## How to calculate force per unit length of a helicopter blade in hover

## How to calculate force per unit length of a helicopter blade in hover

(OP)

Dear members,

I am trying to find out the force per unit length of a helicopter blade while in hovering. I know from blade element theory how to calculate the lift force (1/2*rho*v^2*....)generated by one blade. For example, if i have 4 blades, then, the total lift force=weight of the helicopter=4xone blade lift. Now, if the total weight of the helicopter is 20000 N, can we say that each blade is taking 1/4 of the of the weight ie, 5000 N? On the other hand, from the disk loading, we are getting the pressure and if we multiply that pressure by one blade's planform area, we are getting different lift value per blade. Does it mean that, while rotation, the lift force is distributed through out the disk evenly so that each blade is loaded lightly as can be seen from disk loading? Or the 1/4 sharing rule is correct (this is around 12-15 times greater than disk loading lift)? To calculate he force per unit length of one blade, which formula should we use? And if anyone of the two is selected, then how to calculate the force per unit length? Could you please explain? I appreciate your help.

Thanks

Pratik

I am trying to find out the force per unit length of a helicopter blade while in hovering. I know from blade element theory how to calculate the lift force (1/2*rho*v^2*....)generated by one blade. For example, if i have 4 blades, then, the total lift force=weight of the helicopter=4xone blade lift. Now, if the total weight of the helicopter is 20000 N, can we say that each blade is taking 1/4 of the of the weight ie, 5000 N? On the other hand, from the disk loading, we are getting the pressure and if we multiply that pressure by one blade's planform area, we are getting different lift value per blade. Does it mean that, while rotation, the lift force is distributed through out the disk evenly so that each blade is loaded lightly as can be seen from disk loading? Or the 1/4 sharing rule is correct (this is around 12-15 times greater than disk loading lift)? To calculate he force per unit length of one blade, which formula should we use? And if anyone of the two is selected, then how to calculate the force per unit length? Could you please explain? I appreciate your help.

Thanks

Pratik

## RE: How to calculate force per unit length of a helicopter blade in hover

using the disc area (ie the area swept by the blade) to derive a pressure, and applying that to the blade area sounds unreasonable. Calc the pressure from disc area and apply this to a quadrant (ie the area swept by a blade). Better would by to use disc area and factor the pressure by the activity factor (the area of the blades/disc area).

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## RE: How to calculate force per unit length of a helicopter blade in hover

I doubt the lift is evenly distributed; at least I haven't noticed a significant washout like is seen in propellers.

I will guess that there is some amount of washout to even the loading, but it makes most sense that that would be a cruise condition where efficiency is important, and that hover will not be ideal and will shift the load in some direction. One can assume that the AoA and section are constant, so the lift will vary as the square of the radius. One place I just searched to suggested 6 degrees of washout; you can look at the Cl/Cd charts to make an estimate on the size of the effect for your case.

## RE: How to calculate force per unit length of a helicopter blade in hover

Suggest downloading/reviewing documents from DTIC. Answer probably lies within...

http://www.dtic.mil/dtic/

Search AMCP engineering design handbook helicopter [or possibly the AD number listed]

AMCP 706-201 ENGINEERING DESIGN HANDBOOK. HELICOPTER ENGINEERING. PART ONE. PRELIMINARY DESIGN [ADA002007]

AMCP 706-202 ENGINEERING DESIGN HANDBOOK. HELICOPTER ENGINEERING. PART TWO. DETAIL DESIGN [ADA033216]

Regards, Wil Taylor

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