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There's some folklore in the radar world about the Nazi fighters doing a roll before engagement to mark them to a German radar operator. This makes a lot of sense because it allows you to change your backscatter signature and give a basic "friend or foe" signature to a radar operator.

I've been unable to find an authoritative source on this in the radar context; however, I have found reports of Nazis doing a roll, which leads me to two questions:

  1. Does anyone know of an authoritative source?
  2. Would a roll be executed for some other reason?

Edit: The origin of this folklore is the following: 1) I heard about this from a lecturer when I was first doing basic radar design. 2) I heard about it from an American WWII pilot who was over Germany because I asked him after #1 (note: this was 20-some years ago). 3) I am currently reading a book that discusses radar and it is mentioned but unreferenced. I asked around to other analog designers who do radar-type tracking, and we've all heard this folklore. It's fundamentally how we do RFID tracking and the initial work was 1930s.

To that end, it seems that this isn't a very good question, and perhaps I need a historian. It seems to be a question that if the Nazis rolled all as some initial move and then someone just assumed it was for radar.

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    $\begingroup$ Would a Wurzburg C or D be able to resolve a JU-88 30 miles away well enough to do that? The ground based radars were mainly used to place the night fighters into the bomber stream. Targets were acquired by the on-board radar to within a couple hundred yards, and then visually by detecting the target's exhaust for getting in close enough to fire. $\endgroup$
    – John K
    Commented Sep 8, 2021 at 15:21
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    $\begingroup$ That theory doesn't pass the smell test. If it was that easy it's the first thing that the allies would have copied. $\endgroup$
    – GdD
    Commented Sep 8, 2021 at 15:28
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    $\begingroup$ For a radar to be able to detect/recognize/distinguish the electromagnetic signature of a "roll" requires very sophisticated real-time Signal Processing, that was not available until digital computers were designed into and became part of the radar system, which did not happen until the late 60s and early 1970s (at the earliest). $\endgroup$
    – Charles Bretana
    Commented Sep 8, 2021 at 15:37
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    $\begingroup$ @CharlesBretana That's not true. The roll could cause a "fade and huge blip" due to the capacitance on the line between the tube that is the receiver and the base-band mixdown. I am an analog designer who has made some radars, and as far as technology, I am sure it would have worked circa 1940. If it's folk lore, it is still feasible. $\endgroup$
    – b degnan
    Commented Sep 8, 2021 at 15:42
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    $\begingroup$ In air combat maneuvering, turning is the fundamental tactic. You turn to move your velocity vector off your foe's flight path when you are on the defensive, and you turn to bring your guns to bear on your target when you are on the offensive. High-G combat turns start with a roll. So unless the closure is head-to-head (or very rarely, head-to-tail), the first thing every fighter does to engage (up to the present day) is roll to then execute a turn. $\endgroup$
    – Todd Wilcox
    Commented Sep 9, 2021 at 8:48

2 Answers 2

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For a radar to be able to detect/recognize/distinguish the electromagnetic signature of a "roll" requires very sophisticated real-time Signal Processing, that was not available until digital computers were designed into and became part of the radar system, which did not happen until the late 60s and early 1970s (at the earliest).

It could be conjectured that a very astute radar operator using a WWII radar that displayed raw returns, might be able to recognize the increase and decrease in radar signal intensity as an aircraft rolled, presenting a higher wing planform and radar cross section, in a roll, but that would only occur at beam (90 degree) cross angle, not head or tail on, not to mention that numerous other phenomena (weather, variance in transmitted signal strength, etc.), would probably cause the same variance. So it is highly unlikely that this would have useful or reliable.

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    $\begingroup$ Besides, how fast is the radar sweeping? Even if ( IF ) the operator could see that "this" blip is a friendly, as soon as that aircraft merged with the formation of bombers, it's just a bunch of blips, with no ability to "track" the blip that earlier had rolled. This sounds more like folklore than tactics to me. $\endgroup$
    – Ralph J ♦
    Commented Sep 8, 2021 at 19:16
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    $\begingroup$ Radar aside, do you have any idea why then a roll would be executed? Is there some aircraft related thing like.. dunno, reset some fuel gauge or something? $\endgroup$
    – b degnan
    Commented Sep 8, 2021 at 22:28
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    $\begingroup$ They could identify themselves to other wings. If you assume most german fighters were involved in anti-bomber sorties, they would be up against escort fighters as well. Fighter groups would be scrambled from multiple directions and climb as high as they could before intercepting. During the sorties it'd be very hard to identify friend from foe unless very close. Wasting your whole fighter groups' energy to dive in on a friendly would be quite wasteful and dangerous. $\endgroup$
    – Olaf
    Commented Sep 9, 2021 at 9:50
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    $\begingroup$ @bdegnan, where did you see this roll maneuver mentioned? do you have a source? There may be other info in that source that offers a clue to why... $\endgroup$
    – Charles Bretana
    Commented Sep 9, 2021 at 14:22
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    $\begingroup$ @DavidWaterworth, that makes a lot of sense when entering a dive (in the referenced article with a dive bomber, but also for fighter attacking from above) – you have no downward visibility, so by rolling inverted first you can keep your eyes on the target below before starting the dive. You can also pull more positive G than negative (without fainting). Additionally in Spitfires the fuel system didn't handle negative Gs well, so that's another reason to roll and pull to enter a dive – but German planes didn't have that problem (as they already used direct injection). $\endgroup$
    – Jan Hudec
    Commented Apr 4, 2022 at 7:28
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To answer the question about tactical behaviour, I have no authoritative sources but I have elements. From this elements (that might be partial of course), it could be concluded that this behaviour might not be folklore, but not made before engaging:

Disclaimer: This answer assumes the manoeuver was feasible and efficient as a basic IFF method

Main use of radars by the Germans was for countering British night bombings. This included:

  • Detect the big formation of British bombers
  • Defeat the counter-measures used by them
  • Direct fighters to the bomber formation
  • At some point, let the fighters alone because counter measures do not allow anymore radars to pinpoint a target for the fighters, so it is up to the fighter pilot's eyes and on-board radar to find targets

The tricky point is the third part: the radar, facing a lot of spots that, from time to time, start to mix (because aircraft are closing on each other), needs to discriminate between them. So it could be a method to ask, from time to time, the fighters to execute the "basic-IFF" manoeuver to discriminate them.

However, just after engaging, this would not have sense anymore to ask that. Because of the 4th point, the problem of German fighters was no more to be identified by ground radars, but to escape British night fighters and diligently shoot down the bombers. The fighters would act autonomously, without the helpof ground radars. It could only use the basic-IFF manoeuver again if it had pursued a bomber for a while and thus lost main formations. At that moment, the radar could ask him to identify in order to direct him again to the bombers or to its land strip.

Conclusion:

Probably not folklore, but not use "just before engaging", but more during the approach or recovery phase.

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