P-51 gear door fairings ripping off in high speed dives

[Bremspropeller]

...but not an issue on any of the aiplanes I mentioned, as flutter-speed is in excess of attainable aispeeds.

If critical flutter sets in, merely losing a control-surface is probably the best outcome imaginable.

[Pict]

2 hours ago, Bremspropeller said:

Some airplanes' high-speed limitations (P-47, P-51, Fw 190, etc.) are totally imaginary anyway.

I haven't ever seen a report stating the 190 would shed control-surfaces, neither about the Mustang or Jug or P-38 for that matter. They're Mach-limited for the most part, as drag-rise will cut their terminal velocity short of reaching destructive dynamic pressures. They're also stick-force limited in a way.

 

Indeed, I was just looking at what Eric Brown had to say about this in his book "Wings on my sleeve" and sure enough he describes critical mach as loss of control  not loss of controls, which is another thing altogether

 

Edited December 11, 2019 by Pict

[Legioneod]

10 hours ago, Bremspropeller said:

I'd be more concerned about losing the gear-fairing in the first place.

Then again, I'd probably be scared sh1tless, diving to Vne and not caring about turbulence, slipstreams, wingtip-vortices, etc.

There's a lot of load on those airframes at Vne and I wouldn't want to be the first guy to figure out the wings will come off when going terminal velocity and hitting a rough patch of air.

 

Some airplanes' high-speed limitations (P-47, P-51, Fw 190, etc.) are totally imaginary anyway.

I haven't ever seen a report stating the 190 would shed control-surfaces, neither about the Mustang or Jug or P-38 for that matter. They're Mach-limited for the most part, as drag-rise will cut their terminal velocity short of reaching destructive dynamic pressures. They're also stick-force limited in a way.

 

After all, it's just a game-limitation/ placeholder for trying to keep people from overspeeding their airframes.

This. Shedding control surfaces is a game mechanic that has no basis in real life (for the most part) for the above mentioned aircraft. It most certainly happened under some conditions but it was not caused by speed alone, there were other factors at work (damage, g load, etc)

 

8 hours ago, Pict said:

 

Indeed, I was just looking at what Eric Brown had to say about this in his book "Wings on my sleeve" and sure enough he describes critical mach as loss of control  not loss of controls, which is another thing altogether

 

Exactly but we don't see this in-game, instead we see loss of controls.

 

I can understand why the devs chose to model control loss even if it didnt happen irl but I'd prefer if critical mach was modeled more accurately and control loss due to speed was removed or made accurate.

[336th_Porkie-]

tried p-51 after patch and at 516mph didn't lose door covers. anyone else confirm?

 

[357th_KW]

I was just playing with this, and they came off around 525mph or so - possibly a bit sooner and the animation just takes a moment to take effect.

[MikhaVT]

On 11/27/2019 at 8:10 PM, AndyJWest said:

The thing is with pilot anecdotes is that they come from the pilots who got away with things, not the ones who's misadventures into overspeed territory ended in a smoking hole. Which is one reason why the developers prefer hard data.

 

And no, there is no particular reason to assume that the margin between placarded maximum and actual safe limits should be the same for all aircraft. Differing ideas as to what was acceptable, combined with limited data (often hard to gather from a smoking hole) are likely to result in very divergent numbers.

Not trying to argue either point of this discussion here, just browsing the sources and such but anyways: It's important to always keep in mind that the user manual is just about the "softest" form one can get "hard" data in, specifically because of the safety factor provided for any numbers given. While we might assume that not exceeding the numbers stated in the manual means no problems, we dont know how much safety factor was given to those, and in some cases we dont know how long the item in question was designed to hold up for. As an example: one country might not have stated engine limits in the manual if they dont care about replacing/overhauling the engine after each flight, while another might have very strict limitations because they cant afford that. On top of that, we also have differences in the engineering methodology between countries which might result in country A having much better confidence in their safety factor than another country. Without engineering reports about how the aircraft was designed and the numbers used for limits before safety factor, much of this is hard to get. An alternative would be to consult blueprints if they're available; however, to get useful information out of those would require an engineer to analyze them. 

 

On 12/10/2019 at 6:25 PM, 6./ZG26_Klaus_Mann said:

Why do all the Anecdote Guys completely forget to Mention that your Regular Pitot Tube Airspeed Indicator has an Error of around 12.5% positive at 0.7 Mach, 17.5% posititive at 0.8 Mach and about 25% at 0.9 Mach.

 

That is the Grain of Salt you have to take all of these Speed claims with.

Great question, i never see the Pitot tube accuracy questioned anywhere here. I have also wondered if the Pitot accuracy difference between types of aircraft is modeled or not -- after all the placement of the pitot tube is critical for any accurate readings. 

[357th_KW]

Just as a point of reference, both the 1944 and 1954 version of the P-51D manual have airspeed indicator correction charts that show that the airspeed indicator actually read about 1% slow at 390-400mph which is the top end of the correction charts.  The 1954 manual has a compressibility correction chart (to be applied after your airspeed chart - indicated -> calibrated -> equivalent) as well which shows a correction of about a 1% reduction at 5000ft,  1-2% at 10,000ft, just over 3% at 20,000ft and around 5% at 35,000ft.

 

So if somebody was really seeing 550mph at 10,000ft, once you applied both the airspeed indicator and compressibility corrections from the manuals, you'd be around 545mph EAS or so.  I think a more likely explanation is given in the AAF report describing the dive testing.  In section 2.C it mentions that vibration in a dive begins a .76 mach and in 2.D states that the vibrations become extremely heavy above .80 mach.  505mph indicated at 10,000ft, with the airspeed correction would be 510mph calibrated, which works out to around .791 mach - right in the range where vibration is starting to become a problem.  If the pilot is experiencing heavy vibration like that, it would be really easy to misread the instrument, of for the instrument needle itself to be moving due to vibration, or some combination of those factors and human stress etc.

 

All that being said, the gear door failure mode in the game is a little disappointing.  In the game right now, you basically lose both doors at almost the exact same time right around 525mph or so.

 

In the report, they mention carrying out 31 dives.  In the dives they achieved a max mach of .85 and a maximum calibrated airspeed of 486mph.  They noted deflection of gear and gun/ammunition bay doors and fabric elevator surfaces above 460mph, but they never state that this is what caused the landing gear strut door failure.  Note that only the left side gear door and strut fairing failed on one occasion - and that the various technical reports indicate this was related a maintenance issue with the door locks.  They do however state "in each dive to 0.84 mach or above the vibration became so severe that the airplane was damaged."  They then go on to mention some other issues that resulted from vibration damage - flap skin buckled, radiator cracked, hydraulic line broken. 

 

The evidence seems to point to the gear door failure being a maintenance related issue, and as we can see it occurred once in 31 dives.  It's not entirely clear (though it seems likely) that the failure was due to airspeed as opposed to vibration from high mach numbers.  It may have resulted from a combination of the two.  They clearly state that they never even got to the 505mph placarded limit and so the numbers in the game are clearly an estimate.  However, the structural damage from high mach numbers occurred every time at a known mach number.  It would be really neat to get a more detailed modeling of this is game - say a small random chance at losing a gear or gun/ammo bay door above the 505mph limit (and the possibility of losing just one or many).  Then combine that with adding compressibility vibration that would lead to structural damage at mach .84 and above.  That would make for a really interesting and nuanced damage/dive model.

 

The other big disappointment in the Mustangs dive performance in game is the extreme nose heaviness that it encounters.  In the dive test report I linked above it states "Recovery in any case must be gradual and executed with extreme caution since relatively light elevator stick forces or rapid application of trim may very easily result in the application of excessive load factors. As acceleration is applied at the beginning of the pull-out some increase in vibration may occur. This will gradually decrease as the recovery is completed. In no case is elevator trim necessary to aid recovery."  These statements are mirrored in both the 1944 and 1954 versions of the P-51D pilots manual.  In the game, the P-51 requires nearly full nose up trim and full aft stick pressure to recover from a high speed dive, which is clearly wrong and is warned against in the manuals.

[LLv34_Flanker]

S! 

 

This nose heavyness applies to many, if not all, planes in the game. It would be nice if devs added more symptoms of overspeed than just losing parts. Like the considerable vibrations mentioned for the Pony etc. 

[PainGod85]

On 1/22/2020 at 6:18 PM, KW_1979 said:

Just as a point of reference, both the 1944 and 1954 version of the P-51D manual have airspeed indicator correction charts that show that the airspeed indicator actually read about 1% slow at 390-400mph which is the top end of the correction charts.  The 1954 manual has a compressibility correction chart (to be applied after your airspeed chart - indicated -> calibrated -> equivalent) as well which shows a correction of about a 1% reduction at 5000ft,  1-2% at 10,000ft, just over 3% at 20,000ft and around 5% at 35,000ft.

 

So if somebody was really seeing 550mph at 10,000ft, once you applied both the airspeed indicator and compressibility corrections from the manuals, you'd be around 545mph EAS or so.  I think a more likely explanation is given in the AAF report describing the dive testing.  In section 2.C it mentions that vibration in a dive begins a .76 mach and in 2.D states that the vibrations become extremely heavy above .80 mach.  505mph indicated at 10,000ft, with the airspeed correction would be 510mph calibrated, which works out to around .791 mach - right in the range where vibration is starting to become a problem.  If the pilot is experiencing heavy vibration like that, it would be really easy to misread the instrument, of for the instrument needle itself to be moving due to vibration, or some combination of those factors and human stress etc.

 

All that being said, the gear door failure mode in the game is a little disappointing.  In the game right now, you basically lose both doors at almost the exact same time right around 525mph or so.

 

In the report, they mention carrying out 31 dives.  In the dives they achieved a max mach of .85 and a maximum calibrated airspeed of 486mph.  They noted deflection of gear and gun/ammunition bay doors and fabric elevator surfaces above 460mph, but they never state that this is what caused the landing gear strut door failure.  Note that only the left side gear door and strut fairing failed on one occasion - and that the various technical reports indicate this was related a maintenance issue with the door locks.  They do however state "in each dive to 0.84 mach or above the vibration became so severe that the airplane was damaged."  They then go on to mention some other issues that resulted from vibration damage - flap skin buckled, radiator cracked, hydraulic line broken. 

 

The evidence seems to point to the gear door failure being a maintenance related issue, and as we can see it occurred once in 31 dives.  It's not entirely clear (though it seems likely) that the failure was due to airspeed as opposed to vibration from high mach numbers.  It may have resulted from a combination of the two.  They clearly state that they never even got to the 505mph placarded limit and so the numbers in the game are clearly an estimate.  However, the structural damage from high mach numbers occurred every time at a known mach number.  It would be really neat to get a more detailed modeling of this is game - say a small random chance at losing a gear or gun/ammo bay door above the 505mph limit (and the possibility of losing just one or many).  Then combine that with adding compressibility vibration that would lead to structural damage at mach .84 and above.  That would make for a really interesting and nuanced damage/dive model.

 

The other big disappointment in the Mustangs dive performance in game is the extreme nose heaviness that it encounters.  In the dive test report I linked above it states "Recovery in any case must be gradual and executed with extreme caution since relatively light elevator stick forces or rapid application of trim may very easily result in the application of excessive load factors. As acceleration is applied at the beginning of the pull-out some increase in vibration may occur. This will gradually decrease as the recovery is completed. In no case is elevator trim necessary to aid recovery."  These statements are mirrored in both the 1944 and 1954 versions of the P-51D pilots manual.  In the game, the P-51 requires nearly full nose up trim and full aft stick pressure to recover from a high speed dive, which is clearly wrong and is warned against in the manuals.

 

This is all well and true, but IL-2 does not model any kind of pitot error as far as I know. As such, indicated speeds given in airplane manuals need to be counter-corrected with available PEC charts to get the corresponding speeds and limits in IL-2.

[357th_KW]

On 1/22/2020 at 9:18 AM, KW_1979 said:

The other big disappointment in the Mustangs dive performance in game is the extreme nose heaviness that it encounters.  In the dive test report I linked above it states "Recovery in any case must be gradual and executed with extreme caution since relatively light elevator stick forces or rapid application of trim may very easily result in the application of excessive load factors. As acceleration is applied at the beginning of the pull-out some increase in vibration may occur. This will gradually decrease as the recovery is completed. In no case is elevator trim necessary to aid recovery."  These statements are mirrored in both the 1944 and 1954 versions of the P-51D pilots manual.  In the game, the P-51 requires nearly full nose up trim and full aft stick pressure to recover from a high speed dive, which is clearly wrong and is warned against in the manuals.

 

I stumbled onto an interesting related bit of info the other day in America's Hundred Thousand by Francis Dean.  In the section on Mustang dive handling qualities:

 

"At airspeeds over 450mph IAS longitudinal porpoising often occurred on Merlin powered P-51 airplanes with fabric covered elevators, particularly with some nose up trim.  The porpoising resulted from fabric bulging at high speed, resulting in airflow breakdown and elevator oscillation.  On the P-51D-30 airplane, and as a modification to other P-51D aircraft metal covered elevators were installed to improve high speed flying qualities.  Since the metal covered elevators did not bulge, the normal tendency was for the aircraft to become increasingly nose heavy at very high speeds, and to compensate, the incidence setting of the horizontal stabilizer was decreased from one and a half degrees nose  up to only one half degree nose up, done at the same time as the change to metal covered elevators.  Without the incidence change of the stabilizer the pilot would have observed a sharp pitch forward of the airplane, and a sharp reversal of control stick forces from push to pull as the high speed placard limits were reached.  With these changes the pitch tendency was hardly noticeable ..." 

 

The author goes on to discuss changes to level flight trim characteristics that don't really apply to this discussion.  But this pretty much perfectly describes how our in game Mustang handles in a dive - like it has metal (or at least non-deforming) elevators but with the original horizontal stabilizer angle of incidence.