Parasite and Induced drag on Su-25/T

[Ironhand]

...and whats the deal with these chutes? doesnt Russia know how to make brakes? ....

That's what happens when you give the chute makers lobby an entre into aircraft design...

 

Rich

[Yo-Yo]

Anything slower and she was stalling. That airplane was already behind the power curve at 280. Notice the hard landing. My practice runs were 300-310. What airspeed do you think it should be? Show me that track with an airspeed you think it should be please.... remember.. you also have about 25 knots of crosswind.

 

 

I cannot understand what you find wrong. The Su-25/25T was made very precisely physically and finally was tested by the Su-25 pilots. By the way - it complains standard go-round flight used for RL Su-25's.

If you have problems there are several tutorial records including crosswind landing.

I can only remind some points for approach (1500 kg of fuel):

at 600 m - on downwind with the flaps in maneuvring position holding 55-60% rpm decelerate to 390 kph, then lower the landing gear.

turn on base leg (the general practice is to make this turn to 120-130 degrees not to 90) increasing rpm to 70-75%

while on base leg descending 5-8 m/s at 65-70% and extend flaps fully when you are below 350. Keep 320-330 kph.

After you are on the final approach you must have 290 kph and 55-65% to maintain speed.

Aim the flaring point at 100 m before the threshold and reduce your speed to 260-270 till this point.

When you are above the threshold move the throttle to idle.

Set the landing pitch (Pitot tubes ends are on the horizon).

Enjoy.

[Shepski]

Heres another track i just did at a slower airspeed.

 

Hi Capt.

 

I just checked out your track and the main issue of your problem with slowing down is your initial set up of the mission.

 

You are 6.5nm from the runway threshold at 270 knots and 3700 feet above ground... does this sound like the makings of a stabilized approach? ;) You were at times descending at 20m/sec which is just under 4000 ft/min. I think you might be mixed up in using metric... :)

 

Try this set up in the editor...

 

25 km from the runway at 375 km/hour and 450 meters altitude.

[capttrob]

Hey Shep,

 

Yeah, i pretty much intentionally came in high (typical slam dunk from ATC, even though this was more tame than something you might get at CYUL ;) ). Most approaches (3.0) at the marker are set at 1500 agl. The first left base i didnt expect much deceleration. But i didnt expect acceleration either with speed brakes, flaps and gear.. final flaps came in outside the marker IIRC and i think i was just above GS at that point. Now im dragging full flaps, gear, speed brakes, thrust still idle and shes barely decelerating on a standard GS (if Krans has a STD GS, it actually looked more like 2.5, but hard to tell with pixels.. :)). I really wasnt sure what the VSI was telling me in terms of units. But once near the marker, it looked about right...lol. I just figured it would be slowing down alot more rapidly with the barn doors out and idle thrust once on the GS. Its still workable... as you saw in the track.... :thumbup:

[capttrob]

Heres the same mission with the F-15C. A non-event. Same distance, altitude and airspeed. Now again, i havent flown either model in real life.. .but im thinking the F-15 is a bit more slippery than the Frog...:smilewink: .

 

I know the F-15C in this game is a BFM, and the Cd may be a bit overmodelled for the 15, but note the amount of power i needed with flaps and gear to maintain GS. Whereas the Su-25T was at idle with full flaps (i believe the 25T flaps extend further than the 15, 2 "notches"), gear and speed brake. Im no expert on either of these models, but something aint right.... Also keep in mind the F-15 is a few thousand pounds heavier, at a slower airspeed and less Cl. Also notice i turned directly to the numbers with the F-15 (modified left base), instead of squaring it off like i did in the Su-25T.

 

Imo, the F-15 acts more like a real jet should with speed brakes. :thumbup:

 

(And for others, gotta love that F-15 braking action...... look ma.. no chute.. :thumbup: )

f15xwnd.zip

[britgliderpilot]

. . . . . I'll scribble some numbers down at some point later, I'm too tired right now, but I'm not sure that your assumptions about the F15 are valid.

 

 

Quick few pointers 'bout the F15:

 

- Smaller wing area than the Su25T (edit - smeg, it isn't . . . . looks like it should be, though)

- Supersonic airfoil as opposed to the subsonic/transonic airfoil on the Su25T

- Bugger me, what a big speedbrake

- Two notches of flap does not necessarily mean more flap than a single flap setting, and it certainly doesn't mean more drag.

 

- Reports from Su25 pilots that they NEED the airbrakes just to prevent from overspeeding on attack. It's a heavy bugger, that aeroplane . . . . .

 

 

You muttered about induced and parasitic drag in the thread title - well we've demonstrated here that you're coming in quite fast for the Su25T.

What do we know about speed and induced drag?

 

 

You may still have a point, and without research/scribbling I'm not sure, but at the moment I'm sceptical about the conclusion you draw from your F-15 comparison.

[capttrob]

. . . . . I'll scribble some numbers down at some point later, I'm too tired right now, but I'm not sure that your assumptions about the F15 are valid.

 

 

Quick few pointers 'bout the F15:

 

- Smaller wing area than the Su25T (edit - smeg, it isn't . . . . looks like it should be, though)

- Supersonic airfoil as opposed to the subsonic/transonic airfoil on the Su25T

- Bugger me, what a big speedbrake

- Two notches of flap does not necessarily mean more flap than a single flap setting, and it certainly doesn't mean more drag.

 

- Reports from Su25 pilots that they NEED the airbrakes just to prevent from overspeeding on attack. It's a heavy bugger, that aeroplane . . . . .

 

 

You muttered about induced and parasitic drag in the thread title - well we've demonstrated here that you're coming in quite fast for the Su25T.

What do we know about speed and induced drag?

 

 

You may still have a point, and without research/scribbling I'm not sure, but at the moment I'm sceptical about the conclusion you draw from your F-15 comparison.

 

Smaller wing area makes for a more slippery airplane. You should know that glider pilot.. ;)

 

Supersonic swept airfoil as opposed to a typical straight wing also makes for a more slippery design. Note Mach Crit numbers and Cd relation.

 

The surface area of the -15 speed brake looks big because its one piece. Add all those 4 plates together on the Su-25T (not to mention the "air grabber" on the 25T which the -15 doesnt have) and i'll bet the Su-25T airbrakes are supposed to be more effective. Keep in mind the 25T is also at a lower weight (less momentum). Additionally, the square fuse and all those things hanging off it, plus the fact i had gear and flaps down with the speed brake on the 25T (while i only used speed brake exclusively on the -15, stowed it, then put out flaps and gear later) and you get much more things hanging out in the breeze on the 25T than the little ol' speed brake on the -15.

 

As far as me coming in too fast. Im still waiting for someone to show me the approach and landing slower than 240 (in my mission). ;) I believe i touched down at stall in the second track (230-240). Care to win a beer britgliderpilot? Show me...:D

 

you should be more familiar with 1.3Vso than i.... calculate the approach speed for that airplane in that configuration. Its 300 IAS.

 

edit: and last but not least, have you seen both tracks? :)

[Crusty]

Just another thing to consider......a straight wing v a delta wing of the same area on the same aircraft..the straight wing is less draggy than the delta in landing phase..e.g., the 25 is near a straight wing..f15, close to a delta

the f15 is more draggy on landing

But it still remains that you were using an incorrect appraoch in the first place

 

Also...your assumption that a supersonic airfoil produces lower drag at landing speed is incorrect ....the lower lift coefficiant of the supersonic airfoil often means it needs to use a higher aoa at landing and hence can produce more drag

[capttrob]

Crusty, forgive me for saying this as i dont want to start any flame war.... but... go back to sleep bud... every one of your conclusions are completely opposite of fact.... ;) (beside the fact we arent talking about landing speeds, rather approach speed and the configuration vs. time to achieve). Too many of you are still thinking groundspeed, when we are talking about airspeed. If you choose to believe the 25T has a more efficient aerodynamic profile than the -15C, then you should feel right at home with LockOn. Personally, in all my years of flying jets, there is something definitely wrong. Speed brakes are designed for one of two things... either

 

A. Being vectored for the slam dunk

 

or...

 

B. Screwing up your approach profile because you and your buddy were too busy looking or "gifts" in the cockpit. (Shep and LawnDart know what im talking about)

 

Allow me to retort a bit. A straight wing does have more drag in the landing config than a sweep wing. However, it also produces more lift.... Matter of fact, the straight wing cambered airfoil such as that on the 25T, will produce more drag "and" lift vs. that of the -15. However, the F-15 will out power the 25T. Think flate plate airfoil.... ie. rocket science... ;)

 

the -15 has no problem making that approach, while the 25T acts like the SR-71... imo (needs a chute too... and at less weight than the F-15)

 

 

Ok... let me see if i can put this into terms the average layman can understand....

 

Everyone loves Brownies... right? (im talking about the kind you bake and eat... a la Betty Crocker). So far so good... :smilewink:

 

Those of us that make these brownies knows the force needed to stir the mix. (bear with me for a minute) :D

 

Now, some of us use a fork to mix the product, some of us use a spoon.

 

Imagine a model of these pics below were the only tools available for you to stir the mix (instead of a fork or spoon). A small model of each in scale. Lets go a step further and say your grandmother is making the mix. Which model airplane would you give her to stir the brownie mix? Which of the following models would flow through the brownie mix with less effort while still stirring the mix?

 

 

 

(keep in mind, take the weapons off the F-15. I couldnt find a picture of an F-15 empty)

 

After you are done answering that question to yourself. Now ask yourself, if the 25T had gear, flaps and speed brake out, while the F-15C had only a speed brake out and no gear or flaps... which would you give to your grandma to stir the brownie mix? (keep in mind, you love your grandma and dont want her to put in more effort than she has to....)

 

 

Silly scenario? Absolutely. But if you wanted to stir brownies with as little effort as possible. Which model would you use? Personally, i would want to give my grandma the -15C model because it looks like it would be easier to use to stir the brownies. However, if the above models were designed as they are in LockOn, i'd give her the 25T.

 

 

edit: (excuse me while i make myself another drink....:D .. hiccup... burp...)

[Yo-Yo]

2 capttrob

 

try to calculate two values for F-15 and Su-25:

 

AR = L^2/Sw, where L is wingspan and Sw is wing area.

You will be surprised...

 

Don't mix two kinds of drag. As you know, total drag is the function of two arguments -

 

D = D_0 + A*Cy^2,

at high speed the second part is low because of low Cy required. At low speed before landing the second part is much greater. Coeff A is greater for F-15 because it has less aspect ratio.

 

By the way, the maximal Lift/Drag ratio is about 10-11 for F-15 and 14.5 for Su-25.

Sic!

 

 

P.S. The maximum Lift/Drag speed is about 350-400 kph for Su-25 and about 500 kph for F-15 (clean). As you can see Su-25 is better glider than F-15 at low speed.

[capttrob]

Thanks for your reply Yo-yo.. and although i love Pulp Fiction, hitman had the ultimate signature, "Its the one that say Bad Motha-fuka...". You may want to try going with, "Zed is dead baby... " ;)

 

I admire your input as far as formulas.... however.... (ready for this?) :smartass:

 

Its the old cliche of scientist vs. monkey. You are the scientist, and it looks great on paper. I am the monkey with thousands of hours in actual jets. Combined with tens of thousands of hours watching airplanes. Now its been a while since i studied under my aeronautical degree (im kind of partial to (rhoV^2)/2), but, i've been flying airplanes for a very long time. If this 25T design in LockOn is accurate as endorsed by real 25T pilots, i actually feel for them if they have to fly it. I'd also like to know when their last currency on type was performed. If the F-15C in this game is any relection of the real thing, im starting to doubt american technology (although Sept 11 and the Iraq war has created many doubts for me... not that i want to get political.. :D )

 

My next concern, is why the -15 is a non-event with much less form, parasite and induced drag?

 

Then again, i dont even know why im getting caught up in this since i never fly the 25T to begin with...lol... and now that i have seen its behavior... have no desire to..... :thumbup:

 

The whole reason i even tried to fly the 25T in a crosswind.. is because of this thread.... http://forum.lockon.ru/showthread.php?t=15805

 

Of which isnt even a crosswind landing.

 

Edit: Imo, the 25T is a poor design considering the amount of time designing an AFM. My apologies for being blunt, call me Simon... however, the game still rocks... for me..... despite its bugs..... ;)

[Crusty]

edited out, ..POINTLESS JUST EXPLAINING WHAT i ALREADY SAID...i STAND BY IT 100%

[capttrob]

Crusty, you are so confused on airfoil design, it is the reason i made the humorous remark of going back to sleep. Its a good thing that is your last post. However, if you do choose to post again, please familiarize with A,B,C and D (and even E) category approach mins and why they are different before you post. Also, as britgliderpilot pointed out, refresh your academics with respect to NACA airfoil design, Cl vs. Cd, and the effect speed has on induced drag. Hopefully you'll be able to see a pattern. After you get done with that, you may want to reresh your memory with respect to Mcrit numbers and the difference between swept vs. straight wing. Take particular note to the Cd and where it spikes on different wing design. After you find the spike and see the relation, follow it back to see which NACA airfoil design has more drag at lower speeds. Then, when you are done with all that, study some Vg diagrams and L/D max charts with diferent wing designs. If you initially studied approach speed category.. it will all come together.... Let me know i you need some links to NACA tutorials.... ;)

 

edit: No flames... just facts. :v:

[SimFreak]

If pilots of Su25T confirmed properness of FM, and capttrob had neither, does that make FM wrong?

[capttrob]

All i know is that the F-15C in this game flys this mission as a normal slam dunk in a jet with speed brakes. The 25T flys this mission as an SR-71 or Space Shuttle (exaggeration note.. :megalol: ). If you believe the 25T should not slow down as rapidly as an F-15C despite added drag and lower weight on the 25T, more power to ya....:thumbup:

 

You wont find me flying the thing unless i join a server where i am able to even the teams by flying a 25T.

 

Bottom line, i hate to break it to you die-hard simmers, but there is a serious problem with the 25T and/or the -15C. Shep will tell ya after he see's my 15 track for the same mission.

[diveplane]

su25t has the advanced afm , f15 doesnt

 

 

remember that..., its been known that most russian aircraft have high

landing speeds, hence the drag chutes

[capttrob]

lol

 

 

 

:pilotfly:

 

 

 

Thanks for your input diveplane.. :)

[brewber19]

Hi all,

 

This has been an interesting topic to follow for me, since I almost exclusively fly the 25T/vanilla, and not being a pilot or an aeronautics engineer the value of my input is mere opinion BUT....

 

yes, the Frog is very slick on landing when "clean" but stick any weapons on it and it slows down real nice. For me, its a "realistic" difference between dirty and clean re: weapons. I dont fly fighters, but I assume a simple drag factor is present when clean versus dirty, perhaps its just far less pronounced in the SFM because its...er...simple!

 

Like anything, you get used to flying it. In 1.12a (thank god they reduced the drag of the MPS and Fantasmagoria) the T can now fly at 650/660 fully loaded versus 850 when clean....at 'X' altitude...so it makes sense for it to lose speed slowly when running in on finals. My preferred landing profile is shallow, so if I dont bleed enough speed off, I exchange speed for height...most runways are long enough and I 99% hit the rubber marks on runway.

 

Ultimately though, the discussion is academic, since the A/C "are what they are" - before FC I only flew the a-10...now I only fly the Frog....may be with BS all I'll fly is the chopper - we just need to deal with the limitations of our A/C and have fun....or die trying to develop better tactics.

 

Anyways, have fun all ;)

[britgliderpilot]

Without deliberately trying to sound disrespectful here, captrob, you're sounding awfully cocky . . . . . . ;)

 

If I were you I'd be taking a more matter of fact approach here . . . .

 

Purely as a matter of course, I rarely bother with the 1.3x stall speeds - my approach speeds vary so much with wind that I rarely fly the same approach speed two days in a row.

 

Haven't seen the tracks - I'm on v1.12 still. One day I'll get around to v1.12a, but not today.

From the description, both approaches should be flown at similar speeds, right?

 

 

Anyway - done the scribbling and the F-15 is indeed at a lower Cl than the Su25T - it gets interesting, doesn't it? :)

 

 

Ookay . . . . next step, lift/drag curves . . . . this stuff I do know, but can't be too specific about it since I don't have the two curves here for comparison.

 

As Yo-Yo has kindly informed us, the max L/D for the F-15 is 11:1. Max L/D for the Su25T is 14:1.

 

So if they're both at max L/D, for the same Cl the F-15 will have a rather higher Cd.

 

 

Rough assumption, the F-15 wing will have max L/D at a higher speed than the Su25T - it's a supersonic aeroplane and is supposed to fly at higher speeds than the -25T, no sense in having it any other way.

 

As such, at a similar landing speed the L/D of the F-15 will be less than that of the Su25T - since this speed in the F-15 is further below the max L/D speed than the same speed in the Su25T.

 

 

Conclusion?

Yeah, the F-15 may be at a lower Cl than the Su25T - but it's still possible for the Cd to be greater.

 

Plus it does have a rather large speedbrake on top of it . . . .

 

 

 

Now that I've recounted this theory from memory (making sense to me thus far) . . . . I'll go away and see if I can find suitable graphs to demonstrate the point.

[diveplane]

some 1 wrote .......you're sounding awfully cocky

 

 

i say that your right on that...

[SwingKid]

Adding more fuel to this fire:

 

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

 

:music_whistling:

-SK

[capttrob]

Haven't seen the tracks - I'm on v1.12 still. One day I'll get around to v1.12a, but not today.

 

 

The above quote says it all.........

 

I had typed out a rebuttal. But, you know what? I just erased it. If you feel the 25T is accurate, by all means, fly your heart out.... :thumbup:

 

 

Cocky? absolutely. That is my intention... in a humorous way.....

 

If it offends anyone, let me know, i'll tone it down a bit.... :)

 

edit: Why, oh why, did i even start this thread..... what a waste of time.... lol :)

 

Lets just put it this way... I put a retired Marine, 18,000 hour buddy of mine.. .close to retirement part 121... flying LockOn. He was fed up within 15 mins and wanted nothing to do with it....lol.

 

However, i enjoy the game.

[britgliderpilot]

Humorous?

 

Nah, just sounded cocky from over here ;)

 

 

Hard to have a serious discussion with "I'm-right-I'm-100%-right-and-I-know-I-am" attitudes, after all . . . .

 

 

I don't believe you are right, and I believe you've described your issue sufficiently for me to present an alternative suggestion - but y'seem a bit miffed at the suggestion that you might in fact have missed something . . . . .

[GGTharos]

Sooo anyway ... F-15C ... clipped delta wing ... delta wing=big-ass speed brake.

 

Su-25 ... straight, think wings. Kinda like what they used on the F-104 ... so, who's got less drag at given AoA? ...

 

 

It would be interesting find a real Su-25 pilot to ask about this, and I think the 'should do this' and 'should do that' based ON OTHER AIRCRAFT (or missiles, or hair on the back of your neck) needs to be toned down,

since 'it makes sense' has been proven to yield incorrect results before - at least in avionics, if not elsewhere.

 

In addition, IIRC turbojets allow air to flow through quite nicely compared to turbofans at idle speeds - but don't quote me on that, I'm no engineer ... the F-15 has -significantly- larger wing area due to the lifting body, IIRC, while at it. The frog, on the other hand, is like a flying brick with corrective surfaces to make it steer where you want it to go ... in short, when going -down- you have a whole lot less air mass to contend with in the 25.

 

But, since it only 'makes sense' ... it's probably wrong anyway :P

 

 

my 2 uneducated cents.

[capttrob]

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?