Rudder "snap back" - what am I doing wrong?

[Stevecat]

Haven't played this thing in a good bit but decided to give it another try.  Does anyone else notice after applying rudder - even in minimal amounts - there is an even stronger "snap back" of the nose in the opposite direction?  See the attached vid.  Start in perfect trim: 2550 rpm/44hg/250knots.  Then two slight rudder kicks (I hesitate to even call them kicks - I barely touched them) where I maintain rudder pressure.  Then a little larger one.  Almost no pressure applied in either case and the nose is bouncing back and forth like crazy.  I have my rudder calibration curve set to 25.  Is this normal for the F4U?  I also fly the K4 and D9 and they don't have as wicked of a "snap back" when applying rudder.  Seems like a plane this heavy in real life shouldn't be moving like Mexican jumping bean when the rudder is applied.  It makes getting rounds on target pretty difficult, and heavy rudder maneuvers are ridiculous.  After reading a bit here, I unchecked the FFB box which seems to help overall just a tiny bit.  But am I doing anything wrong?  Am I just too hard on the controls?  I'm trying not to be.

 

Edited Saturday at 10:20 PM by Stevecat

[carbolicus]

I do think that the nose nods from side to side with rudder inputs a lot more than it should.  This is also a problem with other flight sims such as the (generally excellent) Cliffs Of Dover.

The whole point of the fin is to damp this kind of thing; it’s why the fin has a much greater surface area than the rudder.

I’ve obviously never flown a Corsair, but in all the different types I’ve flown IRL, including aerobatic aeroplanes, when you apply rudder by itself the nose swings smoothly together with a bit of roll (secondary effect as the outer wing speeds up a little), and it swings smoothly back when you take the rudder off again.

All the nodding around makes it very difficult to either side slip or do a decent stall turn in DCS - both manoeuvres are easy IRL.

[Art-J]

^ Well, Corsair is a bit of a the-other-way-around outlier here, 'cause its rudder has a much greater surface area than the fin, so I would expect the plane to be very responsive to initial rudder input and have a poor directional static stability in controls-free state.

Still, with virtual rudder released and centered firmly (as in the case of majority of flight sim pedals without FFB), we're back at fixed-controls directional static stability state. Should dampening be so unimpressive then? I don't know. Vertical empennage being unusually ahead of horizontal one doesn't help with figuring it out either.

[carbolicus]

11 hours ago, Art-J said:

^ Well, Corsair is a bit of a the-other-way-around outlier here, 'cause its rudder has a much greater surface area than the fin, so I would expect the plane to be very responsive to initial rudder input and have a poor directional static stability in controls-free state.

Still, with virtual rudder released and centered firmly (as in the case of majority of flight sim pedals without FFB), we're back at fixed-controls directional static stability state. Should dampening be so unimpressive then? I don't know. Vertical empennage being unusually ahead of horizontal one doesn't help with figuring it out either.

True enough re the large rudder, but you’ve still got the surface area of the fuselage providing a damping effect as well.  Also in-flight cockpit videos of Corsairs in the circuit show no sign of the nose nodding around, but the pilot must be using the rudder frequently with all the power changes going on.
But hey, I’m no expert aerodynamicist, and as you say, there are other design factors which might make a difference.  Just find it hard to believe that any real aeroplane would be passed by a test pilot as acceptable for an average pilot to fly when it seems to shear around so much!

Need some input from a real Corsair pilot here I guess!

[GTFreeFlyer]

I have been doing aerodynamics and aircraft sizing and configuration IRL for the past 20+ years and can chime in here.  The size of the vertical surface alone tells you nothing.  Please don’t make any assumptions based on how big that thing looks (insert “that’s what she said” joke here).  You have to take the tail arm into account as well.  This would be the distance between the wing and tail’s aerodynamic centers (tougher to compute, but close to the 1/4 chord of each).  Also, there’s vertical surface ahead of the wing center (lonnnng nose) that will take away from the tail’s effect.  
 

The combination of all this is taken into account when computing the “vertical tail volume coefficient”.  Someone here can research that, and then use three-view drawings to estimate areas and lengths if they want a fun activity.   You can take that coefficient and compare it to some known coefficients of other similar aircraft.  
 

As for damping, that goes along with what I mentioned above, but one more thing to consider is inertia.  In yaw, there’s a big ol’ massive engine being swung around.  If you aren’t slow and smooth with it, it’s going to be tough to tame. 
 

Wish I could fly a real Corsair to verify, but I used to have scale RC Corsair that exhibited similar behavior.  Even when comparing to other scale RC warbirds I’ve flown, the Corsair was always the most “wiggly”.  Yeah, small scale planes aren’t representative of full scale, so take that for what it’s worth.  At least in comparing it to others of same size, the characteristics match my experience in DCS.

 

I haven’t really had any issues keeping the nose steady with some curves on the axis, and some good rudder pedals that allow me to be smooth and precise.  I can see it being an issue with my old cheapo pedals (CH), or even worse, twist sticks.  I definitely suggest playing with the curves and just training the muscles to be smooth, with slow movements.  Push a very little, wait a second, repeat. 
 

I find it not fair to compare to the other warbirds.  Each plane always has its own quirks to learn which is why we all love picking up new planes in the first place.  I think it would be quite boring if they all flew the same, not to mention, the Corsair would never have been so legendary if it was exactly like everything else in the sky.  Every time I hop into the Corsair in VR, I feel like I’m part of that amazing history.

 

Anyway, if you are taking on my “tail volume coefficient” challenge, feel free to ping me for help if you get stuck with something.  

[carbolicus]

Thanks for the info GT, very interesting.  I’ve certainly tamed it with curves and I really like flying it in DCS, so I think the developers have done a great job!