Airborne Early Warning & Control: News & Discussion

saptarishi · Post by **saptarishi** » 03 Jun 2009 01:18

i think IAF should think before going in for new large aircraft procurement,i mean they are going in for A330 MRTT ,EMB-145 for drdo AEW, AND ALREADY have il-78 AND IL-76 PLUS C-130J . so going in for new large platform can turn out to be a logistic nightmare.

by now everybody knows the difference between AEW and AWACS. AWACS planes offer greater advantage in the sense that these have greater number of control operator stations for example basic AEW aircrafts are

1.B737 Wedgetail AEW for australia has 6 operator stations
2.EMB-145 AEW has 5 ,can be increasd to 6
3.CAEWS G550 EITAM has 6 windows based stations
4.TSPs SAAB 2000 ERIEYE has 5 stations
5.E-2D HAWKEYE has 3 stations plus the co pilot station can be used as a station some times

compare that with the AWACS systems
1.IL-76 PHALCON or with russian radar has 10-12 stations
2.JAPANES B767 AWACS HAS 14 stations and can be increased to 18
3.b707 E-3 SENTRY has 14 to 18 stations

THESE are full EARLY WARNING and CONTROL SYSTEMS
full AWACS SYSTEMS are extremely useful for large conflicts involving large number of aircrafts and also when the country is large, the ideal combination for india i think is
1. 6 full awacs ( 3 il-76 based & 3 A330 based phalcons,a330 because of the comonality with IAF's future tankers)
2.3-6 AEW ( DRDO EMB-145 )

indian navy also has a requirement for a 6 shore based AEW systems, CAEWS G550 EITAM and E-2D ADVANCED HAWKEYE seem to be the favourites ,but B737 AEW has an outside chance because of the commonality of platform with P-8I POSEIDONS ie.B737.

as far as IL-76 is concerned, its an extremely good plane but its airframe and cockit systems are old and not good enough for next 25 years, no wonder IAF's decision to go for A330 for tankers prooves it.

Dmurphy · Post by **Dmurphy** » 03 Jun 2009 07:32

^^
Nicely put.

But could somebody explain what those controller stations exactly do? Considering Saab 2000 AEW, it has only 5 controller stations. So what does that mean? How many aircrafts does 1 controller communicate with?

NRao · Post by **NRao** » 03 Jun 2009 08:16

With sensor fusion, I am not too sure that the number of stations will matter as much as they did a few years ago. And, in the future it should be FAR more independent of the stations.

Austin · Post by **Austin** » 03 Jun 2009 08:52

vivek_ahuja wrote:Austin,

The A330 based aircraft is certified to fly on one engine and conduct landings in relatively adverse weather. So are most other twin engined civilian aircraft models.

Also, the massive difference between the endurance and range of the A330 based model versus the IL-76 model should allow the former to be based far into the rear and still not affect its operational deployment or duration.

-Vivek

Vivek I understand your point , but we are not talking about plain vanila civil aircraft , but a military AWACS.

DO we know that a A-330 based aircraft can actually land in adverse weather condition with that BIG Chapati on a single engine ? Not to mention the other special avionics suite which will add up weight.

Why would the IAF want to move away from a proven platform like IL-76 to an unproven AWACS platform like A-330 ? If it works why break for small tactical gains ?

Range may be better but with refueling is that a major concern ?

Victor · Post by **Victor** » 03 Jun 2009 09:38

The A330 based Phalcons may not carry the big chapati and could have them installed on the nose and along the fuselage and tail like the Israeli & Chilean 707-based Phalcon/Condor with almost zero loss of aerodynamic efficiency.

Rakesh · Post by **Rakesh** » 03 Jun 2009 09:38

Austin Bhai....you vigorously defend everything Russian, one would think you are working for the Russian lobby

andy B · Post by **andy B** » 03 Jun 2009 09:44

Victor wrote:The A330 based Phalcons may not carry the big chapati and could have them installed on the nose and along the fuselage and tail like the Israeli & Chilean 707-based Phalcon/Condor with almost zero loss of aerodynamic efficiency.

I think, given the size difference between the A330 and G550 the forward hemishpherical array in the a330 will be fairly larger than the G550

Given the size differences between the array sizes of the G550 in the forward, backward and side arrays would it mean that the forward and backward ones wouldnt be able to look as far as say the side ones?

negi · Post by **negi** » 03 Jun 2009 10:03

This civil platform vs a cargo plane i.e. IL-76 discussion is an old topic (iirc Geet and Paul did post a couple of pages on one of the threads on BRF ).

Both IL-76 and A330 were never designed ground up for hoisting a Radar so aerodynamics and other structural issues are common to both ; coming to the ability to land or even fly on a single engine all modern passenger aircraft are very much capable of doing this so A330 is in no way inferior to IL76 in that regards .

More than the technical specs it is the sheer economics and logistics which imho will govern the choice of the platform in this case since IAF already operates the Gajraj in cargo/tanker config hence it is a natural choice for the AWACS platform .

Drevin · Post by **Drevin** » 03 Jun 2009 12:48

The A330-200 widebody was revolutionary design for its time in the late 1990's. Only recently did Boeing realize this, and have specifically asked that the 787 dreamliner be 15% better than the former in all parameters. So other than the 787 dreamliner no other aircraft will be comparable to the A330-200 for quite a few decades.

However, I hope airbus engineers and Air France clear the air regarding the recent crash due to electrical failure.

Also, i feel Austin has a point when he vouches for the 4-engine jets. O well, I'm confused.

Singha · Post by **Singha** » 03 Jun 2009 16:36

the A330 will certainly have room for more onboard consoles, racks of eqpt and crew rest areas. its quite simply a bigger a/c - same reason why its being considered over midas in tanker role.

in these strategic missions, bigger and longer legs is preferable imho.

alexis · Post by **alexis** » 03 Jun 2009 16:39

Rakesh wrote:Austin Bhai....you vigorously defend everything Russian, one would think you are working for the Russian lobby

Austin bhai seems to have crossed over to the other side. no doubt because of the influnce of vodka and natasha's

First he switched to Mig 35 camp from EF camp; now wants Russian planes for AWACS

Surya · Post by **Surya** » 03 Jun 2009 17:44

I am suspecting Shakarosky's hand in it!!!

vivek_ahuja · Post by **vivek_ahuja** » 03 Jun 2009 18:24

Austin wrote:Vivek I understand your point , but we are not talking about plain vanila civil aircraft , but a military AWACS.

DO we know that a A-330 based aircraft can actually land in adverse weather condition with that BIG Chapati on a single engine ? Not to mention the other special avionics suite which will add up weight.

Yes.

The presence of the big chapati is not as disruptive as it might seem in a cursory glance. And the OEI capability of the A330 is calculated with a full Take-Off weight.

Why would the IAF want to move away from a proven platform like IL-76 to an unproven AWACS platform like A-330 ? If it works why break for small tactical gains ?

That I agree. I see the circus of aircraft in the IAF inventory being formed and I cringe. Some on BR call the Pakistanis names for having a fleet of AEW aircraft made up of small numbers of diverse aircraft types. They should look at the IAF's own plans first. Either the IAF should have all their Phalcon systems mounted on the IL-76 or on the A330 etc. But my gut feeling is that we will, as always, end up subsidizing all of the world's products by making a circus of our own fleet.

Range may be better but with refueling is that a major concern ?

The IL-76 based Phalcon doing mid-air refueling is a major, major concern. Even though the capability exists, the "chapati" on the top and the overall airframe design means that refueling with the another tanker is a nightmare. The vibrations and buffeting effects on the A-50 nearly meant that that particular aircraft never was able to practically use its so called mid-air refueling capability. The same should apply to the Phalcon as well unless some serious modifications have been made. And I don't think the Phalcon has really been tested for mid-air refueling, just yet. We will have to wait and see the IAF experience coming out on that front over the next few months.

The other issue is from the operational standpoint: the tanker limitation in the IAF. Consider the chart below.

It shows where the IL-76 stands with regard to time on station for AWACS patrols versus a similarly mounted A330 model (only as an example). Along the land border with Pakistan and China, we are looking at an AWACS deployment close to their launch bases, probably within 250NM radius. Here, if both the A330 and the IL-76 models are launched, the A330 outperforms significantly when the IL-76 is not refueled in mid air. When the latter is refueled once, it outperforms the A330 (Here we assume that the IAF will have only the hose and drogue system refuelers and that the A330 will not be able to refuel in mid-air from the existing tanker fleet).

Having said this, lets do a quick simulation:

Suppose that a constant 24 hour patrol of an AWACS and four Flankers as escort is needed. We assume that the AWACS will stay up in the air for an average of 12 hour rotations (limited by crew fatigue and maintenance requirements) so that for every day in the war, a given AWACS station will require two airframes on rotation. This means that for a 1500 Km border, roughly six Phalcon airframes are required (assuming a high intensity conflict in the initial days of the war). Now, for an IL-76 based airframe, a 12 hour station will require one refuel from an IL-78 for extending a 6 hour station keeping mission (requiring around 45 tons of fuel) to 12 hours.

Similarly, suppose that four Su-30s are required to stay on station for a total of 9 hours at a time (basically doubling their 4.5 hour endurance on typical fuel loadout in a combat configuration for air-to-air). Thus four SU-30s will require one refuel every 4.5 hours from an IL-78, with a total fuel requirement for the flight coming out to be around 20 tons of fuel.

Now suppose that one IL-78 is put on station to refuel this group. This aircraft carries 85 tons of transferable fuel when it left its home base. No IL-76/78 is allowed to be in the air with less than 5 tons of fuel at any time. Then it requires the fuel to fly back and forth from its base and do the refueling maneuvers in flight. Overall, if the IL-78 was within 200 Km from its launch base, it would be able to offload around 65 tons of fuel and have enough to return to base. This amount means that one IL-78 can refuel one Phalcon and four Flanker escorts for every sortie it makes, having no fuel left for other missions.

For a 24 hour period, then, each Phalcon+escorts group would require around 5 IL-78 sorties to maintain continious station (even though the aircraft themselves can change on rotation). This means that for maintaining three Phalcon groups along a 1500 Km front for 24 hours, three dedicated IL-78s are needed that are not available for any other use. In addition, 24 Flankers and 6 Phalcons are also involved with this specific mission.

The same when applied to a A330+Flankers model, gives a requirement of 3 IL-78s, but without the dedication. In other words, once the IL-78 refuels the four escort Flankers, it still has enough fuel to offload to another 6 Flankers (or larger numbers of smaller endurance fighters like the LCA etc) that are involved in other missions.

Now, against Pakistan, these effects will not be significant mainly because the targets are so close that tanker requirements are low: the tactical aircraft can basically complete their missions even on drop tanks. But when you consider China, you start seeing the numbers becoming a choke point.

-Vivek

NRao · Post by **NRao** » 03 Jun 2009 18:36

Austin wrote:
Vivek I understand your point , but we are not talking about plain vanila civil aircraft , but a military AWACS.

DO we know that a A-330 based aircraft can actually land in adverse weather condition with that BIG Chapati on a single engine ? Not to mention the other special avionics suite which will add up weight.

Why would the IAF want to move away from a proven platform like IL-76 to an unproven AWACS platform like A-330 ? If it works why break for small tactical gains ?

Range may be better but with refueling is that a major concern ?

Because the first TWO options for the Israeli Phalcons were Boeing and Airbus? And, it was only on the insistence of India that the Israelis agreed to the IL-76? IIRC, Boeing was scratched - at that time - because it was a US based product and associated takleef at that time. So, the only viable option was Airbus - from a Israeli PoV of course.

Having said that I suspect that the Israelis prefer a conformal radar unit and not a radome.

Finally, it is all a matter of re-design. After all even this IL-76(TD) is redesigned for the Phalcon. And, redesigns as we all know means funds. Simple. Airbus is therefore no exception. All airbus has to do now is to agree to open MRO in India, as of 2006!!!

Both the Boeing offer for the P-8I and the potential offer for Airbus for AWACS should be based on "commercial" planes - the point being that there is a very large commonality with the commercial planes - the "maintenance" components in the future should not be as big of an issue as compared to the Russia option. I would expect the ILs to be phased out, until then IAF will "rely" on a rather shakey partner to deliver.

NRao · Post by **NRao** » 03 Jun 2009 18:47

DO we know that a A-330 based aircraft can actually land in adverse weather condition with that BIG Chapati on a single engine ?

The "chapati" is designed to carry its own weight - it does not add too much to the flight profile of the AC it is being carried on.

In adverse weather even commercial flights are diverted to other airports, AWACS are no exceptions.

The single-engine scenario is a rather rare case and it should be taken into account in the design phase - for sure.

I just do not think that is an issue.

SaiK · Post by **SaiK** » 03 Jun 2009 19:09

http://news.yahoo.com/s/ap/20090603/ap_ ... azil_plane
A key possibility is some sort of collision with a brutal tropical storm in the area that sent winds of 100 mph (160 kph) straight into the airliner's path.

but it also says:-

"Nobody in their right mind would ever go through a thunderstorm,...If they were trying to lace their way in and out of these things, they could have been caught by an updraft."

? what is the maximum tropical strom winds any airliner can handle?

Kersi D · Post by **Kersi D** » 03 Jun 2009 19:26

negi wrote:This civil platform vs a cargo plane .....

More than the technical specs it is the sheer economics and logistics which imho will govern the choice of the platform in this case since IAF already operates the Gajraj in cargo/tanker config hence it is a natural choice for the AWACS platform .

I do not think so. IAF opted fro IL 76/78 for AWACS as well as tanker because it probably did not have a choice. Today all arms pimps are falling over each other to get a piece of Indian chapati, no puns intended.

No doubt Russia does make very sturdy stuff (Rakesh I am NOT a Russian agent) but today we have a choice. Let us make full use of it

Kersi D · Post by **Kersi D** » 03 Jun 2009 19:30

NRao wrote:
DO we know that a A-330 based aircraft can actually land in adverse weather condition with that BIG Chapati on a single engine ?
The "chapati" is designed to carry its own weight - it does not add too much to the flight profile of the AC it is being carried on.

In adverse weather even commercial flights are diverted to other airports, AWACS are no exceptions.

The single-engine scenario is a rather rare case and it should be taken into account in the design phase - for sure.

I just do not think that is an issue.

The chapati actually contributes to the lift of the aircraft !!!

K

vivek_ahuja · Post by **vivek_ahuja** » 03 Jun 2009 19:44

Kersi D wrote:The chapati actually contributes to the lift of the aircraft !!!

I think Austin was referring to the drag effects as they directly affect the engine requirements. But even so, I agree that the effect is not that severe unless you go for things like Mid-air refueling where the buffeting effects as a result of the radar dome is a severe problem.

-Vivek

Singha · Post by **Singha** » 03 Jun 2009 20:07

thinking in a chankian yahudi way, what prevents yindia and yisrael requesting Airbus to scope out conformal side panel phalcon sized housings on A330 ? far easier to implement than a big new rotodome housing above the fuselage - structurally, center of gravity and aerodynamically. we could fit the same phalcon sized arrays
on the sides and a smaller MESA style top of T thing in a housing below the fat tail.

planB would be a MESA T-housing itself. I am sure the future amirkhan 777/787 awacs would use that arrangement.

yahudis IAI do have some exp in cargo conversions of passenger liners already....

masha-allah
- a MESA beast above the fuselage
- side looking GMTI radar on fuselage near nose
- a powerful sea search radar in the nose
- modular rackable GMTI/AWACS mission eqpt
- multi purpose salesforce.com wyse consoles serving up VDI for mission specific roles and sw

- satcom doghouse blister above cockpit
- long sting in tail for directional radio transmission
- spool-e-shaitan-e-wire unreeled from tail for ELF work

we could have a Bear_mki + E3 + Jstars all in one box using "virtualization" concepts. all tied together
onboard with a pair of redundant catalyst6500 switch.

NRao · Post by **NRao** » 03 Jun 2009 20:32

Kersi D wrote:
The chapati actually contributes to the lift of the aircraft !!!

K

My understanding is that just enough to compensate for itself.

sum · Post by **sum** » 03 Jun 2009 21:13

all tied together
onboard with a pair of redundant catalyst6500 switch.

Small gorilla ads inserted in the AWACs thread...

SaiK · Post by **SaiK** » 03 Jun 2009 22:38

for some reason i was thinking we did sign up for conformal aesa elta radar for our awacs. the talk about A330 modification is in lieu of this requirement or in addition?

dsingh · Post by **dsingh** » 04 Jun 2009 00:06

Hi Vivek,

Nice analysis of IAF tanker's limitations in war scenario. However they can maximize usage of tanker fleet by using 3 tanker's at a time in a Parabolic arc( Like US tankers did in Gulf and Vietnam Wars) along India-Paki border, deployed in parabolic arc, 200 to 300 miles away from their sams and air to air missiles. This way they can support all the operations of fighter escorts, AWACs and all the tactical and Strategic Bombings of " Sacred Pukiland A***" and their " Chinki Grand Ma" if needed. All the 6 tankers at present and future( all Though IAF plans to have at least 12-16) would be used to fill the parabolic arc round the clock as needed. Depending upon their location and deployments, these tankers can support navy ( Mig 29 k and Naval Tejas) operations as well.

Regards,

DSingh.

Naidu · Post by **Naidu** » 04 Jun 2009 00:52

vivek_ahuja wrote: It shows where the IL-76 stands with regard to time on station for AWACS patrols versus a similarly mounted A330 model (only as an example). Along the land border with Pakistan and China, we are looking at an AWACS deployment close to their launch bases, probably within 250NM radius. Here, if both the A330 and the IL-76 models are launched, the A330 outperforms significantly when the IL-76 is not refueled in mid air. When the latter is refueled once, it outperforms the A330 (Here we assume that the IAF will have only the hose and drogue system refuelers and that the A330 will not be able to refuel in mid-air from the existing tanker fleet).

Vivek: Why wouldn't the A330 be able to use a hose and drogue refueling system? I'm assuming the the A330 can get the same long proboscis that I see on the IL-76 Phalcon that was just delivered to the IAF. Thanks, in advance, for your answer.

NRao · Post by **NRao** » 04 Jun 2009 01:37

SaiK wrote:for some reason i was thinking we did sign up for conformal aesa elta radar for our awacs. the talk about A330 modification is in lieu of this requirement or in addition?

The alternative to the radome is conformal - the Condor with the Chilean AF (which has had - per reports - plenty of problems (electrical IIRC)).

And, as we know, an alternative to the large frames is the G550.

Singha · Post by **Singha** » 04 Jun 2009 09:37

Chile is long ago. G550 is deployed with IDF and new.

I'dsay the T-MESA is the best arrangement as it guarantees 360 cover.

conformal stuff might have a certain blind area in front and back just like
the Ereyie and cabs-aew which is || and not T

p_saggu · Post by **p_saggu** » 04 Jun 2009 09:55

Singh Saar,
Why does it have to be side panels or a chapati on top at all? Why can't it be a bar like the Aussie Wedgetail? Two large side looking AESA's and one in the front and in the rear.

It shouldn't be too much toil for a major manufacturer like Airbus to optimise a platform for AWACS operations, it could be done very quickly, and very eagerly since Airbus is now breaking into a new market of defense applications for its airliners.

SivaVijay · Post by **SivaVijay** » 04 Jun 2009 10:31

Here is what I think, it took 5 years for us to get the Phalcon in IL-76, why diversify now and go back to drawing board to change the aircraft config and spend another half a decade. Sticking to IL-76 is good like Vivek sir said in term of logistics and maintenance. Moreover our own platform is under development, though it is not an equivalent to Phalcon now, I believe it will be soon.
I say stick to the IL-76 and redirect the extra cost for adapting the phalcon to the airbus to the indigeneous effort. With all the big talk about self reliance I hope DefMin will see reason.

PratikDas · Post by **PratikDas** » 04 Jun 2009 11:35

Conformal antennas do have some disadvantages as captured in this BR post in the Aero India 2009 forum, viz.

Conformal antenna:
- Large EMI and loss
- Cooling is required... more power reqts and more size also. Also increases maintenance and failure rate.
- Reduced space in fuselage.
- no windows - increased operator fatigue.
- Maintenance and secrecy - maintenance of the aircraft and overhaul will require removal of the Active Antenna Array Units (AAAU) to maintain secrecy, which becomes extremely difficult for a conformal array.
...
Podded Antenna:
- Loss and EMI is minimal, which means a far better performance.
- Cooling can also be accomplished extremely easily, with ram-air cooling, which reduces the weight penalty.
- More space in the fuselage for equipment and/or operator cabins.
- More space and windows also means taht the operators are less fatigued and stressed, a major factor in their performance.
- Secrecy is also maintained, since it is much easier to remove the AAAUs.

Yes, this excerpt has a bias towards the podded antenna. The Phalcon and the Wedgetail are both podded antennas though.

neerajb · Post by **neerajb** » 04 Jun 2009 11:42

NRao wrote:
Kersi D wrote:
The chapati actually contributes to the lift of the aircraft !!!

K
My understanding is that just enough to compensate for itself.

Whenever there is lift generation, there is associated drag penalty. Whether the Chapati generates the lift or the aircraft wings, finally it comes down to aircraft engines to compensate for the generated drag though it doesn't affect the structural limits of the airframe. Apart from induced drag, the big Chapati should generate friction drag too. So IMVHO, the aerodynamic performance of AWAAC will be lower than that of a vanila IL-76/78.

Cheers....

Kersi D · Post by **Kersi D** » 04 Jun 2009 16:55

NRao wrote:
Kersi D wrote:
The chapati actually contributes to the lift of the aircraft !!!

K
My understanding is that just enough to compensate for itself.

What I understand is that it gives a helluva lot of lift. So it is (aerodynamically) designed to give sufficient extra lift to counter extra drag.

Our BR "aerodynamic experts" may shed more information on this issue

K

Samay · Post by **Samay** » 04 Jun 2009 17:22

Why is rotodome cofiguration preferred over side array configuration, when its an open secret that the experimental awacs crashed because they were not able to configure correctly it on the a.c ,
Even if such is not the case then why ?,considering that in Israeli phalcon that radar doesn't rotates?

k prasad · Post by **k prasad** » 04 Jun 2009 17:26

Samay wrote:Why is rotodome cofiguration preferred over side array configuration, when its an open secret that the experimental awacs crashed because they were not able to configure correctly it on the a.c ,
Even if such is not the case then why ?,considering that in Israeli phalcon that radar doesn't rotates?

Read just 3 posts above Samay... dont expect spoonfed answers, at least not on this forum.

Samay · Post by **Samay** » 04 Jun 2009 17:35

k prasad wrote:
Samay wrote:Why is rotodome cofiguration preferred over side array configuration, when its an open secret that the experimental awacs crashed because they were not able to configure correctly it on the a.c ,
Even if such is not the case then why ?,considering that in Israeli phalcon that radar doesn't rotates?

Read just 3 posts above Samay... dont expect spoonfed answers, at least not on this forum.

I was asking what was the reason to increase complexity?,while Israeli awacs ,have different structure

abhiti · Post by **abhiti** » 04 Jun 2009 17:46

Samay wrote:I was asking what was the reason to increase complexity?,while Israeli awacs ,have different structure

Israeli configuration also does not provide true 360 degree coverage resulting in operational complexities (limitations).

neerajb · Post by **neerajb** » 04 Jun 2009 18:07

Kersi ji, I am not an expert but my knowledge says that induced drag is directly proportional to the square of the lift generated.

Whether the wings are generating all the lift necessary for the setup (aircraft + dome) or wing+dome generating the lift, the drag generated by that would be proportional to the sqare of the lift (total lift = Wing + Dome). That's why I said that the more lift you generate the more drag you produce. Not to mention the pressure wave drag and friction drag especially at high subsonic speed regimes.

Cheers....

NRao · Post by **NRao** » 04 Jun 2009 18:34

k prasad wrote:
Samay wrote:Why is rotodome cofiguration preferred over side array configuration, when its an open secret that the experimental awacs crashed because they were not able to configure correctly it on the a.c ,
Even if such is not the case then why ?,considering that in Israeli phalcon that radar doesn't rotates?

Read just 3 posts above Samay... dont expect spoonfed answers, at least not on this forum.

And, if I may, "that the experimental awacs crashed" is a blow under the belt - due to lack of knowledge perhaps. The India AWACS (that crashed) was built on a rather low cost variety aircraft. Not that it was an expected crash, but certainly that was not the aircraft on which the final radar would have been built - that was an "open secret" too.

Israeli configuration also does not provide true 360 degree coverage resulting in operational complexities (limitations).

The G550 I think does. the Condor perhaps does not, but that is aged technology and should not even enter in any discussions.

vivek_ahuja · Post by **vivek_ahuja** » 04 Jun 2009 18:54

neerajb wrote:Kersi ji, I am not an expert but my knowledge says that induced drag is directly proportional to the square of the lift generated.

That ratio is not with lift but with lift coefficient. That is: the induced drag coefficient is proportional to the square of the lift coefficient. This makes the induced drag very small overall unless the chapati is inclined to the airplane central longitudinal axis at a large angle (which it is not). What it will have is perhaps a net effective camber along its diameter that aligns with the aircraft longitudinal axis or if the dome is symmetrical, it may be aligned with perhaps one or two degrees of inclination to the central alignment axis. This ensures enough lift to compensate for the equipment and structure in the dome because these are modified transports that were not designed with that additional lift requirement in the cruise condition in mind (note that the idea is not to consider these structures as part of the payload, since they are exposed to the airflow and therefore must be designed to carry themselves rather than burden the mother aircraft). So you compensate by having the dome produce its own lift.

Having said that, take a look at this simple calculation:
Suppose the weight of the dome is: 1500 Kg
At the cruise speed of the IL-76, the lift coefficient for the dome is: ~0.02
So the induced drag Coefficient is (assume an oswald efficiency factor of 0.7 for now, even though the actual value will be lower): 0.0001818
So induced drag is: ~223 N

This is about 5% of the skin friction value for the same dome.

Now, for the AWACS of today, the major issues with the rotodome is of skin friction and proximity interference drag (other components of drag such as wave drag contributes perhaps around 5% to the overall chapati drag even at high subsonic Mach numbers, and pressure drag perhaps ~1%) and flow disruption over the tail control surfaces.

Skin friction over the chapati surface contributes perhaps 6-7000 N on an airframe that in its transport version ALONE has drag in excess of 350,000 N! That makes the chapati around 1.5% of the overall drag. The other effect of proximity interference may boost up this value siginificanlty in absolute terms, but still leave the chapati drag in the vicinity of a few percent of the original IL-76 drag.

The disruption to the control surfaces is perhaps a bigger issue, but even that has sufficient solutions to a point that design modifications are routine and overall effect negligible.

Victor · Post by **Victor** » 04 Jun 2009 19:38

abhiti wrote: Israeli configuration also does not provide true 360 degree coverage resulting in operational complexities (limitations).

Actually it does. There are two arrays on each side of the fuselage, one in the nose and one in the tail, total of 6 array elements just like our Phalcon for full 360 degrees coverage.