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  #101  
Old 05-08-2012, 09:39 PM
K_Freddie K_Freddie is offline
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Probably something to do with engine type/power and propellor type... Just guessing as I know sweet ....all, you know
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  #102  
Old 05-08-2012, 11:27 PM
Whacker Whacker is offline
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Originally Posted by JtD View Post
But it does. The original poster asked in the topics title why there are no dive acceleration differences. There aren't because gravity is the same for all planes. As soon as you consider anything else, the dive accelerations are different. Had he asked why all the dive accelerations are different, the question would have required a much more complex answer.
The answer was specifically given to the question asked.
Dive acceleration is not solely a function of gravity.

One must also account for frictional coefficients and powerplant thrust as they relate to the specific "dive" profile being discussed, as well as each individual aircraft's operational guidelines and parameters. It's not a simple answer by any means.

If anyone has a direct link to that TAIC study report, please post it, I'd be very interested to read it.
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  #103  
Old 05-09-2012, 02:46 AM
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Crumpp Crumpp is offline
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as well as each individual aircraft's operational guidelines and parameters.
Excellent point.
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  #104  
Old 05-09-2012, 04:37 AM
JtD JtD is offline
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Originally Posted by Whacker View Post
Dive acceleration is not solely a function of gravity...
I know. So once more: If someone asks why dive accelerations are the _same_, he doesn't want to know about thrust, drag and whatnotelse. Because these are reasons for _different_ dive accelerations.
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  #105  
Old 05-09-2012, 09:11 AM
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FC99 FC99 is offline
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Originally Posted by Crumpp View Post
Of course it is Fatcat. This is not the issue and completely irrelevant as to why your FM's would not exhibit any differences in dive acceleration. It does not answer the original poster's question.
Who says that there is no difference in dive acceleration. Whole premises in initial question is wrong. There is a difference in dive acceleration.

Quote:
Take the force triangle for a dive. A component of weight contributes to thrust based on the angle of dive. The difference between the force on the axis of motion in the dive and the force on the axis of motion for level flight is your initial excess force that will move the aircraft to its new equilibrium point velocity. The derivative between that and equilibrium is your average excess force along that vector....
You are missing the point here, people want dive acceleration "fixed" as there is some magical switch that is turned on when plane start to dive but there is no such thing. All you have are Thrust, Lift,Drag and Weight acting on a plane no matter what's the plane attitude.

If you change something you will change plane behavior in all flight regimes not only in dive. If you have plane behavior modeled reasonably well in level flight and climb than there is no reason to believe that dive behavior is wrong.

Where game has its problems are extreme parts of flight envelope but that's not what the thread starter asked.


BTW Long ago I made tests and posted it on CWOS but it's lost now. But anybody can repeat it.It's simple.

1. Start the plane at alt above the initial testing point. Use no cockpit view to get TAS and stabilize the plane at desired TAS.

2. Keep the TAS constant and measure the time required to pass from the start altitude to end altitude.

3. Repeat for all planes you want to test, try it with no power and full power.

4. Compare the results.


As the test measure the time required to get from StartAlt to EndAlt it also measure the distance traveled. Test requirement is that TAS is kept constant so difference in time from let's say 4000m to 2000m will mean that planes passed different distances which in turn means that their diving angle was different.

Plane that needed longest to get to EndAlt is the best diving among the tested planes because it needed smallest help from gravity to keep its speed. Consequently it will dive fastest in a dive that is performed at same diving angle.
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  #106  
Old 05-10-2012, 01:39 PM
BlackBerry BlackBerry is offline
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Originally Posted by FC99 View Post
Who says that there is no difference in dive acceleration. Whole premises in initial question is wrong. There is a difference in dive acceleration.

You are missing the point here, people want dive acceleration "fixed" as there is some magical switch that is turned on when plane start to dive but there is no such thing. All you have are Thrust, Lift,Drag and Weight acting on a plane no matter what's the plane attitude.

If you change something you will change plane behavior in all flight regimes not only in dive. If you have plane behavior modeled reasonably well in level flight and climb than there is no reason to believe that dive behavior is wrong.

Where game has its problems are extreme parts of flight envelope but that's not what the thread starter asked.


BTW Long ago I made tests and posted it on CWOS but it's lost now. But anybody can repeat it.It's simple.

1. Start the plane at alt above the initial testing point. Use no cockpit view to get TAS and stabilize the plane at desired TAS.

2. Keep the TAS constant and measure the time required to pass from the start altitude to end altitude.

3. Repeat for all planes you want to test, try it with no power and full power.

4. Compare the results.


As the test measure the time required to get from StartAlt to EndAlt it also measure the distance traveled. Test requirement is that TAS is kept constant so difference in time from let's say 4000m to 2000m will mean that planes passed different distances which in turn means that their diving angle was different.

Plane that needed longest to get to EndAlt is the best diving among the tested planes because it needed smallest help from gravity to keep its speed. Consequently it will dive fastest in a dive that is performed at same diving angle.
I tried:4.11m Tempest mkv vs p51c engine overheat off, radiator closed

From 3000m altitude to 2000m, keeping speed=700TAS

2minutes and 2 second for tempest mkv
2minute and 30 second for p51c

So, p51c outdives tempest? No. Tempest mkv definitely outdives p51c!

BTW, this kind of "dive" is very shallow, smaller than 10 degree.
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  #107  
Old 05-10-2012, 03:39 PM
BlackBerry BlackBerry is offline
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When P51 dives to 4500 m=15000ft altitude, and reaches 640km/h=400 mph IAS, that is 1.25*640=800 km/h=222m/s TAS, the mach number is equal to 222/322=0.69. That's fuselage speed.

However, the speed of tip of airscrew is far more 0.69 mach.

4-blade hamilton airscrew,10.5 feet diametre, the reduction ratio of airscrew rotating to engine is 0.477. 3000rpm engine, 1431rpm=23.85r/s airscrew, the rotating speed of tip is:3.14*10.5*0.303*23.85=238m/s

So the combination speed is: (238^2+222^2)^0.5=325 m/s.

Unfotunatly, sonic speed at 4500m altitude is 322m/s, that is to say, the tip of airscew is 1 mach. There is no mach number "concept" in il2's model at all, how can I believe that il2 simulates 1 Mach aerodynamics very well?
Quote:
http://en.wikipedia.org/wiki/Indicated_airspeed

In aneroid instruments the indicated airspeed drops-off with increasing altitude as air density decreases. This leads to an apparent falling-off of airspeed at higher altitudes. For this and other reasons never exceed speeds (abbreviated VNE) are often given at several differing altitudes in some aircraft's operating manuals, the VNE IAS figure falling as height is increased, as shown in the sample table below.
Diving below mph IAS
30,000*ft 370
25,000*ft 410
20,000*ft 450
15,000*ft 490
10,000*ft 540

Ref: Pilot's Notes for Tempest V Sabre IIA Engine - Air Ministry A.P.2458C-PN
Tempest could dive to 450 mph IAS below 20000ft, aha, tempest's big rotol 14-ft airscrew, 3800rpm, 450mph IAS, the tip of rotol must be supersonic, so is P51's hamilton, thus the efficiency curves of their airscrews play an important role when they dive to high speed.

Why tempest outdives p51? For more efficiency airscew @ supersonic? Maybe. For much more heavier fuselage? Probably.

All in all, il2's model is lack of supersonic simulation, that's why we couldn't experience what tempest/p51 should be. That's why tempest couldn't outdive dora easily in game.

Last edited by BlackBerry; 05-10-2012 at 04:13 PM.
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  #108  
Old 05-10-2012, 04:58 PM
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FC99 FC99 is offline
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Originally Posted by BlackBerry View Post
When P51 dives to 4500 m=15000ft altitude, and reaches 640km/h=400 mph IAS,
Why 400mph IAS ? Why not any other number?

Quote:
Unfotunatly, sonic speed at 4500m altitude is 322m/s, that is to say, the tip of airscew is 1 mach. There is no mach number "concept" in il2's model at all, how can I believe that il2 simulates 1 Mach aerodynamics very well?
Prop planes can't break sonic barrier so it's not that important for Il2 to have highly detailed Mach model.

Quote:
Tempest could dive to 450 mph IAS below 20000ft, aha, tempest's big rotol 14-ft airscrew, 3800rpm, 450mph IAS, the tip of rotol must be supersonic, so is P51's hamilton, thus the efficiency curves of their airscrews play an important role when they dive to high speed.
Yes it does.

Quote:
Why tempest outdives p51? For more efficiency airscew @ supersonic? Maybe. For much more heavier fuselage? Probably.
It does? Maybe, depends which models you take for comparison .

Quote:
All in all, il2's model is lack of supersonic simulation, that's why we couldn't experience what tempest/p51 should be. That's why tempest couldn't outdive dora easily in game.
It should out dive Dora easily, why ?
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  #109  
Old 05-11-2012, 12:34 AM
BlackBerry BlackBerry is offline
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Why 400mph IAS ? Why not any other number?
It's very common to dive to 400-450MPH IAS for WWII late aircrafts such as P51, P47,Tempest, fw190, etc.


Quote:
Prop planes can't break sonic barrier so it's not that important for Il2 to have highly detailed Mach model.
Prop planes can't break sonic barrier, but Prop planes' propeller CAN often break sonic barrier in a dive, it's an unfortunate fact for il2.

As early as 1904 when Wright brothers made the first a/c, they knew both airscrew and wing are "same thing".

Quote:
The twisted airfoil (aerofoil) shape of modern aircraft propellers was pioneered by the Wright brothers. They realised that a propeller is essentially the same as a wing, and were able to use data from their earlier wind tunnel experiments on wings. They also realised that the angle of attack of the blades needed to vary along the length of the blade, thus it was necessary to introduce a twist along the length of the blades. Their original propeller blades were only about 5% less efficient than the modern equivalent, some 100 years later.[2]

http://en.wikipedia.org/wiki/Propeller_(aircraft)

Their original propeller blades were only about 5% less efficient than the modern equivalent, some 100 years later.........


That conclusion is based on low Mach data, for supersonic airscrew, the story is totally diefferent.


In my opinion, the airscrew theory/simulation is the weakness of il2's FM.

Quote:
It does? Maybe, depends which models you take for comparison .
Tempest MKV 9lbs boost outdives P51B(Mustang III) 18lbs boost.

Quote:
Dive
For the same reasons as the zoom climb, the Tempest pulls ahead. As the speed is increased it does so more rapidly. The fact is it has the best acceleration in the dive yet seen at this Unit.

http://www.hawkertempest.se/TacticalTrials.htm

Quote:
Speed and acceleration in the dive is an essential quality to a successful fighter, but a decisive conclusion on the order of superiority is largely dependant on throttle settings, and the maximum speed in straight and level flight of the individual aircraft. Here again, however, by carrying out a number of tests under different conditions, it is reasonable to assume that the Meteor is well ahead of its rivals, followed by the Tempest, Thunderbolt, Mustang and Spitfire in that order.

http://www.wwiiaircraftperformance.org/sl-wade.html

Last edited by BlackBerry; 05-11-2012 at 12:38 AM.
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  #110  
Old 05-11-2012, 02:06 AM
MadBlaster MadBlaster is offline
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if you simply want to test 'drag' without digging into the code, I think it better to do it with the engine off (i.e., no thrust test).

- go into fmb and set your spawn kph to zero and start a track in cockpit view.

- spawn your plane say 5000 meters and leave the engine off.

- close your rads, neutral your trims, set you prop pitch to 100%.

-push nose into 90 degree vertical.

-end the track when your plane hits the beach at zero alt.

-go back and look at track. look at speedometer at say 20 second mark per track time. (e.g., speed says 400 kph at 20 seconds for this plane)

-repeat with another plane and compare results.

- For the thrust piece, you can use devicelink to get an idea. There is an acceleration parameter that can be graphed/logged. You can see the effect of adjusting throttle and prop pitch. Prop pitch changes and its effect on acceleration is modeled. The csp may be slower to change blade angle than the vdms. At least, that's how it feels like to me. The fw vdm has a torque limiter. I think p factor is also modeled. You can produce de-celeration by adjusting blade angle, according to devicelink.
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