Degraded Su-27 aerodynamic lift

Thanx for pointing out twice that I have only '2D knowledge'

Was I wrong based on what you described?

 

If your wonderful 3D wing stalls above 20deg AoA, why is it that light buffeting starts already at 8deg AoA and turns to medium buffeting at 12deg? Isn't buffeting related to flow separation? Still a long way to reach the stable max AoA of 38deg.

Buffeting on the F-15C's wings at 8 AoA? Here we're talking about the F-15C right, or are have you only been referring to the T-10 (Su-27 prototype) all along? Buffeting is a partial flow separation indeed. If for some reason (usually low Reynolds and very high to infinte AR) you have a buffeting at 8 AoA, there are just a couple more degrees of AoA until complete separation, 12..13 AoA or so, but for our subject which is the F-15's wing and it's critical AoA (which takes place at a quite high Reynolds), regardless of what separately happens over the fuselage, for the AR it has (about 3) a buffeting wouldn't probably (can't know for sure) occur before 14..16 AoA has been reached.

Btw, your description of how LE flaps add lift is wrong and you contradicted yourself when you tried to explain the difference between LE flaps and LE droops...

I haven't contradicted myself anywhere! Check here:

https://surjeetyadav.files.wordpress.com/2014/02/staj-7.png

http://cfile219.uf.daum.net/image/115856534DE23F29347E44

https://ultralightdesign.files.wordpress.com/2017/10/flaps-fig31.jpg?w=660

https://www.theairlinepilots.com/forumarchive/principlesofflight/flapcurve.jpg

The 4th link shows exactly what I am saying about leading edge flaps (which are not droops as you most probably believe) and for your knowledge they have a slot too, yet very small compared to slats, while the droops have no gap.

Now you provide the difference in effects for each of them if you have better info!

...

Btw, the later T-10 version did have leading edge flaps.

This confirms you confuse things and the T-10 never had leading edge devices afterall. That was already the Su-27 that had LE droops!

There are slotted slats (with gaps) and sealed slats without slots, both are noticable different in their basic function compared to LE flaps.

The slat by definition allows the airflow from one side of it to mix with the airflow of the other side of the edge it leads. It's incorrectly said: slats without gaps (or sealed slats as someone invented this term). Again, those that you refer to are: droops only.

Perhaps. But if the function of a slat is to allow air to flow from below the leading edge to the top of the wing to increase allowable AoA, no gap means it would function similarly to a Kruger flap, wouldn't it. Different name but similar effect.

Sorry, but their effects are quite different as their names too. You were perfectly right about their description, but their effects differ, one improving the AoA mostly, the other improving the zero AoA lift mostly.

https://www.theairlinepilots.com/forumarchive/principlesofflight/flapcurve.jpg

Regards;)!

The point is just that slats increase critical AoA, leading edge flaps increase lift. Just my interpretation.

You are among those few who understood them right.

Not quite. Since slats increase wing area and camber, they increase lift due to the increased camber and wing area.

 

If they are slotted the useable AoA is increased which creates additional lift due to the higher AoA.

 

LE flaps increase lift only due to the increased camber.

The man said it just right, sadly you are the one who entangles these devices definitions and functions!

Someone, somewhere, just used the wrong definition for droops by calling them leading edge flaps and now look what happened...! It's so hard now to clean this mess!

I like this thread, I'm learning quite a bit. So the Leading-edge flaps on the Su-27 are meant to generate lift and not to increase the critical AoA of the wing?

The other way around friend. Some folks here found some wrong stuff on the internet and it spreads real quick, lol!

That's what those leading edge devices on the Su-27 (or F-16 or F-18 ) called droops (not flaps or sealed slats or whatever one might re-invent now) are used for..., to increase the critical AoA, so you guessed it right.

It's pretty simple in the end how the masters who built airplanes named these devices based mostly on their general effect. Flaps -> for primarily increasing lift. Slats -> for primarily increasing the critical alpha (AoA). Droops -> for increasing AoA only and are used when selected by designers in order to reduce mechanical weight and to avoid various structural problems. So the droops aren't as good as slats for critical AoA increment, but are a better compromise for designing.

Regards!

Leading edge flaps do increase the camber because when they move, the leading edge actually droops, and with it the leading edge extremity of the chord line, while the mean camber line remains in place for most of the airfoil length:

The airfoil is dramatically bent down to the point the bottom side is now concave and the top side has a larger convexity.

Effect on the lift coefficient:

For the same angle of attack α, CL is decreased, but it is now possible to reach larger α and with them larger CL. It also means lower speeds are required to generate the same amount of lift.

LE flaps are usually found at the outboard portion of the wing.

Source:

LEF

According to this ^ table slats don't increase the critical AoA compared to LE flaps and LE flaps increase the critical AoA more than slots......I don't buy that.

You are right, the slot does help the lower pressure (usually upper surface) area's flow separate at a higher angle of attack than droops (again, not leading edge flaps) do, but in that illustration there were no numbers given, so you can only look at what the picture wants to say.

LE devices can increase lift due to increased chord, increased camber and due to boundary layer control. Out of these three only slotted slats influence the boundary layer and that's the only way to increase the critical AoA.

Again, the leading edge flaps (Kruegers only, cause there is no other type of leading edge flap) also slightly increase the critical AoA besides improving the zero AoA lift. The droops also increase the critical AoA (which is in fact their only purpose). You messed things up again!:(

While LE flaps are most effective with thin sharp nosed airfoils they generally aren't working nicely together with trailing edge flaps because they contribute to a thickened nose and main element wake.

This wake passes over the trailing edge flaps and reduces their effectiveness.

Then why are the A-380's droops used at landing if they do such trouble? The nose (leading edge) of the droops is quite thick, right?

Regards!

Maverick Su-35S, I think of you could reduce your argument down to a few concise points it’d be helpful. I’m having trouble following what topic of discussion is supposed to be here. This thread has turned into an argument for the sake of having an argument.

Sorry, you are right, but..., I sometimes don't know how to say it all in just a few words, so I spread into details very quick;(.

Regards!

The OP's argument that F-15 turns better than Su-27 in DCS is just plain wrong. I don't know where he has gotten this idea and anyone who knows how to BFM can see it themselves by tryin it out (assuming both have realistic combat fuel load that gives similar endurance). I haven't done any accurate measurement if the FM has changed very recently but I cant feel any major difference in it.

Well, then you talk without knowing what's going on in DCS atm. Test, then come back!

Su-27 is superior in a sustained turn contest by a small margin just as it is supposed to be.

Again, re-check and use numbers, not feelings or beliefs. Things have changed very much.

Eagle can counter Flankers STR advantage by not fighting a two circle fight but instead choosing one circle and making a maximum G turn from the merge and use the max G advantage to pull a tighter first turn and use the shotgun like gun to gun down the Flanker from head on. Eagle can make another pass like this and still be able to threaten the Flanker in the head on but after this F-15 has so little speed that it will be a sitting duck for the Flanker if it's still flying. F-15 can still disengage from this merge if the merge is tight though.

 

The Eagles one circle tactic is based on the fact that Eagle can pull more G's than Flanker as it should and it can reach max G faster. G advantage is available only if the fight is started from a reasonably high speed. When the fight gets slow Eagle will lose this ITR advantage as Flanker has better ITR at lift limit than Eagle, as it should be according to OP and as it is in DCS.

You are talking just from what you experience in DCS! In reality, the Flanker puts it's nose and keeps it (although it flies slower) on the Eagle right near the end of the first circle after a face to face merge. The real F-15 pilot won't pull 11..12G constantly (although he normally can even for at least 10 seconds) and will mostly stay at 9. Don't really know if the latest CAS (control augmentation system) on the F-15 still allows the pilot to pull as many Gs as he likes.

Regards!

The CAS has no limitations on the g. There is damping etc but no limitations on g or AOA.

...

 

And here's the absolute error of our ways, both of us:

These aircraft are rated to a specific g limit and the charts don't go much further that this.

You can over-g a light eagle to 12.5g and it won't break, but won't be flying that particular aicraft again.

You'll break a flanker doing this.

This is incorrect technique (as in not practically useful) for both aircraft, and by the same token, so is what I did:

 

So here's my technique for the flanker:

1) Set aircraft to 'immortal' or suffer the consequences, unless you manage to be extremely precise on your stick pull. I might be able to do it but I frankly can't be bothered to waste any more time on this.

2) S 'god mode' on. Trim as required, get comfy.

3) Actually accelerate to 450kts like you did with the eagle

4) Roll, pull, manage AoA. Notice peak TR exceed that of the eagle. If you fail to manage AoA, you will run yourself out of speed and result in a slower average TR.

You are right that I didn't test them at the same G-load, yet still, I've tried it at 9 (what the Flanker's flight controls limit it to) and guess what, the F-15C is still quicker than the Flanker.

Here it is:

F-15's 360 turning time still lower even for the same G.trk

Anyway, even if in DCS the Flanker would finish the quickies 360 turn in less time than the Eagle (but not even this is provable so far), the Eagle has a higher STR than the Flanker at any airspeed for similar fuel percentage, which is utterly wrong and this is what I'm trying to point out more than everything, even more than the fact that the Eagle's ITR is also higher. At speeds below 600km/h, the Flanker should have a better STR (constant airspeed and G-load turn), period!

The eagle doesn't. And FYI the AoA peaks for the eagle are consistent with RL reports.

 

Case closed.

Then something really is wrong with the Flanker's performance simulation, yet I believe it's more with the Eagle although both suffer. How can the F-15 with no lerx, no droops (not slats) have such a great roll control even at 25 to 30 AoA, while the Flanker's roll control (which should be enhanced in comparison to the Eagle) is badly degraded above 17..18 AoA for the same amount of beta (sideslip angle).

For me, it's case not closed until the truth wins!

Regards!

The CAS has no limitations on the g. There is damping etc but no limitations on g or AOA.

I see. So then, for a first turning shot, the Eagle driver would can that in his advantage! Nice things to learn.

...

I don't know what was the reason for this reply as I was saying the same thing. I admit I didn't also tell that the effective AoA decreases (defined by the affected chord line), thus lift coefficient decreases, so I only focused on it's primary role: increasing the critical AoA only (not moving the slope as flaps do) and logically, of course (maybe this I also forgot to say) the maximum lift by prolonging the lift slope.

But..., for what reason do you guys still like or want to call the droops as flaps? Because they act like plain flaps mounted on the leading edge? Seriously, it's not the first time when someone invents a new terminology which is not normally recognized and creates a lot of confusion and later contradictions. I know it's not you guys here who named the droops as leading edge flaps, but someone did that and it's wrong. There is only ONE leading edge device for which the name leading edge flaps came out... and those are the kruegers, nothing else.

LE flaps are usually found at the outboard portion of the wing.

It's not a general rule where they are placed along the wing. They may be placed anywhere along the wing.

In mobile so keeping it short.

the Eagle has a higher STR than the Flanker at any airspeed for similar fuel percentage,

This is correct.

which is utterly wrong and this is what I'm trying to point out more than everything,

And this is not, which is exactly why I keep saying that the only valid comparison is that of the aircrafts data to itself.

To make the point, you opinion on the relative performance here is irrelevant. The performance of the aircraft is what it is, period. You don't judge it by another aircraft's performance.

How can the F-15 with no lerx, no droops (not slats) have such a great roll control even at 25 to 30 AoA,

Because the Eagle has a very effective ARI, huge rudders and a lighter airframe.

Eagle AoA capability is available from NASA studies and was used to tune the FM.

while the Flanker's roll control (which should be enhanced in comparison to the Eagle) is badly degraded above 17..18 AoA for the same amount of beta (sideslip angle).

While the FLANKER has more inertia, may or may not have an effective ARI, longer wings and so on.

For me, it's case not closed until the truth wins!

 

Regards!

No, the case IS closed. While you may want to satisfy your curiosity as to how and why these things are, the performance simply is what it is. The Eagle matches its own data and so does the flanker.

If you want to prove something is wrong, price that the data doesn't match.

I'll reiterate again that opinions of who should be outturning who are invalid for pretty much everything FM related.

There are a lot of publications naming that Leading Edge Flaps but if the exact term is Leading Edge Droops or simply Droops is ok for me.

But there are multiple sources calling that Leading Edge Flaps. Books, internet pages and several publications. Is not me, is just hundreds of references around internet using that terminology.

Why this name is used so often is out of my knowledge but is a fact that LEF is used a lot.

The OP's argument that F-15 turns better than Su-27 in DCS is just plain wrong. I don't know where he has gotten this idea and anyone who knows how to BFM can see it themselves by tryin it out (assuming both have realistic combat fuel load that gives similar endurance). I haven't done any accurate measurement if the FM has changed very recently but I cant feel any major difference in it.

Well, then you talk without knowing what's going on in DCS atm. Test, then come back!

I did a very quick test with "competitive" BFM fuel setting (71% for F-15 and 55% for Su-27) which gives them same endurance on full AB at 5000m (things might have slightly changed since these settings were tested last time) and 2 heat seeking missiles. F-15 could sustain ~18deg/s at ~460KCAS and Su-27 could sustain same ~18deg/s but at ~325KCAS so Flanker could turn as fast as Eagle while doing it inside Eagle's turn circle. If that's not an turning advantage for Su-27 then i don't know what is.

Yes, I'm talking about the F-15C and that buffeting/flow separation starts at 8deg is a fact, straight from NASA/MCD data.

Then tell ED to implement it! In DCS the F-15 finds a constant CL increase up to 35 AoA. How is that compared to the very low critical AoA you suggest? Like I said, you only know 2D aerodynamics!

...your knowledge about aerodynamics and performance is surprisingly limited (despite your signature)and you are apparently trying to prove something by referring to tables and links you don't even understand, like e.g. the wrong statement that Krueger flaps are the only type of LE flaps and how the various LE devices actually work.

I guess you are the one who's limited in aerodynamics knowledge by what you wrote all along, and I'm not the only one contradicting you on things. I do have a background for more than 10 years in this domain and I didn't waste time on just one direction, you only have words so far and regarding the leading edge flaps, I have never head of (although it wouldn't matter much) anything else than kruegers. The droops are normally not regarded as leading edge flaps, but non-slotted slats if I may call them this way (because slats are of only one type = with gap).

This has become a jungle of endless quotes and replies from your side and most likely your next reply will consist of even more quotes and replies. Way too time consuming for me.

For me it's not time consuming as long as good things can also come up.

Regards!

This link contains one of the rather bad descriptions for LE flaps/droops and I agree with Mr.Kämpf who wrote: Your graph shows an exceptionally poor representation of a typical LE device.

If they would work as described they could really be used on the outboard section of the wing, but since they actually don't (noticable) increase the critical AoA they are usually used only on the inner section of the wing e.g. 747, A380.

The droops don't increase the critical AoA...! You are very none sense my friend! Sorry, but that graph is correct! The droops reduce your effective AoA (thus the null AoA lift drops or becomes negative) and increase the critical one. Exactly what you told me (regarding aerodynamics knowledge) matches you in fact...!

Then tell ED to implement it! In DCS the F-15 finds a constant CL increase up to 35 AoA. How is that compared to the very low critical AoA you suggest? Like I said, you only know 2D aerodynamics!

CL should increase up to 40deg AoA.

In mobile so keeping it short.

To make the point, you opinion on the relative performance here is irrelevant. The performance of the aircraft is what it is, period.

I only wish you were right, but I can't agree that the DCS F-15C respects the real plane's wing lift performance cause in the end this is what mostly affects a plane's turning ability (besides T to Drag ratio and wing loading).

You don't judge it by another aircraft's performance.

Why not? I'm forced to use other references such as the F-16C if I haven't seen a chart of the F-15's performance from ED which matches our plane in DCS and which totally differs from what I see on the internet (the charts that we can find).

The best F-16C (GE powered) can't do a 360 in less than 14 seconds, but our Eagle with no LE devices, no lerx is much better than the best, by doing it about +13 at 25% fuel. I trust the true F-15C's performance only so far, not what I see in DCS at the moment, which I find deliberately exaggerated and I believe that someone at ED wanted/liked it this way.

Because the Eagle has a very effective ARI, huge rudders and a lighter airframe.

I was talking about roll rate? There is a basic knowledge problem here if you say that weight affects roll rate. The Flanker doesn't have ARI? The Flanker has even bigger rudders if this is what you're looking at, but that's not what matters for a rolling acceleration and rate to gain a specific value. What matters is the ratio between the resultant aerodynamic moment to rolling inertia moment.

Eagle AoA capability is available from NASA studies and was used to tune the FM.

At the moment, I do not trust that the way it was tuned reflects the same values found by NASA as I do not believe that what I see in DCS reflects the real plane, sorry!

While the FLANKER has more inertia, may or may not have an effective ARI, longer wings and so on.

Again, I was talking about roll rates, not moments and yes, the wing span and the relative aileron area to that of the wing, the relative aileron span to that of the wing and it's position along the wing affect the resultant roll rate, but the general sense tells that if the F-15 has very much lower aileron span and area to wing ratios than that of the Su-27 and if the Su-27's flight controls and aerodynamics are correctly modeled in this area, the F-15's achievable rolling rate should by logic be lower for the same AoA not higher.

 

CL should increase up to 40deg AoA.

Ok, but is this extracted from DCS's FM or is a NASA result?

Regards!

Ok, but is this extracted from DCS's FM or is a NASA result?

 

Regards!

NASA’s test data