Homebuilt Coaxial Helicopter: Ben Dixey, SW UK

skyguynca

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Thanks so much for sharing your research. Questions are being answered that I haven't been able to find answers to, its brilliant!

So it seems I can expect high instability going from ground to hover but once in hover things might get easier. Thinking about it I have experienced this to some degree in a rc model, the transition from ground to hover is always a bit sketchy, then it stabilizes further away from the ground.

I reckon a dynamic roll over with the helitrike would be a major problem with so much weight so high.

Interesting thought on the offset hinge I don't know much about this but looking at the air scooter and Nolan designs I can't work out if they had offset hinges.

on your weight shift machine, is the pilot seat suspended from a single chord like a hanglider?

I need to decide how best to tether the craft to try and make testing as safe as possible. If I ever experienced a dynamic roll over and walked away I think it would be the last time I went near the machine. Its tether versus long poles bolted to the skids or I make some sort of telescopic platform?
Tethering at low power settings, getting use to response to throttle and cyclic rotor movement at low power, low power means not enough to make the aircraft light, sure ok.

Power settings enough to lift the aircraft the tether has pluses and minuses.
Plus, too much lift response whether due to throttle or sudden wind - keeps you from finding yourself way up in the sky.
Minus, if you drift, and you will drift, one tether pulling tight will result in a off center vector you can not respond to, dynamic roll over.

Tethers that are longer say 4 to 5 ft long are ok, just as long as you don't get more than 1 foot off the ground and preferably attached to the aircraft at 4 equal box forming to the aircraft, each tether extends to make an x that would make line through the center of gravity, so no matter which way you drift 2 or more tethers should equally pull on the aircraft and hopefully stop the aircraft without tipping it.

Also training poles that go thru your landing skids that form an x in the center of the landing gear and extend to the total of the diameter of your rotor system will also kind of push your aircraft back to level it you tilt too much in any direction as long as you are less than a 2 ft hover and the aluminum tubes have sufficient wall thickness.
 

skyguynca

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Thanks so much for sharing your research. Questions are being answered that I haven't been able to find answers to, its brilliant!

So it seems I can expect high instability going from ground to hover but once in hover things might get easier. Thinking about it I have experienced this to some degree in a rc model, the transition from ground to hover is always a bit sketchy, then it stabilizes further away from the ground.

I reckon a dynamic roll over with the helitrike would be a major problem with so much weight so high.

Interesting thought on the offset hinge I don't know much about this but looking at the air scooter and Nolan designs I can't work out if they had offset hinges.

on your weight shift machine, is the pilot seat suspended from a single chord like a hanglider?

I need to decide how best to tether the craft to try and make testing as safe as possible. If I ever experienced a dynamic roll over and walked away I think it would be the last time I went near the machine. Its tether versus long poles bolted to the skids or I make some sort of telescopic platform?
Not a cord but a rigid seat assy made from 4130 tubing on a universal joint. . Can't be bouncing around in turbulence, the helicopter would get away from you super fast.

Chuck has done many posts on how the offset gimbal rotor head adds stability and how previous Bensen head (spinal head) did not. That is why Bensen incorporated the same offset gimbal head in the B9.
 

btd1982

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Thanks again for this amazing information. you're a legend!
 

skyguynca

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Thanks again for this amazing information. you're a legend!
Nope, but there are a bunch of them on here...Chuck Beaty, Don Hillberg (temporarily suspended) JazzenJohn, Gyro Jake, Scott Essex just to name a few are the go to guys
 

skyguynca

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Opposite rotation cancels gyroscopic force if the rotating bodies are equal. Each rotating body still generates gyroscopic force against its suspension.
Hey Chuck, one thing I have noticed was that the heads for each rotor are rigid. You think this could have been one of the reasons that they had blade collisions when they went from the original 40 hp engine to the 75hp engine? Also abrupt cyclic inputs (weight shifting quickly) could be another reason since the blades are flying really close to each other?

Another thing that I question if it was a blade collision or maybe first the blade failed near the root just prior to and causing the blades to collide since that there was not a method of handling lead/lag or flap conditions and fwd speed increased?

Could increasing the the distance between blades and using teetering heads prevent the blade collisions? maybe improve performance?
 
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C. Beaty

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The individual rotor doesn’t care what its mate is doing; it goes merrily along acting as it always did. If a blade flexes as a result of rotor tilt, it does so without regard to its brother rotor. As an inanimate object, a rotor isn’t very smart.
A demonstrator with a pair of counter rotating discs is a real eye opener, it seems strange that there is no external reaction to precession.
 

Martin W.

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Bensen incorporated delta-3 angle at the teeter bolts on his Zipster Coaxial helicopter

I have no idea if that pertains to anything you guys are talking about.
 

skyguynca

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Bensen incorporated delta-3 angle at the teeter bolts on his Zipster Coaxial helicopter

I have no idea if that pertains to anything you guys are talking about.
Yep, about 15 degrees if I remember right. It helps keep the rotors separated. You see it a lot on T/R for modern helicopters.

Doesn't play into this though, Ben has not cut any Delta 3 into his rotor heads.

Thanks though for reminding us, it is a consideration for blade separation.
 

btd1982

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I didn't realize that the B9 had a blade collision. Explains why they designed the top rotor with a 4 degree pre-cone and no pre-cone on the lower rotor plus the Delta 3 teeter hinge. The 22ft rotors on the B9 surely wouldn't have helped.
The Nolan 14ft rotors only 13" apart both on standard teeter hinges, I've read didn't have any issues with blade collisions.
I measured the separation on mine and it's currently 21". The teeter stops I need to make and was planning to have them so that the blades couldn't physically get near each other but obviously blade flexing is an issue beyond my control.
 

skyguynca

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The blade collisions was on the hz1 flying platform, sorry for the confusion.

Only 1 B9 flew so far, no one ever built a kit and flew it. I know of only one other B9 at the PRA museum. I will send you pictures of it.
 

btd1982

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Ah ok, my bad!
Thinking about control sensitivity the Helitrike and zipster both use the direct movement of the rotor assembly's via solid bars. There doesn't appear to be anyway of adjusting the control inputs sensitivity. The Helitrike's control on youtube has been described as "twitchy". The Nolan's hydraulic cylinders and the air scooter's cables and push-rods would be adjustable. I wonder if this has some bearing on the crafts success or failure.

Further to that are the controls of a craft like the ones just mentioned going to be similar in sensitivity and reaction to a Robinson? Would it be of benefit for someone like myself with minute helicopter experience to practice hovering in a Robinson? They are very different machines so I wasn't expecting them to be similar at all but I am probably wrong.
 

skyguynca

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Helicopter training would be wise if your wanting to fly one.
 

btd1982

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Just thinking,
If the gyroscopic forces and gyroscopic procession are opposed to each other in a coaxial rotor system the forces are then acting between the two rotors and using the shafts and bearings to transfer the forces. With my solid central shaft being a bit longer than I ideally would like, could this be
vulnerable to bending if a short sharp cyclic movement was applied or are the teeter hinges going to reduce the forces transferred?
 

btd1982

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No problem; as long as each rotor can teeter freely, no gyroscopic moment can be applied to the shaft.
Thanks for the reply Chuck. You really know your stuff! I find it hard to find answers to these sorts of questions and this forum is proving to be a real asset. I've been wondering about the gyroscopic effect in a coaxial helicopter for months and just didn't understand the situation until now. Amazing !!!!
 

btd1982

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My rotor blade video if anybody is interested. Not as good as Joseph Bensen's but practice makes perfect!

 

btd1982

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Bit more info. All four blades are complete now apart from end closures. The first blade took 10days to cure in the cold of my workshop. Using an infrared heater I could get the blade temperature to the 90 degrees Celsius needed to cure in half an hour. Getting closer to spinny time!

 

btd1982

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Found a major problem this morning. Bit gutted as so much work had gone into these blades but better to find out now.

Think me and Mr Tree had better get to know each other a little better as metal blades aren't looking a likely way forward at this point.
 

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If aluminum surfaces are properly prepared prior to bonding, things won’t pop apart as illustrated in the pictures. The conventional way is surface treatment in chromic acid but the self etching primers used prior to painting will work infinitely better than untreated surfaces.
The difficulty in bonding aluminum results from aluminum’s affinity for oxygen; a freshly machined surface oxidizes almost instantly when exposed to air and the oxide coating thus formed flakes away like dandruff.
 
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