vendredi 24 février 2012

Nieuport 161: 09-22-1936, the day at which France had lost the mastery of the 1940 sky (revised 01/ 05 / 2014)

A fatal crash

The Nieuport 161 was rejected after a lethal crash occurred the September 22, 1936, which killed her test pilot, captain-engineer  Coffinet.

For the eye-witness, the crash seemed not linked to any misconception of the fighter.

Nevertheless, all the following events were running as if the crash was the consequence of some faulty conception.

The order of 30 planes (already on the desk of Pierre Cot, then Ministre de l'Air) was a little after forgotten...

A then-official theory for the crash

The only published analysis was the one that General-Engineer Louis Bonte and Jacques Lecarme have written in their book Histoire des essais en vol (Docavia #3, Larivière, 1974). 

It was quietly recopied since that day in every paper on that fighter. 

However, as every scientist, I never accepted to be a "quiet" man, and, because the Morane 406 was hopelessly outclassed by the Bf 109 E, I was interested on the French fighter contest programmed in 1934.

The official theory of Mr Bonte was the crash was due "to the cooling device, which induced - during some particular flying pattern - a stall affecting all the center part of the wings, with a steep slope at 15 to 20 m/s vertical speed."

It exist no historical document supporting the Bonte thesis. I've red an undated and not authenticated document which was clearly written after 1945 in which this thesis appear. But this document never explain the aerodynamic experiences allowing such conclusion.

So, such a final judgement needs, at least, some discussion.

The cooling device of the Nieuport was an original feature of the Nieuport Team.

It has been fitted in each wing, near the fuselage. 

The air intakes were beneath each wing (almost at the mid-chord) and the exit way for the hot air was on the upper side of the wing just in front of the flaps (in order to benefit from the depression created by the air-stream existing on the upper surface of the wing). 

This device was demonstrated in some handbook for French students in aeronautical schools in the early 60's, without any adverse warning.

Document of the author - Left wing of the Nieuport 161 - The cooling device exit is the little black stain on the right part of the wing

I was never convinced by the Bonte's theory  

Several facts are clearly against such a theory.

Even if it would be experimentally proven, it could be possible to fix such a bug by displacing the cooling device beneath the center fuselage as in the Hurricane, the Avia 534 or the Mureaux 170.

It was also possible to put wing fences to limit the stalled zone (if it really existed) only to the cooling device zone itself.

Moreover, two years after the crash, the same Nieuport team developed successfully a family of fighter-bombers to be used on the French aircraft carriers: The Loire-Nieuport 40, 41, 401, 402 and 411.

They were straight forward derivatives of the N 161,  sharing her fuselage and... her cooling device.

All of them were operationally used and fought gallantly with the Aéronavale in May 1940, operating dive bombing as also fighting duties (they accounted one victory on a German bomber). 

Moreover, they have also recorded several landing on the Béarn carrier without displaying any problem.

To my knowledge, such tests on an aircraft carrier were be seen everywhere as the best method to detect any loss of lift!

Another point against the Bonte's theory is given by the Dewoitine D.520-01 fighter prototype. 

The October 2, 1938, when this completely different fighter flew for the first time, she used a very similar cooling device and her test pilot Marcel Doret was very satisfied by her flying qualities which he already evaluated as outstanding.

A good picture may be seen on the very good site of Joseph Bibert. You can see the Dewoitine 520-01 prototype displaying, just to the left of her left Karman, the exit of her left radiator. 

Compared with the Nieuport 161 fitting, this exit was clearly more up-stream.

This cooling device was discarded several flights later, only owing to its poor cooling performances.

Bonte's theory origin

OK, you may now wonder: What would be the Bonte's theory origin?

I've found it while reading a Docavia Book on the Avion Bernard company which developed some very advanced aircrafts for their times, even an absolute World Air Speed record breaker in 1924. 

None of these aircrafts were issued into production because that company was only a system to wash money or funds... 

As usual in such a case, the high staff of the company said to the engineers that the problem was the Nieuport concurrent which have got dishonestly several orders...

These engineers provided Louis Bonte the theory stemming from one of their own experiment: They developed a civilian plane, the Bernard 200 T (T for tourism) in the early 30's. 
From the beginning, this plane displayed abnormal difficulties to take off and, thereafter, to climb. 
The reason lied in the wing structure which can be folded in 2 orthogonal directions, the first one, classical, parallel to the longitudinal axis of the plane, the second one orthogonal to the previous one and parallel to the ailerons.

Personal document of the author - Starboard wing of the Bernard 200 T -
Axis 2 generated  the secondary folding device, creating the slot in the wing

Unexpectedly, this second fold opened a 2 cm wide vertical slot between the lower and the upper surface of the wing, destroying a lot of lift. 

After obstructing of the slot with fabric, the plane flew normally.

Comparing the huge slot affecting each wing of the Bernard T 200 with the small exit of the Nieuport cooling device (see at the two previous figures) seems not relevant at all.

It is also difficult to explain an extension of the stall perpendicularly to the airflow over a large distance so close to the trailing edge.

Logically, the air exiting from the Nieuport device was very hot, so it must be accelerated with a small angle to the airflow and might had induced something like a moderate Coanda effect.

If the lift was reduced, even a little, the climbing speed must have been slow, as it was perfectly measured during the Bernard T 200 trials. 

That was not the case for the Nieuport 161 case, this fighter had, by far, the best climbing performances of all French fighters until the trials of the record breaker Dewoitine 550.

So, I hope you are now convinced the Bonte theory is no more relevant.

A more likely crash theory

Now, I want give my personal explanation to the Captain Coffinet crash.

From the paper of Arnaud Prudhomme in Air Magazine, April 2005, the crash occurred during a gun firing trial, while the pilot was recovering after a dive to shot a ground target.

That test followed the idea that fighters will be able to destroy tanks with their guns (the idea was not bad, but a little too early).

Personal collection of the author - Drawing of the attack of an armored column by fighters
in P. Barjot, l'aviation militaire française en 1939, de Gigord ed.

A question arise for us, ~80 years later: In 1935, the D 500 fighters flew, at most, at 365 kph in horizontal fly. 

So, during a dive, they can reach about 450-500 kph. 

Her fixed undercarriage as her fixed pitch air-screw were powerful brakes. 

The pilots had no difficulties to do such a job.

One year later, all the aerodynamic "landscape" was different. 

The new fighters exceeded easily the 700 kph in dive, the best exceeding 800 kph.

The g's number arose from less than 3 to more than 6: The same sketch became lethal. 

Yet, the Nieuport 161 was the fastest and, by far, the most streamlined among all the French fighters.

If Captain Coffinet followed too exactly the scheme of the test, he was in great danger. 

But, he cannot be blamed for the crash.

So, the only persons to be blamed were the men who ordered such a trial without taking care themselves about the performances of the new fighters which fly very faster than the previous ones.

Other prototypes of this period had also suffered from crash, the most famous one being the Messerschmitt Bf 109, but also a lot of diving planes everywhere.

The French Air Forces Staff ordered the Morane 405... and the History of the French defeat began.

A other post displays the assets of the N 161. You can read it by clicking here.

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