Homebuilt Coaxial Helicopter: Ben Dixey, SW UK

I am guessing the administrators can find a way (and perhaps change the thread title too?)
Given the title of the thread, and the section it's in, no one is [was] going to find all the good info it contains except by luck.
 
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What is the chord length on those blades?
I was going to have some vertical shaper blades/knives made, so I could easily make rotor foil by the foot
for one of my projects, but I am only looking at about 3.00" chord, which is probably too short for your application....
 
What is the chord length on those blades?
I was going to have some vertical shaper blades/knives made, so I could easily make rotor foil by the foot
for one of my projects, but I am only looking at about 3.00" chord, which is probably too short for your application....
Yeah, 4.75" is my chord. I'll be interested to see your blades when you have made them. 👍
 
Part 3 of testing.
Just wondering, the torque balance between the rotors seems about right with a 1 degree pitch difference. Is the lower rotor running at a higher pitch because the angle of attack is being reduced by the downwash of the top rotor ? Is it a combination of angle of attack and turbulent air ?
 
All the counter rotating propellers I have seen had more pitch on the down wind side, if you look at a Volvo counter rotating outdrive for boats, it is quite severe and more critical, water being 800 times more dense than air...
You are correct to assume the lower rotor needs more pitch, the upper rotor is shifting the apparent wind, or relative wind upward and therefore the need for more pitch. There was an Austrian company that use to make a contra rotating prop for ultralights, they said that the distance was critical to performance and had to be tuned to within less than a mm. They said it made a significant noise when even slightly out of spec...
In the RC coaxials I have seen, the separation between rotors is fairly large due to interaction. The full scale coaxial helis seem to have less, so maybe this distance recedes with higher Reynold's numbers and lower rpm?
I flew some coaxial RC autogyros and the rotors needed separation at least a quarter span of the rotor radius. I think this has more to due with having enough space for clean entry of airflow to the top rotor and can probably be less space on a driven system like yours.
How did you arrive at the spacing between rotors?
Also, have you thought about strapping some long spars in a cross formation under the skids to allow longer tethers?
 
All the counter rotating propellers I have seen had more pitch on the down wind side, if you look at a Volvo counter rotating outdrive for boats, it is quite severe and more critical, water being 800 times more dense than air...
You are correct to assume the lower rotor needs more pitch, the upper rotor is shifting the apparent wind, or relative wind upward and therefore the need for more pitch. There was an Austrian company that use to make a contra rotating prop for ultralights, they said that the distance was critical to performance and had to be tuned to within less than a mm. They said it made a significant noise when even slightly out of spec...
In the RC coaxials I have seen, the separation between rotors is fairly large due to interaction. The full scale coaxial helis seem to have less, so maybe this distance recedes with higher Reynold's numbers and lower rpm?
I flew some coaxial RC autogyros and the rotors needed separation at least a quarter span of the rotor radius. I think this has more to due with having enough space for clean entry of airflow to the top rotor and can probably be less space on a driven system like yours.
How did you arrive at the spacing between rotors?
Also, have you thought about strapping some long spars in a cross formation under the skids to allow longer tethers?
Thanks for this insight, you have a lot of knowledge.
The rotor spacing I kept coming across the figure of 10% rotor diameter in things I read. I remember reading an article where a very similar rotor diameter to mine was tested at 10% and 15% spacing, the 10% spacing required less overall torque to produce the same lift.
I don't know how the spacing changes with varying diameter rotors and speeds. I would like to know what spacing Kamov use.
I know the Nolan coaxial used a spacing 100mm closer than mine with the same diameter but in the subsequent airscooter 2 the spacing was increased, to what I don't know. The designers of the VA115 would have done their research but again I don't know the spacing.
The extended skids is something I'm looking at and would happily try that with perhaps longer tethers. I also thought about having the tether anchored to something on the ground that can move, it would allow the helicopter to move in any direction but restricted height as it does now. If the tether was bolted to a framework on bearings would this work? Part of the problem with learning the cyclic is most of the time the tether is interfering, I could possibly go longer with the tethers but I'm not keen on that at the moment. I would have thought extended skids would also interfere with the learning process.
 
Thanks for this insight, you have a lot of knowledge.
The rotor spacing I kept coming across the figure of 10% rotor diameter in things I read. I remember reading an article where a very similar rotor diameter to mine was tested at 10% and 15% spacing, the 10% spacing required less overall torque to produce the same lift.
I don't know how the spacing changes with varying diameter rotors and speeds. I would like to know what spacing Kamov use.
I know the Nolan coaxial used a spacing 100mm closer than mine with the same diameter but in the subsequent airscooter 2 the spacing was increased, to what I don't know. The designers of the VA115 would have done their research but again I don't know the spacing.
The extended skids is something I'm looking at and would happily try that with perhaps longer tethers. I also thought about having the tether anchored to something on the ground that can move, it would allow the helicopter to move in any direction but restricted height as it does now. If the tether was bolted to a framework on bearings would this work? Part of the problem with learning the cyclic is most of the time the tether is interfering, I could possibly go longer with the tethers but I'm not keen on that at the moment. I would have thought extended skids would also interfere with the learning process.

What I have read on coaxial props indicates more efficiency than a single, but only if the spacing is correct, I don't have a clue
on figuring out that part.....

Before the electronic stability, (RC Helis) it was standard procedure to put long rods laterally and longitudinally on the skids when learning to fly the heli.
It would be a lot cheaper than a new set of blades and you could probably make the next project from them...
Have you thought about finding a high time heli pilot to help with the testing?
It might save a lot of time. I use to fly RC sailplanes with a guy who was a test pilot for McDonnald Douglas "No Tars"
He told me about flying beyond VNE until it started bucking, he would then inform the ground crew of the behavior and they would make some adjustments,
then he would repeat the process.
 
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Before the electronic stability, it was standard procedure to put long rods laterally and longitudinally on the skids when learning to fly the heli.
I've been instructing in helicopters for a VERY long time, in aircraft with no governors or stability aids, and I have never, ever seen this done.

Maybe it has been done by RC modellers, but not for anything man-carrying that I've ever seen.
 
I've been instructing in helicopters for a VERY long time, in aircraft with no governors or stability aids, and I have never, ever seen this done.

Maybe it has been done by RC modellers, but not for anything man-carrying that I've ever seen.

I was referring to RC, maybe didn't make it clear though.....
 
Thanks for clarifying - - you had me worried there for a bit!
 
I made an edit to clarify, my practical experience is ALL remote at this point, so I assumed that was common knowledge, my mistake...
For something as small as this heli, "training wheels" that made a 12 to 14ft. platform could eliminate the tether and prevent a rollover if he kept it within about 18" off the deck....
 
On a semi-sequitur note:
The guy who ran a bombing range I spent too much time at ("Johnny, in case you are out there?) told me he piloted Kaman "egg beaters" as rescue platforms in one of our (US) far east excursions and he also flew the first remote controlled versions. The whole team loved this guy, but he was a self confessed electrical "Jinx" as said as long as he was nearby, things would go horribly wrong, so he would leave the site right before we would fly...
 

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I've been instructing in helicopters for a VERY long time, in aircraft with no governors or stability aids, and I have never, ever seen this done.

Maybe it has been done by RC modellers, but not for anything man-carrying that I've ever seen.
Can't remember where on the net I saw a video a few years ago of a single-place heli with lateral pipes out the side and ground tethers.
smiles,
Charles
 
What I have read on coaxial props indicates more efficiency than a single, but only if the spacing is correct, I don't have a clue
on figuring out that part.....

Before the electronic stability, (RC Helis) it was standard procedure to put long rods laterally and longitudinally on the skids when learning to fly the heli.
It would be a lot cheaper than a new set of blades and you could probably make the next project from them...
Have you thought about finding a high time heli pilot to help with the testing?
It might save a lot of time. I use to fly RC sailplanes with a guy who was a test pilot for McDonnald Douglas "No Tars"
He told me about flying beyond VNE until it started bucking, he would then inform the ground crew of the behavior and they would make some adjustments,
then he would repeat the process.
Although it has been suggested a few times about getting a helicopter pilot to try it, it's not something I would do. If something were to go wrong and the person was injured I wouldn't forgive myself. I have done my best to build the machine to the best of my ability but certain parts in the commercial world would be tested on a rig until they failed, then you would have some component life figures to go by. This is cost prohibitive from my point of view so there is a risk I'm taking which only I will take. I used the x pattern long poles to learn rc and it worked fine. I will try it and see what it's like full scale.
 
Although it has been suggested a few times about getting a helicopter pilot to try it, it's not something I would do. If something were to go wrong and the person was injured I wouldn't forgive myself. I have done my best to build the machine to the best of my ability but certain parts in the commercial world would be tested on a rig until they failed, then you would have some component life figures to go by. This is cost prohibitive from my point of view so there is a risk I'm taking which only I will take. I used the x pattern long poles to learn rc and it worked fine. I will try it and see what it's like full scale.

I was thinking more about the testing procedure and the fact a very high time heli guy could probably get it in a 1ft. hover and tell you more about what is going on than you can at this stage because there are so many variables. I wasn't considering the pilot actually flying in a life threatening manner at this point.
Just a ground effect hover..
I think you might find someone interested enough to help without big financial rewards and a high time pilot who would be interested would
certainly be able to pre-flight the system and assess the risk themselves.
Too bad your on the other side of the planet, I knew a couple guys in Phx. Az. who probably would have been thrilled to help out..
Either way, keep it up, it look like progress is being made..
 
I was thinking more about the testing procedure and the fact a very high time heli guy could probably get it in a 1ft. hover and tell you more about what is going on than you can at this stage because there are so many variables. I wasn't considering the pilot actually flying in a life threatening manner at this point.
Just a ground effect hover..
I think you might find someone interested enough to help without big financial rewards and a high time pilot who would be interested would
certainly be able to pre-flight the system and assess the risk themselves.
Too bad your on the other side of the planet, I knew a couple guys in Phx. Az. who probably would have been thrilled to help out..
Either way, keep it up, it look like progress is being made..
Point taken, I can see the benefit of that. I'm curious to find out how this design handles in comparison to a Robinson, if I ever get to experience a controlled hover in this machine I will go back for another R22 lesson and compare the two. I already know the altitude control is delayed, the pedals are less sensitive and probably easier but the cyclic so far feels similar in it's response. I'm just rectifying the excess forward cyclic force needed due to the imbalance of the rotor mast. Can't believe I over looked this, it's also the reason for the 1.8 degree nose heavy flying condition as the control rods push up on the rotor mast from a point behind the CG line on the airframe, lifting the nose. Neither of these design faults were considered so I wasn't expecting them.
Something I'm considering is whether this design is a tilt rotor or weight shift or a combination of the two. The coaxial configuration cancelling gyroscopic forces means I can move the rotor around on the ground while at full rpm which wouldn't be possible if the rotors were spinning the same way. However in the air the cyclic forces reduce further.
 
If it requires excessive forward stick, It would seem evident the CG is off and the rotor head, or seat needs to to be adjusted, assuming you built in that adjust-ability.
As far as "tilt rotor, weight shift, or combination of the two" I have been asking myself that question since I started building RC gyroplanes back in the mid 90s. The gyro plane community refer to "cyclic" which I would associate with a fully articulated rotor head changing the pitch of individual blades causing the the disk to aerodynamically fly in a different plane. But with a gyro plane, fixed blades, you are literally shifting the under carriage through the push tubes, if you swapped the old style drop bar for a triangular frame control bar, it would be identical to a weight shift hang glider.
The part that makes it slightly different is that the rotor doesn't have a lot of mass, so the "Combination of the two" idea comes to light.
I am assuming the control input shifts the disk plane as well as shifts the weight, the ratio would be interesting to find out.
This discussion also reminds me of a hovering platform project I was on the peripheral of in the early 2000s. I may be opening a can of worms, but It brought to light that a rotor with the payload above the disk can be more stable than payload in a pendulous configuration. That one is counter intuitive.
 
The gyro plane community refer to "cyclic" which I would associate with a fully articulated rotor head changing the pitch of individual blades causing the the disk to aerodynamically fly in a different plane. But with a gyro plane, fixed blades, you are literally shifting the under carriage through the push tubes, if you swapped the old style drop bar for a triangular frame control bar, it would be identical to a weight shift hang glider.
I can't agree with that description. Lacking a swash plate is not the same as weight shift and doesn't mean that the control is not aerodynamic.
 
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