Hispano-Suiza 12 Ygrs reduction gear ratio 48 / 51

[YavorD]

It is probably an old question but I am not able to find the answer.
The question is what is the reason to produce an engine with a reduction gear ratio of 48 / 51? What is the reason to install such an engine on Morane Saulnier 405 C1 while far more common reduction gear ratio 2:3 was available (HS 12 Ycrs) and a much better option?

A useful link to another thread French WW2 engines.
Notice descriptive et d'utilisation de l'avion Morane-Saulnier 405 C, moteur Hispano-Suiza 12 Ygrs à réducteur 48/51 [BnF]

 

[Bretoal2]

It is probably an old question but I am not able to find the answer.
The question is what is the reason to produce an engine with a reduction gear ratio of 48 / 51? What is the reason to install such an engine on Morane Saulnier 405 C1 while far more common reduction gear ratio 2:3 was available (HS 12 Ycrs) and a much better option?

A useful link to another thread French WW2 engines.
Notice descriptive et d'utilisation de l'avion Morane-Saulnier 405 C, moteur Hispano-Suiza 12 Ygrs à réducteur 48/51 [BnF]

The choice of a small propeller revving at higher rpm does not only have disadvantages compared to a geared one: better ground clearance owing to the smaller diameter, and a lower moment of inertia, reducing gyroscopic effects. Between 1935 and 1940, there were still many direct-drive aircraft engines.

However, in its "moteur-canon" configuration, the Hispano-Suiza 12 Y obviously could not accommodate a direct-drive propeller.

A 1:1 reduction gear would have been a mechanical aberration, since to evenly distribute gear wear, especially when transmitting very high power, it is better to choose numbers of teeth that are relatively prime numbers (coprime integers). Here, 48 and 51 are "almost" comprime integers.

Hence, a choice that is not entirely absurd, even if it has not been confirmed by history.

12Y Grs propeller diameter: 2.60 m. - Diameter of 12Y Crs / 12Y-31: 3.00 m.

 

[YavorD]

Ground clearance can hardly be a factor for an aircraft which can accommodate 800 kg and something engine-propeller installation. The diameter of 3 meters is good for Morane-Saulnier 405 and 406. Brewster also was able to accommodate such a propeller on F2A-2/3. What is killing the "direct drive" idea is the propeller tip speed. For a high-speed dash a MS 405 geared 48 / 51, the tip speed is supersonic between 2000 and 6000 meters. The larger propeller geared 2/ 3 down does not experience such a problem. Looks like this is the most important factor causing the difference in maximum speed: about 450 km/h for MS 205 C1 48 / 51 and 470 ... 480 km/h on MS 205 C1 with Hispano-Suiza 12 Ycrs (or MS 206 C1 , HS 12 Y-31). My calculations show the difference in propeller efficiency of 5 to 7 % in favour of the larger propeller.

 

[Bretoal2]

Ground clearance can hardly be a factor for an aircraft which can accommodate 800 kg and something engine-propeller installation. The diameter of 3 meters is good for Morane-Saulnier 405 and 406. Brewster also was able to accommodate such a propeller on F2A-2/3. What is killing the "direct drive" idea is the propeller tip speed. For a high-speed dash a MS 405 geared 48 / 51, the tip speed is supersonic between 2000 and 6000 meters. The larger propeller geared 2/ 3 down does not experience such a problem. Looks like this is the most important factor causing the difference in maximum speed: about 450 km/h for MS 205 C1 48 / 51 and 470 ... 480 km/h on MS 205 C1 with Hispano-Suiza 12 Ycrs (or MS 206 C1 , HS 12 Y-31). My calculations show the difference in propeller efficiency of 5 to 7 % in favour of the larger propeller.

Certainly. But perhaps it is not necessary to dwell on a test that came to nothing.

In 1938, the Morane 405 was re-equipped with a 12Y with a standard reduction gear, and the 12Y Grs was put on a test bench... for the development of the Planiol-Szydlowski S39 H3 supercharger.

 

[YavorD]

But perhaps it is not necessary to dwell on a test that came to nothing.

Thank You! May be it was not so obvious at the time the crankshaft speed is going up towards 3000 rpm and the direct-drive is not an option for 500 km/h aircraft