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Richard502

Question to Ju88 experts

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On Ju88s the fixed forward firing armament (FFFA), and the radar aerials, were canted downwards relative to the longitudinal axis of the plane (LA) by about 5 degrees. On most fighters the FFFA is installed parallel to the longitudinal axis, which is almost identical to the line of flight (LoF), so the pilot can aim by pointing the whole plane at the point he wants to hit. (Actually the FFFA is canted upwards so the trajectory of the bullets will intersect the line of flight at a given distance. So why the downward pointing FFFA on Ju88s? Did Ju88s fly in a tail down attitude so that the downward pointing FFFA is indicating the line of flight? If so, would this not increase drag dramatically?

TIA, Richard

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The lift provided by the wing is dependent on the speed and the angle to the direction of flight (angle of attack).  The slower the aircraft flies the higher this angle must be.  The wing is however fixed to the fuselage, so the angle of the fuselage to the line of flight will vary with speed.  The effect of this on drag is fairly small.  Normally this angle is fixed to ensure a short takeoff - the classic case here is the Whitley which has a very nose-down attitude, or perhaps the Stirling which had to have a very stalky undercarriage to ensure reasonable takeoff distances.

 

I'm not familiar with the downward angle of the Ju88, is this a feature of the fighters rather than the bombers?  The guns in the ventral pack do not seem to have this angle.

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Posted (edited)

For the downward angle of the ventral guns see the sideview cutaway drawing in this link. It is necessary to roll down the page.  

https://www.asisbiz.com/il2/Ju-88/Junkers-Ju-88/pages/Artwork-technical-drawing-Junkers-Ju-88-cutaway-0C.html

or this photo:

https://www.asisbiz.com/il2/Ju-88/NJG2/pages/Junkers-Ju-88G-13.NJG2-(4R+BX)-1944-01.html

Richard

Edited by Richard502

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Very interesting! Never really noticed the downward cant of the MG151's or the radar aerials. I have no idea why the aerials were canted from the vertical, but maybe the MG-151's were slightly depressed to minimize blast damage from the guns when fired, and/or allowing the Ju-88 to position itself slightly above the intended target to avoid wake turbulence from the intended victim. Just a guess, you understand!

Mike

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More speculation: how much angle of attack (actually, I mean fuselage angle relative to the horizontal in straight & level flight here) would be needed for a Ju88 to match speed with a fully laden 'heavy' for a radar guided stern attack?

 

Max speed of the 'G', I'm guessing would have been in the region of 300mph* at bomber height, so perhaps a few degrees nose up could be expected at 'heavy' cruise speeds?

 

Regards,

 

Martin  

 

*A quick Google suggests max speed was somewhat more than this for the G.  Doesn't change the argument though.

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As Graham said, with increasing speed lift also increases and this is compensated with nose down flying attitude. The Flugzeug-Handbuch Antti linked to mentions Schnellfluglage 4,5° gesenkt (literally: high speed flying attitude 4,5° depressed) several times. So, at high speed Ju 88 G-1  flew with nose depressed for 4,5°. However, taking 5° mentioned in OP in account, this would also mean that at such flying attitude guns pointed to almost 10° downwards. This sounds excessive to me and table on page 17 as well as Geschoss-Flugbahnschaubild (missile flight trajectory diagram) on page 20 both suggest fixed gun armament angle is actually increased and not decreased. Note Visierlinie (sighting line) on the same diagram, intersected with round trajectory curves, list of distances, at which rounds intersect it for the first time when climbing and then for the second time when falling, greatest heights above line of sight rounds reach etc.

However ... On page 11 of equivalent Ju 188 E-1 armament handbook here it is clearly stated: ˝Bei Einsats des MG151/20 als starr gerastete Schusswaffe ist der Lauf um 4°30' - bezogen auf Flugzeuglängsachse - nach unten geneigt.˝ (When MG151/20 is used as a fixed weapon its barrel is canted down at  4°30', compared to aircraft's longitudinal axis). Certainly Ju 188 E-1 is different aircraft, still ... Any thoughts? Cheers

Jure

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Posted (edited)

The relative angles of the wing and tail and fuselage of the 88 are well optimised for its original early-war role as a bomber of the same speed range as the four-engined heavies  it met later as a night fighter. It has never been noted in the detailed accounts available from seasoned and competent German or Allied test pilots as cruising with the nose significantly up or down, and there is no reason why it should. The basic airframe was noted for its good aerodynamic performance which would certainly have been adversely affected if the designers had not succeeded in getting the beast to point in the same direction it was flying.

 

The angle of incidence of the wing to the fuselage was not changed for the night fighter version, and there was no reason to do so.

 

Looking at that drawing, one reason for angling the guns downwards is to probably avoid throwing a hail of four-ounce cannon shells a couple of inches past the radar antenna at Mach 2.3. The potential for shockwave damage is non-trivial.

 

As a radar-guided night-fighter gun platform the aircraft is intended to shoot at big targets flying fairly straight and level at fairly high level, It offers a good view down over the nose so there's really no problem aiming the guns.

 

In a single-engined type with a lot of engine and prop ahead of the pilot and little or no downward 12 o'clock field of view, it would be difficult to sight a gun aimed down that much even from behind a straight and level target. Deflection shooting in a proper turning visual dogfight would be highly disadvantaged. But that's not the mode of combat in which a Ju.88 is expected to kill a Halifax at night, and it doesn't have anything blocking the view down and ahead. 

 

As a tactical side-effect it occurs to me that it also puts the fighter somewhere other than directly in front of the rear-gunner's face, which probably helps a bit, while being at a lower angle than a mid-upper gunner might be focused on. Although the opposite approach of the muck more rakishly upward-firing Schragemusik is a lot sneakier again for hiding from Bomber Command aerial gunners. 

Edited by Work In Progress

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15 hours ago, Work In Progress said:

one reason for angling the guns downwards is to probably avoid throwing a hail of four-ounce cannon shells a couple of inches past the radar antenna at Mach 2.3.

This drawing has the guns angled downwards on the radar-less Zerstörer version as well.

http://www.airwar.ru/image/idop/fww2/ju88c/ju88c-2.gif

 

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16 hours ago, Work In Progress said:

The relative angles of the wing and tail and fuselage of the 88 are well optimised for its original early-war role as a bomber of the same speed range as the four-engined heavies  it met later as a night fighter. It has never been noted in the detailed accounts available from seasoned and competent German or Allied test pilots as cruising with the nose significantly up or down, and there is no reason why it should. The basic airframe was noted for its good aerodynamic performance which would certainly have been adversely affected if the designers had not succeeded in getting the beast to point in the same direction it was flying.

A quick Google suggests that the Ju88A4 had a max speed of ~290mph at 17000ft.  The wing incidence rel. to the fuse (prob. pretty low?) was no doubt carefully worked out, as you say, to enable the Schnellbomber to reach this speed, suggesting a fairly flat  flat fuse incidence at high speed to minimise drag.  Again from Google, the cruising speed of a laden Lancaster was ~200mph, so assuming a co-speed stern attack, the Ju88 would have adopted a somewhat more nose up angle at this speed to maintain the required wing lift.

 

The main armament of the G, unlike the C, was carried in a ventral gondola (possibly to free up space in the nose, for c.g. issues or to avoid damage to the radar arrays as you say).  I assumed (posdibly wrongly) it was the downward angle of these guns we were discussing.  The gondola guns are certainly canted downwards in the models I've seen.  

 

If the gondola guns are also canted down at the same angle as the nose guns in earlier variants (@Richard502 which fixed armament were you referring to?), this suggests to me at least that the fuse angle rationale may have credence.

 

Good discussion, though, whatever the conclusion.

 

Regards

 

Martin

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48 minutes ago, mike romeo said:

A quick Google suggests that the Ju88A4 had a max speed of ~290mph at 17000ft.  The wing incidence rel. to the fuse (prob. pretty low?) was no doubt carefully worked out, as you say, to enable the Schnellbomber to reach this speed, suggesting a fairly flat  flat fuse incidence at high speed to minimise drag.  Again from Google, the cruising speed of a laden Lancaster was ~200mph, so assuming a co-speed stern attack, the Ju88 would have adopted a somewhat more nose up angle at this speed to maintain the required wing lift.

 

You don't optimise a WW2 bomber for flat-out maximum speed. You optimise it for what it will do most of in their hours of flying. Which is a mid to high speed cruise very similar to that of a Lancaster. The Lancaster I/III is only about 18 MPH slower flat out than a Ju-88 A4

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3 minutes ago, Work In Progress said:

You don't optimise a WW2 bomber for flat-out maximum speed. You optimise it for what it will do most of in their hours of flying. Which is a mid to high speed cruise very similar to that of a Lancaster. The Lancaster I/III is only about 18 MPH slower flat out than a Ju-88 A4

Agreed in general, but the point I'm arguing is whether that was the design philosophy for the Ju88, specifically as a 'Schnellbomber'.  Is there any evidence one way or another?

 

Also, what did you think of the other points in my post?

 

Regards

 

Martin

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"schnell" is about the mission and the design brief, not the way you fly it. It doesn't mean you fly it at max boost and RPM all the way to and from the target. It means fast for its time, a bomber intended to get to and from the target at a higher speed than the previous bomber types, and with the hope of being into and out of the target area before enemy interceptors can catch up with it.

 

My comments on angle related to the  gondola armament of the radar-equipped G in the linked 3-view I was referring to.

Turning to the question of nose armament on the C, as subsequently raised by Richard502, I don;t know a great deal about the C model but note that this site: 

http://www.aeroflight.co.uk/aircraft/types/type-details/junkers-ju-88-fighter-variants.htm

claims this for the initial use for the aircraft (and presumably the role for which it was developed)

"A small batch of early production Ju 88A-1 bombers were converted into Ju 88C-0s during July and August 1939, and used operationally during the invasion of Poland by the Zerstörerstaffel of KG 30 for long-range ground-attack... It was initially planned that the subsequent production variants would be the the Ju 88C-1 with 1,600 hp BMW 801MA air-cooled radials, and the Ju 88C-2 with liquid-cooled 1,200 hp Jumo 211B-1 engines behind annular radiators. In the event, the BMW 801 engines were reserved for the new Focke-Wulf Fw 190 fighter and the C-1 and the proposed C-3 derivative were abandoned. The first production model was thus the C-2 with an armament of one 20 mm MG FF cannon and three 7.9 mm MG 17 machine-guns in a new smooth metal nose section. These aircraft were converted on the production line, and retained the ventral gondola. The C-2s were used for more than a year for coastal and anti-shipping patrol, before another role appeared"

 

If that is correct, then at the time the aircraft was designed and built, the C model fixed nose guns were intended not primarily for air to air use, but mainly for attacking ground targets while flying at low level, in which case a hefty downward deflection is an obvious boon in preventing you from flying into the target.  

 

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3 hours ago, mike romeo said:

which fixed armament were you referring to?

All fixed, foreward firing armament in all types ouf Ju88, as apparently all models had the FFFA maounted in an downward angle.

 

2 hours ago, Work In Progress said:

If that is correct, then at the time the aircraft was designed and built, the C model fixed nose guns were intended not primarily for air to air use, but mainly for attacking ground targets while flying at low level, in which case a hefty downward deflection is an obvious boon in preventing you from flying into the target. 

Good point! Should have though of that, but....

 

3 hours ago, mike romeo said:

The main armament of the G, unlike the C, was carried in a ventral gondola (possibly to free up space in the nose, for c.g. issues or to avoid damage to the radar arrays as you say).

Maybe the "G" carried the 4 MG151s in a ventral gondola because there was no room in the nose? The earlier fighter versions carried a very weak fixed foreward firing armament (1 MG FF, 3 MG17, in the nose, augmented in later versions by 2 more MG FFs in the bomber-style gondola): pitifully small and weak for such a heavy aircraft, but apparently all that could be installed.

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Thanks for that, Richard.

 

When I get a mo, I'll check my copy of William Green's Warplanes of the Third Reich and see if there's any further enlightenment therein.

 

Regards

 

Martin

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Hi again, Richard,

 

William Green had some interesting things to say about the '88.

 

According to him, tlhe Schnellbomber spec. mentioned a requirement for a speed of 310mph (in '36!) which had to be held for 30 mins.  It's max cruising speed was to be 280mph.  No mention is made of a range requirement at cruising speed which, as previously pointed out, would usually help set wing / fuse incidence.  Unless Green left out the range requirement, this suggests to me  that the challenging max speed requirement would drive the need to minimise drag at high speed, with a requirement for a flat fuse angle at that condition, like a fighter.  Perhaps this was due to the tactical doctrine of the Luftwaffe, in support of the Army, leading to less interest in range, per se.

 

Anyway, the stretching max speed and endurance mentioned suggests (to me, at least) that the driving requirement for setting wing / fuse incidence would have been that.

 

So, to recap, I would expect a flat fuse angle at max speed to minimise drag.  Consequently, I would expect a nose up fuse angle at heavy bomber cruise speeds. This is a hypothesis, not fact, and your mileage may vary, but until more evidence surfaces, it'll do for me for now.

 

Green also says that the nose armament on the C was set nose down ~3° and that the G nose armament was deleted due to the cannon flash distracting the pilot.  So, that was a possibility that I hadn't even thought of earlier!

 

Green mentions the 5° downward cant of the gondola armament, but is not forthcoming on the reason for this, darn it!

 

So, in the absence of any firm evidence, I currently believe that the downward cant was to counteract the nose up angle taken by the fuse when attacking at heavy bomber speeds. . .

 

However, it could have been done to avoid damage to the aerials.  I, personally, don't buy that, but I can't prove it wrong.  Who knows? Maybe there were multiple reasons for the downward angle!

 

Hopefully one of the Experten will come along to put us out of our misery.

 

Regards

 

Martin

 

 

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Martin, I still believe that at high speed Ju 88 would fly in slight nose down attitude. Also, according to handbook Antti found (pages 5 and 6) Ju 88 G-1 had two MG 151/20 A installed in the cockpit. I reversed my opinion about fixed nose armament and I agree Ju 88 G-1 cannons had been angled down by 4,5° (slightly less to keep grenades flying reasonably close to aiming line, but in this case we are talking about minutes, not degrees). What I am trying to figure out now is why. I realize that in aerial combat occasions when an aircraft has too much energy (speed + altitude) are rare but generally speaking Ju 88 G-1 had had little trouble catching bombed-up Lancaster, let alone other, slower types. Also, I believe Luftwaffe night fighter pilots, even when flying planes without Schräge Musik armament, preferred attack from below (Prinz zu Sayn-Wittgenstein certainly had). I fail to see how canted down cannons would help in this case. However, when chasing Mosquito attack from slightly above to gain speed (and the use of GM-1 or whatever German pilot also had up his sleeve) would certainly help. Yes, He 219 was the plane for this task, but Milch detested labour and resources intensive Heinkels and argued for increased production of cheaper Ju 88s. Again, any thoughts? Cheers

Jure

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1 hour ago, mike romeo said:

William Green had some interesting things to say about the '88.

 

According to him, tlhe Schnellbomber spec. mentioned a requirement for a speed of 310mph (in '36!) which had to be held for 30 mins.  It's max cruising speed was to be 280mph.  No mention is made of a range requirement at cruising speed which, as previously pointed out, would usually help set wing / fuse incidence.  Unless Green left out the range requirement, this suggests to me  that the challenging max speed requirement would drive the need to minimise drag at high speed, with a requirement for a flat fuse angle at that condition, like a fighter.  Perhaps this was due to the tactical doctrine of the Luftwaffe, in support of the Army, leading to less interest in range, per se.

 

Obviously there was a range requirement  within the RLM Schnellbomber RFP. Every customer statement of needs since the dawn of commercial aeroplane manufacture had has a range requirement and in some cases a flight endurance requirement too. This has to be part of the requirement statement if only for the reason that until you know the required range you cannot make a decision on the necessary size of the airframe. Plus it is obvious that anyone can artificially boost the top speed of an aeroplane at the expense of its usability by eliminating most of the fuel along with the cubic and strength capacity of the airframe required to house it. If they hadn't had a range requirement then every single designer would have been on the phone immediately, enquiring:  "This Schnellbomber thing, what mission radius do you need?"

 

As to what that requirement was, I don't have a copy of the RLM document, but the recently published 'Ju88 Aces of World War Two' (Robert Forsyth, Bloomsbury, Jan 2019) quotes it as "a range of around 2000 km" (page 10).

 

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Hello all,

 

I have a book written by an ex Luftwaffe JU-88 night fighter pilot. He mentions that RAF heavies quite often left the target in a shallow dive to gain more air speed and that in level flight JU-88 got closer very slowly. What is the only possible way to close your target faster? You have to dive steeper than your target. If your guns point slightly downwards when you are approaching high you actually get a longer time for shooting because your aiming point isn't moving forward due to the relative speed. Is this the correct answer? I really do not know but the interception geometry works. I think that also the idea to protect pilot's night vision and/or to keep the grenades a few centimeters further away from the radar aerials make sense.

 

Cheers,

Antti

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Here's another thought (or two). On any aircraft when the heavy armament fires it tends to slow the aircraft. And/or can lead to pitch up or down?

The guns are synchronised with the sights. So,if the guns are fitted nose down then to bring the target onto the sight you trim the elevators until you have it right.

The Aircraft will happily fly along nose down in this trimmed position.

Fire the guns, Aircraft (with nose down guns) will pitch up. Push on the stick, align the target, fire, and repeat until target goes down. Retrim, find next target. 

I could be wrong but some of this makes sense to me at least.

 

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Posted (edited)

If that makes sense it can only be in the absence of relevant piloting experience e.g actual ACM or formation. You don't use the trim wheel to sight aircraft fixed guns on a target. You use the primary flying controls. The only purpose of the elevator trim is to relieve sustained control application forces. 

 

Go out and try joining into any kind of line-astern formation using the control column and rudder pedals for roll and yaw control but the elevator trim for pitch, and see how long you can stay in position... You simply don't have the means to apply the very low-lag small-gain control inputs required when using the trimmer. 

 

Even a landing approach using just elevator trim for pitch (something everyone should practice from time to time) is extremely demanding and that's a much easier target in terms of the pitch axis, as you can generally touch down safely at any point in the first third of the runway length, whereas a Lancaster is a very small target from behind, and putting a round three feet above or below it does not damage it, it just causes it to take evasive action.

Edited by Work In Progress

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In my defence, it was very early in the morning and I'd only had green tea with bourbon biscuits.

Of course if you fly around with nose down trim you will also lose height. Doh.

If only we had a set of pilots notes!

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Pete in Lincs, there is one here, translated to English, but it covers A-1, A-4 and A-5 versions only. Elevator trim handling is mentioned only in connection with pull-out mechanism failure:

˝If pull-out mechanism is inoperational, the pull-out can be performed by using the elevator trim tabs (Approx. for 1 1/2 turns on the wheel to direction "Tail heavy" ("Schwanzlastig"), after which flight is continued and landing performed.˝

Cheers

Jure

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Posted (edited)

If you don't mind a digression, that's an interesting highly specialised and unusual use of trim, with a good reason behind it. 

 

The Ju-87 and Ju-88 are very different kettles of fish as dive bombers. The 87 was purpose-designed for that role with very little compromise, and was absolutely brilliant at it. It could do genuine 90 degree vertical dives all day long, giving excellent accuracy, with speed-limiting dive brakes, and was small enough, draggy enough and strong enough to pull out hard at the bottom. The automatic recovery mechanism was set to provide a 6G pull-out which starting from the 600 km/h it would reach in a dive is enough to black out the crew. A hard pull-out however makes life more difficult for defenders shooting at it, and enables the dive to continue to a lower height than a gentle, say 3G pull-out. Having the whole process automated meant that the crew could grey out ofr black ouit and still pull out safely, and it also eliminates variations of pull-out technique.  But I doubt you would find a way to pull the wings off an 87 no matter how hard you hauled on the stick with both hads, providing you;d git the brakes out properly and were speed-limited.

 

The 88 on the other hand, like any aircraft of its size and speed, was severely compromised as a dive bomber. It could not get anywhere near a vertical sustained dive without zooming past Vne and was restricted to shallower dives, which is somewhat missing the point of dive bombing as you lose a lot of the accuracy potential that comes from a truly vertical dive. It was a much faster, cleaner, heavier airframe and also nowhere near as strong as the 87 (and really couldn't have been without making the wing much thicker or much heavier, thus affecting performance, range, bomb capacity).

 

With the 88 the pull-out mechanism was not really there to enable blacked-out pilots to complete the recovery and get their bearings whie the aeroplane did its own thing safely, an 88 crew would not have blacked out at the lower G loadings. It was there to provide consistency, so that the pilot did not accidentally break the aeroplane hauling it out too hard.

 

If the dive recovery mechanism was bust then using the trim wheel in the manner described is a way of applying a consistent and known amount of up elevator in the heat of the moment. Anyone can take three handfuls of trim (each rotating the trim wheel half a turn) and it will have the same effect every time regardless of the pilot's skill and adrenaline level. Quite clever really.

 

Of course you'd have to wind it off pretty promptly too as the nose comes up through the horizon, but that's easy enough.

 

Those notes are also interesting in that they confirm, as expected, that you could not cruise an 88 at its top speed. As one would expect from normal aviation practice full power is only available for 60 seconds at a time (take-off or emergency). The aeroplane's quoted max level speed would be with climb power, but that is only available for 30 minutes so not for any sustained flight. After that you have to reduce to a lower setting again, which is the maximum you have have available for most of the flight.

 

 

Edited by Work In Progress

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2 hours ago, Pete in Lincs said:

Of course if you fly around with nose down trim you will also lose height. Doh.

No.  Playing with the trim wheel adjusts the attitude of the aircraft so that the stick is centralised for ease of control.  Over these angles, the angle the nose is pointing with respect to the line of flight does not affect whether an aircraft goes up or down.  What makes this happen is the power setting.  More power and the aircraft goes up, less and it descends.  Adjusting the trim changes the attitude of the aircraft in steady level flight not the altitude.

 

This is for steady flight, of course the aircraft will dive if you stick the nose down far enough by moving the stick, and will climb (or perhaps, go upwards) if you pull it back.  But if you don't touch the power settings it will accelerate in the dive and slow down in a climb - this is what is called a zoom climb, exchanging kinetic energy (velocity) for potential energy (height).  If persevered with, the end result of a zoom climb is a stall.

 

To a small order, changing the trim will change the drag and thus at constant throttle the aircraft will change speed, and thus perhaps altitude; the two are kept as required by the pilot adjusting the throttle.  In addition, by altering the angle of attack between the air and the wing, the lift will change.  If flying was easy, anyone could do it.

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