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Rule Of Thumb Or Old Wives Tale?


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It is highly likely that a rule of thumb used for JetRangers is not right. The rule in question is the one that maintains that 1% torque (power) is the equivalent of 32 pounds of lifting capability (thrust).

 

Wikipedia provides an article on momentum theory which is applicable to helicopter rotors. The article provides a formula which states that the power required to create thrust is directly proportional to the square root of the cube of the thrust. This relationship departs from the linear relationship suggested by the rule of thumb by a significant amount.

 

If an amount of torque is required to hover OGE, the amount of torque required to hover OGE at a greater gross weight can be calculated. The ratio of the torques is equal to the square root of the cube of the ratio of the thrusts. A table illustrates this.

 

Thrust Ratio Torque Ratio

1.0-----------------1.0

1.1-----------------1.2

1.2-----------------1.3

1.3-----------------1.5

1.4-----------------1.7

1.5-----------------1.8

1.6-----------------2.0

1.7-----------------2.2

1.8-----------------2.4

 

For instance, if 85% torque is required to hover OGE at 2800 pounds, then 104% torque is required to hover OGE at 3200 pounds. The thrust ratio is 3200/2800 = 1.14 and the corresponding power ratio is 1.22. 85% * 1.22 = 104%. Conventional wisdom suggests that the increase in torque should only amount to about 13% not 19%.

 

A table of values with finer entries for which the thrust ratios vary by 2% instead of the illustrated 10% could easily be constructed.

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Have to remember that it takes around 28% torque to turn blades at flight idle. I think this is called a rule of thumb for a reason. Same machine same weight same pad different day different wind and different torque daily......

Rule of thumb for power is that it usually takes 15% torque to get out of confined area. 15' TOT on 500D to do the same. 1.5%NG on Astar.....

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Have to remember that it takes around 28% torque to turn blades at flight idle. I think this is called a rule of thumb for a reason. Same machine same weight same pad different day different wind and different torque daily......

Rule of thumb for power is that it usually takes 15% torque to get out of confined area. 15' TOT on 500D to do the same. 1.5%NG on Astar.....

Nice rule of thumb Skully.

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It is only a 'rule-of-thumb'.....so use it accordingly.

 

That rule of thumb applies to all helicopters that use 100% torque as a limit,

i.e a Bell 212 has a max. gross wt. of 11,200 lbs, therefore each extra 112 lbs of weight will require 1 percent more torque.

 

It is close, but not perfect.

I won't use it as a guide for Hover OGE operations. If it is that critical, I will use the charts and then do a practice run first to see what the helicopter will really do.

 

However, I have used it as an aid to load the aircraft for a HIGE take-off.

But, as always, don't COMMIT to ANY departure until you are assured of a clear, safe profile.

 

Also remember that the 206B is allowed to lift 3350lbs gross, if externally loaded. But this is still limited to 100% torque. What does this do to the figures quoted above?

 

Some other related thoughts....

-Have you ever actually tried to hover OGE with the weight published in the charts? I have, and it is very rarely that the helicopter will do what is published.

-If you need 100% torque to do a HIGE take-off (at sea-level)....you are probably well over max. gross weight.

-Passengers are extremely heavy!! Don't ever forget it. It seems you can stuff the back seats full with groceries and fly away....but adding a few people will seriously affect performance.

 

OT

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