F-35 vs J-20

[steve2267]

This is off topic but as long as we are talking about aerodynamics I've got a question:

Why does the f35 often seems to be at positive AoA during level flight?

I'll take a stab at this question.

I presume citanon is inquiring about the seemingly high angle-of-attack of F-35s flying at relatively low airspeeds in level flight. This youtube video seems to do a decent job at illustrating what I am trying to say:



Somewhere here on the F-35 Lightning II forum on F-16.net, someone mentioned that the F-35's horizontal tail flies at a positive angle of attack, that is, the horizontal tail is generating positive lift. Conventional aircraft (tube / wing / horizontal tail / vertical tail) horizontal tails typically generate negative lift (i.e. push the tail down) to counteract the nose down pitching moment created by the wing's generation of lift (since all conventional aircraft airfoils - at least with which I am familiar - have a negative pitching moment (i.e. nose down) when positive lift is created). However, the F-35 has significant fuselage and forebody shaping:

1. nose chine
2. air intake chine
3. small leading edge root extension (LERX)

I believe the fuselage/forebody shaping permits the F-35 to generate significant lift, enough to overcome the nose down pitching moment created by the wing's lift generation, if the angle of attack is high enough. Note the nose chine slopes slightly down when the aircraft is sitting level on the ground:



In this next photo, the aircraft is at a significant angle of attack, and it should be obvious that the angle of the nose/forebody chine is now at a positive angle (presumably) to the flight path vector, enabling the nose chine, air intake chine, and LERX to all generate positive lift, overcoming the wing's nose down pitching moment.

If true, then the horizontal tail can either be flown at a nominally zero lift (trimmed) state, or possibly at a positive lift state if the forebody lift is greater than that necessary to overcome the wing's nose down pitching moment.

That's my take on it, anyway. Any confirmation (or correction) is welcome.

[citanon]

Don't know whether your explanation is true or not but it's a fascinating thesis.

[vanshilar]

This is off topic but as long as we are talking about aerodynamics I've got a question:

Why does the f35 often seems to be at positive AoA during level flight?

This comes up, most often with the infamous F-35 vs F-16 Thunderbirds publicity photos.

It's been mentioned elsewhere by somebody else that those photos tend to be taken at low speed, i.e. for example where a helicopter is the camera platform. Thus the reason why the F-35 often seems to be at positive AOA is simply because the flight computers are scheduled that way for low speeds -- to have the plane rely more on body lift (via positive AOA with the body) as opposed to with the wings and the tails.

For example, though it's hard to tell in many of the shots, it seems like the F-35 only has the leading slats down, whereas the F-16 has both leading slats and trailing flaps down for additional wing camber. Thus if you're looking at the AOA in terms of the wing's leading edge to the wing's trailing edge, the AOA of both are actually fairly similar. See for example this image:

http://static4.businessinsider.com/imag ... hunda2.jpg

So if both planes had to go even slower, then the F-16 would have to start relying on more body lift (pointing the nose upward), whereas the F-35 can then choose to use the flaps -- the wing has additional available camber, so to speak.

As mentioned above, the F-35 (like the F-22) have chines that are actually slanted downward (they don't actually generate vortices when the plane is flying level though, because the nose is still expanding outward there). This means that the body needs to be at a higher AOA compared to the F-16 in order for them to generate vortices (and I'm guessing also means that they stay effective at higher AOA than for the F-16).

As for why they chose to schedule the flight computers this way, perhaps it's for stability reasons, or for fatigue reasons (if the body generates some lift then less forces have to be transferred from the wings to the body to keep the body up), or for better maneuverability, or maybe it has a better L/D this way, or whatever. Who knows. But it's how they chose to schedule the plane's aerodynamic surfaces for when the plane is flying at low speeds.

[citanon]

Thanks for that explanation. Makes sense.

[count_to_10]

Interesting point on body lift and the possible effect on structural lifetime.

[sprstdlyscottsmn]

Look at the difference in the H-Stab position in that photo too.

[nutshell]

In just 2 posts in this page there are enough reasons why this board is an amazing source of infos; vastly superior to pretty much every dedicated military "enthusiast" websites

[gta4]

@arian: I'm having fun here. This is the first time I've heard of pedal turns, and while I'd insist it's just an instantaneous turn, even possibly just a yawing action, it's interesting to note this about the capabilities of the F-35.

You are having a mental health condition and I'm not joking. How on earth could it be instantaneous turn while its angular velocity is constant? The turn rate of instantaneous turn drops over time!

viewtopic.php?f=55&t=52965

"Pedal turn" is neither sustained turn, nor instantaneous turn. However, its effect is equivalent to a sustained turn of 28 deg/sec, because it changes its heading at a steady 28deg/sec, and an enemy fighter could not counter it unless it performs a sustained 28deg/sec turn (conventionally or yawing).

[steve2267]

Look at the difference in the H-Stab position in that photo too.

Yes, but...

A photograph is a snapshot at a point in time. I went back and re-watched the youtube video to which I linked above, and I can see the F-16 stabilitor varying angle a lot. So I cannot say with any degree of certainty that the Viper's tail is generating negative or positive lift in this flight regime (i.e. slow, level flight). I recall JohnWill mentioning that the Viper's tail generates positive lift (at times -- supersonic?). Dunno if it also generates positive lift slow and level. I am going to "guess" not, as it would only begin generating appreciable lift where the LERX/strakes begin, whereas the F-35 nose chines, at a visibly larger angle of attack, would seem to be generating more lift closer to the nose, which will generate more of a nose-up pitching moment since it will have a longer moment arm back to the CG compared to to the F-16 strake-CG moment arm. I'd file this comment in the FWIW section...

[sprstdlyscottsmn]

Look at the difference in the H-Stab position in that photo too.

Yes, but...

A photograph is a snapshot at a point in time. I went back and re-watched the youtube video to which I linked above, and I can see the F-16 stabilitor varying angle a lot. So I cannot say with any degree of certainty that the Viper's tail is generating negative or positive lift in this flight regime (i.e. slow, level flight). I recall JohnWill mentioning that the Viper's tail generates positive lift (at times -- supersonic?). Dunno if it also generates positive lift slow and level. I am going to "guess" not, as it would only begin generating appreciable lift where the LERX/strakes begin, whereas the F-35 nose chines, at a visibly larger angle of attack, would seem to be generating more lift closer to the nose, which will generate more of a nose-up pitching moment since it will have a longer moment arm back to the CG compared to to the F-16 strake-CG moment arm. I'd file this comment in the FWIW section...

Of course it's a FWIW comment. The fact that nothing is ever truly static on these planes caught my mind before I even posted. And you bring up many good reasons why it is hard to eyeball. All the reasons you bring up however further point to the plausibility that the F-35 in that still is using more tail lift than the F-16 in that still. Also good to think about is that when TEFs are deployed there is more downwash off the back of the wing further reducing the AoA of the H Stab (increasing potential for downforce). It really looks like, in that still, the F-16 is using primarily wing lift while the F-35 is using a more distributed lift.

Again, FWIW.

[steve2267]

Of course it's a FWIW comment. The fact that nothing is ever truly static on these planes caught my mind before I even posted. And you bring up many good reasons why it is hard to eyeball. All the reasons you bring up however further point to the plausibility that the F-35 in that still is using more tail lift than the F-16 in that still. Also good to think about is that when TEFs are deployed there is more downwash off the back of the wing further reducing the AoA of the H Stab (increasing potential for downforce). It really looks like, in that still, the F-16 is using primarily wing lift while the F-35 is using a more distributed lift.

Again, FWIW.

My "FWIW" comment was a self-deprecating comment intended to caution the reader to take what I was writing with a grain of salt. It was not intended to deprecate or to take a swipe at what you had written.

I had thought to write something very similar to what you had written, but then I thought about the "point in time" characteristic of a photo, and re-viewed the video. When I saw the F-16 stab moving up and down, it gave me pause as to whether or not I could say for sure if the F-16 tail is generating positive lift. I think you have a better handle on that than I.

But I agree for all reasons stated -- forebody shaping, (generally visible) higher angle-of-attack that the F-35 is probably either neutral or positive lift on its stabilators. IMO, a pretty nifty design feat accomplished without canards. That is, the F-35 designers appear to have achieved all positive aspects of a canard, while retaining all the positive aspects of a horizontal tail.