Why still no dive acceleration difference?

[Crumpp]

P47's 3-blade prop drops to 63% efficiency when diving to 0.7 Mach which is subsonic.

How do you figure?

[BlackBerry]

Originally Posted by AndyJWest

You are asking TD to rewrite the entire flight physics modelling based on a single number? Yeah, that's going to happen...

I don't know whether detailed propeller efficiency is calculated in il2 FM. Someone told me it's only a simple value. It's ridiculous that a propeller a/c simulation game dosn't provide accurate efficiency curve. Simply setting 85% for all of CSP ? No, that's totally unacceptable.

People will spend quite a lot time to collect different propeller data such as prop diameter, reduction ratio, airfoil section shape, angle, etc. Next step is to use Xfoil/Ansys(software) to calculate complete efficiency curve for every propeller. It's worthy because <<cliff of Dover>> could also benefit from this work. Don't forget 10% efficiency difference will cause 100-200 HP error.

like this:
a.JPG

BTW, efficiency drops as altitude increases. If a CSP get 85% at sea level, there is only 85%*80%=68% at 6000m altitude(800KM/H TAS).
b.JPG

[MadBlaster]

Well, I know for a fact from playing that it takes more elevator trim to maintain level flight the higher the altitude. So, I think we can conclude air density effects on wing lift are modeled. I will take a guess Oleg somehow imbedded the prop efficiency losses in the air density values as well. In other words, he came up with a simplified way of doing a complex operation to save cpu cycles.

[AndyJWest]

I don't know whether detailed propeller efficiency is calculated in il2 FM. Someone told me it's only a simple value. It's ridiculous that a propeller a/c simulation game dosn't provide accurate efficiency curve. Simply setting 85% for all of CSP ? No, that's totally unacceptable.

So now you want TD to rewrite the physics modelling because of something that "Someone told" you... ?

If you can provide verifiable evidence that IL-2 is wrong, do so. But bear in mind that even if you do, this is a ten-year-old game, and is hardly likely to undergo a substantial rewrite that would make little practical difference in terms of relative aircraft performance - at lest, from the evidence I've seen so far. IL-2 gets it wrong at high Mach numbers: but this isn't news. It seems to be fairly consistent in the 'wrongness' anyway, so why worry about it...

As for CloD benefiting from the work, do you have any information at all regarding how this entirely new simulation models such things?

[JtD]

I think this debate has shown a lot more than just a single number WRT real life performance.
It is, however, wrong to assume that the game's model is overly simple and wrong.

[BlackBerry]

Originally Posted by Crumpp

How do you figure?

http://digital.library.unt.edu/ark:/...dc62616/m1/19/


3-blade Hamilton standard 6507A-2 on P47D, efficiency varies from 83%-63% when TAS is between 0.25-0.7 Mach.


Diagram 1, Cp/blade=0.9, should be at high altitude.

Diagram 2, Cp/blade=0.8, should be at medium altitude.

Diagram 3, Cp/blade=0.6, should be at low altitude.

[BlackBerry]

Originally Posted by AndyJWest

So now you want TD to rewrite the physics modelling because of something that "Someone told" you... ?

If you can provide verifiable evidence that IL-2 is wrong, do so. But bear in mind that even if you do, this is a ten-year-old game, and is hardly likely to undergo a substantial rewrite that would make little practical difference in terms of relative aircraft performance - at lest, from the evidence I've seen so far. IL-2 gets it wrong at high Mach numbers: but this isn't news. It seems to be fairly consistent in the 'wrongness' anyway, so why worry about it...

As for CloD benefiting from the work, do you have any information at all regarding how this entirely new simulation models such things?

"Someone" is a man involved in il2 mod developing.

As for relative aircraft performance, if your Tempest MKV(9lbs) get caught by a la7@3000m altitude with same speed, could you just dive in a 60 degree angle to the ground (<720Km/h)and simply get far away from la7 shooting range and then come back to 2500m with a much better "energy saving" zoom ? No, in my experience, you can't achieve that. How could those light--2.5 tons--tiny aircraft--smaller 3-blade ClarkY prop.---la7 dive with same accelaration as a 5-tons-huge aircraft-much bigger 4-blade prop. Tempest MKV?

When TempestMKV/La7 dives to 720km/h=200m/s=0.59Mach on the deck, what's the efficiency?
la7 Shvetson M-82FN 14 cyl. with 2-stage supercharger and direct fuel injection rated at 1,850 hp at 2,500 rpm. VISh-105V-4 3 bladed controllable-pitch metal prop of 10.17 ft (3,10 m) diameter

only 2400-2500rpm engine, merely 3.1meter prop, I don't know the reduction ratio but let's assume 1350rpm for propeller. We know 3-blade 1350rpm 4m Hamilton efficiency is 77% or so at 0.59Mach @ low altitude. Advance ratio for Hamilton=2.22, advance ratio for la7 VISh-105V-4=2.87. wow, 2.87? this ratio is for P47 @0.77Mach.

It's very reasonable for la7 to get only 50% efficiency in this situation. La7 loses extra 35% efficiency? lost 650HP? A piece of Sh*t for La's high speed diving. This soviet monster shows his weakness, haha.

I just suspect 10-year-old il2 FM how to treat prop efficiency. If "someone" tell me what formular il2 uses in FM, everything will be clear.

[Crumpp]

3-blade Hamilton standard 6507A-2 on P47D, efficiency varies from 83%-63% when TAS is between 0.25-0.7 Mach.

Not in the envelope of the aircraft. It is a CSP.

[BlackBerry]

Originally Posted by Crumpp

Not in the envelope of the aircraft. It is a CSP.

CSP could maintain constant efficiency roughly when speed of propeller tip is below 0.85. Please check 3-blades efficiency curve, when advance ratio is far greater than 2.2, namely when prop tip is approaching 1 Mach, the story changes. A CSP will lose efficiency inevitably at high speed diving(a/c noise louder and louder).

Btw, CSP will also lose efficiency when TAS is very very low.

[Crumpp]

CSP could maintain constant efficiency roughly when speed of propeller tip is below 0.85. Please check 3-blades efficiency curve, when advance ratio is far greater than 2.2, namely when prop tip is approaching 1 Mach, the story changes. A CSP will lose efficiency inevitably at high speed diving(a/c noise louder and louder).

Btw, CSP will also lose efficiency when TAS is very very low.

You are correct but don't misapply it as it has little bearing on the game shapes in IL2.

The efficiency is nearly constant in any portion of the envelope that design can sustain flight....

That is the beauty of a CSP.

The very nature of power producers is such that the faster they go, the less thrust they produce. The reverse is also a characteristics of power producers. The lower the velocity, the more thrust they produce. That efficiency drop occurs because the propeller blades are stalled just like in very high speed flight. The reason is different but believe me, both realms, high and low speed, produce stalled blade portions. In the low speed realm, we are looking at speeds at taxi and the first part of take off but our thrust force is extremely high at low velocity. Therefore, in the scheme of things, it is a useless detail to include the reduction in efficiency in a dive. The performance is not sustainable in the first place and our reduction in thrust with velocity is already well approximated by:

Crumpp says:
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....

In other words, the details are included when you make the standard assumption of .85 efficiency.
You could also incorrectly conclude that all subsonic propeller theory violates the very definition of lift because it does not include the fact lift force develops at right angles to the relative wind. This means that in all propellers, regardless of blade stalling will not produce thrust. Why? As the velocity increases the relative wind gradually shifts and eventually lift produced by our propeller no longer parallels the flight path but is deflected upward.
Fortunately we don't have to do that or at least we would not be adding any accuracy by deriving our own approximation of the effect. It is one more thing rolled up in our standard formulation.