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HeliTorque :: View topic - Doors Off VNE / Cyclic Stick Reversal
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HeliTorque Forum Index » Flight Dynamics

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Doors Off VNE / Cyclic Stick Reversal Goto page 1, 2, 3, 4  Next
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animalsticks
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PostPosted: Fri Feb 13, 2009 12:34 am    Post subject: Reply with quote

'Control reversibility' - is this the same thing that limits the b206 r44 etc back doors off speed?
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PostPosted: Fri Feb 13, 2009 10:00 am    Post subject: Reply with quote

Animal

No nothing like it the 'Control Reversibility' or Jack Stall problem is one where the hydraulic jacks can simply not provide enough force to oppose the force applied to them by the rotor blades throuugh the pitch links, if they were meatier (my term) and pushed harder the problem would not exist (other problems might at those kind of loadings, but this one wouldn't).

I have stolen a quote from Shawn Coyle from another forum where he succinctly explains the 206 problem of stick reversal.

Quote:
The reason is quite simple.
On the bell 206A, at very light weight and aft CG, with the doors off, the aircraft has negative static stability - ie when you start from 60 knots, you have to push the stick forward to go faster, but when you stabilize at 70 knots the stick will be slightly farther aft than at 60.

Negative static stability was not permitted by the regulations, but the choice of words is awful - stick reversal is not a term that is used anywhere else, and leads one to think that if you are in that situation that pushing forward on the stick will bring the nose up, which of course is patently wrong.

The Canadian Air Force did some tests on the Bell 206BII, which is heavier and when fitted with radios and an interior has a more foreward CG. They did tests to 100 knots and found the handling acceptable.
Because of commonality, and a desire to keep costs down, the doors off supplement from the Bell 206A has been kept.

But I often wondered if some lawyer wasn't going to take Bell to the cleaners over the 'stick reversal' term.


I hope Shawn doesn't mind me nicking this, but its a brilliant explanation from a Test Pilot of something that is surrounded by myth in the 206 world.

Is there a 'Stick Reversibility' issue on the R44 ?
Or is there a link being made up between the two doors off VNEs by instructors who need to find something to say about it ? I don't know.
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PostPosted: Fri Feb 13, 2009 11:10 am    Post subject: Reply with quote

Thanks veeany nicely written piece.
yeh i understood the principles laid out in the AAIB report re; jack stall, servo-transparency, control reversibility - whatever you want to call it.
your article cleared up my uncertainty regarding the b206. if the case isn't the same with the 44 why's the vne lowered for doors off too?
recirculation perhaps?

Very Happy
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PostPosted: Fri Feb 13, 2009 11:39 am    Post subject: Reply with quote

Without checking, I seem to remember that the R44 limits were something to do with the structural integrity of the airframe.

....

Having now checked my Robinson Safety Course diary (4 years ago) I noted the following:

Quote:
For the R22, VNE is simply determined by the forward cyclic travel. For R44, it is to do with fatigue loading, which is why there are several different VNEs.


Not especially detailed, but enough for me.
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PostPosted: Fri Feb 13, 2009 12:39 pm    Post subject: Reply with quote

Yeh that's what i remembered too - do you think that its the air pressures through the cabin creating the loads on the airframe, can't imagine the doors giving much more strength to the frame?
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PostPosted: Fri Feb 13, 2009 1:26 pm    Post subject: Reply with quote

What springs to my mind is that the extra drag created by not having the doors would require a more thrust from the rotor system to overcome. That of course puts extra loads through the drive and engine systems.

But that would be about the extent of my guess - I'm a computer programmer, not an aerodynamicist!
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PostPosted: Fri Feb 13, 2009 3:26 pm    Post subject: Reply with quote

James T Lowe wrote:
But that would be about the extent of my guess - I'm a computer programmer, not an aerodynamicist!

Neither am I. Laughing

Unfortunately, my rep. over at Bell is out of the office, but I have a call in to him.

It is an interesting question for which I have no definitive answer at this time.

It could have something to do with the "maximum" tilt allowable on the rotor plate when in forward flight. Doors off creates more drag which necessitates more forward cyclic. It could be the reduced Vne is simply a safety cushion to limit the possibility of mast bumping.

Who knows. I only fly the things, not build um. Laughing Laughing

gotta go now.... Helicopter
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PostPosted: Fri Feb 13, 2009 3:36 pm    Post subject: Reply with quote

AB

Its the Robinson answer that elludes us not the Bell one, Shawns text that I posted on the last page is from quite a bit of experience of test flying in Bells.

GS
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PostPosted: Fri Feb 13, 2009 3:40 pm    Post subject: Reply with quote

veeany wrote:
AB

Its the Robinson answer that elludes us not the Bell one, Shawns text that I posted on the last page is from quite a bit of experience of test flying in Bells.

GS

Yes, I know, but both helicopters have the same type restriction, so the prnciple should apply to either.

My Bell Rep just called and said he does not believe it to be a structural restriction, but does not have an exact answer. He will take the question to the engineers who put the restriction into the manual.

I can also call a contact over at Robinson for their view if you wish.

By the way, I did find Shawn's explanation excellent.

Wonder if Bell and Robinson will agree? This should be interesting. Laughing
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Last edited by afterburner on Fri Feb 13, 2009 3:55 pm; edited 1 time in total
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PostPosted: Fri Feb 13, 2009 3:44 pm    Post subject: Reply with quote

I was going to call a Robinson Tech man but you are in the same country and will probably have more chance of getting through (and its cheaper).

If you wouldn't mind i'd be very interested to see what Robinson say.

I'll be really surprised if its for the same reason (doesn't mean it wont be, just that I'll be surprised).
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PostPosted: Fri Feb 13, 2009 3:54 pm    Post subject: Reply with quote

veeany wrote:
I was going to call a Robinson Tech man but you are in the same country and will probably have more chance of getting through (and its cheaper).

If you wouldn't mind i'd be very interested to see what Robinson say.

I'll be really surprised if its for the same reason (doesn't mean it wont be, just that I'll be surprised).


Veeany,
Ok. I would try now, but there is a time difference between NY and Calif. The office is still closed. As soon as they open, I will put the call into them and post later on today.
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PostPosted: Fri Feb 13, 2009 4:17 pm    Post subject: Reply with quote

BELL HELICOPTER VIEW ON Vne, rear doors off
The main reason for reduced Vne with the doors off is to lessen the likelihood of seat cushions or belts being pulled outside the aircraft and possibly disloging, thereby creating a serious risk of ingestion by the tail rotor system. The "reduced" speed was that speed at which this condition did not significantly present itself.

Now if that doesn't sound like "legal" jibberish....what does.

However, it does make "some" sense. So, it's not structural, it has nothing to do with drag, nor does it pertain to mask bumping. (at least in a Bell). Laughing

I'll call over to Robinson after lunch. Rolling Eyes
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Last edited by afterburner on Fri Feb 13, 2009 4:22 pm; edited 1 time in total
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PostPosted: Fri Feb 13, 2009 4:22 pm    Post subject: Reply with quote

afterburner wrote:
It could be the reduced Vne is simply a safety cushion to limit the possibility of ......gotta go now.... Helicopter


Oh heck, at least I had a part of it right. Laughing Laughing Laughing Laughing
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PostPosted: Fri Feb 13, 2009 6:01 pm    Post subject: Reply with quote

I am going to stick my neck out now and suggest that your Bell man is mistaken.

I will report back shortly either way.
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PostPosted: Fri Feb 13, 2009 6:27 pm    Post subject: Reply with quote

I got an email back from Shawn who told me where to look and where he got it from.

Its in the flight manual supplement that covers doors off operations, FMS 24.

Quote:
This supplement will permit flight operations with one, all or any
combination of forward and/or aft cabin doors removed. The
limitation imposed by this supplement SHALL BE ABSOLUTELY
ADHERED TO FOR SAFETY REASONS
, as fore and aft cyclic
control deteriorates and cyclic stick reversal can occur.


He was told about it by the Test Pilot who did the certification flying with the doors off on the 206.


Sarah is it worth spliiting this thread up and leaving the Colin Mcrae stuff where it was and giving the Doors Off / Stick Reversal stuff a new one with a link to it ?
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