Tech Pro Blades Support                              

 

This page has information supporting our Tech Pro Blades product.

PURCHASE Last updated on: 8-30-09
 

 
   

Specificatons

Tech Pro blades are PVC covered wood blades and come in five colors and three sizes. 

Colors:  White, Yellow, Red, Black and Blue

Sizes: 315mm, 325mm and 335mm

Airfoil: Fully symmetrical

Configuration: Constant cord 31.5mm wide with a weight range of 16.5 grams to 19.5 grams

Weights: 2 lead strips along outer leading edge totaling 3.1 grams

Wood Species: Alder

Maximum safe RPM rating is 2400 (See Testing section below)

 

Construction

The blade root has both the top and bottom plastic grips glued on and screwed in place. 
Each blade is formed from a single piece of wood and sanded smooth.

 
   

Blade root construction
 

   

Top side blade tip
 

   

Lead weights (3.1 grams) are installed on the underside (glued in)

Testing


We built a test fixture called the "Blade Ripper" to determine the force required to pull the blade off the blade holder bolt.
The jack simply applies force through the blade which is measured by a digital force gauge (scale).  The scale has a
peak hold function which records the maximum force before the blade gives out.

 
   

 
     

 

Blade Testing Results


Our testing has shown that the Tech Pro blades and the Align Pro blades are about the same strength.
For each, the minimum breaking force was about 212 pounds and maximum was about 233 pounds.
We can say that the static breaking force is above 200 pounds for both brands of blades that were tested.

Note:  These numbers are based on a total sample of 20 blades tested.

 

Blade Force Calculations - Centrifugal Force

The force applied to the blade root (behind the bolt in flight) can be calculated with the formula below.

Fb = W / G x Ω x Rcg

Where

Fb   is the force in lbs
W    is the weight of the blade in lbs
G     is the acceleration of gravity = 32.2 (ft/sec)
Ω     is the rotor speed = RPM x 2π / 60 radians/sec
Rcg is the distance from the center of the main shaft to the CG point on the blade.

 
Example 1  Given:  325mm blade, 19 grams, RPM = 2400

W     = 19 / 454 = 0.041 lbs
Ω      = 2400 x 6.283 / 60 = 251.33 radians/sec
Rcg  = 0.64 ft (for a TRex 450 with plastic head)

Fb = 0.041 / 32.2 x (251.33) x 0.64 = 51.47 lbs

The Force at 2400 RPM is about 52 lbs

  Example 2  Given:  325mm blade, 19 grams, RPM = 3100

W     = 19 / 454 = 0.041 lbs
Ω 
    = 3100 x 6.283 / 60 = 324.62 radians/sec
Rcg  = 0.64 ft (for a TRex 450 with plastic head)

Fb = 0.041 / 32.2 x (324.62) x 0.64 = 85.87 lbs

The Force at 3100 RPM is about 86 lbs


Summary

The testing was performed on a relatively small number of blades (20) so there may be smaller minimums and larger maximums
with respect to the breaking force.  There is no practical way to completely test these blades without damaging them.
Wood is a natural material that is not homogeneous.  Defects can be present which weaken the blade and cause it the break sooner.

The centrifugal force is not the only force on a rotor blade although it is the major one.  These other forces are dynamic and vibratory
in nature and although smaller in magnitude they are considerable since they have a jack-hammering effect. 
A safety margin must be allocated for them.

All wood blades deteriorate (lose strength) with use.  The harder they are worked the weaker they will get.

The centrifugal force is not a linear function with respect to RPM, it is exponential.  If you double the RPM you will quadruple the force.

These blades are made in a factory overseas.  We have no control of production or quality control.  Because of this, we cannot be held
responsible for any damages or injuries.  Use these blades at your own risk.

Given all of these factors, we recommend that the maximum RPM ratings on these blades be followed for safety.

 

Going forward

Our blade testing fixture can handle up to 3000 lbs of force which is limited by the scale.  We would like to test some of the
composite blades out there like the SAB etc.  Also, it would be informative to test some heavily used and abused blades.  
Accurate records of the blades history (# of flights etc.)  is required to extract any value from the test data.

We welcome your contributions to this effort. 

 
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