Degraded Su-27 aerodynamic lift - Page 12 - ED Forums
 


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Old 02-21-2018, 08:45 AM   #111
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Old 02-21-2018, 11:07 AM   #112
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Originally Posted by bbrz View Post
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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Old 02-21-2018, 01:05 PM   #113
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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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Old 02-21-2018, 06:39 PM   #114
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Originally Posted by bbrz View Post
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 ) regarding the effects of LE flaps on pitch authority vs AoA:
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Old 02-23-2018, 01:45 PM   #115
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Originally Posted by Esac_mirmidon View Post
So you say we shouldn't care if things are wrong in this simulator (being an internet game).?!
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Old 02-23-2018, 01:56 PM   #116
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C'mon.

Its only a little of humor.

Why so serious?
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Old 02-23-2018, 01:56 PM   #117
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Originally Posted by Alfa View Post
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.

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Originally Posted by Alfa View Post
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!
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Last edited by Maverick Su-35S; 02-24-2018 at 10:05 PM.
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Old 02-23-2018, 02:14 PM   #118
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Originally Posted by GGTharos View Post

...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.

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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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Old 02-23-2018, 02:34 PM   #119
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Originally Posted by bbrz View Post
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!
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Last edited by Maverick Su-35S; 02-24-2018 at 09:54 PM.
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Old 02-23-2018, 02:38 PM   #120
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Originally Posted by GGTharos View Post
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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