Evolution of the Dewoitine D.520 - (Corrected 09 / 09 / 2015)

Very often, I have read some papers about the wonderful planes the Nazis could possess if they were not defeated... 

The only little problem was that their defeat was absolutely unavoidable: 

• They had spent all the resources of all the countries they occupied, 

• they had lost several millions of their own soldiers, 

• they were under fire of the most powerful nations in the world and 

• their leaders had wasted all their diplomatic capacities. 

They had no choice at all.

The situation of the Dewoitine 520 mass production in June 1940

On the contrary, when Pétain imposed a cease fire to the French Armies, he had others choices, as Charles de Gaulle demonstrated it.

Yes, France suffered a huge defeat, but France could have continued the fight.

At the mid of June, 1940, after the excellent testing of the prototype, the Dewoitine plant was preparing the mass production of the Dewoitine D.523. 

This new fighter was a D.520 fitted with the Hispano-Suiza 12 Y 51 engine, yielding 1000 hp at altitude and 1100 hp for take off.


The first D.523 fighters were scheduled for July, before a lot of aerodynamic refinements could be applied to this aircraft, allowing to get more performances with the new power available.


At the same moment, the prototype of the D. 524 was prepared to her first flight with the Hipano Z engine.


The 3 tables displayed start from the real speed measurements on 3 different aerodynamic versions of the Dewoitine D.520. 

The others data of each table result from the computation of the speeds which would be reached by a given aerodynamic version with others engines.


The Hispano 12Y  was a very reliable engine (900 hours when used without heavy maintenance done in other place than the mother plant, as wrote Admiral Thabaut in the excellent Icare review).

The engines used in the operational D 520 were all built by Hispano-Suiza:

•     The 12 Y 31 delivered 860 hp  at  3250 m.

•      -    12 Y 29        -        920 hp at  3,600 m.

•      -    12 Y 45        -        920 hp at  4,200 m (S 39 supercharger)

•      -    12 Y 49        -        910 hp at  5,500 m (S 40 (?) supercharger)

•      -    12 Y 51        -      1000 hp at  4,200 m (S 39 supercharger)

•      -    12 Z             -      1200 hp at  4,750 m

Configuration I

The first fighter - the only one ready to be put into operational service - was the best choice for all the first year of the WWII, unfortunately, the rejection of the 100° octane fuel rejected her too. 

       Recorded speed with 920 hp:     527 kph      

 Estimated speed with 860 hp:       508 kph  

                 

Estimated speed with  950 hp:       532 kph  

Estimated speed with 1030 hp:     547 kph  

Table I - All results are given for an altitude of 5,000 m.

First configuration of the D.520 with a Hispano-Suiza 12 Y 29 engine - 

The results for 950 hp and 1030 hp are not related to any new engines but to the introduction of individual exhaust pipes for each cylinder in place of those gathering 2 successive cylinders (as for the Hawker Hurricane).

The D 520 with this engine could fight as easily against the Messerschmitt Bf 109 E than the D 520 S which was built, she was likely a bit more easy when flying close to the stall. 

The first production fighters could have been in operational squadrons before the first days of WWII. 

In such conditions, the number of fairly equipped squadrons could have been far superior for the May 10, 1940.

Configuration II

This configuration is the standard configuration of the series D 520 in 1940.

       Recorded speed with 920 hp:     535 kph         

 Estimated speed with 1,000 hp:      556 kph     

                 

Estimated speed with 1,100 hp:     560 kph    

Estimated speed with 1,200 hp:     585 kph    

Table II - aerodynamic configuration of the operational  D 520 S -

The first line displays the speed attained at 5500 m by the #2 D.520 exiting the production line .

All the other lines give the speed for an altitude of  5500 m

The value obtained for 1000 Cv are not far from the 562 kph of the Dewoitine D 523 (using a HS 12 Y 51 engine obtained in May 1940 by the famous test pilot Marcel Doret, 950 m above the critical altitude of the preceding version). 

Climbing times:

• 4000 m was reached in 5 minutes 5 seconds,

• 8000 m in 11 minutes 46 seconds.

For July or August 1940, such performances were very good. 

The first D 523 series fighters were scheduled to exit the assembly lines at July, 15, 1940.

Table II shows, also, how ridiculous were the assertions of French officials who claimed that the Morane 406 fighter needed only a more powerful engine to transcend the Bf 109 E fighter.  

With the Klimov (Hispano) M105P of 1100 Cv, the more aerodynamic Mörkö Morani reached only 525 kph, a value to compare to the 585 kph that a D.520 S could have reached with the same engine!

Configuration III

Table III displays the speed the D.520 #465 really reached with her aerodynamic refinements experimented in wind tunnel in Spring 1940 but experimented only in 1942-43 on the real fighter.

With a Hispano-Suiza 12 Y 51 engine, the speed could be of the same order than that of the Macchi 202.

       Recorded speed with 920 hp:    570 kph     

 Estimated speed with 1,000 hp:   588 kph

                 

   Estimated speed with 1,100 hp:   607 kph  

      Estimated speed with 1,200 hp:   625 kph     

      Estimated speed with 1,250 hp:    634 kph     
   

Table III - Third aerodynamic configuration of the Dewoitine 520 fighter -

- All the lines were computed for an altitude of 7000 m - The two last lines are a bit optimistic (which see). 

For the later period, the aerodynamic refinement of the D.520 - # 465 could, with more powerful engines, have given a very interesting fighter, matching with the best German ones, especially with the Bf 109 F.
                                                                                     

The 2 last speeds don't no take into account the 160 kg weight in excess of the 12 Z engine over the 12 Y 45/51 nor the significantly larger air intake of the liquid cooler. 

Nevertheless, it was possible to Hispano-Suiza to develop a more powerful variant of the HS 12 Y 51 able to deliver 1200 hp, as Klimov obtained with his M 105 used in the Yak 3 fighter.

It have been also discovered the more powerful engines needed an increase of the vertical part of the tail by 28 cm for a better control at slow speed. 

The Dewoitine engineers expected, at the take of weight of 3150 kg, a top speed of 660 kph at 9000 m. That speed imply a rather sophisticated air-supercharger.

The expected climbing times were:

•  4' 10" for 4000 m,

•  8' 00" for 8000 m,

• 14' 20" for 11000 m, the service ceiling being 12000 m.

The armament was expected to use 3 Hispano 404 canons.