What (roughly) is the time period of the oscillation? Is it similar in time constant to hover instability? This is mostly in roll?
It sounds to me like a more extreme version of the hover dynamic instability from the delay in roll response to a lateral cyclic input. As the blades are operating at higher pitch than hover then the rotor (optimised for level flight) produces more lateral flapback for any given sideslip. So the dynamics of the whole rotor-fuselage interaction change to the point where stability augmentation is required.
Out of interest do you find that the longitudinal trim position for Vy changes with climb when compared with level flight? If what i mentioned is correct then i would expect cyclic trim position to be more rearward in climb.
Last edited by Graviman : 3rd July 2009 at 13:40.
Reason: Corrected terminology: Higher pitch for climb but same Rel_AOA to lift same mass (and same total pressure increase through rotor)...
DennisK
Yup Hughes300's will demonstrate it very well, never really sure what causes it, Iam convinced it is the wind and main rotor vorticee hitting the horizontal stab ( not horizontal so causes the problem for the tail) and then going into the tail rotor that causes the problem. Teach my students to stick the nose forward and pull power and it goes away.
What's happening with the Schweizer and others in the low-airspeed environment wouldn't qualify in the classical sense as Dutch Roll - it's a weird oscillation where the response in roll gets out of phase with the pilot inputs.
Just wondering if this might be the cause of several otherwise unexplained accidents in the hover / low airspeed environment...
Sorry, but you all have it wrong as there is only one Dutch roll - Ham en Kaas, and some foreigners were always spluttering in their best attempts at Cloggie demanding Geen Kaas! But give 'em their due - 20 years ago they had DDAFCS, Pratt & Whitneys, FMS and floppy downloads of flight logs ...... munch on that.
I am surprized that no one has referenced the installation of the ventral fin on the Super Puma (all variants) as compared to the standard Puma without the fin.
This was to counteract the D/R characteristics which existed on the standard Puma and could certainly be felt in cruise (fishtailing) when occupying the most rearward seats.
The ventral fin on the Super Puma was added in part to counteract the problem found at aft cg. If you rolled in to a turn and the ball was not centered (i.e. it was to the outside of the turn) in the original Puma, adding pedal would roll the helicopter quite quickly. Lateral cyclic would not stop the roll...
Aerospatiale called this 'le virage Anglais' - (the English turn) as it first happened to a Royal Air Force Puma which was fitted with the very large particle separators well in front of the CG.
The issue with the Puma has always been described as a 'yaw/roll divergence' rather than a dutch roll since it is very unstable.
I think the Puma fishtailing is just a case of yaw instability and a desire to weathercock because of the extra area forward of the C of G; this is why the large ventral fin stops it but it doesn't appear to match the classic dutch roll motion.
Carholme - there's no need to be like that, if you have more experience on a type than others here then say so, we are not mind-readers and most of us are always looking to learn and improve our helicopter knowledge.
What do you mean, "don't be like that"? All I did was offer my knowledge of the 330/332 with reference to D/R and you and Coyle subtly countered that to imply other reasons for the ventral fin.
If you feel those are the reasons for the fin, as I said, "have at er". I am not here to impose my experience or knowledge or impress anybody.
I am sure there are many pilots and engineers who have studied the autopilots in both aircraft and are familiar with their levels of capability and why the ventral fin was introduced on the 332.
Carholme - what I meant was that you made a statement about the ventral fin which to you was the absolute answer but, to people reading the post, was just the opinion of another guy on PPRuNe who may or may not know what he is talking about.
If you want everyone to know you have extra inside knowledge of the Puma/332 when you make a post then mention it at the time.
As a tech rep then are you a pilot/test pilot?
As Shawn says, this phenomenon has been widely experienced by many pilots and test pilots and the Aerospatiale answer for the Puma was, from what I have been told, yaw/roll divergence aggravated by the particle separators, fixed on later models with the large ventral fin.
You are referencing helicopters equipped with large particle separators installed however the 330 had this problem long before the separators were installed as experienced with Schreiner and Sealand 330s, both without separators.
I am a licensed engineer with UK and Canadian licenses and do not feel for one minute that my comments require one to be a test pilot for confirmation.
Carholme - so did the 330 receive the ventral fin modification before the particle separators were fitted or did the separators aggravate the existing yaw instability enough that something had to be done about it?
Ref the test pilot point - I was just trying to ascertain if you had any practical flying experience on which to base your comments on handling qualities - this is a professional pilots rumour network after all
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