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Test data: level flight envelope


4d
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I collected some data this morning, thought I'd share.

 

eaNWo5Nx.gifb6s0A2Iy.jpg

 

Compare with between the STD and STD+10c lines on the flight manual, as I used default weather to collect data (20c, which is STD+5c). Also note I was about 1500lbs lighter, and carrying empty weapon pylons.

 

Speed does not peak at 36000ft in dcs

Ceilings are some 15500ft (AB) / 5500ft (MIL) too high in dcs

dcs is about 0.5 mach too fast

under 38000ft, dcs has roughly two linear lines, where the flight manual shows a smooth curve

 

Other notes:

With the weather set to -50c, I only reached mach 1.99. At +50c, mach 2.634. This is the opposite affect the flight manual shows.

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Note the capability remaining detail; if you are flying raw to the 68,000' plateau, you're not observing the data as provided, thus your comparison is invalid for the altitude peak.

 

Further, "Standard Day" is 15C. Compare accordingly, and to the notices, and let's get apples with apples with actual bite, rather than differing fruit.

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Also note that the F-15 FM is not completed.

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Maybe I should have said 'faster' and 'higher' instead of 'too fast' and 'too high', as the they are slightly different conditions and aircraft. I wasn't trying to say that the flight model is incorrect or bad, rather just noting the differences. Anyway, the main reason I collected this data is to use it in combat. For example it's useful to know that peak speed is a little above 36000ft.

 

I collected data either by holding altitude (<= 62000 for afterburner) or mach (<= 2.1), and letting the other stabilize with time acceleration and unlimited fuel. There were no zoom climbs.

 

How much more thrust do the -220 engines have? This site suggests they produce the same static thrust.

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They'll produce up to 25000lbs static. Besides, static is merely a yardstick, and doesn't tell you much about what happens with a moving aircraft.

 

Did you use the same techniques as the real test pilots to create your chart?

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Again, you're stating a difference is there *without acknowledging why*. The charts do not show altitude or speed LIMITS; they show performance based on a specific amount of EXCESS REMAINING PERFORMANCE.

 

Thus, they're not actual "differences" as definable within the confines of the chart. You must fly as the chart was *constructed* to find where the variances exist, not simply drive to limits.

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Oh ok. I just assumed that the line defined where excess power was 0. I missed the little note at the top saying that the ceiling line is with 500fpm climb rate. The right edge of the says acceleration of 0 and 0.03g. Gathering ceiling data at 500fpm climb rate instead of 0 would make it quicker, and the same when gathering top speed data with an acceleration of 0.03g. They must have done some extrapolating/fudging when making the chart too, for changing fuel loads, etc.

 

Why does the 0g acceleration 'line' blend into the ceiling 'line' if one is with excess power of 0, and the other with 500fps?

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I collected some data this morning, thought I'd share.

 

eaNWo5Nx.gifb6s0A2Iy.jpg

 

Compare with between the STD and STD+10c lines on the flight manual, as I used default weather to collect data (20c, which is STD+5c). Also note I was about 1500lbs lighter, and carrying empty weapon pylons.

 

Speed does not peak at 36000ft in dcs

Ceilings are some 15500ft (AB) / 5500ft (MIL) too high in dcs

dcs is about 0.5 mach too fast

under 38000ft, dcs has roughly two linear lines, where the flight manual shows a smooth curve

 

Other notes:

With the weather set to -50c, I only reached mach 1.99. At +50c, mach 2.634. This is the opposite affect the flight manual shows.

 

Also, weight has absolutely nothing to do with top speed. And, if you didnt have weapons on the pylons at takeoff, DCS acts as if there were no pylons on it at all. The 3d model just doesnt change.

[sIGPIC][/sIGPIC]Weed Be gone Needed

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"Also, weight has absolutely nothing to do with top speed"

 

Not actually correct. Strictly speakin Weight will have an influence on max speed, though 1500lbs or so will be minimal

 

How? weight should only affect speed as a function of time. It should not have any bearing on maximum speed, only Thrust vs Drag should affect top speed.


Edited by USARStarkey

[sIGPIC][/sIGPIC]Weed Be gone Needed

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Vmax (and a few other things) is a function of Excess power or "Ps"

 

Ps= V(T-D)/W

 

So weight is in the formula. The Ps=0 line that is the boundary of the charts above represents Ps=0 that is steady state performance.

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I went back and tested the ceiling with a 500fpm climb rate, and it was only about 1000ft lower. That difference would be barely visible on the chart. So to me it still apears that the ceiling difference between dcs and the flight manual is over 10 000 ft.

 

Temperature has such a significant and backwards affect in dcs that it could be the cause of much of the differences in altitude and top speed.

 

In various other simulators, I noticed that usually increasing weight increased top speed, because the angle of attack was negetive at such high airspeed (most wings produce lift at 0 AoA). Adding weight can help align the fusalage and wings with the airflow. This needs to be concidered when designing aircraft, selecting the wing, and its angle of incidence.

 

I did a quick test in DCS, and weight had no measurable affect on top speed. Top speed was mach 2.606 at ~1deg AoA, with both 0 fuel and full internal fuel. I was unable to produce a negetive AoA at 1g, even after diving to the ground and exceeding Vne. Going inverted at top speed resulted in about -1deg AoA. These tests suggest the F-15 wing as modeled produces near 0 lift at 0 AoA.


Edited by 4d
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