The program : a bomber which could be as nimble as a fighter
Another constraint was the use of an engine chosen by some state deciders in place of self chosen engine. Why?
The French Republic was always delaying what it was buying. Worst, by the bias of systematic devaluations, the price was automatically lowered...
Were the members of these commissions very proficient? Obviously, all of them claimed they were. However, the only perfect validation is, allways, given a posteriori by the History itself. In the actual case, the defeat of June 1940 demonstrated how there claims were illegitime.
A very innovative prototype: A good case to use untested engines for trial flights?
That decision was the worst possible one. Ok, Gnome et Rhône experienced some problems with its own aircooled radial engines 14 Kirs / Kjrs and ordering engines to Hispano-Suiza was a method of pressure on the French specialist of air cooled radial engines. But this order was very shortsighted.
At the origin, the Hispano-Suiza company had bought some licences for Wright radial engines.
The STAé (service technique de l'Aéronautique) commissions was lacking completely of any adaptation capability to the real situations.
Nevertheless, after a lot of work, the very good engineers of Hispano-Suiza achieved to finish fairly the 14 Aa engine, which obtained good results on two prototypes: The excellent Latécoère 570 and the Koolhoven FK 58. It was too late.
This history had an adverse consequence on the developpment of the Hispano-Suiza liquid cooled 12 cylinder in Vee, because the very experimented engineers needed to that work have been send to reinforce the team working on the 14 Aa. That explained how long was the time to finish the 12Y51 needed for the Dewoitine 520.
A tactically erroneous conception
The major idea of this bomber program was the new bombers being faster than the projected French fighters, they were absolutely able to outrun all fighters in the world. Amazing !
Even if I acknowledge that some French fighters - as the Nieuport 161, for example - were very well designed, that did not garantees that the ennemy fighters cannot be faster.
Using the data of some Caudron racers flying at 500kph at sea level with only 300 hp, it was easy to project a 860 hp fighter able to fly at 550 kph at 5000m.
No, a true bomber, once in the air, cannot be sure to go back home: She is the natural prey of the ennemy fighters. General Giulio Douhet was only reasonning as if the bombers were battleships. But they are not...
A fast good bomber may kill the naughty fighter by shooting strait between the twin rudders...
Another exacting was the twin fins-and-rudders of the B4 bombers to allow the rear shooting of a 20mm cannon.
- At low speed, with important angles on wind stream, the fins-and-rudders were masked by the engine cowlings, or the landing gear box, that giving a very difficult aerodynamic stiring control.
- The twin fins-and-rudder was mechanically a complicated structure, because they are situated downstream the engines cowling. At high speed, the vortices induced by the engines produce vibrations which are dangerous from a structural point of view. To overcome these vibrations, on must strengthen the assembly of the tail, then one increases substancially the mass of the bomber.
At last, I need a cannon!
The French bureaucracy had decided another juicy nugget: The rear defense used a cannon!
The chosen weapon, the Hispano-Suiza HS 404 was indoubtedly a very good one, with a impressive destructive power and very good ballistic properties.
If that weapon had a muzzle velocity twice as great as the one of a machine-gun, a gunsight allowing a very great precision, the bet may have been a winner one. It was not the case.
Worst, the number of shells was to small. Initially, the cannon had 60 shells (6 seconds!) in the drum, after, it had only 30 shells drums (14 kg). In a bomber, which is manoeuvring to escape ennemy firing, the number of g might increase from 1 to 3, even 4, increasing the drum weight up to 60 kg. The French gunners were not Hulk!
One or two belt-fed riffle machine-guns were a very better solution.
The perfect bomber?...!
That bomber was the Lioré-Olivier 451 or LéO 451.
An aircraft I saw personnaly in a hangar of an airfield (Boufarik, in Algeria) when I was 6 or 7 (1952).
Around me, the peoples were chating together, mezzo vocce, and said "it is a warplane".
Obviously, I did not understand anything, but she appeared me as malicious.
The aerodynamics study of the LéO 451 was up to date, with very thin wings (15 to 11% relative thickness), the engine cowlings designed by engineer PE Mercier were very optimized for the highest possible speed (allowing 40 kph more than the previous ones).
The fuselage was elegant allowing a good visibility to the pilot, the bomber and the gunner.
The performances were outstanding for its time, achieving, with a bomb load of 1350 kg and all fuel, 502 kph at 5100 m and 640 kph IAS in very shallow dive (the finish of the tested bomber was particularly good, the others flying at 495 kph).
That was 25 kph faster than a Ju 88, 55 kph faster than a He 111, etc.
The climb to 5000 m in 14 minutes displayed that this bomber was a bit too heavy.
The reason for that was the suggestion made by Jacques Lecarme, a skilled engineer and test pilot who thought that a bomber could be used as a fighter (!).
The flying qualities were very good in flight, with an outstanding manoeuvrability.
A big take off concern!
Nevertheless, some problems occurred when taking off.
The origins of these problems were in the weight excess, in the too small fins-and-rudders and in the landing gear box.
From the September 3, 1939 to the Mai 10, 1940 (included), 20 crashes destroyed at least one bomber.
So, the losses, without any ennemy action, accounted ~20%.
The test pilots - too skilled to be representative of common operationnal ones (?!) - never anticipated such a behaviour.
Lately, since the March 18, 1940, Mr Jacques Lecarme went in the bomber squadrons to explain the pilots his own take off method.
Nevertheless, there was still some crashes, but post-take off and probably due to air screw problems or excess of weight.
The first 5 LéO 451 were built in 4 months.
And none were ready to fight. From September 1, 1939 and May 10, 1940, about LéO 451 were taken into account by the French Air Force, an average of 12 bombers a month.
It's impossible to see that as a sucessful result of our nationalization.
Even if Jacques Lecarme answered to an article published during the 60's in Aviation Magazine, that the SNCASE company had delivered 450 bombers at the June 24, 1940, only the half part of these aircrafts were taken into account by their operational aircrews.
The efficiency?... only average
Among the 247 LéO delivered for Mars 1st, 1940, only 108 were accepted! So, the May 10, 1940, few pilots and crews were able to use this bomber with the maximum skill.
Ok, this aircraft allowed loops and roll, but that was quite useless.
The losses rate was 31 bombers among the 63 bombers available (49%).
That was the same rate as the one for the Bréguet 693 and similar to the one of the RAF Fairey Battle.
For all the Battle of France, 80 Léo 451 were lost (for any reason) among the 180 used in operational units.
The rate of losses was progressively lower.
Facing the Wehrmacht, the results of such bombing were likely a bit disapointing
That was the hard lesson of the hasty order of an unperfected bomber.
Just after the French defeat, the fins and rudders of all LéO 451 bombers were enlarged with the amazingly very fast agreement of the German commission.
After the invasion of the Vichy France, the Germans captured hundreds of these bombers they transformed as cargo for fuel or as troops transporter, each LéO 451 carrying up to 17 soldiers in their modified bomb bay.
After the French Liberation, the remaining LéOs were used to various tasks (mostly in Sea Air Rescue duties) and the lasts were flying up to 1956.