AFM for Su25/25T at 30% dive

[4c Hajduk Veljko]

I don’t know what to say. In my view, the statements from Shepki’s and my books are very clear.

 

Yet, I am not a native English speaker and I might be misunderstanding some of the more complicated English sentence structures.

 

I wonder, if there is anybody from the Russian side with some original Russian books that could contribute to this discussion?

[Guest EVIL-SCOTSMAN]

Page 36, Sukhoi Su-25 Frogfoot

 

 

how did you do that, i mean, how do you get a link to display other text like you have done, so you dont see the http://www.abcd.com and see sukhoi su-25 frogfoot instead.

 

ive always wondered how it was done, but ive never got round to asking anyone...

 

so, how do ya do it ?

[Ironhand]

Easy. As an example, here's the link for this discussion after you click 'Okay' in the URL dialogue box:

 

{URL="http://www.forum.lockon.ru/showthread.php?p=178526#post178526"]http://www.forum.lockon.ru/showthread.php?p=178526#post178526[/url}

 

{URL=http://www.forum.lockon.ru/showthread.php?p=178526#post178526]ENTER YOUR TEXT HERE[/url}

 

After you click 'Okay' in the URL dialogue box, the text you change is actually highlighted. You might have to scroll right to see it.

 

Rich

[4c Hajduk Veljko]

Hi Scotsman,

 

Also, in the bottom left corner of this page, you can click on vbCode link and get the help screen that explains all html coding that you can use on this forum. Examples are provided as well.

[Shepski]

Page 36, Sukhoi Su-25 Frogfoot

Quote:

"As a consequence of the aircraft's combat expirience, the Soviet Air Force set an additional performance requirement - the speed during 30 degrees dive should not exceed 700 km/h (378 kt)."

 

I take the above as meaning that the pilot is not to exceed 700 km/h in a dive. You can't make the aircraft do it on it's own. :) This limitation, after combat experience, is probably due to high G loads on the airframe and the pilot during the pullout of the dive.

 

I think the problem might come from the translation of a Russian book to English.

[nscode]

I think the problem might come from the translation of a Russian book to English.

 

Now, where is the guy who wrote to Su? We need him again :D

[Fish]

If you look at the size of the air brakes, you can see why they would be ineffective at removing acceleration at steep angles of attack.

[Guest EVIL-SCOTSMAN]

cheers hadjuk and ironhand ;)

 

mucho gracias....

[4c Hajduk Veljko]

Somebody mentioned in the other thread that the engines of the Su-25T as modeled in FC produce 30% of the thrust when running at idle? Could that explain the fact the Su-25T accelerates in a dive even with speed brakes and full flaps deployed?

 

How can we measure the thrust levels of Su-25t in FC?

[GGTharos]

I'm not sure. However you shouldn't take the 30% nuebr as gospel, it was said that this was just a numebr tossed out there and is not necessarily accurate.

 

A more accurate statement would have been 'the engines generate some thrust at idle' which is probably true of all engines.

 

Not sure how to measure the thrust unfortunately unless ... ah, perhaps you can extract the forces via LUA.

[Kenan]

Could that explain the fact the Su-25T accelerates in a dive even with speed brakes and full flaps deployed?

 

I think there's nothing wrong with that. It's the basic law of physics: objects accelerate while falling.

 

For your bombing runs you should try something else, the 'tactic' I'm using when I'm too fast and too close to the runway but have to land as fast as possible: temporatily shut down your engines, wait for 2-3 seconds and crank them up again!

 

During this time, your speed -will- definatelly go down and with the deployed airbrakes, your plane should be quite steady there.

 

I didn't try it with a Frog and with its AFM, but I think it should work.

Actually, it should work IRL too, except the engines may suffer if it's done too often.

[nscode]

And it may or may not restart. Bet your life on it?

 

Not sure how to measure the thrust unfortunately unless ... ah, perhaps you can extract the forces via LUA.

It would be soooooo nice if we could display force vectors on the airplane, like on a car in LFS :)

[4c Hajduk Veljko]

I didn't try it with a Frog and with its AFM, but I think it should work.

Well, guess what, it doesn't.

 

I tried level speed to 700 km/h, turn the engine off, apply the speed brakes and went to approximately 25 degrees dive. Su-25T accelerated with turbine turning at 20-30% (due to air flow in dinve).

 

So with the engines off, half full fuel tank, no weapons under the wings, Su-25T will accelerate in 25% dive with speed brakes on.

[SwingKid]

Hmm, it seems that the moderators closed the other discussion about this topic, so I guess we should continue here...

(at least they were kind enough not to delete it...)

 

In the previous discussion, I studied only the aircraft deceleration, without airbrakes.

 

Without airbrakes, a 12254 kg Su-25T in a vertical dive accelerates about 7.22 m/s^2 through 700 km/h speed (~1000 m alt).

From this we caluclated a cross section-referenced drag coefficient Cd of about 0.15, which seemed reasonable when compared to internet sources.

 

Now, WITH airbrakes, a 12254 kg Su-25T in a vertical dive accelerates through 700 km/h at a reduced rate of about 5.83 m/s^2.

 

This means that at 700 km/h, the 1.8 square-meter airbrakes are producing a deceleration force Fd = (7.22 - 5.83) * 12254 ~= 17033 N

This gives a cross section-referenced drag coefficient of:

 

Cd = 17033 / (0.5 * 1.225 * 194.4^2 * 1.8 ) = 0.4

 

Ok, I don't want to make any bets this time, but now it looks again like Hajduk might have a point. Comparing to some common shapes:

 

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

 

...we can see that the AFM "claw" air brakes seem to be a little bit more slippery, than if they were shaped like spheres!

I'm not sure if I made a mistake somewhere, but it seems possible to me that maybe the AFM Su-25T airbrakes are too weak. Such flat shapes generate a lot of form drag and should probably be designed to have a Cd that is about 2 or 3 times higher.

 

I hope Yo-Yo reads this topic also..

 

 

-SK

[Shepski]

.

I'm not sure if I made a mistake somewhere, but it seems possible to me that maybe the AFM Su-25T airbrakes are too weak. Such flat shapes generate a lot of form drag and should probably be designed to have a Cd that is about 2 or 3 times higher.

 

More like a little too small... look at the size of the brakes on the A-10... The Su-25 just doesn't have big enough speed brakes to be super effective like many think they should be. Have you been testing the effectiveness of the A-10 brakes for a comparision?

 

The speedbrakes on the B-737 are not very effective and they have a lot of surface area. On my next flight I will test their effectiveness and report what type of descent rate they will slow the jet up at a particular weight when under 10,000 feet. The Learjet 35 has very effective spoilers but it also has a small wing and is is fairly light at a GTOW of 18,300 lbs.

[SwingKid]

More like a little too small... look at the size of the brakes on the A-10... The Su-25 just doesn't have big enough speed brakes to be super effective like many think they should be. Have you been testing the effectiveness of the A-10 brakes for a comparision?

 

I don't think it would be a good comparison. Besides the fact that A-10 isn't AFM in the sim, it also needs much larger air brakes because they operate at a lower speed. I wouldn't expect the Su-25 airbrakes to be very effective at A-10 speeds and I'm not testing them there, but rather at the upper end of its envelope, around 600-800 km/h. In this range I have the feeling that they aren't producing enough effect.

Besides, what would be proven by comparing one simulated aircraft to another simulated aircraft? They could both be off.

 

-SK

[SwingKid]

Ok, I took a look at the A-10A dive brake anyway... After a very rough calculation, it also seems to have a Cd of about 0.4.

 

-SK

[Shepski]

Ok.. I just did a quick test in the -25T...

 

With a weight of 12,000 kilos and clean in a constant 10 degree dive from 2800m to 800m. I started at 3000m, thrust to idle, and speedbrake out then began the dive.

 

Brake retracted...

 

@2800m the speed is 711 km/h

@800m the speed is 820 km/h.

net gain of 109 km/h.

 

Brake extended...

 

@2800m the speed is 682 km/h

@800 the speed is 687 km/h.

net gain of 5 km/h.

 

Does the brake still seem overly ineffective to you?

 

Now... I always use the external outside speed because the indicated airspeed on the HUD will increase as altitude decreases so you will get a false acceleration reading using indicated airspeed.

[4c Hajduk Veljko]

With a weight of 12,000 kilos and clean in a constant 10 degree dive from 2800m to 800m. I started at 3000m, thrust to idle, and speedbrake out then began the dive.

Why did you choose 10 degree dive? The book I am quoting is referencing Russian Air Force requirement for Su-25 not to accelerate @ 30 degree dive.

 

Does the brake still seem overly ineffective to you?

It is not about me. It is about the fact that even at 10 degree dive in your test it accelerated through the air brakes.

[Shepski]

You have to understand that there is no way the Su-25 will not accelerate in a 30 degree dive with that small of a of speed brake.

 

A 10 degree dive is just under a 10,000 foot/minute descent. What do you think a 30 degree dive descent rate is? It's over 23,000 ft/min.

 

The 700 km/h number is a limitation that the pilot should not exceed 700 km/h in a dive and the speed brakes are used to help decrease the rate of acceleration because the jet will accelerate. Didn't you read my earlier posts about this with reference to my Su-25 book?

[4c Hajduk Veljko]

Didn't you read my earlier posts about this with reference to my Su-25 book?

Yes, I read it and can not comment because that quote does not clearly state facts that I am looking for.

 

Here is another quote from ”Sukhoi Su-25 Frogfoot”, page 27, bottom left hand paragraph:

 

Quote,

“The efficiency of the layout was improved by around 60% by adding auxiliary surfaces of 0.6 meters sq, (6.45 sq feet) overall area, connected to the main airbrake surfaces by a kinematic linkage, the combined surface area and effectivness of the brakes permitting steep dives at less then 700 km/h (378 kt) without acceleration in the dive”

End of quote.

 

Now, it might be that book authors did not have correct data.

[GGTharos]

Remember that the Su-25's engines still produce thrust when idling ... it may be that this is changing the performance.

[Shepski]

I would bet on a translation error more then anything.

 

Look at the pic below...

 

look at the difference between the form drag of the A-10(engine cowls, fan baldes, landing gear housing/tires/dual tail fins and speedbrakes. Compare it to the form of the Su-25T with the speedbrakes extended. Do you notice the huge difference? In Lock On, the speedbrakes will hold the A-10's airspeed in a 30 degree dive using the SFM.

 

How do you figure with the AFM, the tiny speedbrakes, and sleek airframe of the Su-25T that it will not accelerate in the 30 degree dive wich is quite steep?

 

You just have to look at it to realize that there is no way those small brakes will stop any acceleration in a dive.

 

Throw the gear and full flaps out and then you will see the difference of what the extra form drag will do.

 

[Shepski]

Remember that the Su-25's engines still produce thrust when idling ... it may be that this is changing the performance.

 

The A-10's engines will also produce some thrust at idle too... I'm sure it's modeled in the SFM. Just the ram air will increase the speed of the turbo fans to produce some residual thrust.

[4c Hajduk Veljko]

Remember that the Su-25's engines still produce thrust when idling ... it may be that this is changing the performance.

It accelerates even with engines off. The rate of acceleratoin is somewhat slower, however, it does accelerate.