A few more HUD inaccuracies.
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Actually, nose down tendency is incorrect in BMS - it should rise a little. However, its not necessarily due to lift. On a FLCS equipped aircraft, it can be due to lift - but an increase lift can result in a nose down effect if accompanied by the right torque - such as that caused by the landing gear.
Note that the flaps and gears lift and drag are modeled from a real F-16 in a wind tunnel and are correct. What is incorrect is the transient forces on the aircraft during the gear movements. The end result should be the nose staying level or rising a little (according to the BEM anyway).
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What is incorrect is the transient forces on the aircraft during the gear movements.
Or that.
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Could it be that gear drag is on/off and the flaps increase with the angle? Or perhaps that is what you wrote?
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I just discovered where exactly in the HUD in real life does the FPM gets crossed out, there is this amazing feature where the HUD symbology get “zoomed out” in the tapes when the HUD is in LANTIRN mode in a night mission:
http://i794.photobucket.com/albums/yy226/Y2Jericho619/HUD%20FPM%20X%20Real%208_zpsppxim4aa.jpg
The quality is low, so is the fps of the video, but it’s obvious where the limits of the FPM actually are. This is from desert storm 1990/1991, a real combat operations video… The block is most probably block 40 since it was the LANTIRN capable block in the 4th TFS at that time (it’s the block which Keith Rosenkranz flew, these videos are of his flights, you can even hear him say his tail number in the vids ;)), but it’s my opinion that these are HUD standards present in all blocks. Someone more knowledgeable than me did actually confirm that.
Link:
… are you sure that the FOV of the AVTR is the same than the HUD? … because those records do not comes from a HUD, but from AVTR and there might be some difference in the display footprint.
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I can’t say I am sure… I am not an engineer of f-16 huds… but common sense would say that the very hud the pilot sees and the one projected over the video tapes come from the same source… This will need confirmation.
@Fenrir:Could it be that gear drag is on/off and the flaps increase with the angle? Or perhaps that is what you wrote?
The flaps extend to max angle afaik… if I understood you correctly
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I meant that perhaps the gear drag goes from 0 to 100 as soon as you press G and the flap lift increases as the angle increases, in bms that is? And that’s why fpm goes down?
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I can’t tell you, that’s something a bms programmer or dev knows. I don’t know what causes the behaviour exactly
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… are you sure that the FOV of the AVTR is the same than the HUD? … because those records do not comes from a HUD, but from AVTR and there might be some difference in the display footprint.
FOV is not the big thing to question though Deejay… the thing to note is this: That the HUD symbology is overlaid on top of the AVTR video, and is recorded separately. As long as the HUD symbology is aligned correctly, the symbols should be ‘crossed out’ when they leave the HUD FOV. It doesnt matter when the leave the video FOV - when they become crossed should be the point where they are in the edge of the HUD FOV.
Because the FPM is always displayed in the HUD at the corner, and ‘blanked’ using the X symbol inside the FPM, the point where it becomes ‘blanked’ on the video is the point where it should get blanked on the HUD in the sim.
I meant that perhaps the gear drag goes from 0 to 100 as soon as you press G and the flap lift increases as the angle increases, in bms that is? And that’s why fpm goes down?
The flaps extend to max angle afaik… if I understood you correctly
Below 240 KIAS, the flaperons are lowered to 20 degrees deflection. Roll inputs will cause one or the other to be raised, causing the roll. If implemented correctly, the end result should be that the lift is not changing at all, except if you lower the gear above 240 knots - in which case, the TEFs are lowered automatically, even above 240 knots. In that case, the total aerodynamic reaction should be changing as the TEFs lower and increase the camber of the wing. It is clear that the drag should be changing as the position changes. The exact behavior of that change seems like it would be fairly complex. Its not detailed in the NASA paper that the BMS F-16 aero model is based on. I dont know how its modeled at present, either. Seems like the devs could have come up with a convincing simulation of the exact effects with a virtual wind tunnel software and a detailed F-16 software model. Dont know if thats happened (I would guess not, but anythings possible).
I dont think the FPM lowers due to the gear drag being all or nothing and the TEFs effects being graduated, simply because that would cause the effect to only be present during LG extension at airspeeds higher than 240 knots.
Edit: Oh and I almost forgot this one: when lowering the gears you’re also lowering the flaps, which apparently has more overall lift (at least initially) than the gears have drag (a lot more in fact, so far that in the dutch video the pilot had to pitch down about 5°’s to offset it) as evidenced by these videos:
Last one’s got loads of low approaches you can just watch him lower the gears over and over until landing.
(Clarification: In the sim, when you lower the gear your FPM goes down meaning you are losing lift (airspeed), I guess it was modeled with the assumption that the gears has more lift spoiling drag/deceleration than the flaperons have lift, but according to HUD tapes the opposite is true. This one may be more related to flight model not the HUD.)In the dutch video, he drops the gear doing 254KIAS. His TEFs would have already been mostly extended anyway, at about 18° (slightly less) - dropping the gear would have just further extended them all the way to the 20° point. TEFs are only fully retracted at/above 370 KIAS.
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I’d say a bit lower than 240, the dutch F-16 got the FPM up behaviour still when he lowered gears at ~233 knots or so. But yeah it does seem the FPM remains neutral when gears are downed at lower speeds.
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The FLCS on CCIP block 40/42/50/52 aircraft schedules the TEFs fully down to 20 degrees deflection at 240 knots. Seeing as its a dutch F-16, its not a CCIP aircraft. I dont know if there is a FLCS difference there.
Keep in mind like I said before - changes in lift due to TEFs are not the only thing that can cause nose movement. Its a FBW jet. You can have nose movement for no other reason than the fact its programmed to do that. When you extend the LG, the FLCS changes from cruise gains to takeoff/landing gains. Gear drops, TEFs extend if they werent already (i.e., faster than 240 knots). Lots of things happen all at once. Could even be due to a quirk in the transition between the FLCS operating modes.
Edit: HAF F-16 blocks 50/52+ also schedule TEFs the same way - full deflection by 240 knots.
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I’m now getting confused by what you mean by “nose”. If by nose you mean the FPM and not the Gun cross, then I don’t understand how the FPM moves due to FLCS programming & not due to the actual change in the Flight Path of the jet.
(Minutes, Seconds linked), which I’m assuming is an American F-16, but I have no clue if it’s CCIP upgraded.
Also the behaviour in the Dutch jet also happens in this one -
Well by nose I mean the movement of the nose of the aircraft. The gun cross is a good reference to see movement of the nose I guess. The FPM depicts the flight path of the jet (or as best as the jet can work out).
If you see the nose raise during LG extension below 240 knots, that proves that the change in nose position is not due to the TEFs, as they dont get extended any further when the gear are extended below 240 knots.
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Well by nose I mean the movement of the nose of the aircraft…
If you see the nose raise during LG extension below 240 knots, that proves that the change in nose position is not due to the TEFs, as they dont get extended any further when the gear are extended below 240 knots.Nope, I meant, and always have, that the FPM position changes with gears down, and not the nose ;).
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just watch the videos at the times I linked to see what I mean: gears down above 225 or so knots, the FPM/Meatball/VV rises up.
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Ah. So to clarify, the assertion is that the lift generated with the LG down in a steady state, is too little? That there should be less angle of attack with the gear down?
I was describing only the brief state while the gear are in motion, not the steady states of either LG extended or retracted. I am under the impression that the data BMS uses for those steady state conditions is based on wind tunnel data and is thus pretty solid. Mav-jp would be the one to ask about that.
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I’m describing the brief state mainly as well :P. The steady state needs more studies/calculations than simply noticing behaviour on HUD tapes, which would need a lot more time and more knowledge than I have obviously ;).
Summary: Pilot lowers the gear handle: Velocity Vector Rises up in a matter of one or two seconds. -
Bump! I’ve updated the post to make it more readable and more organised and also added one more point. Congratulations everyone and BMS team on the deal with Tommo! :).
I’m a bit sad my notes on the HUD were not implemented in the update though I hope you guys do consider these on your to do list. -
3. Edit: Oh and I almost forgot this one: when lowering the gears you’re also lowering the flaps, which apparently has more overall lift (at least initially) than the gears have drag (a lot more in fact, so far that in the dutch video the pilot had to pitch down about 5°’s to offset it) as evidenced by these videos:
Last one’s got loads of low approaches you can just watch him lower the gears over and over until landing.
(Clarification: In the sim, when you lower the gear your FPM goes down meaning you are losing lift (airspeed), while in real life the FPM goes up. I guess it was modeled with the assumption that the gears has more lift-spoiling drag/deceleration than the flaperons have lift, but according to HUD tapes the opposite is true. This one may be more related to flight model not the HUD.)That first video (dutch) features a pitch down of 2°, not 5°. The nose lowers, the FPM lowers, the AoA decreases. Lowering the gear also cost 20 knots (235 to 215).
Second video, nose lowers, FPM goes down.
Third video, nose lowers, FPM comes up. Could be relative winds, could be local air density, could be lots of things. Id suggest that any work in this area would do well to set up a virtual wind tunnel and see what kind of forces they get. -
Heya Blu3wolf, still in the argumentative spirit eh :P. Well lets just say I am as well, we’re just not seeing this the same way, hehe.
In the dutch vid the plane was slightly above 5° (about 6°’s), when pitching down he got down to about 1° or slightly above. 2nd vid it kinda doesn’t move, but the pilot pitches down a couple of degrees. 3rd it can’t be a coincidence in the 3-4+ times he lowers the gears each time the fpm goes up. I have loads of other HUD tapes that show this behaviour, those 3 were just an example.