At very high airspeed, engine shuts off

[D4n]

Hey, anyone knows whether Mikoyan knew about this 21Bis issue and why they didn't take measure to avoid engine shutdown at very high speed (Mach 2)? (at +1 g)

[D4n]

edited, meant speed, not G obviously.

[Jonne]

There is a reason for a maximum speed. In case of the MiG-21 its speed is limited by the engine. There are other aircraft, like the Mirage, where airframe temperature is the limit.

[D4n]

Mirage 2000C ? that temperature-damage even modelled in DCS?

Also, your reply sadly is no explanation for why the engine shuts down. (I heard it's due to not enough air getting into the engine, but then why didn't they increase fuselage-size a little bit?)

[Jonne]

At least on old MiG-21 versions the capacity of the fuel pump was limiting the top speed.

 

For the Mirage, yes that is modelled.

[D4n]

Wow, thanks! Any idea how pilot in DCS will notice the Mirage critical wing temperature? (What Mach, what altitude?)

[Art-J]

Extremely simplified and short explanation:

 

a) Axial compressor of turbojet engine needs subsonic airflow at its first stages to operate well, or even operate at all (both stator vanes and rotor blades are designed to compress air at subsonic flow. Yes, I know there are some Russian engines with partially supersonic first stages, but they're exception rather than the norm);

 

b) thus for supersonic flight, an inlet is required with adjustable device (cone for circular inlet, semi-cone for semi-circular inlet, flat plate for rectangular inlet), which, along with the inlet lip, creates at least one oblique + one normal shockwave pattern to slow down the airflow to subsonic speed (and compress it somewhat at the same time before it reaches engine compressor);

 

c) there's only so much airflow volume and speed that you can control with adjustable inlet and bleed valves which will provide optimal engine parameters within both high but also and low-speed regimes (takeoff, maneuvers, landing). For MiG-21, Mach 2 capability from such an inlet was actually pretty good for 1960's era design, not many frontline fighters from that time were faster.

[D4n]

Very interesting, thanks. So Mikoyan probably did know about it but back then was like "ok, Mach 2 is enough compared to other fighters of this time, so we're not gonna spend even more money to make it's engine more stable" ?

Also very strange that MiG-21-93 still has exact same engine as 21Bis. They could've made the -93 even faster with better engine/inlet-technology I guess? Maybe length of inlet is deciding factor?

[Jonne]

Very interesting, thanks. So Mikoyan probably did know about it but back then was like "ok, Mach 2 is enough compared to other fighters of this time, so we're not gonna spend even more money to make it's engine more stable" ?

Also very strange that MiG-21-93 still has exact same engine as 21Bis. They could've made the -93 even faster with better engine/inlet-technology I guess? Maybe length of inlet is deciding factor?

 

From reports of German MiG-21 Pilots, maximum achieved speed and altitude was nearly always limited by available fuel. So there really is no point in changing design, if you cannot reach the speed with the fuel you have anyway. I however cannot find any reference to a flameout due to insufficient air.

[Fri13]

As the MiG-21 was designed as short range interceptor, that is located at various roadbases, airbases etc all across the country, there was no need to try to make it fly as SR-71 (that was MiG-25 and MiG-31 task) or like a Su-27 for long range fight or long time patrol.

 

A small, very fast, agile interceptor with enough weapons to take down 2-4 bombers that you can can get take-off in 30-60 seconds and maintain its maximum speed for the range to get missiles out and bug off.

 

Each alert group meant to be ready and sit engine running for the last seconds before take-off for the optimal interception. So all the fuel you carried, was there enough for the range at the needed time to get on location and engage the bombers, and then another alert group was already on its way when you are ready return to base.

 

The MiG-21 can be seen as a manned S-200 or S-300 SAM battery, you calculate the interception point, launch the interceptor for that long range course, and then while on intercept point, get the very agile small missiles at the target and their long range escort fighters. The IR missiles being the primary and radar missiles for the all-weather-all-day interception.

 

And this is something that MiG-21 can do exceptionally well. Set up a missions where you are doing exactly this, and it will shine in the task. Far better than F-5E (bomb truck), Mirage 2000C (similar but more modern and somewhat moved work to the pilot from GCI) and so on.

 

Now think about being a long range strategical bomber, with a long range fighter escorts, and your 500-1500 km distance to the target, you get intercepted by MiG-21 groups of 4-6 between every 50-100 km. You really can't much start changing the course or engage them as you don't have fuel nor weapons or tactical positioning to defend against those.

[QuiGon]

I however cannot find any reference to a flameout due to insufficient air.

Hmm, that makes me wonder if the high speed flameouts of our DCS Mig-21 are even realistic? :huh:

[Sarpalaxan]

Also a small hint. You don't really want to look at the Mach indicator to warn you about impending Flameouts. The mig doesn't really care about tas. What you need to look at is the AIS Indicator on the upper left. Keep the IAS below 1200 and you are a happy camper. You can push it to 1300 if you feel lucky and if you know how to manipulate the nose cone manually to 1400.

[Hiromachi]

Hmm, that makes me wonder if the high speed flameouts of our DCS Mig-21 are even realistic? :huh:

 

I know of a few events where young pilots experienced flameout during high speed flight above 15.000 m, when trying to reach as high as they can. Incorrect throttle operation and young pilot at 18.000 m caused flameout, however he managed to stabilize flight at 10.000 m and restart the engine at 4.000 m, than successfully landing.

The above is a story mentioned by one of the pilots on polish forum back in the day. Aircraft involved was no other than this MiG-21bis:

[D4n]

I know of a few events where young pilots experienced flameout during high speed flight above 15.000 m, when trying to reach as high as they can. Incorrect throttle operation and young pilot at 18.000 m caused flameout, however he managed to stabilize flight at 10.000 m and restart the engine at 4.000 m, than successfully landing.

Wow interesting, thanks!

 

Btw does MiG-21(Bis) still provide hydraulic pressure (run the pump) from battery power on engine failure? Or did that polish pilot slowly lose hydraulic pressure until the engine reached it's operating RPM again after engine-restart?

[Drotik]

Hmm, that makes me wonder if the high speed flameouts of our DCS Mig-21 are even realistic?

 

An interesting subject. Until someone with actual experience on the MiG proves me wrong, I (without any real knowledge of the 21) would say no. With my understanding of jet engines an flow mechanics I can't find a real explanation why an engine designed for supersonic operation would quit simply because of a too high speed. In the event described above improper throttle operation is mentioned and that is a simple explanation. Those old engines with all hydromechanic control systems are easy to maloperate.

[Jonne]

From a certain altitude upwards it is actually forbidden to use anything but Full AB for that reason, I cannot remember the exact altitude though.

 

One thing I could imagine beeing a limit would be engine speed with increasing altitude and speed, but that would cause engine destruction in the worst case.

[corn322]

From a certain altitude upwards it is actually forbidden to use anything but Full AB for that reason, I cannot remember the exact altitude though.

Looking through the PDF "MiG-21 Pilots Flight Operating Instruction", point 23 on page 23 reads as follows:

 

23. Engine run in flight is allowed at all sustained and transient power settings at airspeed of not less than 400 km/h (on takeoff, during approach to land and in going around the airspeed is not limited), limitations are as follows:

 

(a) reheat shall be cancelled after takeoff when airspeed is not less than 600 km/h;

 

(b) selection of reheat is allowed:

(i) at any altitude, when it is selected from full throttle power setting, at airspeed not less than 500 km/h;

(ii) at altitude of or below 15,000 m, when engine is being accelerated, at airspeed of not less than 500 km/h;

(iii) at altitudes above 15,000 m, when engine is being accelerated, of airspeed not less than 600 km/h;

 

© it is allowed to accelerate engine to full throttle power and to throttle it down from reheat or full throttle setting to any required setting, at altitudes above 15,000 m, when airspeed is not less than 600 km/h;

 

(d) at altitudes above 18,000 m, engine run is allowed at reheat settings; and it is permissible to cancel reheat by moving throttle lever to FULL THROTTLE at airspeeds not less than 500 km/h.

 

 

Btw does MiG-21(Bis) still provide hydraulic pressure (run the pump) from battery power on engine failure?

From the Emergency Procedures part of the same PDF, under Approach and Landing with Dead (Windmilling) Engine, one paragraph reads:

 

- in gliding, avoid (if practicable) abrupt deflections of the control stick; in the case of insufficient windmilling RPM, hence an insufficient pressure in the hydraulic systems, disengage the aileron boosters;

 

Further reading through this section, it seems if the engine is windmilling, you will get some hydraulic pressure. But do keep an eye on the pressure. If it drops, abandon the aircraft. The electric booster pump seems to only be for the booster hydraulic circuit, which runs the stabilizer two-chamber booster and the aileron boosters?

[D4n]

Ok the flight operating instruction is good, but doesn't tell what might happen if not following those rules

[Jonne]

Well there are not many options of what can happen with wrong throttle operation:

 

-Flameout

-Surge

-Destruction

 

Manuals rarely state direct consequences of not adhering to limits, so it might be, that this information is not available and may not even have ever been tested.

 

The only high altitude engine problem I have found from pilot reports was a flameout during high altitude and speed intercept and was caused by the wake turbulence of the target. Other than that, the limiting factor for either altitude or speed maximum was always the remaining fuel.

[backspace340]

Wow, thanks! Any idea how pilot in DCS will notice the Mirage critical wing temperature? (What Mach, what altitude?)

 

You'll notice because the wing snaps - happens at Mach 2.3-2.35 or so.

[Frederf]

Wow interesting, thanks!

 

Btw does MiG-21(Bis) still provide hydraulic pressure (run the pump) from battery power on engine failure? Or did that polish pilot slowly lose hydraulic pressure until the engine reached it's operating RPM again after engine-restart?

Yes the NP-27T pump can power the booster system from battery power. Obviously this pump is not powerful enough to supply lots of hydraulic energy continuously and relies heavily on the the accumulators and light use.

[AeriaGloria]

If doing dead stick turn off aileron boosters to save from early hydraulic loss

[Argo Navis]

The maximum speed limit is due to aerodynamic reasons, and the engine has nothing to do with it besides burning the necessary fuel to go faster. The engine could accelerate the aircraft beyond mach 2 easily had it had more fuel. Above mach 2 however, the aircraft starts to suffer from stability problems that can lead to total loss of control mostly because of directional stability decreases. Loss of control above mach 2 could easily lead to in-flight breakup. For this very reason people didn't try to break personal speed records in this aircraft but stuck with the rules. Even on maintenance test flights, which required a test pilot, the max speed limit was M2.05.

 

Why the devs chose to cut the engine at these high speeds I am not sure. Maybe it's there to give you a second chance.

[Jonne]

Do you have any source for that? While this does apply to the MiG-23, I have never heard about that for the MiG-21. I am however open to learn something new.

[Argo Navis]

Hi Jonne,

It is in the russian Mig-21Bis manual. The manual does not go into detail about what happens if you overspeed or exceed limitations, and it is not supposed to anyway. However, there is a graph in the manual that tells a lot about directional stability. It also says that directional stability decreases but remains adequate up to Mach 1.9-2.0 with two rockets or clean, and up to Mach 1.4-1.5 with one central droptank. The emphasis is on the word adequate. The whole reason for this is that the shock cone interferes with the airflow around the vertical stabilizer as you accelerate towards Mach 2.

In real life, if the aircraft started to yaw and oscillate left and right lightly at these high speeds even before reaching mach 2, you had to abort the exercise and decrease speed. A friend of mine who flew this aircraft says he could only hit Mach 2 once or twice. Even though he attempted the max speed exercise countless times he had to stop before reaching mach 2 most of the time due to fuel constraints. A lot depended on atmospheric conditions like tropopause levels, temperatures etc, and it was easier to get to Mach 2 during the winter months. Here is the graph. Directional static stability coefficient against mach number: