Parasite and Induced drag on Su-25/T

[SwingKid]

Good link Crusty,

 

What in the...

 

 

Which one is the REAL Crusty?!

 

-SK

[TucksonSonny]

Does it have any programmed into the design?

 

I checked out the Su-25 and 25T today. This thing is programmed more slippery than an F-15 or a MiG. lol... I was practicing crosswind landings today to check out the weather generator in this program. It takes like 5 miles dragging everything out (airbrake, flaps, gear) power at idle in level flight, just to get this thing to slow 20 km/h (numbers arent exact, but you get my point). For something so dirty looking even when its in the clean config, it sure is slippery. Whats the deal? Is there an in flight drag chute or something? lol. I thought my power was stuck so i double checked my throttle settings. Is there a button to slow this thing down?

 

Sorry if this argument was already covered in the huge thread (this night I don’t have time to read all of it).

In my opinion you can’t compare the 15 with the 25T.

F-15C:

Wing span: 13.05 m

Wing area: 56.5 m2

Su-25T:

Wing span: 14.52 m

Wing area: 30.1 m2

Conclusion: the F-15 almost doubles the wing area (56.5 vs 30.1)

Considering that during the landing (2 wheels down, nose wheel up) the wing area operate like a huge air-break which is almost 2 times more effective than with the 25T.

[Yo-Yo]

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.

 

 

 

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

 

I am surprized... your ignorance shocked me. ANY DATA FOR ANY PLANE uses STANDARD AREA named WING AREA to make standard drag comparison of the planes. The drag of the plane is measured then this force is divided by dynamic pressure and THIS AREA. This standard Cd tells about the plane aerodynamics.

Have you ever see polar charts? I am glad to report you that L and D values are the coefficients obtained by this way.

 

Yes, AFM DOES USE this area. But not for calculations - only for reference to obtain total drag FORCE airframe must provide. And this area is used only during AFM elements design. Each element has its own area, coefficients amd many, many other things to calculate.

[capttrob]

In all this has turned into an interesting thread to read, though some of the formulas make my eyes cross. As far as the expression "half again as much" is concerned, I've lived in America all my life and it's an expression I'm very familiar with. So it's definitely not just a British thing.

 

Rich

 

 

lol.. there is a very real possibility i have heard the expression, but always interpreted it as "half as much". Thank goodness im not a bartendar or i'd be shorting customers who asked for "half as much again" alcohol in their drink.... ;)

[capttrob]

I am surprized... your ignorance shocked me. .

 

Yo-Yo, do you work for ED? I see that ED Team is on your avatar, but not sure if that means you work for ED. I was a bit conused about the "ED Tester Team" signatures as well till Alfa clear it up for me that they are just regular customers like everyone else, chosen to do beta testing.

 

If you do work for ED, and Swing Kid is a customer, did you just call your buying customer ignorant? Sorry, it just took me by surprise....:doh:

[GGTharos]

I believe Yo-Yo is the FM programmer. While I wouldn't go calling anyone anything, being called on something shouldn't particularely be surprising eithe r...

 

ED members get the 'you're doing it wrong' stick thrown at them all the time (I'm guilty of it too) Niceness probably wears down.

 

I would - guess - that Yo-Yo might be frustrated with people effectively saying that he doesn't know what he's doing.

 

Anyway - as you can see, he gave you an answer: They use the wing area as fart of the coefficient, and add fuselage and other parts to the coefficient calculations.

 

Now, that doesn't mean there isn't a bug somewhere, but it does mean that as far as Yo-Yo is concerned, the calculations should be correct, and you'll need to do a lot of work to prove him wrong. :)

 

He's the expert after all, right?

 

It may feel wrong to you, and it may -be- wrong, or maybe it isn't.

 

But he's the guy doing all the math, and the simulation is -all- about math, so ... you need to show where the math's going wrong conclusively. And so far ED has had Su-25 pilots (AFAIK) tell them that their model is pretty good.

 

So at this point, about the only way to resolve this is to get a Su-25 pilot to figure this one out.

 

I suggest that you look at reasons for what's portrayed being POSSIBLE ... I think showing both sides can bring about a better conclusion/understanding.

[capttrob]

Its perfectly reasonable to get frustrated in a good debate and fact finding process. However its another thing to call a paying customer ignorant, no matter how ignorant they are, if you want to make money in your business. Maybe im the one who is ignorant, but it just doesnt make good business sense to me. And no matter how accurate he is, Fighter Ops is looking more and more attractive every day. Unless of course Yo-Yo has a hand in their development. As far as im concerned, the customer can call me names, tell me im a liar, or tell me my product sucks as much as he wants, and i'll smile back and say thank you every time if he is buying my product.

 

I understand other possibilities which do make sense for the 25T behavior, such as the idle thrust settings we have discussed. I have no doubt that Yo-yo knows alot more about the 25T than i...However. for something that needs to be pretty precise as a weapons platform/bomber, it sure is a handful to get the thing to slow down.

[GGTharos]

Yep ... it's a pig. PIIIIG! :D

 

*runs away from frog drivers*

 

No, we're not talking about getting frustrated in a good debate and fact finding process. We're talking about hearing the same thing quite often ... ED hears it here, they hear it on the Russian forums ... and quite frankly, I don't understand why those who dish it out can't take it ... regardless of wether they are buying customers or not.

 

Understand that while he called you ignorant directly, you called him incompetent back-handedly. To be honest, I really do -not- see where being a 'buying customer' gives anyone the right to do this, yet is seems to be socially acceptable to bash the producer, but if you're a paying customer! Oh man! Untouchable!

 

Get over it man, it's not like he railed on you and got into a flame war with you. We all have better things to do than to hang on words, no? :)

As for Vapor Ops, I'll call it Fighter Ops when it produces something errr ... fun :)

 

 

Anyway:

 

Su-25: Pig. Now, it -is- stable ... and unlike a fighter, rapid decelaration is probably not part of its requirements (As you can see, a lot of stuff is not 'part of its requirements' ... ) :P

[capttrob]

He never called me ignorant, he called Swing Kid ignorant, when Swing Kid included "With all due respect" in his post.

 

 

Im over it, but like i said before, i wont be recommending it ;)

 

So, who gets hurt here?

[britgliderpilot]

Its perfectly reasonable to get frustrated in a good debate and fact finding process. However its another thing to call a paying customer ignorant, no matter how ignorant they are, if you want to make money in your business. Maybe im the one who is ignorant, but it just doesnt make good business sense to me. And no matter how accurate he is, Fighter Ops is looking more and more attractive every day. Unless of course Yo-Yo has a hand in their development.

 

I understand other possibilities which do make sense for the 25T behavior, such as the idle thrust settings we have discussed. I have no doubt that Yo-yo knows alot more about the 25T than i...However. for something that needs to be pretty precise as a weapons platform/bomber, it sure is a handful to get the thing to slow down.

 

SwingKid's not just a paying customer, he's a respected member of the forums who's knowledgable and resourceful - reference the dynamic campaign he's been working on . . . . .

 

He's well known around here and is usually pretty good - I seriously doubt he's going to run off through a comment like that ;)

 

 

The other possibility that makes sense for the Su25T is that the thing's modelled correctly!

 

If you're only interested in a sim that agrees with your opinion of what something should be, without other reference . . . . shrug.

 

 

You talking about Fighter Ops, btw?

 

(edit - swear that said Flight Ops when I clicked reply . . . .)

[GGTharos]

He never called me ignorant, he called Swing Kid ignorant, when Swing Kid included "With all due respect" in his post.

 

 

Im over it, but like i said before, i wont be recommending it ;)

 

So, who gets hurt here?

Then you should leave it between Yo-Yo and SK ... The exchanges between between beta testers and ED are a little different ... debates can be more agressive there for a variety of reasons. And I'm pretty sure I've seen SK take a 'tone' as well.

 

As for who gets hurt ... there are a whole bunch of people who -will- be reccomending it ... frankly ED has put a lot of work into this, the devs come to the english forum to talk to people despite it not being their first language, and for a single incident like this, which is rather minor, to go talking about not reccomending it ... okay, well. Good luck with other devs. Haven't seen a team yet who doesn't occasionally say something off-color. Unless, of course, you end up talking to their PR guy, and not the devs themselves!

 

So, you're obviously not over it, and you're not considering all the facts ;)

 

Anyway that's my opinion and I'll leave this here - if you want to continue this particular conversation for any reason feel free to PM me, I'm not here to attack your character, I just think there are far too many kneejerk reactions here, on both sides.

[Red Star]

As for Vapor Ops, I'll call it Fighter Ops when it produces something errr ... fun :)

 

I'll call it Fighter Ops when they actually release the fighters. Until then it's Trainer Ops. ;) :lol:

[capttrob]

 

 

You talking about Fighter Ops, btw?

 

(edit - swear that said Flight Ops when I clicked reply . . . .)

 

It did. I edited it. We have a fractional operator here in the states called Flight Ops, force of habit.... :D

[SwingKid]

Yes, AFM DOES USE this area. But not for calculations - only for reference to obtain total drag FORCE airframe must provide. And this area is used only during AFM elements design. Each element has its own area, coefficients amd many, many other things to calculate.

 

Well, then I'm confused. In one message you tell me that I shoulnd't be using cross-section as a reference area, because I should be using wing planform instead.

 

Now you say that in fact AFM doesn't use wing planform, but rather "each element has its own area."

 

Ok, so what is the area used to calculated parasitic drag from the Su-25T main body? And what Cd, does it have?

 

I'm not talking here about the induced drag from the wing, that varies with angle of attack - I'm talking about the parasitic form drag from the aircraft body, that generates zero lift. It seems to have Cd of about 0.05 in Flaming Cliffs, using the body's own cross-sectional area as a reference. Can you show a different calculation?

 

I don't think that we can use the wing planform as a reference for parasitic drag Cd, because the wing planform induced drag changes rapidly with angle of attack, while parasitic body form drag doesn't.

 

-SK

[SwingKid]

Thanks for kind words guys, but I'd really rather have the dollars. Usually at the end of fifteen pages - somebody - or the developer or the tester - agrees that he was wrong.

 

So come on, there may be only two more pages to go - place your bets! Yo-yo or SwingKid?

 

;)

-SK

[Shepski]

Its perfectly reasonable to get frustrated in a good debate and fact finding process. However its another thing to call a paying customer ignorant

 

Relax already!

 

"ignorant" is the same thing as "unaware", "not knowing", "not having knowledge."... it isn't an insult.

 

Tone it down please.

[Shepski]

Fighter Ops is looking more and more attractive every day. Unless of course Yo-Yo has a hand in their development.

 

Now you've insulted a member of the development team and that's not acceptable... maybe Fighterops is where you should go because you are now taking a hoilday from here.

[SwingKid]

Ok, if we insist on using wing planform as a reference area, fine. Going back to this calculation, let's use 30.1 m^2 planform for S, instead of 9 m^2 cross-section:

 

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

= (11028.6 N) / (0.5 * 1.225 kg/m^3 * (194.4 m/s)^2 * 30.1 m^3)

Cd ~= 0.016

 

Now, let's compare this wing-area-referenced Cd to the comparable wing values that Crusty referenced in this post:

 

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

 

The Flaming Cliffs Su-25T is:

more slippery than a Cessna

more slippery than a Learjet

more slippery than a Starfighter

more slippery than an X-15

 

In fact, there's only one aircraft (on of the list of 17) that is less slippery than our Flaming Cliffs Su-25T.

 

:confused:

 

-SK

[britgliderpilot]

Hmmn.

 

 

That aforementioned 30% power at idle coming in again?

 

If the engines are still chucking out some thrust, there's more drag than we think.

 

 

Purely out of interest, what's the result like if the engines are switched off altogether? Any different?

[SwingKid]

Here's a trk that might be interesting:

 

SwingKid's Su-25T, out-accelerating his own bombs,

with engines off.

 

http://www.ecf.utoronto.ca/~pavacic/lomac/bombrace.zip

 

 

:confused:

-SK

[S77th-konkussion]

brake................. NOW!

[Kenan]

Here's a trk that might be interesting:

 

SwingKid's Su-25T, out-accelerating his own bombs,

with engines off.

 

:confused:

-SK

 

Maybe because thr Frog is a lot heavier then those FABs? :D

[SwingKid]

Ok - despite how funny that image looks, in order to prevent Yo-yo from exploding, I should admit that that's actually just drama. The problem with that image/trk is not so much with the Su-25T AFM, but rather, the LOMAC bomb flight model is a little bit "tweaked" - LOMAC bombs have helium in them, did you know? ;)

 

Now that I look at the .trk again, with engines off, the empty Su-25T does seem to decelerate more than my original calculation showed - it now seems to fall about 8% slower than an artillery shell, rather than 28% faster that I originally estimated. I guess I should check now the 30-degree dive also, with engines off...

 

-SK

[SwingKid]

Winners: Yo-yo, GGTharos, Shepski, britgliderpilot, Crusty&Crusty, Weta, and the rest

 

The idle thrust seems to be about 23% maximum bench static - 1900 kgf! And that was throwing off all my calculations, more than I expected.

 

I WAS WRONG.

 

Good that nobody took my bet!

 

For Yo-yo:

 

 

And now, after capttrob was so nice and concerned about my feelings, I abandon him to the wolves. :rolleyes:

 

-SK

[GGTharos]

Woah woah woah ... where's the wolf picture? ;)