Why geared engines ?

[HagarTheHorrible]

There were engines that spun slowly (1400), there were engines that spun fast (2100) and there were engines than spun fast, but drove slowly (2100/ 1170), but why have a fast engine that drove slowly ?

 

The Hisso engine that powered the Spad XIII was geared to turn the prop at 1500 rpm despite the engine running at 2100 rpm while the Dolphin, that used the same engine, was supposed to have gearing that turned a four blade prop at 1170, but depending on engine producer, might actually have a gearing ratio that turned the prop (2 bladed) at either 1333 or 1500.  

 

Despite, not insignificant,“teething” troubles the Allies continued to use geared engines, the question I have is, what were the advantages of a geared engine, that encouraged the Allies to persist with them despite reliability concerns and the potential option of direct drive options ?  

 

I suppose I might also ask, why specify an engine that has an rpm of 1400 or 2000 ?  Was it simply a case of “that”s what we have available” and it would take too long to develop an entirely new engine ?

[BB5000]

Maybe the engine torque wasn't enough to spin that prop at higher revs ?

[J2_Trupobaw]

My guess: engine revolving at given RPM does not mean it has horsepower to rotate any prop you want at this RPM. Gearing means you can use engine with prop pitch you want, rather than pitch that fits engine power at "native" RPM. With high engine revs and relatively low power, they would be limited to very fine pitched props, no?

[NO.20_Krispy_Duck]

Prop speed varies in efficiency depending on how fast the prop is turning. A faster prop is not necessarily a more efficient prop. There's a sweet spot of efficiency, which depends on pitch, speed, and what the driving engine is doing. So you use reduction gear to "tune" the prop and engine to get a particular prop speed and efficiency. You can use direct drive, but as the engines became capable of higher and higher RPMs, you started to drive the prop to a point where it became increasingly inefficient.

 

Part of why they persisted is that the design behind the Hispano Suiza 8B figures in reduction gearing, 0.75:1, if I recall rightly. But reduction gearing can involve a great deal of stress on the reduction mechanism. Your design, your tolerances, and your metallurgy all have to be good for reliable, long-lived reduction gearing. Times were still early in 1917-18, and the reduction gearing on the Hispano Suiza 8B broke down to the point that many pilots were resorting back to 180hp Spad VII aircraft, with a direct drive 8Ab engine. Others resorted back to Nieuports.

 

We don't deal with any of this in FC, RoF, or WOFF - the problem that your Spad XIII might be grounded (yet again) with a broken reduction gear. The mechanics of 1917-18 must have had plenty of work fixing those broken reduction gear systems.

[No.23_Triggers]

3 hours ago, Krispy_Duck said:

or WOFF

WoFF does have 'random' engine failures - but I don't think they deal in specifics (although apparently you can randomly catch fire! Never happened to me though). I don't know if it's more likely for more 'unreliable' engines but it certainly made for interesting gameplay when your engine cut just before a big scrap (or in one!!!) 

Edited June 7, 2020 by US93_Larner

[NO.20_Krispy_Duck]

I was under the impression it was a random failure element and not linked specifically to historical reliability stats. I could be mistaken. I do know the random failures feature adds a little something extra to career mode certainly.

[Chill31]

Gearing allows you to have more torque with accompanying power increases.

 

Let's say they did direct drive on the same propeller that they were gearing. If the engine and prop turn at 1400 rpm, the engine may only produce 200 hp. If they gear the motor, let it rev up and get a little more torque and power out of it, they might get 1700 rpm out of the same propeller. Translates to better performance!

[US103_Baer]

From memory for Hispano Suiza Type 35s.

 

2000/1500 for scouts

2000/1333 for twin-engined aircraft