Issues with the JAS 39 Gripen flight model

[Mytho-GR1]

@FeetPics

The Gripen in-game already matches Eurofighter performance numbers from what I have seen

It doesn’t its below them

[amApollo-psn]

I think the best way to go about this thread is to avoid/ignore non-productive comments and stay focused on the people actually providing meaningful input on the Gripen FM - goes both ways

[Mytho-GR1]

Fair enough i shall wait for what @Giovanex05 has to say as i find it interesting

[SlowHandClap]

@Giovanex05

Doing a sustained turn test at 700kph and 50% fuel we find that currently the sustained turn rate is ~22.3 degrees per seconds at that speed, at least 1.3 deg/sec higher than what it should be, but this is far from what makes the gripen such a good fighter, especially it does not explain why it loses much less energy than any other aircraft when tightening down a turn.

Your test was at 700 kph, which is the bottom of that range.

[FeetPics]

@Giovanex05

The reason why I idid all this crap math up there is that lifting line theory does a good job at approximating the polar diagram with steady and non compressible flow, but at higher AoAs the flow is more and more unsteady and at very high AoAs upper flow even starts detaching from the wing (which causes the lift coefficient to start actually decreasing all the way to 0 while the drag coefficient increases (that’s why aircraft will stall at high AoAs and also why Cobras aren’t useful in 99% of the situations).

I have friend who made flight model analysis tool. I have the polar plots and the CL/CD plot for the Gripen if you want it for your bug report.

[MiG_23M]

@CorporalApollo

I’m sure I read of several people who tested the sustained after the nerf and claimed it performed to the ∼20-21° gaijin claimed, didn’t you also do this @MiG_23M ?

I tested prior to the drag change for sure, however I have been absent and haven’t made any reports in some time.

@SlowHandClap

Your test was at 700 kph, which is the bottom of that range.

So, 2.3 deg/s higher than expected?

[Giovanex05]

@CorporalApollo

Anyways @Giovanex05, you seem like you know what you’re talking about, this is a complex matter still and admittedly I’ll have to take a look at your math again when it’s not 1am and I’m half-awake before I can make any informed comments on it, but regarding this:

Before we go further, i like to point that while I do have education on how to math like this (currently studying mechanical engineering), i’m not studying aerospace/aerodynamics so there may be stuff that I am missing on where and how certain formulas to approximate stuff can be applied.

@CorporalApollo

Isn’t this exactly what the delta-canard excels at? Making the flow more steady at very high AoA, and delaying the detaching from the wing, increasing lift? I think this is relevant if we’re using non canard-delta (different aerodynamic configurations) as reference for calculations? What’s your take on the whole canard-thing? Thank you :)

All moving certainly do have a benefit at lower speeds together with the unstable configuration, but they can’t outright prevent unsteady flow and flow separation, so the polar of a delta canard fighter, while better in the low speed department than a ”conventional” configuration, is it impossible for it to be literally equal to what is basically an ideal perfectly steady flown that never detaches.

The reference of the calculation is not a delta wing, but an ideal elliptical wing as it present a perfectly even lift distribution (which again is the best case possible as with the flown assumptions before), and then e_0 simply scales that efficiency back to another wing (that’s why the 700kph test calibration was important, there’s no other way to find the current e_0 the gripen has in game).

In shorter terms, the calculation I did should proove that the gripen in game follows that approximated curve that assumes what is basically an ideal flown. Now we need to see how much is it overperforming, as the real gripen curve will certainly be below current levels, but will most likely be above the ones of the MiG-29 or the F-16.

[Giovanex05]

@SlowHandClap

Your test was at 700 kph, which is the bottom of that range.

Since that STR value is higher than what was reported by devs it is still too high. The reason i picked 700kph is that this was (and is) not the absolute best rate speed (wanted to have some margin) and there’s no risk to get close to transonic regime where i have no idea if Lifting line theory works.
Thinking back thought i could have done 750-800kph and be fine… current function I did in topic predicts 760-790kph as peak rate speed, it would be cool to find out if it is true.

@FeetPics

I have friend who made flight model analysis tool. I have the polar plots and the CL/CD plot for the Gripen if you want it for your bug report.

That would be very welcome indeed, if his Cl/Cd plot matches mine we would have a really solid base. Obviously need to know also how he did the testing (or datamine if it extrapolated them from there)

[FeetPics]

@Giovanex05

That would be very welcome indeed, if his Cl/Cd plot matches mine we would have a really solid base. Obviously need to know also how he did the testing (or datamine if it extrapolated them from there)

I will DM them to you.

[SlowHandClap]

@MiG_23M

So, 2.3 deg/s higher than expected?

The image used in your bug report shows that 20-21 isn’t the peak though afaik, and we don’t know if that was at 700 or 900kph.

[Metrallaroja]

@Giovanex05

• Around 20-21 deg/sec sustained turn at: sea level, speed: 700-900kph and 50% fuel
• Engine thrust curve should be somewhat accurate

Doing a sustained turn test at 700kph and 50% fuel we find that currently the sustained turn rate is ~22.3 degrees per seconds at that speed, at least 1.3 deg/sec higher than what it should be, but this is far from what makes the gripen such a good fighter, especially it does not explain why it loses much less energy than any other aircraft when tightening down a turn.

Thanks, I asked several times for this to be tested and none did but kept arguing.
Can you also do a vid at 900kph or 800kph?

[Malekitth]

R-73 still gets flared in rear aspect >%50 of the time <1KM VS afterburning gripen

[Metrallaroja]

Also, please dont fake flag posts just because you dont agree with them.
You are just wasting time of the forum moderators and making the forum a worse place to discuss.

[SlowHandClap]

@Metrallaroja

Thanks, I asked several times for this to be tested and none did but kept arguing.

You’re more than welcome to test yourself.

@Malekitth

R-73 still gets flared in rear aspect >%50 of the time <1KM VS afterburning gripen

I’m not having any problems with R-73s. Any reason you’re launching from within 1km? Depending on your airspeed that’s potentially inside the minimum engagement range.

edit: unhiding my posts after mig23m flag abuse.

[Necronomica]

@Giovanex05

If this calculation was done to approximate what the real life gripen would look like I think we would not be able to get further than ~5 degrees AoA analytically (basically untill canards start moving extensively) as to simulate actual canard and wing interaction i think you need to do a CFD or a straight up wind tunnel.
(Idk thought, i am studying mechanical engineering so maybe some way exists that allows to approximate that analytically for steady flow)

https://sci-hub.ru/https://arc.aiaa.org/doi/abs/10.2514/6.1991-3195
here is some information from the pre production trials if you are interested :)

and here is some general comparisons to generic delta canard configurations and their relative performances :)

ADA063819.pdf

[Necronomica]

i think you are talking about different things here, you are saying lift, he is saying lift-coefficient.

you are both correct in your statements just talking about different things and interpreting each other to mean the wrong thing.

[Necronomica]

@FeetPics

We also know that the plane is unstable and has a canard. There is nothing in my research that suggest that the canard is inherently more efficient than the conventional layout of the F-16; in both planes they basically function as additional wing area to an extent. In fact I think the strength of the canard layout is that unstable designs are able to fly at higher AoA limitations without risking deep stalls…i.e falling out of the sky.

if you are interested here is some general comparisons to generic delta canard configurations and their relative performances compared to wing sweep (mirage and gripen have different wing sweep) as well as comparison within the same wing sweep but with and without canards (F-16 and gripen has about the same sweep but gripen has canards)
:)

ADA063819.pdf

a canard has the possibility to gain up to 30% more efficiency in lift compared to the same added wing area but to main wing instead of canards (page 6-7 of this document). i’m not saying the JAS39 has maxed this efficiency but still. add to this that the canards are fully movable and thus able to optimize efficiency for any given AoA.

the fact that gripen also weighs about 2/3 that of the F-16 also helps, as it needs less lift converted into turn force to achieve a specific radius compared to F-16 that needs more lift converted into turn force for the same radius.

if you dumb it down into a basic circular motion (not really realistic i know but it shows my thoughtprocess):

the centriputalforce needed for a specific speed at a specific radius is:
a=(V^2)/r
and F=ma gives a=F/m
so
F/m=(V^2)/r gives F=(m(V^2))/r
which means that the force needed at a specific velocity and radius to maintain a constant circle depends on mass. since the mass of Gripen is about 2/3 then generally (and WAY simplified) the force needed to make that specific turn will be 2/3 of what the F-16 needs.
which would also mean that Gripen needs less AoA (and thus less induced drag) to make the same radius turn at the same speed as the F-16.

Now this is as previously stated WAY to simplified and there are LOADS more factors to consider. so its probably not even close to 2/3 the turn-force.
but it still shows something i don’t think i’ve seen others talk about before.

That being said:
Gripen in game is still very much overperforming, but i don’t think it is as egregiously overperforming as people think. like if it is now a 8/10 and people claim it should be a 4/10 i personally think it might be closer to a 5 to 6 /10. (numbers taken out my **** just to show what i ment).

Otherwise a very well written post! GJ! :)

[amApollo-psn]

Good stuff. Nice to get a better understanding of the actual relation of weight, aerodynamics/drag and thrust to performance, it’s not all linear and 1:1 as some people make it out to be.

The Gripen is widely known for its ability to “cut through the air like butter” and its advanced aerodynamics. A Gripen pilot also told me regaining / energy retention is where its advantage lies over the F16, not a proper source I could use in a report but still interesting context.

I firmly believe there’s truth to this. It’s a very complex subject, but I don’t think it’s fair to ignore the delta-canard configuration if bug report is going to be based off comparisons and referencing conventional designs. Especially when it seems that the issue in question is very much something the delta-canard would inherently have an advantage in… Do we have any info on the Rafale perhaps to use? @Giovanex05

[Necronomica]

asligtly off topic but still gripen.
recently released video of an intervew with a Gripen pilot WHILE IN THE AIR ON A SIMULATED COMBAT MNISSION!!!

Gripen in NATO: Sweden's Anti-Russia Fighter

[amApollo-psn]

@CorporalApollo

Really cool video

Oh yeah, great video :) learnt some new stuff in it