206 Doors off Fore and Aft cyclic stick reversal

[jemax]

Morning,

Doing a bit of doors off work in the jetranger tomorrow, just checking my VNE (69kts) doors off and the flight manual warns of cabin buffeting, I get this but what exactly is fore and aft cyclic stick reversal and why does it happen, just curious.

Thanks

[170']

Not as drastic as it sounds Jemax...

The airflow change with the front doors off plays with the aerodynamics, and what you end up with in a light all up weight condition, is that the cyclic can be further forward or aft than it's previous position after accelerating or decelerating... so if you fly 40 and accelerate to 60, the stick might be displaced further back, even though your going faster...We're talking millimetres not inches...More learned folks can explain the technical side, but from your point of view, not much happens...

External load Pilots have been known to exceed 69kts in jetbangers with the doors off and don't really notice the effect...So I'm told :rolleyes:
(observation not recommendation)


Have fun...170'

[SASless]

170'.....amazing what one can learn after reading the book.....operative word being "after".:O

For buffetting....try a 500E with front doors off at 130 knots! Gets rather noisy due to the wind slapping about the front end.

[inmate]

This is from some time age so hope it helps:

The Reason

--------------------------------------------------------------------------------

Okay this question was asked at Bell a few years ago and this is the answer recieved, hopefully sometimers hasn't dulled my memory.
My apoligies for the long winded answer.

Firstly when one reads the statement of "cyclic reversal" one thinks of all sorts of sound barrier, Chuck Yeager events, clear your minds rotorheads nothing to do with it.
Also remember this was a lot of years ago and helicopter flight envelope was still fairly new.

This explanation is in three parts, all intertwined but need to be laid out seperately.
Firstly:
A helicopter traveling forward at any speed requires imput of the cyclic to change the disc. Speed is increased by more forward cyclic imput, this places the nose in a very low nose down attitude and you hanging on the shoulder straps. Not a very comfortable position to be in, so to compensate for this Bell added the horizontal stabalizer. By inverting the aerofoil shape they actually applied a downward force which in turn brought the nose back up to a more comfortable position in cruise flight, (it also compensates for other flight characteristics but not for this discussion)

Secondly:
In forward flight with all the doors on the airflow over the body of the helicopter is smooth and in turn flows evenly over the horizontal stabalizer thus creating the lift required. However if you remove the doors (especially rear) the airflow at higher airspeeds (above 69knts) does not flow smoothly but actually burbles back toward the tail and therefore disrupes the even lift over the stabalizer. Due to this disruption you can experience a loss in lift and therefore in theory as speed increase, so lift decreases (over the stabalizer) and the nose will tuck requiring aft cyclic to correct.

Thirdly:
Keeping in mind the above you need to draw a graph with the vertical line representing 10% segments and the horizontal line airspeed in 10mph segments.
What bell did was, on the ground place the cyclic in the far aft stop which then represented zero/zero on the graph.
They then measured the % of forward cyclic movement required to maintain a hover, then at 10mph,20mph and so forth.
As an example if you say at the hover it takes 15% cyclic, 10mph=20% and so on you can see that the graph line climbs as the cyclic is moved forward and speed increases.
Now the theory at that time was that at a certain speed lift started to decrease (second explanation) so lets say its the magical 80 mph and to stop the nose tucking the cyclic was required to be pulled aft.
On the graph this will be seen as a reversal in cyclic movement as speed passes the magical 80mph thus the explanation of the statement.

It probibly could have been explained a little better but I'm an old fart,grumpy and going off shift.
Fly safe.

[Shawn Coyle]

The term 'cyclic reversal' is extremely misleading.
The correct term should be 'negative static stability' - it means that having noted the stick position at 60 KIAS and with constant collective, accelerating to 70 KIAS that the stick position to maintain 70 KIAS will be slightly aft of that to hold 60 KIAS. If you tried to go to 80 KIAS with the same collective position the stick would be farther aft to maintain 80 KIAS. The amount is very small, and hardly noticeable by most pilots (since there is no force gradient or built-in stick position indications).

It's a certification requirement.

It most certainly does not mean that if you go to 70 knots and the nose starts to go up that you should pull back to recover....

[jemax]

Curiosity satisfied, many thanks

[delta3]

Adding to inmate's explanation

Second point: I personally think that the drag increase of the hull is the most important factor. This makes the noose go down and makes the horizontal stabiliser work harder. Loss of efficiency of the horizontal stabiliser because of turbulence looks more remote to me.

m2c, d3

[Shawn Coyle]

The problem evidently only exists at very light weight in a 206A model. If you're heavy, it's not a problem.
The Canadian Air Force looked at this problem on their 206BII trainers in the mid-80's and went considerably faster and found no problem.

[delta3]

Beats me...

If you are light, the rotor disk will tilt a lot more (requiring also more forward cyclic) at the same (high) speed,
The heli noose down pitch will also be higher.
Would indicate to me that the stabilisers are near stall (have no B206 data, so answer is qualitative)

Any small changes in the stabilisers between models?

d3

[CYHeli]

I assume gents that we are still suggesting that pilots should fly the A/C within the limits specified within the FM/POH?

Considering the history of rotory flight and the advancements made over time, it's a pity that some of these limits can't be reviewed.

It's now possible to take all doors off an R44, an AS350 and a B206 and the R44 will get there first! But on the B206, at least, the limit is related to old, possibly changed data.
Am I missing something here?
I understand that with the finish of the B206 series, that there is no value in Bell putting the current model through a new lot of tests, but does the 407 have the same doors off limits?

[407 too]

407 - 100 kts

[Pilot DAR]

For buffetting....try a 500E with front doors off at 130 knots! Gets rather noisy due to the wind slapping about the front end.

Is the 500E approved for two front doors off? I had to specially approve the 500D for two front doors off, as that was not an approved configuration, and I limited the Vne in that configuration.

Pilot DAR

[rotors88]

The Kiowa OH-58 does not have the doors off VNE (OH-58 is basically the same airfame as B206). I know of a number of Ag Pilots who routinely fly at 100+knots in the ole Jetbanger with no probs.;)

[ramen noodles]

Shawn Coyle has it right - the cyclic reversal is just a small feature of the handling that most pilots would not be able to detect. Basically, in the condition where there is negative static stability (cyclic "reversal") the airspeed woul seem slippery and hard to control. If the pilot glances away from the airspeed indicator for several seconds, the speed would slip up or down a bit and seem squirlly.

This is surely due to the fact that bad air spills around the open doors and causes the horizontal tail (the source of the static stability) to lose effectiveness. As that turbulent air washes over the aft fuselage, it disrupts the smooth stream of air that the horizontal tail needs to work its magic.

The FAR and JAR require positive static stability, so the doors open condition has a max speed, above which the airspeed stability is poor.

Frankly, static stability is perhaps the least important handling standard, because most pilots have no earthly idea where the stick is trimmed, they fly (rightly) so that a forward push moves the nose down and accelerates the speed, and an aft push does the opposite. This is still true even if horrendously negative static stability is present.

Most of the stick behavior FARs are hold-overs from the time when regulations asked helos to fly like airplanes, an impossible dream, at best.