MiG-25P (Foxbat-A) "proof of concept" tweak

[+nele 0]

This is "demo", or proof-of-concept modification/WIP of MiG-25P (Foxbat-A) that can cruise over 20,000m/65,000ft.

 

Original model by Mig-25 package by wpnssgt, usafmtl, tomcat1974, sundowner,ghostrider, deuces and moonjumper, from the "Foxbat" package)

 

When I checked "Foxbat" in original form from the package, I found out that it has more/less adequate behavior for the large,heavy interceptor but its performance at altitude needs significant improvement. It suffered from too much "Mach tuck" that is hard to fix in the normal way, since there is no trim. Also, it was obviously hard to incorporate high-speed aerodynamics, and the "ordinary" shape of Foxbat airfrane can be a mislead. Truth is, Foxbat does not behave "normally" at high speed/altitude, having both airframe and engines in mind!

 

This is not the final product-this is a "proof-of-concept", (re)built almost solely by feel&performance understanding "match" and needs a lot of refinement. Basis for altitude engine perf is R-35 chart (which I took from MiG-23 to speed the things up). Many things were taken from MiG-25RB aerodynamics manual, which is basically the same airframe (performance is relaxed a little, due to competely different mission).

 

First-the engines. R-15B engines are, considering operational speed, ramjets augmented with turbojet core. Quite opposite of other afterburning turbojets! The role of turbine part (and first afterburner stage!!!) is to bring AB2 section in operational zone, which is +11,000m/+M1.5. AB2 switches on automatically, and is fed by air compressed by intakes, fed into turbines that are heating and further "conditioning" the airflow and serve it to AB2. Such powerplant produces almost 6T of thrust (each!) at peak performance (+M2.8). Maybe it does not look much, but the drag, temperature and specific fuel consumption are much lower than of "normal" turbojet, which would melt itself on that speed if constant, or simply "suck" all the fuel. Also, normal AB turbojet is on decline of performance chart when this type of poweplant just starts to "breathe".

 

I couldn't find the way to put these into one (i.e. two engines) so I added two more "dummy" engines that basically take over the propelling of "Foxbat" at high altitude. If you've seen the charts (of MiG-25RB), it is only viable solution! ALso, R-15B is 2-stage AB engine where "staging" is cruical. That cannot be done, or cannot be done easily using just one set of engines.

 

Bigger problem was the trim. Foxbat flies straight at +20,000m not just because of engines, but because of aerodynamical qualities. The AC/CG (aerodynamic centre/centre of gravity) shift is quite huge, which is countered by large all-stab authority (-23.50/-9.15 deg total range of stabilizers, perpendicular to airstream), but even this would not be sufficient if there was no intake-fuselage lift.

 

Many people don't know that canards were ment to be installed to Foxbat during design phase. The long, heavy nose and was the main concern then. However, it was discovered that intake lips make much lift that render canards unnecessary. They shift AC forward and reduce trim drag. Some Western types have such quality, wanted or not, (like F-15 Eagle), but Foxbat really needs it. Also, huge AC shift happens when engines are "revved" at altitude, i.e. when aircraft accelerates/decelerates. Pilot seldom holds "Foxbat" straight with his muscles-there are "autopilots" that are common in Soviet airplanes, but with the raised priority in hi-perf aircraft like MiG-25.

 

I did this version with "dummy" lift surfaces on the nose that lift the nose at high speeds and act as intake lip lift. As the Mach increases, so does the lift of those surfaces (which is neglible at lower speeds). It is exgeratted, I know, but it has to serve the non-existant trim function as well! I made another version with "titled" thrust of those AB2 dummy engines for 45 deg up and roughly recalculated thrust again (c2=2a^2) so it gives the same thrust and upward vector). The result was good, but there was a little too much "oomph" (pitchup) when AB was lit. Maybe I wil tilt thrust a little less to get desired "oomph" result in final product :)

 

I had to "shut" braking chutes-I really got frustrated when I saw aerodynamic brakes were excluded on this model (from the "package") and re-instated chutes (and they are 2-part brakes on the true thing, upper used only above M1.1/7,000m). I adjusted the drag/Mach behavior to the one from the manual, and you-really-need-it now, no matter how small they are!

 

Flying the "Foxbat".

 

First, the climbout; it has to be done properly: roughly, best way is to press throttle to full Ab, release the brakes, and switch on the autopilot. Once gear and flaps are up, you can take over and point it into general desired direction, -but- keeping the altitude below 1,000m and airspeed under 1,100kph. MiG-25 has a "hard limit" of 1,100 level/1,150KPH climb IAS at low altitude, due to adverse aileron effect on earlier machines, and it was kept on newer models with differential stabilator to prevent pilots to have too much "jollies" and keep engine life up (air is dense, and it easily goes outside temp limits).

 

Achieving "on" speed, pull to 30 deg pitch. It is a steady pull. Your goal is 5,000m+ deck, where you level out and wait it to hit M1.2 or close to 1,000kph IAS. Then pull 25 deg and keep that Mach number (or with tendency to go little higher) to 12,000m, where you level out again and aim again to 1,000kph. You will notice the speed rise significantly over M1.5 (AB2 on!), pull 20 deg , and watch MiG Mach-accelerate through the climb! Passing 14,000m start level-out and ease throttle to cca. 80% at 17-18km alt. It is easy to overshoot or "undershoot" 20-21,000m (which is atmospherical change in "TK's world") so you will have to practice.

 

Once you are level at cca. 20,500m (wing leveler on&steady), when speed is M2.35 you can chop thrust to 68-69 percent (still AB!) that will keep the speed up (more/less, real Foxbat also either accelerates or decelerates, since the smooth regulation of AB2 is not possible).

 

Performance&fighting

 

Maybe some of you have read about Iraqi Foxbats, and that the elite fighter unit of Iraqi dictatorship was not Fulcrum unit-but Foxbat unit. There is a reason-flying Foxbat needs alot of competence, not because the type itself is (overly) unforgiving, but because -keeping- it within usable envelope requires more than party ties! Pilot has to think ahead -waay ahead- and to remember that if speed and altitude goes down it is just a big, clumsy target. The general rule is that it is outperformed by virtually everything under 5,000m (limited to subsonic speeds), about equal to "Phantom" between 7-12,000m and M 1.2-1.3 and absolutely superior in performance to everything in range +12,000m and over M1.5, including the mighty Eagle!!! SR-71 flies outside the range of "ordinary" airplanes and overlaps "the curve" only with "Foxbat". Cca. 3g allowed turn at those altitudes is extremely fast, and there is simply no target that can get outside the "g" envelope of "Acrid" missiles (because there is simply no air dense enough to allow any conteporary fighter more than 3.5-4g turn).

 

Stay above 6,000m and M1.2. Whenever you get close to these numbers, level out and pick up speed and altitude. You will get into controlability problems once speed around 1,100kph-"Foxbat" wings flexes a lot! It is easy to over-g the real aircraft, though, newer models have coupled alpha and g-limiter that prevent this. To get down from 20km altitude, the procedure is to go max dry thrust (2nd, "supersonic" dry thrust), slow down to M2.2, and descent keeping the IAS the same. You can zoom the fighter up to cca. 27km altitude, but it is too sensitive and lazy on controls up there. Brakes are effective, but at too high speeds they mess up the airflow around the aircraft too much (not forbidden, but not recommended either). After firing two missiles from the same wing, you will get quite of bank which has to be (smoothly!) countered or you will make an unintentional roll. Before introduction of differential stabilator, "Foxbat" exibited uncouterable auto-roll after firing "Acrid" at high IAS (called "Kazaran effect", after the pilot who "discovered" that flaw).

 

"Smerch" radar is jam-proof but not too much range and with much clutter under 1,000m alt, no "lookdown-shootdown". Missiles are all-aspect (IR, too!), but IR model is very sensitive to the "heat clutter" (real thing is restricted 30 deg sun/moon), so no "shootdown" with it either. Missile is designed in '60es and was, actually, quite a performer. Snap-up is normal procedure. The true "Foxbat" has a unique digital targeting computer (for its time) that analyzes target speed, altitude and trajectory, "Foxbat" relative position, altitude and speed compared to the built-in missile speed/turn performance aerodynamic ability (calculates PK) and will not give firing solution unless those parameters are satisfied and PK is viable, regerdless that target is acquired by seeker... former "Foxbat" pilots write in forums that they had quite a number such "guaranteed lock-ons" on SR-71's!

 

Enjoy-and I will try to get the rest of series soon!

 

Zip file containing modified ini attached, backup your existing MiG-25P_DATA.INI and replace it with the one in this file. Note that "Foxbat" entered service in 1971-72, so you may have problem with the missiles (due the date). The model can "act" as early export "Foxbat" to Lybia, too, as they were equipped with "Smerch" radars, but they can carry R-60's.

 

Nele

MiG_25P_DATA.zip

[+Crusader 2,082]

now this is interesting ....

[Nicholas Bell 117]

Nele,

 

I thoroughly enjoy your analysis of what you're doing "behind the scenes" in the sim and how it relates to real world aircraft performance. Looking forward to all your FM mods.

[Lexx_Luthor 57]

Doc this is really neat. Very interesting indeed. I have the -25 manual in PDF, but I don't read Russian.

 

I recently did experiments with JATO. I used the Mach table to greatly increase thrust below Mach 0.2, and it works to get off the runway in less than a jiffy, and the thrust returns to "normal" as Mach 0.2 is approached. The only problem this may offer far too much thrust when you get into trouble at very low airspeeds, potentially allowing too easy escape from dearly departed flight. But, I like operational abilities from shorter airfields, so I'll be using this. I haven't thought how to do JATO grafix limited by speed though...eww...maybe the vortex grafix methods can offer a clue.

 

--

 

Doc...you disabled the exhaust of the 2 "bonus" engines, but you forgot the the extra 2 contrails -- (( edit...I think the ExhaustPosition defaults to 0,0,0 if its disabled )).. Four (4) contrails are generated now. and they can all combine to form overly dense trails, and this can effect framerates under some conditions. To make sure these are eliminated, move the bonus engines' exhaust position...

 

 

ExhaustPosition=1E+9 , 0 , 0

ExhaustPosition=-1E+9 , 0 , 0

 

This puts the contrails and any other exhaust grafix 1 million kilometers distant, astronomically far beyond grafix rendering distance.

 

This is really neat stuff. What sim offers the ability to explore this, and the upcoming B-70 by NGHENGO with streakeagle on the FM. I hope the Mach 3 modders club grows without limit.

Edited May 10, 2007 by Lexx_Luthor

[+nele 0]

Lexx, tnx for the input. I run at low-res since I have "fastest among slowest" GF6150 onboard VC :) Did you consider putting "extra" engine for JATO's? Basically, all you need is to make them active on the sea-level only to 100-200m altitude (using AltitudeTableData) and DryMachTableData to 0.1-0.2. Inertia will do the rest, or you can make graduated table. Also, you can adjust the trust rate separately from the main engine(s).

 

Careful, Soviet airplanes use JATO to conserve fuel and often make takeoffs in -dry- thrust. Also, it seems that flight engine makes some type of "curve" if two adjecent values are too different, so it is better to make "denser" table values with mid-values.

 

When I was testing addon engines, I was getting "flameouts" when I pressed AB since the inlet temp of "dummy" engines is actually exaust temp of "true" ones! There is one more idea for you to disable JATO after takeoff, if you can place dummy engine not to shut "real" engine, too :)

 

The idea of "addon" engines emerged in my head not from JATO's but from the fact that there are types with AB WEP. For example, those are MiG-21bis ("cheresvychayniy", or emergency regime) and IAI Kfir ("combat plus") that can produce increased thrust, but only under certain speed/altitude conditions. It is actually much easier than what you are trying to do, making dry thrust of WEP "engine" zero, and placing AB thrust section far away to the right stop (and possibly shorting one of the true engine).

 

Nele

 

P.S. is that manual of RB or fighter you have, I would make a use of fighter one...?

[+kreelin 2]

As I already said in another post and board, it's really great to have you around Doc ;) ...

 

When you said:

 

...I did this version with "dummy" lift surfaces on the nose that lift the nose at high speeds and act as intake lip lift. As the Mach increases, so does the lift of those surfaces (which is neglible at lower speeds). It is exgeratted, I know, but it has to serve the non-existant trim function as well! I made another version with "titled" thrust of those AB2 dummy engines for 45 deg up and roughly recalculated thrust again (c2=2a^2) so it gives the same thrust and upward vector). The result was good, but there was a little too much "oomph" (pitchup) when AB was lit. Maybe I wil tilt thrust a little less to get desired "oomph" result in final product :) ...

 

You know there is a "trick" to simulate TRIM. For those who flies in HARD mode maybe the Foxbat would be the first mod where you NEED to use the trim for more immersion ;) . ..

In despite of that, It fully makes a sense to add Aerocoefficients on those big air intakes even if it's not very simple to estimate their values (a lot of trial and error would be needed ;) ). So if you want to "autotrim" the bird, your solution is not "exgeratted".

Nevertheless, I think there is another way to take in account the air intake effect but without an hard calculation to estimate correctly the aerocoefficients values. In fact, you can consider the air intake area as a LEX for the wings. So you can you use (for a good approximation) the standard formulas to get the CLa of the "merged" wings. I used this method in the Mirage Factory's F-15A FM. Don't know which of both methods is the best but I just choose the one who saved me times :D

 

 

OTW, I have another questions about engines.

 

1 - I'd like to know if there are some infos in the manual confirming this

 

ThrustAngles=-2.00,-2.50,0.00

 

for each engines ?

 

 

2-And How did you estimate the extra thurst generated by the RAMJet part of the engine ?

Edited May 10, 2007 by kreelin

[+nele 0]

Hm, the tweak is rather "empyrical". I have mixed in some data from the manual and tweaked other thata until it started behaving properly

 

Yes, the engines are titled for -2 deg (nozzles are facing each other) and up 2.5 related two the main axis (engine installation drawing).

 

For the engines, the proper way would be to draw a "curve" and then divide it into two for "normal" jets and ramjet effect (some thrust left on the main engines), but I took the R-35 curve and adjusted it as necessary (as it is more detailed than one of R-15). In another words, flies right, but numbers are "wrong". There is probably a little bit too much thrust. Thrust of real R15B engines (in AB2) is around 5.900kg each at M2.83/20km and has an /upward/ tendency on the chart, but the "wetmach" curve is really a guesswork.

 

You, see, I "made" the Foxbat in my head (engines/intake lift) and I wanted to test it ASAP, so I ended/will end doing things backwards You may notice that I wanted to put aileron reversal over 1240kph and drop "artificial" hard speed limit, but it went to nowhere.

 

I also wanted to make "self-adjustable" trim, as the true MiG has a nice trim-effect (and autopilot) by "mounting" stabs on a dummy vertical-swiveling surface (like swing wings but in vertical plain), but it didn't work :umnik2:

 

Nele

[+kreelin 2]

Ok.... Thanks for these insights ;-)

[Lexx_Luthor 57]

Thanks nele. You made me think -- a bonus engine would be best for JATO. Some mods (B-29B, B-35) use engine pods carrying engines (this also causes doubling the contrails but can be fixed). Perhaps, engine pods could have the equivalent of landing gear "max deploy speed" and fall off after takeoff, thus allowing for JATO grafix; something like that. Another fun application would be zero length rocket blastoff from the ZELL (MiG-19, F-100), although giving standard modded aircraft a high nose attitude before launch might not be so fun.

 

nele::

Careful, Soviet airplanes use JATO to conserve fuel and often make takeoffs in -dry- thrust.

I've read that was the normal peacetime procedure, but they could do the full blast off if needed, particularly from rough wartime airfields.

[Lexx_Luthor 57]

Forgot one -- La-9 with ramjets. I figure no thrust until some solid Mach number and then the thrust builds up in the same style as R-15B here, but remains subsonic. These ramjets could boost speed maybe 100km/hr, but did they offer a useful altitude boost? It would be interesting if we could vary thrust with angle of attack.

[+nele 0]

Well, AltitudeTableData, DryMachTableData and WetMachTableData can sort when will the engine "work", especially as there is no engine drag (i.e. turbine aerodynamical drag) involved. For a test, add one "clipped" engine similar to the one in "Foxbat" POC (same location as existing or bit after), but put inlet temps of that engine way up. Make Altitude table "dense", or example:

AltitudeTableNumData=20 (200?)

AltitudeTableDeltaX=100.0

AltitudeTableStartX=0.0

AltitudeTableData=1,0.5,0,0,0...etc. this may be tricky because of AGL...

 

WetMachTableNumData=10

WetMachTableDeltaX=0.02

WetMachTableStartX=0.0

WetMachTableData=1,0.5,0,0,0,0,0,0,0,0

 

Anyway, it is easier for me to build WEP for MiG-21 then for you to make RATO that will work on elevated airports.

 

You can use large multiplier instead of huge installed thrust to get the impulse. Watch it-thrust pushes nose down a lot!

 

JATO (actually, RATO) was used in many ways, but its usage was actually not that popular installation; India never used it on Su-7s, for example. "Fishbed" was tested and can takeoff from muddy airstrips, but a genius that could install wipers on "Atoll" IR-seekers has never been found!

 

Ramjet La-9 was not a successful aircraft-pods were draggy so it took forever to accelerate, and they added to much drag to make significant increase due to ramjet thrust, and they sucked too much fuel.

 

Campini (piston driven turbine) of MiG-13, however, is viable, and the type served sucessfully in coastal units. Anybody to make this "missing link"...?

 

Nele

[+ghostrider883 526]

JATO (actually, RATO) was used in many ways, but its usage was actually not that popular installation; India never used it on Su-7s, for example.

 

Nope, some of our Su-7s used them certainly , specially when Su-7 detachments were deployed in forward airbases with semi-prepared runways. I have a pic of an IAF Su-7 performing a RATO-assisted take-off somewhere on my HD. Will try and find it.

[+nele 0]

Hm, that is a new information! I use Bharat Rakshak webpage for information about aircraft in IAF service, as they seemed liked to IAF to me (very nice, descriptive articles). They were explicit about that information there:

 

/quote/ "Two JATO solid-propellant rocket units can be attached under the fuselage to assist in take-off, but the IAF has not adopted this procedure... ...The cockpit, although spacious, is cluttered with a large number of instruments and switches, many of them redundant as the IAF did not adopt the JATO booster..." /unquote/

 

It was peculiar to me not using JATO in humid/hot days in India, or at least to save some fuel for long-range missions. Makes more use to me than the "mud airstrip" original capability (as Soviet-type "hard" airstrips are made of replaceable blocks anyway, not multilayer tarmac).

 

Can you put a frame or two from the movie somewhere and link it here for documentary purposes? And send it to Bharat-Rakshak to make a correction to the article, some of us Europeans know what's happening with IAF only from what they tell us

 

Nele

[+ghostrider883 526]

It was peculiar to me not using JATO in humid/hot days in India, or at least to save some fuel for long-range missions. Makes more use to me than the "mud airstrip" original capability (as Soviet-type "hard" airstrips are made of replaceable blocks anyway, not multilayer tarmac).

 

Can you put a frame or two from the movie somewhere and link it here for documentary purposes? And send it to Bharat-Rakshak to make a correction to the article, some of us Europeans know what's happening with IAF only from what they tell us

 

Nele

 

Actually I think i found this pic at BR, but can't find the link to it . Its shows a post-1971 war three tone camo Su-7 doing RATO from Bareilly AFS.

Apparently the Su-7 was quite useless in terms of range.There were cases where Su-7s suffered fame-outs due to fuel starvation on landing approach after missions.

In IAF service, it was a made to perform roles which it was not designed for ie. low level CAS. Many Su-7s fell to AAA while doing this, they made repeated passes over enemy troop concentrations and some of the Su-7 pilots paid with thier lives unfortunately.But it was robust as well, capable of taking punishment. It was one of the few aircrafts of the IAF which survived a direct missile hit(the tail section of one such a/c is on display at a Delhi museum.).

A flight of Su-7s from IAF's elite TACDE squadron performed accurate airstrikes at night making the PAF(and a famous American officer) believe that these mission were flown by Russian pilots and that they were guided onto their targets by a Tu-126 Moss AWACS.

THe aircraft had a relatively short service life in the IAF when compared to other aircraft of that era. Our Canberras(PR.57/67/B(I)58/T.Mk.54s of No. 106 SPR Sqdn) were retired just a week back after 50+ years of service with the same sqdn.

Edited May 15, 2007 by ghostrider883

[Lexx_Luthor 57]

Thanks nele.

 

PVO is going to need JATO in my SF mold because the airfields, especially forward airfields, will be under constant threat from SAC bombers and long range escorts / fighter bombers. Smaller airfields should be longer lived airfields -- hence the need for standard use of JATO. I'm exploring La-9 ramjets because they did work, and the tough penalties may be acceptable if it means La-9 can catch high altitude lightened B-29s or early B-50s until better things become available such as MiG-9.

 

MiG-13, or I-250N, I was not planning to use. Yefim Gordon says the thing just didn't work out, and the Derwent and Nene were becoming available to designers. 50 were ordered but VVS and later NAVY cancelled. Apparently, VVS documents say MiG-13 was taken out of service, but Yefim sees these as the few (about 8?) aircraft made for Stalin's 1946 flypast (canclled by weather). Still, it seems possible the VRDK was simply never given high resources, so assuming throwing more resources could make it work, theoretically. The mixed prop/jet compressor concept worked for the Ryan Fireball, and it would be nice to see it used. Lots of neat stuff available to experiment with assuming the 1947-1967 PVO finds the same motivation as the western Luftwaffe did in 1943-1945....

 

 

Yak-19 (my favorite jet of this era, use Dassault Ouragon model, add afterburner and other huge data file changes)

La-9 with ramjets.

MiG I-224 ... this is a very interesting high altitude interceptor and might could be made in small numbers for 1947, or maybe just a later Yak-9PD type of variant.

La-15 (model by Pasko, assume MiG-15 eats up all VK-1 production, or VK-1 factories destroyed, etc... I have much dynamic campaign programming to do later!!)

Aleksyev I-211 ... lets assume Lyulka TR-1s have the same "reliability" as the Luftwaffes' BMW's and Jumo's which were used operationally during wartime.

MiG-9 with afterburning ... I'm thinking that one campaign variable would be Britain selling Nenes. If not, what would happen? Another campaign variable could be the licensing issue over Merlin engines in F-82.

okay.... MiG-13. A campaign variable may allow a rare appearance of a few examples of this bird. Use Polikarpov I-16 external model!

Yak-15

etc...

All the way to using a single R-15 in Ye-152M in about 1964 or so -- I added a big fat fuselage spine to Pasko's Su-11 by using large Tornado drop tanks. We saw the Luftwaffe was willing to use unreiliable engines also in desperation.

 

The models I need most are...

 

 

Yak-19

Tu-128

B-45

B-36

B-70 (wip)

F-101

R-80 missile

MiG I-224

etc...endless list.

 

Alot of fun might be a fantasy IL-28I with data file changed for nose turret for interception. History and things like historical use of JATO can only be a guide here, as I am exploring the most Fantasy story of them all -- strategic nucular warfare which both US and Soviet air forces devoted most of their budgets preparing for during the early Cold War years (along with US Army's huge strategic SAM budget). And of course, the MiG-25P will play a starring role in the latter stages of a campaign. Thanks nele, your -25P FM is AwSim.