Project Logs

Collapse

• 3rd Gen Rotor Design

  Peter McCloud • 03/24/2017 at 03:25 • 0 comments

  The design for the 3rd gen rotor has been finalized and is shown below. The upper portion of the picture shows the rotor with the machining supports present. This is what the rotor looks like coming out of the CNC Router. The lower portion of the images shows the finished rotor with the supports removed. The airfoils for the tip and the root are highlighted.

• RC series Airfoils

  Peter McCloud • 03/02/2017 at 05:09 • 5 comments

  The previous rotor designs used generic NACA 2412 airfoils. These airfoils were developed over 80 years ago, but they are well documented and good general purpose airfoils. However to get the most thrust, airfoils specifically designed for rotors are required. Searching around led to the RC series of airfoils

  RC series airfoils

  The RC series of airfoils are specifically designed for rotorcraft applications. The naming scheme is RC(X)-YY where X is the number in the series and YY is the airfoil thickness in percent chord. Two of the airfoils, designed for the inboard section of rotor blades are the RC(4)-10, RC(5)-10. Data for these airfoils is found in:

  NACA TP 3009 Aerodynamic Characteristics of Two Rotorcraft Airfoils Designed for Application to the Inboard Region

  The other airfoil is the RC(6)-08 airfoil, designed for rotor tips. Data for this airfoil is documented in:

  NASA TM 4264 Aerodynamic Characteristics of a Rotorcraft Airfoil Designed for the Tip Region of a Main Rotor Blade
  

  Applying the new airfoils

  Previously the rotor.py code was only written to use a single airfoil for the entire rotor. Recent updates allowed using two different airfoils, one a the tip and the second at the root. Changing the tip airfoil from the NACA 2412 to the RC(6)-08 means a 25% reduction in the chord. Since the bending moments are low at the tip, the thinner section doesn't affect the structural strength significantly.

  Changing the root airfoil from the NACA 2412 to the RC(5)-10 airfoil means a 17% thinner airfoil. It may not seem like much, but the bending strength scales by the thickness cubed. Keeping the chord constant and changing from a 12% to 10% thick airfoil means a 42% reduction in bending strength.

  To keep a similar strength with the thinner airfoil, the root chord will be increased from 4.5 to 5.4 inches to keep the overall thickness the same at the root. This leads to a negligible decrease in aerodynamic efficiency, but a big difference in the structural design.

  So how big is the difference?

  Rotor hover performance is measured by Figure of Merit (FM) vs. Efficiency. Hovering isn't doing any useful work so efficiency doesn't really apply. The FM is the ratio of the required power vs. the ideal required power. The previous rotors had a FM of about 60%. Analysis suggests the new rotors using the RC airfoils will have a FM of 80%.

• Rotor Blade Element Analysis (RBEA)

  Peter McCloud • 02/24/2017 at 04:20 • 0 comments

  Code Updates

  Recently, the rotor designs were re-visited to double check some of the design assumptions. In the process the code was polished up and some improvements were made.

  Bug Fixes

  Found that the code would not converge for some cases and the user wouldn't know. Not so much a bug as poor programming. Added logic to notify the user if the case doesn't converge

  Improvements

  Broke out the inputs into a file (inputs.txt), keeping the code cleaner. Additionally switched to using splines for the airfoil data instead of fits done in a spreadsheet. Added support for Mach and alpha airfoil coeffs vs just alpha.

  Additions

  Code for calculating blade moments

  Python Code (repo location)

  import sys
  import math
  import numpy as np
  import scipy.interpolate
  # Rotor Blade Element Analysis
  # Python Script for determining Rotor Performance using Blade Element Theory
  # Definitions
  # theta - The physical angle the airfoil is rotated from the plane of rotation
  # phi	- The change in flow angle due to the induced airflow.  Makes the angle of attack smaller
  # alpha_rad	- The effective angle of attack the airfoil experiences
  # alpha_rad + phi = theta
  pi = math.pi
  # Read inputs file
  execfile('inputs.txt')
  # Determine airfoil method
  try:
   airfoil_name
  except:
   # airfoil_name is not defined, see if tip & root airfoil names exist
   try:
    tip_airfoil_name
   except NameError:
    print 'Neither airfoil_name nor tip_airfoil_name in inputs file'
    print 'Exiting!'
    sys.exit(1)
   else:
    # tip_airfoil_name exists, see if root_airfoil_name exists
    try:
     root_airfoil_name
    except:
     print 'root_airfoil_name is not defined in the inputs file'
     print 'Exiting!'
     sys.exit(1)
    else:
     airfoil_type = 'blended' 
  else:
   airfoil_type = 'single' 
  # Read airfoil data
  if airfoil_type == 'single':
   airfoil = np.loadtxt('./airfoils/'+airfoil_name+'.dat',skiprows=1,delimiter=',')
   # Create splines
   mach_data = airfoil[:,0]
   alpha_data = airfoil[:,1]
   cd_data = airfoil[:,2]
   cl_data = airfoil[:,3]
   cm_data = airfoil[:,4]
   # Determine Mach Range
   if min(mach_data) == max(mach_data):
    # Single Mach Number present, Create 1D splines
    airfoil_data_type = '1D'
    cd_spline = scipy.interpolate.splrep(alpha_data,cd_data)
    cl_spline = scipy.interpolate.splrep(alpha_data,cl_data)
    cm_spline = scipy.interpolate.splrep(alpha_data,cm_data)
   else:
    # Multiple Mach Numbers present, Create 2D splines
    airfoil_data_type = '2D'
    cd_spline = scipy.interpolate.bisplrep(mach_data,alpha_data,cd_data)
    cl_spline = scipy.interpolate.bisplrep(mach_data,alpha_data,cl_data)
    cm_spline = scipy.interpolate.bisplrep(mach_data,alpha_data,cm_data)
  else:
   # Tip
   airfoil = np.loadtxt('./airfoils/'+tip_airfoil_name+'.dat',skiprows=1,delimiter=',')
   # Create splines
   mach_data = airfoil[:,0]
   alpha_data = airfoil[:,1]
   cd_data = airfoil[:,2]
   cl_data = airfoil[:,3]
   cm_data = airfoil[:,4]
   # Determine Mach Range
   if min(mach_data) == max(mach_data):
    # Single Mach Number present, Create 1D splines
    tip_airfoil_data_type = '1D'
    tip_cd_spline = scipy.interpolate.splrep(alpha_data,cd_data)
    tip_cl_spline = scipy.interpolate.splrep(alpha_data,cl_data)
    tip_cm_spline = scipy.interpolate.splrep(alpha_data,cm_data)
   else:
    # Multiple Mach Numbers present, Create 2D splines
    tip_airfoil_data_type = '2D'
    tip_cd_spline = scipy.interpolate.bisplrep(mach_data,alpha_data,cd_data)
    tip_cl_spline = scipy.interpolate.bisplrep(mach_data,alpha_data,cl_data)
    tip_cm_spline = scipy.interpolate.bisplrep(mach_data,alpha_data,cm_data)
   # Root 
   airfoil = np.loadtxt('./airfoils/'+root_airfoil_name+'.dat',skiprows=1,delimiter=',')
   # Create splines
   mach_data = airfoil[:,0]
   alpha_data = airfoil[:,1]
   cd_data = airfoil[:,2]
   cl_data = airfoil[:,3]
   cm_data = airfoil[:,4]
   # Determine Mach Range
   if min(mach_data) == max(mach_data):
    # Single Mach Number present, Create 1D splines
    root_airfoil_data_type = '1D'
    root_cd_spline = scipy.interpolate.splrep(alpha_data,cd_data)
    root_cl_spline = scipy.interpolate.splrep(alpha_data,cl_data)
   root_cm_spline...

  Read more »

• Rotor Repository

  Peter McCloud • 07/22/2015 at 05:08 • 0 comments

  A new rotor design github repository is being created to store the rotor information separate from the Goliath design repository. This new repository will contain the design files for the previous rotor design (alpha) and the new rotor design (beta). The repository will also contain the python program developed to predict the rotor thrust.

• Documentation

  Peter McCloud • 10/23/2014 at 11:15 • 0 comments

  I'm starting a separate project for the propellers that I've been building for #Goliath - A Gas Powered Quadcopter. As Goliath has progressed, the documentation of all the elements is becoming a bit unwieldy and the rotors are a project on there own. The plan is to more thoroughly document the rotors here than they have on Goliath's project log and also describe the project in more generic terms for anyone to apply towards their own applications.

View all 5 project logs