vanman

My Feedback: (2)

I looke far and wide to see if this had been questioned before adding yet another Honey Bee forum.......

I carry the Honey Bee Heli's in my shop and I have seen a couple of things that so far we have not been able to correct.....

When the Honey Bee takes off and is put in a hover, it starts out smooth and then the tail begins to bounce. Softly at first and then continues to increase until it is completely out of control. I have seen 2 that do this all the time, 1 that does it occasionally and 2 that don't do it at all.

The one that didn't do it at all is probably the best flying, most stable electric heli I have seen... The ones that do begin to bounce start in to the hover fine, and then become un-flyable.....

We have looked at and adjusted just about everything that we can think of...... has anyone else seen this problem? What was the correction?

Thanks to the collective mind of RCU in advance....

Bill

quaker1

It sounds like the main blades are too loose, this happens. Tighten the screws a little bit at a time and Im sure this will solve the problem

racemiata

Same thing happens to my esky honey bee. I have everything balanced as separate components then as an assembly but for some unknown reason, the blades don't unfold perfectly by themselves like they used to so I have to tighten the blades a little bit. Although this fixes the problem, a light tap of the skid on the pavement can trigger the vibration again. All I have to do is land the heli and lift off again then the vibration goes away. This is not too annoying but at one point this problem absolutely didn't apply to my heli so I know it's fixable.

At one point, I also had my rotating components balanced separately and then balanced the whole assembly and tracked a set of higher-speed blades (they're just stock blades with the tips shortened -- actually "broken" is more descriptive). The heli went so balanced that it requires significantly less tail power that I had to trim down the mix. I even tried running the blades so loose that even on a smooth slow-down on the main motor, the blades would fold themselves. Even with the main blades that loose, there was absolutely no vibration, even if I deliberately tap the skids on the pavement. I suspect that the stock head speed is a bit low and is prone to harmonic resonance. I suspect increasing the head speed will prevent this harmonic resonance (but of course you'll have to have every rotating parts balanced FIRST).

quaker1

absolutely right, my head speed is very high but my blades are very loose and all is fine. If you have the stock head speed which is very slow you have to have the blades on the tight side I find, and it also helps to have them starting off in a slightly advanced position.

camra88

Whats it mean to be "slightly advanced position"?

quaker1

Left one normal, right one advanced.

racemiata

Interesting! I found exactly the same. When I have to tighten the main blades a bit more to prevent this tail oscillation, it helps to start with manually "unfolding" the blades to balance on the flybar. The oscillation won't come back until after a few bumping such that the main blades shift angles on the grips. And it helps even better to start with manually "unfolding" the blades to balance on the flybar but having both blades advancing a bit. In that case, the heli can withstand even more bumping before the tail starts oscillating. I wonder if I file down the leading edge of the blades in order to move CG further behind, will the centrifugal force automatically unfold the blades a bit more advance and prevent this oscillation from happening?

quaker1

I think the reason it works is because when spinning up the blades they can retard into a good position, so even if you file them you will still have to advance them to get the same effect.

racemiata

...but if my theories are right centrifugal force should bring the blades to their ideal positions. I'd prefer that solution than turning up head speed. The problem with low head speed (without reducing lift) is not just because it falls into harmonic resonance speed range. At low head speed, air resistance could be significant enough to prevent centrifugal force from bringing the blades to their ideal positions. So theoretically, blades with CG further behind should let the direction of the centrifugal force on the blade cancel the pull from air resistance. Maybe I'll try putting some tape on the trailing edges of the blades to change the CGs and see if my theories are right.

camra88

what is harmonic resonance?

racemiata

An oscillation occurs when an object goes back and forth with a fixed pattern. If somehow an external force, even though very small, contantly nudges this object in the direction it travels in each stroke (in harmony), the magnitude of the oscillation can be substantially increased(harmonic resonance). The key is to have the external force change direction exactly as frequent as the oscillation itself and act in the same direction the object travels. On our micro helis, this external force most probably comes from the chassis flex. The frequency is probably somewhat below stock headspeed. A proof can be found when you attach training kit to the skids and amplify vibration. Because of that, I removed my training kit way before I was able to confidently hover. Also, I deliberately pull the skids from the chassis a bit in order to leave some slacks to reduce harmonic resonance thru' the skids and batteries.

However, this harmonic resonance is not what I meant about the tail bouncing which occurs at ~2cps (cycles per second) with the tail going up and down (or tail swirling in circles if gyro setting is low). That is very different than the vibration from an imbalanced main rotor assembly at about headspeed (~10X the frequency). The reason I brought up harmonic resonance is that it might trigger the tail bounce.

I've ruled out the cause of tail bounce from any tail rotor imbalance as 2cps is too low to come from the tail rotor, especially my DD tail. I suspect the tail bouncing comes from some kind of a delay/slack adding to the stability design on the main rotor.

If I have time tonite, I'll try to see whether the flybar mechanism is able to keep its rotating plane while the tail bounces. That might suggest whether the problem lies between the servos and flybar mechanism, or between the flybar mechanism and the main blades.

racemiata

Tail bouncing fixed!

Interestingly, it requires a simple but effective mod -- "The rubberband mod!". More on it later.

Friday nite, I swapped back-and-forth my stock blades and high-speed blades to reproduce the problem. With the high-speed blades NOT tightened, I noticed the tail bounce went away by itself. With the stock blades, I tried to hover eye-level in order to notice whether the flybar would stay in the same rotating plane while tail bounce occured. Turned out it was quite a difficult and dangerous stunt (tap heli on ground then fly up eye-level and stay there risking the heli flying into me). I decided to use trial-and-error diagnostics instead. Putting a piece of tape on trailing edge (or leading edge) didn't make a difference. I then tried rubberbanding various couple of parts joint by a ball-link to see if any loose ball-link is the culprit.

I ended up with a simple but effective mod to successfully eliminate tail bounce. With the main blades folded towards their leading flybar paddes, I slipped a rubberband thru' each main blade and the flybar such that when the main blade is unfolded, the rubberband creates a very small force (not enough to fold up the freely dangling blade when stopped) to help cancel out air resistance overly pulling each blade backwards. Seems like in the meantime, the rubberbads double-duty as taking out the slacks on the ball-links connecting blade holder and flybar outer frame, and also the fore-aft rotating slacks of the flybar hub (inside the rotor centre hub). All these must be helping stabilize the flybar rotating plane.

I could tell the difference even before lift off. Before the mod, I used to have the skids light and then found the flybar rotating plane shifting around without cyclic input. When that happened, the heli could tip over any direction any time so I had to quickly lift off. Now with the rubberband mod, the flybar rotating plane is much more stable even though I run the main blades very loose. I initiated the tail bounce and saw it went away even quicker than with my high-speed blades but no rubberband mod.

Like with my high-speed blades, now I can run my main blades dangling loose and have no worry about tail bounce. Whether the main blades get folded forward or backward after a crash, I don't have to manually unfold them. Centrifugal force will take care of it. I verified that the rubberband mod doesn't throw off the rotor balance or blade tracking. The rubberbands don't rub on moving parts and the rubberbands don't get constantly stressed-and-relaxed during normal operation.

quaker1

Looks interesting, I think they have something like this on full size heli`s.

racemiata

Next, I need to make the mod less ugly. [&:] Maybe I'll use some much skinnier rubberbands in single-loop and perhaps paint them black. They aren't asserting enough force to require those thick rubberbands I'm using anyways.

racemiata

Update to my rubberband mod:

Although still not perfect, the newer smaller rubberbands I use now are more stealthy (especially after having them painted black). Check out my pics below. Notice the main blades in the first pic there're scratches from hitting the flybar paddles during crashes. On the last 2 pics you can see that the rubberbands are not too noticeable. [link=http://www.imagestation.com/album/?id=4286125085&p=4132108261&idx=18&mode=invite]Check out my video here to compare the heli vibration after my mod (the part with "zero-gyro" towards the end ).[/link] Warning: Big file of 37MB and long video. You may have to do a right-click and "save as" instead of streaming the video. As you can see the heli is even more stable with zero-gyro setting than before where I had to run 60-80%.

JAG Hobbies

Has anyone figured out the cause of the oscillation yet? I have just received my first Honey Bee for evaluation and I have the same problem. I have balanced the blades, tightened them up, added rubber bands, advanced the blades and aligned the paddles. Still no luck. It seems that every time the helicopter is hovering, it reaches it's natural frequency, oscillates until it's uncontrollable then it must be landed. (Usually crash landed.) It's very stable until it hits that point.

The only thing I haven't tried is to cut down the blades. I believe all this does is increase the frequency beyond the natural frequency. Is there some part that may be misalligned or out of balance that could be causing this? Could it just be the slop in the linkages?

Please help.

Thanks,
Jim

JAG Hobbies

Problem solved!! It's the main rotors. Since they are injection molded and cool differently, not all are the same. If the pitch of the blades don't match, the aircraft will oscillate. This can also happen when the blades are too loose. The pitch can change and will have the same effect.

To fix the problem, replace the blades and tighten the screws down where the blades are not too free. It worked for me. I didn't like cutting down the main rotors because it would reduce the flight time.

Since I think I have the problem resolved, I'm going to stock the helicopters and parts.

Jim
JAG Hobbies
www.jaghobbies.com

tanasit

Instead of the rubber band, try o-ring. My ornithopter uses several o-rings to hold tail to servo horn and wing to main frame (this is the stock items not modify).