Hurricane Speed Test in game

[bongodriver]

Annoyingly without going into graphs it seems Crumpp is right, if FTH in standard atmosphere is 17,500ft density altitude, then if ambient temps are ISA + then density altitude is higher than pressure altitude therefore boost drop off should happen lower than 17,500 ft on the altimeter.

There are simpler graphs showing a density altitude conversion from pressure altitude just using ISA deviation temperature, again Crumpp may have a point about dry lapse rates as I think density alt is based on it, relative humidity comes in and complicates matters.

Crumpp, your working out was done with a standard 2 degree lapse rate, I believe a 3 degree lapse rate would work in 'favour' of performance in that case.

19C + 3 degree per 1000 ft lapse rate = 17500/1000 = 17.5*3 = 52.5 = 19C - 52.5C = -33C

using -33C at 17,500ft pressure altitude I find a density alt of 16,000ft so in effect I could go to more than 17,500 pressure alt before boost drop off.

I will look at this again tomorrow.

p.s. I forgot to add....the key to this would be to test and find the in game actual OAT (outside air temp) at 17,500 ft pressure altitude and see what the deviation is from a 3 degree lapse rate.

p.p.s. I think the test is worth doing using standard setting on the altimeter 1013mb, which RAF aircraft use an OAT gauge?, might be worth doing the test as a co op so someone can check temp at various pressure altitudes.

P.p.p.s reverse engineering the games atmosphere model at 1 in the morning is nuts

[Crumpp]

Find Pressure ratio,

Delta at sea level is 29.29”/ 29.92” = .978944

Find Temperature Ratio,

19C =66.2 F

Theta = T/To at sea level , where T = Actual Temperature in deg K or Deg R and To = Standard sea level Temperature in deg R or deg R.
At altitude Theta = (66.2F + 460R)/(59F + 460R) = 1.01387

Find Density ratio,

Sigma = Delta / Theta = .965552

Find Standard Means Of Evaluation:

SMOE = 1/SQRT(sigma) = 1.01768

Hurricane Mk I sea level TAS @ standard conditions = 263mph TAS = 263mphEAS

Under game atmospheric conditions with a standard lapse rate:
263mphEAS * 1.01768 = 268mphTAS @ Game Sea Level

The Hurricane Mk I should be in the vicinity of 268mphTAS at sea level in the game under the atmospheric conditions recorded.

[bongodriver]

With a decent graph you don't need to find any ratios, some geek has already done it for you.

[MiG-3U]

The full throttle height is function of pressure altitude as seen directly from altimeter, given the altimeter base level setting. Supercharger increases manifold pressure and that ability depends on outside pressure and dynamic pressure on intake regardless density of the outside air. Air density is, of course, funtion of pressure and temperature but supercharger only sees the pressure.

BTW Gaming wise it's more important that the relative performance of the planes is about right and that is pretty easy to test regardless atmospheric model of the game, given that all the planes can be tested in the same conditions.

BTW2 I just looked in my crystal ball a saw that soon my sayings will be denied and scans from various books will be posted with thick, red underlinings

[bongodriver]

BTW2 I just looked in my crystal ball a saw that soon my sayings will be denied and scans from various books will be posted with thick, red underlinings

I'd hate to dissapoint so here goes.

No it is a function of density altitude, a supercharger produces an excess of pressure which it can only maintain until it reaches a certain level of air density due to volumetric efficiency,atmospheric density is very much affected by temperature and atmospheric pressure is not, if conditions were ISA exactly then density altitude would equal pressure altitude, inevitably temparature varies and whenever things are on the hot side performance is reduced and vice versa, so it stands to reason performance is dependent on density, you only have to look at aircraft performance charts to see this is true, jet engines have the same problem, after all a supercharger is just a centrifugal compressor just like you find on some jets.

[MiG-3U]

Originally Posted by bongodriver

I'd hate to dissapoint so here goes.

No it is a function of density altitude, a supercharger produces an excess of pressure which it can only maintain until it reaches a certain level of air density due to volumetric efficiency,atmospheric density is very much affected by temperature and atmospheric pressure is not, if conditions were ISA exactly then density altitude would equal pressure altitude, inevitably temparature varies and whenever things are on the hot side performance is reduced and vice versa, so it stands to reason performance is dependent on density, you only have to look at aircraft performance charts to see this is true, jet engines have the same problem, after all a supercharger is just a centrifugal compressor just like you find on some jets.

I'm glad to be corrected on this, preferably in layman's terms without underlined scans

My thinking is that supercharger keeps constant pressure up to the FTH and the mass flow through the engine and power is at maximum at FTH. Below FTH intake air is warmer and hence density at the intake manifolds is lower at given pressure and mass flow and power lower than at FTH, above FTH supercharger can't keep the pressure which reduce mass flow and power.

Let's assume that pressure altitude is keeped constant but temperature is higher. Then the mass flow through engine is reduced due to lower density and less mass flow is needed through supercharger to keep constant pressure due to same reason. The power is, of course, lower at this situtation, however, pressure ratio between manifolds and outside is still unchanged and hence the FTH unchanged as well.

[Crumpp]

it is a function of density altitude

Correct

I'm glad to be corrected on this, preferably in layman's terms without underlined scans

It would be nice if people would not stick to their guns when they are wrong so that such things are not required.

My thinking is that supercharger keeps constant pressure up to the FTH and the mass flow through the engine and power is at maximum at FTH. Below FTH intake air is warmer and hence density at the intake manifolds is lower at given pressure and mass flow and power lower than at FTH, above FTH supercharger can't keep the pressure which reduce mass flow and power.

Let's assume that pressure altitude is keeped constant but temperature is higher. Then the mass flow through engine is reduced due to lower density and less mass flow is needed through supercharger to keep constant pressure due to same reason. The power is, of course, lower at this situtation, however, pressure ratio between manifolds and outside is still unchanged and hence the FTH unchanged as well.

Unfortunately, that is not the case and people continue to argue their point.

[Crumpp]

'Full throttle height' is defined as that altitude above which the supercharger is unable to compensate for the reduced air density and both the BHP and airflow decrease in direct proportion to any further increased altitude.

http://www.keenzo.com/showproduct.asp?ID=3192182

Most of the flight data and engine data you guys argue about is converted to standard day conditions.

On non-standard day's FTH will change true altitude to maintain the same density altitude.

[IvanK]

Courtesy of Klems data tool and his work it has been established that the basic CLOD atmospheric temperature lapse rate is 1.97 deg C per 1000feet (corrected to 2 decimal places). This has now been measured up to 26,000feet. So its only 0.01 deg C per 1000 feet out ... not much and well and truly in the ball park to the ISA value of 1.98 deg C per 1000feet.

[bongodriver]

Originally Posted by IvanK

Courtesy of Klems data tool and his work it has been established that the basic CLOD atmospheric temperature lapse rate is 1.97 deg C per 1000feet (corrected to 2 decimal places). This has now been measured up to 26,000feet. So its only 0.01 deg C per 1000 feet out ... not much and well and truly in the ball park to the ISA value of 1.98 deg C per 1000feet.

You beat me to it but a squad member from the dogz and I have been doing a little experimenting and found something interesting, much like you we have found the lapse rate to be in the 2 degrees per thousand feet but in addition we stumbled upon an element in the FMB that changes the the atmospheric temperature not sure of its function but it is the mission parameters box, there is an option for 'Breeze/Thermal Activity =0/10' with a sliding scale, default seems to be 0 and this gives an ambient temparature of ISA +10, but if the slider is set to 10 the temp almost lines up with ISA, so far no indication that the temps can be brought to an ISA - figure.

I have attached a screenshot of the FMB tool and a graph showing the results of our quick test today.

p.s. I mean to add the test mission was done on a clear day at 12:00 midday, no other tests done to check for diurnal variations.

p.p.s. also added a usefull graph for density altitude calculations.