Aerodynamics thread

[thunderchief]

That argument doesn't work, since wing loading is a bad metric for gauging a modern fighter aircraft's performance. Indeed, J-20 is designed to be better than J-10, not only in speed but in maneuverability as well. This is evident from the degree to which vortex lift is used. You have the right to be absolutely positive about your assumption, but that can't make reality agree with said assumption.

Au contraire, you made assumption about J-20 being superior in any possible way without any evidence. My assumption is made based on information we have .

Vortex lift is additive. That is, if a leading edge extension and a canard can each enhance lift by the same amount, then combining them together would result in twice as much additional lift.

What you failed to understand is that both of these work only at high AoA and they use kinetic energy of aircraft to increase lift , therefore increasing drag .

The diagram shows canard generates additional lift when the aircraft's angle-of-attack is close to zero. The reason is that canard still generates vortex in that situation, thereby contributing to lift.

Nope. Diagram CL(alpha) shows almost no increase when alpha (AoA) is low.

You are contradicting yourself. The reason this happens is because you are making up theories as you go along. The diagram shows canard can generate more lift with less drag, simply because the C[sub]L[/sub] versus C[sub]D[/sub] curve for canard is situated higher within the graph.

We don't have CD(alpha) diagram , but you could extrapolate it form CL(alpha) and CL(CD) diagram . It will show that at same AoA , configuration without canards has less drag and less lift.

That's the purpose of a wing too, and it does not support the argument that canard contributes to more drag.

Smaller wings means less drag and less lift. For example, rockets don't need lift (they have T/W greater then 1) so they don't need wings.

Vortex energy is only loss when the energy is not recaptured. In the case of vortex lift by canard, the energy is recaptured to enhance lift. Even with an efficiency of 80%, a canard is still better since the efficiency would be worse without the canard. This is evident in the graph.

There is always loss when you convert energy. More vortices means more loses , pure and simple. Rule of thumb, more maneuvering aircraft does , more energy it loses .

 

[Engineer]

Au contraire, you made assumption about J-20 being superior in any possible way without any evidence. My assumption is made based on information we have .

Your assumption is not based on any fact, since it is an assumption. It also defies common sense since PLAAF would have no need for the J20 in the first place if J10 is better.

What you failed to understand is that both of these work only at high AoA and they use kinetic energy of aircraft to increase lift , therefore increasing drag .

Nope. The diagram clearly shows canard producing lift at low AOA as well.

Nope. Diagram CL(alpha) shows almost no increase when alpha (AoA) is low.

Nope. The diagram clearly shows that C[sub]L[/sub] versus alpha curve is higher with canard than without.

We don't have CD(alpha) diagram , but you could extrapolate it form CL(alpha) and CL(CD) diagram . It will show that at same AoA , configuration without canards has less drag and less lift.

Nope. C[sub]L[/sub] versus C[sub]D[/sub] curve is higher with canard than without. That says drag is lower with canard.

Smaller wings means less drag and less lift. For example, rockets don't need lift (they have T/W greater then 1) so they don't need wings.

Wing is still just a mechanism to convert kinetic energy into lift, which is the same as what you are saying with regards to canard. Hence, your argument does not work.

There is always loss when you convert energy. More vortices means more loses , pure and simple. Rule of thumb, more maneuvering aircraft does , more energy it loses .

The graph shows there is less loss with canard than without. It is that simple.

 

[SamuraiBlue]

The diagram shows canard generates additional lift when the aircraft's angle-of-attack is close to zero. The reason is that canard still generates vortex in that situation, thereby contributing to lift.

This will not generate any lift since the bottom half and top half will both generate a vortex that will reduce pressure on both side of the main wing making it a zero sum gain.
If you actually understand how canards works you'll find that it has very little if not zero effect since a canard to work properly requires the cutting plane to be symmetrical creating identical phenomenon on both top and bottom surface and since the canard wing surface is one piece once attack angle is moved the vortex generated will mostly not flow on the surface of the main wing making it worthless.

 

[latenlazy]

This will not generate any lift since the bottom half and top half will both generate a vortex that will reduce pressure on both side of the main wing making it a zero sum gain.
If you actually understand how canards works you'll find that it has very little if not zero effect since a canard to work properly requires the cutting plane to be symmetrical creating identical phenomenon on both top and bottom surface and since the canard wing surface is one piece once attack angle is moved the vortex generated will mostly not flow on the surface of the main wing making it worthless.

I've been thinking about this problem a bit, and figured maybe the J-20's design employs the lerxes in part to be a handoff vortex generator once the canards have to depart for pitch control. If that's the case the use of dihedral canards in combination with large lerxes could present an interesting possible form of optimization of the designs t lift curve at different angles of attack.

 

[Quickie]

Actually, when an aircraft begins to increase its positive angle of attack, the canards goes increasing higher than the wing so that the vortex generated are always above the wings. Even at zero angle of attack, the vortex could still be above the wings if the canards are located at a higher level than the wing like the J-10, Rafale etc. J-20 has, just in front of the canards, vortex generators at the side engine inlets for both the older and newer prototypes.

 

[F-15]

Nope. Diagram CL(alpha) shows almost no increase when alpha (AoA) is low.

Correct, the increase seen with the canard on configuration at low Alpha (AoA) is just given by the canard and wing area sum but there are loses because the drag is higher due to down wash, you are correct, there is only a real gain once the aircraft is at high Alpha

We don't have CD(alpha) diagram , but you could extrapolate it form CL(alpha) and CL(CD) diagram . It will show that at same AoA , configuration without canards has less drag and less lift.

Correct

Smaller wings means less drag and less lift. For example, rockets don't need lift (they have T/W greater then 1) so they don't need wings.



There is always loss when you convert energy. More vortices means more loses , pure and simple. Rule of thumb, more maneuvering aircraft does , more energy it loses .

Correct, that is basic thermodynamics and entropy and it is translated into wing stall

 

[Engineer]

It's just like you to distort facts and cherry-pick figures to try to make your argument look good.

Oh, you are too modest. I can't half compete with you even if I did cherry pick. For one, I am not the one who uses imaginary value that can subject to change in the next five years to argue against hard figures currently published by US Air Force. For another, I am not the one who argue about aircraft performance based only on WVR engagement.

You're comparing LRIP figures for the F-35 to production figures for the F-22. Of course the F-22 will be cheaper for mass than the F-35, if you compare LRIP to production. Now, if you were actually doing an apples to apples comparison, you'd note that the F-22's initial LRIP phase cost 189 million in 1998 dollars. That's 273 million a piece in 2014 dollars, giving you a rough ratio of 17:10.

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Besides that, the LRIP argument is extremely sound.

There is nothing sound about using LRIP as a premise, because the issue of contention is not whether scale of economy can drive down cost. The issue is whether smaller fighter aircraft have lower unit cost compared to larger ones, which is an entirely different concept to that of economy of scale. Moreover, economy of scale isn't unique to F-35, and if F-22 had production up to thousands of units, we would see lower unit cost for F-22 as well. In other words, economy of scale doesn't help your argument.

The F135, as I've mentioned before, is just a derivative of the F119, except it gains a high bypass ratio, drops supercruise, and loses TVC. The F135 does boast higher T/W ratios than the F119, but the thing to note is that the F119's flat nozzles eat up about 1/6th of its total thrust. That means that once you factor out the F119's flat nozzles and use axisymmetric nozzles, the F135 has about the same T/W ratio as the F119, so the engines are essentially identical.

Yet the F135 costs $30 million a pop, making about 20% of the cost of the F119s, while the F119s cost only $10 million per engine on the F-22s, also making 20% of the total cost. Except for the LRIP aspect, there is no reason the F135s should cost so much, as the engines are not at all more capable than the F119s, hence the currently inflated cost of the F-35s is due to its LRIP / low-scale production status.

You can go on and on about specifics of the two engines, but the F-35 cannot super-cruise and has a lower thrust-to-weight ratio than F-22. That's go against your generalization that a smaller fighter is more optimized for WVR than a large fighter.

As far as optimizing for WVR vs optimizing for BVR, the problem is that you're imagining that a single airframe can do everything. That is exactly why the F-35 project is having such grotesque troubles, because they're trying to make a small fighter take over the roles of more than 5 different aircraft (F-18, F-16, A-10, EA-6B, F-111). The thing is, a small, lightweight fighter doesn't need to be designed so that it can handle a BVR specialist by itself; by its very nature, as we've noted, smaller fighters are disadvantaged in BVR. Instead, you use other aircraft with different designs and functionalities to deal with the BVR phase of the engagement. Jammer aircraft, for instance, can degrade the effectiveness of enemy BVR systems so that they lose their advantage in BVR combat, while your own BVR specialists can screen off enemy BVR craft from being able to engage your WVR-oriented fighters.

The flaw of the above statement is the assumption that fighters don't have to be good in both BVR and WVR. A fighter aircraft that's optimized for BVR engagement can at least choose to avoid getting into WVR, whereas a fighter aircraft that's optimized for WVR engagement has to survive through BVR first. So, a fighter can have the best performance in the world at WVR engagement, but that's useless if that aircraft gets blown up at BVR and can't make use of the advantage.

 

[Engineer]

Correct, the increase seen with the canard on configuration at low Alpha (AoA) is just given by the canard and wing area sum but there are loses because the drag is higher due to down wash, you are correct, there is only a real gain once the aircraft is at high Alpha

Incorrect.

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. Downwash from canard actually improves performance.

 

[Engineer]

This will not generate any lift since the bottom half and top half will both generate a vortex that will reduce pressure on both side of the main wing making it a zero sum gain.

If you actually understand how canards works you'll find that it has very little if not zero effect since a canard to work properly requires the cutting plane to be symmetrical creating identical phenomenon on both top and bottom surface and since the canard wing surface is one piece once attack angle is moved the vortex generated will mostly not flow on the surface of the main wing making it worthless.

If you actually understand how canard works, you wouldn't have even attempt to claim "canard does not generate lift". Each canard generates a vortex, just one, at the tip. This is a result of pressure differentiation between the upper and lower surface, and has little to do with airfoil geometry. These vortices from properly positioned canard generates lift because they are made to pass above the wing, and only one side of the wing does pressure change.

 

[Engineer]

I've been thinking about this problem a bit, and figured maybe the J-20's design employs the lerxes in part to be a handoff vortex generator once the canards have to depart for pitch control. If that's the case the use of dihedral canards in combination with large lerxes could present an interesting possible form of optimization of the designs t lift curve at different angles of attack.

The main purpose of the leading edge extension on the J-20 is to reinforce the vortices from the canard, so that a long-coupled canard retains the advantages of close-coupled canard. This resolves a design conflict in the longitudinal positioning of canard.