AFM for Su25/25T at 30% dive

[4c Hajduk Veljko]

I would bet on a translation error more then anything.

It could be ...

 

Look at the pic below...

A10 is heavier then Su-25, yet (with SFM) it does not accelerate in 30 degrees dive.

 

Su-25T (with AFM) is lighter and it accelerates even at 10 degree dive.

 

How do you figure with the AFM, the tiny speedbrakes, and sleek airframe of the Su-25T that it will not accelerate in the 30 degree dive wich is quite steep?

I don’t. However it does make sense for the air brakes to be more effective then what they are in FC. In real life, Su-25 pilots (use to) visually acquire relatively small targets on first pass, then fly around it maintaining visual contact and then destroy it. That’s how it worked until appearance of the Stinger.

 

Well for that to work, I assume the Su-25 would have to have very effective air brakes to engage targets from low altitudes and do not accelerate in steep dives towards the target. Again, this is only my assumption.

 

 

Throw the gear and full flaps out and then you will see the difference of what the extra form drag will do.

Thats what I do on regular basis.

[Shepski]

It could be ...

 

A10 is heavier then Su-25, yet (with SFM) it does not accelerate in 30 degrees dive.

 

Su-25T (with AFM) is lighter and it accelerates even at 10 degree dive.

 

I used the same weight in my tests but the empty weight of the A-10, with 20% fuel for each, is less then the empty weight of the Su-25T... in Lock On 10,360(A-10) and 12,250(Su-25T)

 

I don’t. However it does make sense for the air brakes to be more effective then what they are in FC. In real life, Su-25 pilots (use to) visually acquire relatively small targets on first pass, then fly around it maintaining visual contact and then destroy it. That’s how it worked until appearance of the Stinger.

 

If they are acquiring and maintaining small targets visually then they would have to be quite low and not very fast. They would also probablly hardly spend time in a dive at those low altitudes... my assumptions. :)

 

Well for that to work, I assume the Su-25 would have to have very effective air brakes to engage targets from low altitudes and do not accelerate in steep dives towards the target. Again, this is only my assumption.

 

I think they would not be initiating the dive from a very fast speed to begin with at such low altitudes. Also, if the real speedbrakes create as much turbulence as modeled in Lock On then I doubt they would even keep them extended while lining up the pipper. I hardly ever use the speed brakes in Flaming Cliffs when running in but I will use them to slow down prior to begining the dive if needed. I also use the first flap setting when low and slow attacking targets.

[SwingKid]

Ok.. I just did a quick test in the -25T...

 

With a weight of 12,000 kilos and clean in a constant 10 degree dive from 2800m to 800m. I started at 3000m, thrust to idle, and speedbrake out then began the dive.

 

Brake retracted...

 

@2800m the speed is 711 km/h

@800m the speed is 820 km/h.

net gain of 109 km/h.

 

Brake extended...

 

@2800m the speed is 682 km/h

@800 the speed is 687 km/h.

net gain of 5 km/h.

 

Does the brake still seem overly ineffective to you?

 

Some data is missing fror a proper Cd calculation, e.g. we don't know what was the AOA or the descent time (assuming 0 AOA it would seem about 60 seconds).

So, my math from this example somehow resulted in a section-referenced brake Cd of 0.15, which is even lower than my previous estimate.

 

I think Cd = 0.4 is a slightly more reliable measurement, and yes I still think it might be too low, based purely on the shape of the airbrakes.

Coincidentally, I also think Lock On's A-10 airbrakes might be too slippery now, for the same reason, but I'm less sure about those because they have a different shape.

 

Is it possible that ED had aerodynamic data available only about the less-effective old two-panel airbrakes of early Su-25s, and is using it to model the four-panel airbrakes of later aircraft?

 

-SK

[Shepski]

Some data is missing fror a proper Cd calculation, e.g. we don't know what was the AOA or the descent time (assuming 0 AOA it would seem about 60 seconds).

So, my math from this example somehow resulted in a section-referenced brake Cd of 0.15, which is even lower than my previous estimate.

 

Clean: AoA = 0-1 degree. Time was 42 seconds.

 

Brakes out: AoA = 1-2 degrees. Time was 45 seconds.

[SwingKid]

Ok, something's slightly off.

If with the brakes out you were averaging 685 km/h, that's about 190 m/s (assumed TAS).

That means you covered 190 * 45 = 8550 m slope.

If the slope was 12 degrees (10 degree pitch + 2 degree AOA), then you should have descended only

8550 * sin(12) = 1778 m, instead of 2000 m.

Assuming the 45 s and the speeds are correct, then asin(2000/8550) = 13.5 degree dive (10 deg. pitch-down + 3.5 degrees AOA?)

 

With the brakes retracted, we have an average speed of about 766 km/h = 213 m/s,

42 seconds gives 8946 m slope,

asin(2000/8946) = 12.9 degree dive (10 degrees pitch-don + 2.9 deg AOA?)

 

Ok to find drag force without the brakes, let's see,

engine thrust = (9.8 N/kgf) * (1900 kgf) = 18620 N

gravity component = (9.8 N/kg) * (12000 kg) * sin(13) = 26454 N

measured acceleration = (820-711 km/h)/(3.6 kms/mh)/(42 s) = 0.72 m/s^2

drag force = (18620 N) + (26454 N) - (12000 kg * 0.72 m/s^2) = 36434 N

Cd = 36434 / (0.5 * 1.225 * 212.6^2 *8 ) = 0.165

 

So the body drag at 685.5 km/h should be:

body drag = 0.165 * 0.5 * 1.225 * 190.4^2 *8 = 29310 N

measured acceleration with airbrakes = (687-682 km/h)/(3.6 kms/mh)/(45 s) = 0.03 m/s^2

brake force = (0.03 m/s^2) * (12000 kg) + (29310 N) - (18620 N) - (26454 N) = 15404 N

Cd(airbrakes) = 15404 / (0.5 * 1.225 * 190.4^2 *1.8 ) = 0.385

 

Not far off from the ~0.4 figure I calculated from my own measurement (thanks for the extra data).

The airbrakes still seem to be only about as draggy (in shape) as a "boxy road car".

Consider also these shapes, especially the "flat plate":

http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html

 

-SK

[Shepski]

I would use 0.5 degrees AoA for the clean config and 1.5 for the dirty one. At the start of the dive the AoA was 1 then decreased to 0 for the clean config and for the dirty is started at 2 then reduced to 1.