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Degraded Su-27 aerodynamic lift


Maverick Su-35S

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

Both increase aoa and lift ☺ slaps are just more eficient than kruger (or sealed flap) but need more space in wing


Edited by =THRUST=
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That's wrong (and it wouldn't make sense to call slats LE flaps and vice versa). Different design, different movement and a different way to provide additional lift.

 

Quick search came up with this:

 

http://www.atpforum.eu/forum/technical-subjects/-081-principles-of-flight-a/14192-leading-edge-flaps

 

The only reason why I joined this thread is that Maverick Su-35S mentioned that the 'slats' of the Su-27 increase the critical AoA by 6-7deg and that's exactly what LE flaps don't do.

 

Not the Su-27, but here is a description from the Luftwaffe MiG-29 flight manual concerning the function of LEF:

 

"With the flaps in the position UP, the LEF operate automatically as a function of AOA and airspeed. When the AOA increases to 8.7 degrees or above and the airspeed is below M0.8, the LEF extend automatically. The LEF retract when the AOA decreases to 7 degrees or airspeed increases above M0.8. The exact Mach number is dependant on the switching point of the Mach sensor installed."

 

Sounds to me that "increasing critical AOA" is exactly what they do?

JJ

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picture-41.png

 

:D

 

Maybe, but still, some basic terminology rules have to be respected when talking about ANY tech subject, aviation related or not, otherwise more confusion follows because of people describing & understanding different things - might as well use different language altogether.

 

We've got guys mislabelling LE flaps as slats, and if that wasn't eyebrow rising enough, there's a joker from the North Pole, who doesn't seem to have idea what a Krueger flap is, or at the very least didn't look at the pic a couple of posts above.

 

It's not going to help in discussion of wing design features of two planes, is it?

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:D

 

Maybe, but still, some basic terminology rules have to be respected when talking about ANY tech subject, aviation related or not, otherwise more confusion follows because of people describing & understanding different things - might as well use different language altogether.

 

We've got guys mislabelling LE flaps as slats, and if that wasn't eyebrow rising enough, there's a joker from the North Pole, who doesn't seem to have idea what a Krueger flap is, or at the very least didn't look at the pic a couple of posts above.

 

It's not going to help in discussion of wing design features of two planes, is it?

 

It’s not just us “guys” mislabeling. We’ve got the DCS Su-27 manual mislabeling LE flaps as slats too apparently.

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Leading+Edge+Devices+and+Boundary+Layer+Control.jpg

 

This ^

No slot, not really a slat

 

Both from your illustration, and because t's a pre-existing word taken into aviation terminology, and slats have gaps between them.

 

slat

(slæt  )

 

Word forms: plural slats 

 

countable noun [usually plural]

 

Slats are narrow pieces of wood, metal, or plastic, usually with spaces between them, that are part of things such as Venetian blinds or cupboard doors.

 

 

picture-41.png

 

Come on - we've all been late to dinner over that one...

Cheers.

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Leading edge devices exist to keep airflow better attached to the wing at high angles of attack. This has many benefits when operating at high AoA:

1) Reduced buffeting for a given AoA

2) More lift for a given AoA

3) A higher AoA limit.

4) Reduced drag for a given lift coefficient since the air flow is smoother and a lower AoA can achieve the same desired lift.

5) Improved stability.

Of course, one can argue about the magnitude of these improvements versus the cost in weight and complexity, but seeing how most modern fighters have some form of leading edge device it must be somewhat cost-effective.

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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.

? Nowhere in my image does it state that. I think you are focused too much on the comparing graphs directly to eachother.

The graphs show the movement of the curve depending on the type. And slat or slotted Krueger increase AoA and CL (hint: check vectors) over the LE flap and unslotted Krueger.

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Btw, I'm presently not aware of an airplane which uses slotted Krueger flaps. In fact I didn't even know that these kind of Krueger flaps exist.

Top of my head.. 747 uses a variant from such family. Boeing calls them Variable Camber Krueger. I know also that Airbus has few patents on slotted versions.

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Agreed and in this case it doesn't make sense that a LE flap can cause such a signficant increase in critical AoA.

 

Btw, I'm presently not aware of an airplane which uses slotted Krueger flaps. In fact I didn't even know that these kind of Krueger flaps exist.

 

Here’s an interesting snippet (I can’t find the source now :doh:) regarding the effects of LE flaps on pitch authority vs AoA:

goa4Irj.jpg

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picture-41.png

 

So you say we shouldn't care if things are wrong in this simulator (being an internet game).?!

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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Leading Edge Flaps(LEFs) is the same thing as "slats" and yes the Su-27 has them :)

 

Wrong! The leading edge flaps are one type of LE devices. The slats are another type of LE devices and finally droops are another type of LE devices. In terms of aerodynamics only, disregarding their structural disadvantages / advantages, they are all quite different in performance output.

 

The LE flaps only slightly increase maximum CL, with little effect on critical AoA (only move the lift slope upwards due to higher CL0 with small AoA increment). The droops (non-slotted LE devices) increase the critical AoA not affecting the lift slope (as a result the maximum CL also increases). The slats (slotted LE devices) re-energize the boundary layer by rapidly increasing the airflow on the appropriate region (usually the upper surface of an affected airfoil) and provide the highest amount of critical AoA and maximum CL increment.

 

The presence of LERX induced vortexes increase the critical AoA, thus the maximum CL, with low effect on lift slope.

 

It doesn't have separate trailing edge flaps and ailerons - the functions of those are combined in single devices known as "flaperons".

 

Who talked about flaperons?

 

Regards!


Edited by Maverick Su-35S

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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...The only problem that can exist is an eagle turning faster than an eagle.

 

Which happens only in DCS and is not only in my mind. As you provided the Su-27's chart which already contains the Eagle and I doubt that the F-15C's performance in that chart are wrong, can you provide a more real chart for the F-15 then? We'd appreciate it.

 

I hate to break it to you, but the F-15 FM is very well validated. And yes, it was designed to be an excellent dogfighter.

 

Well validated? Beating even the better powered F-16 in constant turn rate at any airspeed? And about being an excellent dogfighter, sorry, but that can't be more wrong! If you already know to build a plane which excels in aerodynamic lifting performance (F-5 which has LERX and droops or F-14 and F4 which have slats), who would you make a clean wing and with low AR (draggy in turns as compared to the provided lift).

 

Again, even an F-16C powered with the worst engine variant (F100-PW-220) always outturns the Eagle (F-15A and C) in constant turning at all speed ranges, but in DCS the F-15 turns better even than the best F-16. I trust real data only and not what I see now in DCS flight models. Sorry, but I kind of start to lose my trust in how DCS planes performances are being dictated.

 

It's not a problem for me if a plane outperforms another in reality in any way, it's a problem for me because I only wanted to fly a simulator which respects real life data, but what I find is way off...!

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When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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Quite a bit of essential info missing in your comparison.

 

1. What are the airfoil differences between the F-15 and the Su-27?

2. What's the difference in washout?

3. The F-15 doesn't have slats but the Su-27 doesn't have slats either.

4. Do you mean 20° AoA? I don't know any airfoil on which flow stays attached at such a high AoA.

 

For point 3, was too quick on that word "slats"! Yes, droops, not slats, but even so the critical AoA increment advantage should be there and in reality it is great as compared to F-15C.

 

1. How much of a difference in output performance do you believe would occur if one would have a symmetrical airfoil while the other has the most highly cambered airfoil ever used for high lift? You confuse what happens in 2D with what happens in 3D. In 2D, yes, the differences are high in CL0 (zero AoA lift coefficient), CL vs AoA (lift slope) and critical AoA (which decreases for the more cambered foil), but in 3D (and I understand you don't know) the differences exponentially decrease (can't tell an exponent but it's at least 2) as the AR (aspect ratio) decreases. So to answer your question, taking into consideration what I've just said and the fact that the airfoil differences for these aircraft aren't great (when the Flanker's droops are retracted), there can't be any advantage for the F-15 in this area, but rather a disadvantage when the Flanker's droops fully extend (which increase the camber at a position very close to the leading edge).

 

2. What does a washout help you with if you mentioned? In this domain, the F-15 has a disadvantage rather than an advantage.

 

4. Same as for point 1, you only seem to have 2D airflow knowledge. For infinite span or AR, the critical AoA won't usually go beyond 13..15 AoA (depends on camber). For 3D (limited span and AR), the critical AoA exponentially increases (can't tell an exponent, but the function is non-linear for sure) as the AR decreases.

 

From my knowledge, the F-15's wings only (not vortex washed fuselage) stall above 20. This is the truth and it was perfectly simulated when the F-15C just came out as PFM. So above 20 AoA, you can't have any more aileron effectiveness on a real F-15 and have to use beta angles (obtained by rudder) in order to command a roll and the lift coefficient normally remains flat (lift vs AoA slope becomes null) up to a couple of degrees of AoA more and then find a shallow decrease (negative lift to AoA slope) up to around 30+ AoA. Above 30+ AoA the small energy vortex created between the Eagle's engine inlets starts to break away and by the time the AoA reaches 35, the lift slope starts dropping fast.

 

Kind regards!


Edited by Maverick Su-35S

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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I must suck at flying this eagle, I can't get it to turn around in under 14 sec.

 

PS: Actually I can make it happen in about 13 in the eagle with an insane 500kt start and a near 12g turn.

 

I can get the flanker started at 850kph and do it in about 12 sec with similar insanity. So, does the flanker turn better? Yep!

 

It's all good if your pilot survives. Generally speaking you'll run yourself out of AoA must sooner in the eagle than you will in the flanker so, while the flanker's still turning, you'll be stuck trying to get your nose around 90 deg before him.

 

That's if you have g simulation on and you try to make a sensible ITR turn.

 

1% fuel, too.

 

 

Sorry, but one of us isn't true here, and I'm not the one. I've just provided a track, did you check it? How can it be that in my test, the Eagle did a 360 in 12.00 seconds at 1% fuel starting from 200meters MSL and 450 KIAS and you can't get it below 14, while for the same conditions but starting the fastest turn from 750km/h IAS, the Su-27 can't do it in less than 13.8?

 

Again, I use the other reference, the F-16C. How can our DCS F-15C have a better STR than the F-16C? Somewhere, there's a nonsense, period!

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When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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Why would you use that scenario for comparison? the Eagle with 1% fuel isn't realistic. It would burn this fuel in less than 5 seconds. Eagle can't be flown with 1% of fuel in real life, if you think DCS FM is wrong, and you like to compare it with real life, you should at least use a realistic loadout.

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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.

 

Su-27 is superior in a sustained turn contest by a small margin just as it is supposed to be. The real advantage in this kind of fight that Su-27 has is that it can pull a long energy losing turn to get behind the F-15 and still has enough energy to maneuver for the gun shot and force the Eagle to jink and lose the energy advantage it had. F-15 can't do this because it will bleed so much energy tryin to do the same that it cant turn well anymore to stay away from Flankers nose and it also cant jink the Flankers gun when the Flanker gets behind the Eagle.

 

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.

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Wrong! The leading edge flaps are one type of LE devices. The slats are another type of LE devices and finally droops are another type of LE devices. In terms of aerodynamics only, disregarding their structural disadvantages / advantages, they are all quite different in performance.

 

AFAIK "slats" are also known as "leading edge slotted flaps", but I could be wrong :) .

 

Anyway, all I meant was really just the presence of LE devices without specifying the exact type, but I realise that this was what bbrz was talking about and that my use of the term "slats" was incorrect in regards to the LE devices on the Su-27.

 

The LE flaps only slightly increase maximum CL, with little effect on critical AoA (only move the lift slope upwards due to higher CL0). The droops (non-slotted LE devices) increase the critical AoA not affecting the lift slope (as a result the maximum CL also increases). The slats (slotted LE devices) re-energize the boundary layer by rapidly increasing the airflow on the appropriate region (usually the upper surface of an affected airfoil) and provide the highest amount of critical AoA and maximum CL increment.

 

The presence of LERX induced vortexes increase the critical AoA, thus the maximum CL, with little effect on lift slope.

 

Thanks for the explanation :) . I am not very familiar with the terminology of aerodynamics and didn't know the specific meaning of "critical AOA".

 

Who talked about flaperons?

 

I did :D - just a side remark in regards to the wing design of the Su-27 in contrast to that of the Su-33 :)

JJ

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You're right, it's 12 seconds flat, so I had to check what's going on.

 

I'm going to point one thing out here right from the start: You bring up F-anything or Su-anything other than the aircraft type you're actually looking at, I end the convo with you, personally. I couldn't care LESS about what other aircraft do. In case it hasn't sank in yet, there's only one valid comparison: The aircraft to itself.

 

So let's get back to basics: You turned the eagle in 12 sec flat. I turned the flanker in 10 sec flat.

 

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.

 

Now I'll stress that these extremes are a complete non factor in the game because you cannot sustain these g's.

 

But what, then, of the flanker's flight control systems? Perhaps there we have an issue, perhaps not. The flanker limits you in AoA unless you use direct control, which in and of itself is quite dangerous.

 

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

 

Case closed.

 

Sorry, but one of us isn't true here, and I'm not the one. I've just provided a track, did you check it? How can it be that in my test, the Eagle did a 360 in 12.00 seconds at 1% fuel starting from 200meters MSL and 450 KIAS and you can't get it below 14, while for the same conditions but starting the fastest turn from 750km/h IAS, the Su-27 can't do it in less than 13.8?

 

Again, I use the other reference, the F-16C. How can our DCS F-15C have a better STR than the F-16C? Somewhere, there's a nonsense, period!

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Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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