Dont use airplane prop tachs on helicopters

[Dood]

Im upset that some online helicopter retailers even sell these things.
They arent designed for helicopters. Dont use it for your helicopter.
Its dangerous, and completely wrong.

what this MORON does is also completely wrong and dangerous.
http://i175.photobucket.com/albums/w155/DJB78_bucket/0003.jpg

[DebianDog]

I wish I could see him underneath the helicopter holding the stick. :lol:

[Dood]

I wish I could see him underneath the helicopter holding the stick. :lol:

Oh. he's not really holding the stick.
But check out a wider view, notice he has a tether tied to his "restraint" as a just in case
http://i175.photobucket.com/albums/w155/DJB78_bucket/0001.jpg

[Dood]

I felt this thread needed to be in the safety forum.
It is essentially a response to this thread: http://www.helifreak.com/showthread.php?t=57224

Its not an issue of whether or not those cheap airplane prop tachs work or not.
Its an issue of what lengths guys will go through to MAKE IT WORK.

When someone (in particular a beginner) is shopping at an online store,

if they see a few optical heli tachs for $80 to $100, then the $20 tach sitting right next to it, honestly, which one do you think he is going to buy?
Seeing this for sale: http://www.readyheli.com/LCD_Digital_Mini_Tachometer_p/hcap0401.htm
It pisses me off, especially since they dont sell airplanes.
They should at least include a warning, "NOT FOR HELICOPTER USE"

[fogger]

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

[Dood]

But this is reasonably safe, he's got the machine bolted down,


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

[fogger]

hmm, I don't recall seeing that stipulation in the AMA safety rules, but at least one person fervently believes it to be the truth, so I guess it must be so...

What exactly can happen to a heli while strapped down that can't happen to one just sitting there on it's own?

-Fog

[sleddog7]

It is stupid and it isn't.

Fog and dood have their points, but as long as the guy doing this doesn't go over 6 or 7 degrees pitch he might be alright... going full pitch however, can release the skids for the frame... not many screws/bolts hold them on.

I'll stick to the optical tach myself though.

[MarkD]

Used to be the norm way of setting up the engine/head speed a few years back. I used a black and decker workmate with the heli strapped to the top to set it up years ago

[Finn Erik]

So, strapping down the heli to set it up and make adjustments isn't safe?
And reaching under a rotor at top speed to, say adjusting the needle on a heli sitting loose on a table or on the ground isn't?
Come on folks.

And what on earth did the vids above have to to with this subject??

Then again I'd rather have my heli strapped down throwing parts around than the whole thing coming after me. Been there once, though that was a link coming off in flight and not me flying but scary.

[Dood]

So, strapping down the heli to set it up and make adjustments isn't safe?
And reaching under a rotor at top speed to, say adjusting the needle on a heli sitting loose on a table or on the ground isn't?
Come on folks.

And what on earth did the vids above have to to with this subject??

Then again I'd rather have my heli strapped down throwing parts around than the whole thing coming after me. Been there once, though that was a link coming off in flight and not me flying but scary.

http://www.helis.com/howflies/groures.php

The videos I linked to, were that of a helicopter STRAPPED to the ground.

[mjws]

I believe the heli is ACE BIRD'S. I believe he spent a significant amount of time building the test stand and ensuring safety. On nice soft grass (no ground resonence possible) with a dolly (and I think even a kill switch) is about as good as it is going to get if you need to run up a heli on the ground facing wicked intermittent interference or a gasser that just won't tune and will cost you a grand to crash.

I'm all for safety, but strapping a tach to a paint stick and spooling up over it is hardly a capital offence.

Mike

[fogger]

For all we know they may have purposely imbalanced the rotors on that chinook and / or varied the rpm to exascerbate/demonstrate a potential problem. That machine with it's squishy tires and landing struts is probably a thousand times more likely to experience ground resonance than a model rigidly strapped down. Not a very convincing argument so far... Let's see a vid of this phenomenon on a 2 bladed model heli rigidly bolted to the ground. Talk about an apples to carrots comparison. (IE not even both fruit... :) )

-Fog

[Ginvent]

http://www.helis.com/howflies/groures.php

The videos I linked to, were that of a helicopter STRAPPED to the ground.

Dood, ground resonance IS a real issue for larger RC helis. particularlly those with scale fuses. One thing you dont realize is the ground resonace doesnt effect the heli if positive picth is applied. those that see resonace in their scale ships usally have a nrmal curve with 2 degrees or more of positive picth at the bottom so the heli doesnt shake itself to bits.

Find a example of a RC heli NOT in a scale fuse that is experiencing resonace. I have yet to find one.

[N11634]

Now let's take a closer look at the heli setup. We have a Predator Gasser with some sort of hideously modified frame on it. Plus, let's do away with those push-pull linkages and just put a single airplane clevis on there. Is that fluel tank even mounted on there? Looks like rubber bands holding it on! I'm sure that Co-Pilot helps him out a lot, too. This is the poster-heli for a safety thread.

BTW....both crashes on my Pred Gasser were right about $285 per.

[cudakid03]

http://www.helis.com/howflies/groures.php

The videos I linked to, were that of a helicopter STRAPPED to the ground.

"It cannot occur in a two bladed semi-rigid see-saw type rotor system, because the blades do not lead and lag." --- This is a quote from the page you linked to. Would this not apply to most of our rc birds. I am somewhat of a noob, and i personally like the idea of strapping the heli down for testing after a fresh build. Just my opinion.

[Skiddz]

Dood, ground resonance IS a real issue for larger RC helis. particularlly those with scale fuses. One thing you dont realize is the ground resonace doesnt effect the heli if positive picth is applied. those that see resonace in their scale ships usally have a nrmal curve with 2 degrees or more of positive picth at the bottom so the heli doesnt shake itself to bits.

Incorrect. Ground Resonance has nothing to do with the amount of pitch. It has to do with blades getting out of phase due to a hard landing - usually hitting on one corner of the landing gear before the rest of the gear in on the ground. Helis with wheels and/or oleo struts are most succeptable. The fix, should you get into this, is to pull pitch and get the heli back in the air to let the blades get back into phase.

Find a example of a RC heli NOT in a scale fuse that is experiencing resonace. I have yet to find one.

Any RC heli on spool up with the shakes as the blades phase IS in resonance.

[Old Sloppy]

You're supposed to tape it to the top of your tailboom, then hover it at eye level.
Kind of like checking the mainblade tracking.
I do not see any danger in this photo. (but I do see a moron in action)

Harry

[Skiddz]

"It cannot occur in a two bladed semi-rigid see-saw type rotor system, because the blades do not lead and lag." --- This is a quote from the page you linked to. Would this not apply to most of our rc birds. I am somewhat of a noob, and i personally like the idea of strapping the heli down for testing after a fresh build. Just my opinion.

Unless you've bolted your blades on so tightly they cannot move, they can still get out of phase. What the quote above references is full scale semi-rigid systems where there is no pivot point where the blade attaches. The blades on this type of system are free to feather (i.e. change pitch) and flap. Acceleration forces compensated for by lead/lag hinges/dampers on fully articulated heads are compensated for in semi-rigid systems by flapping and the underslung teetering hinge.

[Swoop]

What is that sitting on top of the boom over the tail servo? It looks to me to be an onboard tach. Is that a rubber band holding it on? I have more issues with strapping something under the main rotor with a rubber band.

[fogger]

That is a optical horizon sensor for an AP autopilot system I think...

[tz250w]

I think I'd be more worried about the pop-riveted and soldered frame coming apart from the weed whacker engine's vibration than anything else... :shock:

[squeakywaffle]

this:

http://www.heli-chair.com/aerodynamics_102.html

is a great explanation of ground resonance.

My best explanation is that on a helicopter where the blades are free to lead and lag (like most models), striking a hard surface on one skid (touching down while not level) can deflect both blades to that side (if they are more or less parallel to the axis of the boom) such that one leads and the other lags, making it so that the center of mass is no longer on the axis of the main shaft.

This imbalance can shift it to the other skid, which then hits the ground much like the first skid did. If the rotor has rotated about any number of revolutions plus an additional 180 degrees, this impact will lead/lag the blades even more in the same direction, moving the center of mass further from the rotor's axis of rotation (If it has rotated a whole number of turns, the blades would lead/lag back toward their original positions).

If the interaction is 'in phase' (which depends on a lot of different parameters like the lead/lag damping, wheel damping, etc), this will happen over and over again until the center of mass of the rotor has moved quite far from the axis of the main shaft. As it continues to rotate, this causes a load perpendicular to the main shaft in the direction of the center of mass, which is rotating around the main shaft with the rotor. You know this force as centrifugal force (don't let anyone tell you it's not real! :YeaBaby: ).

This force quickly grows as the resonance increases the displacement of the rotor's center of mass, eventually causing enough radial load on the main shaft bearings to either tear them apart or cause significant structural failure elsewhere.

So yes, RC helis can suffer from this (actually they are more susceptible to it because they have no lead/lag dampers). But given the higher ratio of maximum main bearing radial load to rotor disk and body mass that's present in models, especially little ones like my t-rex, the heli doesn't fall apart, it just dances around.

I've never seen it but I can totally imagine that a larger model could have some serious problems if this were to happen. Under the right (wrong?) conditions this could happen very quickly.

I'm actually glad I thought about this... I'm trying to write a flight sim but I got bogged down in the equations of motion for the helicopter. The rabbit hole goes pretty deep.

[fogger]

I've actually seen this happen a couple weeks ago on my concept 30, which has a pretty loosey-goosey flapping head setup. It was not strapped down but on landing it started shaking violently until I either lifted back off or shut it down. Pretty crazy. But on the newer models with practically solid mounted blades it is far less likely IMO, as I've never seen it in a trex 450 or 600, or any of the other 3D models at the fun flys I've been to... Based on that experience I would say that blade flapping is more likely to cause this than lead-lag, but that is just based on my observations

-Fog.

[archiebald]

My Raptor is very well balanced and running carbon blades.

And yet, by spooling up slowly and holding it at a certain rpm I can put it into ground resonance at will and if kept there would probably shale itself to bits or topple over. Naturally, as soon as I see it, a dab of throttle clears it but it CAN and DOES happen.

Most people don't see it because they don't try to find it.

No way will I tie down a heli for setting up.