Parasite and Induced drag on Su-25/T

[GGTharos]

IIRC the 15's speedbrake is supposed to be -extremely- effective, according to McDD ... they originally had a shorter (smaller area) brake extending out to 60deg, but that created too much turbulence for the vertical stabilizers, so they lengthened the brake and made it entend out to 45 deg to correct the problem.

 

I think if you measure the actual area of the 15's and 25T's speed brakes you might find that the total area is similar, but the F-15 has a single big surface which -may- or may not cause more drag. I don't know. I figure placement is also important.

[capttrob]

I agree the F-15C speed brake is effective. Its a speed brake. ;)

 

However, i didnt think that one speed brake would be more effective than gear, flaps AND speed brakes combined, in the same dive, on an airplane weighing 8000lbs less. Thats one damn effective speed brake! :thumbup:

 

Side note: For those getting confused that a heavier airplane produces more drag. Thats not neccessarily true. And in an idle thrust dive, your weight is now your thrust vector. So, basically, the F-15 has 8000lbs more thrust than a Su25T in the same dive at idle power (to keep it simple, will vary with dive angle). Again.. thats one effective speed brake on the F-15 (if the Su25T is accurately modelled).

 

 

Next test when i get the time (or if someone else wants to try it). Start both aircraft at same altitude and speed. Nose over into a 90 degree vertical dive. Clean configuration for both. Note the speed upon ground impact. Whichever airplane has the highest gain in speed (Impact speed - top of descent speed) is the more slippery (this way we can see how effective that 15 speed brake really was). First i'll try it at the weights tested previously where the Sui-25T is 8000lbs less. Then i'll try it at same weights (if i can get it close), even though the 25T will have some weapons on the wing. Although i can probably reduce F-15 fuel load by 8000lbs. Will have to check....

 

Unless, someone in the know wants to chime in here and tell us that ground pounders are designed to be more slippery than a fighter. Might save us some time....;)

 

apologies for all my edits My "f" key is starting to go on my laptop... :)

[GGTharos]

Hmm, that might be a worthwhile test :)

 

Though it's worth noting that one has AFM, the other has SFM.

 

I wonder how Su-25 compares to S-25T? Probably too similar to matter ...

 

One other thing ... the MiG-25/31 is pretty 'square' ... and they hang some huge missiles offa these things too.

[chris2802]

Whichever airplane has the highest gain in speed (Impact speed - top of descent speed) is the more slippery

 

This will only work if both planes begin at precisely the same speed and hit the ground before the 25T hits Mach1 and becomes uncontrollable. Drag coefficients are logarithmic, it will take more energy to accelerate from 500-600kph than it would for 300-400

 

Chris

[britgliderpilot]

Side note: For those getting confused that a heavier airplane produces more drag. Thats not neccessarily true. And in an idle thrust dive, your weight is now your thrust vector. So, basically, the F-15 has 8000lbs more thrust than a Su25T in the same dive at idle power (to keep it simple, will vary with dive angle). Again.. thats one effective speed brake on the F-15 (if the Su25T is accurately modelled).

 

 

You haven't bothered to read the Yo-Yo/BGP explanation, have you? ;)

 

Sigh . . . . .

 

 

Wonder if I can import a spreadsheet showing that Cd for the F-15 can be greater than Cd for the Su25T at these kind of speeds . . . . and the F-15 has more wing area to be draggy with . . . .

 

I'm still noticing a lack of any aircraft types in my PM inbox, btw.

[britgliderpilot]

Unless, someone in the know wants to chime in here and tell us that ground pounders are designed to be more slippery than a fighter. Might save us some time....;)

 

You're obsessed with this "groundpounders can't possibly be more slippery than a fighter" idea . . . . .

 

 

They can be - and the Yo-Yo/BGP demonstration will show you why.

 

It's fairly basic aerodynamics . . . . you should know this stuff. Come on, make an effort to understand it . . . . .

[Crusty]

You're obsessed with this "groundpounders can't possibly be more slippery than a fighter" idea . . . . .

 

 

They can be - and the Yo-Yo/BGP demonstration will show you why.

 

It's fairly basic aerodynamics . . . . you should know this stuff. Come on, make an effort to understand it . . . . .

 

I tried this point some time ago....but the guy has a degree in aerodynamics so when he tells me in no uncertain terms im wrong....;)

[capttrob]

I was able to fit in the test now. Just did it....

 

The Su25T gained 113 knots

 

The F-15C gained 120 knots.

 

Both airplanes were set to 269 airspeed and 4000 feet in the menu.

 

In game, The Su25T HUD indicated 430 km/h. 640 km/h upon impact.

 

The F-15 hud indicated 255 KIAS. 375 upon impact.

 

Both were nosed over to full deflection until 90 degree dive established.

 

Lowering the F-15 fuel load by 8000lbs to match 33,700 weight of the Su25T didnt seem to make a difference. The F-15 impacted at 375 for both weights.. 33,700 and 41,652.

 

In conclusion, the F-15 gained 7 knots over the Su25T in the same dive, clean configuration.

 

For ground pounder comparison purposes, i tried the A-10 as well.

 

The A-10 gained 100 KIAS in the dive. 20 knots less than the F-15. The A-10 weight was maxed out with full fuel at 31,398.

[capttrob]

You're obsessed with this "groundpounders can't possibly be more slippery than a fighter" idea . . . . .

 

 

They can be - and the Yo-Yo/BGP demonstration will show you why.

 

It's fairly basic aerodynamics . . . . you should know this stuff. Come on, make an effort to understand it . . . . .

 

ground pounders can be... yes. I should have clariifed my statement. My bad....

 

I should have said, i dont believe the Su25T in the dirty config is more slippery than the F-15C in the clean config with speed brake deployed...

 

or wait... maybe i did say that before. :D

 

I also didnt think the F-15C speed brake could be significantly more effective on an airplane weighing 8000lbs more, than gear, flaps and speed brake combined on the Su25T.

[britgliderpilot]

ground pounders can be... yes. I should have clariifed my statement. My bad....

 

I should have said, i dont believe the Su25T in the dirty config is more slippery than the F-15C in the clean config with speed brake deployed...

 

or wait... maybe i did say that before. :D

 

I also didnt think the F-15C speed brake could be significantly more effective than gear, flaps and speed brake combined on the Su25T.

 

The F-15 airframe clean is more draggy than the Su25T clean at these speeds . . . . . might that be why you're getting confused?

[capttrob]

 

I'm still noticing a lack of any aircraft types in my PM inbox, btw.

 

 

I mentioned in an earlier post i decided against sending you my resume. Sorry bud....

 

:smilewink:

[capttrob]

The F-15 airframe clean is more draggy than the Su25T clean at these speeds . . . . . might that be why you're getting confused?

 

 

Doest seem so in my dive test. They both performed pretty much equally.The F-15 gained 7 knots over the Su25T, same dive, clean config.

 

Edit: But yes, i do understand your point that the F-15 design should get more draggy at lower speed in level flight. This is due to induced drag (higher AoA for a slower speed on the 15). However, you reduce the induced drag variable when performing dive tests (like changing the relative wind angle in a wind tunnel). And you're also right that im getting too tangled up in something i really could care less about since i dont even fly the 25T ;)

[britgliderpilot]

I mentioned in an earlier post i decided against sending you my resume. Sorry bud....

 

:smilewink:

 

 

however, if you really need to know, feel free to pm me and i'll send you a copy

 

 

How much does it matter whether I ask by PM or post? ;)

[capttrob]

How much does it matter whether I ask by PM or post? ;)

 

huh?

 

 

I posted yesterday that i would send my resume to you. In a later post yesterday, i told you i decided against it. Today you ask me again for my resume. Today i tell you that i had posted earlier i decided against it. All caught up to speed?

[britgliderpilot]

But yes, i do understand your point that the F-15 design should get more draggy at lower speed in level flight. This is due to induced drag (higher AoA for a slower speed on the 15). However, you reduce the induced drag variable when performing dive tests (like changing the relative wind angle in a wind tunnel).

 

(to section in bold)

 

Thing is though . . . . . . you don't ;)

 

 

AoA is chord line of wing to velocity vector of the aircraft, has little correspondence to pitch.

 

Your pitch angle rotates towards the ground in a dive, but so does your velocity vector. You are not suddenly flying at a lower angle of attack, you are flying at a lower angle of pitch.

 

Angle of attack . . . . . is the same.

 

Lift . . . . is the same.

 

Drag . . . . is the same.

 

"Thrust" increases, since you have your weight component acting forwards . . . . but you don't magically reduce the lift the wing is generating by aiming the nose downwards.

[capttrob]

Also, my apologies to anyone who was offended by my cocky posting yesterday... I had a few in me and i tend to get a bit sarcastic while underway.... :drink:

 

Parents... dont let your kids DWP! (drink while posting) :thumbup:

[britgliderpilot]

huh?

 

 

I posted yesterday that i would send my resume to you. In a later post yesterday, i told you i decided against it. Today you ask me again for my resume. Today i tell you that i had posted earlier i decided against it. All caught up to speed?

 

I do apologise, I must have missed that - I've skipped over again and still can't find it, but then I am skipping.

 

 

Puzzled as I am by your U-turn, if you did say you decided against it then it's my bad.

[SwingKid]

The aerodynamic drag force may be calculated from the equation:

 

Fd = Cd * 0.5 * p * V^2 * S

 

where Fd is the drag force, Cd is the drag coefficient, p is the air density, V is the speed, and S is the cross-section area.

 

Let's consider the Flaming Cliffs 20% fuel Su-25T in a 700 km/h, 30 degree dive at idle power (no speed brakes).

 

The accelerating force due to gravity would be (sin(30) * 9.8 N/kg) = 4.9 N/kg.

The aircraft's measured acceleration in the dive is only about 4.0 m/s^2, so the aerodynamic drag force must be (4.9-4.0) = 0.9 N/kg.

For a total aircraft weight of 12254 kg, this gives us a drag force of about (0.9 * 12254) ~= 11028.6 N = Fd

 

V = 700 km/h ~= 194.4 m/s,

p ~= 1.225 kg/m^3,

and from diagrams, I estimate the frontal cross-section area of a Su-25T to equal about S ~= 9 m^2.

 

So the measured drag coefficient in Lock On is:

 

Cd = Fd / (0.5 * p * V^2 * S)

= (11028.6 N) / (0.5 * 1.225 kg/m^3 * (194.4 m/s)^2 * 9 m^3)

Cd ~= 0.05

 

So, is the Flaming Cliffs Su-25T "slippery?"

 

Consider:

"a slippery road car has a Cd of about 0.32. A chunky one is 0.38."

 

http://www.insideracingtechnology.com/tech102drag.htm

 

The Flaming Cliffs Su-25T (Cd = 0.05) seems to be about 6 times more slippery, than a "slippery" road car (Cd = 0.32). So what's wrong, to say that it's "slippery?"

 

-SK

[capttrob]

(to section in bold)

 

Thing is though . . . . . . you don't ;)

 

 

AoA is chord line of wing to velocity vector of the aircraft, has little correspondence to pitch.

 

Your pitch angle rotates towards the ground in a dive, but so does your velocity vector. You are not suddenly flying at a lower angle of attack, you are flying at a lower angle of pitch.

 

Angle of attack . . . . . is the same.

 

Lift . . . . is the same.

 

Drag . . . . is the same.

 

"Thrust" increases, since you have your weight component acting forwards . . . . but you don't magically reduce the lift the wing is generating by aiming the nose downwards.

 

 

Have you ever seen a L/D max chart? Why do you think induced drag increases with decrease in speed. Why do you think Parasite drag increases with speed? And what is the speed at which they cross? What happens to your speed when you point the nose down at idle thrust?

 

AoA does not remain the same.

 

Thrust only increases as you roll the nose over. It remains the same once established in the dive, however, you are accelerating. That doesnt mean thrust is increasing.

 

Drag definitely DOES NOT remain the same. You are accelerating. Where did you get this from?

 

You dont reduce lift by pointing the nose down, it will actually increase depending on airfoil design. So i dont know where you got that one from. What i was referring to is that you reduce induced drag as you increase speed (because you are diving). But you also increase parasite and form drag. As speed increases further, you'll start to get compression drag and wave drag. There are actually some high aspect ratio wing designs that will have a "negative" angle of attack at higher speeds to maintian level flight (or flight path/velocity vector)

 

All better now?

[Crusty]

Swingkid, , ...That figure actually may be pretty close

 

http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml

[Weta43]

While suspecting that I'm just inviting an artfully crafted putting in my place - & this may have been mentioned but I can't remember it's being... are these tests being done by throttling back in the dive?

Something to take into account is that as modelled in LO - at zero throttle the F15 produces zero thrust (which is why it sits still on the runway), while at zero throttle the 25T produces (from memory E.D. said - & it's 05:32 here so I can't be bothered searching) 30% thrust (which is why it doesn't). Even if the residual thrust is lower than 30%, any residual thrust is going to distort the figures from minizap & make it appear more "slippery" than it is. To compare apples & apples you'd have to actually kill the engines on both.

[capttrob]

The aerodynamic drag force may be calculated from the equation:

 

Fd = Cd * 0.5 * p * V^2 * S

 

where Fd is the drag force, Cd is the drag coefficient, p is the air density, V is the speed, and S is the cross-section area.

 

Let's consider the Flaming Cliffs 20% fuel Su-25T in a 700 km/h, 30 degree dive at idle power (no speed brakes).

 

The accelerating force due to gravity would be (sin(30) * 9.8 N/kg) = 4.9 N/kg.

The aircraft's measured acceleration in the dive is only about 4.0 m/s^2, so the aerodynamic drag force must be (4.9-4.0) = 0.9 N/kg.

For a total aircraft weight of 12254 kg, this gives us a drag force of about (0.9 * 12254) ~= 11028.6 N = Fd

 

V = 700 km/h ~= 194.4 m/s,

p ~= 1.225 kg/m^3,

and from diagrams, I estimate the frontal cross-section area of a Su-25T to equal about S ~= 9 m^2.

 

So the measured drag coefficient in Lock On is:

 

Cd = Fd / (0.5 * p * V^2 * S)

= (11028.6 N) / (0.5 * 1.225 kg/m^3 * (194.4 m/s)^2 * 9 m^3)

Cd ~= 0.05

 

So, is the Flaming Cliffs Su-25T "slippery?"

 

Consider:

"a slippery road car has a Cd of about 0.32. A chunky one is 0.38."

 

http://www.insideracingtechnology.com/tech102drag.htm

 

The Flaming Cliffs Su-25T (Cd = 0.05) seems to be about 6 times more slippery, than a "slippery" road car (Cd = 0.32). So what's wrong, to say that it's "slippery?"

 

-SK

 

Can you do that for the Flaming Cliffs F-15? I'd be curious to see the difference since they both performed about the same in the dive test.

[capttrob]

Something to take into account is that as modelled in LO - at zero throttle the F15 produces zero thrust (which is why it sits still on the runway), while at zero throttle the 25T produces (from memory E.D. said - & it's 05:32 here so I can't be bothered searching) 30% thrust (which is why it doesn't). .

 

And there it is! I always said it felt almost like i had a tailwind when landing the 25T, when it was the thrust acting like that tailwind. If the net thrust vector is 30% higher than the F-15, that explains everything! Well, at least it starts to make more sense.

 

Mind you, we are talking about net Thrust vector and not thrust setting as seen on the guages, because they both show about 40% thrust at Idle IIRC.

 

As far as being able to sit on the runway. Rhetoricallly speaking, If they were both producing 30% thrust, The F-15 will stay still because it is heavier while the 25T will need some brakes.

 

All my dive tests were conducted at idle thrust.

[Yo-Yo]

The aerodynamic drag force may be calculated from the equation:

 

Fd = Cd * 0.5 * p * V^2 * S

 

where Fd is the drag force, Cd is the drag coefficient, p is the air density, V is the speed, and S is the cross-section area.

 

Let's consider the Flaming Cliffs 20% fuel Su-25T in a 700 km/h, 30 degree dive at idle power (no speed brakes).

 

The accelerating force due to gravity would be (sin(30) * 9.8 N/kg) = 4.9 N/kg.

The aircraft's measured acceleration in the dive is only about 4.0 m/s^2, so the aerodynamic drag force must be (4.9-4.0) = 0.9 N/kg.

For a total aircraft weight of 12254 kg, this gives us a drag force of about (0.9 * 12254) ~= 11028.6 N = Fd

 

V = 700 km/h ~= 194.4 m/s,

p ~= 1.225 kg/m^3,

and from diagrams, I estimate the frontal cross-section area of a Su-25T to equal about S ~= 9 m^2.

 

So the measured drag coefficient in Lock On is:

 

Cd = Fd / (0.5 * p * V^2 * S)

= (11028.6 N) / (0.5 * 1.225 kg/m^3 * (194.4 m/s)^2 * 9 m^3)

Cd ~= 0.05

 

So, is the Flaming Cliffs Su-25T "slippery?"

 

Consider:

"a slippery road car has a Cd of about 0.32. A chunky one is 0.38."

 

http://www.insideracingtechnology.com/tech102drag.htm

 

The Flaming Cliffs Su-25T (Cd = 0.05) seems to be about 6 times more slippery, than a "slippery" road car (Cd = 0.32). So what's wrong, to say that it's "slippery?"

 

-SK

 

Do you know what the idling engines produce thrust?

By the way the standard area for the plane is its wing area. I use it to calculate drag from your example. Cd = 0.016. This value is WRONG for Su-25. The real Cd_0 is at least 1.5 times of this value.

 

Take a look to ALL graphs in the thread - maybe it will help you.

http://www.forum.lockon.ru/showthread.php?t=12463

 

And by the way - your method to compare the car and the plane is very amazing... Just to laugh - take your very SLIPPERY car (Cd = 0.32 as you wrote corresponding to 9 m^2) and try to correspond it to 30.1 m^2 of Su-25 wing area. AND YOU WILL SEE THE PLANE WITH Cd = 0.096.

Please find an example of such UNSLIPPERY plane. :D

[SwingKid]

Swingkid, , ...That figure actually may be pretty close

 

http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml

 

Indeed, as Yo-yo points out we have to be careful. In the link you provided, they use the wing plan area as a reference, not the cross-sectional area, so the Cd values are not compatible.

 

My reason for using cross-sectional area is because at low angles of attack, the wing doesn't produce very much drag compared to the aircraft body.

 

By the way the standard area for the plane is its wing area. I use it to calculate drag from your example. Cd = 0.016. This value is WRONG for Su-25. The real Cd_0 is at least 1.5 times of this value.

 

With respect, I'm not sure what you are saying here.

 

Does AFM use the "standard area' for its calculations? i.e. - it includes the drag from the wings, but not the drag from the aircraft body? In that case, it might explain why the Su-25T seems "slippery."

 

What do you mean, "my example"? My example is the Flaming Cliffs Su-25T. I would agree that it's wrong, and the real Cd should be much higher. Are you agreeing?

 

-SK