Roll odd behaviour at high speed

Hi, just wanted to know if this is normal (I don't think so)...when at high speed (500+knots) and wing sweep full aft (manual or auto) when I roll at full lateral stick deflection, the plane starts to make odd roll oscillations and I can see the horizontal stabs moving up and down very hard even when I am not doing such movements...is there some issue with the AFCS?? I dont think this should be happening.

I posted a track with the issue though, Im not sure it will work correctly as the tracks with the Tomcat are broken at the moment but here it is...note how doing RCtrl+Enter you can see the stick is steady at full deflection and these oscillations appear even then.

thanks!

f14 roll stutter.trk

I believe this is referred to as "ratcheting" and is a side effect of the AFCS, It goes away if Roll SAS is deactivated.

I believe this is referred to as "ratcheting" and is a side effect of the AFCS, It goes away if Roll SAS is deactivated.

So you think it is also happening in the real aircraft?, then it's just fine...yeah, disabling the AFCS should help, thats why I though it could be an issue with it but if it is also happening in the real thing then it is ok :thumbup:

yeah, disabling the AFCS should help, thats why I though it could be an issue with it but if it is also happening in the real thing then it is ok :thumbup:

It's the reason that disabling roll SAS is in the fence-in checklist for the real aircraft.

It's the reason that disabling roll SAS is in the fence-in checklist for the real aircraft.

Looks like so, according to the manual :"Roll SAS should not be used for situations involving flight at AOA above 15 units and should therefore be set to off for combat maneuvers."...however, it says "above 15 units of AoA", but the stuttering in the roll is happening at high speed and wings sweep full aft...guess it is also best to disable it when at high speeds although I noticed a big roll rate decrease when doing so at high speed.

thanks!

Looks like so, according to the manual :"Roll SAS should not be used for situations involving flight at AOA above 15 units and should therefore be set to off for combat maneuvers."...however, it says "above 15 units of AoA", but the stuttering in the roll is happening at high speed and wings sweep full aft...guess it is also best to disable it when at high speeds although I noticed a big roll rate decrease when doing so at high speed.

 

thanks!

The ratcheting is because available roll rate markedly increases in the F-14 with speed and increasing sweep. As the wings sweep, roll ramping and inertia markedly decrease due to the centralization of mass and reduction in wing surface area. The relative power of the horizontal stabilizers also increases due to airspeed and airflow changes. The net effect is way more roll authority and roll rate becomes high enough that the effects of roll SAS are quite apparent. Roll SAS restricts maximum roll rate to 180 deg per second, but the maximum available roll rate is higher so the rate will ratchet as SAS periodically reduces horizontal tail deflection. Part of the reason for reducing rate is that the torque created by the power of the tail surfaces and roll rate is hard on the airframe that joins the wings to the tail, especially since only the horizontal stabilizers provide roll control with the wings swept above 62 deg.

It is worth noting that this is only obvious when performing sustained rolls, which real pilots really didn't do since there is no real benefit to the maneuver. Most routine maneuvers involve rolling about 90 deg at a time and the roll rate is noticeably faster with speed and sweep under these conditions since roll SAS generally won't restrict rate for only 90 deg of roll.

Turning off roll SAS for high AOA maneuvers was a standard practice for any jet aircraft with swept wings and conventional roll SAS. Since most swept wing jets experience roll reversal above a specific AOA (related to the adverse yaw effects of the down deflected aileron creating drag while the up going aileron is blanketed or less effective).

It was also standard procedure to turn off Roll SAS in the F-4 Phantom (all variants) for maneuvers above 15 units as well.

Basically the roll reversal means that SAS inputs will have a paradoxical effect and lead to a sustained input, increasing the risk of departure.

-Nick