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Whitestone

Bell 206B Weight And Balance Envelope.

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Can anyone explain why once you take a door off the Bell 206B the center of gravity envelope is restricted? It has always perplexed me as the door weights maybe 11 pounds. Perhaps someone has even gotten an answer from Bell at some point?


Thanks for your thoughts,


W.

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Can anyone explain why once you take a door off the Bell 206B the center of gravity envelope is restricted? It has always perplexed me as the door weights maybe 11 pounds. Perhaps someone has even gotten an answer from Bell at some point?
Thanks for your thoughts,
W.

 

Do your weight and balance calcs with no door with say a 200 lb. load on the hook and you'll find that your long is over 110 pretty much all the time. Any load over roughly 40 lbs. will put you into that restricted area. This of course depends on the C of G of said ship. In the 20 or so 206's I've flown, pretty much every one of them had this issue. The grey question here is do we factor the weight and arm of the external load into the ship... I'd say of course but this means we must keep our doors on.

As for your question, a Bell guy explained to me it's not the weight but how the airflow is effected around the fuselage at speed. Not really an issue when long lining of course which is probably why we've always overlooked the whole thing I went on about earlier. How C of G works with speed... the faster you fly, the more forward cyclic is required. Your neutral cyclic position while hovering is governed by your C of G. I'd say it's restricted due to the new fore and aft limits of the cyclic.

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From the Bell 206 book......

 

The speed restrictions for flying with 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.

 

The cyclic is not necessarily at its most central point when in the hover. In fact, it is more likely to be central at around 40 kts.

 

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.

 

In forward flight with all the doors on, the air flows evenly over the horizontal stabiliser to create the lift required to keep the nose up in forward flight (the horizontal stabiliser is an inverted aerofoil). However, if you remove the doors the airflow at higher airspeeds (above 69 kts) now burbles back toward the tail and disrupts the lift over the stabiliser. Thus, as your speed increases, the lift created by the stabiliser decreases, which means that the nose will be too far down, requiring aft cyclic to correct it.

 

Because of this, the problem got called Cyclic Stick Reversal, but there has never been a situation where you had to push the stick forward to make the nose go up.

 

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. The Canadian Air Force apparently found in later models than the 206A that there was no problem out to much higher airspeeds, but if you nudge the envelope in this area, in a descending right turn, with doors off and 120 kts IAS, the adverse yaw will almost tip you out the door.

 

phil

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Thanks for the replies, i am collaborating on an excel spreadsheet weight and balance program for the 206B and L and was pondering the doors off limitation. It always seemed a bit pointless to me as i suspected it had something to do with airspeed but if you were to fly at your line limit speed it shouldn't be a problem but it would be nice if they amended the manual to reflect that.

 

Whitestone

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Forget the restrictions put on CofG with doors off, how about that ridiculous chart in the Aeronautical ***. supplement for the range extender. In our company we have two JetRangers that had to have an extraordinary amount of lead put in the nose to keep the aircraft within limits statically as per that specific chart. Needless to say range extenders had to be removed otherwise the aircraft was put into an extreme nose heavy situation.

Any others had to deal with this?

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