Wheels effect upon touchdown
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I’ve noticed an interesting effect on touchdown when landing in (heavy) crosswind conditions. When coming in crabbing and de-crabbing just before touchdown with proper cross controls, aligned the jet with the centerline, wings level (or upwind wing slightly down) and smooth touchdown, the aircraft suddenly turns massively in upwind direction, whilst keeping rudder deflection. I’ve never experienced this in any other sim nor in real life. It seems there is a big discrepancy between the aerodynamics modeling of the jet in the air vs when in contact with the ground without smooth transition. Any ideas on this? Is it modeled that the grip of the wheels slowly transitions from 0% to 100% upon landing?
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@Bowser Well, for starters, you do not decrab the F-16 before touchdown, you land crabbed. That may help the experience.
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@Bowser said in Wheels effect upon touchdown:
I’ve noticed an interesting effect on touchdown when landing in (heavy) crosswind conditions. When coming in crabbing and de-crabbing just before touchdown with proper cross controls, aligned the jet with the centerline, wings level (or upwind wing slightly down) and smooth touchdown, the aircraft suddenly turns massively in upwind direction, whilst keeping rudder deflection. I’ve never experienced this in any other sim nor in real life. It seems there is a big discrepancy between the aerodynamics modeling of the jet in the air vs when in contact with the ground without smooth transition. Any ideas on this? Is it modeled that the grip of the wheels slowly transitions from 0% to 100% upon landing?
Hehe
Simple and that is a reason why it’s forbidden to to this in the real f16
When you decrab, you use cross control to remain wing level, doing this you use aileron and rudder in the same time.
The problem is that the ARI is disconnected when Wow on main gear, so as soon as you trigger the wow, the rudder position brutally changes as it is no more coupled with your previous aileron input, that’s why you have a sudden yaw movement
That is the reason you don’t decrab the f16 , ARI kicking off can be dangerous
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“This is a feature, not a bug”
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@Mav-jp Is this true for both trim, and control inputs?
In BMS at least, it feels like I can apply yaw-trim (and a bit of counter roll-trim) to de-crab, and I don’t notice any terrible transients upon touchdown.
In RL I have no idea if this is ever done… maybe other issues, like differential ground-effect dynamics when the ailerons aren’t balanced, are a danger.
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@Mav-jp Even without de-crabbing, the aircraft makes an abrupt change of direction upon touch down in crosswind conditions. E.g. when still in the air the center line is followed in a crab position. Then upon touchdown the aircraft instantly moves in the direction of the main wheels. This is physically impossible due to inertia and the fact that the wheels have close to zero grip upon touch down due to the airflow over the wings. There should be a slow transition in which the wheels first skid over the runway in landing direction (so center line) and then when speed decreases the aircraft will slowly start going in the direction of the main wheels if no rudder is applied.
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@Bowser - Might want to check that your NWS is not selected ON at touchdown…shouldn’t be, but you might want to check. I’ve not noticed this, but I always aero brake until the nose falls, and when it does I’m still NWS disengaged, but positively down.
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@Stevie I’m talking two-point on touch down, so has absolutely nothing to do with NWS
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@Bowser Maybe a video (from external cam angle) would help explain what you’re seeing / experiencing?
The mg wheels are well behind the center of gravity… so as they contact the tarmac, the sudden onset of friction force will pull the rear of jet to be in-line with the runway…
Then once the mg wheels are freely rotating (assume no braking) that friction decreases greatly, and the tail becomes a weathervane, causing the nose to drift back toward upwind side.
This “feels” right to me. (But I haven’t tried observing a crosswind landing with chase-camera pov… so idk.)
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@airtex2019 Just try a crosswind landing with 25 kts perpendicular to the RWY and you’ll see it yourself. The sudden change of direction upon touch down should not be possible due to inertia.
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@Bowser Sure, I’ve done plenty, and as I say it “feels right” to me. But I’m not a fighter pilot. I’m more accustomed to landing a single-engine Cessna in crosswind. Much lower inertia.
We might be able to find some videos of crosswind landings, to roughly calibrate the yaw-acceleration, IRL vs BMS. Not sure how much it would vary, due to things not visible like fuel in wings… and engine rpm (rotational inertia).
This vid has a long stream of examples, and you may be right. Yaw transient appears barely noticeable as they touch down.
There was another recent thread about brake-channel and anti-skid behavior. I think I read there’s a difference in “touchdown anti-skid” (only channel 1) vs “wheelbrake anti-skid” (available on both channels)
Does F-16 landing gear have a mechanism to get the wheels rotating, with passing airflow, before touchdown? That would greatly reduce any transient upon touchdown.
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@Bowser - I don’t get you…even if you are countering a crosswind your mains should both be on deck before your nose, and NWS should not be engaged before the nose touches down - if it is, you could be touching down with your nose wheel cocked…which it likely would be if NWS is engaged and you are countering a crosswind with rudder.
What do you mean by “two point”?
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@Stevie Only main gear touching the RWY.
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The gear has no mechanism to get the wheels spinning prior to touchdown.
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@Bowser said in Wheels effect upon touchdown:
@Mav-jp Even without de-crabbing, the aircraft makes an abrupt change of direction upon touch down in crosswind conditions. E.g. when still in the air the center line is followed in a crab position. Then upon touchdown the aircraft instantly moves in the direction of the main wheels. This is physically impossible due to inertia and the fact that the wheels have close to zero grip upon touch down due to the airflow over the wings. There should be a slow transition in which the wheels first skid over the runway in landing direction (so center line) and then when speed decreases the aircraft will slowly start going in the direction of the main wheels if no rudder is applied.
Open your AFM file , reduce lateral friction of the gear 1 and 2 . Have fun
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@Bowser - ok. That’s what I was thinking.
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@Mav-jp said in Wheels effect upon touchdown:
Open your AFM file , reduce lateral friction of the gear 1 and 2 . Have fun
Out of interest, What’s the current default lateral friction for the main wheels?
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I don’t see where the danger is or why not de-crab with the BMS model. I did the crab landing, the sideslip, and the decrab, all of which was possible. It was a clean jet on weight, winds blowing 28kts westerly.
Edit: I guess what was mentioned above about Crab only is for the real thing.
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" no reason to ever use the rudders on the F16 is my recollection."
I was chatting with Keith Rosenkranz “Vipers in the Storm” last year and this was a direct quote.Mud
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