Very abstruse questions

[plane00]

1. How much vulnerable to FOD are su-27, mig-29s ? How well do mesh(or screen or guard or protection) of air intake(or duct) protect engine from foreign objects ?

 

How fatal if FODs are inserted(or inhalated or sucked) into engines?

 

2. because of low position of engine (su-27, mig-29) , below center of gravity, pitch effect when full throttle(afterburner) may occur ?(like vertical climb or stall situation)

Edited September 25, 2010 by plane00

[mvsgas]

How fatal if FODs are inserted(or inhalated or sucked) into engines?

Depends on the engine, the type of F.O. or D.O. and other conditions, A small screw being sucked into the engine while aircraft is taking off could damage the first stage blades. Once the first stages get damaged, pieces that may come off, get added to the screw, so the second stage deals with what's left of the screw plus any debris that came of the 1st stage blade. This will continue troughs all the stages, more and more debris added, so it could be very damaging.

 

2. because of low position of engine (su-27, mig-29) , below center of gravity, pitch effect when full throttle(afterburner) may occur ?(like vertical climb or stall situation)

I do not think the engine are places below the center of gravity. Center of gravity on aircraft is not strait forward, I would have to see SU-27 weight and balance manual before I could tell you where the center of gravity is, then there is always calculating the moment etc.. Also, engine may be angle (by design) to correct some things like that. Look at F-4 engines, they are pointed inboard (front) and nose up while in the frame.

At any rate, it does not make since to me to designed an aircraft that when AB is selected, you have a chance of the aircraft departing because of un-commanded pitch up due to thrust miss alignment.

Edited September 25, 2010 by mvsgas

[flydragon126]

about the second question, I think A-10 is an obvious example of engine thrust not aligned with center of gravity.

[plane00]

I think ,

su-27, mig-29 are air to air fighter, so they must climb or sometimes be at very low speed in urgent situation like CAC, so generating effects like pitch-up with afterburner should be disadvantageous.

Edited October 6, 2010 by plane00

[vanir]

In the MiG-29 at least anyway, isn't there a bar that extends and literally pushes the stick forward in excessive pitch situations? It can be disconnected by the pilot for high AoA emergency manoeuvres, but normally it functions as part of the automatic control system (I think it engages at something like 26o pitch up, where western FBW correct at more like 22o and can't be disconnected).

 

Also the MiG does have angled thrust a bit like a Phantom.

 

From what I understand supersonic flight moves the centre of gravity. Not much of an issue in a dartlike design that has variable geo wings like the Flogger, but it would cause significant nose down in 4th gen fighters mostly built for transonic/subsonic performance, which would have to be dealt with in trim, limiting their high mach performance.

 

I don't know what the supersonic handling of the MiG is like but its subsonic/transonic handling is famously neutral.

The Flanker has a downward angle for the cockpit such that level flight is at 3o AoA but this isn't for supersonic trim, it's just because the design was centred around short field performance (you can operate Flankers off airfields that could never support an Eagle), the nose down attitude was entirely for pilot view during landing I believe.

 

Still because of this it is fairly possible the Flanker's thrust line is slightly misaligned on purpose, since neutral level flight is 3o pitch up and putting it at 0 AoA makes you dive.

 

My guess, not without some reference, is a very neutral supersonic attitude though, where most other fighters have to trim some pitch up (handled by automatic flight systems iirc but it would cause some increase in drag).

 

This is probably where the Flanker is getting its pretty good high Mach performance from considering it's a big, weighty aircraft and engine thrust isn't quite as spectacular as their fuel efficiency.

 

So yeah I'm guessing you get a little pitch up when you hit burners on a Flanker, but the control system compensates immediately with trim and it only happens at slow subsonic.

IIRC from German pilots, the MiG tends to go right for some reason.

 

 

note, I should add I am by no means any kind of expert and am just rambling about what I've read/heard/assumed from various sources. I could be way off or just plain wrong.

Edited October 6, 2010 by vanir

[ApacheDoctor]

FOD, regardless of what aircraft, is potentially deadly. I work on US Army helicopters, and they are ANAL about making sure FOD stays where it belongs, well away from aircraft. We do FOD sweeps after every break, and when we are working on compenents we have to ensure there is nothing on us but our clothes, we just wear our boots, trousers and undershirts and thats it...nothing else. No rings, tags, watch, etc...nothing can go in there.

 

Think about it, axial compressor engines have progressive stages that get smaller and smaller as the air runs further down the engine. Anything that gets wedged in there is going to cost a pretty penny. Hence, NO FOD period...it can kill and costs lives.

[vanir]

It was one of the things that apparently most impressed the west Germans about the Fulcrum, where its actual aerial performance has strengths and weaknesses, a lot of weaknesses compared to NATO contemporaries, the one thing frequently remarked on is that trick intake system for rough field operation.

They said, when operating F-16s you have ground personnel and aircrews all walk down the runway line abreast to make sure there are no foreign objects on the runway before flight operations are allowed. With the Fulcrum you taxi straight out onto a very roughly maintained strip while everyone's still in bed and rest easy.

According to the Germans this factor of Russian aircraft comprised a tactical advantage that helped even out east-west technological disparities.

 

The Frogfoot doesn't even need any modern facilities, just a piece of flat ground and a couple of trucks with some diesel fuel will do just fine. It even has its own service kit in a cargo hatch for combat operation in forward areas without any ground facilities, just some flares laid out on a clearing and a truck with reloads. The engines will run on kerosine, diesel fuel, anything you can get your hands on.

It's the whole famous thing about the Frogfoot, hard to represent in a flight sim but that was the thing that had western analysts really sit up and take notice of the Su-25.

[mvsgas]

From what I understand supersonic flight moves the centre of gravity. Not much of an issue in a dartlike design that has variable geo wings like the Flogger, but it would cause significant nose down in 4th gen fighters mostly built for transonic/subsonic performance, which would have to be dealt with in trim, limiting their high mach performance.

I think you are confusing center of gravity with center of lift.

http://www.allstar.fiu.edu/aero/flight43.htm

[mvsgas]

They said, when operating F-16s you have ground personnel and aircrews all walk down the runway line abreast to make sure there are no foreign objects on the runway before flight operations are allowed. With the Fulcrum you taxi straight out onto a very roughly maintained strip while everyone's still in bed and rest easy.

 

We don't check the runway every day. We do walk the parking areas and they do checks on the flight line but we don't FOD walk the runway every day.

 

The Frogfoot doesn't even need any modern facilities, just a piece of flat ground and a couple of trucks with some diesel fuel will do just fine. It even has its own service kit in a cargo hatch for combat operation in forward areas without any ground facilities, just some flares laid out on a clearing and a truck with reloads. The engines will run on kerosine, diesel fuel, anything you can get your hands on.

It's the whole famous thing about the Frogfoot, hard to represent in a flight sim but that was the thing that had western analysts really sit up and take notice of the Su-25.

See, I believe the SU-25 will still need more than dirt to take off with a combat load. That thing is pretty heavy, and if only only dirt, I think it would sink with a full combat load.

I have seen a video of it going thru a puddle of water but it had nothing on the pylons. Same with the video of the jaguar landing and taking off from a field, just some empty practice bomb containers.

[plane00]

I recently found good fighters which high position of air intake far above(from) ground.

 

sepecat jaguar

panavia tornado :thumbup:

Edited October 12, 2010 by plane00

[vanir]

I think you are confusing center of gravity with center of lift.

http://www.allstar.fiu.edu/aero/flight43.htm

 

quite right, I'm mixing up my terminologies with a poor explanation, without visiting the link it was about the area and distance between centre of lift and centre of gravity (relating to stability of flight) which is a part of design engineering, so the issue is with centre of gravity but it is the centre of lift which moves.