Related to this question Why did the Ju-87 Stuka have a siren?
I would like to know the reasons why they have not changed the engine of the Stuka when they realised it was easy prey for Hurricanes and Spitfires in 1942/43.
Couldn't they have replaced the engine with a bigger one?
It is a heavy aircraft, but certainly could have gone through some improvements in order to increase its speed and manoeuvrability.
Ok it is a bomber, not a fighter, but still...
The Ju-87 Stuka did undergo engine change during its life.
The initial Ju-87A version used the Junkers Jumo 210, an engine, which produced 602 hp. This was almost immediately replaced with the Jumo 210D, which produced around 661 hp.
The first production version, the Junkers Ju-87 B-1 used a Junkers Jumo 211D generating 1,184 hp. Note this version had the fuselage and landing gear redesigned from the 87A.
The next major version, the Ju-87D, used a Jumo 211J delivering 1,401 hp, after briefly dabbling with Daimler-Benz DB 603 engines.
The final operational version of Stuka, the Ju-87G, used the Jumo 211J engine.
This is more or less the same path followed by other combat aircraft of the era. For example the Hawker Hurricane MkI used a Merlin III engine producing 1,310 hp, while the last major version, the MkIV used the Merlin 24 (or 27) engines, producing 1,620 hp.
However, the main thrust in engine change in the Ju-87 was increasing the payload, rather than increasing speed or maneuverability like the Hurricanes. For example, bomb carrying ability was nearly quadrupled from 500 kg in the B-version to 1,800 kg (max load) in the D-version.
Another reason was the aircraft design itself. It was designed to be a rugged dive bomber, and increase in speed would've done it no good (it is to be noted that the fixed landing gear added a lot of drag, though it was sturdy enough to mount a cannon) and it had limited maneuverability at the best of times when loaded (though it had good turn performance at low altitudes). The airframe had already reached its limit.
The only reason the Stuka served till the end of the war despite its limitations becoming apparent by the Battle of Britain was simply because no better replacements were available (though the process started in the early 1940's), though the Focke Wulf 190 took over some of the duties of Ju-87.
The engine was replaced when newer versions were introduced: The first production model (Ju-87 A) used a Jumo 210 with 690 HP takeoff power. This was raised in the B version to 1200 HP by switching to the improved Jumo 211. From 1941 the D version with the more powerful Jumo 211 J (1420 HP) was introduced.
But that is all besides the point. No powerplant would had made it faster than the contemporary fighters, not even a jet engine. The design placed ruggedness first, and as a consequence had a fixed landing gear which could be used on unprepared fields. The airplane had a lower wing loading than fighters, had the very effective Junkers Doppelflügel (offset flaps) and could turn much better, so it was not easy to shoot down. The downside was an inherently higher drag which could not be overcome with a bigger engine (which would had made the aircraft heavier).
Detail of the Ju-87 wing flaps from the operating manual (picture source). By using those, a Ju-87 could fly much slower and turn much tighter than contemporary fighter aircraft.
The idea was to optimize the Ju-87 for ground support and let the Bf-109s and Fw-190s take care of the fighters. This worked well during the Blitzkrieg of 1940 and on the Eastern Front when the airplane was used as designed. This is the same strategy the US Air Force uses for the Fairchild A-10 attack airplane. Being designed as a dive bomber, the Ju-87 airframe was very sturdy and robust. There are many stories of Ju-87 pilots returning with heavily damaged planes.
Battle damage on a Ju-87 (picture source)
Damaged Ju-87 wing (picture source)
Especially Hans-Ulrich Rudel used the Ju-87 with great effect. If you read his memoirs (Amazon link), you will learn that he once was surprised by fog and could not fly back to his base. So he landed on a road and taxied the 70 km back. He almost made it back to base; only an impassable railroad crossing could stop him from taxiing all the way home.
At several occasions, he landed next to a shot-down Ju-87 and took the crew onboard. Try that with a Hurricane or a Spitfire.
In 1944 he was chased by a Russian fighter pilot. His observer, sitting in the rear seat, facing backwards, could tell him every time what his opponent was up to, so he could evade the attacks successfully, flying close to the ground and at minimum speed, flaps down. Suddenly, the Russian fighter was gone: Unable to fly as slowly as the Ju-87, he had stalled and crashed. Later, Rudel learned that this had been one of the most highly decorated Russian aces. The Ju-87 with a skilled crew was more than a match for him.
After the war, he consulted Fairchild during the development of the A-10.
Replace the engine with... what, exactly? Bear in mind that at low-mid altitude (definitely below 10,000ft) the Ju 87D had a more powerful engine than any Bf 109 until the autumn of 1943.
After mid-1944 every high-power engine generally had good high-altitude performance, with low-altitude performance being mediocre in the absence of MW 50 injection. Every such engine was needed in a Bf 109, even with Fw 190Cs or Bf 110s theoretically waiting in the pipeline somewhere.
The Ju 87 was quite manoeuvrable, and performance with a bomb is worse in airplanes regardless of their installed power or role. If the enemy makes you jettison your bomb s to dogfight them, even if you do so in a high-performance fighter-bomber, they have already won.
You need to understand that attacking ground targets and attacking planes entail rather contradictory priorities. Fighters need to fly fast and to climb well to bounce the enemy and to retain as much energy as possible over the course of the dogfight. Ground attack missions require planes to fly low and slow so as to find and prioritise targets properly. This means that most of the time ground attack planes are inherently vulnerable to fighters. Thus to properly survive in a fighter-heavy environment a ground-attack plane needs to accelerate quickly to a speed approximately equal to that of a fighter and then be a reasonable match to their opponent in manoeuvrability and (very importantly) rate of climb so that repeated/successful bounces by the enemy fighters are not likely. They need to do so while carrying their ordnance, be it large guns in pods or bombs. These are extraordinary requirements and quite unreasonable ones at that, especially as the Germans had an inability to make very powerful engines in quantity from factors rather beyond their control. And of course if your performance is that good even with a bomb you have a plane that would make a wonderful fighter in a clean configuration so chances are many planes would be diverted from ground attack detail for this reason...
One can expand the question: why didn't they replace the Ju 87 outright? Well they tried that too, but invariably ran into one problem or another that had nothing to do with the 87.
The Me 210 was a fascinating aircraft. It had a window that ran down the nose of the aircraft so the pilot could see ground targets throughout the bomb run. It could dive at "reasonable" angles, perhaps 30 to 40 degrees, had useful dive brakes, retracting gear, very good performance and an internal bomb bay. It also was unstable, and required massive redesign before emerging as the 410 right as swarms of P-51s were showing up.
The Arado 240 was a similar design and had similar stability problems. It was also ridiculously complex due to a strange arrangement of the fuel tanks.
The Ju 88 was also equipped with glide-bombing capability, although with a much shallower dive angle. Much has been written about how this aircraft was ruined by the addition of this capability, but I'm not convinced this was the root of the problem, compared to indifferent engines and odd design criterion for defensive fire.
So they had all these aircraft that were supposed to either supplant or replace the 87, but none of them matured, while the 87 kept being upgraded. This is far from the only example; the German war diary is filled with similar stories from Bomber B/Jumo 222 to their entire tank production effort.
