Parasite and Induced drag on Su-25/T

[Weta43]

this site:

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

gives Cd min for a variety of aircraft & seems to indicate that a Cd of 0.05 is not particularly slippery for a jet. (learjet .0216, Cessna 620 .0322, F4 .0205 - subsonic & .048 supersonic) Admittedly this is clean, but using your formula above if there's still 600kg thrust from the engine at idle the Cd is actually 0.056

[capttrob]

So, let me see if i can understand this. There is a possibility the Cd of wing area is taken into account for the LOFC 25T design... but not all the other stuff like the 10 pylons side by side, the engine cowls sticking out, the square fuse? Plus it nets 30% thrust at idle? Oh man..... lol

[SwingKid]

this site:

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

gives Cd min for a variety of aircraft

 

Those aren't aircraft, those are wings.

 

-SK

[GGTharos]

I think you guys are misinterpreting Yo-Yo ... the AFM actually seems to use a 'thrust and tow' model, where the engine 'tows' parts of the aircraft - this is why if you lose a heavy weapon, you roll/yaw, or if you lose part of a wing ... the AFM takes those forces into account. So I don't see how, judging from this, you could conclude that some big and important drag coefficients are not modelled.

 

Let's put it another way: It's more likely that YOU are wrong, than Yo-Yo ;)

 

They've for the flight performance charts to compare this thing to, and, granted, due to the non-linearity of the equations it -is- possible that there are some strange effects once you depart certain configurations but it's hard to think there was such a gross oversight when a huge attempt at such detail was made.

 

It's possible sure, just rather unlikely ;)

 

 

PS: HOWEVER! This isn't meant to have a 'chilling' effect on the conversation. Sometimes it happens that bugs or oversights that are HARD to spot creep in ... and sometimes it takes a while to even realize that they really are errors, so ... I suggest digging up as much proof as possible.

 

A lot more proof than 'I think this should be draggier' ... and I think SK was on the right path for this.

[SwingKid]

So, let me see if i can understand this. There is a possibility the Cd of wing area is taken into account for the LOFC 25T design... but not all the other stuff like the 10 pylons side by side, the engine cowls sticking out, the square fuse? Plus it nets 30% thrust at idle? Oh man..... lol

 

Yes, that's my question - does Flaming Cliffs AFM Su-25T drag include the aircraft body, or only its wing?

 

-SK

[Weta43]

No - he's saying the site linked to only looks at drag from the wing. also don't take the 30% as gospel - the point is mainly that there is residual thrust for the Su-25 but not for the F-15 & that wasn't factored in by SK to work out the Cd above but as modelled using SK's formula if the engines are still putting out 600kg the MODELLED Cd is 0.56 (which is close to the figure expected by YoYo & the more residual thrust the higher the Cd of the model - compared to a learjet at 0.0216 - seems decidedly unslippery to me.

[SwingKid]

Let's put it another way: It's more likely that YOU are wrong, than Yo-Yo ;)

 

Do you even know what Yo-yo is saying? I can't even tell if he disagrees.

 

If he does, then I'm ready to take bets. ;)

 

-SK

[SwingKid]

- compared to a learjet at 0.0216 - seems decidedly unslippery to me.

 

If this is what you think is the Cd of an entire learjet, then what do you think is the Cd of its wing alone?

 

 

-SK

[britgliderpilot]

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?

 

 

You are accelerating not because drag is decreasing, but because a component of your weight is now acting forwards through the airframe.

 

That component acting forwards means your aircraft is no longer in equilibrium - forces acting forwards now are larger than forces (drag) acting backwards. You accelerate.

 

 

My lecturers, my instructors, and me all call this "thrust" for a glider . . . . we make sure we include the quotation marks, but even so I suppose it could puzzle a power pilot. Thrust is only from engines for you guys, right? ;)

[capttrob]

You are accelerating not because drag is decreasing, but because a component of your weight is now acting forwards through the airframe.

 

I never said this. Where are you getting this from? Induced drag is decreasing because you are accelerating, not the other way around. Parasite drag is increasing, also because you are accelerating. The over-all combination of drag is less than the thrust vector because you are accelerating. But drag does not remain the same acellerating in a dive as you suggested earlier. Over-all drag increases as you accelerate any moving body. Drag will increase (as you accelerate) where it eventually catches up with the constant thrust force and equals out. In a dive, this is known as Terminal Velocity. Lift and AoA, depending on airfoil design, also does not remain the same while accelerating in a dive. As you also suggested earlier.

 

That component acting forwards means your aircraft is no longer in equilibrium - forces acting forwards now are larger than forces (drag) acting backwards. You accelerate

.

 

agreed, until drag catches up with thrust and you hit terminal velocity, or impact the ground, or rip your plane apart. ;)

 

 

My lecturers, my instructors, and me all call this "thrust" for a glider . . . . we make sure we include the quotation marks, but even so I suppose it could puzzle a power pilot. Thrust is only from engines for you guys, right? ;)

 

No, i believe i stated earlier that the F-15C had an 8000lb thrust advantage over the 25T (speaking in simple terms), in an established dive at idle power, because the F-15C is 8000lbs heavier in my test. However, it didnt seem to have an effect since they both peformed rather equally in the clean configuration.

[GGTharos]

I seem to recall a highschool physics lesson where you drop a heavy ball and a light ball at the same time, see which ones makes it down to the ground first ...

 

Anyone know the answer?

 

Anyway, it would seem that for the same shape, the drag seems ... -seems- to have a fairly minor effect, even for very different masses (unless the difference is quite radical, like an empty paper ball vs. an iron one) ...

 

So I think an 8000lb mass difference is proportionally negligible here. Of course I'm comparing small objects to rather big ones, so things might be quite different (like wind tunnel tests) ...

[britgliderpilot]

I never said this. Where are you getting this from? Induced drag is decreasing because you are accelerating, not the other way around. Parasite drag is increasing, also because you are accelerating. The over-all combination of drag is less than the thrust vector because you are accelerating. But drag does not remain the same acellerating in a dive as you suggested earlier. Over-all drag increases as you accelerate any moving body. Drag will increase (as you accelerate) where it eventually catches up with the constant thrust force and equals out. In a dive, this is known as Terminal Velocity. Lift and AoA, depending on airfoil design, also does not remain the same while accelerating in a dive. As you also suggested earlier.

 

Gotcha, you were the step ahead of me.

 

Misunderstanding - I thought that this:

 

However, you reduce the induced drag variable when performing dive tests (like changing the relative wind angle in a wind tunnel).

 

Meant you thought the drag reduced simply because you were in a dive, not because you were accelerating.

 

 

 

I believe i stated earlier that the F-15C had an 8000lb thrust advantage over the 25T (speaking in simple terms), in an established dive at idle power, because the F-15C is 8000lbs heavier in my test. However, it didnt seem to have an effect since they both peformed rather equally in the clean configuration.

 

It is indeed 8000lbs heavier - only a component of that is going to act as thrust, but that's an aside.

 

 

Let me fiddle with the old spreadsheet for a bit . . . .

[britgliderpilot]

I seem to recall a highschool physics lesson where you drop a heavy ball and a light ball at the same time, see which ones makes it down to the ground first ...

 

Anyone know the answer?

 

Anyway, it would seem that for the same shape, the drag seems ... -seems- to have a fairly minor effect, even for very different masses (unless the difference is quite radical, like an empty paper ball vs. an iron one) ...

 

So I think an 8000lb mass difference is proportionally negligible here. Of course I'm comparing small objects to rather big ones, so things might be quite different (like wind tunnel tests) ...

 

Yeah, the heavy ball and the light one were independent of air resistance.

 

We did the trick with a coin and a feather - in air, and then in a vacuum.

[SwingKid]

I seem to recall a highschool physics lesson where you drop a heavy ball and a light ball at the same time, see which ones makes it down to the ground first ...

 

Anyone know the answer?

 

The ball shaped like a Su-25T, of course. We already know it falls faster than an artillery shell. :rolleyes: ;)

 

-SK

[britgliderpilot]

Hmmn, can't upload an Excel spreadsheet.

 

 

I need better L/D values for it to work properly, but if you take maximum L/D, the Drag force for the F-15 at 300km/h is half as much again as the Su25T at 300km/h. Just due to the difference in L/D . . . . .

[capttrob]

 

Misunderstanding - I thought that this:

 

 

 

Meant you thought the drag reduced simply because you were in a dive, not because you were accelerating.

 

 

.

 

Drag is decreasing when you are in a dive, induced drag (Induced drag decreases as speed increases). Parasite drag increases. Over-all drag increases as you accelerate. It doesnt remain the same...

 

Are you familiar with the difference between induced and parasite drag? Just curious, not trying to be sarcastic.

[capttrob]

Hmmn, can't upload an Excel spreadsheet.

 

 

I need better L/D values for it to work properly, but if you take maximum L/D, the Drag force for the F-15 at 300km/h is half as much again as the Su25T at 300km/h. Just due to the difference in L/D . . . . .

 

And that makes sense. If the drag acting on the F-15 is half as much as the 25T at that speed, why does the F-15 slow down more rapidly than the 25T having double the drag, plus gear, flaps and speed brakes.

 

I suppose the F-15 speed brake creates all that drag by itself... ?

 

Edit: The residual thrust advantage on the 25T does lend to a better understand of why it seems slippery with all that drag hangin out though.... if in fact its 30% more than the 15 at idle.

[GGTharos]

Half as much again, he means 1.5x, not 0.5x I believe.

[capttrob]

Half as much again, he means 1.5x, not 0.5x I believe.

 

Hmmmm, and just as it was starting to make sense.... oh well...lol

 

"Half as much again" means 1.5x? Ok.. must be a british thing. Never heard that expression...:)

 

I figured the "again" was a typo he forgot to backspace.... see what i get for thinking? ;)

[GGTharos]

Caught me by surprise when I've heard it the first time, too, in fact it still does.

 

I don't understand why people can't say '50% more' or one and half times ...

[britgliderpilot]

Drag is decreasing when you are in a dive, induced drag (Induced drag decreases as speed increases). Parasite drag increases. Over-all drag increases as you accelerate. It doesnt remain the same...

 

Are you familiar with the difference between induced and parasite drag? Just curious, not trying to be sarcastic.

 

Yeah, I missed out the word "induced" from that sentence . . . . whoops.

 

 

I am familiar with it (drag's a fairly essential thing in soaring, we concentrate on it quite a bit), but I call it parasitic drag . . . . what is it they say about America and Britland? Two countries separated by a common language ;)

 

 

Half as much again does mean 1.5x, yes - but without the exact L/D values at a given speed it's not exactly accurate.

 

At 300km/h I guess the F-15 is likely to be a bit further below best L/D than the Su25T is . . . . . but I can't prove it without the charts.

[169th_Crusty]

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

 

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

 

Good link Crusty, I found it on FO forum and... "The rocket scientist" is

already fed up with me:P

I`ve never done any "test" flying in a Frog. Not my cup of tea. I did fly it

quite a few times just to get familiar with weapon systems. The landing

approaches were... pretty much what I expected them to be. Hell, I

deviated from the SOPs and haven`t used the chute on most of my rolls.

Never ended up in the overrun and never blew a tire... yet. So, now that

we know that I`m the best pilot there is...:lol:

 

Capttrob, you wrote few pages back that you had a ex-Marine flying FC

(Frog?) and he left laughing... It`s funny coz... recently I saw Hornet

driver (also a Jarhead) doing some FC flying. Well, he has it`s own copy

of FC so it wasn`t the first time he did that. He flew the Mig and the Eagle.

As he was doing some FCLPs at Sochi (hehe) in an Eagle he was smirking

at the AOA. He was landing it at exactly the same AOA as in Hornet.

A dragged in approach and no flare... and he liked it:noexpression: (he`s a

Jarhead) - I hope he`s not reading it right now...

Back to Frogs:doh:

[capttrob]

 

I am familiar with it (drag's a fairly essential thing in soaring, we concentrate on it quite a bit), but I call it parasitic drag . . . . what is it they say about America and Britland? Two countries separated by a common language ;)

 

 

.

 

lol... good one... never heard that before either.

 

Parasitic is the propery terminology. I guess i picked up a few bad habits since my CFI days... ;)

[capttrob]

 

Capttrob, you wrote few pages back that you had a ex-Marine flying FC

(Frog?) and he left laughing... It`s funny coz... recently I saw Hornet

driver (also a Jarhead) doing some FC flying. Well, he has it`s own copy

of FC so it wasn`t the first time he did that. He flew the Mig and the Eagle.

As he was doing some FCLPs at Sochi (hehe) in an Eagle he was smirking

at the AOA. He was landing it at exactly the same AOA as in Hornet.

A dragged in approach and no flare... and he liked it:noexpression: (he`s a

Jarhead) - I hope he`s not reading it right now...

Back to Frogs:doh:

 

Yeah, he didnt fly the Frog though. I had him doing some practice in the F-15. He's too used to transport category aircraft now that i guess it was pointless to sit him down in front of LOFC.

[Ironhand]

..."Half as much again" means 1.5x? Ok.. must be a british thing...

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