F-15C poor performance under 350kts

[Hummingbird]

Every modern design? So the F-15 isnt modern?

 

I said every modern fighter designed these days, the F-15 was designed in the early 70's. Pretty much every fighter design since then, incl. the Su27, features LE devices, and for a good reason.

[USARStarkey]

I said every modern fighter designed these days, the F-15 was designed in the early 70's. Pretty much every fighter design since then, incl. the Su27, features LE devices, and for a good reason.

 

Ok, "these" days then. Still doensnt change the ludicrous nature of specific performance comparisons based on general aero design characteristics.

[EAKMotorsports]

I have the TM Warthog what recommendation curve will be for flying thx.

[JetBLASTER]

I have the TM Warthog what recommendation curve will be for flying thx.

Ae EAK eu tenho o TM Warthog e nao uso curva .Trabalha muito ben .Valeu

Foi so de sacanagen.

I have the Warthog and I don't use any curve .Work very well with F15 and A10C.All the best.

Edited September 28, 2014 by JetBLASTER

[Exorcet]

The gap is infact huge

I guess it would depend on what we're talking about. Further reading your posts it sounds like you're referring to raw lift/CL and fair to say that the Flanker is better off there. That doesn't necessarily translate to significantly better sustained turning, which is what I was referring to more. I'd say that the Flanker should be better in this area at low speed anyway, but not to the point that an Eagle would be hopeless fighting in that regime.

 

, and mostly due to the blended wing design of the Su-27, which is obvious even at first glance.

 

The Su-27 is basically one giant wing with two engines strapped underneath:

The F-15 isn't too far off. The biggest difference would be that the F-15 lacks the center tunnel of the Su-27. That's definitely an advantage for the Flanker, but the Su-27 is still heavier. Also, The bigger LERX might translate to more drag at certain angles of attack. When it comes to nose pointing I think the Flanker will win though.

 

 

 

Forget about LERX, something the F-15 has exceptionally little of anyways.

All you need is enough to keep air flowing over the wings and top of the fuselage. Doing that with the smallest vortex might even be advantageous.

 

The critical thing that the F-15 lacks are wing mounted LE devices, either in the form of slats or LE flaps. The other fighters I mentioned all feature this, including the Su-27 which has much more comprehensive LERX as well, not to mention much higher body lift.

Alright, I misunderstood what you were talking about.

[Hummingbird]

Ok, "these" days then. Still doensnt change the ludicrous nature of specific performance comparisons based on general aero design characteristics.

 

No, what's ludicrous is to deny the obvious advantages that devices such as LEF's provide.

[Hummingbird]

I guess it would depend on what we're talking about. Further reading your posts it sounds like you're referring to raw lift/CL and fair to say that the Flanker is better off there. That doesn't necessarily translate to significantly better sustained turning, which is what I was referring to more. I'd say that the Flanker should be better in this area at low speed anyway, but not to the point that an Eagle would be hopeless fighting in that regime.

 

In a sustained turn fight the T/W ratio plays a role about as important as the L/W ratio, thus far we agree. However the Su-27 enjoys such a large advantage in L/W ratio (thanks to its LE devices and blended wing design) that the small advantage the F-15 enjoys in T/W ratio simply can't make up for it. It's not even close, which I have so say is quite evident when you watch the two aircraft being put through their paces at airshows. The Su-27 clearly enjoys a great advantage in available lift in comparison to its weight.

 

 

The F-15 isn't too far off. The biggest difference would be that the F-15 lacks the center tunnel of the Su-27. That's definitely an advantage for the Flanker, but the Su-27 is still heavier.

 

I'd say the F-15 is indeed quite far off, which is obvious when you add up the various features that it lacks in comparison to the Su-27, and most of these things really are evident at first glance: such as the F-15 being of the conventional winged fuselage design where'as the Su-27 is a blended wing design, which in itself is a big advantage fpr the Su27 - at least when it comes to lift.

 

That having been said there's no doubt that the F-15's boxy fuselage produces lift, it does, however to nowhere near the extent as that of the Su-27's blended fuselage which was designed specifically with body lift in mind. Then when you add in the LEFs & LERXs the Su27 really starts to pull away.

 

Also, The bigger LERX might translate to more drag at certain angles of attack. When it comes to nose pointing I think the Flanker will win though.

 

The bigger LERX should actually translate to more lift at high AoA's, the extra drag being a byproduct of this, however at the same time the L/D ratio increases, which is the whole logic behind applying LERX. In short it improves both instantanous and sustained turn performance at the cost of a slightly larger wetted area.

 

All you need is enough to keep air flowing over the wings and top of the fuselage. Doing that with the smallest vortex might even be advantageous.

 

To be honest I'm not even convinced that the F-15 features LERX, at least not in the proper way as we see it on the Su-27, F-16 & 18, all of which employ large LERX starting far forward and benefit greatly from it.

 

Alright, I misunderstood what you were talking about.

 

Yes and these LE devices really are quite crucial as they're a big reason as to how aircraft with a high wing loading, such as the F-16 & F-18, still manage to be highly nimble in the air, even at lower speeds.

Edited September 30, 2014 by Hummingbird

[Exorcet]

In a sustained turn fight the T/W ratio plays a role about as important as the L/W ratio, thus far we agree. However the Su-27 enjoys such a large advantage in L/W ratio (thanks to its LE devices and blended wing design) that the small advantage the F-15 enjoys in T/W ratio simply can't make up for it. It's not even close, which I have so say is quite evident when you watch the two aircraft being put through their paces at airshows. The Su-27 clearly enjoys a great advantage in available lift in comparison to its weight.

The T/W ratio means nothing without drag. At high AoA, induced drag is huge and the Flanker could very well suffer to the point of inferiority because of it. The Eagle will stall first though, so go low enough in speed and the Flanker wins by default. Do you have airshow videos in particular that you think show the performance disparity? I don't really like using them and wouldn't consider them terribly reliable as far as ultimate performance goes.

 

 

 

 

I'd say the F-15 is indeed quite far off, which is obvious when you add up the various features that it lacks in comparison to the Su-27, and most of these things really are evident at first glance: such as the F-15 being of the conventional winged fuselage design where'as the Su-27 is a blended wing design, which in itself is a big advantage fpr the Su27 - at least when it comes to lift.

I wouldn't even call the F-15 conventional in design, but this could just be semantics. The F-15 is worlds away from something like the F-86. That is conventional. The entire F-15 upper surface, except the nose is wing. The lower surface suffers in comparison to the Flanker because there is less in the way of spanwise flow control to my eyes. The air probably rolls off the corners of the intakes lowering pressure on the underside. The F-15 is a giant wing as much as the Flanker, but it's producing less CL.

 

That having been said there's no doubt that the F-15's boxy fuselage produces lift, it does, however to nowhere near the extent as that of the Su-27's blended fuselage which was designed specifically with body lift in mind. Then when you add in the LEFs & LERXs the Su27 really starts to pull away.

As far as I'm aware the F-15's fuselage was designed specifically with body lift in mind. It looks different because it's optimized for higher speed where you don't need extremely high values for CL and don't need to go to extremes in turns of AoA. The Flanker is the opposite. This makes it a clear choice for instantaneous maneuvers where raw lift/weight is all that matters, but it's less clear for sustained maneuvers because drag enters the picture. The LERX's in particular could be huge sources of drag. LEF's are good all around though and the Flanker's wing should be better at low speed.

 

 

 

The bigger LERX should actually translate to more lift at high AoA's

Yes, this is why I think the Flanker wins noise pointing.

 

the extra drag being a byproduct of this, however at the same time the L/D ratio increases, which is the whole logic behind applying LERX. In short it improves both instantanous and sustained turn performance at the cost of a slightly larger wetted area.

Compared to stalling, the L/D is better, but bigger vortex doesn't translate to better performance. If you can keep air flowing over the wings with a smaller vortex you're generating just as much momentum exchange without viscous losses. If that wasn't the case, every plane would probably use a delta wing with 70 degree+ sweep.

 

 

To be honest I'm not even convinced that the F-15 features LERX, at least not in the proper way as we see it on the Su-27, F-16 & 18, all of which employ large LERX starting far forward and benefit greatly from it.

I don't have specific information on the F-15, but the inlet fairings should act just like the LERX's on other aircraft. You'll get lateral flow separation as the air tries to go around the decreasing radius of the fairing and the top of the inlet acting like a LEF probably helps in flow separation.

 

EDIT

 

Here's an image showing a vortex down the length of the entire fuselage of a F-15E

 

Edited October 2, 2014 by Exorcet

[USARStarkey]

No, what's ludicrous is to deny the obvious advantages that devices such as LEF's provide.

 

Nobody has denied that LERX provide an advantage. You seem to think however that you can declare precise relative performance claims from aerodynamic generalizations. Like I said before, nobody is declaring the Eagle a better low speed turner, but declaring the difference is huge is just stupid. First of all what the heck does "huge" mean? Second, once defined, you arent using any math or performance tests to prove your point, your just screaming: hey look guys ---> "TECHNOLOGY" and expecting anyone to take that seriously.

[Hummingbird]

The T/W ratio means nothing without drag. At high AoA, induced drag is huge and the Flanker could very well suffer to the point of inferiority because of it. The Eagle will stall first though, so go low enough in speed and the Flanker wins by default. Do you have airshow videos in particular that you think show the performance disparity? I don't really like using them and wouldn't consider them terribly reliable as far as ultimate performance goes.

 

Induced drag is a direct byproduct of lift, thus how much induced drag is generated is entirely dependant on how much lift is being generated, and the Su-27 doesn't need to pull CL_max to outmaneuver the F-15.

 

 

I wouldn't even call the F-15 conventional in design, but this could just be semantics. The F-15 is worlds away from something like the F-86. That is conventional. The entire F-15 upper surface, except the nose is wing. The lower surface suffers in comparison to the Flanker because there is less in the way of spanwise flow control to my eyes. The air probably rolls off the corners of the intakes lowering pressure on the underside. The F-15 is a giant wing as much as the Flanker, but it's producing less CL.

 

Obviously the F-15 is of a conventional design by 1970+ standards, not 1940's or 1950's standards.

 

As far as I'm aware the F-15's fuselage was designed specifically with body lift in mind. It looks different because it's optimized for higher speed where you don't need extremely high values for CL and don't need to go to extremes in turns of AoA. The Flanker is the opposite. This makes it a clear choice for instantaneous maneuvers where raw lift/weight is all that matters, but it's less clear for sustained maneuvers because drag enters the picture. The LERX's in particular could be huge sources of drag. LEF's are good all around though and the Flanker's wing should be better at low speed.

 

Thing is LEF's are usable at all speeds and it's one of the reasons the F-16 waxes the F-15 in WVR combat despite a noticable difference in wing loading. The F-18 with it's more straight wings does even better in turns.

 

 

Compared to stalling, the L/D is better, but bigger vortex doesn't translate to better performance. If you can keep air flowing over the wings with a smaller vortex you're generating just as much momentum exchange without viscous losses. If that wasn't the case, every plane would probably use a delta wing with 70 degree+ sweep.

 

It's the speed/strength as well as the position of the vortice that matters, and here the Su-27, F-18 & F-16 all enjoy an advantage over the F-15.

 

I don't have specific information on the F-15, but the inlet fairings should act just like the LERX's on other aircraft. You'll get lateral flow separation as the air tries to go around the decreasing radius of the fairing and the top of the inlet acting like a LEF probably helps in flow separation.

 

EDIT

 

Here's an image showing a vortex down the length of the entire fuselage of a F-15E

 

 

Not really convinced that this vortice is the result of the wing fairings, but more importantly it's not directed over the wings which is where it is needed the most. Additionally it is quite small and not very visible despite the obvious high speed (and high humidity) at which the maneuver is being carried out judging by the large amount of condensation being generated over the wing. (The image was taken in Scotland IIRC)

 

A large/powerful vortice generates a large amount of visible condensation even at rather low speeds where none is being generated by the wings, something which you can observe on both the Su-27, F-16 & F-18 when they are performing maneuvers at lower speeds.

 

 

 

 

 

Edited October 5, 2014 by Hummingbird

[Exorcet]

Induced drag is a direct byproduct of lift, thus how much induced drag is generated is entirely dependant on how much lift is being generated, and the Su-27 doesn't need to pull CL_max to outmaneuver the F-15.

CL isn't all the matters, the wing plan form does too. The higher the AR, the lower the induced drag. A wing with infinite AR has zero induced drag ever at infinite CL. LERX's are going to produce a lot more drag for a given amount of lift, so the more you depend on them, rather than the wings, the higher your drag will probably be. The Flanker doesn't need to maximize CL to keep up with the F-15, but it does need to produce more lift. The F-15 needs (28000 lbs + fuel + weapons)*g of lift to turn. The Su-27 needs (36000 lbs + fuel + weapons)*g of lift to turn. What that means for drag is harder to say. At low AoA the Flanker is probably more efficient. At high AoA, the LERX's will allow more maximum lift, but it will probably be draggier as well.

 

 

 

Thing is LEF's are usable at all speeds

Yes, agreed.

 

and it's one of the reasons the F-16 waxes the F-15 in WVR combat despite a noticable difference in wing loading. The F-18 with it's more straight wings does even better in turns.

If I recall, the F/A-18 suffers in sustained turns despite the wings due to weight (drag) and lack of thrust. Mostly because it's a Navy plane though.

 

 

 

 

It's the speed/strength as well as the position of the vortice that matters, and here the Su-27, F-18 & F-16 all enjoy an advantage over the F-15.

This is my fault because of wording. I was taking strength into account. The strength of the vortex is going to create additional drag along with the additional lift. It's never going to be as efficient as generating lift with more laminar flow. The flatter/sharper LERX's of those planes will produce stronger vortices, I agree. Those vortices will also produce more drag though.

 

 

Not really convinced that this vortice is the result of the wing fairings, but more importantly it's not directed over the wings which is where it is needed the most. Additionally it is quite small and not very visible despite the obvious high speed (and high humidity) at which the maneuver is being carried out judging by the large amount of condensation being generated over the wing. (The image was taken in Scotland IIRC)

It might not be a result of the wing fairings, but I think more important is that it is attached to the fuselage. It will tend to draw surrounding air along with it, meaning the upper fuselage surface will act like a wing surface.

 

It is small, and I think that is an advantage for the F-15 as long as there is no major flow separation over the fuselage. Doing that with the smallest vortex possible should lead to the least amount of drag being produced.

 

A large/powerful vortice generates a large amount of visible condensation even at rather low speeds where none is being generated by the wings, something which you can observe on both the Su-27, F-16 & F-18 when they are performing maneuvers at lower speeds.

Yes and those vortices are very similar to the tip vortices coming off the wings. They're a source of drag, they're less efficient at making lift than the wings, and I don't think you're maximize sustained turning by relying on the LERX's to generate the bulk of your lift. For sustained turns, you just want to use them to keep air flowing over your more efficient lifting surfaces.

[Hummingbird]

CL isn't all the matters, the wing plan form does too. The higher the AR, the lower the induced drag. A wing with infinite AR has zero induced drag ever at infinite CL. LERX's are going to produce a lot more drag for a given amount of lift, so the more you depend on them, rather than the wings, the higher your drag will probably be.

 

The Su-27, F-16 & F-18 don't rely on the area of the LERX to generate lift, rather on the extra lift that the wings will generate because of the vortex stemming from the LERX.

 

In short, yes LERX does add drag, but the increase in lift they provide for over the wings more than offsets this.

 

The Flanker doesn't need to maximize CL to keep up with the F-15, but it does need to produce more lift. The F-15 needs (28000 lbs + fuel + weapons)*g of lift to turn. The Su-27 needs (36000 lbs + fuel + weapons)*g of lift to turn.

 

That is obvious, however the more lift pr. weight you have available the less energy you're also going to need to complete a similar turn, which is what I'm trying to get at :)

 

What that means for drag is harder to say. At low AoA the Flanker is probably more efficient. At high AoA, the LERX's will allow more maximum lift, but it will probably be draggier as well.

 

Of course if the Flanker pulls max AoA it will, along with generating more lift, also generate more drag and hence slow down faster, as it should, but it will change direction (swing its nose around) a lot quicker as well :)

 

With that in mind the Flanker doesn't need to pull near its limit to stay with the F-15, if it just follows in the same circle then it will be flying at a lower AoA, suffer less drag and thus loose energy slower and eventually catch up in the circle.

 

If I recall, the F/A-18 suffers in sustained turns despite the wings due to weight (drag) and lack of thrust. Mostly because it's a Navy plane though.

 

Agreed, the F-18 does suffer in sustained turns due to these reasons.

 

A NAVY pilot's opinion on the F-16 vs F-18:

http://defence.pk/threads/f-16-vs-f-18-a-navy-test-pilots-perspective.169261/

 

This is my fault because of wording. I was taking strength into account. The strength of the vortex is going to create additional drag along with the additional lift. It's never going to be as efficient as generating lift with more laminar flow. The flatter/sharper LERX's of those planes will produce stronger vortices, I agree. Those vortices will also produce more drag though.

 

Absolutely.

 

 

Yes and those vortices are very similar to the tip vortices coming off the wings. They're a source of drag, they're less efficient at making lift than the wings, and I don't think you're maximize sustained turning by relying on the LERX's to generate the bulk of your lift. For sustained turns, you just want to use them to keep air flowing over your more efficient lifting surfaces.

 

Very important: None of the aircraft with LERX rely on it for anything near the bulk of their lift, they (LERX) are there simply to increase the lift generated by the main wing area itself, thus the added drag that the LERX constitutes is very low - esp. in view of the increase in overall lift they provide for, which is the logic behind their application.