Thrust vectoring at 400 knots

[VCheng]

Hi,

It maybe related to rate of change of speed----but that is not what we are addressing----. We are talking about a rapid change in the direction of the aircraft and what transpires during that rapid process---.

We are talking about a snap shot---a quick draw---the strike of a cobra or a mamba or a rattler---so speed does matter----because time is of essence.

Are we talking about planes or snakes?

The physics of the process as I have described it is correct.

 

[kbd-raaf]

1) Simply, it is Altitude Control by different Techniques. Russians use Gas Dynamics, Western use Fluid Dynamics.

A gas is a fluid.

 

[Viper0011.]

A gas is a fluid.

Air is a Fluid......literally? Hydraulics / Actuators (Hardware) and Air Pressure / Propulsion Pressure (hot gas) are the same to you? I don't think I want to write more....

 

[VCheng]

Air is a Fluid......literally? Hydraulics / Actuators (Hardware) and Air Pressure / Propulsion Pressure (hot gas) are the same to you? I don't think I want to write more....

Sir, the field of fluid dynamics covers both gases and liquids. Yes, air is a fluid. You probably are the confusing the terms fluid and liquid.

 

[Viper0011.]

Sir, the field of fluid dynamics covers both gases and liquids. Yes, air is a fluid. You probably are the confusing the terms fluid and liquid.

I think anyone reading this and knowing the field would automatically know the Fluid is being used for Liquid and the other part is related to Gas....not sure why there had to be three posts on it when it was well understood by all mature and smart people on here

 

[VCheng]

I think anyone reading this and knowing the field would automatically know the Fluid is being used for Liquid and the other part is related to Gas....not sure why there had to be three posts on it when it was well understood by all mature and smart people on here

Well, when someone says something like "Russians use Gas Dynamics and the West uses Fluid Dynamics" it does indicate a certain basic flaw in understanding of the topic being discussed, wouldn't you say? Hence the repeated and simple sentences explaining the basics, not just for that someone, but others too.

 

[Viper0011.]

Well, when someone says something like "Russians use Gas Dynamics and the West uses Fluid Dynamics" it does indicate a certain basic flaw in understanding of the topic being discussed, wouldn't you say? Hence the repeated and simple sentences explaining the basics, not just for that someone, but others too.

In this context, Fluid is taken as Liquid. You, I and almost everyone reading it, knowing basic science knows that. However, you want to pick a point and blow it out of the proportion vs. looking at the content of the post. I've had the same issue with you before. Keep gong, this was my last response as I am wasting my time and the forum's disk space....

 

[gambit]

Gentlemen,

Thank you for your posts regarding the TV---. That is what I have been saying in my older posts---TV is a very slow speed maneuver---like in the range of 120 to 150 knots---pretty much stall at speeds---.

Stall speeds are a death sentence for any aircraft in combat---.

The airframe and human body can only take so many G forces---either + or - .

Very good, sir.

Aside from exotics, all aircrafts, civilian and military, uses aerodynamic exploitation, such as spoilers or ailerons, to effect ATTITUDE changes. An attitude change is essentially reorienting the nose to a different direction.

For example, if we want to point the nose up, we move (deflect) the rear horizontal stabilators to leading edges (LE) down, or trailing edges (TE) up. How fast and how much we deflect the rear horizontal stabs depends on how much we want to reorient the nose and how fast do we want to do it. In the old days and for less sophisticated aircrafts of today, the burden for that calculation -- how much and how fast to deflect the surfaces -- rests solely on the pilot, aka 'flying by the seat of your pants'. But as aircrafts gets more sophisticated that made them fly faster and higher, we became the hard limits for aircraft performance. So we developed automation, aka flight control avionics, to make those calculations for us.

These are the major factors involved: altitude, airspeed, attitude, load, and surface area.

At several hundreds kts airspeed, to execute a 9g turn will not require much surface deflections, in fact, those deflections will be barely visible to the human eye. As airspeed decreases, those deflections will be increasingly visible. This is because as airspeed decreases, there is less force on those surfaces. Load, or weight, equals to inertia. Surface area means how much available aerodynamic force can be exploited. When all of these are in combination -- per aircraft design -- we will have different displacement rate and degree for each design, meaning no aircraft designs in existence are identical in their flight control systems.

Thrust vectoring (TV) is about effecting attitude changes when aerodynamic forces on flight control surfaces are too weak to make the requested changes. We call that a 'stall' condition. Thrust vectoring follows the same rules as aerodynamic exploitation regarding how much and how fast to redirect thrust. So if current airspeed is several hundreds kts and there is plenty of aerodynamic forces on our flight control surfaces to give a 9g maneuver, what need is there for TV at that speed ? None at all.

As someone who know what an F-16 induced 9g maneuver feels like, constant and not instantaneous, I can say that any additional feature to make my F-16 go beyond what my body can handle is not needed. But say that we have a 'super pilot' formula that will give pilots constant 15g physical strength, if we want ALL F-16s to have 10g or more capable, why not just reprogram the FLCC to deflect the rear horizontal stabs a little bit faster and higher degree at X airspeed and Y altitude combination. It would be cheaper and faster than installing a new engine with TVC capability.

Those who criticize the F-22 for having only 2D TVC are ignorant of even how basic flight controls system works. The -22's FLCS, which includes physical features such as flight control surfaces and the mathematics that governs their actions, is enough to make it the dominant 'dogfighter' in the sky today. Better than the F-16, and the -16 is already tough to par, let alone to beat. The Raptor does not need more than 2D thrust vectoring. Now add in a pilot that is well familiar and trained in the aircraft and to make the aircraft the best 'dogfighter' in the world.

So everyone should keep in mind that TVC is not even about supplementing aerodynamic exploitation because over 90% of any flight there is enough aerodynamic forces on all flight control surfaces for us to make maneuvers, including the maneuvers powerful enough to kill us, so why the need to supplement such capability ? Rather, TVC is about giving us a reserve capability to effect attitude changes when aerodynamic forces are too weak for us to use.

 

[MastanKhan]

Are we talking about planes or snakes?

The physics of the process as I have described it is correct.

Hi,

No---it is not as simple as that---from 200 knots to 500 knots the force multiplier of air resistance increase by a multitude. For a similar change in angle in a given time at 200 knots and 500 knots would have catastrophic effects on the fuselage.

Basically the maximum induced angle attained at 200 would not all be attained at 500 knots in the same time----the aircraft will tear apart---the structure will face a total failure. It is basic physics.

 

[VCheng]

Hi,

No---it is not as simple as that---from 200 knots to 500 knots the force multiplier of air resistance increase by a multitude. For a similar change in angle in a given time at 200 knots and 500 knots would have catastrophic effects on the fuselage.

Basically the maximum induced angle attained at 200 would not all be attained at 500 knots in the same time----the aircraft will tear apart---the structure will face a total failure. It is basic physics.

You are correct in pointing out that, just as keeping the physiologic limits for the pilot in mind, the FCS needs to keep in the mind the stresses on the airframe as part of any high speed maneuver.

 

[Capt.Popeye]

Well, when someone says something like "Russians use Gas Dynamics and the West uses Fluid Dynamics" it does indicate a certain basic flaw in understanding of the topic being discussed, wouldn't you say? Hence the repeated and simple sentences explaining the basics, not just for that someone, but others too.

LOL; is there a difference between Fluids and Gases?
I'll need to turn my "High School Physics" upside down.
Now where are you Mr.Feynman?

Very good, sir.

Aside from exotics, all aircrafts, civilian and military, uses aerodynamic exploitation, such as spoilers or ailerons, to effect ATTITUDE changes. An attitude change is essentially reorienting the nose to a different direction.

For example, if we want to point the nose up, we move (deflect) the rear horizontal stabilators to leading edges (LE) down, or trailing edges (TE) up. How fast and how much we deflect the rear horizontal stabs depends on how much we want to reorient the nose and how fast do we want to do it. In the old days and for less sophisticated aircrafts of today, the burden for that calculation -- how much and how fast to deflect the surfaces -- rests solely on the pilot, aka 'flying by the seat of your pants'. But as aircrafts gets more sophisticated that made them fly faster and higher, we became the hard limits for aircraft performance. So we developed automation, aka flight control avionics, to make those calculations for us.

These are the major factors involved: altitude, airspeed, attitude, load, and surface area.

At several hundreds kts airspeed, to execute a 9g turn will not require much surface deflections, in fact, those deflections will be barely visible to the human eye. As airspeed decreases, those deflections will be increasingly visible. This is because as airspeed decreases, there is less force on those surfaces. Load, or weight, equals to inertia. Surface area means how much available aerodynamic force can be exploited. When all of these are in combination -- per aircraft design -- we will have different displacement rate and degree for each design, meaning no aircraft designs in existence are identical in their flight control systems.

Thrust vectoring (TV) is about effecting attitude changes when aerodynamic forces on flight control surfaces are too weak to make the requested changes. We call that a 'stall' condition. Thrust vectoring follows the same rules as aerodynamic exploitation regarding how much and how fast to redirect thrust. So if current airspeed is several hundreds kts and there is plenty of aerodynamic forces on our flight control surfaces to give a 9g maneuver, what need is there for TV at that speed ? None at all.

As someone who know what an F-16 induced 9g maneuver feels like, constant and not instantaneous, I can say that any additional feature to make my F-16 go beyond what my body can handle is not needed. But say that we have a 'super pilot' formula that will give pilots constant 15g physical strength, if we want ALL F-16s to have 10g or more capable, why not just reprogram the FLCC to deflect the rear horizontal stabs a little bit faster and higher degree at X airspeed and Y altitude combination. It would be cheaper and faster than installing a new engine with TVC capability.

Those who criticize the F-22 for having only 2D TVC are ignorant of even how basic flight controls system works. The -22's FLCS, which includes physical features such as flight control surfaces and the mathematics that governs their actions, is enough to make it the dominant 'dogfighter' in the sky today. Better than the F-16, and the -16 is already tough to par, let alone to beat. The Raptor does not need more than 2D thrust vectoring. Now add in a pilot that is well familiar and trained in the aircraft and to make the aircraft the best 'dogfighter' in the world.

So everyone should keep in mind that TVC is not even about supplementing aerodynamic exploitation because over 90% of any flight there is enough aerodynamic forces on all flight control surfaces for us to make maneuvers, including the maneuvers powerful enough to kill us, so why the need to supplement such capability ? Rather, TVC is about giving us a reserve capability to effect attitude changes when aerodynamic forces are too weak for us to use.

@gambit: that was about the most coherent and cogent explanation of TVC and its context as one can get.
I tip my hat to you.

 

[SQ8]

Those who criticize the F-22 for having only 2D TVC are ignorant of even how basic flight controls system works. The -22's FLCS, which includes physical features such as flight control surfaces and the mathematics that governs their actions, is enough to make it the dominant 'dogfighter' in the sky today. Better than the F-16, and the -16 is already tough to par, let alone to beat. The Raptor does not need more than 2D thrust vectoring. Now add in a pilot that is well familiar and trained in the aircraft and to make the aircraft the best 'dogfighter' in the world.

So everyone should keep in mind that TVC is not even about supplementing aerodynamic exploitation because over 90% of any flight there is enough aerodynamic forces on all flight control surfaces for us to make maneuvers, including the maneuvers powerful enough to kill us, so why the need to supplement such capability ? Rather, TVC is about giving us a reserve capability to effect attitude changes when aerodynamic forces are too weak for us to use.

The F-22 does 28 Degrees per second at 20000 feet without TVC is what is the public has been told.

And that 90% is where all air combat usually occurs. Anyone stupid enough or desperate enough to get themselves into that 10% area is already in trouble.

 

[Didact]

I was just asking you---I have asked many of our indian colleagues about the speed at which thrust vectoring can be deployed effectively during flight SAFELY.

Thrust vectoring at speeds can be compared to a high speed collision. The car travelling forward at 60 mph comes to a sudden stop due to impact---but the driver is still moving forward at 60 mph----he gets retrained by his seat belt----but then the internal organs are still travelling at60 miles per hour---sometimes the seat belts break due to force exerted on them.

So---with a sudden change in direction---in case of thrust vectoring---the plane changes direction exerting extreme duress on its structure---the pilots is still moving in a different direction even though being held back by the seat belts---and the body organs and fluids moving in another direction.

Even though pressurized suits are a big help and stronger seat belts work better---human body can take so much.

S---I have yet to get an answer from indian members----what is a safe speed for thrust vectoring?

There appears to be quite a lot of confusion amongst members about G-Forces. As such I'll try explaining the basics. After that, using those basics I'll answer your question, so please bear with a longish post.

Well first up is acceleration. Acceleration is really the rate of change of velocity w.r.t time. Note that it's not rate of change of speed but velocity. This difference is significant, as you will soon understand.

Now acceleration of a body in motion can occur either when the body is moving in a straight line, or moving in a non-linear path. For both cases, the Linear acceleration (even for non-linear motion) is given by the simple equation:

where a = linear acceleration,
v = velocity of motion
r = radius of circle traced by the body by its motion.

(Note that this equation is valid only for constant speed, am skipping the variable speed to simplify the explanation).

Now, for a straight line of motion, r = infinite. So as long as your speed is constant, you do not experience any acceleration. You could run faster than sound and you'd still feel no acceleration.

However, for a non-linear motion, there is always some value of r. So no matter what speed you choose to travel, you will feel some acceleration. This is because the velocity is changing, even though the speed is not.

For simplicity, let us consider a fighter plane moving in a circular motion, with the radius of circle being 1000 meters. If the plane travels at say 100 m/sec, he will experience an acceleration of 10 m/sec^2, i.e. 1G.

If the radius of circle was doubled, to 2000 m, the pilot will experience acceleration of 5 m/sec^2 , or 0.5G. Thus even though his speed is constant, by changing the radius of his circle of motion, he can change the acceleration he experiences.


Now I can work on your question:

The human safe tolerance is estimated to be 12G (I'm not an expert on the human body so I'll just assume that figure as the limit)

12G = 120 m/sec^2. Now let's assume a constant speed of 300 m/sec for the plane in the example above.

The pilot will experience 12G's if his circle is 750m, which for an aircraft at nearly 0.9x the speed of sound is quite tight. Now I'll take it a step further.

The circumference of a circle of radius 750 m = 2xPix750 = 1500xPi m.
At 300m/s, the plane completes one revolution of the circle in = (1500xPi)/300 s = 15.71s.
As such, it's angular velocity is w = 360/15.71 = 22.92 degrees/sec.

Thus the pilot would reach his peak G if he were to execute a turn at 22.92 deg./sec. If the speed of the pilot was 600 m/s, the maximum turning rate of the plane would be 11.5 degrees/sec.

To put it simply, the wider the radius of the circle, the greater the speed the plane can carry without reaching G limits. Conversely, the tighter the circle, the slower the pilot has to approach to reduce the G experienced by him.

So coming back to your main question about the maximum safe speed for TVC, your question is erroneous. The combination of speed and radius of turning will decide whether the TVC is safe or not.

 

[narcon]

Even rockets at high velocity have TV = What is the problem talking about 400 knots or what not?

 

[gambit]

Even rockets at high velocity have TV = What is the problem talking about 400 knots or what not?

View attachment 42946

Structurally speaking, even an airliner is more robust than an ICBM even though both are essentially hollow tubes. Just that the hollow tube that is the airliner is much more complex and stronger because the airliner is designed to be reuse many many times. Precisely because the rocket is not as robust as the aircraft, it simply cannot maneuver the way an aircraft can. A rocket uses thrust vectoring in lieu of aerodynamic exploitation to execute GRADUAL attitude changes. The rate of thrust redirection in a rocket is not as fast as how an aileron may need to deflect to effect attitude change in an aircraft.