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HagarTheHorrible

Engine brake

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I posted this in "SE5a Woes" but thought it maybe deserved a wider audience, if not it's very own thread.  It all started with Larners "SPAD Dive Capability" thread, asking myself why an aircraft running a geared prop didn't over rev an engine quicker than a direct drive engine when it dived down.

 

If you cast your minds back to when RoF first came out and the SE5a was introduced into the game it came with a glass engine, that quickly overreved in a dive.  The developers revised the flight model so that this overreving wouldn't happen so quickly. I don't know what they did and I can't remember what effect it had on normal engine running rpm's or aircraft performance.

 

Yesterday i was reading the "Haines" manual for the SE5a and on page 29 it talks about "An engine decelerator was also fitted for emergency use".  That got me thinking about how they did that which in turn lead me to thinking about "engine braking". This can be achieved by restricting the flow of air to the cylinder heads, this slows the piston from moving in a downward stroke (put your finger over the open end of a syringe, sans needle, and then try to pull the plunger out).  This in turn got me thinking that part of the problem with overreving might be an unrestricted amount of air getting to the cylinders under normal working conditions. 

 

One of the first things to break in the FC aircraft, usually, when trying to dive fast, is the engine.  Maybe there should be more of an engine braking effect, either within the normal airflow capacity to the cylinder heads or through means of a seperate flow restrictor, if historically fitted.

 

I just couldn't quite square the circle between annecdotes of pilots fast diving their aircraft during WW1 and the climb in rev's that we get in FC when we try and do the same, in the past I had only ever thought in terms of propeller drag when considering limitations to engine rpm.

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I don't think I'm technically-minded enough to contribute any meaningful answer...but an interesting topic nonetheless!  

It is quite curious to think about the way some engines tend to break in FC, vs. pilot accounts of "Diving with the engine full on", etc, etc. So far, the most durable engine in a dive in FC (as far as I see) is the geared HS 8Ba, like you point out, which can do a max of (IIRC) 86% throttle without failing at any attainable speed by the aircraft it powers. The big contrast is the D.IIIa engine, which smashes itself even at 0% throttle if you dive for too long! 

Interested to see where this thread goes...! 

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9 hours ago, US93_Larner said:

I don't think I'm technically-minded enough to contribute any meaningful answer...but an interesting topic nonetheless!  

It is quite curious to think about the way some engines tend to break in FC, vs. pilot accounts of "Diving with the engine full on", etc, etc. So far, the most durable engine in a dive in FC (as far as I see) is the geared HS 8Ba, like you point out, which can do a max of (IIRC) 86% throttle without failing at any attainable speed by the aircraft it powers. The big contrast is the D.IIIa engine, which smashes itself even at 0% throttle if you dive for too long! 

Interested to see where this thread goes...! 

 

 

Indeed, it does seem strange that aircraft, who’s main tactic, was a diving attack, or diving escape, such as the Albatros and Pflaz D.IIIa, should have such a high gain in rpm in a dive ?

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I’d like to know more about this ‘engine decelerator’ fitted to the SE5a.   I’m not sure how you could restrict air passing through the cylinder head of a turning engine.  You could certainly do that by restricting the inlet but air would be drawn in through the exhaust valves.

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Posted (edited)
3 hours ago, DD_Arthur said:

I’d like to know more about this ‘engine decelerator’ fitted to the SE5a.   I’m not sure how you could restrict air passing through the cylinder head of a turning engine.  You could certainly do that by restricting the inlet but air would be drawn in through the exhaust valves.


When your sucking, you’re not blowing, any girl can tell you that.  When your sucking in air fuel mix the exhaust valve is closed is it not.  I watched a YT video explaining engine braking and why you should use it (trucks).   When I think about it, the vast majority of braking that I do when driving my farm loader/handler is done with the engine brake, I hardly ever touch the actual brakes.

Edited by HagarTheHorrible

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Er yes. I am familiar with the working of internal combustion engines.  I'm sure too that you are aware that nature abhors a vacuum. Think about it.

 

Also, engine braking works on a road vehicle due to the resistance of the wheels on the road surface acting as a brake on the reciprocating parts of the engine.  Is this applicable to an aero engine driving (or not driving) a fixed-pitch propeller through......a gas?      

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Posted (edited)

What is Engine Braking and Why you Should do it

 

"In standard petrol (gasoline) engines, engine braking works by restricting airflow (by releasing the accelerator) which causes a high manifold vacuum that the cylinders have to work against.

This has the effect of sapping energy from the engine which is what gives that sudden sense of deceleration and drop in power.

While some of the braking force is due to friction in the drive train, the majority is caused by the manifold vacuum created by the lack of air."

Edited by HagarTheHorrible
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I stand corrected👍. I always thought the majority of engine braking was down to friction loss through the transmission - and I’ve built engines, lol!  Still very intrigued how this ‘engine decelerator’ worked?

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I suppoose a question that might be asked is; 

 

Does an in-line aircraft engine, with a fixed pitch propeller, restrict airflow, as well as petrol, to reduce power, or is it all down to the petrol supply?  Is the propeller completely ungoverned ?

 

Presumably, governing rpm through restricted airflow wasn't unknown to the engine designers of the period as it is essentially part and parcel of how a rotary engine is governed (Block Tube).

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