J2M Raiden combat history

[DaveBB]

You can change the compression ratio just by changing the timing.

[lyric1]

https://www.scribd.com/document/133052864/Mitsubishi-J2M-Raiden

[lyric1]

[morfiend]

You can change the compression ratio just by changing the timing.

  I'm unsure how this could happen?

  I always thought the compression ratio was the difference in volume at bottom dead center compared to the volume at top dead center and was a fixed ratio given the difference. It can be changed by either increasing/decreasing the volume at either TDC or BDC but not sure how adjusting the ignition timing could effect it.

  Now there are variable compression engines but these werent used in any airplane that I know of.



   

[DurrD]

Changing the level of boost in a forced induction engine effectively changes the compression ratio.  This is something I haven't seen anyone mention yet.  It is true that high octane fuel in a car that doesn't require it has no real effect on increasing performance, in a plane of these types where you can control manifold pressure different dynamics are in play.  I presume that when using the lower octane fuels, even if the aircraft were not artificially limited on the ground by the maintainers to prevent higher MP settings, the pilots would be forced to when they started getting detonation.  That said, you could probably get away with it for a short time in an emergency I would think, albeit with the potential for engine damage?

[colmbo]

  I'm unsure how this could happen?

I don't believe it could happen.

Nor does changing boost alter the compression ratio.

[MiloMorai]

If increasing the boost changes the c-r then why were Merlin engines always 6:1 c-r when the boost went from 6lb to 25lb boost?

[Hajo]

Before you boost anything you have to have the pistons and other internals be able to hold up to the boost.   Timing would have to be changed to allow the valves to operate to wider/longer.  Lift and duration.  Or valve size would have to be changed.  Designing an engine from the get go with planed boost is a piece of cake. However if you have insufficient fuel quality it's not going to make much of a difference.  Heads, pistons and timing the key.  Along with other forged internals.   Just an opinion, with a Raiden over 7000 lbs, a small laminar flow wing. I would say I would have to call BS on the climb rate with the power plant that didn't compare to an R2800.  Just say'in.

[Devil 505]

Just an opinion, with a Raiden over 7000 lbs, a small laminar flow wing. I would say I would have to call BS on the climb rate with the power plant that didn't compare to an R2800.  Just say'in.

The wing loading is the only issue with the J2M. It's weight and engine power are marginally better than a 109K-4. The wing is the only bit holding it back - expect horizontal acceleration to be at K-4 levels.

[Krusty]

The TAIC teams only had 2 flights in it, after which the engine died and they dead sticked it. During those two flights the engine ran rough and it is noted that not only was the engine "brought back to operation" for the tests but that the prop was swapped out from a hydraulic to an electric. This to me sounds like significant work was done and it was not operated in any way like the actual Japanese units would have run it.

This has been my point all along: You can't trust the TAIC reports in terms of performance. Especially when they only flew it up to 325mph (or so, they don't mention top speed, just when the controls start getting hard to manage) and didn't even try to push it to its limit. I doubt they could have considering the notes on engine performance.

Given this, even with the TAIC report they give it 370mph top speed at 5400m, which is slower than a Bf109G-6 at the same altitude.

Do NOT expect K-4 acceleration, is what I'm getting from all of this.

[bustr]

At the time of the report, why was the number 325mph so important for pilots reading to know versus ever mentioning higher speeds by number?

[Devil 505]

The TAIC teams only had 2 flights in it, after which the engine died and they dead sticked it. During those two flights the engine ran rough and it is noted that not only was the engine "brought back to operation" for the tests but that the prop was swapped out from a hydraulic to an electric. This to me sounds like significant work was done and it was not operated in any way like the actual Japanese units would have run it.

This has been my point all along: You can't trust the TAIC reports in terms of performance. Especially when they only flew it up to 325mph (or so, they don't mention top speed, just when the controls start getting hard to manage) and didn't even try to push it to its limit. I doubt they could have considering the notes on engine performance.

Given this, even with the TAIC report they give it 370mph top speed at 5400m, which is slower than a Bf109G-6 at the same altitude.

Do NOT expect K-4 acceleration, is what I'm getting from all of this.

So where did TAIC come up with the 1,850 hp figure, then?

My comparison to the K-4 is based solely on power-to-weight, with the J2M being better in this regard. The fuselage, which seems to be higher drag, would probably make the K-4 the better accelerating plane, but the J2M should be in the ballpark based on the published data.

[Krusty]

They probably quoted what they already had on-file or from captured pilots' handbooks about the engine, which was used in a lot of aircraft. However, as you know horsepower doesn't denote actual thrust sometimes. It took a special flat wide wooden prop blade to make effective use of the Fw190D-9's horsepower. Many engines have had prop/governor/gearbox issues which lessened their effective horsepower output. Or, even just the exhaust pipe routing can strip a lot of horsepower or add supplemental thrust, depending on the setup.

Also that 1800 hp was only a very short emergency takeoff setting and dropped to 1600 at a couple thousand meters and change and dropped to 1500 at FTH of 5000 meters and change. I think the overall design balance of the K-4s prop, engine, governing controllers, drag, weight, and all that will make it far and away a better accelerator. IMO engine problems, stalling engines, failing fuel sources and problematic prop extension shafts tend to make something not accelerate like you want when you pour on the gas. Off the top of my head I can't think of one "hot-rod" super accelerator that had as many engine problems as the Raiden.

[FBKampfer]

Regarding compression ratio and boost pressures.

You can't alter an engine's compression ratio without changing pistons or heads. Period, stop, end of story. This is a fixed number, inherent to the engine, much like (and directly related to) bore/stroke figures.

However, it isn't the ratio that causes detonation. You can take gasoline and artificially lower its octane to 10, and inject it into a 1:400 CR engine at 1/400 atm, and suffer no ill effects whatsoever, since you're not compressing the fuel mixture beyond atmospheric pressure.


By this same measure, you can take an engine and fuel pairing that run perfectly fine under atmospheric pressure, and induce detonation by simply putting it under forced induction.


Remember, the only thing that matters to the fuel is the total pressure it's under. This is the only thing the engine can do that affects the fluid properties, and thus how a reaction (ignition) propagates through the medium. The ratio between TDC and BDC doesn't matter a lick, what matters is the pressure at time of detonation. This, however, can be altered by changing the MAP and ignition timing, without affecting CR in any way.

The only way CR would need to be changed to detune an engine would be if your fuel is such crap, and your CR is so high, that you're getting compression ignition regardless of the spark plug timing.

[morfiend]

 I think you may have misread what I said.  CR is calculated by the difference in volume at TDC and BDC,so if you have a 100cc volume at BDC and a 10CC volume at TDC you get a 10:1 CR....


  Just a side note,the BMW powered DviiF used and engine that would destroy itself it you used full MAP at SL,it had 2 throttles,the usual one and an altitude throttle that once you were above about 5000m you could safely open it to full.
 The BMW engine had an unheard of CR of about 6.5:1!  This combined with the fuels of the day would cause the engine to disassemble itself if you used WOT for both throttles,our Dvii must be a Merc powered model as it only has the single throttle.



     I personally have destroyed several small 2 stroke engines....   Usually from running them too lean.