Homebuilt Coaxial Helicopter: Ben Dixey, SW UK

[btd1982]

Thanks Chuck. I appreciate your comments. I remember reading the data sheet and it saying about etching as an alternative to abrasion. I assumed etching was for un-abraded bonding and the strengths would be similar. This is the price you pay for not testing and lack of experience. A lesson to be learnt! I was also worried about the surface area to be bonded was much smaller than other bonded blades I had seen. I also wish I went for asymmetric blades, I only went for symmetrical based on an article on redback aviation's site about the Nolan. If I put some adhesive plastic wrapping the entire leading edge both sides would I be able to do some tethered testing or do you think this would end in disaster ?

 

[C. Beaty]

As nearly as I can tell from the pictures, the flat head screws attaching skins to spars ought to hold for the test run when used with an adhesive film overlay.

 

[btd1982]

As nearly as I can tell from the pictures, the flat head screws attaching skins to spars ought to hold for the test run when used with an adhesive film overlay.

Ok, thank you.
After three years of building it would be nice to just spin something if only to test other parts of the drive and get some sort of lift.

 

[btd1982]

My next stupid mistake.

When I starting making these blades my thoughts were, as the rotors were fixed in pitch the chord balance and mounting points were not that important as the feathering forces were not an issue. I thought as long as the center of mass is ahead of the aerodynamic center I shouldn't get blade flutter.

I did a chord-wise balance today and the com falls at 28% but my mounting holes and therefore feathering axis (although fixed in pitch) is at 13% due to where the spar is strongest.

I can now understand that this will result in a nose downward twisting force being applied to the tips as lift increases. However will the blades also flutter due to the balance in regard to the mounting holes being so tail heavy?

I know I have to re-make these blades anyway but would shifting the holes in the blade straps so that the chord-wise center of mass co-insides with the feathering axis solve blade flutter or would it make no difference?

I thought building a helicopter with no previous knowledge of them would be difficult but it's amazing how much you can get wrong.

Any feedback is appreciated. Thank you

 

[C. Beaty]

Three of the most important things for rotary wing airfoils are that the chordwise CG and aerodynamic center coincide and that the pitching moment be as nearly zero as possible. The aerodynamic center for conventional airfoils in normally at or near the ¼ chord point.
It is silly to try to invent your own airfoil; the literature is full of proper airfoils designed by aerodynamicists and tested extensively in a wind tunnel.

 

[btd1982]

Three of the most important things for rotary wing airfoils are that the chordwise CG and aerodynamic center coincide and that the pitching moment be as nearly zero as possible. The aerodynamic center for conventional airfoils in normally at or near the ¼ chord point.
It is silly to try to invent your own airfoil; the literature is full of proper airfoils designed by aerodynamicists and tested extensively in a wind tunnel.

Understood, I will do better next time.

I was careful to copy the naca 0012 profile as accurately as I could but the construction method was my own and hence the bolt holes in the blade are nowhere near where they should be.

thanks

 

[C. Beaty]

An NACA-0012 is a safe and proper choice for any rotorcraft. Slightly better performance can be had with modern cambered airfoils with a reflexed trailing edge to cancel the nosedown pitching moment.
The NACA-8H12 used by Bensen and clones was an attempt at a laminar airfoil that didn’t make the cut but it has a flat stall which eases hand starting.

 

[btd1982]

An NACA-0012 is a safe and proper choice for any rotorcraft. Slightly better performance can be had with modern cambered airfoils with a reflexed trailing edge to cancel the nosedown pitching moment.
The NACA-8H12 used by Bensen and clones was an attempt at a laminar airfoil that didn’t make the cut but it has a flat stall which eases hand starting.

I was thinking it would be best to go for wooden blades on my next attempt as I really can't see how I can make metal ones with very low inertia and keep the blade attachment bolt pattern in the chord wise CG. I was planning on going for a cambered airfoil as it could be more suited to my fixed pitch but not sure at the moment. I understand there is a lot of info on the making of wooden blades on this forum and I will be going through it all. Thanks

 

[btd1982]

Bit more progress.

The teeter blocks have phosphor bronze bushes pressed in and the teeter studs screw into the aluminium rotor hub with a m14 course thread. These will be secured with loctite and wire locked as well. The other end of the studs I have screw cut a m14x1.5mm thread for the castle nuts. The Studs are made from m14 12.9 grade bolts. The top teeter Pin has been made from EN24T pressed into the rotor shaft hub and again m4x1.5 screw cut threads with castle nuts. The Rotorhubs have some play to allow for some 0.1mm steel shim washers so I can adjust the chord balance of the rotors.

I've been reading about adjusting chord balance in this way on gyros. It seems a chord adjustment on a gyro is made then a test to see if it's improved the vibration level is carried out. I plan to make the tool that Rotorway use to get there hub in the middle of the rotor shaft by using a DTI clock and comparing both sides.

Fine adjustment can now be made of the blade feathering.

I have applied Helicopter tape to the leading edge joins on my blades in order to perform a ground test and I'm going to offset the holes in the blade straps so that the chord balance of the blades aligns with the axis of rotation.

There was a bit of a hold up as I had my Bridgeport mill slideways re-ground but it's nearly back together now. Sure did miss it and the DRO on the mill is so much easier.

Next, re-make the blade straps to align blade chord balance, make the teeter stops, make blade closures, spanwise balance the blades.

 

[btd1982]

Finally to the stage of balancing the rotors.
I decided instead of making some more blade straps to move the rotor blades chord balance alignment by leading them. This was achieved by machining slotted holes in the outer feathering blocks. I know this is not the best way to fix the problem but hopefully it will allow me to carry out a ground run.

I was very kindly sent some information from another forum member on how to do a double angle balance on the blades to establish the COM. With my blade straps and bolts also attached in the process this made the blades balance at 23% chord instead of 28% without the straps attached.
I can understand that the weight added from the straps ect at the route of the blades rely's on the torsional rigidity of the blade to have the desired effect.

The first set of blades span-wise balanced within the weight of one m8 washer placed on one end, which I'm quite happy with.

I've got to string line the blades but it's quite easy to see from balancing like in the photo where the chord balance of the rotors lies. By leading or lagging a blade it moves the chord-wise balance point visibly. I wondered if anybody had made a tool to accurately set the chord balance in this way? From what I've read on gyro rotor balance it's a case of move the lot 0.1mm at a time and monitor the stick shake during a test flight.

When it comes to locating and measuring vibration I've got a lot to learn but coaxial's are going to be even more of a challenge I think.

Ben

 

[btd1982]

update on progress.

I have spun the blades up to a maximum of 850 rrpm and they ran pretty smooth without any adjustment from the static span-wise and chord-wise balancing I did. I increased the pitch from 2 degrees to 4 degrees and it lifted the front of the helicopter off the ground but would not lift the whole machine. I was sat beside it, not in it I must add. At 4 degrees pitch and full throttle I could only achieve 700rrpm with a tip speed of 516ft/sec. Increasing the pitch to 5 degrees had little effect, at 6 degrees it was too much and the clutch was slipping. The Engine is not producing is full power at the moment I think because of the gearing and my exhaust not being tuned. I have thought of a way to change my gearing without masses of time and expense so this will probably be required.

The next test will be with the tilting head unlocked as it's currently been restrained from moving.

I am testing the helicopter on a large trailer with four chains allowing a couple inches of lift.

I'm encouraged by the testing and would like to thank the people on this forum who have offered advice and shared knowledge. I can say with certainty that it made the first tests successful ones. Thank you.

 

[btd1982]

Some testing of the cyclic left and right. All seems ok. The force required to move the stick is harder than expected but I think I can improve this.

I've got the machine to lift an estimated 90-95% of the craft including myself but that's it for now. A tuned exhaust is next.

 

[btd1982]

latest test, so pleased it's capable of flight, now things get interesting playing with cyclic response. I'll extend the tethers but not too much, I might construct some sort of test platform, not sure at this point. Perhaps difficult to tell from a short tether but altitude control is quite responsive to throttle input.

 

[j4flyer]

This is a very interesting project. Please keep posting your videos. Your testing method seems very professional. Good job

 

[btd1982]

This is a very interesting project. Please keep posting your videos. Your testing method seems very professional. Good job

Thank you, happy to post more videos as they develop. Have been looking at the Rotorschmiede TR115 training platform, certainly looks a nice safe way to learn the machines characteristics. Think I would swap the caster wheels for linear rails though, would be much less resistance to movement. Plenty of exciting things to think about now.

 

[btd1982]

Not an exciting testing video but hopefully the next one will be.

 

[btd1982]

Here's the latest test video, I couldn't be happier with the machine and there's nothing to modify for the moment. It's tethered at 4 corners with 12" long wire rope to ground anchors. Im not at full throttle at any stage. The cyclic felt good and reacted well, I didn't get chance to play with the pedals. Changes in altitude isn't like collective pitch and there's a delayed response particularly when descending, this is going to make learning to fly this machine more of a challenge but achievable in my opinion.

 

[btd1982]

Next video. I might go to a single tether and see what that reacts like, the four tethers at each corner are ok but interfere with the natural movement quite a lot. I fitted a radiator fan so I can take more time and not be rushed. It's not going to be easy teaching myself to hover and it will take some time, it will be interesting to report back on how the cyclic feels/reacts when compared to a Robinson.

 

[Tyger]

All very impressive, so maybe this thread should be moved out of the "Welcome Mat" section at some point?

 

[btd1982]

All very impressive, so maybe this thread should be moved out of the "Welcome Mat" section at some point?

Thanks, yes I did think that I started a thread in builders corner at the beginning but it didn't go anywhere . Can it be moved ?