Homebuilt Coaxial Helicopter: Ben Dixey, SW UK

btd1982

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Devon
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Home built helicopter
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Here are the next two progress videos for anyone who hasn't seen them. Current blade pitch is 5.5 degrees on the top rotor and 6.5 degrees on the bottom rotor. Believe I can go to 7 degrees for maximum lift but not sure on this. I've not tried full throttle since the pitch increase so I don't know what height it can now climb to but don't want to go higher than 3ft at the present time. I am beginning to understand more about helicopter flight control and it's definitely better higher off the ground as I was told. I'm happier to test in windier conditions next time and see how it goes, as long as it's not gusting.

I was told in a comment about the effect of the vertical position of the centre of gravity. Too high and the helicopter becomes much harder to control and that's why Robinson's ect have the rotor up so high to bring the CG down. I'm not sure about this but wondered if anybody had any thoughts? My argument was the hiller flying platform seemed to work well.


 

Aerofoam

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I have done some experiments with the payload above the rotor and as odd as it seems, it is more stable than below the rotor if it is within a certain ratio of the disk radius. A friend worked on a a fairly expensive large project that confirmed this.
In a simple, but incomplete description, the weight under a disk will create a pendulum effect. If it doesn't have aerodynamic drag, it will easily try to come back to center, but while tipped, the whole platform slid towards the low side, so depending on the surface drag of the payload, this can set up an oscillation that increases with each cycle unless counteracted by pilot input.
With the weight on top, tilting the disk results in the disk trying to slide back under the payload.
You can play with this concept with one of those little rotor toys you spin between your hands....
 

btd1982

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Devon
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Home built helicopter
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I have done some experiments with the payload above the rotor and as odd as it seems, it is more stable than below the rotor if it is within a certain ratio of the disk radius. A friend worked on a a fairly expensive large project that confirmed this.
In a simple, but incomplete description, the weight under a disk will create a pendulum effect. If it doesn't have aerodynamic drag, it will easily try to come back to center, but while tipped, the whole platform slid towards the low side, so depending on the surface drag of the payload, this can set up an oscillation that increases with each cycle unless counteracted by pilot input.
With the weight on top, tilting the disk results in the disk trying to slide back under the payload.
You can play with this concept with one of those little rotor toys you spin between your hands....
Great info, thanks, can't beat real experimentation. That explains the stability of the flying platform. 👍
 

Eric S

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Dec 22, 2012
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Kingsland, TX
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Sport Copter Single & Tandem
Here are the next two progress videos for anyone who hasn't seen them. Current blade pitch is 5.5 degrees on the top rotor and 6.5 degrees on the bottom rotor. Believe I can go to 7 degrees for maximum lift but not sure on this. I've not tried full throttle since the pitch increase so I don't know what height it can now climb to but don't want to go higher than 3ft at the present time. I am beginning to understand more about helicopter flight control and it's definitely better higher off the ground as I was told. I'm happier to test in windier conditions next time and see how it goes, as long as it's not gusting.

I was told in a comment about the effect of the vertical position of the centre of gravity. Too high and the helicopter becomes much harder to control and that's why Robinson's ect have the rotor up so high to bring the CG down. I'm not sure about this but wondered if anybody had any thoughts? My argument was the hiller flying platform seemed to work well.


I just watched Part 7 and while watching your view from the pilot's seat I thought "choke up on the stick".

I over controlled my more sensitive single place gyro after 2-place training and the advice given to me was to rest my forearm on my right leg, then make an OK out of my thumb and forefinger wrapped around the stick. It really helped me and looks like it could help on your coax heli. This way you only move your wrist to help make smaller, more precise inputs.
 

btd1982

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Devon
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Home built helicopter
Total Flight Time
0.5h
I just watched Part 7 and while watching your view from the pilot's seat I thought "choke up on the stick".

I over controlled my more sensitive single place gyro after 2-place training and the advice given to me was to rest my forearm on my right leg, then make an OK out of my thumb and forefinger wrapped around the stick. It really helped me and looks like it could help on your coax heli. This way you only move your wrist to help make smaller, more precise inputs.
Thank you for the advice, I think you're right it that helping, I will either grip the stick lower down or add a knee pad to my leg to raise it high enough. Definitely something to try on the next test and I'm sure it will make a difference. 👌 I'm just sorting the cooling system at the moment, think the water pump is too slow at 7ltrs/minute, it's rated at 15ltrs/minute but small coolant holes in the engine restrict the flow. I suspect 25ltrs/minute it somewhere close to adequate.
 

btd1982

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Devon
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Home built helicopter
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I suppose there is a need to go through legal procedures to fly anything.
Yes, I believe I can only operate under a tether from a CAA perspective but haven't found any written documentation to verify this. So far the machine hasn't had the lift to get out of ground effect, in my eyes that makes it a ground effect vehicle and is unrestricted. I believe experimenters should be allowed to experiment within the boundary's of their own land with restricted height without the need for a dangerous tether. I'm all for protecting the public but don't agree I should be protected from myself, that's my choice.
 

Asker

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İstanbul
Yes, I believe I can only operate under a tether from a CAA perspective but haven't found any written documentation to verify this. So far the machine hasn't had the lift to get out of ground effect, in my eyes that makes it a ground effect vehicle and is unrestricted. I believe experimenters should be allowed to experiment within the boundary's of their own land with restricted height without the need for a dangerous tether. I'm all for protecting the public but don't agree I should be protected from myself, that's my choice.
You can try and find an indoors arena like a gymnasium to fly untethered but it is a little risky to fly indoors.

FAA think like that but very few other nations do the same.
 
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Barney Bahle

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So far the machine hasn't had the lift to get out of ground effect,
Not necessarily true. I can't speak for coaxial's but when a helicopter has forward speed and achieves effective transitional lift it requires less power to fly. Given the right conditions a helicopter that doesn't have enough power to hover can still get airborne.
 

btd1982

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Not necessarily true. I can't speak for coaxial's but when a helicopter has forward speed and achieves effective transitional lift it requires less power to fly. Given the right conditions a helicopter that doesn't have enough power to hover can still get airborne.
Yes Good point, I would expect coaxial's to be the same. Perhaps that's why they couldn't be considered ground effect vehicles even with limited power. Do you think the helicopter could gradually accelerate from ground effect hover into transition and keep a safe distance above the ground? Surely some lift would be lost propelling the craft forward.
 

wolfy

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Even without the power to get off the ground at all translational lift can be had from a wind and also from a running take off, while light on the skids simply start sliding forwards on the skids until airspeed is high enough for effective translational lift. Better learnt with an instructor though.

wolfy
 

btd1982

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Even without the power to get off the ground at all translational lift can be had from a wind and also from a running take off, while light on the skids simply start sliding forwards on the skids until airspeed is high enough for effective translational lift. Better learnt with an instructor though.

wolfy
Another good point Wolfy, yes forgot about the wind. Don't worry, no plans to try moving forward at all until I have sufficient hovering control, it's nowhere near at the present time. I'm looking forward to the next R22 lesson should be interesting to compare the cyclic.
 

btd1982

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Devon
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Home built helicopter
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An engine cooling update and an R22 lesson, it was great to compare the machines, the R22 is more sensitive on all three controls.
Despite on the coaxial the cyclic being very different to blade feathering it behaves very similar in flight. I don't think it's more stable or easier to fly than the 22 as far as cyclic is concerned. The control of yaw and altitude is easier on the coaxial I would suggest.
I was able to feel the interaction between each control on the R22 and although is seems daunting to hear people say that altering one control effects all the others you just seem to react to it without thinking to much. It was a great experience and hope to do more of it.

 

btd1982

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Devon
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Home built helicopter
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The next two videos. Funny, if you put dodgy in the title how it increases views with lots of comments saying how dodgy it was. Whether it was or wasn't doesn't matter the suggestion is all it takes. Plenty of nice comments as well but it's the negative ones you remember. The pros and cons of YT 😉

Dodgy Moment, Homemade Coaxial Helicopter Learning to FLY


BALLS UP Homemade coaxial helicopter learning to FLY
 

All_In

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Way to make it happen!
 

Mayfield

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The next two videos. Funny, if you put dodgy in the title how it increases views with lots of comments saying how dodgy it was. Whether it was or wasn't doesn't matter the suggestion is all it takes. Plenty of nice comments as well but it's the negative ones you remember. The pros and cons of YT 😉

Dodgy Moment, Homemade Coaxial Helicopter Learning to FLY


BALLS UP Homemade coaxial helicopter learning to FLY
I'm not certain, but I think "Balls up" has a different connotation in the United States from the way you folks use it.

To many of us, "Balls up" would mean to crash as in "He balls up his helicopter every time he flies." Or, "he balled it up when he tried to fly."

If I'm interpreting your usage correctly it means to mess it up or make mistakes.

Anyway, your build is very, very impressive.

Jim
 
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