# AIM-120 turn rate at 60k feet

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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?

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

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

Edited by garrya
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Bonus questions can anyone please translate or explain this graph ?  ##### Share on other sites

Looks like flight envelope of mig 25

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Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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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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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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Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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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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Like I said, NASA got their F-15 up there, but I'm also pretty certain that they modified the engines. :-)

[sIGPIC][/sIGPIC]

Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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