Agile F-35 High Wing Loading

[wrightwing]

And when the simulation takes fuselage shape and aerodynamic attributes into account, and the virtual fighter still behaves like a 'draggy F-104'?

You'd find that X-plane doesn't conform to what is programed by the user, but actively predicts how an aircraft's shape, weight distribution, and thrust affect performance. Even a version of X-plane on the ipod touch takes fuselage shape into consideration for lift and drag forces.

This virtual F-35 is not a pig, but it's certainly not as nimble as an F-16, save a high T/W ratio.

This is precisely what I was getting at before, with regards to wing loading not telling the whole story. It doesn't account for the lifting body design, which in effect serves to lower the actual wing loading, from a practical standpoint. The F-104 derived all of its lift from its wings, whereas the F-35 will be getting lift from both wings, and the fuselage. When you combine that, with an unstable design, good T/W and T/D, it makes for a nimble aircraft. Then factor in the fact the the aircraft won't be adversely affected by the drag of external stores(or having to overcome. The inertial effects of having the weight out on the wings), when rolling/turning.

[cola]

battleship..... X-Plane uses Blade Element theory to resolve the forces on the aircraft. The problem with the F-35 in X-plane is it assumes the lift that the fuselage creates is very low to no lift at all. So what you end up with is an airplane that flys like a draggy F-104 which isn't how the F-35 flys.

http://wiki.x-plane.com/Appendix_A:_How_X-Plane_Works

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'cowboy,
you can't base your arguments on video games.

It's really simple, small AR bodies (small span, long chord) have lower Cl buildup, but higher max alpha, than bodies with higher AR (larger span, shorter chords).
So, in terms of instantaneous turning, the F35 COULD have advantage over F16, but in terms of sustained turning (in presently disclosed configuration), it's not even close.

[wrightwing]

battleship..... X-Plane uses Blade Element theory to resolve the forces on the aircraft. The problem with the F-35 in X-plane is it assumes the lift that the fuselage creates is very low to no lift at all. So what you end up with is an airplane that flys like a draggy F-104 which isn't how the F-35 flys.

http://wiki.x-plane.com/Appendix_A:_How_X-Plane_Works

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'cowboy,
you can't base your arguments on video games.

It's really simple, small AR bodies (small span, long chord) have lower Cl buildup, but higher max alpha, than bodies with higher AR (larger span, shorter chords).
So, in terms of instantaneous turning, the F35 COULD have advantage over F16, but in terms of sustained turning (in presently disclosed configuration), it's not even close.

He wasn't the one basing his argument on the X-plane simulation- that was Battleshipagincourt.

[battleshipagincourt]

This is precisely what I was getting at before, with regards to wing loading not telling the whole story. It doesn't account for the lifting body design, which in effect serves to lower the actual wing loading, from a practical standpoint. The F-104 derived all of its lift from its wings, whereas the F-35 will be getting lift from both wings, and the fuselage. When you combine that, with an unstable design, good T/W and T/D, it makes for a nimble aircraft. Then factor in the fact the the aircraft won't be adversely affected by the drag of external stores(or having to overcome. The inertial effects of having the weight out on the wings), when rolling/turning.

Cola made a good point as it is, but you simply repeated your last statement. Given as X-plane DOES take fuselage shape into consideration, what's your point?

I'll admit that I can't base a judgment on a program with statistics that may or may not be accurate to reality. All you know for absolute certainty is that the F-35 is a very heavy fighter with stubby wings and no thrust vectoring. While LM is claiming it's at least on par with the F-16, I really hoped it would have gone one step further. This is a fifth generation fighter, right?

And even if it's not as agile as it's hyped to be... that still doesn't mean its other advantages can't compensate for only moderately good agility. The Panavia Tornado is such an example of a fighter/bomber not built to dogfight like the F-15/F-16, but it proved to be a very decent multi-role aircraft in most regards. I just can't see how it could achieve such remarkable turning performance without thrust vectoring.

No, adding thrust vectoring isn't simply a matter of agility, but giving a pilot much more to work with in tight situations. At high altitudes, thrust vectoring could make all the difference in a WVR fight. It also takes stress off the wings in a sharp turn, extending the life of the airframe. And having another set of control surfaces means that you would have more freedom in the design of the fighter. Even the navy design would have done very well with this feature for carrier landings.

And I'll give this issue a rest for a while. I just hope I'm wrong about this.

[gtg947h]

Cola made a good point as it is, but you simply repeated your last statement. Given as X-plane DOES take fuselage shape into consideration, what's your point?

X-Plane's fuselage calculations are nothing more than "I have a lump about this big, so here's a rough lift and drag calculation based on the projected area of that lump and a user-input Cd0 value". You're just fooling yourself if you think you can model up a fuselage with a 16-sided polygon 20 segments long, and get accurate CFD results from it (nevermind that XP doesn't do CFD). Wing performance is based off of user-input 2D airfoil performance (so who knows if that's right on your model) at a "low" and "high" Reynolds number, with textbook undergraduate approximations applied for compressibility effects. And none of that will handle complex fuselage-wing interaction at high AOA, evermind that the author of the model almost certainly did not have access to real data (wind tunnel, flight test, blueprint, or otherwise) to compare to.

X-Plane is designed and optimized to replicate light aircraft (remote control, on up to King-Air size) with conventional wing-and-fuselage construction under reasonably uncompressible flow. It will give reasonably good approximations of that performance under those conditions. Oddball configurations, or anything outside the sim's "comfort zone", are immediately suspect.

Further, the "FAA Certified" hype doesn't mean you can crap out just any old airplane in your spare time and say "see, I can certify this because X-Plane said so!" Like all other aspects of certification, that applies only to very specific configurations. To certify it for a given aircraft, you will need to prove (with actual documented testing) that your design matches the flight characteristics. You're also going to need to be very specific about implementing your design and have very good airfoil and test data (from windtunnels or a full-scale aircraft) to get it right. Just because your airplane looks like the real one in planemaker is no guarantee whatsoever that it will actually fly like one.

To put it bluntly, claiming you can accurately predict how an F-35 will handle based on a freeware F-35 you downloaded from the internet is like claiming you can predict how well a real car will drive based on the Hot Wheels car you picked up at the store.

[exorcet]

The thing to remember about all simulators is that they aren't accurate. For the last month, I've been doing CFD just about every night. I'm trying to simulate flow around a car. A low quality simulation will last up to 30 minutes. A high quality simulation takes about 6 hours. And this is the car driving in steady state - without wheels because that would make the CFD take 3 times as long. It's simply impossible to have accurate flow physics in real time.

Flight simulators are good at replicating well behaved planes. A 787 at cruise is pretty easy to do, in fact you could make an approximation of its performance on pen and paper (it will take a while, but it's do able). When you start to get to turbulence and high alpha flight, the simple models just don't work. The truth is, we don't even understand turbulence. None of my CFD simulations do turbulence right because science hasn't figured it out yet, and it would take too much computer power anyway. The vortices I see forming under the car are estimates, as are the drag and other numbers. And this is after hours, or days of simulation.

Computer flight sims can be used to give you estimates and ideas on performance, but you have to be careful with what you take out of them. If the F-35 is cruising along at Mach .6 in level flight, that could be fairly accurate. If it’s at 40 degrees AoA in a dogfight, you’re using a very simple model, even in the best sims. The difference between a $20,000,000 sim and PC sims is, or at least I would guess, probably in that high AoA/turbulent regime. The PC sim may get it 40% right, the 20M sim may get it 65% right, CFD may get it 80% right.

[wrightwing]

Cola made a good point as it is, but you simply repeated your last statement. Given as X-plane DOES take fuselage shape into consideration, what's your point?

I'll admit that I can't base a judgment on a program with statistics that may or may not be accurate to reality. All you know for absolute certainty is that the F-35 is a very heavy fighter with stubby wings and no thrust vectoring. While LM is claiming it's at least on par with the F-16, I really hoped it would have gone one step further. This is a fifth generation fighter, right?

I just can't see how it could achieve such remarkable turning performance without thrust vectoring.

No, adding thrust vectoring isn't simply a matter of agility, but giving a pilot much more to work with in tight situations. At high altitudes, thrust vectoring could make all the difference in a WVR fight.

It also takes stress off the wings in a sharp turn, extending the life of the airframe. And having another set of control surfaces means that you would have more freedom in the design of the fighter. Even the navy design would have done very well with this feature for carrier landings.

-it does NOT take the body lift into account. As has been said many times already, the folks that wrote X-plane don't have access to the information, that would allow them to have that level of realism.

-LM/USAF claim superior performance to either the F-16 or F-18, in ITR/STR and in acceleration, as well as better high AoA performance

-it has already been said that TVC doesn't turn the plane faster(it only points the nose faster, at the expense of bleeding energy). I agree that at high altitudes, and at high speeds it is useful, as is it in the post stall regime, but you're mistaken about pre stall advantages.

-TVC doesn't take stress off the wings/airframe or pilot. It is useful when the aerodynamic control surfaces don't provide enough lift, to control the aircraft.

[Raptor_claw]

If one were to observe the F-35 in flight, it clearly is nose heavy. Just watch footage of it doing a sharp turn or on takeoff

The F-35 is unstable, just like F-16/F-22. You can't use takeoff as a reference, as the flap configuration tends to move the center of lift aft, and besides you are always gonna need nose-up tail to rotate. Have no idea what you're looking at in "sharp turns", but yeah, you may need a little nose-up tail at higher AOA's and G, but it shouldn't be much.

What thrust vectoring allows is to use the enormous power of a jet engine to increase rate of pitch without increasing the G-forces exerted on a pilot.

Only in a universe where the laws of physics are different.

I just can't see how it could achieve such remarkable turning performance without thrust vectoring.

Only if you are talking about very high altitude. As others have already said, vectoring has almost no effect on turning performance anywhere near the "heart" of the envelope, where max turn rates happen.

... thrust vectoring ... also takes stress off the wings in a sharp turn, extending the life of the airframe.

No, it doesn't. Vectoring does not create lift. Vectoring allows improved control of pitch rate at the "extremes", which in turn means improved control of AOA. Lift still comes from the wing/body, vectoring or no.

Even the navy design would have done very well with this feature for carrier landings.

Again, no. Approaches to the ship are AOA limited (for many reasons) and the AOA is well below where vectoring really adds anything. Furthermore, you're at a very low power setting during the approach - IDLE or slightly above. Vectoring does very little when thrust is low, for obvious reasons.

(Edit: Sorry for the duplication, ww. You obviously posted while I was typing.)

[sewerrat]

And let's not forget the case where an F-16 had to use the afterburner to keep up with a non-afterburning F-35.

What was the fuel loading of each aircraft? Without that information, this comment is without meaning. If the -35 had FULL internal fuel, then that would be one thing; and if only a few thousand pounds, thats quite another thing altogether.

[wrightwing]

And let's not forget the case where an F-16 had to use the afterburner to keep up with a non-afterburning F-35.

What was the fuel loading of each aircraft? Without that information, this comment is without meaning. If the -35 had FULL internal fuel, then that would be one thing; and if only a few thousand pounds, thats quite another thing altogether.

Presumably neither aircraft are taking off without a full load(especially considering that the chase planes usually have to bring an external tank, just to be able to keep up).

[exorcet]

I'm pretty sure I heard at least one reference to a "long" test flight made without full fuel in the F-35 by one of the pilots. Don't know where it is off the top of my head. On fuel tanks, while they are pretty draggy, a F-16 with just one tank is probably better off drag wise than one loaded for combat. While the clean F-35 is more or less exactly like a combat outfitted F-35. Weapon weight matters less too since the F-35 is heavier.

[spazsinbad]

Raptor_claw said: "...you're at a very low power setting during the approach - IDLE or slightly above. Vectoring does very little when thrust is low, for obvious reasons."

There is a misunderstanding about engine power/rpm for a naval aircraft carrier approach. The power/RPM needs to be high (around 85%) to aid engine acceleration / responsiveness. In older aircraft with speedbrakes these were used to further add a couple of RPM percentages for this purpose. This is a general range but not clear what the RPM range will be for the F-35C 'The very long thread' has some info about these issues onesuch is: http://handle.dtic.mil/100.2/ADA399988 (1Mb PDF) The Influence of Ship Configuration on the Design of the Joint Strike Fighter which mentions the issue in passing but is clear about the low speed handling requirements which necessitated the changes that form the F-35C.

There are other 'general' PDFs about this engine response issue for other naval aircraft. References will be added when found. Here is a good overall look at the X-35C and testing, especially in FCLP but exact engine RPM settings not mentioned. I'll guess that soon enough there will be an updated article with the F-35C numbers featuring etc.:

X-35C Navy Flight Testing by Eric Hehs - Code One Magazine Vol.16 No.2 - 2nd Qtr 2001

http://www.codeonemagazine.com/archives ... index.html

This article is no longer at the URL....

[Raptor_claw]

Raptor_claw said: "...you're at a very low power setting during the approach - IDLE or slightly above. Vectoring does very little when thrust is low, for obvious reasons."

There is a misunderstanding about engine power/rpm for a naval aircraft carrier approach. The power/RPM needs to be high (around 85%) to aid engine acceleration / responsiveness.

Way too high for this jet....
Besides, you're always gonna have cases where the guy gets a little fast and needs to slow down. The last thing you want is for a/c control to fall apart just as he's trying to do that.

[spazsinbad]

Raptor_claw, I don't know why you say what you say. I hope it is clear I have no way of knowing what an F-35C nominal carrier approach rpm will be - until it is made public - whilst the 85% was just a generalisation for older carrier aircraft. Drag is maximised to achieve the highest RPM for Optimum Angle of Attack where IAS varies according to all up weight (within limits). Pity we cannot read the old Code One Magazine article. I could make it available as a PDF here I guess they would not mind. Anyway it is clear that the APC Approach Power Compensator will make the auto throttle a great feature of the F-35C in carrier landing configuration.

[spazsinbad]

Raptor_claw also said: "Besides, you're always gonna have cases where the guy gets a little fast and needs to slow down. The last thing you want is for a/c control to fall apart just as he's trying to do that."

I don't know if you know what you have written there. Anyway carrier approach flying is a lot different to USAF airfield flying. I won't claim to know how the F-35A is flown to an airfield but I have RAAF basic flying training experience when taught prop and jet landing techniques. In a nutshell decreasing airspeed to a margin above stall airspeed for a flare with reducing power on a constant glideslope. Then I went back to the RAN FAA to learn carrier approach flying techniques which are in a nutshell the reverse of conventional technique. If you can imagine flying a constant airspeed short field (short stopping distance) fixed glideslope USAF approach then that will approximate an aircraft carrier approach.

There is no way a Navy pilot will be too fast in the groove that the aircraft cannot be slowed by throttle. With engine RPM high any slight throttle movement brings a precise power response. In this case it is for position on the glideslope referenced to the meatball in the mirror / IFLOLS datum lights. Power controls glideslope position whilst AoA/nose position controls 'fast/slow' with the ideal being Optimum Angle of Attack; which usually is easy to achieve in a carrier aircraft - trimmed - dirty - everything hanging out for drag effect.

What is required is a fast throttle response and excess power for waveoff / bolter performance. It is clear from pilot reports for the X-35C and now the F-35C that the aircraft has 'excess power' for excellent performance in this regard. Perhaps this is what you have misunderstood? The X-35C did 250 FCLP approaches so it is well known what the handling characteristics were then and equally the F-35C will be pounded around the FCLP circuit at Pax River to get all the numbers.