Spin, Spin Spin.....

[SmirkingGerbil]

22 hours ago, Diesel_Thunder said:

Underboost is not too terrible in itself, but can easily lead to the engine being driven by the prop, which is horrible for the main bearings. Here’s why:
 

With radials, each row of cylinders only has one crankpin. In the R-2800, this means 9 cylinders share one crankpin.
 . . .

Back to the radial.  experience: https://www.avweb.com/features/pelicans-perch-78-props-driving-engines/

 

Thank you sir, magnificent and easy to understand.

Now I get why pulling the throttle back too far on a steep dive to compensate for engine torque (nose wobble) can lead to damage or worse.

As to all the other posts regarding how the P-47 handles.

Yeah, when I mention torque exclusively, I figure most folks would understand you must have dealt with all the other pitch and roll axis, just like any other aircraft.

I fly the 47 with a short stick, and regardless of any OB patch, once you get a feel for the influence of torque, trim (axis), throttle speed, and engine settings you can easily hands-off fly this bird all day. Short stick, long stick, or tuned curves. The 47 can and does fly like a dream hands off. Smooth and trouble free.

Edited January 4, 2021 by SmirkingGerbil

[Whisper]

On 1/1/2021 at 12:40 AM, CoBlue said:

NO it doesn't! You can clearly see it in the pics, look at controls indicator at full back stick, 1:st pic is with with no Saturation, 2:nd with 65 Y Saturation. So you're LIMITING your CONTROL SURFACES MOVEMENTS with using Saturation!

Who want's to have maximum pitch authority? I do in ex. dogfights & slowflight.

 

 

 

Ha, I see the pics now.

 

Well, limiting is the whole point since it also decrease the sensibility. And like I said, if I want a max performance turn, I don't pull the stick all the way because doing so goes wayyyyy past the AoA limit of the aircraft.

Quick & dirty example here :

 

Look at the "Right VPC Stick MT-50 - Axe Y" to check my stick position during the tests. For example, my first turn is gradually pulling the stick, I "lose control" (left wing drops, AoA goes past 23-25 ) at 0:17 when the stick is at around 70% of its max deviation.

During the various tests (and others I did), between this 70% and max stick pulling, my G load or turn rate stays the same. Which means I don't gain anything by pulling even more.

And you know what? This was recorded with a 70% Y Saturation.

What I gain by pulling at max between a 100% saturation and a 70% saturation is absolutely marginal (I'm already in the 40° AoA with 70% saturation), and moreover, if I ever need such extreme numbers during a dogfight, it means I did horribly wrong long before that to end up having to resort to these extremes.

So in the end, dumping down my saturation just may make me lose marginal numbers in max AoA in extreme situations which I must avoid at all cost anyway.... In exchange, I gain stability in 99.9% of the rest of my flight....

 

So I'm really eager to learn something everyday (really, I'm not trolling here), so if there is indeed a gain in going to these extremes by pulling your stick 100%, I'm all ear, but I've yet to encounter one myself.

Edited January 4, 2021 by Whisper

[Jonne]

On 1/3/2021 at 2:54 PM, Diesel_Thunder said:

Underboost is not too terrible in itself, but can easily lead to the engine being driven by the prop, which is horrible for the main bearings. Here’s why:
 

With radials, each row of cylinders only has one crankpin. In the R-2800, this means 9 cylinders share one crankpin. One cylinder in the row will have a master connecting rod that connects the piston to the crankshaft, the other 8 in that row will have articulating rods that bolt to the master rod. The key thing here is that all cylinders in a row share the same crank pin. 
 

This is unlike most engines in use today in cars and trucks, where each cylinder has it’s own crankpin. In the case of V engines, the crankpins are typically shared by 2 cylinders. The forces on the crankshaft are reduced because of this, as the bearing surfaces only have to handle one power stroke per revolution. 
 

Back to the radial. Because all rows share one crank pin, that surface has to take the force of 4 or 5 power strokes in each revolution. Nearly all radials have rows with odd numbers of cylinders, 9 per row in the case of our R-2800. This is to help simplify the firing order of the cylinders in each row, which is every other cylinder (1, 3, 5, 7, 9, 2, 4, 6, 8 ). The cylinders are set 40° apart (360/9=40). Every 80° of crankshaft rotation, there is a power stroke in each row of cylinders, and that power stroke pushes on the same spot on the crankshaft. That crankpin takes a lot of force in that one area each time a cylinder fires, and is heavily built for that reason.
 

One other thing that the engine builders did to help those bearings and crankpins stand up to these forces was to drill a hole into the crankshaft that would supply pressurized oil right between the crank pin and master rod at that spot where the power stroke would push to keep the two well lubricated, and more importantly, keep the two from ever touching (metal on metal) and keeping the engine and pilot airborne and happy. 
 

Now, if the engine is at a low power setting, and with decent airpeed, the prop now becomes a windmill and starts driving the engine. Here’s where the problems start. The forces on the crankshaft are now reversed. The crankshaft is pushing the pistons around, and with that the oil hole is on the wrong side of the bearing in this instance. The lubrication on this side of the crankshaft is considerably less, and with the prop driving the engine, usually isn’t enough to prevent metal on metal contact. This is what will wreck the bearings (combat damage not withstanding). 
 

How long an engine can tolerate that is a matter of how hard it is being driven by the prop. High RPM and low power in a steep dive would be more harmful than low RPM and low power in a shallow dive. The key takeaway is not allow the prop to drive the engine as best possible, and to minimize the potential for that to happen. Keeping some power on in a dive would keep the crank shaft loaded properly and mitigate the problem entirely. 
 

Here’s a great link where one could learn more about this with math, diagrams, and an author with loads of experience: https://www.avweb.com/features/pelicans-perch-78-props-driving-engines/

 

 

I am very sorry, but the article and your post are based on the wrong assumption, that the force during the expansion cycle changes direction when it actually does not. Even with a windmilling piston engine, during that cycle the pressure in the cylinder is still higher than in the crankcase and thus the force is still pointing from the piston towards the crankshaft. Secondly the oil supply holes on a radial crankshaft are where they typically are for a piston engine, which is where the stress is the least and not where it is the biggest like you and the unnamed author of the article describe it. Also holes for oil supply are not a special radial trick, but a standard feature of a crankshaft, or a plain fluid bearing in general. Those bearings tend to work best at speed and low force, which is exactly the situation you have with a windmilling engine.

[grafspee]

1 hour ago, Jonne said:

 

 Even with a windmilling piston engine, during that cycle the pressure in the cylinder is still higher than in the crankcase and thus the force is still pointing from the piston towards the crankshaft. 

Yes, but this does not change fact (pressure is higher yes, it provide some force and induce some acceleration, but this is not enough to match 2700rpm so majority of force come from prop windmiling), that crank shaft starts pulling pistons down, so master bearing is loaded from other side. But this is not a big anomaly for every engine, because intake stroke is exactly the same, cranks shaft pull piston down and here we have even less help because throttle is closed and pressure is very low on intake side.

So this damage from under boost must came from some other reason.

Maybe this intake stroke is reason why master bearing fails, when crank shaft pulls piston down when intake pressure is very low (throttle closed) at high rpm it can get to maybe 10". So crank shaft fights against higher pressure in crank case, maybe master bearing isnt prepared for that.

Edited January 4, 2021 by grafspee

[SmirkingGerbil]

4 hours ago, Jonne said:

 

I am very sorry, but the article and your post are based on the wrong assumption, that the force during the expansion cycle changes direction when it actually does not. Even with a windmilling piston engine, during that cycle the pressure in the cylinder is still higher than in the crankcase and thus the force is still pointing from the piston towards the crankshaft. Secondly the oil supply holes on a radial crankshaft are where they typically are for a piston engine, which is where the stress is the least and not where it is the biggest like you and the unnamed author of the article describe it. Also holes for oil supply are not a special radial trick, but a standard feature of a crankshaft, or a plain fluid bearing in general. Those bearings tend to work best at speed and low force, which is exactly the situation you have with a windmilling engine.

 

3 hours ago, grafspee said:

Yes, but this does not change fact (pressure is higher yes, it provide some force and induce some acceleration, but this is not enough to match 2700rpm so majority of force come from prop windmiling), that crank shaft starts pulling pistons down, so master bearing is loaded from other side. But this is not a big anomaly for every engine, because intake stroke is exactly the same, cranks shaft pull piston down and here we have even less help because throttle is closed and pressure is very low on intake side.

So this damage from under boost must came from some other reason.

Maybe this intake stroke is reason why master bearing fails, when crank shaft pulls piston down when intake pressure is very low (throttle closed) at high rpm it can get to maybe 10". So crank shaft fights against higher pressure in crank case, maybe master bearing isnt prepared for that.

 

Well, that's helpful. Two diametrically opposed descriptions, and one that explains something in between.

Guess I just need to slam the throttle to zero, do dives from 15K feet one after another, and see how many times I destroy the engine?

[Andy1966]

 

     Hi, The thing to remember is the minimum maneuvering speed is 150 knots, and P-47 weighs almost 7 tons. it gets really mushy(sluggish) under 230 Kn with a clean configuration. also it IS a stick and rudder aircraft! meaning it does not want to fly coordinated turns naturally, the opposite rudder is counter-intuitive, and depending on power settings, can be extreme, but just remember to keep the ball centered with the rudder.   Practice flat 90 degree break turns at 275 Kn, first one axis at a time; gently, as the controls are more sensitive the faster you go, meaning roll, then pull gently, to do the roll then the turn, one input at a time, keeping the ball centered in a coordinated turn and exit the turn with the same airspeed as when entering the turn. once able to do that in combat Don't turn, zoom climb, level at 275 kn to extend out, then turn, and re-engage, use the weight of your aircraft to maintain your energy, think of it is kind of like a pendulum, when you run out of, or get low in E , then turn and swing the pendulum the other way.

[CoBlue]

On 12/30/2020 at 9:15 PM, Whisper said:

I lower the Y Saturation on the pitch axis..

People will tell you to adapt and be super precise, but this is shooting yourself in the foot for zero reason, you'll be more precise and won't lose anything in the range of motion of your bird virtual stick, this will NOT hamper any capacity of it in any way.

 

On 12/31/2020 at 5:20 PM, Whisper said:

I just re-tested to be sure. Spitfire on the ground, 75 Y Saturation : my elevator still reaches max deflection (easily seen from the outside, the elevator stops moving) before my stick reaches its max

Well, you said lowering the Saturation won't effect elevator travel & I proved you wrong. If I see somebody posting completely wrong information I will correct it.

Now, if you don't need the full elevator deflection that's your business, but it's there for a reason. I'm not going to post all the reasons when you need it but as I said, slow-flight dogfights & aerobatics among others. Of course you're not going to need it when flying fast, all this is basic aerodynamics.

We fly PvP dogfights all the time & yes I need full deflection in certain cases.

[Whisper]

2 hours ago, CoBlue said:

 

Well, you said lowering the Saturation won't effect elevator travel & I proved you wrong. If I see somebody posting completely wrong information I will correct it.

Now, if you don't need the full elevator deflection that's your business, but it's there for a reason. I'm not going to post all the reasons when you need it but as I said, slow-flight dogfights & aerobatics among others. Of course you're not going to need it when flying fast, all this is basic aerodynamics.

We fly PvP dogfights all the time & yes I need full deflection in certain cases.

"Well, you said lowering the Saturation won't effect elevator travel & I proved you wrong. " Indeed.... congratulations, for what it's worth. At least in the positive deflection, I tested too quickly and wrote something wrong.

It is still useless and I've still yet to see any aerodynamic difference. I keep trying different maneuvers at max stick pull, with different speed, attitudes, shapes, etc.... I never see any difference in the outcome, in cockpit or in tacview, between doing it with 100% Y saturation and 70% Y Saturation in a Spit...

Edited January 5, 2021 by Whisper

[Yo-Yo]

On 1/3/2021 at 3:54 PM, Diesel_Thunder said:

Underboost is not too terrible in itself, but can easily lead to the engine being driven by the prop, which is horrible for the main bearings. Here’s why:
 

With radials, each row of cylinders only has one crankpin. In the R-2800, this means 9 cylinders share one crankpin. One cylinder in the row will have a master connecting rod that connects the piston to the crankshaft, the other 8 in that row will have articulating rods that bolt to the master rod. The key thing here is that all cylinders in a row share the same crank pin. 
 

This is unlike most engines in use today in cars and trucks, where each cylinder has it’s own crankpin. In the case of V engines, the crankpins are typically shared by 2 cylinders. The forces on the crankshaft are reduced because of this, as the bearing surfaces only have to handle one power stroke per revolution. 
 

Back to the radial. Because all rows share one crank pin, that surface has to take the force of 4 or 5 power strokes in each revolution. Nearly all radials have rows with odd numbers of cylinders, 9 per row in the case of our R-2800. This is to help simplify the firing order of the cylinders in each row, which is every other cylinder (1, 3, 5, 7, 9, 2, 4, 6, 8 ). The cylinders are set 40° apart (360/9=40). Every 80° of crankshaft rotation, there is a power stroke in each row of cylinders, and that power stroke pushes on the same spot on the crankshaft. That crankpin takes a lot of force in that one area each time a cylinder fires, and is heavily built for that reason.
 

One other thing that the engine builders did to help those bearings and crankpins stand up to these forces was to drill a hole into the crankshaft that would supply pressurized oil right between the crank pin and master rod at that spot where the power stroke would push to keep the two well lubricated, and more importantly, keep the two from ever touching (metal on metal) and keeping the engine and pilot airborne and happy. 
 

Now, if the engine is at a low power setting, and with decent airpeed, the prop now becomes a windmill and starts driving the engine. Here’s where the problems start. The forces on the crankshaft are now reversed. The crankshaft is pushing the pistons around, and with that the oil hole is on the wrong side of the bearing in this instance. The lubrication on this side of the crankshaft is considerably less, and with the prop driving the engine, usually isn’t enough to prevent metal on metal contact. This is what will wreck the bearings (combat damage not withstanding). 
 

How long an engine can tolerate that is a matter of how hard it is being driven by the prop. High RPM and low power in a steep dive would be more harmful than low RPM and low power in a shallow dive. The key takeaway is not allow the prop to drive the engine as best possible, and to minimize the potential for that to happen. Keeping some power on in a dive would keep the crank shaft loaded properly and mitigate the problem entirely. 
 

Here’s a great link where one could learn more about this with math, diagrams, and an author with loads of experience: https://www.avweb.com/features/pelicans-perch-78-props-driving-engines/

 

Very detailed review, but the things about master bearing are a bit worse: the overall loading is tuned to have all forces including inertial  (centripetal and reciprocated) and gas pressure forces in relative balance. If one of them - gas pressure - disappears, the inertial instant forces become higher and overload the master bearing. 

[SmirkingGerbil]

56 minutes ago, Yo-Yo said:

Very detailed review, but the things about master bearing are a bit worse: the overall loading is tuned to have all forces including inertial  (centripetal and reciprocated) and gas pressure forces in relative balance. If one of them - gas pressure - disappears, the inertial instant forces become higher and overload the master bearing. 

Alright, that seals it.

When Yo-Yo weighs in, concurs, but also elaborates on how the forces are balanced against different sources of cylinder pressure and explains that one being out of sync or outside of nominal conditions, then you will overload the main/master bearing during hard windmilling and low throttle.

I am going with this.

Thanks everyone for the time and detail, fascinating and learned even more about my favorite Warbird.

[Voyager]

On the original subject, as I understand it, the P-47 has a noticeably lower max AoA than its contemporaries, so while it may have been forgiving is a standard stall, for what I've seen, it is quite prone to nasty accellerated stalls. 

 

On the power settings and impacts of props driving engines. I need to go find the early P-47 split-s manuals, and check to see exactly what it said about throttle settings. I'd recalled that it said to "idle" the engine before entering, but I may be mis-remembering what it actually said. 

[grafspee]

1 hour ago, Voyager said:

On the original subject, as I understand it, the P-47 has a noticeably lower max AoA than its contemporaries, so while it may have been forgiving is a standard stall, for what I've seen, it is quite prone to nasty accellerated stalls. 

 

On the power settings and impacts of props driving engines. I need to go find the early P-47 split-s manuals, and check to see exactly what it said about throttle settings. I'd recalled that it said to "idle" the engine before entering, but I may be mis-remembering what it actually said. 

Are you sure that max AOA for p-47 is lower then contemporaries, as far as i know p-47 have very good wings. Second thing, as far as i know that all wings has similar critical AoA of that era.

Another thing, accelerated stall is just stall with more then 1g load, at leas how i understand that. It is more abrupt, but it is just as 1g stall.

[PL_Harpoon]

At least in my experience it's better at high G turns that the Mustang and Dora.

[Tiger-II]

P-47 has some quirks for sure!

 

I'm a "jet guy", but I love the P-47 because she's hard to fly (IRL) due to her mass.

 

I think the P-47 was the heaviest fighter of her type/size?? Not to be confused with "heavy fighter" (role).

 

I totally missed that it was released over Christmas! I'll pick it up tomorrow.

 

As for handling: if you don't fly coordinated, she'll spin easily.

 

IMHO, DCS over-exaggerates p-factor/yaw inertia seems a little too high.

 

I remember the DCS:P-51 had quite a large problem with this upon initial release. She'd ground-loop far too easily, and would increase yaw-rate from a very very slow yaw indeed.

 

Maybe a similar thing is happening with the P-47?

 

I'll pick it up and write back with what I find! I don't have my rudder pedals available at the moment so I'll be flying with the T16000.

 

"Prop driving the engine" is a very real issue! Everything is "back-loaded" and components that are stressed for operation in one direction are harmed by being loaded in the opposite direction.

 

The reason this is an issue for bearings is because when being loaded the wrong way, they are SIMULTANEOUSLY suffering oil starvation (often there is a low oil pressure condition). This is why the damage occurs.

 

It can also damage prop hubs.

 

If you see the prop RPM start to decay at high speed, you've throttled back too far.

 

When throttling up particularly, don't slam the throttle.

 

For takeoff, don't use max power. From what I recall, initial takeoff MP is 40", increasing once airborne to reduce directional control problems on the ground.

Edited January 18, 2021 by Tiger-II

[Mars Exulte]

On 1/4/2021 at 3:27 PM, SmirkingGerbil said:

Well, that's helpful. Two diametrically opposed descriptions, and one that explains something in between.

 

 Welcome to the forums =) To be fair, we're talking about aerodynamics and complex mechanical engineering. It's kind of complicated, even actual professionals don't necessarily ''know it all'' =)

[Voyager]

On 1/5/2021 at 3:07 PM, grafspee said:

Are you sure that max AOA for p-47 is lower then contemporaries, as far as i know p-47 have very good wings. Second thing, as far as i know that all wings has similar critical AoA of that era.

Another thing, accelerated stall is just stall with more then 1g load, at leas how i understand that. It is more abrupt, but it is just as 1g stall.

 

Haven't been able to find the reference again yet, but the P-47 uses the Seversky S-3 airfoil, not one of the NACA airfoils that nearly every other WWII aircraft used. Severky was a bit of a wild designer and would sometimes just do things. As I recall the S-3 air foil was one of them. 

 

I recall it had a very low induced drag, comparable to the later NACA laminar flow wing designs, but only in certain angles of attack. It was weird. 

[SmirkingGerbil]

18 hours ago, Tiger-II said:

P-47 has some quirks for sure!

 

I'm a "jet guy", but I love the P-47 because she's hard to fly (IRL) due to her mass.

 

 . . .

 

I remember the DCS:P-51 had quite a large problem with this upon initial release. She'd ground-loop far too easily, and would increase yaw-rate from a very very slow yaw indeed.

 

Maybe a similar thing is happening with the P-47?

 

. . .

 

For takeoff, don't use max power. From what I recall, initial takeoff MP is 40", increasing once airborne to reduce directional control problems on the ground.

 

Thanks for all this!

 

I will say one thing about the P-51 with "ground loop" on take off (if that is what you mean). I discussed this effect with my wife's Uncle who works for the FAA, and also a crash site reconstruction expert. He rebuilds Luscombe's as a hobby.

He recounted what he knew, from a friend that flew the P-51 in WWII (This was several years ago, when the P-51 first released), and he told me how to set the rudder trim, and other surfaces for take off. From that day forward, I never "ground looped" into the dirt again.

Not saying that the effect wasn't too exaggerated on release, but I always found it fascinating I could describe the aircraft behavior, and a pilot, that knew a pilot who had heard of how he dealt with it in real life, and viola!! It worked for me. After that, I found new respect for our friends at ED and DCS, may not have been on the mark, but it was close enough that word of mouth allowed me to fly it and not flip right into the dirt on take off.

Edited January 19, 2021 by SmirkingGerbil

[Voyager]

naca-wr-l-439 Full-Scale Tunnel Investigation fo the Presure Distribution over the Tail fo the P-47B Airplane.pdf@grafspee

 

Found where I'd seen the stall angle referenced: NACA L-439 notes that the P-47B stall angle is just before 17.1 degrees AoA. As I recall most of the contemporaries were in the 20 degree range. Not sure if I have a test report where they actually tested it though.

[grafspee]

Still AOA is only one figure here. The best way to compare wings would be just to know how much G will induce 5,10,15 AOA at 250mph. That would tell a lot more.

Edited January 25, 2021 by grafspee