Wondering if shorter blades might just help with this or shorter fronts 315 vs 325 ?? Move the two tip vortex’s into different positions ? Nose up flare to slow down is smothering the rear rotor in dirty air. Or recirc off the front is double compounded - drowning the rear during the flare ?
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Over the past 2 months, I've been testing things a lot with the Twinrex mechanics. No matter how tight I get the gear mesh, the aircraft constantly has +/- 15 deg play in the cross shaft between the fore/aft rotors. I can slightly reduce that to +/- 10 deg with strategic replacement of gears and bolts, but after 1 flight it works its way back. The "sinking/VRS/settling with power" issue never goes away either. It is less noticeable/ more easily recoverable with a very forward CG, but that's all that seems to help.
One clue I've had a few times to explain this phenomenon, has been scuff marks and scratched paint on the top of the forward rotor blades, and bottom of the aft blades. There is also a very distinct sound, and audible growl when this sinking occurs. This leads me to conclude that the cause of the sinking is the blades leading/lagging enough to end up over the top of each other. This seems to also explain why this issue is not seen in other tandem models, nor in real tandem helicopters. If this is the case, switching to a 2 blade head should resolve the issue by providing more clearance between the rotors even with the drive train slop. This will be the next step in the troubleshooting process.
If this works, it means I'll need to re-engineer the drive train to work similarly to other tandems, with larger drive gears on each mast and a smaller gear connecting to a lower torque/high RPM interconnect drive shaft. This should dramatically reduce the allowable slop between the rotors.
For now, its just a theory, but its all I've got to go off of.Comment
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hmmm thats a real pity as the three blade head is what the orginal used and provides more lift, any easy way to raise the rear mast and or lower the front mast another 1/2" or so ??? Scrape marks though indicate blade contact surely ??? yikes !!!
what about longer rear blades (345) and shorter fronts (315) ?? assuming both are on 325mm now ???Comment
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not sure if you have seen this guys videos, really interesting;
couple of explanations on VRS
https://www.youtube.com/watch?v=P30SyNldEy4.
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A few updates..
Kiwi, to me the issue didn't seem to actually be VRS, it just behaved similarly to it and was a more stable (harder to exit) condition. I'm a full scale helicopter pilot and flight test engineer on experimental helicopters, so I'm quite familiar with VRS. I've experienced it a few times in my other (generally heavier) rc scale helis as well. Usually recovery is pretty quick and easy assuming you recognize what is going on quickly and get some forward or lateral speed to pop out of the vortex and into clean air. My theory remained that there was enough play between the rotors, plus enough lead/lag motion of the blades, that the rear blades were crossing over the forward blades in certain flight conditions. This seems to be backed up by the paint scrape marks on the blades, and the audible growl that is produced when the condition occurs.
I spent the weekend going through the TwinRex mechanics and shimming gears at the advice of another person who has the same mechanics. I changed out all the 2mm gear through-bolts for 2mm steel pins. Also added some shims to get the gear mesh set better. Play between the rotors was dramatically reduced from +/- 10-15 deg to +/- ~2 deg. I took it for a quick flight in the backyard and didn’t see any of the sudden sinking behavior I’ve seen before. Still needs some gain tuning, but it’s a definite improvement. Hoping to get a bit more gain tuning done this week and start looking at mounting the mechs in the fuselage soon!Comment
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Hi F106DeltaDart
Beautiful helicopter and very original.
To limit the play between the front and rear rotors wouldn't it be possible to put larger gears (with more teeth)?
It would be a shame to put two-bladed rotor heads.
On the Chinook Vario we can clearly see the positioning of the blades in flight
If this can help you, here is a picture of the mechanics
DanielComment
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delta , awesome to hear it's more of just a a mechanical phasing issue, and that you have been able to get it sync'd better. I've learned allot from that website on the basics of heli flight thats for sure. I really like interesting technical problems, the solution is always so much sweeter when its nailed down from a solid understanding of the systems etc. Glad to hear that it looks like an easy fix !
Really look forward to seeing some flight videos in due course, it's such a sweet design, I'd like to build one myself. Also in dialogue with the guys over a Sikorsky H-5, thanks for the assistance there.Comment
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Update, several years later.. I might have a potential explanation for my issue now. Will need to put it back together and test it to see if I’m right. The theory is that some of the issues I was having may relate to delta-3. This is the offset between the ball link and flapping hinge along the grip axis. This distance is pretty important in understanding blade dynamics especially at high flapping. If your ball link is inboard of the flapping axis (for leading edge control) this results in a negative delta-3. This means that when a blade flaps up, its angle of attack actually increases. This can under certain circumstances, drive you towards an unstable system producing a lot of lead/lag motion. If I switch to trading edge control, it will become a stable system, ie flapping up, results in a reduction in blade AOA. Will be giving this a shot soon on the TwinRex. Hopefully it solves the issue, otherwise I’ll need to reengineer the bevel gears and drive system to resolve it.Comment
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