AIM-120 turn rate at 60k feet - ED Forums
 


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Old 12-30-2015, 11:02 PM   #1
garrya
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Default AIM-120 turn rate at 60k feet

So someone over F-16 made an estimation of maximum turn rate AIM-120 could make at 60k feet when motor burn out
His conclusion is that AIM-120 can make an instantaneous 21g turn at 60k feet, is his estimation acceptable? or too inaccurate?


http://www.f-16.net/forum/viewtopic....28234&start=60
Quote:

https://www.google.co.uk/url?sa=t&rc...10151844,d.d24





AIM-120C-5 have body diameter of 180 mm and total wing span of 482 mm ( 0.482 meters ) which mean the span of each fin is ( 482-180)/2 = 151 mm ( 0.151 meters )
This is AIM-120C fin


here is the fin size relative to the total body


The photos shows that Aim-120's fin have trapezoid shape ,
For the rear fin : the big base length is around 2.5 times the height
the small base length is around 1.5 times the height
the height (wing span ) as we measured earlier is around 0.151 meters , so the big base is 0.3775 meters , small base is 0.2265 meters
Solve for area we have 0.046 square meters for each rear fin


For the frontal fin : the big base length is around 2 times the height
the small base length is around 0.5 times the height ,the height (wing span ) as we measured earlier is around 0.151 meters so the big base is 0.302 meters , small base is 0.0755 meters
Solve for area we have 0.029 square meters for each frontal fin

we have total of 4 frontal fins and 4 rear fins so total fin area on aim-120 is :
0.046*4 + 0.029*4 = 0.3 m2
( this number is 3 times bigger than my estimation earlier in this thread because before I only estimate area of horizontal fin and ignore vertical fins )

For missiles body i put picture of Aim-120 into paint and use the ruler app to calculate the ratio between the cone length and total missiles length
the pointy part of the cone accounted for 1/9.4 of total missiles length , so the cone length is 388 mm ( 0.388 meters ), missiles length is 3262 mm ( 3.262 meters )
missiles have diameter of 180 mm so the radius of the base is 90 mm ( 0.09 meters )
solved for area ( put number in formula )we have :
lateral surface area of Aim-120 body is : 1.84 square meters
lateral surface area of Aim-120 pointy nose cone is : 0.11 square meters


So the total wetted area of aim-120 is 1.84 + 0.11 + 0.3*2 = 2.55 square meters




According to the model graph in pdf file
At altitude of 5 km they estimated the CL value for wetted area of Aim-120 to be 0.11 at AoA of 30 degree and speed of 1200 m/s
Also from the graph when altitude rise by 15 km the CL value at same AoA reduced by around 10%
From data above it safe to assumed at altitude of 60-65K feet ( 19-20 km ) , speed of 1200 m/s and AoA of 30 degree then CL value for wetted area of Aim-120 is around 0.099 - 0.1


put everything estimation value above into equation
Air density at 60K ft = 0.12 kg / m3
AIM-120 total weight is 152 kg , 52 kg of solid propellant so at burn out aim-120 weight will be 100 kg
at 60K feet speed of sound is 293 m/s
let say aim-120 max speed at burn out is mach 4
CL value at AoA of 30 degree , 60K feet = 0.1
total wetted area of aim-120 is 1.84 + 0.11 + 0.3*2 = 2.55 square meters

Lift = 1/2 * rho * v^2 * S * CL
at maximum instantaneous G turn then lift must be equal weight* acceleration so :
100*10*( G-load ) = 1/2* 0.12* (293*4 )^2 * (2.55)* 0.1
G-load = 21 G

Quote:
thrust is 16771 N
lift equal thus*sin (AoA )
the lift from thrust when missiles fly at AoA of 5 degrees = 1461 Newtons
the lift from thrust when missiles fly at AoA of 10 degrees = 2912 Newtons
the lift from thrust when missiles fly at AoA of 15 degrees = 4340 Newtons
the lift from thrust when missiles fly at AoA of 20 degrees = 5736 Newtons
the lift from thrust when missiles fly at AoA of 25 degrees = 7087 Newtons
the lift from thrust when missiles fly at AoA of 30 degrees = 8385.5 Newtons

Last edited by garrya; 12-30-2015 at 11:04 PM.
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Old 12-30-2015, 11:05 PM   #2
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Bonus questions can anyone please translate or explain this graph ?

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Old 12-30-2015, 11:11 PM   #3
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Looks like flight envelope of mig 25
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Old 12-30-2015, 11:16 PM   #4
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Quote:
Originally Posted by GGTharos View Post
Looks like flight envelope of mig 25
yes it is, but i don't understand Russian so i don't know what they write in it
Btw, how can Mig-25, Mig-31 fly much higher and faster than Su-27, F-15, Mirage IV?
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Old 12-30-2015, 11:24 PM   #5
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Different type of engine designed to operate up there. Turbofans probably wouldn't be very happy in such thin air, though NASA had flown their f-15 to M2.7 ... Don't recall the altitude.

Anyway regarding the graph, looks like flight envelopes for a bunch of different payloads.

Can't make out too much more than that :-)
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Old 12-31-2015, 12:30 AM   #6
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Quote:
Originally Posted by GGTharos View Post
Different type of engine designed to operate up there. Turbofans probably wouldn't be very happy in such thin air
but D-30F6 on mig-31 is a turbofan engine, and with very high bypass value too ( similar to F-135)
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Old 12-31-2015, 12:33 AM   #7
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Like I said, NASA got their F-15 up there, but I'm also pretty certain that they modified the engines. :-)
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