There is a simple test for how badly an aircraft designer has run out of room. Look at where the instruments ended up.

On the Henschel Hs 129, some of the engine gauges were bolted to the inside faces of the engine nacelles, out in the slipstream, where the pilot had to look through the side glass to read them. The gunsight was mounted on the outside of the nose. The instrument panel itself was tucked so far down under the windscreen that it was nearly invisible from the seat.

This was not a prototype quirk. This is how the Luftwaffe’s dedicated tank-buster went to war.

Built Around a Bathtub

The requirement came out of Spain. The Condor Legion had shown that aircraft sent specifically to attack ground targets were worth having, and in April 1937 the Reichsluftfahrtministerium sent a specification to four manufacturers. Henschel’s answer was the P 46, which became the Hs 129 in April 1938. It first flew on 29 May 1939.

The design started from survivability. Wikipedia’s description of the structure is exact: the aircraft was built around a single large “bathtub” of steel sheeting that made up the entire nose area, with the fuselage sides angled inwards to shed fire.

That bathtub is where all the trouble came from. Armour is heavy and it is thick, and the space inside a wedge of armour plate is not shaped like a cockpit.

Nowhere to Put the Dials

The pilot sat in a slot with two small side windows and two flat angled blocks of glass in front of him. There was no room for a conventional instrument panel in his eyeline, so it went underneath.

It is worth sitting with that for a moment. A pilot flying at low level, being shot at by everything on the ground, had to glance sideways through a small window at gauges mounted on the engine cowling to find out what his engines were doing.

The Version Nobody Wanted

The Hs 129 A was a failure. It was overweight, underpowered on its 465 PS Argus As 410 engines, and the visibility was poor. Worse, the controls went nearly solid as speed rose. The V2 prototype crashed on 5 January 1940 because, in the blunt wording of the record, the stick forces were too high for the pilot to pull out.

When the aircraft was put in front of operational pilots in late 1940 they declined to accept it.

A French Engine Saved It

The fix came from the fall of France. Henschel reworked the aircraft around captured Gnome-Rhône 14M radials, which gave 700 PS instead of 465. Two converted A-1 airframes flew in early 1941 and the difference was obvious.

The B-series that followed got a redesigned canopy with more glazing and electrically actuated trim tabs, which made it flyable if never pleasant. Fifty B-1s had been built by May 1942, when B-2 deliveries started.

The Gun Kept Getting Bigger

The B-1 carried two 20 mm MG 151/20 cannon and two 7.92 mm MG 17 machine guns, later swapped for 13 mm MG 131s. Against infantry and soft-skinned vehicles that was plenty. Against a T-34 it was nothing.

By late 1942 the 30 mm MK 101 had been found wanting, so the B-2/R2 got the more powerful MK 103. Then came the B-2/R3 with a 3.7 cm BK 3,7, which worked but carried almost no ammunition. And then Henschel stopped being subtle.

The Hs 129 B-3 carried a 7.5 cm Rheinmetall PaK 40 — a towed field anti-tank gun — adapted to fire automatically as the BK 7,5. It weighed 1,200 kg. It needed a hydraulic recoil-damping system. It fed from a twelve-round rotary magazine housed in the rear wing root, and the gun and its pod took up a large part of the space under the fuselage.

Testing began in May 1944. Around twenty-five B-3s were finished before production stopped in September that year.

Here is the parent weapon on the ground, which gives some idea of what the airframe was being asked to absorb.

Where It Fought

The Hs 129 entered service in April 1942 with I./SchlG 1, whose 4th Staffel was the first to be declared operational on the type. It went to Tunisia, and from May 1942 to the Eastern Front, beginning in the Crimea. By July 1943 five Luftwaffe units were flying it, and it stayed in action into 1945. Hungary and Romania flew it too.

Total production was 865 aircraft, peaking in 1943 — a small number for an aircraft that has such a large reputation.

The Idea Outlived the Aeroplane

Strip away the armour bathtub and the misplaced gauges, and the Hs 129 was an attempt to answer a question that has never gone away: how do you kill a tank from the air without getting shot down doing it?

The answer the Americans arrived at thirty years later was the A-10 Thunderbolt II, an aircraft designed around a 30 mm rotary cannon, with a titanium bathtub around the pilot and engines mounted where the airframe shields them. The parallel is not an accident. It is the same problem producing the same shape of answer.

The Soviets got there first and got there better with the Il-2 Sturmovik, which they built more than thirty thousand of. Henschel built 865 Hs 129s and hung a field gun under the last two dozen of them.

Der Hs 129 was not a good aeroplane. It was an honest one: every awkward thing about it was the direct consequence of a decision to put armour first and work out the rest afterwards. The gauges on the nacelles are what that decision looks like from the pilot’s seat.

Sources: Wikipedia (Henschel Hs 129).