B206 Forward Doors Off Vne 69KIAS

[ANCIENT]

FM WARNING
Airspeeds in excess of airspeed limitations door(s) off will cause cyclic fore and aft stick reversal and fuselage buffeting.

Why?

Thanks

[Disguise Delimit]

Why?

Because at some stage of the initial test flying and exploration of the flight envelope, they found that a restriction was necessary.

You will find, if you nudge the envelope in that area, that a descending right turn, doors off, IAS of 120 kt, the adverse yaw will almost tip you out the door. Never found any fore/aft control reversal, but the yaw behaviour can be exciting. Buffeting can be as advertised, too.

Not that I have ever exceeded the FM limit, of course........:8

[paco]

The restrictions for flying with front doors off are due to possible controllability problems when the C of G is too far aft, or where aircraft response will not always follow a predictable pattern for a given cyclic input. In addition, the back doors, with the original handle design, tended to blow open in flight, and the changed airflow over the static ports may cause the altimeter/ASI to misread slightly.

In other words, the anticipated rate of speed increase or decrease with movement of the cyclic forward or backward may not be achieved with doors off, due to disturbed airflow. It is a carry-over from early versions of the 206A which, when flown at very light weight with the doors off, had negative static stability, meaning that the fuselage did not try to return to its original position after a disturbance (helicopters are normally statically stable). For example, if you settled down at 60 KIAS, then accelerated to and held 70 KIAS, the longitudinal cyclic position would actually be slightly aft of the 60 KIAS position. Because of this, the problem got called Cyclic Stick Reversal, but there has not been a situation where you had to push the stick forward to make the nose go up. The negative static stability may arise from the effect of airflow on the horizontal stabiliser.

As the 206 series increased in empty weight, or if you flew it at something other than empty with nearly no fuel, the problem went away, but it stays in the supplement as a restriction.

Does that help?

Phil

[NickLappos]

The stick reversal is probably longitudinal static stability, a property that you can hardly feel, but which achieves near religious importance to qualifying agencies, because it is in the regs and is easy to test. It is almost impossible to feel unless you conduct the test, which is not natural and relates only slightly to actual helo flight.

To conduct the test, stabilize at a speed, take your hand off the collective, and now push to 15 knots faster. If when you stabilize at 15 knots faster, your stick is slightly aft of where it as at the other speed, you have "stick reversal." Of course, if you have a high drag configuration, your rate of descent will be about 800 to 1000 feet per minute, so the fuselage will be at a big angle of attack, and the horizontal tail will get an up force, so it will tuck the nose a bit, and this will make you pull the stick back a bit, and everybody will die. (no I am kidding, it feels perfectly normal). If you dont fly with cyclic trim on, you cannot feel the stick reversal, and if you need more speed cues than the horrific rate of descent, you are numb and hopeless.

Why do I say these things? Because static stability never drive a pilot's workload, since the Dynamic Stability of the helo is so awful he never gets to feel the static stability.

[Shawn Coyle]

Personally, I think the 'stick reversal' wording is potentially damning. What if someone took it to mean that if I push forward, the nose goes up?
The 'negative static stability' wording is much more correct, and I know the Canadian Air Force found that in later models than the 206A, that there was no problem out to much higher airspeeds.

[Scissorlink]

My question is why 69 knots..why just not make it 70 ??

SL

[Aesir]

69 kts = 80 Mph

[lup]

Flew about 3000 hours on the 206 in the Middle East with front door off, sorry but was the only way to stop frying!!!

Never had a problem myself, also power boat races with all doors off, with some hairy manouvers, no problems.

In answer to the 69 knots, have you not noticed how many limitations are odd hard to remember numbers, to keep you on the ball for those check ride questions?

[inmate]

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.

[Aesir]

*inmate* Well explained now I remember it to from Bell school.

[paco]

Yes, thank you - I will add it to my stuff

Phil