# How did the gyro gunsights of WW2 get the range and lead of a target?

I just read this article on the gyro gunsight, but I don't understand how it computed range/position to the target and then calculated how much lead is needed.

Surely it must have had radar to get range to the target? But I could only find one mention of radar in the article, and it only pertained to one particular model of the gyro gunsight. Even if so, I don't understand how a radar return is integrated with a gyro instrument.

I understand that a gyro can tell you your roll angle and rate of turn in any or all axes, but the main thing I'm missing is how did it know range and position of the target? It seems only radar can do such a thing and I don't know how radar data could be integrated with a gyro way back in the 1940's. The wiki article even says that these gyro gunsights were developed "just before the Second World War". But I also noticed the article is not that well-cited.

• I think the sight has an circular indicator that the pilot can adjust it's size. The pilot adjusts it to match the wing span of the target then the sight would set the distance accordingly. Sep 15 '17 at 16:35
• @mins No I hadn't, but thanks to the answers, now I understand that when wikipedia says "the pilot had to set the wingspan of the target", this means tell it 10 m (or whatever) and keep the reticule at the right diameter. In other words, it needs the absolute size and relative size. Having both of those is enough for a trig calc, no radar necessary. Sep 16 '17 at 3:13

You are seriously over estimating the technical capabilities of the time.

The gun-sight is not much more than an intricate slide-rule. You had to feed it information about the target size/aircraft type and it used the gyro information from your own aircrafts turn rates to project a reticule circle on the glass.

The position of the projection is offset from the guns aiming point according to the gyro factors, the set range, and the fall rate of the ammo. When the reticule circle lines up with the wingtips of the target you are at the right distance and angle to the target and you push the trigger.

It assumes you are following the same flight pattern as the aircraft in front of you. That is.. chasing it in a dog fight. That makes it fairly limited in use and effectiveness and would be no replacement for a seasoned pilot.

Aviators Database

Axis History Forum

Video: Gyro Gunsight Mk IID

CORRECTION:

Upon watching the video it is apparent that the implication that the aircraft must be following the same flight pattern as the target is misleading. In reality the target reticule, and in effect the pilot's aircraft, needs to be turning at a rate to track the target's flight pattern. Further, the range can be adjusted while continuing to track the target. That all sounds a bit like rubbing your head while patting your tummy and I am sure took quite a bit of practice and an accommodating target that maintained a constant heading or turn rate in front of you.

• When I read "input the target wingspan", I just thought it was something like, push buttons to tell the device that the wingspan is 10 meters. That alone cannot tell you anything about range. Now I see that, in addition to that, the apparent size of the target also has to be input by twisting a dial to make the reticule encircle the target. Both of those things, absolute size and apparent size, are enough info for a trigonometric calculation of range. Sep 16 '17 at 2:26
• @DrZ214 indeed, it is a type of visual range finder. The video does not mention it but if you were catching a target the procedure would be to set the range short so you don't have to keep adjusting it and then wait till the target filled the circle. Sep 16 '17 at 12:17
• @DrZ214 if you notice in the image, setting the wingspan was really just a matter of selecting the target aircraft type on the dial. Popular targets had marks for them. like Junkers-88 (Stuka) , Messerschmitt 110,119 etc. Sep 16 '17 at 12:22

Other answers here describe the functionality of such gunsights; the "calculation" was done by mechanical analog computer, made up of gears, cams, shafts, and linkages.

Here's part of a Sperry K-3 gunsight computer from a B-17 (inherited from my father, who liked collecting strange gadgets, now decorating my kitchen):

Additional information on the mechanical gunsights and bomb sights of the WWII era can be found here.

• Gotta love the true computers :-)
– yo'
Sep 15 '17 at 20:05
• @yo' The concept isn't at all new, either; the Greeks had things like the Antikythera mechanism
– JAB
Sep 15 '17 at 20:18

I don't understand how it computed range/position to the target and then calculated how much lead is needed.

Reading the Wikipedia article you linked, it didn't, but both assertions are unsourced.

The distance was estimated visually by the gunner through a visual aid:

a reticle to match to a target plane's known wingspan (to adjust the sight for the target's distance)

i.e.: the gunner would know the target wingspan, and its size in the reticle at a given distance, the mismatch would allow the estimation of the distance.

The lead was based on gyro information (hence the instrument name):

It is important to note that the information presented to the pilot was of his own aircraft, that is the deflection/lead calculated was based on his own bank-level, rate of turn, airspeed etc. The assumption was that the flightpath was following the flightpath of the target aircraft, as in a dogfight, therefore the input data was close enough.

Personally, rather than "calculated" I expect that some sort of mechanical linkage was moving the floating reticle around based on the relative position of gyro and airplane.

The video found by Trevor confirms that this is the case.

Starting at 2:00 we are told that:

• given the range, the gyro corrects only for the rate of turn
• the pilot has to manually adjust the gyro correction for the range (at 4:15 we are told that the renge adjustment does not alter only the position of the reticule, bu also its diameter, to reduce the errors deriving from manually estimating the range)
• the pilot can also select the base size of the reticule to match the aircraft being chased (4:35)

The full procedure with the correct sequence of settings and adjustments is described starting at 5:42