Homebuilt Coaxial Helicopter: Ben Dixey, SW UK

btd1982

Gold Supporter
Joined
Jan 28, 2021
Messages
157
Location
Devon
Aircraft
Home built helicopter
Total Flight Time
0
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

Active Member
Joined
Dec 2, 2020
Messages
227
Location
Az.
Aircraft
Pteradactyl, AC 447/503, too many UAVs
Total Flight Time
Over 3k....(From the ground !)
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

Gold Supporter
Joined
Jan 28, 2021
Messages
157
Location
Devon
Aircraft
Home built helicopter
Total Flight Time
0
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

Junior Member
Joined
Dec 22, 2012
Messages
211
Location
Kingsland, TX
Aircraft
Sport Copter Vortex & AAI (Sparrowhawk) RAF
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

Gold Supporter
Joined
Jan 28, 2021
Messages
157
Location
Devon
Aircraft
Home built helicopter
Total Flight Time
0
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.
 
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