I - P U R P O S E
    In his letter (ref) dated 01/23/1940 about the practical experimentation of the Dewoitine 520, the Général commander in chief of the Air Forces requested in particular that the combat of this aircraft versus the Messerschmidt 109 should be studied.  
    Moreover, the Général inspector and commander of fighter command, in his letter (ref) dated 04/25/1940, asked that the CEMA (air equipment experimental centre) performed flight tests with the Messerschmidt 109 and the French fighters Dewoitine 520 and Bloch 152 (late variant), in order to compare their respective characteristics concerning:

		at sea level
- Speeds	in level flight	at service altitide
		at high altitude
	in climb
	in dive
- Manoeuvrability at various altitudes

    The CEMA established an experimental programme given in this document. The Messerschmidt 109 was also compared in flight with the modern obervation and reconaissance aircraft Bloch 174 and Potez 63-11(...)
    The CEMA Daimer-Benz DB 601 engined Messerschmidt 109 was delivered in late March to the CEAM (air testing facility). A Munerelle breathing kit and a Debris gun camera were fitted.
    The testing took place during April 1st - 21st. Initiated in Bricy (near Orléans), it was eventually completed in Marignane (near Marseille ) with help from the Experimental Squadrons.

II - P R O G R A M M E
The program consisted in the study of the following items:
A/ - COMBAT CAPABILITY
    This study inteded to research the best possible usage of the already known favourable characteristics of the aircraft

- Messerschmidt 109	- level speed
	- climb rate
- French aircraft	- Manoeuvrability, especially at great angles and high speeds

    To prevent the test from turining into a confuse melée, from which little conclusion could be drawn, the mockup combat was studied as separate offensive or defensive situations.

a) Surprised aircraft
- Determine the best possible break manoeuvre
- Attempt to break away from combat, or establish a favourable position
b) Aircraft not surprised
- Determine the best possible manoeuvre to establish a favourable position
- Evaluate the choice to accept or refuse the turning fight.
- Evaluate the capability to break away from combat after the engagement.

   Because of problems encountered with the Messerschmidt 109 engine at very high altitudes, testing was limited to the aircraft's service altitude (approximately 5000m). It is reasonable to assume the relative manoeuvrabilities remain the same at 8000m.

B/ - SPEED COMPARISON
This comparative study was to complete the already available results from the CEMA experimentations.(...)

III - P R A C T I C A L  E X P E R I M E N T A T I O N  O R G A N I Z A T I O N
Standard schedule
1 - Simultaneous climb to 8000m - measured times to 1500 and 5000 - level flying at full throttle at 800, 5000 and 8000 metres.
2 - Engagement at average service altitude (between 5000 and 6000 metres)

A/ - Surprised aircraft
    The attacker starts with 500 metres altittude advantage, dives and positions in the defender's rear sector. The latter begins his manoeuvre when the attacker is approximately 300 m away from him.

a) Surprised Messerschmidt Dewoitine 520 attacking
1/ Break by optimum climb
2/ Break by turning fight (continuous tight turn). The defender tries to reverse the fight
3/ Break away by steep dive

b) Surprised Dewoitine 520  attacking Messerschmidt
Same manoeuvres as above.

B/ - Non surprised aircraft
The test starts when the two aircraft cross each other at similar altitudes.
The engagement was done twice on each side. (ie crossing left- or right-hand side)

(...)

IV - E X P E R I M E N T A T I O N S  V A L I D I T Y
One cannot draw definitive conclusions from the results obtained with a single Messerschmidt 109 example, in particular the fuel system proved troublesome at high altitude. Test were limitied to 6000 metres, as below this altitude the engine and aircraft seemed to perform perfectly. In regard to manoeuvrability, it should be noted that the pilot was possibly not at ease with some characteristics of the Messerchmidt such as the extremly narrow cockpit and the inverted throttle(...)

V - R E S U L T S

I) MESSERCHMIDT 109 VS. D.520
A - SPEEDS
a) Level flight, full throttle, low altitude (600 m)
Similar speeds. The Messerschmidt kept its radiator flaps open during the test. With the flaps closed it would have had a slight advantage (20 to 30 km/h)
b) Climb to 5500 m at full throttle
The Messerchmidt's climb rate was superior. Its cooling system was efficient, whereas the D.520 had to reduce throttle once during each test run in order to let its engine cool down (see note ) (at 4500 m on the first run, at 3000 m on the other run). The Messerschimdt reached 5000 m in approximately 6'20''.
c) Level flight at 5500 m
Very similar speeds with a slight advantage to the Messerschimdt 109.

B - COMBAT MANOEUVRES
A/ Surprised Messerschmidt
a) Left spiral break
The Dewoitine 520 could have fired for a few seconds, but the Messerschimdt pulled away rather quickly thanks to its superior climb rate.
b) Turn break
The Messerschimdt slowly puled away.
c) Dive
Speed reached by the Messerschimdt: 700 km/h IAS. The D.520 followed it (it should be noted that the Messerschimdt's speed indicator is optimistic compared with the French systems).
After each steep dive, the D.520's landing gear extended during the pullup, and could not be retracted again. Another incident ocurred with the hydraulic systems leading to a flaps failure.

B/ Surprised D.520
a) Right spiral break
The Messerschimdt easily kept its aim during this manoeuvre.
b) Break by cutting throttle and giving yaw
If this manoeuvre is attempted at the moment the Messerschimdt is about to open fire, it is certainly very efficient, whatever the defending aircraft type is. The Messerschimdt could avoid to overshoot, and the Dewoitine was able to get in favourable position.
c) Dive
The Messerschimdt was able to follow the D.520 in the dive. However, the latter remained more manoeuvrable at high speeds.

C/ Fight with no surprise (4500 - 6000m high)

               D.520

                  |

                  \/

   /\

   |

Me 109

a)The D.520 comes from the Messerschimdt's right
    Right-hand turning fight. There was an indecisive fight for quite a long time; each adversary getting in a favourable position for brief moments. Suddenly the D.520 brutally stalled several times to the left, doing 1/2 or 3/4 rolls at full throttle. The Messerschmidt tended to stall too, but its auto-rotating movement was slowed down by the extending of the leading edge slats, that cause big shocks in the control column.
    After these stalls, that were recovered from quickly, the D.520 lost ground and the run ended with clear advantage to the Messerschmidt, who got in favourable situtation without having been himself in the D.520's gunsight.
b) The D.520 comes from the Messerschmidt's left
    Left-hand turnning fight. First part of the fight was a draw, each adversary gets in favourable position for rather long moments.
    As the manoeuvres got tighter, the D.520 stalled to the left. Instead of recovering from the stall, the pilot decided to complete its snap roll. The Messerschmidt, because of the strong reactions felt in the controls, was unable to follow that manoeuvre and lost sight. Then the D.520 got and stayed in firing position, in spite of his opponent's evasive manoeuvres.
    This run ended with D.520 as the winner, as it could have fires whereas the Messerschmidt was never able to put the Dewoitine in its gunsight.


        C O N C L U S I O N S

A - Speed
- Messerschmidt slighly faster in level flight at tested altitudes
- Messerschmidt's climb rate clearly superior, which can certainly be used for good in combat. The Dewoitine pilot should avoid any long climbing pursuit.
    It seemed that the Dewoitine pilot should set the propeller pitch control to "manual" and slighly reduce the pitch while in combat. This issue must be verified by an operational unit flying Dewoitines.
- Dive speeds: both aircraft are approximately equal.

B - Combat manoeuvres
- The Dewoitine 520, in combat tight turning manoeuvres, tends to stall earlier than the Messerschmidt, the latter benefiting from its leding edge slats. The Dewoitine's stall is very sharp and is always to the left; therefore it is more an issue during a right-hand turning fight, as the Dewoitine will perform a full snap rool. As a conclusion the fight should be engaged to the left, if it is possible for the Dewoitine pilot to choose.
- After these turning engagements, that all ended with the D.520 stalling, it appeared that this stall, however easy to recover from, causes a temporary loss of control and, therefore, a loss of ground to the adversary. It is dangerous, especially if the Messerschmidt 109 tends to be in a favourable position at this very moment, to try to control the stall and resume the turning fight. Rather, it should be attempted to turn the stall into a sharp reverse of directions that the Messerschmidt will probably be unable to follow. The stall can therefore be used as a start to a break manoeuvre, that in some cases can enable the Dewoitine to get into a favourable position.
- This stalling issue aside, it was concluded both aircraft have very comparable manoeuvrabilities: all the engagements lasted for a long time before one side took the advantage. Nevertheless the D.520 should be considered as more manoeuvrable as its controls are smoother, especially the elevators at high speeds: the Messerschmidt's pilot has to re-trim in pitch frequently, and this operation is difficult and awkward. In contrast, the Dewoitine pilot can leave his pich trim in middle position, or use it by small amounts only, which is an asset as far as high-speeds alternate dives and climb manoeuvres are concerned.
- No particular observation was made on both aircraft's stability as a firing platform.

    The Messerschmidt seems reliable in combat conditions. In contrast the Dewoitine suffers from insufficient cooling ( see note below ), and troublesome landing gear and flaps.

2) MESSERSCHMIDT 109 VERSUS BLOCH 152

To be continued...

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The evaluated Dewoitine 520 was s/n 2, a pre-production model, partially converted to the defitive production standard.

Early production D.520s suffered from insufficient cooling. A redesign of the cooling system was applied to new examples, and retrofitted to pre-production examples. The modifications included a new coolant circuit, an enlarged water radiator scoop, and an additional Y-shaped part near the landing gear wells deviating the airflow from the oil radiator and preventing this warm and turbulent airflow to enter the water radiator.

D.520 s/n 2 did not feature the last two modifications. For the comparative tests, it was fueled with type D aviation fuel (100 octane), whereas the standard fuel for the HS 12Y45 engine was 92 octane (type C in French military terminology). Source: "Le Dewoitine D.520" by R. Danel and J. Cuny, Docavia publishing.