Me262 question: Did it have WEP?

[dtango]

If you've ever flown a Lear 25, you would be completely aware of the "mach tuck" related by the makers of the me262.

My question is....."Who discovered this unrecoverable "tuck?".....and "Did he survive the event?"

Surviving mach tuck vs. surviving it while hitting & exceeding supersonic speeds are two different things .

[dtango]

Charge:

Regarding solving stability issues related to compressibility, no WW2 aircraft I know of ever did.  The similar compressibility control & stability problems that plagued the P-38, P-47, P-51, & P-39 were never fully solved.  The best that could be done was to increase the critical mach number through various means or various ways to try & increase control effectiveness (e.g. P-38, P-47 dive flaps to increase trim lift coefficient, etc.).  Once at critical mach and beyond the only chance you had was to hope to God that there was some means of reducing your mach number below critical mach (e.g. denser air at lower altitudes, slow down, etc.) and/or to regain enough control of your airplane without destroying it in the process - all this hopefully with enough altitude to do something about it before you lawn-darted into the ground .

For grins here is the time history collected from P-47 compressibility dive tests:



In a vertical dive once the P-47 was beyond it's critical mach compressibility caused the nose of the P-47 to tuck away from pure vertical (this occurs because shockwave induced boundary layer separation changes the wing center of lift resulting in a nose tuck pitching moment).  Also the elevator lost effectiveness to change the attitude of the aircraft.  As can be seen on the figure the elevator deflection angles ranged between +3 to -3 degrees.  At the high airspeeds of the dive usual loads from those amount of elevator angles would have produced loads in excess of 20 to 30 g's, easily destroying the aircraft.  As can be seen however due flow separation from sonic shock waves due to compressibility the elevator effectiveness only produced a 1/2 g instead which was pretty much was control freeze to a pilot diving at that speed.  At about 15,000 ft the air got dense enough increasing the drag and slowing the airplane down enough to reduce or eliminate the compressibility shock separation so that elevator control came back.  However if the pilot was continue to pull hard back or had the tab set to full trim up this would result in a violent recovery which could easily destroy the aircraft.

Anyone interested in the fascinating details about the dive tests themselves, you can check out this post:
P-47 Dive Tests - Major P. Ritchie

[dtango]

my mistake on the hogs - the -8W engine was introduced from late-model -1As onwards so only AH's -1 doesnt have ADI. Only the 47M/N used ADI though, the others didnt. I havent checked any of the allied buffs, my guess none used ADI (strangely WEP is only modelled on the A-20, B-25H and TBM iirc.)

I know the Germans were keen ADI users, not sure about Japanese and Russian aircraft though.

I'm at work at the moment so don't have access to my references.  Two comments:

1) P-47D-10's and beyond had the R2800-63 engine (or something like that).  According to Graham White's engine book these versions of the 2800's had ADI built in.  Checking AHT for listed engine types for P-47 models this would mean all our AH versions of the P-47 had ADI.

2) Graham White in discussing ADI states that for allied aircraft ADI was installed primarily for fighters and purposely limited on bombers.  I can't remember the reason.  I'll check when I get home.

[Krusty]

I'd concur on P-47s' use of water. I believe they have water tanks onboard just for that. It was retrofitted to older models as well, in the field.

However, there are still a vast number of planes that had no secondary substance, they simple ran at higher power.

I think we all agree on that, yes?

[RTHolmes]

hmm my copy of the 1943 pilot notes for B, C, D and G models doesnt mention injection at all, however it only has charts for the 2800-21. other sources indicate the 2800-59 was used for late C-D models, which does use injection, so I guess our jugs do all use ADI

[dtango]

However, there are still a vast number of planes that had no secondary substance, they simple ran at higher power.

I think we all agree on that, yes?

Take out "vast number of airplanes", then I'd agree .

2ndly I think it's is over-simplifying things by saying "they simply ran at higher power".  My ADI-only-WEP mistake aside, the concept to enable higher power output with a piston engine aircraft by increasing maximum allowable compression without detonation I think remains consistent but I need to do a little more research to fully comment on implications to engine limits & reliability.

[Krusty]

It's not about reliability.

It's about the established limits.

P-51s were limited to 5 minutes. Not because they only had 5 minutes of water onboard. It wasn't going to fall apart the second you went past 5 minutes, but that was the dictated limit.

The 262 limit (if it can be verified) is a dictated limit. These engines didn't fall apart (reportedly) until 3000 RPM. There is mention of 2900 rpm in one place as a possible experiment/test rating. 2700 was decided as a maximum and for only 10 minutes.

So, I fail to see the issue you have with that. The issue *I* have is "Is this right? Why doesn't HTC do this? Do they have differing info?"

I'm not debating engines wear or tear or reliability, which are long-term issues that HTC doesn't model.

[EagleDNY]

Eagle: That's a foolish thing to say. Every WEP limit in this game was imposed for heat and damage related reasons. The reason you don't have unlimited WEP on any plane in this game is the same: There were limits put in place. Those limits were not to be exceeded per some T.O. or the pilot handbook. Here's a case of a pilot handbook. So what's the problem? What's the different between this and any piston plane's WEP limitation? HTC strives for historical accuracy, no?

Krusty, I hate to burst your bubble here, but that is entirely INCORRECT.  There are many piston engines that could run practically forever at WEP, the real limit is the amount of additional 'stuff' in your boost tank.  For example, the German MW50 contained enough mixture for 2 10 minute boosts before being empty.  The GM-1 nitrous system was about the same, but you could extend it because there were 3 different settings for how much nitrous was added.  The American 'Water-Injection' systems are just another name for the same thing - they are injecting a 50/50 mixture of water and alcohol (either methanol, or ethanol works too) for the anti-detonation cooling effect.  WEP without a water injection system is typically them just injecting extra fuel to do the same thing, but it not as efficient. 

The PW2800 in a P47 was fed by a 30 gallon water-methanol tank.  You had about 20 minutes worth, and once that was gone, say goodbye to WEP.  To certify the engine for the US Government as being 'qualified' to use WEP, Pratt & Whitney had to run a 2800 continuously for 7.5 hours: 5 hours with alternating 5 minutes on / 5 minutes off WEP cycles, and then 2.5 hours continuous at full WEP.  This supposedly wasn't much of a problem since PW ran their engines for 100 hours at full WEP anyway.  Here's a link to a bio on one of PW engineers that talks about this (he helped develop the water injection system for the Americans): http://www.enginehistory.org/Frank%20WalkerWeb1.pdf
In AH - we aren't modeling this limit (that is what I meant by unlimited WEP cycles) - I can run 10 minutes on / 5 Minutes off in a 109K4 as long as I have fuel.  The MW50 tank never goes dry.  Same for a P-47, a FW190, or any other ride using WEP through a boost tank of 'stuff'. 

In P51s, (somebody check because I don't think they used water injection until the H model came out), using WEP required a full teardown inspection once you got back on the ground.  That didn't mean your engine was going to fail after 5 minutes (I agree with you here), but the 'established limit' is there.  That doesn't keep our P51s in AH from using 5 minutes of WEP over and over again, because we aren't modeling engine reliability limits.

Which brings me back to the 262 - originally the Jumo 004B was run at 9,000rpm - but was limited to 8,700rpm full military power for reliability reasons, and the manual tells the pilot to limit himself to 10 minutes usage.  Since a Jumo 004B was going in for a full teardown at about 10 hours, I can see why they might try to get the pilot to limit his use of full military power.  Since we have no worries about the engines spitting out a turbine blade because we aren't modeling reliability, I just don't see a reason to make the 8,700 rpm setting some kind of WEP thing.  If anything, that might make the 262 modeling more 'unrealistic' - WEP in AH adds boost instantly, where the jet engine needs time to spin up.  WEP in jet engines is an afterburner, and I don't want to see the 262 suddenly acting like it has a 300rpm afterburner for 10 minutes at a time.   

[RTHolmes]

Krusty, I hate to burst your bubble here, but that is entirely INCORRECT.  There are many piston engines that could run practically forever at WEP, the real limit is the amount of additional 'stuff' in your boost tank.

really? if thats you're right why is my WEP limited in a spit? no tank to drain, I should be able to run WEP settings continuously right? do you really want AH's WEP limited to planes with ADI tanks of water/ethanol/whatever?

[FuFiter]

all this talk about 262's makes me want to go fly one...

[Krusty]

You're flat wrong eagle... MW50 tanks had enough for over 30 minutes of constant use. That's longer than your gas will hold out at 2x fuel burn. And other stuff I really don't have the energy to list. You're getting hung up on injected additives. Many simply RAN HIGHER... that's it. The Fw190A5 had no MW50. Nor did the Bf109E. Nor did the P-40E. Nor did most of the planes in this game. A notable amount did, yes, but this game has over 100 rides in it.

Scrolling down the AH planes list I count 42 off the top of my head that have WEP but no injected substances.

I only count 14 that have it, namely 4 German rides, the F4us (minus 1), the P-47s, and the F6F.

You're obsessing, seriously.

WEP is a SETTING. It's a rating that is limited in duration because it exceeds certain standards. Doesn't matter what why where when or how, it's a limited setting that exceeds limits.



EDIT: I see what you're saying about the instant boost though. Too bad we cant have an overheat option on the 262. P.S. The fears for the Jumo engine gave it those short overhaul times, but apparently they were unfounded. If the engine passed the tests (i.e.if it didn't destruct) it was fine. They found that the overhaul limit really wasn't needed. Also the 9000 rpm wasn't a reliability issue... It was a structural failure issue. Literally the engine flew apart. The 8700rpm was implemented after studying the resonant harmonics of the materials and determining this was the max level before it starts flying apart. Literally. They didn't have microfracture detection back then, which we use now to avoid any flaws or minute defects in turbofan blades.


EDIT2: It occurs to me we could have the throttle still work with WEP. The WEP just ups the maximum reach of the throttle... So if you're at 8400, you cannot progress further (there is a deadband on the throttle). Hit WEP and that throttle deadband becomes active, allowing you to throttle in that zone as you can now. After 10 minutes that WEP mode turns off and the deadband kicks back in. Just a thought.

[STEELE]

It's kind of the same type of deal as the F4U1A, its supposed to run at a lower RPM or Manifold pressure lower than that of AH's 1A MIL power, but our engines don't blow up unless leaked dry, so...

[dtango]

Limiting the Jumo 004 power settings based on the POH which is a valid argument.  But there are at least two other arguments for NOT limiting power output that are just as valid.  For the sake of readability & time I’ll just limit my response to just one of them.

The reason for the engine limits for non-ADI piston engines vs. the Jumo 004 turbojet are different.
Piston engine non-ADI WEP usage is limited by unabated engine heat increase leading to certain engine destruction in minutes.  The Jumo 004 (and turbojets in general) isn’t limited by unabated engine heat increase but the lifespan limit of the turbine blades & combustors in hours and days.

Non-ADI Piston Engine WEP:
Piston WEP for non-injected engines is limited by the inability to cool the charge heat and/or engine enough to avoid detonation.  Increased compression for greater power output raises temps beyond what the cooling system can balance out resulting in un-stopped rising temperatures for a fixed compression / manifold pressure.  Eventually the charge heat or cylinders reach a temperature where fuel-air pre-ignition or detonation is unavoidable.  The probability of destroying your engine in this scenario is 100% in a relatively short amount of time.  That’s why WEP usage is time limited for non-ADI piston supercharged engines.  

That’s also why per EagleDNY’s response theoretically for ADI WEP if you had an unlimited amount of ADI you would not have a WEP time limit because ADI also lowers charge & combustion temperatures thus avoiding unabated engine heat increase.


Turbojets & the Jumo 004:
Normally operating (i.e. not malfunctioning) turbojet engines however don’t have the same phenomenon of unabated temperature increase.  The isentropic gas expansion process to create thrust means the engine must deliver a specific, steady temperature to produce a specific thrust.  For a given compression ratio (RPM), the temperature at the turbine inlet remains constant. There isn’t the same process creating engine overheating like what occurs with non-ADI piston engines at WEP.  

Turbojets are designed around the maximum temperature the turbine inlet (and combustors) can stand.  How is this related to RPM’s for a turbojet?  The turbine inlet temperature is a function of the temperature of the air due to compression plus the temperature from the heat transfer from combustion.  The engine is designed around this maximum temperature constrained by turbine material strength.  Unlike non-ADI WEP operation, there isn’t an unabated temperature rise resulting in certain engine failure in matters of minutes.  Instead it’s the long term subjugation of the turbine to these designed temperatures that is the cause of concern for failure.  Depending on the material strength, this can be a life span of 10’s to 100’s of hours.

A closer inspection of issues with the Jumo 004 reveals that turbine material compromises because of supply constraints resulted in a service life of 10-25 hours.  Running the Jumo 004 below rated maximum RPM makes sense in light of prolonging service life of the engine.

[EagleDNY]

really? if thats you're right why is my WEP limited in a spit? no tank to drain, I should be able to run WEP settings continuously right? do you really want AH's WEP limited to planes with ADI tanks of water/ethanol/whatever?

WEP is limited in a Spit for the same reason that it is limited in a P51D - the Merlin (or Packard Merlin) runs on a higher octane fuel and increases power by adding more fuel and additional boost pressure instead of an additive.  As Krusty pointed out, the engine isn't going to blow up after 5 minutes, but the pilot manual recommends that limit and requires an inspection after WEPing the engine.  Why?  Because running an engine this way raises cylinder head temps and you start getting things like oil leaks, burned up rings, or cracking.  Once you let the engine cool back down, its WEP on again - the only limit is how much the engine can take before something fails.  It might be a little or a lot, but since we are not modeling that in AH we don't have to worry.  

What you guys are losing is the purpose behind the additives - they are trying to increase HIGH ALTITUDE PERFORMANCE.  Rides that were never intended to go up to extreme altitudes (like the P40E, 109E, A6M, etc. etc. etc.) didn't get these systems.  Further, the British and Americans also brought in higher octane fuels to accomplish the same thing.  Your spitfires aren't using water injection because they are using higher octane fuels as the war went on - they started at 87 and went all the way up the 100/150 octane fuels later in the war to get better high altitude performance.  The Germans, lacking higher octane fuel were stuck retrofitting 109G6s with MW50 to get better high altitude performance.  The very late war rides are the extreme examples that would take high octane fuel AND additives to get as much performance as possible at extreme altitudes - this is where you get stuff like the TA152 with C3 fuel and the MW50 or GM1, or a P51H running high octane and water boost in a Packard Merlin with a two-stage blower.   P47M/N - they are running the highest octane fuel and water injection to get performance numbers of 462 Mph up at 38,750 ft - not where we normally see them in AH.

I'm not trying to say that AH needs some different WEP limits for every different ride - what I am saying is that the WEP we have now is just an abstract boost of the engine power to the maximum rating (as determined by HTC) for 5-10 minutes depending upon the ride, and there is no extreme realism in it.  Further, HTC is not modeling the limit on the use of full military power in piston engines - planes routinely run around at full military power all the time here, while IRL there were limits on the maximum continuous power.  For example - a P51H (just happens to be the chart I found first) has a Takeoff/Full Power setting of 61" boost, but that is supposed to be limited by the pilot to 15 minutes.   The maximum continuous power is supposedly 46" boost, with a dry WEP setting of 67" for 5 minutes, and a 'wet' WEP (water injection on) setting of 80" for 7 minutes.  Also - on some engines, the amount of boost at "full military power" is regulated at low altitude: in the P47N pilot manual, the limit is 54" full military power at zero altitude, but the regulator ups it 1" for every 6,000ft of altitude increase - I don't think this feature is modeled either.

Finally - IMHO just leave the 262 alone.  Letting it run around at full military power of 8,700rpm is no different than letting the piston engined rides run around endlessly at their full military power, and certainly no worse than letting rides with RL limited WEP have an unlimited WEP cycle.

[RTHolmes]

wow 2 walls of text! I'll keep it simple:

the 262 appears to have had time limits for max power operation according to the pilot notes, in the same way as most WWII fighters did.

therefore this max power operation should be modelled as the AH WEP setting, in the same way as most of AH's WWII fighters.



btw Eagle, AH models only one WEP setting, many aircraft had several time-limited power settings IRL (as you mentioned.) HTC have done a pretty good job of selecting the "Mil Power" and "WEP" settings for the mostpart, although there are some glaring ommissions (eg. no WEP for the Lanc.)