Parasite and Induced drag on Su-25/T

[4c Hajduk Veljko]

Adding more fuel to this fire:

 

http://forum.lockon.ru/showthread.php?t=14854

 

:music_whistling:

-SK

 

I was quoting reputable source. :thumbup:

[britgliderpilot]

I believe i stated more than once i am by no means an expert on these models. However, with that said... i agree one of us here "missed something" (as you stated)... perhaps it had something to do with stating opinion without even watching the evidence? Hmmmm... let me see.. who was that? Well.. it couldnt be me.. considering i made the tracks....:thumbup:

 

Im not saying im 100% right. However, in my experience flying jets with speed brakes, comparing a dirty 25T to a clean -15 at a higher weight in LockOn... "something" is wrong (as i have said before). What is wrong? I have no idea.... Do i care? Not a bit. I have already over extended my stay in this thread.... ;)

 

 

Lets try to keep it civil... whaddayasay?

 

 

Based on the description you provided of your flight profile, I contend that nothing is wrong in the first place - perhaps your tracks will show me something you didn't describe, perhaps not.

 

 

Thus far the only step towards not being civil I've made is to suggest you're being cocky - which I believe you are ;)

 

 

On a civil note and out of interest - which jets do you fly? :)

[capttrob]

britgliderpilot,

 

I'd prefer not to bust out my resume in this thread, however, if you really need to know, feel free to pm me and i'll send you a copy. If you need to know each jet i have flown, i guess i can scan my logbook and send you pics. But, i wont.. ;)

 

I said it before and i'll say it again. The F-15C is a non-event in my mission in the clean config. The 25T flys like a slick platform with "air grabber" speed brakes in the landing config. It doesnt seem accurate to me. What you do with my opinion is up to you. It almost seems like you are offended that im stating a negative opinion on the 25T. Is this your favorite airplane or something? Did you contribute to the design? I dont mean to offend anyone.... so my apologies if i have done so....:D

 

I am stating my opinion based on experience. My experience tells me that the 25T is a poor design and something is wrong. You dont have to agree, and you certainly dont have to pass judgement.

[Shepski]

Ok, looked at your -15 track Captrob...

 

You flew the approach differently in each... in the -25T you held altitude and after a few seconds extended the brake and then started the descent. In the -15 you popped the brake and dove for the ground. I have attached 2 tracks where I tried to do the same approach profile for each and the results, considering the different types of aircraft seemed reasonable to me.

 

A few notes...

 

The -25 is at 15.3K kilos and -15 is at 18.8K kilos, thus the -15 will require, based on weight alone, more thrust to maintain an approach speed.

 

The speedbrake surface area of the -15 is much larger then the -25 and, as we already know, the speedbrake of the -25T is used primarily for keeping the rate of acceleration to an acceptable level while the jet is in a dive to depoly it's weapons. The speedbrake in the -15 is purely a highly effective brake whereas the -25T brake is not.

 

The -25T has a straight fat wing with leading edge devices which will allow it to produce more lift for a given speed whereas the -15 wing is a thin super critical wing with no leading edge devices and is much less efficient at producing lift then the -25T... especially in the approach speed range.

 

IMO, I don't think there is as big a problem as you think. :)

 

The above -25T brake discussion was an interesting one. :)

Shep-xwind.zip

[capttrob]

Hey Shep... good to see you back....

 

 

I'll keep this brief.

 

The 25T accelerated in a 4000 fpm descent with gear, flaps and speed brake "air grabbers" out at idle thrust, square fuse, numerous pylons and a lower weight than the F-15C and squaring the turn to final.

 

The heavier -15C decellerated when i "dove" for the ground at almost 6000 fpm with the speed brake out.

 

Thats it.... :smilewink:

 

I also disagree that the 15 speed brake is "much" larger than the 4 plates combined which make up the speed brakes on the 25T. Not to mention the 25T essentially has 8 plates for speed brake when you include those intrusive "air grabbers" that the 15 does not have. I would almost put a bet that the 25T speed brakes combined amount to a larger surface area sticking out in the breeze than the solo 15 brake. I would also bet that the 25T "air grabber" speed brake design creates more drag than the 15 spoiler design. But, i could be wrong.. .and i dont intend to research it... ;)

 

 

Lets keep this simple shall we?

 

Basically, you guys are telling me the Frog has a more slippery design in the landing config with 4 speed brake air grabbers at a lower weight than a swept wing F-15C in the clean config with a spoiler design speed brake deployed... got it.... lol.. thanks for clearing it up or me...

 

ok.. .so it wasnt so brief.... ;)

[britgliderpilot]

britgliderpilot,

 

I'd prefer not to bust out my resume in this thread, however, if you really need to know, feel free to pm me and i'll send you a copy. If you need to know each jet i have flown, i guess i can scan my logbook and send you pics. But, i wont.. ;)

 

I said it before and i'll say it again. The F-15C is a non-event in my mission in the clean config. The 25T flys like a slick platform with "air grabber" speed brakes in the landing config. It doesnt seem accurate to me. What you do with my opinion is up to you. It almost seems like you are offended that im stating a negative opinion on the 25T. Is this your favorite airplane or something? Did you contribute to the design? I dont mean to offend anyone.... so my apologies if i have done so....:D

 

I am stating my opinion based on experience. My experience tells me that the 25T is a poor design and something is wrong. You dont have to agree, and you certainly dont have to pass judgement.

 

Hmmn, modesty ;)

 

OK, PM it to me, I'd quite like to see :)

I don't wish to see pics, a type name will suffice - I'm not kidding, I really am interested!

 

 

Have watched the tracks and still don't see any major showstoppers - assuming they're playing back properly on my version, you got the Su25T down to 220km/h at one point, but I didn't see you get the F-15 below 150knots (roughly 300km/h) at any point.

 

You also don't dirty up the aircraft totally until quite late on in the approach - when you do, it seems to slow down quite fast.

 

And an aside which has nothing to do with the subject under discussion - you don't compensate for the roll coupling when kicking the rudder to slew the aircraft onto the line of the runway. Makes for some truly frightening looking landings . . . . were you being lazy? ;)

 

 

The Su25T isn't quite my favourite aircraft - I think it's an overweight, underpowered, draggy SOB . . . . . as you might guess, I don't exactly have much truck with that sort of thing!

 

It's a challenge to fly it precisely, though, so I do fly it for fun value. IMO the big Su25T campaign is the best of the lot :)

 

 

I'm unsure at this point whether you're questioning the real Su25T design or the modelling of it in Lomac - care to clarify at all?

 

 

You say you have an issue with Su25T modelling based on your experience . . . OK, that's fine.

 

But you posted about it, so I guess it's not too unfair to assume you'd like input about the topic . . . . I'm giving an opinion.

[britgliderpilot]

Lets keep this simple shall we?

 

By all means.

 

Basically, you guys are telling me the Frog has a more slippery design in the landing config with 4 speed brake air grabbers at a lower weight than a swept wing F-15C in the clean config with a spoiler design speed brake deployed... got it.... lol.. thanks for clearing it up or me...

 

 

"Slippery design"?

 

 

I'm telling you that I believe it to be entirely possible that the F-15 has a higher Cd in the landing config, than an Su25T in the landing config, at the same speed.

 

 

Swept wing does not equal less drag at low speed . . . . .

[Shepski]

Did you even look at my tracks?

 

In both cases, I dirtied up the jets, brake/gear/flaps/half flaps for the -25T and dove to try to get to an altitude that would allow me a good stabilized approach.

 

In the -25T, at a descent rate indicating 9800 ft/min I accelerated 40 km/h with only half flaps and when full flaps were set as I started to level off, the jet immediately started decellerating and I retracted the brakes.

 

In the -15, at a descent rate of 6000-7000 feet/min I accelerated only about 1 or 2 knots and retracted the brake as I leveled off. Both these allowed me for a stabilized approach on the glideslope using the proper amount of thrust to maintain it.

 

It all looks normal to me.

 

You're giving the speedbrakes alone on the -25T way too much credit for slowing the machine down I'm afraid. They aren't nearly as effective as you think.

[capttrob]

lol... who cares....

 

enjoy your 25T :thumbup:

[Shepski]

If you're not willing to look at my tracks after all this discussion then this is pointless.

[Force_Feedback]

Give it F-404 AB engines from the F-18, then we'll talk about the su-25t.

 

It's quite the compromise now, you either have this draggy pig that won't do anything except in clean config with 30% fuel. Then you have the pig's father, which has a better acceleration and is quite bearable to fly, but when you mess up, or get hit by a Strela you either won't see the damage, or shutting down the damaged engine won't do you any good, as the fire will relight again and your plane goes BOOM.

 

Let's hope we will finally have something agile with AFM in 1.2 ;)

[capttrob]

Shep,

 

The -15C decellerates in a 6000 fpm descent with only speed brake deployed.

 

The 25T accelerates in a 4000 fpm descent with gear, flaps, and 4 "air grabber" speed brakes deployed at a lower weight.

 

I think something is wrong. You guys disagree. Its cool... lets move on... You guys fly the 25T whenever you like. I choose not to fly it... ok? :thumbup:

 

No hard feelings... group hug.... :D

 

edit: PS Brit, i decided against sending you my resume... sorry bud... maybe next time ;)

[britgliderpilot]

lol... who cares....

 

 

Clearly you do - you started a topic about it :P

 

 

And now have no interest in fighting your corner - or, apparently, PM'ing me a brief list detailing a couple of types you have hours in . . . . . which I'm interested in as an aside to this discussion.

[capttrob]

Clearly you do - you started a topic about

 

I know... what a mistake that was.. huh? Wont happen again.. promise...:thumbup:

[Shepski]

Attention to detail seems to be the problem with your findings Capttrob...

 

I just flew your two track files and with a fully dirty -25T and a 20 m/sec descent(4000 ft/min) it did not accelerate.

 

In the -15 with just the speedbrake and a 6000 feet/min descent the jet accelerated slightly... you seem to be misreading something.

 

Anyway... time to leave this waste of a thread.

[SwingKid]

Well, I have to admit that my suspicions have been raised.

 

Flying a minimum-fuel Su-25T in Flaming Cliffs with no payload at 1000 m altitude, I'm getting about 2.5 m/s2 acceleration in an idle-thrust 30 degree dive at 700 km/h with airbrakes, about 4 m/s2 without.

 

Meanwhile Bedretdinov's book also claims the airbrakes "provided for steep dive without considerable acceleration at flight speeds of less than 700 km/h."

 

A frictionless aircraft would only experience 4.9 m/s2 in a 30-degree dive.

 

So, I'm not an aerodynamics expert, but considering that even an artillery shell can barely break the sound barrier in a 45-degree dive, I also have the sense that the Su-25(T) in Flaming Cliffs is "slippery" - with or without airbrakes.

 

-SK

[diveplane]

Anyway... time to leave this waste of a thread.

 

 

totally agree,

 

i say leave the job to ed the experts and real pilots they have in asking for ref to help aid design and test the models....in lockon..

 

leave to think this thread is closed....

[SwingKid]

For comparison, I used my "miniZAP" missile aerodynamics simulator:

 

http://www.ecf.utoronto.ca/~pavacic/missiles/minizap.zip

 

Instructions:

(1) Click the drop-down "Missile Parameters" combo-box in the upper left

(2) Select "105 mm Artillery Shell (test)"

(3) In the middle column, in the "Intercept Controls" box, set "Launch Loft Angle" = -30

(4) Set "Launch Speed" = 700 km/h

(5) Set "Launch Altitude" = 1000 m

(6) Click "OK" button

 

In the right column, under "Trajectory calculation", look down to the last few boxes. Acceleration is in two boxes near the bottom - "turning" acceleration and "linear" acceleration. Observe that the linear acceleration of this artillery shell is only 2.9 m/s2

 

Meanwhile, our lightly-loaded Su-25T in the same conditions is out-accelerating the artillery shell, at idle thrust, with 4 m/s2!!

(reduced to a mere 2.5 m/s2 if we employ speedbrakes)

 

Ok, I'm becoming even more persuaded, that the AFM Su-25(T) is too slippery.

 

:(

-SK

[capttrob]

Drag test. Same mission as other tracks.

 

Route - straight ahead in a controlled descent.

 

Su-25T

 

Start Airspeed - 430

End airspeed - 450

Configuration - Flaps, Gear, Speed brakes deployed at start of mission

Vertical speed - 20m/s descent (4000 fpm) till ground crash.

Power - Idle

Weight - 33,700

 

 

 

F-15C

 

Start Airspeed - 255

End Airspeed - 195

Configuration - Speed brake deployed at start of mission

Vertical speed - 4000 fpm descent till ground crash.

Power - Idle

Weight - 41,652

 

Tracks are saved if anyone wants to see.... :v:

[peterj]

Accelerating faster than an artillery shell in a dive seems a bit off, however

the engines delivers some power at idle, it will slow from 800 to 700 in half the time with the engines off as opposed to idle.

 

OT what's the small surface in the rudder for? it usually goes in the opposite direction when you apply rudder, some source said it had a two-piece rudder for reliability.

[iVIPER21]

Interesting discussion. Anyways, if you were "slam-dunked" into an approach, why not get creative and revert back to your private pilot days - use a forward-slip to landing? I'd be interested to see how that plays out in a full dirty config with crosswind.

[4c Hajduk Veljko]

OT what's the small surface in the rudder for? it usually goes in the opposite direction when you apply rudder, some source said it had a two-piece rudder for reliability.

I believe it is a yaw dumpener.

[ARM505]

Maybe the Su25T's airbrakes are like the 737's - lots of rumbling, but not much else. At least at approach speeds!

[Yo-Yo]

Well, I have to admit that my suspicions have been raised.

 

Flying a minimum-fuel Su-25T in Flaming Cliffs with no payload at 1000 m altitude, I'm getting about 2.5 m/s2 acceleration in an idle-thrust 30 degree dive at 700 km/h with airbrakes, about 4 m/s2 without.

 

Meanwhile Bedretdinov's book also claims the airbrakes "provided for steep dive without considerable acceleration at flight speeds of less than 700 km/h."

 

A frictionless aircraft would only experience 4.9 m/s2 in a 30-degree dive.

 

So, I'm not an aerodynamics expert, but considering that even an artillery shell can barely break the sound barrier in a 45-degree dive, I also have the sense that the Su-25(T) in Flaming Cliffs is "slippery" - with or without airbrakes.

 

-SK

 

Your comparison with artillery shell is very dramatic :) and can screw away somobody's brain... not very proof to such imaginations.

But you should have known that there is a thing named "BALLISTIC COEFFICIENT" . I mean that if you take in consideration .22 bullet and 380 mm shell in 45 degree fall their speeds will be very different. You should have known that if you increase the mass of the shell its drag increases too, of course, but not proportionally so the dive speed increases for heavy bodies.

Do you remember what "Tallboy" and "Gran Slam" invented for?

 

I am sick and tired to declare that the flyable planes in LockOn is tuned so close to their real prototypes as we can do regarding the data we have those time. Either SFM or AFM especially.

 

If somebody prefers to declare "I don't beleive that XXX can fly so slow (fast, low, high)" - it's a matter to discuss somethere in the church.

If somebody wants to declare that something wrong - please PROOVE it using not MURZILKAs (MURZILKA was a popular Soviet magazin for elementary school children) but something like FM.

 

I don't care what Bedretdinov wrote - it could be wrong interpreted info or something similar. I only know that REAL clean Su-25 with flaps up and airbrakes extended does not exceed MACH limit while diving from 3500 m up to 40 degree dive angle. Try it in LockOn.

You can also estimate what Cx must airbrakes provide to make 30 deg 700 kph IAS staedy diving. Don't forget to calculate their area first...

 

 

P.S. My thanks and respect to britgliderpilot. You did my job... :) Your explanation of F-15 and Su-25 drag was very good and clear.

[capttrob]

I also have the sense that the Su-25(T) in Flaming Cliffs is "slippery" - with or without airbrakes.

 

-SK

 

The Su-25T is more slippery with speed brakes, gear and flaps extended, 10 pylons hanging off the wings side by side, and square fuse than an F-15C with its speed brake extended only. The F-15C single spoiler speed brake alone is significantly more effective than gear, flaps, and four air grabber speed brakes combined on a Su-25T weighing almost 8000 lbs less.... in this game... you decide....

 

That is all... :music_whistling: