MIG-21Bis moments of inertia

[Maverick Su-35S]

Hello,

 

 

After doing a research on some flight dynamics constants of the MIG-21, this is what resulted:

 

For a given weight of 8700kgf, which corresponds to a clean loadout, full fuel + the pilot, the moments of inertia should be as follows:

 

Ixx (rolling moment of inertia) = 6147 kg*m^2 or 145880 lb*ft^2

Iyy (pitching moment of inertia) = 67510 kg*m^2 or 1602052 lb*ft^2

Izz (yawing moment of inertia) = 77091 kg*m^2 or 1829398 lb*ft^2

 

The following are the corresponding gyration radii of inertia:

 

Rolling radii of gyration: 0.8406m or 2.758ft

Pitching radii of gyration: 2.786m or 9.14ft

Yawing radii of gyration: 2.977m or 9.767ft

 

If one may introduce these constants in the flight model file of the MIG-21Bis in DCS will finally have this bird responding authentically.

 

Right now, the pitch and yaw moments of inertia seem appropriate (by how the plane responds to sharp full elevator and/or rudder deflections), but the rolling moment of inertia is still quite lower than expected. You can make the plane roll left and right so quickly at 200km/h as if it were at 300km/h...!

 

Another thing is that the ailerons are as efficient way above stall AoA as they are for low AoA, which is abnormal. Above stall AoA, the ailerons efficiency should become greatly degraded if not null, especially if the leading edge has no droops/slats to enhance the flow.

 

My only goal is to enhance or to help enhance the flight behavior of aircraft in DCS.

 

 

Best wishes, good day!

Edited October 24, 2017 by Maverick Su-35S

[Bogey Jammer]

Hello,

How have you got those figures ?

About the ailerons' efficiency, we need to get their aerodynamic characteristics first. If they are as effective at both low and high speed, checks are needed if they highly deflect at low speed but just need to deflect a little at higher speed for the same roll performance because of the airspeed influence (or overcome hydraulic pressure).

 

Post stall AoA behavior look strange though, but we are not sure it is not a real property of this kind of delta wing.

 

This topic is going to be very interesting :)

[Maverick Su-35S]

...

 

If they are as effective at both low and high speed, checks are needed if they highly deflect at low speed but just need to deflect a little at higher speed for the same roll performance because of the airspeed influence (or overcome hydraulic pressure).

 

Post stall AoA behavior look strange though, but we are not sure it is not a real property of this kind of delta wing.

 

This topic is going to be very interesting :)

 

Hello,

 

The moments of inertia about the 3 axis with their respective radii of gyration have been personally calculated after splitting the plane into the major components that take part in the plane's general weight by knowing their 3D positions and mass. I could not find the real value anywhere and this determined me to try and estimate them. I cannot guarantee how highly accurate these determined values are, but giving the fact that I've used the same model to determine those of an F-16C, for which I had comparison data and the mean error was around 2% (98% accurate), I am confident that in this case they cannot be lower than 94-95% of the real values.

 

About the ailerons, yes, as far as I know they are being driven by some clutch systems which are designed to maintain a relatively constant aerodynamic load on the ailerons. Because the aerodynamic loads vary with the square of the equivalent airspeed, the ailerons deflections will increase with a square function of decreasing airspeed or the deflections will decrease with a square root function of increasing airspeed. Yet I still believe the roll rate to be too high at low speeds between 100 to 400km/h.

 

As compared to an F-15 in DCS, the DCS MIG-21's roll rate at 200km/h with it's small ailerons is 43% higher. It takes 2.1 seconds for the MIG-21 to complete a 360 roll at constant roll rate at 200km/h, while for the F-15 which has elevons (that should further enhance the roll rate) it takes 3.7 seconds to complete a 360 roll at constant roll rate at 200km/h, which seems exaggerated for the MIG-21 to roll so fast. Only by putting into balance or comparing the ratio of aileron area to wing area of the MIG-21 to that of a Su-27 (I took the extreme examples as a better proof) combined with the ratio between the MIG-21's full ailerons deflection and the Su-27's flaperons + elevons deflections, one can notice that something doesn't add up for making the MIG-21 reach such roll rates. Although the ratio between the aerodynamic rolling moment of the MIG-21 and it's rolling moment of inertia may give a rolling acceleration higher than that of an F-15 and Su-27, this ratio (roll angular acceleration) still seems quite too high. Even when flying at 80km/h (using a ballistic trajectory) and jerking the stick full from side to side, the rolling response looks obviously short, the plane would act as if it's very light.

 

As far as I remember, I had given a link of a real MIG-21 which performed a 360 roll after takeoff at around 450-500km/h. That video could've been used as an inverse engineering to determine the rolling moment of inertia and roll rate with quite good accuracy. If we don't have any other reliable data, the videos, if used with precise measurements can give very accurate true data, but of course, this depends on whether we want to make it better or not.

 

Here is the track regarding the MIG-21's roll rate vs airspeed and rolling inertia:

 

Roll rate and rolling inertia problems.trk

 

About the highly swept deltas (the mig-21 has 49 degrees at the 25% chord spanwise line), I am very sure that the flow separations close to the ailerons gradually reduce the ailerons lift vs deflection slope as the wing's AoA gets closer and closer to critical and render them completely useless (lift vs deflection becomes almost null) by the time the wing reaches critical alpha. Tip stalls which usually engulf the ailerons as well, occur a lot more prematurely with higher sweep angles. For the MIG-21, the ailerons stall/ineffectiveness is a true fact. This is the main reason why the pilots are told to use rudder instead of ailerons for controlling the roll as they approach the critical alpha. Not only that the ailerons lift effectiveness should decay with increasing alpha (both positive and negative) and become null once the whole wing is stalled, producing differential drag only, but as it is right now, the ailerons produce about the same differential lift between the wings when fully deflected at angles of attack as high as 50 as they would at produce at low AoA (0..10 degrees), which is abnormal to say the least.

 

Here is the track to review the abnormal aileron lift effects at angles of attack more than double that of stall:

 

High AoA ailerons response is same as for low AoA.trk

(Now, I wish you luck with the tracks, because they don't seem to follow exactly what I did so the plane may crash randomly at each retry to replay, although I didn't crash and showed everything that's not alright)

 

Again, take into comparison the DCS F-15's roll response when the wing as at or near critical AoA. What do you get? The wing (which is a delta with an even lower sweep than that of the MIG-21) is still capable of slightly increasing the lift coefficient between 20 (30 on the indexer) to 25 AoA (35 on the indexer) while the ailerons effectiveness has already become very low by the time 20 AoA was reached. The same goes for the Su-27. You can't say that ED had simulated them wrong or copy pasted one's values to another. By tests, they match each real individual plane remarkably.

 

Another discussion which has the highest priority of all is that of the true critical AoA of the MIG-21, which is in no case 15 degrees. Even high sweep and relatively high aspect ratio wings (for fighters) like those of the F-86, MIG-15, etc. have a critical AoA of +18 AoA. The delta which always has offered a benefit in critical AoA increment should in fact give the MIG-21 around 20-21 degrees critical AoA.

 

Another discussion would be that of the MIG-21's wing lift at zero AoA and also lift slope (CL vs AoA).

 

I am personally involved in enhancing/correcting the flight behavior of various aircraft in another simulator (not giving name) and I feel an urge to see owned aircraft in DCS respect real data, and if real data is not found/available, use all the engineering methods that you have to determine the data or at least get closer and closer to it by whatever means..., of course, if our goal is to have that aircraft behave as close as possible to the real thing.

 

My wish is that at least one after another, as the time permits, all these aspects will be revisioned.

 

Kind regards!

Edited November 1, 2017 by Maverick Su-35S

[Bogey Jammer]

Your tracks are fine, and clearly makes the aileron behavior at low speed hilarious, as if the MiG was made out of cardboard. That moment when it was perfectly perpendicular to the trajectory but still able to roll like it was straight at 300km/h was epic :megalol:

 

Hopefully such situations in combat often result to a defeat.

However I feel it's risky to compare the MiG-21 wing with so different in nature ones of F-15 and Su-27 because unlike the MiG-21, the lifting surface is the whole aircraft, not the wings only. They still can fly without a wing but the limited lifting surface of the MiG-21 is so precious that using ailerons at critical AoA has probably a much more sensitive impact on the total loss of lift of the wing (but that only would proves that the sim is wrong) rather than on roll effectiveness. The mass and architectures are also too different.

I'd better suggest to correlate the results with the Mirage 2000, notorious for its extremely high roll rate and its true delta wing.

Like you said, critical AoA of delta wings are higher because, if I correctly understood the principle, they generate out of their high swept leading edges some vortexes helping the airflow to not separate as easily as standard wings. So the behavior of control surface might be more effective at low-speed and high AoA. How better ? I don't know.

[Maverick Su-35S]

...

 

I feel it's risky to compare the MiG-21 wing with so different in nature ones of F-15 and Su-27 because unlike the MiG-21, the lifting surface is the whole aircraft, not the wings only.

 

Please let me know how would you find these differences affecting what I'm trying to say. Let's openly discuss about it:thumbup:!

 

They still can fly without a wing...

 

Sorry to contradict you here, but this can only be physically possible (for a Su-27, F-15,etc.) at an AoA quite low or at a G-load far below 1 while holding full aileron and rudder just to neutralize the rolling tendency, so all the flight becomes nothing but a long plunge towards earth. Don't listen too easily on what's being said of that Israeli F-15 flying back home with a whole wing gone. If this was true, then it can only be physically possible if the lift produced by the wings is far below the one produced by the fuselage, which is absurd. Most definitely that wing was only about half gone, not completely gone as people lie about just to make it look like a wonderful story or to make the F-15 look like some magical craft defying physics. I trust only physics and validated numbers as they won't lie to me, not fructified stories from Youtube.

 

...but the limited lifting surface of the MiG-21 is so precious that using ailerons at critical AoA has probably a much more sensitive impact on the total loss of lift of the wing (but that only would proves that the sim is wrong) rather than on roll effectiveness. The mass and architectures are also too different.

 

Because the MIG-21's wings lift is so precious might drive you to believe that the ailerons have a huge impact on the lift differential on the wings, when in fact they don't. Maybe on drag, yes, they have a huge impact generating the known yawing moment due to roll inputs which looks correctly simulated, but not on lift. If the aileron would have an effect on the wing at stall AoA that would would take place only on the aileron's region, not affecting the whole wing, but only a small area (less than a quarter of the aileron's span/lenght) slightly inboard of the ailerons. These are facts, not aberrations. Not what you're saying is aberration, but what's being simulated at this moment.

 

I'd better suggest to correlate the results with the Mirage 2000, notorious for its extremely high roll rate and its true delta wing.

 

Extremely high roll rate? Compared to DCS MIG-21, it's twice as worse under same conditions of AoA and IAS, not to mention how badly the Mirage's roll rate decays when approaching critical AoA and keep in mind the huge difference between the Mirage's aileron area to total area ratio and comparable deflection and the aileron area to total area of the MIG-21 and both aircraft comparable rolling moment arm (arm between the aileron's center of pressure and plane's rolling axis), plus the fact that the Mirage has a some sort of ARI (aileron-rudder interconnection) system which deflects the rudder towards aileron input. All these factors combined affect the resultant roll rate. It should be clear that there's something still in need of work on the MIG-21 and not on the Mirage-2000C which behaves remarkably accurate. The JAS-37 also flies remarkably realistic and does not develop the strange aerodynamic forces present on the MIG-21. So far, the 21's aerodynamics still look like a WIP.

 

Initially, when the MIG-21 came in DCS, it was far more realistically behaving than it does now. Some aspects have been fixed, but a lot more got worse somehow. If tweaks made it worse, then tweaks can also save it and make it better.

 

 

Like you said, critical AoA of delta wings are higher because, if I correctly understood the principle, they generate out of their high swept leading edges some vortexes helping the airflow to not separate as easily as standard wings

 

Correct, but it's much more important which areas get wetted (as our discussion seems to lead to) by these vortexes and their intensity for a particular AoA or somewhat to say, the intensity of the vortex versus AoA slope, which drastically affect the vortex breakdown AoA, known as supercritical AoA. If such a powerful vortex can "wash" the wing as far outboard as where the ailerons are, then the ailerons effectiveness decay would be delayed and take place at a higher AoA, closer (yet below) to the vortex breakdown AoA. After passing the supercritical AoA, any aileron movement should find almost (yet not null) no roll response or as life tests show, the ailerons lift vs deflection slope becomes about 10..15% of what it was below stall. But there is NO such thing as high energy vortex developed on a MIG-21's wing and furthermore not possible for the tiny vortices developed along the wing's leading edge that cannot wash out the nasty boundary layer separations and flow reversals that take place very close to the ailerons.

 

Here, this might be very useful and you may find out that there isn't a very big difference between the general wing shape (delta or swept wing) on how it affects the general airflow and flow separation / flow reversal patterns. When a wing becomes more swept, these effects will be less and less different for different wing types. The flow separation effects that take place closer to the wingtips are more affected by wing sweep angle rather than planform shape:

 

"The videos are a valuable tool of information"

 

...So the behavior of control surface might be more effective at low-speed and high AoA. How better ? I don't know.

 

If they were to be more effective due to vortexes (which you presume might reach the ailerons and beyond) then how come we don't see at least the same amount of aileron effectiveness at AoAs above 20 (look how low it is...) on aircraft such as Su-27 which has a far more effective vortex spreading across it's wing as compared to the MIG-21's wing. It's all about the contradictions that emerge from this. Even the Mirage-2000's wing (which you thought to be rather comparable) with slats (which drastically improve the airflow on the ailerons) still has a roll response very comparable to that of an F-15 (slatless wing as the MIG-21) of Su-27. Same goes with the JAS-37. Do you want to say that somehow, irrationally, the MIG-21's ailerons are washed by a tremendously well stabilized and firmly attached vortex which keeps the aileron's lift versus deflection unaffected by alpha? There's definitely something out of the ordinary going on right now and because this didn't happen in previous patches, it's certain that this can be fixed.

 

 

Kind regards,

Mav.