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Dear Bletchley (and anyone else who might know),

 

As you know, I have been working on FM improvements for several OFF planes. One thing I noticed in the stock FM's is that while most planes have a reduction gear setting of 1.00, several have reduction gear settings less than 1.00. My understanding is that a reduction gear is used to reduce the high rpm's of the engine so that the propeller rotates at a slower and more efficient rpm. My question is: did any WWI engines or planes utilize reduction gearing? My guess is most, if not all, did not, with the possible exception of the Spads and the Fokker DVIIF.

 

I'm not really sure that the reduction gear setting actually has any effect in OFF, but I am thinking of setting it to 1.00 for every plane to "level the playing field" unless I can find any evidence of the use of a reduction gear in WWI aircraft.

 

Thanks for your help. I also plan to look in the forums over at the aerodrome site to see if they might have the answer.

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Bletchley would be the guy - but Se5a HS for example has reduction gear, as per OFF's version. Hence problems until they got the Viper engine with direct gear (hence 1.0) for example (note the prop changes direction from models which is a clue). Several aircraft used gear including French craft etc such as SPADs as in OFF. Not that is is noticeable much. From what I saw it can affect some things such as the spinning of the prop on landing, spin speed of the prop etc but other factors can too.

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It's scary how much you guys know about this stuff. Every last little detail that you'd think was long lost to history. I can't imagine how many details will be in OFF 2 that make it historically accurate that most folks (like me) wouldn't ever notice but you put it in there just because that's how it was.

 

Hellshade

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Yes, several WWI aero engines were geared: the RAF1a (BE2) and, I think, also the RAF4a (RE8)and Rolls Royce Eagle. And the German 8-cylinder Daimler Mercedes D.IV.

 

Bletchley

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Yes, several WWI aero engines were geared: the RAF1a (BE2) and, I think, also the RAF4a (RE8)and Rolls Royce Eagle. And the German 8-cylinder Daimler Mercedes D.IV.

 

Bletchley

Not wishing to go off topic here, but have been seriously exploring building a replica...I'm very far from an expert, but from what I've been able to determine, reduction gearing offers the following benefits with a fixed prop:

  • As a propeller tip speed increases the efficiency decreases.

  • The larger the diameter of the propeller the more effective disk area it has.

  • To increase disk area is to increase thrust.

  • It is common practice to decrease the diameter of the propeller to increase its rpm to get more horsepower from the engine.

  • As a general rule of thumb if you increase the engines rpm that will increase its horsepower.

  • If you reduce that rpm by half thru a reduction unit you will double the torque output of the engine.

  • Horsepower is not the determining factor for sizing a propeller.

  • Torque is the determining factor for sizing a propeller.

  • It is more efficient to go with the larger diameter propeller at a lower rpm.

  • If you increase a propellers diameter from 60" to 84" it will increase its effective disk area by 96%.

  • Any increase in the props effective disk area will increase its output.

The big question: if the above is true, why didn't more planes utilize it? From what I can determine, they sized the prop to maximize thrust, based on the torque provided by the engine. Was gear reduction simply a cheap and easy way to maximize the use of their existing inventory?

 

Edited by BirdDogICT

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Thank you for the replies, gentlemen. The more I play around with the FM, the more I learn and the more mysterious it gets! It was only recently that I started to notice the relationship between prop diameter and reduction gearing. In OFF, as in real life I assume, a smaller diameter prop gives more speed, but perhaps less torque. In general, it appears the Allies favored slightly smaller diameter props, while the Germans favored somewhat larger diameter propellers. This seems to allow most Allied planes to have more speed, but the German's better climbing ability.

 

As stated by Pol and Bletchley, only a few planes in OFF utilize reduction gearing. Up until a few weeks ago, however, many of the planes in my FM mod were also "using" a reduction gear. I had to therefore go back into the mod and eliminate the gear in planes that did not historically have it. And, of course, that meant I had to retest all of the planes that were changed! That took awhile, but now I think I can actually see some light at the end of the FM tunnel.

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I believe there are also some disadvantages to having a geared engine, BirdDog - there is some loss from the gearing (but not much), some extra weight, but I think most important for these early aero engines there were often issues of reliability (as Pol mentioned with the HS).

 

B.

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I believe there are also some disadvantages to having a geared engine, BirdDog - there is some loss from the gearing (but not much), some extra weight, but I think most important for these early aero engines there were often issues of reliability (as Pol mentioned with the HS).

 

B.

Gearing seems to be more common with modified VW engines, something I'm not interested in. Leaning more toward a 9-cylinder Rotec engine.

I need to take my time in planning this purchase...haven't even completed the Spousal Approval Certificate yet. I'm pretty certain that any reputable replica kit supplier will require one.:grin: May take more time than the FAA approvals.

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In a german mechanics book from 1920, i have just read that the WW1 "Renaultmotor" with 8 and with 12 cylinders (both V-arrangement) had an elongated camshaft as a propellorshaft, so the propellor turned with half of the revolutions of the engine, making a geared drive superfluous.

Did not know that before, which engine was that, exactly ? Could it by any chance be the one used in the RE8 ?

Thanks and greetings,

Wels/Catfish

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It is not the length of the camshaft, but that the camshat itself is being used to fix the propellor on - the camshaft turns 1/2 of the engine's crankshaft revolutions. The elongated camshaft is just protruding out of the engine block to mount the propellor on it - usually the crankshaft sticks out like in every car or usual engine, but not on this one.

For 4 cycle engines, no matter how many cylinders, the crankshaft turns twice for every turn of the camshaft, so fixing the prop to the camshaft is a good idea to reduce (indeed halve) prop revolutions, without using a gear. Engine 3000 revs, fix prop on camshaft = propellor 1500 revs.

I am just not sure about the exact engine of Renault the german book is speaking about. There's a bad photo though -

 

Thanks and greetings,

Wels

Edited by Wels

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