F-15C poor performance under 350kts

[Bushmanni]

During design leading edge slats were considered and tested on a F-4 testbed but ultimately found unnecessary for F-15. Because of the tests the F-4 got the slats though.

[TAW_Blaze]

Incorrect once again, there's a reason pretty much every fighter since features such devices, incl. the F-16, F-18, F-22, Su-27, Rafale, Gripen, Eurofighter, PAK FA etc..

 

In short all the evidence is against you.

 

Not really, no.

 

There's a rule of thumb between pilots that "rate kills". Now the best turnrate tends to be around high speeds and the lower you go the far worse your sustained turnrate gets. There are tricks to beat someone with higher turnrate but in case of a fair fight between equal opponents it simply won't work.

 

I don't think I have to explain flaps are useless at high speed if not straight blocked out by the FCS, and slats are a marginal benefit when using excessive aoa. They are just a band aid to help the pilot in the worst case scenario.

[USARStarkey]

I dont know if the F-15 is spot on or not at low speeds, but I dont understand how its current performance could be called poor. Quite frankly, I feel like its more agile that it was. Instantaneous turn rate seems much higher at all speeds, and low speed turn rate is still pretty good, you just dont have a perfect Fly by wire system anymore that prevents your from exceeding your limits. Maybe its more agile, I dont know, but Its certainly not bad right now, the AFM in my opinion is a all round improvement over the old plane. For example, the new F-15 is about 17 knots faster at SL.

[Ktulu2]

Low, the PFM is much better, but high it can hardly turn without going into a serie of rolls... I've seen multiples opinions on this so in the end I'm not too sure if its normal (I understand limit AoA should be lower, but not that few G's turns should result in complete loss of control).

[Python]

Do you have external tanks on when this roll takes place? Sometimes when I go into a fight it feels like I'm in an ice cream van... then I remember to jettison the tanks and all is good again.

[Ktulu2]

All clear but amraams, usually full fuel internal.

[Exorcet]

Not even close. Even without LE devices the Flanker's blended wing lifting body design produces a lot more lift than the Eagle's conventional winged fuselage design, there's absolutely no doubt about it. With without LE devices the Flanker would outmaneuver the Eagle at low speeds without trouble.

The gap between the two should not be huge. The Eagle doesn't just have body lift, it has the same vortex lift that the Su-27 has. The fairings on the sides of the intake will have the same effect, and the Eagle's swept wing will probably produce some vortex lift as well, possibly more than the Flanker's wings (though the wings on the Su-27 have the higher lift slope and less drag). The Flanker is also the heavier plane by a considerable amount, and that's going to eat into some of its performance.

 

 

 

Incorrect once again, there's a reason pretty much every fighter since features such devices, incl. the F-16, F-18, F-22, Su-27, Rafale, Gripen, Eurofighter, PAK FA etc..

 

In short all the evidence is against you.

LERX's have been around since the F-5, the the F-15 didn't miss the boat.

[USARStarkey]

Low, the PFM is much better, but high it can hardly turn without going into a serie of rolls... I've seen multiples opinions on this so in the end I'm not too sure if its normal (I understand limit AoA should be lower, but not that few G's turns should result in complete loss of control).

 

Do you mean High alt or high speed? Because High speed turns feel tons better to me in the PFM. The F-15's inst turn is crazy. One of my favorite tactics is always to be over 400knots and when I merge with a Flanker I whip that thing around faster than he can for that first turn, which usually gives me a missile shot. If you meant High alt, it doesnt seem much different from the SFM, although I feel like it stalls a bit less.

[Ktulu2]

I meant high, but i guess I just try to pull as much as when I'm low...

[GGTharos]

I meant high, but i guess I just try to pull as much as when I'm low...

 

Known issue.

[Hummingbird]

The gap between the two should not be huge. The Eagle doesn't just have body lift, it has the same vortex lift that the Su-27 has. The fairings on the sides of the intake will have the same effect, and the Eagle's swept wing will probably produce some vortex lift as well, possibly more than the Flanker's wings (though the wings on the Su-27 have the higher lift slope and less drag). The Flanker is also the heavier plane by a considerable amount, and that's going to eat into some of its performance.

 

The gap is infact huge, 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:

 

 

 

LERX's have been around since the F-5, the the F-15 didn't miss the boat.

 

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

Edited September 28, 2014 by Hummingbird

[USARStarkey]

said gap is in point of fact, not huge.

 

 

ED SFM: quite comparable:

 

[Hummingbird]

said gap is in point of fact, not huge.

 

 

ED SFM: quite comparable:

 

 

Based on Russian estimations from the 70's regarding an aircraft they do not possess?

[USARStarkey]

Ill take that over your vague guesswork about wing design.

[GGTharos]

The graph isn't correct for STR, but the difference isn't huge. The instantaneous turn difference on the other hand is pretty big, and it would make a huge difference in a fight. Until you start fighting at a high altitude.

[USARStarkey]

Inst turn is better for the Eagle is it not?

 

when I made this statement I was referring to high speed turns, which both graphs show the eagle being better or equal at.

Edited September 28, 2014 by USARStarkey

[Hummingbird]

Ill take that over your vague guesswork about wing design.

 

Not vague at all, based on provable facts, something you can even go observe at an airshow if you're ever in doubt.

 

There's a reason the Su-27 delights in being thrown around the sky at low speeds in airshows, whilst the F-15 most certainly does not.

 

But hey, if you want to base your opinion on a 40 year old document speculating about enemy aircraft performance based on nothing but fears & heresay, be my guest!

[Hummingbird]

Inst turn is better for the Eagle is it not?

 

Nope, other way round. Plane with the most amount of lift pr. weight features the higher instantanous turn rate, and that would be the Su-27 by a big margin.

 

During sustained turns the power to weight ratio starts to influence things.

[USARStarkey]

Not vague at all, based on provable facts, something you can even go observe at an airshow if you're ever in doubt.

 

There's a reason the Su-27 delights in being thrown around the sky at low speeds in airshows, whilst the F-15 most certainly does not.

 

But hey, if you want to base your opinion on a 40 year old document speculating about enemy aircraft performance based on nothing but fears & heresay, be my guest!

 

You did zero math. You made vague comments about the aerodynamic design of a plane and then made conclusions about it. I am hardly saying anyone's estimates are perfect, or even correct, but its quite a bit more scientific than your rudimentary analysis of these two aircraft's design.

[Hummingbird]

You did zero math. You made vague comments about the aerodynamic design of a plane and then made conclusions about it. I am hardly saying anyone's estimates are perfect, or even correct, but its quite a bit more scientific than your rudimentary analysis of these two aircraft's design.

 

Alright, let me show you how big a difference the simple addition of LE flaps makes on the lift produced:

 

That's a ~50% increase in lift pr. wing area that the Su-27 enjoys over the F-15.

 

Now add the extra lift the Flanker gains from more comprehensive LERX and the airfoil shaped fuselage and you should get the idea.

[USARStarkey]

Alright, let me show you how big a difference the simple addition of LE flaps makes on the lift produced:

 

That's a ~50% increase in lift pr. wing area.

 

Now add the extra lift the Flanker gains from more comprehensive LERX and the airfoil shaped fuselage and you should get the idea.

 

Lol really? What, you post more vague data regarding the general characteristics of aero design features and were supposed to take that as the holy grail on Su-27 vs Eagle comparison...much less assume a "huge" difference. Thats like posting a chart on how slots work and then telling me that the 109 can supposedly out turn a spitfire.

[Hummingbird]

Lol really? What, you post more vague data regarding the general characteristics of aero design features and were supposed to take that as the holy grail on Su-27 vs Eagle comparison...much less assume a "huge" difference. Thats like posting a chart on how slots work and then telling me that the 109 can supposedly out turn a spitfire.

 

Yes really, and that's a bad comparison on your part.

 

First of all we're talking jets here, so no added lift from air accelerated over the inboard part of the wing by the propwash. Secondly the Su-27 features full span LE devices, thus we know the lift increase is the same across the span.

 

Because of this it can be rather safely assumed that the Su-27 enjoys a ~50% advantage in lift from its available wing area due to the addition of LE flaps, as anything else would be rather mysterious. The larger LERXs and blended wing design only adds to this this advantage, and the result is rather clear when you observe both birds doing aerobatics at airshows, the Su-27 being capable of significantly tighter maneuvers, esp. at slow speeds.

 

By comparison, everything else being equal, the addition of outboard mounted LE slats, like on the 109, will only yield an increase in overall lift of approx. 12-15%. I'm pretty sure the Spitfire enjoyed more than a 15% advantage in available wing area.

Edited September 28, 2014 by Hummingbird

[Hummingbird]

There's a reason every modern fighter designed these days features LE devices: The benefits are huge and the cons nonexistant.

[USARStarkey]

Yes really, and that's a bad comparison on your part.

 

First of all we're talking jets here, so no added lift from air accelerated over the inboard part of the wing by the propwash. Secondly the Su-27 features full span LE devices, thus we know the lift increase is the same across the span.

 

Because of this it can be rather safely assumed that the Su-27 enjoys a ~50% advantage in lift from its available wing area due to the addition of LE flaps, as anything else would be rather mysterious. The larger LERXs and blended wing design only adds to this this advantage, and the result is rather clear when you observe both birds doing aerobatics at airshows, the Su-27 being capable of significantly tighter maneuvers, esp. at slow speeds.

 

By comparison, everything else being equal, the addition of outboard mounted LE slats, like on the 109, will only yield an increase in overall lift of approx. 12-15%.

 

So you completely missing the point, which was you cant just spit out general qualities and the make specific performance predictions. the percentages between the comparisons are irrelevant. For one, nobody is arguing that the flanker doesnt turn better, just that the difference in sustained turns is not "huge." Since were making general statements, the F-15 is significantly lighter and has a better T/W, both of which would play a role in overall turn performance. This issue is too complicated for your to make such broad statements based on nothing more that "hey look guys! LERX and blended wings! It clear the flanker has a huge (whatever that is) advantage!"

[USARStarkey]

There's a reason every modern fighter designed these days features LE devices: The benefits are huge and the cons nonexistant.

 

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