are windmilling or free-wheeling that means that AoA will become negative and it will cause reverse thrust. And that means an additional vector to the drag direction. Which means that at high speeds a windmilling or free-wheeling prop will cause more drag than fixed prop.

 

Edit: Here is a diagram of it.

Edited December 29, 2017 by II./JG77_Kemp

[Holtzauge]

???

 

You don't need to get anything for free. You can trade kinetic energy to potential energy etc.

When forward velocity vector is greater than rotational velocity vector and you are windmilling or free-wheeling that means that AoA will become negative and it will cause reverse thrust. And that means an additional vector to the drag direction. Which means that at high speeds a windmilling or free-wheeling prop will cause more drag than fixed prop.

 

Edit: Here is a diagram of it.

 

Yup, this is my take on this as well.

 

I think you can understand that the spinning props generates more drag like this:

 

Roughly the flat plate drag coefficient for a stationary prop perpendicular to the flight path will be Cd=1 meaning to get the drag of a stationary prop you multiply D= 0.5*ro*v^2*Cd*S where S is the prop blade area. So this is the reference case.

 

Now when the prop is turning, the relative wind will be the sum vector of the forward v and the rotational speed vr so it will be bigger than v. Here is the first component of an increased drag since the drag increases with the square of the speed. In addition, the vector v+vr will give an angle of attack on the prop which will give you both a Cl and and a Cd and the Cl vector will be roughly in the direction of flight which if you consider that Cl will most likely be larger than 1 will give you a higher combined Cn which is the second component of the increased drag.

 

So given this I’m not surprised the drag is higher: You have the prop blade both going at a higher speed and the Cd in the formula substituted for a Cn (factored for the angle relative the flight path) which altogether should mean a higher drag (on the plane in the direction of flight) than for a stationary prop as far as I can see.

[ZachariasX]

I don't speak German but I believe the terms you're looking for (with the airplane engines in this sim currently) are Indicated Horsepower and Brake Horsepower?  Indicated Horsepower is a theoretical maximum output of an engine without considering any power losses due to friction in engine operation, but when these losses are taken into account (aka Friction horsepower) you are left with Brake Horsepower, the actual horsepower available for use when it reaches the propeller.

Indicated power for the theoretical maximum, Thnx!

 

„Schleppleistung“ („tow power“ litterally) is basically the difference between indicated and brake horsepower and something innate to a test bench run.

[ZachariasX]

Yup, this is my take on this as well.

 

I think you can understand that the spinning props generates more drag like this:

 

Roughly the flat plate drag coefficient for a stationary prop perpendicular to the flight path will be Cd=1 meaning to get the drag of a stationary prop you multiply D= 0.5*ro*v^2*Cd*S where S is the prop blade area. So this is the reference case.

 

Now when the prop is turning, the relative wind will be the sum vector of the forward v and the rotational speed vr so it will be bigger than v. Here is the first component of an increased drag since the drag increases with the square of the speed. In addition, the vector v+vr will give an angle of attack on the prop which will give you both a Cl and and a Cd and the Cl vector will be roughly in the direction of flight which if you consider that Cl will most likely be larger than 1 will give you a higher combined Cn which is the second component of the increased drag.

 

So given this I’m not surprised the drag is higher: You have the prop blade both going at a higher speed and the Cd in the formula substituted for a Cn (factored for the angle relative the flight path) which altogether should mean a higher drag (on the plane in the direction of flight) than for a stationary prop as far as I can see.

I gave example how easy the prop can spin up if you lose control over the pitch governer and it goes to full fine in cruise flight. You quickly can add Mach effects to the drag too. But if you lost oil pressure in the engine and the governor, chances are that the bearings are not lubed either, overheat and the prop falls off. Best glider configuration like that (if you are not on fire by then). But then tell the cabin crew to roll all trolleys to the front. You make up for balance then and you can have a beer.

[unreasonable]

As you see in the table, the difference between free-wheeling and fixed gets smaller when the speed increased. At some point it will catch up and pass it. Basically, when the props forward velocity becomes bigger than rotational velocity, angle of attack will become negative and create reverse thrust, so actively slowing down the plane.

 

You may be right, but the table does not show that at all. I understand Z's point about the last two columns in the table - essentially they are not just measuring the drag of the propeller, strictly speaking, but also the friction of the engine components being turned by the prop.

 

But the freewheeling case excludes that - it is not a dead engine, but an engine disconnected from the propeller. This is explicitly discussed in the text (page 429). This may have no bearing on our WW2 planes (did any of them have a clutch pedal?) but it is interesting just to try to understand the propeller drag.

 

So I am comparing the column titles "Locked Propeller set 17 degrees" with "Freewheeling Propeller set 17 degrees". The ratio of the two numbers is almost constant: certainly given the base numbers are rounded I would be reluctant to attribute a trend in the ratio on the numbers alone.

 

The text says that the difference between locked and locked-freewheeling is dependent on the blade angle: more than 15 degrees, in the case of this propeller, the drag of the freewheeling prop is less: as the table shows. I freely admit that I struggle to interpret the diagrams in the report.

 

Quote from the conclusions of the report  " 4. The drag of a free-wheeling propeller is slightly less than that of a locked propeller in the normal range of blade-angle settings."

 

I understand the point about forwards vs rotational velocity: but would not a genuinely free-wheeling propeller find (or try to) the rotational speed at which it is neutral in thrust? Would it not just speed up (edit, or slow down) in the scenario you paint? The report thinks so: it states that to maintain negative thrust at very low blade angles the engine must be used to maintain rpms. 

Edited December 29, 2017 by unreasonable

[Ribbon]

You never want a prop to windmill. It produces most drag then. Why? spinning the prop requires energy, energy that is taken from the planes forward momentum. As soon as the prop comes to a halt, this SIGNIFCANT bleed of energy is stopped and all that is remaining as drag is the exposed frontal section of the prop. This is far less of a brake than making the prop turn and further bleeding energy.

 

Cranking the engine takes a lot of power. Crank one by hand and you see how much that is. Now think of how much that takes to crank a prop at 4000 rpm. In terms of power, in a todays car, you can expect about 25% of the power being lost before it reaches the wheels. In an airplane there is less mechanics in between, but I would be surprised if the internal loss would be less than 10%. So say, just as example, in a Beechcraft Baron, one engine (guessing now) produces 300 hp on the prop at 3000 rpm. In my example this makes 30 hp internal loss (and the engine effectively produce 330 hp). Now, if one engine had problems and you get a runaway prop, say to 4000 rpm, this produces 40 hp of power then. In addition to the drag of the prop blades, you get a 40 hp engine pulling that engine backwards. As soon as you stop the prop (at same prop pitch), you lose this 40 hp pull „in the wrong direction“.

 

In German, there is the term „Schleppleistung“ (anyone knows the proper English term?) that is used to measure the power output of a car on a test bench and that is referring to this internal loss of power. What you do is you measure the power transferred from the spinning wheels to the test bench. After you have that, you cut the power of the engine and see how much power was that was required to keep the wheels spinning at the rpm producing given rated power. This difference is your „true“ power, the one that should be listed in the spec sheet of the car. So if you have bought a 400 hp car, it might well have to have an engine delivering 500 hp for you having 400 hp. And don‘t put your car on a test bench for such. You‘ll be disappointed. Horses do escape.

 

Now this is the Euro-diesel-gate to do it. In my American car, I have „rated bhp“ which were solely good in determining the taxes for the car, but that have nothing to do with the engine, and certainly nothing at all with the car at all.

I don't have experience with piston engines but i assume it require a lot of energy to crank the prop unlike in turbo prop engines where slight wind breeze will start spinning the prop which is unwanted effect which can cause damage to engine and propeller assy.

When free spinning parasite drag is partly transformed into torque, centrifugal and other forces which has different vector of forward thrust force aircraft is moving, so that could explain energy bleed while parasite drag is still present.

 „Schleppleistung“- word we borrow from German and it is used for "tow-force".

[Holtzauge]

I gave example how easy the prop can spin up if you lose control over the pitch governer and it goes to full fine in cruise flight. You quickly can add Mach effects to the drag too. But if you lost oil pressure in the engine and the governor, chances are that the bearings are not lubed either, overheat and the prop falls off. Best glider configuration like that (if you are not on fire by then). But then tell the cabin crew to roll all trolleys to the front. You make up for balance then and you can have a beer.

 

Well as I glider pilot I have no need for a prop anyways be it rotating or fixed: A real man gets around without burning any fossil fuels.

[Guest deleted@50488]

Well as I glider pilot I have no need for a prop anyways be it rotating or fixed: A real man gets around without burning any fossil fuels.

 

That makes two of us :-)

 

Yet, flying in my friends who are also "spin pilots", specially in some of their CS prop bombs, I see how noticeable it is when they "firewall" the prop lever during base leg / final.... You do feel the "braking power" of the fine prop settings - on some of the aircraft it feels like a brick !

 

BTW: I do find this aerodynamic braking very well modelled in IL2, and I often use it for high steep descents, even in Axis fighters where we can opt for manual prop pitch.