Forum: Technology

Another group of questions from me.

1) If two aircraft (of the same type) are flying side-by-side and each are sending out radar waves, would they be able to detect the presence of an enemy aircraft more easily since more radar energy is being reflected off of the target than if only a single aircraft was present? If so, how much does detection ability increase? It's not like I would expect detection range to be cut in half, but is it enough to be useful?

2) I know that AESA radars can have LPI modes, but is LPI an ability inherent in their design or does it have to be included as a side consideration? Can PESA radars and mechanically-steered arrays have LPI modes?

3) Deflection angles of thrust-vectoring nozzles seem, to me at least, to be rather shallow compared to what they could be. For example, the most I've ever heard of is 20 degrees (for the F-22 and planned for the Su-47). What is the reason TVC nozzles don't deflect more (say, 45 degrees)? Is it because the forces induced by such high degrees of deflection would put too much strain on the airframe during a turn? Perhaps these higher degrees of deflection would expose the nozzle to too much of the exhaust heat and they would be damaged by the temperatures?

4) Which would be easier for an infrared sensor to detect: a moderately hot engine nozzle with a very large cross-section, or a very hot engine nozzle with a moderately-sized cross-section? In other words, is it better to be "cooler but larger" or "hotter but smaller"?

5) Both bombers and transport aircraft are designed to haul large payloads (although of different types). Have any aircraft ever been designed that could perform both roles? It seems like it might be a good investment to develop a single baseline model for an aircraft that could be modified into two different versions: one for hauling cargo and the other for dropping bombs. Since there would be a lot of commonality between the two designs (the biggest differences being internal), it seems like it would be a cheaper alternative to designing two completely different aircraft for the roles. Afterall, aircraft like the B-52 and the C-141 seem to have a lot of planform similarities (aside from proportions); large, shoulder-mounted wings with anhedral and low sweep angles, non-afterburning engines on four mounts hanging below the wings, and a relatively blunt nose.

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I'm no expert of those topics so I can only make additions to some of those questions;

1) Isn't picking-up other radars' reflected emissions be some kind of jamming on your own radar? No matter your buddy a/c is flying right on your side or somewhere else. After all isn't it what the enemy will do in order to jam your radar? i.e. Sending you simulated echoes? Or something else?

3) Deflection angles must be considered in conjunction with TV elements' surface area and estimated thrust during the TVing all along with material properties, hinges, actuators etc. Once again optimum design must have been the case. Also I think of an adverse effect on the engine's operation but seeing that f35 STOVL version nozzle movement (which is also some kind of TV), I doubt? I presume that the flight computer will use TV as a final resort when other flight control surfaces are insufficient???

4) We used thermal cameras during my military obligation duty in SE Anatolia, Turkey. Thermal cameras detect temperature differences rather than total heat emissions. Do the IR sensors on the a/c works similar or not?

3) More deflection angle = greater back pressure on the engine, and reduced nozzle velocity. Just like putting water through a bent pipe, flow will suffer. The trick is to get the most deflection without affecting the nozzle's basic job of accelerating the mass from the back of the engine.

5) Such things have been "explored" I've seen drawings of 747 tanker/cargo/cruise-missile carriers, and also B-1 drawings for tanker and other purposes.

Keep 'em flyin' Thumb

TEG

Question 5

Been there, done that.

http://www.air-and-space.com/xc99.htm

Check out the XC-99 cargo version of the B-36. The B-36 was a Convair-Fort Worth project, but the XC-99 was built at Convair-San Diego. Some of the detail design work was done in Fort Worth, and my dad was a design engineer on the XC-99 at Fort Worth in 1943 - 44. I was 4 years old at the time.

I toured the XC-99 in San Antonio in 1983 and saw the parts he had designed 40 years earlier, part of the built-in cargo handling gear.

Care for another question?

6) LERX (Leading Edge Root Extensions) provide improved lift and control when an aircraft is at high angles of attack (or so I believe). That being said, what function do TERX (Trailing Edge Root Extensions) provide? You can see TERX on several propellor planes from WWII (such as the Spitfire, Dauntless, and Zero) as well as the C-5 Galaxy.

Kryptid wrote:

Another group of questions from me.

1) If two aircraft (of the same type) are flying side-by-side and each are sending out radar waves, would they be able to detect the presence of an enemy aircraft more easily since more radar energy is being reflected off of the target than if only a single aircraft was present? If so, how much does detection ability increase? It's not like I would expect detection range to be cut in half, but is it enough to be useful?

Radars are designed to listen for their own emissions. Due to timing differences they would most likely mis-interpret anybody else's signal. This is why airplane working together use different frequencies to avoid confusing (jamming) each other.

Quote:

2) I know that AESA radars can have LPI modes, but is LPI an ability inherent in their design or does it have to be included as a side consideration? Can PESA radars and mechanically-steered arrays have LPI modes?

LPI typically have two features...low side lobes and low power. PESA arrays can be LPI but it has to be a design option.

Quote:

4) Which would be easier for an infrared sensor to detect: a moderately hot engine nozzle with a very large cross-section, or a very hot engine nozzle with a moderately-sized cross-section? In other words, is it better to be "cooler but larger" or "hotter but smaller"?

First off, at range all engine sources are viewed as point sources anyway so size is irrelevant. Second, sensors detect radiated energy, not temperature, although temperature is part of the equation (blackbody sources are all about temperature, but real-world emitters are not blackbody so...). Key therefore is reducing the radiated energy. Sensors work better at high temp, because high temps (engine metal) radiate a lot of energy. The lower the temp, the lower the energy and the higher tech the sensor must be. Typically, hot engine parts can be seen with low tech, uncooled sensors. Engine plumes need a cooled sensor in the 3-5 micron band, and skin friction is the lowest and hardest to detect (8-12 micron).

So this is the underlying principle of flare use. Certainy the flare(s) emit LESS energy (heat) than the engine of an a/c but being a very high temp (hot) spot, IR seeking missiles are expected to follow the flare.

OK one question now;
I'm under the impression that AESA radars work like -for example- an insect's eye and classic radars work like a bird's eye (not in shape of course but in their functioning). The nature has a place for both of them.
Is this correct and if so can we make a comparison of the two radars in a natural point of view?

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

#1: I think non-EE people tend to think of radar as analogous to sonar, that is, ping (wait...) pong. In reality the pulse and echo occur within milliseconds of each other. Radar travels at around 186,000 miles per second, so it would take .0003 seconds to reach the target and another .0003 seconds to return.

With modern pulse Doppler radars there are ways to avoid interference. But also with pulse Doppler radar signals, when you don't have access to the other radar's stable local oscillator you can't do anything too useful with the returns because to you they're not coherent.

What AESA does is allow for incredible flexibility. By adjusting the phase of the transmitted signal from one side of the antenna array to the other you can actually steer the transmitted beam. For example if you want to steer the beam right, you decrease the phase of the transmitted signal from the right side of the array to the left side of the array. With active phased arrays (where there isn't a separate RF source, each element is a transmitter/receiver) you can do all sorts of wacky stuff like dividing the array up and use different sections to generate separate beams steered at individual targets. You can still make it behave like a traditional radar where all the output is focused at one point at a time, the advantage is that isn't the only option like it is with mechanically scanned array.

r2d2 wrote:

So this is the underlying principle of flare use. Certainy the flare(s) emit LESS energy (heat) than the engine of an a/c but being a very high temp (hot) spot, IR seeking missiles are expected to follow the flare.

Temperature is not the primary issue...most standard flares put out more radiant energy than the aircraft, that's what makes them more attractive to the missile.

Energy emitted but emitted in the IR band.
The higher the temperature the higher the emission within the IR band (but not necessarily a higher total energy emission).

I got it better now, thanks.

Quote:

5) Both bombers and transport aircraft are designed to haul large payloads (although of different types). Have any aircraft ever been designed that could perform both roles? It seems like it might be a good investment to develop a single baseline model for an aircraft that could be modified into two different versions: one for hauling cargo and the other for dropping bombs. Since there would be a lot of commonality between the two designs (the biggest differences being internal), it seems like it would be a cheaper alternative to designing two completely different aircraft for the roles. Afterall, aircraft like the B-52 and the C-141 seem to have a lot of planform similarities (aside from proportions); large, shoulder-mounted wings with anhedral and low sweep angles, non-afterburning engines on four mounts hanging below the wings, and a relatively blunt nose.

As it's been stated, it has been done, like Tu-114 and Tu-95. For a while in the 40's to early early 50's, it was pretty common, actually. Of course after the early 50's bombers became too specialized to be readily adapted into cargo aircraft as air defense became more effective. A B-58 or B-2 would not great haulers make.

In addition, bombs are denser than most other cargo, so you're wasting space in the bomber version which translates to higher drag and lower range than a specialized airframe. Take a look at the C-97 and B-50.

Interesting you should mention the B-58. For those not familiar with it, the large pod under the fuselage contained both a nuclear weapon and a large quantity of fuel. Thus the only mission the airplane had was nuclear attack. Convair (later General Dynamics) realized they might sell more airplanes if it could perform more missions. Four weapon hardpoints were added near the wing root to allow carriage of a variety of bombs.

Then some genius decided that the Air Force might like to have a supersonic transport for its top brass, so they actually designed a people pod to fit under the fuselage, using the same attachments as the weapon/fuel pod. As I recall, it would have carried fourteen people for the ride of their lives, strapped under a Mach 2 bomber.

As often happens in cases like that, sanity prevailed and no hardware was actually built.

Nav on intercom: "Hey pilot, what was that bump?"

Pilot on IC: "I think I just accidentally jettisoned the generals."

Shocked

regards, OL

Although my 6th question hasn't been answered, I have a 7th question:

7) I hear a lot about the AMRAAM as if it were the premiere air-to-air missile of the USAF. However, AMRAAM stands for Advanced <i>Medium</i>-Ranged Air-to-Air Missile. Are there any <i>long</i>-ranged AAMs in use by the USAF? If so, why haven't I heard them discussed as much as the AMRAAM? If they don't exist, why is this? Perhaps launching a missile from long range as opposed to medium range allows the opposing aircraft a greater opportunity to defeat the missiles by maneuvering and using countermeasures? Perhaps a longer-ranged missile would require more propellant and would therefore be larger and heavier than the AMRAAM, meaning that fewer can be put onto an aircraft? On the same note, what defines short, medium, and long range by military standards?

you have even answered your question:the NEZ(NoEscapeZone).

to launch a missile from 200M is just wasting time and energy,cauz the target will be aware of danger and have enough time to maneuver and to escape.(except the missile flies at 10 mach)

another issue is the IFF,most current systems only give the OK within certain range...that´s why the medium is still the must(and only efficient solution)for BVR.

Forum: Technology

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