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May 30, 2004, 12:24 PM
Blade Springs "Rock"

Rotor Dynamics


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May 30, 2004, 06:48 PM
Blade Springs "Rock"

Box Stock.....


I flew it box stock with the springs today. Well basically box. The stock blades, motor, and tail rotor blade. I still mounted the stock blades on top of the rotor head simular to the like 90 setup, even without the stiffener plate, the spring and 2mm spacer plus the extra washer stayed in the grip. Did it fly? Yes, and very nicely in fact. A stock Aerohawk with the springs flew as good as the upgrades, or the Like90 package......I guess for about two extra dollars you to could get this Aerohawk as smooth as Duck Butter.........This photo shows the 5/8 bolts, I changed them to 1/2 and it looks better. The flight was good, no crashes, although I still feel the stock setup needs some work...but is pretty good with the springs....not bad.......for out of the box, and the spring dampener.....
Last edited by Geoffrey109; May 30, 2004 at 07:07 PM.
May 31, 2004, 08:16 PM
Registered User
Scott, one thing to consider in diagnosing the wobble is that the frequency of the oscillation is much much slower than the rotor head speed. I posted a video before/after the "elastomeric" fix for the AeroHawk a long time ago. The Wobble is ~1Hz whereas the head speed is ~1020/60 = 17 Hz. This leads me to believe there is more to the situation than just a rotor mass imbalance (from the blades being biased toward one side from lead/lag asymmetry). Also, the Disk motion appears as a rotation about an axis that is not normal to the disk. Imagine a rotor disk tilted from the horizontal plane by 5-10 degrees and spinning about a vertical line at 1 rev /sec. The motion of the tail boom is such that the aft end orbits in a circle in a vertical plane normal to the axis of the shaft.

I haven't strained my brain enough to figure out if having the lift force (from a lagging blade) at LESS than 90 degrees from the (rigid) fly bar/paddles leads to an unstable cyclic pitch control system? (fuzzy visions of Root-Locus plots in my head) The wobble diverges so strongly that I am convinced that there is an pitch driven aspect to it.

Its also true that when the blades lead the fly bar by more than 90 degrees (ie: lead slightly), then no wobble developes, even when the lead is significantly asymmetric (though 16 Hz vibration will occur to some extent).
May 31, 2004, 08:50 PM
Blade Springs "Rock"
The key, I have found is to not let the blades lag. Power changes, spin up, centrifical force will act upon light weight blades and cause them to lag, or put the helicopter into a wobble, this is a condition the spring prevents. I have flown both the stock, and the like90's blades, without adverse flight conditions, or control, exactly as photo'd. This imbalance of blade lag is solved. You would not believe how smooth, and how much control this little device has provided. It is simply amazing.......I wonder if the hobby shops have any left by now....???
Jun 01, 2004, 12:09 PM
Registered User
[QUOTE=Geoffrey109]The key, I have found is to not let the blades lag. QUOTE]

I knew that before you : ) With a heli aerodynamicist reading the thread (Scott), I figured it was time to find out why it mattered and produced the motion that is observed. I attempted to steer the conversation in that direction by proposing a mechanism (call it a hypothesis). I and others have a hard time with the "retreating blade stall" explanation, because it simply misuses the commonly held meaning of those words.

If my quest for greater understanding cuts into your coil spring self-promotion, I'd be happy to start a new thread to discuss the physics aspects exclusively.
Jun 01, 2004, 12:47 PM
Blade Springs "Rock"
Not at all, I added the link to Rotor Dynamics so I could brush up myself on the understanding of what is happening. My confusion, or Relationship between what is happening with Retreating Blade Stall and collective pitch is the systoms are simular, in that a pitch up/yaw will happen to the helicopter. I would very much like for you to continue.....
Jun 02, 2004, 12:49 AM
Registered User
Retreating and advancing will happen in a hover. I saw someone put that it only occurs in flight, not true. Any amount of wind from any direction over the rotor system increases the effiency of the rotor system. So For instance if your heli rotor rotates cw and was facing north and a wind was from the east the front blade would be the advancing blade, into the wind, and the aft blade would be the retreating blade, away from the wind. This would cause a diff in lift, even while in a hover. I think this is why we get the nose up effect in forward flight, just like in a real heli. The advancing blade produces more lift, ie when it is on the left side of the heli into the relative wind. Do to gyroscopic precession and phase lag, effects occurr 90 deg later, and you get the nose up. When we fly real heli's we compensate with a little left and fwd cyclic and so use to it we never notice we are doing it.

Hope this helps some, and sorry for the typing errors it's late and I don't want to spell check! Ha, Ha!!
Jun 03, 2004, 08:55 PM
Blade Springs "Rock"
I have tried to relate a "Retreating Blade Stall" condition, as in real collective pitch helicopters to a fixed pitch helicopter. The term/word Retreating, should've be lagging. The blades lag due to torque, causing the dreaded wobble. Vibrations and Wobbling are cured by installing the springs. Well at least in my Aerohawk, for both a stock, and Like90 setup. I found more control, and better, longer flights.

The Wobble Cure:

1. Tighten Blades, Cures some helicopters, leading the blades prior to take off.....
2. Light blades: Fall into lag, and sometimes do not catch back up....wobble...
3. Install Rubberbands: Works! Proven! Safe! They do keep the blades just forward and centrifical force keeps them in perfect alignment, and prevents lagging....
4. Springs: Cure all.

The Dynamics of the Rotor System are surely discovered. Collective Pitch and the Forward flight are proven. Fixed Pitch, and it's many problems and cures are surely discoveries, in micro flight, to still be discovered and explained. I believe that my blades were lagging causing the wobble, I have not had a wobble, or imbalance since I installed the springs. It's so far proven to me to be: Wobble free, and Vibration free....the funny part is that it cost me a whole $1.69 to find out. Now that is a cheap upgrade.....and for some:
Quote:
Originally Posted by RickyT
I've made blade springs out of paperclip for DragonFly/Hummy.
Tested with both stock & Like90 blades ---> no more vibration !!!
Regards
almost free........a paper clip......go figure......
Last edited by Geoffrey109; Jun 03, 2004 at 08:59 PM.
Jun 19, 2004, 10:47 PM
micro mechanic
aerohawk1's Avatar
Thanks Geoffrey!
They work great! A must for forward flight!
Jul 06, 2004, 04:10 PM
Really Cool User
Silverstreak328's Avatar
Why hasn't anyone taken into consideration the springs are also creating a gyroscopic effect?
Jul 06, 2004, 04:16 PM
Registered User
Lynxman's Avatar
What do you mean?

The springs are too close to the center of the rotor to create any significant gyroscopical effect with the low mass that they have.
Jul 06, 2004, 04:27 PM
Really Cool User
Silverstreak328's Avatar
Since your the expert I guess so. But if I had to guess I'd think the metal springs weight would add a decent gyroscopical effect :dunno:
Jul 06, 2004, 04:30 PM
micro mechanic
aerohawk1's Avatar
no,springs rock...

Period!
Jul 06, 2004, 05:50 PM
Aerohawk in London
Fragster2001's Avatar
Could someone show how they are mounted on a stock setup? Ive seen the two pics of them on stock setups but I cant see how they're held in place.
I wanna keep the blades in the normal holding position, unlike Geoffrey109 who has mounted them on top.

Thanks for any help
Jul 06, 2004, 06:09 PM
Blade Springs "Rock"
I don't have a macro function on my camera, but you can see in this stock setup, blades in the grip, I secured the springs but adding a longer 4-40 bolt, then using a washer and nylonloc nut to secure it......perfect every time......
Last edited by Geoffrey109; Jul 06, 2004 at 06:19 PM.


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