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Stupid questions about aviation


Griffin

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Post your stupid questions here! There was one on a Finnish aviation forum so why not here too?

In almost all of the places where I have worked in aviation business, the professionals have always told us that there is no such thing as a stupid question. Rather ask a stupid one than misunderstand and screw something up for a stupid reason.

Many people have alot of questions they are embarrassed to ask so here's a place to vent it out! Some things seem too hard to even google and it doesn't have to be even a stupid one but... ask anything!

 

Here's one. The first AESA or synthetic aperture radar ground mapping images I saw I thought were just normal pictures photoshopped to look radarish due to the shadows I see there. A radar can't see shadows, right? But seeing such images from official sources as the one below have to be real. So what's the reason why we see shadows on radar images? :huh:

 

apg81sar.jpg

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Radar can't look through things. The point of view does not need to coincide with the sensor. You can create an artificial perspective from the scanned scene, but the places the radar waves didn't reach remain blank, obviously, hence the shadows.

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To perhaps be more illustrative - the picture above is not necesarily a "true" image of what the equipment is seeing - rather the return signal has been processed to display a top-down "satellite" style view even though the "picture" was actually taken from a different perspective.

 

So the "shadows" are not the shadows of the sun, it's radar shadow that becomes visible due to how the signal is being presented to the viewer.

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Semi-actives "simply" home in on the return from the illumination. So the host aircraft illuminates the target, the missile sees the return, and goes for it. Think of it like a laser-guided missile which does sort of the same thing - looks for the reflected laser light and goes for it. (But obviously they can do a lot of other things with them as well to make sure they get the most effective flight path etcetera.)

 

The difference between semi-active and active is that the active not only looks for reflected radio light, it also emits radio light on it's own (has it's own emitter), while the semi only has a receiver.

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Semi-actives "simply" home in on the return from the illumination. So the host aircraft illuminates the target, the missile sees the return, and goes for it. Think of it like a laser-guided missile which does sort of the same thing - looks for the reflected laser light and goes for it. (But obviously they can do a lot of other things with them as well to make sure they get the most effective flight path etcetera.)

 

The difference between semi-active and active is that the active not only looks for reflected radio light, it also emits radio light on it's own (has it's own emitter), while the semi only has a receiver.

 

So could your wingman illuminate the target which the semi picks up and goes for it? Like a buddy lase but with a radar.

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Griffin - AFAIK this has been tested with AIM-120's (unsure which exact version) on Eurofighters as well. (Successfully.)

 

Jona33 - Not sure. I don't see why it would be impossible, but I don't know nearly enough about the deep innards of those things to be able to say.

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ECM isn't just a question of "burning through". The FC2 implementation is extremely simplistic. Real ECM can do things like trick the radar into thinking the return it's seeing is just clutter (and thus reject you), or give false ranges, misidentify and so on and so forth. Basically, any ECM implementation you have ever seen in a consumer-availble product barely even scratches the surface of what real ECM and ECCM is all about.

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So could your wingman illuminate the target which the semi picks up and goes for it? Like a buddy lase but with a radar.

 

Not sure if I got it correctly, but in a squad where similar aircrafts operate with only semi-acitve vapabilities, each aircraft has a little bit different frequency so they can attack at the same time. So I guess that's exactly a technique to avoid the possibility.

 

However, the opposite goes for active-players with datalinks. E.g. the Gripen can act as a host aircraft for 3 more wingmans and guide any of their missiles while they have their radar turned off.

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ECM isn't just a question of "burning through". The FC2 implementation is extremely simplistic. Real ECM can do things like trick the radar into thinking the return it's seeing is just clutter (and thus reject you), or give false ranges, misidentify and so on and so forth. Basically, any ECM implementation you have ever seen in a consumer-availble product barely even scratches the surface of what real ECM and ECCM is all about.

 

Yes the ECM is like another sector which doesn’t tell you where the SEAD missile will fly, but that’s a bit different when guiding a semi active missile from external source.

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Not sure if I got it correctly, but in a squad where similar aircrafts operate with only semi-acitve vapabilities, each aircraft has a little bit different frequency so they can attack at the same time.

 

Probably not a different frequency, but a different code modulated onto the carrier wave, much like laser codes in a TGP.

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So could your wingman illuminate the target which the semi picks up and goes for it? Like a buddy lase but with a radar.

 

I've never heard of it. Certainly not something I can see being considered, after all, who uses SARH missiles these days? ARH with 2 way data link is what it's all about.

 

Griffin - AFAIK this has been tested with AIM-120's (unsure which exact version) on Eurofighters as well. (Successfully.)

 

Indeed, we tested it on our Typhoons quite successfully. Such capabilities are a major aspect of BVR tactics for gen 4.5 & 5 fighters.

 

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I've never heard of it. Certainly not something I can see being considered, after all, who uses SARH missiles these days? ARH with 2 way data link is what it's all about.

 

Yeah I was just wondering really.

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Probably not a different frequency, but a different code modulated onto the carrier wave, much like laser codes in a TGP.

 

Yep, but it would still eliminates the chance of guiding each others' munitions, right? In TGPs it works like selecting from 4 or what number of options, right?

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So could your wingman illuminate the target which the semi picks up and goes for it? Like a buddy lase but with a radar.

 

I thought that the MiG-31 could do that using its Zaslon radar and its datalink. But I'm not sure.

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Is it more economic to fly helicopters at a gigher altitudes as it is for fixed wing aircraft?

I would figure that is the same for helicopters too as they play by the same laws of physics but hey, it's a stupid question. :)

However would it make a big difference if I was low on fuel and climbed to over 3000 m for example?

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Is it more economic to fly helicopters at a gigher altitudes as it is for fixed wing aircraft?

I would figure that is the same for helicopters too as they play by the same laws of physics but hey, it's a stupid question. :)

However would it make a big difference if I was low on fuel and climbed to over 3000 m for example?

 

 

Short answer, no. It is not more economical for helicopters to fly at higher altitudes.

 

Long answer;

 

While planes and helicopters do indeed have to obey the same laws of physics, the construction and operating principals of airplanes and helicopters are very different.

 

Airplanes have engines to generate thrust and wings that use that thrust to create lift. When a plane climbs into higher thinner air it becomes easier for the engines to push the plane. This allows the plane to go faster, which increases the lift generated by the wings. So, even though the wings technically produce less lift in thinner air the speed increase makes up for it. So the plane is now easier to push through the air, is travelling faster and has just as much lift as it would in denser air. That adds up to better fuel economy.

 

Helicopters, on the other hand, have a spinning rotor that is responsible for generating lift and thrust. At high altitudes the loss of lift created by the thinner air forces the engines to work much harder to make the rotors spin faster. Eventually you can get to a point where the engines can't spin the blades fast enough to generate more lift. This is why helicopters can't hover at higher altitudes and tend to have a much lower service ceiling than airplanes.

Helicopters get the best fuel economy when flying fast enough to generate translational lift at altitudes where the air is dense enough for the rotor blades to efficiently generate lift.

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Ok but that doesn't make complete sense to me. Engine governors try to keep the rotor RPM constant withing a certain range. If the RPM is constant then the blade pitch is what determines the amount of lift. Also shouldn't it be easier to spin the blades at the same RPM in thinner air?

That's why it makes sense to me that the laws of physics should make it more economic.

 

Some other things on the other hand also determine the fuel efficiency:

 

- OAT; if we use ISA and it's +15 C at the sea level and ~ -3 C at 3000 m, would the temperature drop be enough to call the engine efficiency increase worthwile?

 

- Max IAS with the increase of altitude. Your maximum speed will decrease with altitude and according to my last sortie, the max IAS is lower than what the EPR gauge cruise power setting would be. Ie, I can't reach the cruise power setting because my maximum speed warning will go off. Also the maximum speed at an altitude is lower than the cruise speed at sea level that is equivalent to cruise power setting. I hope that made sense. :)

 

Additional question came to mind;

 

Would increasing the governor RPM setting allow you to fly higher (because increasing blade pitch would make them stall without additional speed) as long as the rotor RPM is within flight manual limits?


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Also shouldn't it be easier to spin the blades at the same RPM in thinner air?

 

No, thinner air means more AoA to produce the same lift.

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