the dreaded ground resonance. need help

[Jackass1277]

So i go to maiden this blue turder and as im spooling the head up with my fenstron going at a 90% linear throttle line i come into the worst ground resonance i have ever encountered. so bad that literally disassembled almost the entire helicopter for me and even threw the canopy off. now on the good side i hadnt painted it yet. on the bad side i have to re epoxy damn near everything back on.. guess i need better epoxy. but with that being said i want to hear some tips or tricks from the veteran scalers on how to avoid this or build it out...

[Keyrigger]

Good to hear it's not totalled. I have had two very bad times with two scale helicopters. One was 50 size Airwolf and the other was my own CH-139 after I crashed it in Louisiana. The trouble with the Airwolf came down to a motor wobbling, a bent but brand new main shaft, a very sloppy set of main shaft bearings, a bent torque tube (again, brand new), an incorrect setting of the spool-up speed time and power, and a bent tail shaft and you guessed it, brand new. I had two weeks of the worst possible troubleshooting with that helicopter and in the end, it exploded in mid-air, about a foot off the ground.

When all those parts were replaced, the helicopter spooled up just fine and it was one of the best flying helicopters I have ever built. The issue of the speed controller was that when you change speed, you can have the change happen fast, medium, or slow and in the case of this esc, it was set to fast. Torque happens fast and that, combined with the other faults made for one really bad vibration resonance.

With my CH-139, I had thought that the damage done in the crash was not as bad as it really was. I had a bent boom, a tweaked torque tube, a bent tail shaft, a bent tail rotor hub, and a set of blades that were perfectly balanced for weight but statically, would not balance together. I used a simple Align balance checker and added .4gr to the light blade (remember that these blades weighed exactly the same before the balance checker). Once the parts were replaced, the helicopter was back to its old self once again and my nerves finally settled down. I take it that your BT uses separate motors for the tail and main but do have a look at the torque tube used to drive the tail fenestron. High speed buzzes might add to resonance but look for shafts running out of true and maybe a motor that does not run perfectly true and you have two of them. Make sure your drive for the tail is perfectly inline so there is no side-loading of the torque tube. Hope this helps.

Don

PS: Try Hysol or PL Premium for internal woodwork and Hysol for adding detail parts to the outside. If there are things like weapons wings, PL Premium can be used as well. Even slow set epoxy tends to not be able to bind to set fibreglass resin and PL Premium may be better than Hysol according to a few jet builders I know.

[Andy01]

From what I have read (and not personal experience thankfully) some of the things to watch out for may include (apart from the obvious checks to make sure that everything is balanced and smooth);

- Securing the top of the mechanics to the upper parts of the fuselage (so it is secured top and bottom).

- Avoiding spooling up on the ground at slightly negative pitches. I usually start the spool-up and move the stick to a position that gives me around 1-2° of positive pitch. It usually wobbles a bit but that is pretty normal with most helis.

- Avoiding sudden changes in speed in the ESC. Set everything (spool-up rates, change of speed rates etc) to slow if possible.

- If you have wheels (rather than skids) make sure that they are not too "springy". It is better to have them fully compressed under the heli's weight.

- Avoid moving the cyclic stick (if you have a FBL controller) when on the ground.

I would say that the first one is the most likely to cause problems.

To prove the balance etc I generally spool everything up in stages with a new heli;

- bare mechanics with no head or tail rotors (or swashplate)
- mechanics with bare rotor heads (no blades)
- as above with tail blades fitted
- as above with all blades.
- check all screws, nuts, fixtures etc between tests to make sure nothing is shaking it loose.

I usually limit the speed (either Set rpm governor slowed right down, or simply with linear throttle curve) starting out at a few hundred rpm, and working up from there. By the time I got to lifting off recently with my EC-145, I had probably run the mechanics for 10-15 minutes at all speeds (I took the bare mechanics up to around 1500rpm according the data logs). This proved that nothing was bent or vibrating long before I ever got close to being airborne.

Cheers
Colin

[Keyrigger]

One thing I should mention about the Airwolf debacle was that the chassis few perfectly with no vibrations present at any time when pulled out of the body. It was nasty to trace it down and took the helicopter exploding in mid-air to start the vibration chase process. Second, the bent bits in the CH-139 were purely my fault from not tearing it down to check everything out the mechanics, thoroughly.

Don

PS: Good tip about pitch, Colin.

[goodhunting]

I agree with the pitch issue. It can destroy real helicopters as well...

I spooled up my Trex 700 without the blades for a desktop test.... And only had one main blade screw in the grip.... The vibes were such that the whole heli was bouncing on the floor. Put in the other screw and the mechanics spooled up without any visible vibes...

Gh

[Jackass1277]

Hysol... 9460. The epoxy of gods.. what I have noticed after the shake rattle and thump is now the mainshaft is no longer true so I have to order and new one. And as for bracing. I brace every where possible I want my mechs to feel solid to the fuse. As for negative pitch I'm using 4 to 5 degrees max. Is that too much?

[Keyrigger]

In Mode 1 and 2, -4 to -5 is not too much and is normal for most. What we are referring to is when you start the helicopter, there should be no negative pitch in Normal Mode and should gradually increase to your hover pitch at mid-stick. None of my Normal Mode pitch curves have any negative pitch in them. Hope this explains it.

Don

[Andy01]

The resonance tends to occur when you transition from negative pitch to positive pitch because you are going through a phase where there is no load on the blades.

That is why it does not really matter if you spool up with -12° or -2° - it is when it changes from -1° to +1° that the heli is at risk.

Colin

[mbutler6063]

good good stuff

[Mercuriell]

-TAke off from grass -never a hard surface
- Don't have blades too loose - should be stiff in the holders
- Spool up and down with zero pitch
- Make sure shaft straight - blade grips tracking true and blades balanced
- Consider a training gear setup for test spoolups in future

If you want to see world classs resonsance check https://www.youtube.com/watch?featur...&v=jvdE_uKfTdE!

[Jackass1277]

I've narrowed my f up to using negative pitch during spool up. I never even considered that going from negative to positive collective could cause such an interaction but now that I have thought about it I just keep saying to myself... dumb ass what were you thinking.

[goodhunting]

No need to treat yourself harshly. You don't know what you don't know. Consider yourself lucky as "now you know"...

Gh

[StripedGear]

+++1 I love this thread.. Great discussion and great explanations. Thank you Don, Colin and all.. I have never encountered this myself but I know now that I have been flirting with disaster for a long time. I have done a 100 or so static run ups at 0 pitch for 5 minutes at a time to see esc temps, voltage and power out and so on castle logs... No need for that since I've learned that flight logs are what I need to see.. I guess I have been one lucky sob.
Thanks guys