AFM for Su25/25T at 30% dive

[4c Hajduk Veljko]

According to “Sukhoi Su-25 Frogfoot” book, the air brakes on Su-25 were so effective after adding auxiliary surfaces, that the airplane did not accelerate even in steep dives of 30 degrees and speeds less then 700 km/h.

 

I tested both Su/25/25T and both of them accelerated with air brakes deployed even at much lesser speeds and dive angles. And this is with fuel at half full (me optimist) and no weapons under the wings. Maybe, the AFM needs to be adjusted little bit, increase the drag factor when air brakes are deployed.

 

When I find a target reach area, I deploy both stages of flaps, air brakes and lower the landing gears.

[SwingKid]

According to “Sukhoi Su-25 Frogfoot” book, the air brakes on Su-25 were so effective after adding auxiliary surfaces, that the airplane did not accelerate even in steep dives of 30 degrees and speeds less then 700 km/h.

 

I tested both Su/25/25T and both of them accelerated with air brakes deployed even at much lesser speeds...

 

Why much lesser speeds?

 

I think you understood the book wrongly. In such aircraft specifications, "speeds less than 700 km/h" = "speeds 698 km/h and higher".

 

e.g. the technical requirement was that the brakes must work as slow as 700 km/h, and the engineers say, "ok, we satisfied this minimum requirement, and then some extra - the brakes will work, even if the speed is 698 km/h!"

 

It doesn't mean that the brakes will now work at ALL airspeeds lower than 700 km/h! On the contrary, brakes are more effective at higher speeds. That's why the technical requirements are only concerned about the performance at the slow end of the scale, which in this case is not higher than 700 km/h. At slower speeds, the brakes will be less effective.

 

-SK

[RonnieFrown]

That thing always bothered me too. Deploying airbrakes is supposed to feel like as if an imaginery hand smashed the airplane in the nose. In lock on the airplanes fly like gliders with or without airbrakes.

[Hawg11]

Depends on the plane...

[4c Hajduk Veljko]

Why much lesser speeds?

Here's a quote from the book, page 27:

"...the combined surface area and effectivness of the brakes permitting steep dives at speeds less then 700 km/h (378 kt) without acceleration in dive."

End of quote.

Above statement clearly states that with speeds below 700 km/h the airplane should not accelerate in steep dives. "Steep" means (was the russian ariforce requirement) as 30 degrees and is somwhere else in the book (can't find it now).

I do understand that the airbrakes efectivness drops at lower airspeeds.

 

I think you understood the book wrongly. In such aircraft specifications, "speeds less than 700 km/h" = "speeds 698 km/h and higher".

It could be that I understand it wrong. Although I tried with speeds higher then 700 km/h, and the airplane again accelerated.

 

Regards,

[SwingKid]

Here's a quote from the book, page 27:

"...the combined surface area and effectivness of the brakes permitting steep dives at speeds less then 700 km/h (378 kt) without acceleration in dive."

End of quote.

Above statement clearly states that with speeds below 700 km/h the airplane should not accelerate in steep dives.

 

I don't think that's very clear at all.

 

Why 700 km/h? What happens above 700 km/h? The aircraft accelerates? That doesn't make any sense, since the air brakes are more effective at higher speeds, than at lower speeds. So, why mention 700 km/h at all?

 

Also - 700 km/h, at what altitude? Is it 700 km/h airspeed or ground speed? Are the flaps lowered? What is the weight of the aircraft? Why does it say that "it does not accelerate", rather than saying clearly that it should slow down?

 

I'm not saying that the airbrake in Lock On is correct, but there were some cases where the AFM programmer had more precise information, than we were able to provide from such books.

 

-SK

[hitman]

You should take into account of newtons laws of gravity. Naturally this means the planes not going to stop dead in the air like some bugs bunny cartoons...and if the plane was flying at around 10 kmph...it would still accellerate until it reaches its own terminal velocity.

[nscode]

Nobody says it will stop dead hitman..

 

Why does it say that "it does not accelerate", rather than saying clearly that it should slow down?

 

Why should it? When it's in a dive, a normal thing would be to accelerate. When brakes are deployed their task is to prevent that so there are 3 possibile outcomes: 1) they are not effective enough, so it still accelerates 2) they are too efective at given situation, and it slows down, and 3) just a thing you want in a dive to a target - with good settings they keep the speed constant.

[4c Hajduk Veljko]

Why 700 km/h? What happens above 700 km/h? The aircraft accelerates?

Yes, that is what the statement from the book implied.

 

That doesn't make any sense, since the air brakes are more effective at higher speeds, than at lower speeds.

Yes they are. However, once their capacity is reached they will not provide “more” braking even when the speed is increased.

 

So, why mention 700 km/h at all?

I assume because that was Russian Air force requirement.

 

Also - 700 km/h, at what altitude? Is it 700 km/h airspeed or ground speed?

I also noticed that the authors did not mention whether this was air or ground speed. In my test, I tried diving from 1000 meters above see level. This would be favorable condition for Su-25/25T.

 

Are the flaps lowered?

Flaps were not mentioned. However, the authors clearly stated that the air brakes were extremely effective after adding additional surfaces.

 

BTW, the first Su-25 prototype did not have air brakes at all.

 

What is the weight of the aircraft?

Authors did not specify the weight when stating effectiveness of the air brakes. In my test I tried very favorable for Su-25 scenario weight wise.

 

Why does it say that "it does not accelerate", rather than saying clearly that it should slow down?

“Does not accelerate” does not necessarily means slow down.

 

I'm not saying that the airbrake in Lock On is correct, but there were some cases where the AFM programmer had more precise information, than we were able to provide from such books.

I fully understand and respect this statement. I am sure (hope) the AFM programmer has quality info on air brakes on Su-25/25T.

 

The fact is with no weapons, fuel tanks half full, diving with air brakes applied from 1000 meters barometric altitude, the Su-25/25T will accelerate.

 

Regards,

[4c Hajduk Veljko]

...bugs bunny cartoons...

Bugs bunny is not mentioned in the book! :icon_mrgr

[Gazehound]

Hajduk I would go with SK on this one - I think its maybe a way of phrasing things in english which has you confused.

For example an advert:

'xxxx car avaliable for less than £5000'....

When you look, its available for £4998

Or maybe:

'Less than one gram of fat!'

- it has 1.9g, on average.

 

You see? Perhaps more clear would be 'Works at speeds lower even than 700km/h' - Stops dive acceleration down to about 695km/h :D

[4c Hajduk Veljko]

Perhaps more clear would be 'Works at speeds lower even than 700km/h' - Stops dive acceleration down to about 695km/h :D

Let me try understanding book authors statement in a way that you’ve just described.

 

What you’ve just said, does that mean that the Su-25 should NOT accelerate beyond 695km/h when diving from for example 1000m?

 

In other words, the starting speed is 650km/h, the air brakes are fully deployed, the altitude 1000m and the airplane goes in a dive, it accelerates to about 700 km/h and it does not accelerate beyond that speed?

 

Regards,

[SwingKid]

Yes, that is what the statement from the book implied.

 

Yes they are. However, once their capacity is reached they will not provide “more” braking even when the speed is increased.

 

Source?

 

-SK

[Gazehound]

Let me try understanding book authors statement in a way that you’ve just described.

 

What you’ve just said, does that mean that the Su-25 should NOT accelerate beyond 695km/h when diving from for example 1000m?

Regards,

It means at starting speeds of 695 and above, acceleration will not occur in a dive while the brakes are deployed.

 

So if you were to fly level at 700km/h (or 900km/h even) and then dive with brakes out, you would not accelerate. There will be a minimum speed at which this will occur and all the book does is indicate that it is below 700km/h - probably not that far below

[4c Hajduk Veljko]

Source?

Can you provide a source that will oppose to what I said?

 

Thanks,

[4c Hajduk Veljko]

It means at starting speeds of 695 and above, acceleration will not occur in a dive while the brakes are deployed.

 

So if you were to fly level at 700km/h (or 900km/h even) and then dive with brakes out, you would not accelerate. There will be a minimum speed at which this will occur and all the book does is indicate that it is below 700km/h - probably not that far below

 

Good. From what I remember the Su-25/25T accelerated under the conditions you described above. I am at work now, however this afternoon I will perform another test to confirm.

 

Thanks,

[SwingKid]

Can you provide a source that will oppose to what I said?

 

If you Google "aerodynamic drag", the first link gives you this equation:

 

"Aerodynamic drag = 1/2 D x A x Vsquared

 

In this equation, D is the density of the air, A is the frontal area of the moving shape, and V is its velocity relative to the air."

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

 

So, the drag force on the aircraft is proportional to the square of the aircraft's velocity.

i.e. the faster it goes - the more the airbrakes will slow it down. The slower it goes, the less effect will the airbrakes have.

 

Where does the "capacity" arrive in the equation? :confused:

 

-SK

[Shepski]

So, the drag force on the aircraft is proportional to the square of the aircraft's velocity.

i.e. the faster it goes - the more the airbrakes will slow it down. The slower it goes, the less effect will the airbrakes have.

 

That is correct... and propbably the biggest factor on air brake effectiveness is weight of the aircraft... the more inertia the harder it is to slow down.

 

Look at the descent rate in a 30 degree dive and look at the surface area of the air brakes... do you think the small surface area of the brake will prevent acceleration at a descent rate of over 100 m/sec?

[Mizzy]

That is correct... and propbably the biggest factor on air brake effectiveness is weight of the aircraft... the more inertia the harder it is to slow down.

 

Look at the descent rate in a 30 degree dive and look at the surface area of the air brakes... do you think the small surface area of the brake will prevent acceleration at a descent rate of over 100 m/sec?

 

Yes I think I agree with that, if I understood what all this Fm business was all about. Seems every time the subject comes up it's polarised positions and no room for compromise.

 

The games got good graphics though :)

[4c Hajduk Veljko]

So if you were to fly level at 700km/h (or 900km/h even) and then dive with brakes out, you would not accelerate. There will be a minimum speed at which this will occur and all the book does is indicate that it is below 700km/h - probably not that far below

 

I am back from work and as I prommised I retested the scenarios just as you explained them above. Both Su-25 and Su-25T accelerated under the following conditions:

 

1. Fuel tank at half full

2. initial altitude 1000m and 2000m

3. initial speed 700 km/h and 800 km/h

4. no weapons loaded under the wings.

5. dive angle between 25 and 30 degrees

 

Just as expected the rate of acceleration on Su-25T was noticable higher then on Su-25.

[4c Hajduk Veljko]

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)."

End of quote.

 

The text then goes on explaining how that was achived by adding additional deflecting surfaces.

 

The above statement is now very clear and all confusion about understanding is removed.

[4c Hajduk Veljko]

Yes I think I agree with that, if I understood what all this Fm business was all about. Seems every time the subject comes up it's polarised positions and no room for compromise.

 

Hi Mizzy,

 

Handsome little boy you got there! When you posted his pictures, the first thing I did I called my wife to take a look and we both sad “ahhhhh”. Those pictures of you and your son brought the memories of our two sons when they were little babies.

 

Going back to the original subject of the topic, in my view these kind of technically challenging discussions are always good for me and for Lock On community. Ever since I started playing this game I learned so much about airplanes, however, there is much, much more that I don’t know and that I want to learn about.

 

Conclussion: polarized positions are extremely good, as long as both sides use facts. I try every day (not always successful) to eliminate the phrase “I think” and replace it with “that fact is” phrase.

 

Regards,

[4c Hajduk Veljko]

Where does the "capacity" arrive in the equation?

Capacity is a braking ability of those air brakes. With the aerodynamic formula you provided and the total combined area of the air brakes being at 1.8 square meters (each?), you can calculate how much drag force those brakes can produce.

 

However the point is that everything has a “capacity”. And if the air brakes are not big enough then their capacity will be overrun and the airplane will accelerate. The book clearly states that Su-25 has air brakes big enough not to allow the airplane to accelerate beyond 700 km/h at 30 degrees dive angle.

 

Maybe the book is wrong. I don’t know.

 

So, the drag force on the aircraft is proportional to the square of the aircraft's velocity.

i.e. the faster it goes - the more the airbrakes will slow it down. The slower it goes, the less effect will the airbrakes have.

You have made a point that I agreed upon that the higher the speed the more effective the brakes are.

 

However, that is not what we are talking about. We are talking about the fact that in real life, according to the “Sukhoi Su-25 Frogfoot” book, the airbrakes are capable (have a braking capacity) to maintain the „Грач“ speed to 700 km-h at 30 degrees dive angle.

 

I don’t know if the data in this book is right or wrong.

 

Regards,

[Shepski]

I think I see where some confusion in all this lies...

 

In my book... "Su-25 All Varients" by 4+ Publication in the Czech Republic it states the following about the air brakes...

 

"The airbrakes were changed from a 2 piece to an improved 4 piece W-design, allowing steeper dive angles not exceeding maximum speed over 700 km/h limit."

 

To me, this means that the Su-25 has a design airspeed limitation of 700 km/h in a dive and with the aid of the 4 piece air brakes the speed can be better controlled in a dive so as the limit of 700 km/h is not reached as fast as it had been previously with the 2 piece brake.

 

This make sense guys?

[Shepski]

However, that is not what we are talking about. We are talking about the fact that in real life, according to the “Sukhoi Su-25 Frogfoot” book, the airbrakes are capable (have a braking capacity) to maintain the „Грач“ speed to 700 km-h at 30 degrees dive angle.

 

To me this is impossible... a 30 degree dive will give a descent rate of over 15,000 feet/min and there is no way that those little air brakes will stop the plane from accelerating. They will help reduce the acceleration though.

 

Look at the A-10... it has a much higher drag profile compared to the Su-25 and it has very large speed brakes... The A-10 will accelerate in a 30 degree dive with the brakes out.

 

30 degrees nose down is a very steep descent!