Well, the day finally has come when I am ready to give up on this endeavor. My goal was to build a manned, internal combustion quad copter.  I will not say the project was a failure because I learned so much on my journey, but I just see other avenues which seem more promising.

Of note, from the project I did successfully build a variable pitch, bearingless rotor blade, and if that is of interest to anyone, watch my YouTube channel for build info, or contact me.  I would be glad to share what I learned.  

During the project I also came up with a redundant servo design which I believe will have applications in any high value vehicle. 

The last test I performed on my rotor system failed because of a fracture in my test fixture frame.  The belt also fell off.  I don't know which was cause and which was effect, but I just got to the point where the attraction of other ideas seemed to outweigh the benefit of this pursuit.  And I realize with my limited amount of time I'm spending on the project, and the number of obstacles ahead, it would be unlikely I would see a full scale vehicle in the air.

The "other" idea I saw which sealed the deal was Blackfly.

When I saw it, I instantly knew this was a really great idea.  So much better than many of the other manned multirotor vehicles.  So now, I'm following this development and it has gotten me shifted into looking at the primary obstacles of electric flight.  

Thanks to those of you who offered support, and useful comments through this process.

Randy

Test Video

I was finally able to perform a test of the new drive test stand.  The previous test stand used a 4-stroke engine and a less rigid test frame.  This test stand consists of a 6.5 HP, 2-stroke DuraForce engine coupled to a V-Belt centrifugal clutch with a custom made drive shaft.  Couplings between the engine/drive-shaft and drive-shaft/clutch are stainless steel flex-couplings similar to the type used on the Robinson tail rotor shaft drive.  I added a large flywheel to the engine to attenuate the torque pulses from the engine.  The flywheel also makes starting the engine much easier.

The variable pitch mechanism has the following characteristics:

I chose to NOT include a dedicated follower link which forces the upper half of the "swashplate" to follow the speed of the rotor.  Instead, the attachment of the pitch links to the control horn on the rotors has only one degree of freedom, and thus uses the pitch links to transmit torque to the upper (rotating) portion of the swash plate to make it follow the rotor.  I plan to improve this design on my next rotor build, but for proof of concept, this seemed to work reasonably well.  The final product will have 4 blades on each rotor, so I will just need to make sure of proper clearance between the control horn of one rotor, and the flex-element of the neighboring blade.

The lower half of the swashplate is connected to a giant scale servo at only one point.  I was aware that this actuation force, being applied offset from the centerline of the rotor shaft, could induce some binding or sticktion in the variable pitch collective control.  I wanted to test this approach because it so greatly simplifies the connection between the servo and the swashplate.  To combat this tendency to bind, I designed the upper portion of the swashplate as a quite long "sleeve" which slides up and down on the OD of the rotor shaft. 

The "stickiness" issues I experienced during this test I believe are due to the following effects and design choices:

• The upper swashplate "sleeve" material is 6061 aluminum which has a quite high coefficient of friction with steel.
• The rotor blades, in their relaxed state, have only a slight pitch angle (3 degrees) but the design pitch is around 15 degrees, so the pitch servo and linkage must overcome the torsion of the 3/8" fiberglass flex-element in order to achieve the design pitch of the rotor blades.
• Additionally, when operating at design RPM of 1410, this blade induces approx 1600 Lbs of centrifugal force, or tension, on the fiberglass rod flex-element.  This force tends to make the blades want to straighten out, back to their relaxed position, and thereby increases the amount of force/torque required to achieve the design blade pitch.

To mitigate these issues, on the next hub design, I will attempt to:

• Lengthen the upper sliding section of the swashplate.
• Use lower friction brass or teflon sleeves as linear bearings.
• Build the rotors with the blades at flight operating pitch in their relaxed state.
• Keep the pitch servo attachment to the swashplate as close to the centerline of the rotor shaft as possible.

Another problem encountered during the test was the detection of false triggers from the engine RPM photo-interrupter.

The blue line is the engine RPM and the orange line is the clutch RPM.  You can see all of the high spikes which go off the chart for the engine RPM.  This means I was picking up false triggers on the photo-interrupter input.  My circuit does include Schmitt Triggers on the tachometer signals, which seems to be working well on the other two photo-interrupters, but was getting some noise on the engine tach.  I will have to hook up an oscilloscope to see what the issue is.

FYI, the Grey trace is the rotor RPM (approx 2.8 : 1 reduction from the clutch),

The cyan trace is the pitch servo signal in microseconds (inverted)

The yellow trace is the throttle servo signal in microseconds.

So while I discovered some issues (engine power, and pitch mechanism stickiness) I had basic success of proving that I could operate a home-made set of rotors, with an internal combustion engine, without excessive vibration.

I would still like to over-speed my rotors to ensure the safety factor ( I used a 4:1 safety factor over the static tensile strength of the flex-element fiberglass rod).  I would like to take the rotor up to 2000 RPM, which would put the tensile force at 3200 Lbs, - double the normal operating force, but still approx 1/2 of the ultimate tensile strength of the material.

Next steps:

• New carburetor for engine to get the power/RPM I need.
• Tuned exhaust for engine (again, for power)
• Run rotor at 2000 RPM to verify strength / safety factor.
• Add lower friction sleeves to swashplate to reduce binding
• Tilt rotor at speed to observe gyroscoping effects on vibration.
• Attempt to control/balance test stand attitude using closed loop control from a gyro. 

 For those who think this is an ill-advised project, please refrain from negative comments.  I readily acknowledge the difficulties that await me down the development road.  I am working this project for my own satisfaction, not for a commercially viable transportation vehicle of the future.  I am enjoying the process and the knowledge and experience I'm gaining at each step of the way.  If you have questions or encouragement, I welcome those comments.

Thanks!

I continue to develop my "full scale" gas powered, variable pitch quad-copter.

After watching the video below at 

Amazing DIY Projects

I realized (more vividly) how vulnerable a quad is to any fault in the control system.

Then I saw his video here --> Redundant flight controls where at around 23:30 in the video he informs that he will use quintuple redundancy.  That got me thinking about how I could follow the same approach using multiple independent flight controls to provide redundancy for the control system aspect of the project.  His project is different in that he has many outputs (~76 electric motors), but I only have 4 collective "swash plates" that I have to move, so the challenge was to come up with a linkage to connect multiple servos in a way that they won't fight each other, but will also be able to retain control if one of the servo's fails.

Below is an image of the non-redundant setup I'm using for collective pitch.

I struggled quite a bit with many different ideas, but finally arrived at this which I think is elegantly simple.

The servo arm is connected to a rod-end which is screwed into the bottom of the lower (non-rotating) portion of the swash plate. If the angle of the servo sweep is kept reasonable (60 to 90 degrees total) this arrangement does not create too much binding or bending of the servo arm.

Looking back at the first image, you can see that I've connected the servo arms together in an equilateral triangle arrangement, with the rod-end at the center of the triangle.  The motion path of the rod-end is exactly the same as it would be with a single servo.  Of course, this still needs to work if one of the servos fails. If that happens, lets assume the servo just stops moving, and stays in place (gear friction is greater than the linkage torque so servo doesn't "freewheel").

The issue here is that the rod-end only moves one-third the distance of any given single servo movement.  So with one failed servo, the rod-end will only move 2/3 the distance of the remaining two active servos.  For this reason, the servos should be adjusted so that they have 50% extra travel (only use 2/3 of their range when all 3 servos working). 

Another issue is tuning:  The system will have lower gain when one servo fails, so I will need to test the flight stability in both cases (all servo's working, and again with a failed servo) to ensure the PID loops will be able to safely control in both situations.

This design works best if the failed servo doesn't freewheel.  I'm don't think it will work if the failed servo doesn't freeze in it's failed position.

The idea here, is that each of these servo's would be controlled by a separate flight controller, so the whole control system would be tripley (word?) redundant.  Actually, this linkage concept could be extended to make it 4,5 or even 6x redundant, but only 3 servo's can be directly connected to each other by the "Y" linkage.  Beyond 3, I will need to join multiple linkages together.  

Below is a photo of my non-redundant collective linkage on my most recent test stand.  Getting ready to test the vibration of a 2 cycle engine driving a variable pitch 2 blade rotor.

Always interested to hear comments!

This video shows the fabrication of a prototype bearingless rotor hub/blade design.

The rotor diameter is 93 cm (36.6 in) and will operate at 1,410 rpm generating 2,000 g's of acceleration at the rotor tip, and developing 1,600 Lbs of tension in the flexelem rod due to centrifugal force.  The blade (pair) is designed to generate 104 Lbs of lift with a power consumption of approx 6.5 HP.

This is part of an "ultralight" quad-rotor design that I've been tinkering with over the past 18 months.  After running this rotor through some basic structural tests (40% overspeed) the true testing of pitch angle stability, pitch control forces, and flapping stability in forward flight will be tested on a test stand

Bearingless Rotor Assembly

This is an analysis of the variables which affect the ability to control a multi-rotor as the size is scaled up.

This will show why at some point, variable pitch must be introduced.

I hope to follow this with another article which includes Phase Margin and Gain Margin, but right now I'm trying to refresh my memory from 30 years ago on Laplace transforms and Bode Plots.   Ugh!

I hope this makes some sense.

MultiRotorControllability.pdf

Here is a concept shot of what is a continual evolution of designs.  This is way off in the future, but is the end goal of what I'm working towards.  Now that I'm starting to acquire actual parts and pieces I'm a bit concerned about the weight.  But first things first, and one step at a time.  I still have yet to build even one good rotor blade, but I do have a plan now for the first rotor set, and the variable pitch rotor head.  Drive system will need lots of testing but that is phase 2.  Phase 1 is just doing some performance testing on a single rotor head.  Determine lift, drag, power, roll rate, etc.

Here are some photos of my progress on the rotors for my full scale quad.  This quad design targets a 200 kg gross wt with 4 rotors of 2.5m diameter.  It's mot much progress, but if you're interested here are the pics.

Rotor Tip Weights

Rotor Tip Weight against leading edge (fiberglass) rod.

Layup of leading edge portion of airfoil.  Wrapped with 2 layers of 6 oz cloth at 45 deg angle to provide torsional rigidity,

Also, the back end of this section (or this whole assembly) creates a spar which gives rigidity to the blade.

Preparing to glass the tip portion of the leading edge spar section.  This will envelope the lead tip weight in glass.

Attaching the rotor tips (shaped wood pieces that rounds the sharp edges to reduce corner stresses on the fiberglass cloth skin.Main foam core placed in position behind the leading edge spar section.  Trailing edge is 3/4 x 1/16" aluminum strip with very thin tapered wood strips to give correct taper.

Glassed rotor (tip end).  You can see rotor tip cap (wood) and foam core with body filler in areas to smooth the surface.

Flashing of excess fiberglass skin has not been trimmed from trailing edge yet.  This glass was wrapped from end to end so as to give strength along the length of the rotor and support the centrifugal loads.  Tension in the glass skin is transferred to the inboard tip cap which rests against a composite beam, which in turn is bonded to the FlexElement bearingless hub element.

This is the FlexElement end.  Quilted blanket is part of my curing tent setup which allows me to cure the slow-set epoxy at 55C without melting the styrene foam core.

This is a picture of an assembly that is part of my full scale (human size) quad-copter rotor blade.

For background, the FlexElement is the structural member that will connect the rotor blade to the rotor hub or head. (shaft). The leading edge rod is an internal structure to give the leading edge of the rotor blade airfoil durability for small impacts.  The composite beam is a composite layup which holds it all together, but mainly its function is to transfer the centrifugal forces of the rotor blade (which extends to the left off the page) to the FlexElement.  

Originally I did a simple calculation to determine how much surface area I needed for the glue joint between the FlexElement and the composite beam.  This was based on the shear strength of the epoxy resin I was using.

A = F / U  where U is the maximum stress I want the glue to experience.

In my case F=1600 Lb

U = 5000/4 psi (Lb/in^2) (divide by safety factor of 4:1)

A (minimum) =  1600 / 1250 = 1.28 in^2

My FlexElement rod has an OD of 5/16" or 0.3125", so the circumference is pi * .3125 = 0.982"

So every 1" of bond length along the rod, will give 0.982 in^2 of bond area, so my bond length needs to be at least:

1.28 / 0.982 = 1.30" long.  That's how much the rod must insert into the composite beam to provide enough bonding area.

In actual fact, I made the joint length about 4" because of other design factors, so I theoretically had 12 x more area than I needed (safety factor of 4 * (4"/1.3") ~ 12)

I was VERY surprised when this joint failed in simple tension at well below 1600 Lbs of tension.

Lets look why.

My simple bond joint area calculation made one very innocent, but deadly assumption:

That the force or stress would be evenly distributed across the full area of the joint.

WRONG WRONG WRONG WRONG!!

Lets look at a simpler geometry to help understand.

Here is a  cut-away of a simple rod glued into a larger cylindrical rod.

The problem with this design is that it produces a concentration of stresses in the glue joint at the point where the rod first enters the cylinder.  This is because of the huge, and sudden difference in cross sectional area between the rod, and the rod/cylinder combination at that point.

All materials have elasticity and deform (stretch or compress) when a force is applied to them.

The rod is being pulled in tension, so it is stretched longer than it's normal relaxed length.

Since the glue joint between these two parts is very very thin, we assume that the rod and cylinder dont move relative to each other. But that is not the case here, because the rod is a small diameter, so it is under much greater stress (force per unit cross sectional area) than the green cylinder.  So... the rod will be stretching more, so it MUST deform more than the cylinder which is under less stress because the tensile force is spread across a much larger cross sectional area.

When you pull the rod in tension, you quickly cause a stress concentration in the glue joint at the spot indicated.

When the rod pulls to the right, all of that force is concentrated to a small portion of the glue joint.  That section of the glue joint fails, and the rod stretches a minute amount.  But now, the portion of the glue joint just to the left of the stress concentration area becomes the new stress concentration area, and so on until the joint failure/ stress concentration has moved all the way along the glue joint, and the rod simply pulls out of the cylinder.

This is very hard to describe and visualize because it happens essentially instantaneously, but seeing the results (failure of the joint WAY below what was expected) make it impossible to deny.

THE SOLUTION:

The solution to the problem is to design the two parts (or for simplicity in this case - the cylinder) so that there is a very gradual change in cross sectional area, so that there is a gradual change in the stress, and hence a gradual change in the stretch (strain).  

Care must be taken to make this transition gradually so that at no point along the glue joint, does the stress excess the shear strength of the cured glue material.

So back to the stress concentration area, the glue joint only needs to transfer enough force to the "cylinder" (now a cone) to make that tiny, thin green area on the far right to stretch along with the rod which is under full stress.

This force transfer keeps happening along the length of the glue joint until eventually all of the force is tranferred to the green cylinder.

Thanks for taking the time to "bond" with me.  :)

This is an "exploded" view of my rotor design.  Leading and trailing edge foam cores moved out of position to show internal details.

This design features a cylindrical bearingless rotor root, with droop stop.  Airfoil shaped pieces on inboard and outboard ends of rotor airfoil provide an equidistant path for fiberglass cloth layup, to minimize wrinkles where fabric goes around end of airfoil.  You can see the (lead) tip weight at the most outboard portion of the leading edge airfoil.  These tip weights were sand casted in the LE shape with a concave groove on the front to help position the weight against the leading edge rod.  Leading edge foam core, tip weight and leading edge rod will be wrapped with fiberglass at +/- 45 deg layup pattern to provide torsional rigidity to transfer torsional moments from the pitch control horn (extension of leading edge rod) along the length of the airfoil.  Will also include at least 2 layers of 0 deg layup to provide rigidity in bending, forming a "spar" out of the leading edge portions of the rotor.

The brownish section is a composite beam which distributes the large centrifugal tension forces from the inboard end cap (which is being pulled outward by the fiberglass fabric that will be wrapped around it - not shown) to the cylindrical flexelement which connects to the rotor hub (not shown)

Trailing edge will get a thin strip of alum inserted into a slot in the foam core to give some durability to the TE.

Below is a timelapse of the rotor tip casting, and a pic of the final product

https://youtu.be/kFUzCT0zQjM

Picture above is first prototype of fiberglass bearingless hub design.  The idea of this design is to allow flapping and twisting (pitching) of the rotor, but have limited flex in lead/lag. 

This rotor hub is 24" long and 4" wide in the center.  Was made with Qty 10 - 24" x 2" strips of 6 oz plain weave fiberglass cloth.  Bonded with Fiberglast series 2000 epoxy resin with 120 minute cure time.  

After layup, I inserted pins (small finishing nails) into the holes of the mating aluminum plates.  The idea is to distribute the tensile forces across the area of the plate, and also to minimize damage to the individual glass fibers.  While they are pushed to the side slightly by the pin, severing of the fibers is minimized, thus maintaining maximum strength.

This was a messy process and in hindsight, I would do it differently.  Yes, a learning process.  I needed to get the pins through the layup before the epoxy set, but the protruding pins created a bit of a problem for the vacuum bagging process.  Many layers of duct tape later, I was finally able to get a bit of a seal on the vacuum bag.  But I know that it could have been better because I only achieved 10" Hg vacuum with a pump that is capable of 20".  Leaks abound!

From this point I intend to measure the flexibility of the hub in 3 directions (flap, pitch and lead/lag).

After that, I hope to do a tensile test up to 200% of the design operating tension to verify my 4:1 safety factor.

If all goes well in the tensile test, and I still have a functional part, I will begin cycle testing, by flexing the part in the flap and pitch axes while under design tension load.

Here are some additional photos.

This is the end plate of 1/8" thick aluminum on each side of the fiberglass layup.  There are small steel pins in each of the holes that transfer tension from the fibers to the aluminum plates.

This is the center hub which will eventually connect to the rotor main shaft.  You can see the tops of the finishing nails protruding from the holes.  There is a flat alum plate opposite these aluminum channels that will attach to the shaft hub.

This "wishbone" design was seen in a technical paper on bearingless rotors, and seemed to be the simplest to manufacture, which is why I'm trying it first.

As an asside:  While laying up this part, I calculated the resin required to get a 50% resin by weight composite.  However, I did include a bleeder cloth to distribute the vacuum pressure along the length of the part, but it ended up soaking up a LOT of the resin to the point that it entered the vacuum tubing and flowed in the tubing toward the pump.

I did not have a "drop out" chamber to protect the pump from induced resin, and I was very lucky that the resin did not make it all the way to the pump.  Especially since this pump is borrowed!!  In the future, I will use a drop-out pot between the part and pump.

Till next time.

My rotor hub design continues to evolve as I learn more about the complex forces and motions of a rotor system.

One of my goals of this project was to keep it simple.  Ideally as simple mechanically as a small electric quad, but as discussed in a previous post, that is not the desired approach, so I'm designing a "collective only" variable pitch rotor head, not unlike what might be found on a conventional helo's tail rotor.  As you can see from the image, I'm trying to leverage stock structural materials such as standard architectural aluminum channel and square tube.

The design above consists of a "double wishbone" (for lack of a better term) composite strap made of approx 11 layers of 6 oz. plain weave fiberglass fabric with about 50% by weight of Series 2000 epoxy resin with 2120 slow cure hardener (FiberGlast Resin). This should give a tensile strength of about 5500 Lbs which is almost 4x the anticipated centrifugal blade tension of 1500 Lb.

My big challenge here was to come up with a way to attach mechanical structures to the composite flex-element without weakening the composite.  My approach here is to use a large number of small diameter "shear pins" which will pierce through a sandwich of aluminum plates and the composite.  The idea is to spread the load across a large area of the composite and avoid force concentrating geometries which will exceed the local strength of the fibers.

A simple test of a 1/16" diameter nail through 2 layers of this composite matrix is surprisingly strong.  

A variation of that test was to line up 4 nails in a line (spaced about 1/4" apart) along the axis of tension, and even though these nails were all aligned, the strength went up sufficiently that it was able to support my full weight (~160 Lbs).

This tells me that the composite matrix is doing a good job of spreading the load to neighboring fibers.

So, back to the rotor design....The vertical shaft is the main rotor mast.  The channel and plate at the upper end of the mast comprise a sandwich which holds the composite "wishbone" on center, and receives upward forces from the thrust generated in the blades (not shown).

The wishbone shape is a copy of a shape observed in several articles I've read.  The idea is to give flexibility for the rotor blades to flap (up and down due to dissymetry of lift), lead/lag (oscillate forward and backward in the plane of rotation) due to correolis forces generated as a result of flapping, and pitch (or feather) the blade to increase/decrease the angle of attack of the blade to control the lifting thrust of the rotor.

Based on this flat geometry, this particular design will not have a tremendous amount of flexibility in the lead/lag direction, however, I am hoping this is ok because of the measures I am taking to limit flapping.

The pitch control horn protrudes forward of the blade leading edge, and a rod extends this toward the center of rotation a bit.  The idea here is that I will connect my pitch control links to the rod at a point which is outboard of the effective flapping hinge point.  In this way, when the blade flaps up, it's angle of attack will be reduced (because the pitch control link won't flap up), and will tend to limit the amount of blade flapping.  Now I've been around control systems long enough to know that if I get too aggressive with this (by making the pitch control horn too short, or by connecting the pitch link at a point that is too far outboard) the system could become unstable and the flapping could actually be exaggerated to the point of destruction.

Another interesting observation relating quad copters to single rotor copters:

On a single rotor copter, the cyclic forces cause the rotor disc to tilt, and this is transferred to the helo airframe through the rotor hub and mast.  The airframe FOLLOWS the rotor disc.

On a quad copter it is quite the opposite.  The difference in thrust of the 4 rotors causes the airframe to tilt, which causes an effective "flapping" of the rotor discs relative to their masts new position.  Moments transferred through the mast and hub then bring the rotor disc axis into alignment with the mast.  The rotor disc(s) FOLLOWS the airframe.

This is one reason I'm so excited to try this flapping compensation technique with the forward extending pitch control horn as it promises to greatly reduce cyclic bending stresses on the mast, rotor, and blade roots.

A few months ago, I read the Wright Brothers biography by David McCullough. It inspired me to create. I have always been interested in flight and have enjoyed RC airplanes in the past. The widespread popularity of the quad-copter has sparked a new interest for me.

I am a mechanical engineer, and work in the process control industry. Part of my job involves understanding the inter-relationships between process variables in order to better design a control strategy for the process. Along those lines, one day I got to thinking about the equations governing flight of a helicopter.

At it's most basic level, lift is produced by transferring momentum to the surrounding air. The lift is proportional to the rate of momentum transfer which is proportional to (p)(A)(v^2) where p is the density of air, A is the area of the rotor disc, and v is the velocity of the air downdraft produced by the rotor. Additionally, I learned that the power required to produce this lift is proportional to (v^3). What this meant to me, is that theoretically, I should be able to build a very low power helicopter if I made the rotor disc large enough. Of course, reality intervenes, as it often does, and practical limitations (such as weight, strength, of rotors) steer us to some reasonable compromise between all of the competing design parameters.

With that said, my goal is to design a full scale, low power, quad copter, and then with any luck, build it, and fly it. One of the drivers for this idea was the prospect of eliminating the complication, and tremendous vibration and fatigue associated with the cyclic pitch control of a "traditional" single rotor helicopter. I want to make something more simple, like the small quads. I believe that simple somehow inherently associates with "reliable". Not always true, but a worthy goal.

The basic parameters of this project are (currently) as follows:
Rotor Diameter = 4 meters
Number of Rotors = 4
Empty Weight = 96 kg
Gross Weight = 200 kg
Rate of Climb = 2.5 m/s

Initially, after finding that the theoretical power required to hover 50 kg with a 4m rotor was around 2.7 HP, I was thinking that I could just put a compact power pod consisting of a small 2 cycle engine, gearbox and rotor at the end of each of the 4 arms, and away I would go. Then I started reading about "why we don't have large quad-copters", and realized the inertia of the large rotor makes control an issue as the individual rotors cannot respond with the quickness that a small plastic propeller can. This lead me to the conclusion that I would have to use variable pitch (collective pitch) control rotors in order to get quick response from the aircraft. That gets messy, and is anything but simple, but since my hope is to one day hop aboard this creation, safety is a significant concern, and without control nothing else much matters. So after concluding I need variable pitch rotors,

I thought some more about the 4 engine design, with the 4 engines located at the rotor hubs. I did a quick estimation of the aircrafts moment of inertia in the various axis (roll, pitch, yaw). NOT PRETTY. If I lost an engine, I would be upside-down, and whirling around in a fraction of a second. No time to cut power to the other engines, just a big airborne tilt-a whirl. YIKES! So that revelation let me to a single engine design. OK, not as radical as the 4 engine design, but certainly more sensible.

As mentioned before, I'm basing my full scale quad design around a 4m rotor diameter. That makes each rotor blade 2 m long. I've decided on a NACA0012 airfoil which is commonly used for full scale helo's. This is a symmetrical airfoil with a glide ratio of about 60:1. I'm planning to build the rotors of composites with E-glass fiberglass cloth, epoxy resin and extruded polystyrene foam core. My budget is a meer trickle so I am selecting lower cost (and lower performance) materials than might be desired if my budget were larger. For example, I might use urethane foam and carbon fiber cloth for the rotors. However, I believe I can meet the needs using these lower performing materials. The symmetrical airfoil will allow me to build all 8 rotor blades essentially the same which should simplify things a bit. I have purchased some 6 oz/yd standard weave fiberglass cloth, and Series 2000 epoxy resin with 2 hour working time hardener from http://www.fiberglast.com/ . I am currently experimenting with the fiberglass layup process as this is a new area for me. The 2 hour cure hardener requires elevated temperatures to cure, so I glued up a small part and put it under a work lamp to cure. (suggested temperature is 120-130 Deg F for 12-14 hours. I guess I put it a bit close to the lamp because it melted a crater into the polystyrene foam. In hindsight, I learned the working temp of the Owens Corning Foamular 150 polystyrene board is 165 F. I exceeded that.

I'm working towards building a small section (12" long) of rotor spar comprised of a foam core, 5/16" diam fiberglass rod leading edge (for impact durability and rotor CG management) wrapped in two layers of 6 oz glass at +- 45 deg to hopefully give it strength against shear as the web of the rotor "beam" formed by the upper and lower skins.

The chord of the airfoil is 20 cm, and this spar is the front 30% of that chord, or 6 cm.  In my research I've seen this approach (a wrapped leading edge acting as a rotor spar) used for rotor blade construction.  After building this small section, I will test how much bending moment it can withstand before breaking or kinking.