Dont use airplane prop tachs on helicopters

[jared_of_atlanta]

What's the difference between ground resonance and a heli with an unbalanced head? At a very low rpm, my small heli won't vibrate, as I increase it starts to vibrate, as I go to hovering speed, the vibration stops. I KNOW the main shaft is slightly bent.

[lrrps]

Dood,

Your avatar is scary, why don't you replace it by a nice chick's pic ?

[Tonystott]

Dood,

Your avatar is scary, why don't you replace it by a nice chick's pic ?

Maybe a real photo of him is even more scary? (shudder)

[Old Sloppy]

Dood,

Your avatar is scary,

I thought that was him........(Dood)

Michael Jackson's evil twin.....

Harry

[archiebald]

What's the difference between ground resonance and a heli with an unbalanced head? At a very low rpm, my small heli won't vibrate, as I increase it starts to vibrate, as I go to hovering speed, the vibration stops. I KNOW the main shaft is slightly bent.

All rotating systems have a natural frequency. If you happen to be slightly unbalanced and your rpm matches the natural frequency of the system, then you get resonance. If you are off the ground, this will show as a period of vibration at certain revs. But if you are on the ground, it is possible for the interaction with the surface to cause an increase in the oscillation. Once you are clear of the ground, and stop interacting with it, the vibration only gets as far as the skids.

At least thats the way I understand it.

[Cory101]

Maybe a real photo of him is even more scary? (shudder)


It would be. :shock:

[billbill242000]

I use one on occasion, never an issue. I velcro it to a battery and set it down below my rotor disc. I can read it just fine from a safe distance, even hover over it to get lightly loaded hs. Sorry but I've seen far more eggregious methods of tying down a heli, like the guy who strapped one on a ladder and tuned it from below. Now that is nuts. But this is reasonably safe, he's got the machine bolted down, the tach is not going anywhere, and how do you even know how heavy that deck he built is? Could be filled with concrete for all you know.

If you don't want to use one then don't, but it's not the horrific safety violation you make it out to be IMHO. It's not like the guy is holding the tach under the heli by hand or something. THAT would be incredibly stupid...

Going a little overboard there.

-Fog
Actually in the older Ray Hosteller books he shows mounting your heli to a ladder and staking the ladder to the groung to tune it so there is room for you to get at the needle from under the heli and be away from the blades.

[tconners]

ACE acctualy sold a tach that straped to the tailboom of your heli.
You came in close to read it and it would hold your max rpm until you landed.
I almost got one but decided the xcell tach was better.

[quartz001]

Hate to say it but my LHS told me that the digital tach worked fine, but the optical was better. I haven't used it yet so I may return it. My biggest concern was there are no directions on how to use either one. I've already been bit by one of my heli's and really don't want any body part near it, especially at full throttle to find out what the head speed is. I am interested in what the head speed is just don't know how to do it (other than the tables). So if I got the heli out of ground affect wouldn't that take this issue away? so the strapping to a ladder would do that. Believe me I'm not going to do it but in theory???:confused:

[WillJames]

Hate to say it but my LHS told me that the digital tach worked fine, but the optical was better. I haven't used it yet so I may return it. My biggest concern was there are no directions on how to use either one. I've already been bit by one of my heli's and really don't want any body part near it, especially at full throttle to find out what the head speed is. I am interested in what the head speed is just don't know how to do it (other than the tables). So if I got the heli out of ground affect wouldn't that take this issue away? so the strapping to a ladder would do that. Believe me I'm not going to do it but in theory???:confused:

The optical tach is extremely easy. Get someone to hold it and look through the little window while you are hovering/flying. They adjust the speed of the tach up/down until the blades appear stopped. Then look at the display and read off your speed. Setting it a little lower (say 400-500 RPM) than what you think your HS is before handing it to them makes them only have to press up and makes it easy for even your wife or somebody to do it. They can be at a safe distance away from you and the heli.

[Old Sloppy]

My Min Air XCell tach came with an instruction sheet.

Personaly I didn't need instructions, it is real easy to use.

but you need an assistant to take readings.

If you must fly alone then use an airplane tach because,

the airplane tach gets duct taped to your tailboom.

You hover it at eye level and take a reading.

[JMoore12]

Personally i agree with dood on the fact you should not tie down a helicopter.. You have to think about what is holding the landing gear in place. Four little tiny screws and four small (usually) plastic tabs mounted on the bottom of the frame.. waay to easy for somthing to break.

[r40734]

Well, I've never tried to tie one down for testing in that manner, but I've always figured those that do anchored it by the main frame. At least that's what common sense tells me how it should be done. Is it the norm to just anchor it by the landing gear?

[ShinOBIWAN]

Personally i agree with dood on the fact you should not tie down a helicopter.. You have to think about what is holding the landing gear in place. Four little tiny screws and four small (usually) plastic tabs mounted on the bottom of the frame.. waay to easy for somthing to break.

What nail are you flying for it to be built like that? On my own heli ,and for what your suggesting might happen, the lift would have to either break 4x M4 Hex bolts or rip through 2mm thick carbon fibre and nearly 20mm of plastic from the frame and skids. This ground effect is something I've never seen despite anchoring my own heli to the ground just a few days ago.

Scaremongering and misinformation spring to mind.

As for that video, an RC heli is going to have a much higher resonance frequency than full scale chinook with all that mass.

Daft thread.

[corona007]

I pretty sure he's talking about the Resonance that's created by the tied down heli. I have see it live before and they don't liked being tied down, They get mad and start to dissinagrate like that. Kinda reminds me of when I put my R30 on a scale with full neg pitch and then I got too scared to go look at the scale........Thinking to myself .
"Stupid is as stupid does"

[ShinOBIWAN]

Has anyone got a single shred of video or photo graphic evidence of this being a real phenomenon on regular sized RC heli's (ie 90 size or less)?

I've probably watched more vids on utube than is healthy and never seen anything like the Chinook video where oscillation created by an out of phase rotor become self reinforcing and magnifies in amplitude. There's shit load more mass involved and where the mass there's a lowing of reasonance. Higher mass and low resonance = lots of stored energy.

My blades weigh 185g and the heli is 4.8kg. Not hundreds of kg worth of blades and a couple of tons of heli.

Did a little more research and it appears twin rotor heli's such as the Chinook are FAR more suspectable to ground resonance than single rotors.

But sure, if you don't like it, don't do it.

[let60]

When checking the speed just dial you pitch to 0* and th curve to 100% at full. Know you have no lift at all. This takes lift out of the picture and give you your max head speed.

[Spawnmen]

its not in the least bit safe. You dont restrain a helicopter.
http://www.youtube.com/watch?v=ORjbMRNyGnE
http://www.youtube.com/watch?v=-LFLV47VAbI

Yes man, you are right, but you forgget one important thing - this effect could only happen to helicopter, with TWO main rotors. With normal heli model, this is impossible, there could be some crash, but the problem is somwhere else .-).

[Skiddz]

It'll happen with a pod and boom heli as well... Only takes one blade on the head to get out of phase and ground resonance can occur. The way to to get out of it is to pull pitch and get the heli off the ground. The blades will swing back into phase once the heli leaves the ground.

[Spawnmen]

Ae you sure Kevin ? One old man, told me that and I trust him, because he isn´t novice, he has a lot of expiriences ... So, when me told that ... It has something to do with the rotor head on our models, why it can´t occur ... ? ..

[Skiddz]

On our models, it's not as much of an issue because they're for all intents and purposes, rigid systems. And leading/lagging is dampened by the other blade since the blades are mounted relatively tight in the grips.

On a full scale heli, with a fully articulated head with lead/lag hinges, each blade can lead lag independently of the others in the head and because of this, a hard landing on one corner of the gear can get the blades out of phase. Continued contact with the ground keeps the blades from getting back into phase so if RPM is sufficient, picking back up into a hover and letting the blades sort themselves out is the proper recovery procedure.

http://en.wikipedia.org/wiki/Ground_resonance

[krush]

from the dead. I was reading the resonance argument and from my knowledge of vibes and resonance, I'd say that you'd have to have a LOT of variables occur in perfect harmony to get a damaging ground resonance.

The impact would have to deflect the blade CG just the right amount to send it into the crazy residence inducing chaos. The resonance freq would also change with RPM.

[Quah]

WOWW
What an UGLY helicopter!!!!!!!!!

[joediamond]

For all the time and money spent engineering and building the test rig it would have been simpler just to get the proper optical tach.