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Posted by XXXmags | Oct 26, 2015 @ 04:49 PM | 1,334 Views
A low Reynolds number airfoil should be very thin. While this is wise from an aerodynamic point of view, it's a poor choice from a structural perspective. A rotor blade can be thought of as a long, thin strip. This shape has the characteristic of being very torsionally flexible about it's length. Making this strip even thinner, as the aerodynamics recommend, makes the strip even more flexible. This poor torsional rigidity makes the choice of airfoil a critical consideration. As was seen before, a cambered airfoil is desirable for good rotor performance. This type of airfoil produces a torque that will bend a rotor blade to a lower angle of attack, the effect increasing with airspeed.

Low torsional rigidity of a blade and camber pitching moment

This reduction in angle can be very detrimental to the power a rotor can extract. As this power extraction directly affects how much lift a rotor can produce, the blades should be as rigid as possible to avoid this effect. There are two solutions for this: The blades are constructed out of a very rigid material, or they are somehow made thicker. Two questions arise from this though:

1) What material is rigid enough, yet light enough to be used sizeably, to make a thin cambered strip stiff in torsion?

This directly points to a composite material, carbon fiber seeming like the first choice. As I didn't have carbon cloth and resin, the only sources of sizeable CF I had were a 0.5mm thick sheet, and a set of broken T-rex 450 blades....Continue Reading
Posted by XXXmags | Oct 26, 2015 @ 04:20 PM | 1,285 Views
The force an autogyro rotor produces can be assumed to be perpendicular to the plane of the disk. This force increases with the angle of the disk relative to the velocity of the air past it. I'll call this angle of attack.

Rotor disk angle of attack

This force can be divided into two components, one perpendicular to the velocity (lift) and one parallel to it (drag ). As a result, at very low angles, most of the force it will produce lift. At larger angles, more of the force is redirected backwards, increasing the drag.

Rotor disk force components

The lift to drag ratio compares how much lift is produced for a given drag, as such, it is a direct indicator of rotor efficiency. Consider this:



L = Lift force
D = Drag force
LD = Lift to drag ratio

For a gyroplane to fly, the lift must equal the weight of the machine:

L = w = m*g


m = Mass of gyroplane
g = Acceleration due to gravity

The drag must be counteracted by an equal force acting in the opposite direction. Otherwise the machine would slow to a halt and fall. As it needs to be moving in order to fly, it will require power:

Power = Force * Velocity

P = F*V

F = D -> P = D*V


P = Power required to fly
F = Force being applied

Looking at the definition for LD, it can be seen drag can be written in terms of lift:

LD = L/D -> L/LD = D

D = L/LD

As power depends on drag:

P = V*D = V*L/LD

As can be seen, LD is diving the...Continue Reading
Posted by XXXmags | Oct 26, 2015 @ 03:35 PM | 1,315 Views
After knowing the Reynolds number range, it was time to select an airfoil. I didn't find many papers regarding 50k Re. Most of what I found related to airfoils at Re < 10k or Re > 200k, both far away from my point of interest. I did find one paper, by Thomas J. Mueller, which was very good and targeted the 50k range in detail. He experiments with the effect of camber as well as trailing and leading edge shape. By his tests a circular arc, 4% cambered plate with 1.93% thickness performed well at 60k. This is very noteworthy as it wouldn't be the first time someone made the same comment.The rest of my research came from reading RC groups.

Threads that stood out on RC groups:

I need a glider airfoil for very low Reynolds number
Very low Re airfoil wind tunnel data?
Micro airfoils
The perfect airfoil for micro stuff..
Airfoil Performance at Very Low Reynolds Numbers
Why is flat airfoil good for indoor 3D????
Sources for Peanut and Bostonian Airfoils <- This thread contains some very good data on thin airfoils. Highly recommended.
Airfoil for 24" wingspan biplane. This thread contains Jef Raskin's description of the 4% cambered airfoil. A very good read.

Papers of interest:

The paper by Mueller:
Aerodynamic measurements at low Reynolds numbers for fixed wing micro-air vehicles

Michael Reid thesis on Low Reynolds number airfoils (Very useful information. Pages 21-22 are very interesting ):

Very slight camber seemed to provide the best results. To prove this for myself, I set out to do an experiment.
Posted by XXXmags | Oct 26, 2015 @ 03:23 PM | 1,319 Views
Originally posted here:


In this blog I'm going to post my experiences with making light ( sub 120 gram ) autogyros out of materials and equipment I have stored over the last few years. A lot of what I will post will be in retrospect as I've already done it, but the last updates of this thread will be more recent. I will slowly post all the steps I've taken up to the point I get something to fly.

This is the second time I've ever tried making a model autogyro, my first attempt was in 2008 with a micro ( sub 30g ) RET single rotor gyro. It never flew. Since then I've played a lot with helicopters, and this has certainly helped me with autogyros.

One of the things I did (and still do ) is play a video game called Garry's mod. Although it isn't purposely designed for this, one can build and play with mechanical systems that are mostly accurate with real life. I spent a lot of time attempting, and eventually getting an autogyro fly to in the game. One of the things I learned was the critical importance of rotor blade lift to drag ratio. This was the key to successful flight in-game.

Based on that experience, it seemed reasonable to me that it should be the first thing I should focus on. After reading comments online, many of those in Mnowell's aerodynamics thread (Thank you for posting this), I made up my mind that designing the blades should be the first step.

The first thing I thought was: At...Continue Reading