Force on a rotor blade

[joe@tech-mp]

I am trying to calculate the force seen by the blade root (area behind the blade grip bolt) of a helicopter rotor blade.
I would like to have these calculations verified as well as the approach by qualified individuals. Thank you in advance.

Given: W = 25grams (0.055 lbs); weight of the blade
L = 325mm (1.066 feet); length of blade
g = 32.2 ft/sec^2; acceleration of gravity
Z = RPM * 6.28/60; Rotor speed


The formula I found is F= W/g *Z^2 * L

So for 2400 RPM I get

0.055/32.2 * 63101 * 1.066 = 115 lbs

and for 3100 RPM I get

0.055/32.2 * 105278 * 1.066 = 192 lbs


Joe

www.techmodelproducts.com

[Jurgen Heilig]

Hi Joe,

Your calculation would be correct, if all the weight of the blade (25g) would be concentrated at the blade tip - which it isn't of course.

You need to calculate with the real radius of the CofG of the blade from the center of your rotor head (including the blade holders!).

As you calculated with almost twice the radius, your calculated forces are almost twice as high as in reality.

:) Jürgen

[joe@tech-mp]

Thanks Jürgen,

So I will find where on the blade the spanwise CG is located measure this length and then add the additional length to the center of the main shaft. This will give the blade length for use in the calculation.

Joe

www.techmodelproducts.com

[Jurgen Heilig]

Thanks Jürgen,

So I will find where on the blade the spanwise CG is located measure this length and then add the additional length to the center of the main shaft. This will give the blade length for use in the calculation.

Joe

www.techmodelproducts.com

Correct. Of course there are also other loads on the blades apart from centrifugal forces, but this is the major load for the blade root.

:) Jürgen

[JCaste]

Thanks Jürgen,

So I will find where on the blade the spanwise CG is located measure this length and then add the additional length to the center of the main shaft. This will give the blade length for use in the calculation.

Joe

www.techmodelproducts.com

If you want to do it well you should also consider the CG of blade holders and such.
You are calculating the centrifugal force of the spinning ensemble (which includes blade, blade holders, bolts, etc.) so if you aim for the maximum precision, you should consider the CGs of all those items respect the axis perpendicular to the rotation plane (= lenghtwise). The force is calculated with the CG of the ensemble, but what do plan to do may be a good aproximation.

BTW, yes, it's a lot of force! :D

[Jurgen Heilig]

If you want to do it well you should also consider the CG of blade holders and such.
...
BTW, yes, it's a lot of force! :D

If he just wants to know the force on the bolt holding the rotorblade, the weight of the rotor head, blade holder etc. does not matter.

:) Jürgen

[JCaste]

If he just wants to know the force on the bolt holding the rotorblade, the weight of the rotor head, blade holder etc. does not matter.

:) Jürgen
I assumed he meant the shaft. :o Anyway the blade holders may have some influence, quite small in any case, but some after all. :p