DCS: P-47D-30 Discussion

[NineLine]

Can anybody confirm or deny if our Jug is going to have the dive flaps?

 

I was talking to a WWII enthusiast and he swore on his life we wouldn't be getting them. My understanding is anything above block 28 had them.

 

 

If the version we are getting has them, I wouldnt see why we wouldnt get them.

[Pocket Sized]

Isn't the compressibility of the P-47 quite good?

 

Not sure what you mean by "good," but the controls will still stiffen considerably in a steep dive. IIRC one pilot said that they'd practice diving from 42,000 ft, the stick got so hard to pull that the best recovery method was to wait for denser air to slow you down.

[Vitormouraa]

Not sure what you mean by "good," but the controls will still stiffen considerably in a steep dive. IIRC one pilot said that they'd practice diving from 42,000 ft, the stick got so hard to pull that the best recovery method was to wait for denser air to slow you down.

 

In terms of diving and still having all the controls of the aircraft, and it doesn't seem to be the case :D

[Echo38]

All high-speed prop fighters had problems with compressibility at high altitude. The Thunderbolt was one of the ones least affected. The P-38 had (by far) the hardest time with it, due (in large part) to the aerodynamic shape of the rear of the gondola. The 109 I'm not sure about; it did have a harder time with recovering from dives than the P-47 and P-51, but that could have been natural control stiffness rather than actual compressibility.

 

Off the top of my head, I'd place them (from least to most hampered by compressibility) like this: P-47, FW 190, P-51, Me 109, P-38. That's an educated guess from a dodgy memory. Take it with a chunk of salt.

 

It is fact, however, that none of them (not even the P-38) had a problem with compressibility below ~15,000 feet (depending on weather conditions), except when flying over terrain that is high above sea level (e.g. Maj. Ben Kelsey's bail-out from a P-38 during a compressibility test dive over the Rocky Mountains—he'd have been fine in that dive, if the mountains hadn't been in the way of his pull-out).

 

The dive flaps on the P-47 & P-38 improved lift during the dive, preventing the compressibility stall. This increased safe diving speed. Secondary effect (not intentional, but a natural result of the dramatic angle of the dive flaps) was greatly increased drag, which helped prevent exceeding VNE. It must have reduced dive acceleration, but I'd wager that this effect was minimal.

Edited April 21, 2017 by Echo38
ack! I meant least to most. Also, a few more tidbits

[DSR_T-800]

Can anybody confirm or deny if our Jug is going to have the dive flaps?

 

I was talking to a WWII enthusiast and he swore on his life we wouldn't be getting them. My understanding is anything above block 28 had them.

 

Yes we will be getting dive flaps.

[DSR_T-800]

All high-speed prop fighters had problems with compressibility at high altitude. The Thunderbolt was one of the ones least affected. The P-38 had (by far) the hardest time with it, due (in large part) to the aerodynamic shape of the rear of the gondola. The 109 I'm not sure about; it did have a harder time with recovering from dives than the P-47 and P-51, but that could have been natural control stiffness rather than actual compressibility.

 

Off the top of my head, I'd place them (from least to most hampered by compressibility) like this: P-47, FW 190, P-51, Me 109, P-38. That's an educated guess from a dodgy memory. Take it with a chunk of salt.

 

It is fact, however, that none of them (not even the P-38) had a problem with compressibility below 10,000 to 15,000 feet (depending on weather conditions), except when flying over terrain that is high above sea level (e.g. Maj. Ben Kelsey's bail-out from a P-38 during a compressibility test dive over the Rocky Mountains—he'd have been fine in that dive, if the mountains hadn't been in the way of his pull-out).

 

If I remember aright, later models of P-47 had (like the P-38) dive flaps fitted for superior control during dives. They improved lift during the dive, preventing the compressibility stall. This increased safe diving speed. Secondary effect (not intentional, but a natural result of the dramatic angle of the dive flaps) was greatly increased drag, which helped prevent exceeding VNE.

 

Negative, the P-47 were one of the plane that were most affected by compressibility. Second to the P-38 for a matter of fact. The P-51's max permissible speed 505mph is due to structural limitations/concerns while the P-47 is due to compressibilty. So when you start seeing ailerons and elevators pop off at 500mph in your P-47 prepare to cringe. Structurally, I'm sure the P-47 structurally, is fine far above 500mph. The P-47N's maximum permissible IAS at seal level is 564mph. However the wings were strengthen on the N, giving it a 576lbs heavier empty weight.

[Echo38]

the P-47 were one of the plane that were most affected by compressibility. Second to the P-38 for a matter of fact. The P-51's max permissible speed 505mph is due to structural limitations/concerns while the P-47 is due to compressibilty.

 

Ah, but is the P-47's critical Mach lower than the P-51's? Was its VNE truncated by compressibility because it has a greater tendency toward compressibility effects, or because its greater structural strength enables it to reach higher speeds?

 

The P-38 had a much lower critical Mach than the P-51. (IIRC, 0.67 and ~0.8, respectively.) How about the P-47? That's the key figure here.

 

As I understand it, a P-47 reaches compressibility "sooner" (in terms of time, not critical Mach) than the P-51, due to superior acceleration during the dive, and might enter deeper into the realm of compressibility, due to higher VNE (resulting from superior structural strength). But that's still a higher VNE & less overall vulnerability to compressibility effects.

 

Is this understanding wrong? I don't actually know the P-47's VNE (or the P-51's, for that matter), or its critical Mach, but everything I've heard suggests that the P-47 could out-dive the P-51 under all conditions (excluding being damaged), even without the dive flaps. The critical Mach is the thing that matters when determining how much a fighter is limited by actual compressibility. Other problems, such as natural control stiffening or the P-38's bizarre shockwaves-trapping-elevator problem, aren't technically compressibility effects, but are accompanying problems resulting from other transonic phenomena etc.

Edited April 21, 2017 by Echo38

[Pocket Sized]

The 47 is a generally much bigger airplane than the 51. Am I wrong in saying that this would amplify the effects of compressiblity due to the larger control forces?

[Echo38]

The 47 is a generally much bigger airplane than the 51. Am I wrong in saying that this would amplify the effects of compressiblity due to the larger control forces?

 

Actual compressibility is a problem with the wing, not the elevator. Larger control surfaces mean higher stick forces, all else equal, but not greater problems with compressibility.

 

(The P-38's extreme compressibility problem was a combination of the wing shape and that of the rear of the gondola, which had unintended aerodynamic effects.)

 

Again, I'm not sure what the P-47's actual compressibility situation was. The critical Mach would be the defining factor, and I don't have a good source handy for that. Anyone?

Edited April 21, 2017 by Echo38

[Pocket Sized]

Actual compressibility is a problem with the wing, not the elevator. Larger control surfaces mean higher stick forces, all else equal, but not greater problems with compressibility.

 

(The P-38's extreme compressibility problem was a combination of the wing shape and that of the rear of the gondola, which had unintended aerodynamic effects.)

 

Again, I'm not sure what the P-47's actual compressibility situation was. The critical Mach would be the defining factor, and I don't have a good source handy for that. Anyone?

 

Oh. I know all about supersonic flow separation on top of the wing, but all I've ever heard about compressibility is the huge control forces it causes.

 

What I'm trying to say is, what's compressibility like from the cockpit of a WWII aircraft? Spontaneous wing drops due to the unstable airfow over the wings? Complete lack of available lift? Lack of lift combined with stiff controls?

 

Edit: Nevermind, it seems that compressibility is just another name for Mach Tuck?

Edited April 21, 2017 by Pocket Sized

[Echo38]

it seems that compressibility is just another name for Mach Tuck?

 

Strictly speaking, yes, that's the effect of compressibility on a prop fighter in a dive. It's confusing because even real pilots (often even test pilots) use the term "compressibility" to describe accompanying effects as well as the primary one. I've been known to do this as well. TBH, I'm not 100% sure even as to how accurate my assessment of the terminology is.

 

The idea is, "true" compressibility effect is simply the loss of lift during a transonic dive, which will cause the airplane to fail to respond to attempts to raise the nose. Meanwhile, because of the high speed of the airflow over the elevator, the stick forces also are quite high, meaning that the pilot can't pull significant back-stick without trimming. But even should he trim full positive (reducing stick force and allowing him to pull more back-stick), he still won't be able to raise the nose, because the wing is stalled. As the fighter enters denser air, usually between 15,000 and 20,000 feet, lift begins to return. If the pilot still has the stick pulled back far (with positive trim), when lift returns, the aircraft can abruptly begin responding to the "commanded" pull-out, resulting in excessive G-forces and airframe failure (this is what killed Milo Burcham, IIRC).

 

So, the compressibility effect is technically just the "Mach tuck," the loss of lift resulting in the high-speed stall while diving. The stick forces are an accompanying phenomenon (as are the transonic shockwaves from the P-38's wing trapping the elevator in place). Each of these phenomenon independently fights the pilot's attempts to pull out of the dive, but don't share the same root causes (other than all being a result of the high Mach number). Feel free to correct me, aerodynamics experts. This is just my hazy understanding of it based on recollection of my moderate-level research bender ~ten years ago. : )

Edited April 21, 2017 by Echo38

[Pocket Sized]

I'd say you summed it up pretty well!

 

A bit OT, but is this modeled in the P-51? I've never encountered it, but I've never done any high speed dives from high altitude. It seems like going fast the only thing you have to worry about is pulling the wings off.

[rel4y]

Actual compressibility is a problem with the wing, not the elevator. Larger control surfaces mean higher stick forces, all else equal, but not greater problems with compressibility.

 

Compressibility is a problem for any airfoil including the elevator. The actual effect making the elevator near useless is flow seperation caused by a shock wave at the transition point of supersonic to subsonic flow over the airfoil. The thickness of the airfoil being the main factor in this effect, hence modern supersonic airfoils being very thin.

[MegOhm_SD]

Compressibility is a problem for any airfoil including the elevator. The actual effect making the elevator near useless is flow seperation caused by a shock wave at the transition point of supersonic to subsonic flow over the airfoil. The thickness of the airfoil being the main factor in this effect, hence modern supersonic airfoils being very thin.

 

Dont believe we have to worry about transition from supersonic to subsonic with the P47D.

[Pocket Sized]

Dont believe we have to worry about transition from supersonic to subsonic with the P47D.

 

The airplane doesn't have to be supersonic for Shockwaves to form on top of the wing, if that's what you mean.

 

Also, check out my sweet new profile pic :D

[Vitormouraa]

The airplane doesn't have to be supersonic for Shockwaves to form on top of the wing, if that's what you mean.

 

Also, check out my sweet new profile pic :D

 

Yeah, just like a propeller blade when it's spinning too fast, you aren't at supersonic speeds, but the blade tip is.

 

And I can't really tell what I see in the picture...

[Pocket Sized]

[Echo38]

is this modeled in the P-51? I've never encountered it, but I've never done any high speed dives from high altitude.

 

Which explains why you've never experienced it; at low and medium altitudes, the air is too dense to enter compressibility. (Which is why it always drove me crazy when the older sim/games portrayed the P-38 as going into compression at 5,000 feet; that simply couldn't happen IRL.)

 

As to your question, I don't know; I'm not much of a test pilot, and I don't remember if I ever properly tried to enter compressibility in DCS. All I remember specifically testing for was to see if it happened below 15,000 feet, which it doesn't (which is correct).

[SmokeFan]

Hi...Have there been any WIP pics of the Jug?

[rel4y]

Dont believe we have to worry about transition from supersonic to subsonic with the P47D.

 

Local airflow over an airfoil will be supersonic way before the whole aircraft will move at supersonic speeds. If the aircraft goes beyond critical Mach, the first part of the airflow over its airfoil will start to be supersonic and form a shockwave at transition point to subsonic speeds. Generally the thicker the airfoil is, the lower the critical Mach number is going to be, the earlier it is going into "compressibility". Simply speaking, the larger the displaced air volume in front of the wing, the faster it has to get out of the way eg by getting supersonic in the process.

 

Thats actually one of the problems of fluid dynamics scientists in the 30s/ early 40s didnt quite understand yet.

 

Ineffective elevators btw can be avoided completely if the whole airfoil changes AoA. That is why nowadays supersonic aircraft dont have classic elevators, but instead move the whole stabilizer. A trick they have in common with german WWII fighters (in theory).

[jvanhoog]

@SmokeFan. WIP pics of the P47 cockpit in the Feb 2015 newsletter.

[Pocket Sized]

 

 

 

 

I actually had no idea these existed. The cockpit is looks really nice, and these are WIP shots from over two years ago!

 

Edit: Is that a water injection switch I see on the throttle?

Edited April 22, 2017 by Pocket Sized

[Echo38]

Too many levers!

 

If I had to choose one of the DCS fighters to fly in real combat, with my life dependent on it, the choice would almost be a toss-up between the P-47D and the FW 190D. But the sheer complexity of the P-47's engine management (compared to that of the FW 190) causes me to suspect that the FW 190 was, overall, the superior fighter (if range isn't a consideration, anyway).

[Thorvald]

Yep, 11 minutes of pure power... till the water runs out.

 

Hopefully we get to manage the Turbo as well, I'd love to see a fully detailed P-47 where you actually have to FLY the aircraft.

[Vitormouraa]

Yeah it has water injection, which is amazing.