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View Full Version : Oleg was right, and I was (choke) wrong.



XyZspineZyX
11-07-2003, 05:51 AM
Oleg:

I extend to you my humblest apologies. I now think you were right about the P-51D. I now too believe the radiator air exit shutter was closed when it achieved it's top speed of 437 mph.

I argued that the P-51D could not have possibly accelerated to 437 mph on combat power with the radiator air exit shutter closed without over-heating. As you know, the P-51D achieved 437 mph at a little under 25,000 feet. And if the Mustang was running on true combat power at that altitude, I would maintain my position. But it really wasn't running at true combat power at 25,000 feet. In reality, it was simply running at full throttle.

Combat Power in the P-51D was defined as a manifold pressure of 67" of mercury. And 67" of mercury can be maintained by the P-51D up to about 19,000 feet. At that point, manifold pressure begins to drop off dramatically. At 25,000 feet, manifold pressure is down to about 45" hg.

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Well, 45" hg is not Combat Power. In fact, it's is a little less than Normal Power. At 25,000 feet, at full throttle, the P-51D could develope at best Normal Power. And I don't think the P-51D would have necessarily overheated at normal power at 25,000 feet with the radiator door shut.

So, I grovel for forgiveness.

You were right, and I was wrong (I think).





Regards,

SkyChimp

http://members.cox.net/rowlandparks/sigstang.jpg

XyZspineZyX
11-07-2003, 05:51 AM
Oleg:

I extend to you my humblest apologies. I now think you were right about the P-51D. I now too believe the radiator air exit shutter was closed when it achieved it's top speed of 437 mph.

I argued that the P-51D could not have possibly accelerated to 437 mph on combat power with the radiator air exit shutter closed without over-heating. As you know, the P-51D achieved 437 mph at a little under 25,000 feet. And if the Mustang was running on true combat power at that altitude, I would maintain my position. But it really wasn't running at true combat power at 25,000 feet. In reality, it was simply running at full throttle.

Combat Power in the P-51D was defined as a manifold pressure of 67" of mercury. And 67" of mercury can be maintained by the P-51D up to about 19,000 feet. At that point, manifold pressure begins to drop off dramatically. At 25,000 feet, manifold pressure is down to about 45" hg.

http://members.cox.net/rowlandparks/dash3_1.jpg


Well, 45" hg is not Combat Power. In fact, it's is a little less than Normal Power. At 25,000 feet, at full throttle, the P-51D could develope at best Normal Power. And I don't think the P-51D would have necessarily overheated at normal power at 25,000 feet with the radiator door shut.

So, I grovel for forgiveness.

You were right, and I was wrong (I think).





Regards,

SkyChimp

http://members.cox.net/rowlandparks/sigstang.jpg

XyZspineZyX
11-07-2003, 05:54 AM
WOW! We lived to see it!

Seriously good on you Air primate! http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif

XyZspineZyX
11-07-2003, 05:57 AM
But you know what this might mean don't you?

If the Mustang was developing Normal (Maximum Continuous) Power or less from 25,000 feet on up, then at 25,000 feet on up you should be able to stay at full throttle without the coolant overheating.

What say, Oleg?

Regards,

SkyChimp

http://members.cox.net/rowlandparks/sigstang.jpg

XyZspineZyX
11-07-2003, 06:00 AM
I see you posed a logic problem for your usual post-nemeses, right Chimp? LOL!


Neal

XyZspineZyX
11-07-2003, 06:08 AM
WWMaxGunz wrote:
- I see you posed a logic problem for your usual
- post-nemeses, right Chimp? LOL!
-
-
- Neal


And without question, he'll attempt to tackle it. /i/smilies/16x16_smiley-very-happy.gif



Regards,

SkyChimp

http://members.cox.net/rowlandparks/sigstang.jpg

XyZspineZyX
11-07-2003, 06:40 AM
Why wouldn't it overheat? It's at maximum combat power. If the chart were for operation without overheating, wouldn't it say max continuous at the top?

I'm out of my element here, just seems against common sense.

--AKD

http://www.flyingpug.com/pugline2.jpg

XyZspineZyX
11-07-2003, 06:53 AM
Like chimp is saying, it's not a maximum combat power, just full throttle, and the engine is producing significantly less power. Much less fuel being burned also, so proportionately less heat produced.

Barfly
Staffelkapitan
7./JG 77 "Black Eagles"

http://www.7jg77.com

XyZspineZyX
11-07-2003, 07:16 AM
Panelboy wrote:
- Like chimp is saying, it's not a maximum combat
- power, just full throttle, and the engine is
- producing significantly less power. Much less fuel
- being burned also, so proportionately less heat
- produced.
-
- Barfly
- Staffelkapitan
- 7./JG 77 "Black Eagles"
-

This is where I'm a bit confused, because that chart says in very clear letters "maximum combat power settings."

I understand the engine cannot continue to maintain the maximum boost pressure above its critical altitude, but why does this negate the heat the engine is producing when operating at its max limits?

--AKD

http://www.flyingpug.com/pugline2.jpg


Message Edited on 11/07/0306:29AM by A.K.Davis

XyZspineZyX
11-07-2003, 08:41 AM
At altitude, would the radiator not become grossly less effective as the volume of air passing through it is far less? Therefore less efficient colling... therefore more heating of the airplane?

XyZspineZyX
11-07-2003, 09:15 AM
I think that one key element Herr SkyChimp is missing, is that, regardless of what the engine's supercharger is able to provide at 25,000 feet, if you run around with the throttle wide open, you're dumping a bunch of fuel into that engine, and it's getting ignited! It will definitely heat up trying to do what you're asking it to, even though it's only able to produce "normal" power. At 25,000 feet, it's working it's little squirels to death just to get that 45 inches!! You're not gettin' something for nothin' http://ubbxforums.ubi.com/infopop/emoticons/icon_wink.gif

XyZspineZyX
11-07-2003, 09:34 AM
Sure SkyChimp, after I sink time into my take on an explaination on the original thread, you go and post this new one. Seriously, thanks for this post. I hadn't caught the power difference in the chart the first time you posted it.

Waldo.Pepper, you have a point. At 25,000 feet, the air is less dense, and can absorb less heat from the radiator as a result (compared to denser air at lower altitude, both compared at the same velocity and shutter settings.) The engine's producing less heat than at full throttle at sea level, but because of forced induction, it's still producing a large percentage the heat it would at its Combat setting in denser air. The cooling system has no such forced induction to increase air density (and that would only lead to other thermodynamic problems, anyway), so it looses cooling efficiency faster than the Merlin looses its capacity for generating heat (as Razor said.)

The engine will probably still overheat for this combination of altitude/power/speed, but I'm guessing it will overheat more slowly than at sea level. And until the coolant temp gets above 90 deg. C., the shutter is closed and the Meredith effect is in full force.

Blotto

"Only the spirit of attack, born in a brave heart, will bring success to any fighter craft, no matter how technically advanced." - A. Galland

"Look, do you want the jets, or would you rather I slap the props back on?" - W. Messerschmitt

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XyZspineZyX
11-07-2003, 09:56 AM
I don't think you'll see a significant difference in volumetric efficiency of the engine at altitude even though is still has some forced induction- I'm sure the system is designed to provide an effective fuel/air ratio for combustion. That means, less fuel is provided for the lower volume of air provided by the supercharger to the induction system.

I'm also fairly sure that the throttle is not directly connected to the carburation system - it must go through something similar to a modern fuel control unit, that meters fuel according to manifold pressure etc. (leans out the mixture)

In any case you are producing less power, and certainly proportionately less heat. And although you are travelling through a less dense air mass, doesn't the extremely low temperature improve cooling efficiency somewhat?

Barfly
Staffelkapitan
7./JG 77 "Black Eagles"

http://www.7jg77.com

XyZspineZyX
11-07-2003, 10:03 AM
there are a lot of facts that make engine keep heating even when manifold pressure begin to drop...

but it is obvious (well, it was for me at least http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif ) that the pony have no advantage over any plane in this case. While i can understand how the original discussion began about the thrust effect of the p51 radiator, i don't see where this radiator, or the clever use of the meredith effect would affect it in a way much more beneficial than for a 109, a yak or a P47...

The heat building process should remain similar, and consistent with what we already have in the sim...

<center><img src=http://hoarmurath.free.fr/images/sighoar.jpg></center>

XyZspineZyX
11-07-2003, 10:57 AM
The shutters on the radiator are on the back end, it builds up a backpressure so dammit the pressure is going to be higher inside when the shutter is not fully opened as well as less drag from slower moving air inside.

Less air for the same amount of fuel... the engine is running rich which is used as a cooling method in how many engines?

Wheeeee! All the "FM agendas" operating... if only ignoring pertinent data could be cut and sold, this board would make a profit!


How LONG could the P-51 maintain 100% throttle at 25,000 ft? If not for as long as the gas supply held up then maybe by golly the engine was building up heat? That simple. No need to measure gaps or flows or speculate on squat.


Oryx, I think it was but maybe someone else (like, OLEG) wrote about how near the end of the war the Germans and Russians were experimenting with the Meredith effect and getting airscoops out of the boundary airflows for better efficiency. Yeah, I think is was Oleg, the post is in the Mustang radiator thread.


Neal

XyZspineZyX
11-07-2003, 11:30 AM
It shows the Measure of a man when he can honestly apologise. I have new respect, SkyChimp.

"Tis better to work towards an Impossible Good, rather than a Possible Evil."

SeaFireLIV.
(Spitfire & Escape Whiner Member).

XyZspineZyX
11-07-2003, 12:14 PM
WOW - can I trust my eyes? Respect!

XyZspineZyX
11-07-2003, 01:06 PM
Hmmmmmm, somehow I believe this is not the end of it.

SkyChimp is on to something, he is just hiding his cards for now /i/smilies/16x16_smiley-wink.gif .

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agh burzum-ishi krimpatul

XyZspineZyX
11-07-2003, 03:05 PM
its much colder up there, so that would have a effect

XyZspineZyX
11-07-2003, 03:34 PM
IV_JG51_Razor wrote:
- I think that one key element Herr SkyChimp is
- missing, is that, regardless of what the engine's
- supercharger is able to provide at 25,000 feet, if
- you run around with the throttle wide open, you're
- dumping a bunch of fuel into that engine, and it's
- getting ignited! It will definitely heat up trying
- to do what you're asking it to, even though it's
- only able to produce "normal" power. At 25,000 feet,
- it's working it's little squirels to death just to
- get that 45 inches!! You're not gettin' something
- for nothin' http://ubbxforums.ubi.com/infopop/emoticons/icon_wink.gif
-
-

The thing you are overlooking is that full throttle does not equate to full power.

If you can not reproduce ground-level atmospherics at altitude, the engine never reaches ground-level power output.

Sure, the throttle is crammed into the firewall, but the super-charger can't feed it enough air, and without enough air, you can't add more fuel. At high altitude, you aren't able to make max power, and this equates to less heat.

I think that the radiator flap has to be set to auto to get max speed. The coolant system is 'tuned', so at different air speeds, and at varying air densities, the output flow must be matched to the speed/density of the air.

Look at ram-air on a car... it only works at a specific speed. Same with 'cowl induction'.. you don't see a boost until the car is above a specific speed. Its all about the size/volume of the flow, and in the case of the P51, the opening must be changed with air speed in order to take advantage of Meredith Effect.

<font face="Courier New">

‚ ‚ ‚ ‚ ‚ ‚ _____ | _____
‚ ‚ ‚ ‚ ‚ ‚ ‚ _\__(o)__/_
‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚ ./ \.

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XyZspineZyX
11-07-2003, 04:32 PM
it's colder for all planes, not only for mustangs.

<center><img src=http://hoarmurath.free.fr/images/sighoar.jpg></center>

XyZspineZyX
11-07-2003, 06:10 PM
You can always add more fuel up till you choke the engine. You just won't completely burn it all! It's called running a rich mixture. You run a bit lean to get more efficiency and heat, a bit rich to help cool the engine which everyone did but somehow with the Mustang it should be more fuel = more heat.

Perhaps the Mustangs had automatic mixture controls and full throttle was... ?


Neal

XyZspineZyX
11-07-2003, 06:26 PM
SkyChimp wrote:
- But you know what this might mean don't you?
-
- If the Mustang was developing Normal (Maximum
- Continuous) Power or less from 25,000 feet on up,
- then at 25,000 feet on up you should be able to stay
- at full throttle without the coolant overheating.
-
- What say, Oleg?
-
- Regards,
-
- SkyChimp

I think i can underline this from 1.2 RC coolant stays very cool around 10000 m and 100 % with about 50 degrees C‚? coolant temp 3100 rpm and 42 hg manifold radiator closed.

Regards,
Hyperion

XyZspineZyX
11-08-2003, 02:40 AM
One thing that is worth considering is this:

A Mustang pilot (or any other pilot) had three temperature issues to worry about: oil temperature, coolant temperature and cylinder head temperature.

By far, the one that required immediate action if the temperature went too high was cylinder head temperature.

Excessive oil and coolant temperature can both have a detrimental effect on the engine, but unless there is system failure, such as from combat damage, the onset and effects are gradual.

Excessive cylinder head temperature effects can be almost immediate. Excessive cylinder head temperature leads to detonation. And a few seconds of detonation and literally destroy an engine.

The time limits given in pilot's handbooks for a given power settings - such as 5 minutes for Combat Power or 15 minutes for Military Power - were given primarily out of concern about cylinder head temperature, not coolant temperature.

Exceeding these time limits could have led to excessive cylinder head temperature, detonation, and near immediate engine destruction. A substantial safety margin was built into the time limits.

It was possible to overheat the cylinder head without overheating the coolant, and vice-versa. Detonation is not modelled in FB, and I think it should be. I'm not sure what Oleg used to determine when a US plane should overheat. But the time it took the coolant to overheat is not necessarily commensurate with the time limits given in pilots manuals.

Just some food for thought.

Regards,

SkyChimp

http://members.cox.net/rowlandparks/sigstang.jpg

XyZspineZyX
11-08-2003, 04:24 AM
Thanks SkyChimp. Interesting. But I hope Oleg doesn't bother with such modelling before working over the air war environment for FB or FBoB...especially AI. Its a disaster and no amount of detailed engine modelling will help combat gameplay. Now a pure test pilot sim is another story, one with a smaller market maybe. But it would still be Awsum.

XyZspineZyX
11-09-2003, 09:07 PM
Bump

Regards,

SkyChimp

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XyZspineZyX
11-10-2003, 04:35 AM
Bumparoo2

Regards,

SkyChimp

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