View Full Version : When blades collide...

09-11-2006, 11:44 AM
So, what happens when one doesn't use the proper amount of thread locker on the head bolts and the blades de-synchronize?

Well, about a month ago, I found out. And it was caught (albeit briefly) on video.

In short, the forward head bolt ("Jesus bolt") loosened during hover runs. (Since the forward head turns clock-wise, there will be a propensity for the bolt to loosen.) The bolt backed out enough to allow the forward head to lift off of the shaft just far enough to allow the steel pin (that keeps the head linked to the shaft) to escape. Within a split second, the aft blades, still under power, smashed into the forward blades, and the rest was... interesting.


09-11-2006, 12:32 PM
Case in point is when you feel something strange just stop flying.
Is it two engines with a syncronizing shaft or one engine with a shaft that goes to both rotor heads.

Maybe using left handed threads would solve that problem. Depending on threadlock seems like a design flaw and should probably be rethinked.

09-11-2006, 01:13 PM
I agree; if it doesn't feel right, land. I attributed the bouncing to the wind, and thought it was normal.

As for the design, yes, reversed threads would be better, but the maching/parts requirement would be expensive and tricky to package and mark to eliminate confusion on the user end. Frankly, I have to say 'mea culpa' on this one since I've never had it happen before or since. Of course, I am much more careful about the amount of threadlock I use and I check the tightness of the bolts before each flight. Just one more item to add to the checklist.

As for the drive, the Hirobo tandem uses a single engine up front driving the forward shaft through a standard main gear/ clutch-pinion system. The forward shaft is connected to the aft shaft via a twisted, cogged belt. (The twisted belt gives the counter rotation between the shafts) It's a robust, proven drive system that is fairly bullet-proof as long as you set the tension properly.


09-11-2006, 01:30 PM

Nice job on the articles for MHT!

09-11-2006, 02:22 PM
Thanks, Andy! I appreciate the feedback.

I'm working on a new review for the next issue. And I promise that you won't get "stung" by this next review... :wink:


09-12-2006, 06:30 AM

Similar happened to my sea knight. In my case first flight(sofar my only flight) I selected heading hold in the solitary gyro on cyclic pitch after being somewhat uncontrollable in non hh and it reared up until the rear disk was eating dirt, then once this happened it was about 3-5 seconds later it slipped a few teeth and then six blades all got to know each other intimate like. so ugly when 6 blades die at once !! currently having a breather and playing with my new girlfriend, sofar a little cheaper !
Also I have had the same jesus bolt come loose but spotted it before it caused trouble ,as said it's now part of my preflight too !!

so I have a few questions for you tandem fliers

1.what head speed do you run, as i understood around 1300 , but at this speed even full pitch (around 8 on the front) it wont lift off immeadiatly after selecting hold
2.what pitch difference do you run between the back and front heads
3.what pitch range do you run on the front and back disk
4.what pipe do you reccomend (im running a 0s46)

I also understand NHP UK will do a matched set of 3 razor carbons for those of you who have cash burning a hole !

regards Don

09-12-2006, 10:15 AM
Hi Don,

To answer your questions:

1. I run 1600 RPM. The manual recommends 1500-1600 for hover, and 1600-1700 for forward flight. Being a simpleton, I fly 1600. I think the higher headspeed will give a bit more control authority for you.

2. The forward head on my bird has one degree MORE pitch than the aft head. I've tried equal pitch a couple of times now, and it just doesn't work for my model. Each time, I end up back at one degree more in the front.

3. My hover pitch is 6 on front, 5 on back. In normal mode, my pitch is 10 on front, 9 on rear (85% curve point). For throttle hold, I run 12 on the front, 11 on the rear (100% curve point).

4. The Hitori pipe that comes with the current Hirobo kit is very nice. I recommend it, but that's the only pipe I've used on this model.

Maybe we can get Jeff Green and some of the other long-time tandem flyers to give us their input.


09-12-2006, 11:25 AM
I'd think the 46 would make that kit under powered, it sounds like you are pushing a hyper when you factor in that you have to run it rich to keep it cool.

09-12-2006, 05:29 PM
Hot dog man here. I have had my vertol now since 1990 ( ver. 1.00 ) . I ran the stock muffler with my OS 46 sf-hs ( rear cone-start). It ran ok but I had over heating problems caused by Hirobo's inadequate cooling shroud-duct. It was designed for the 32 and stopped short of the nearest cylinder cooling fin. I adapted a 60 size fan using a cooling shroud designed by Century for their Falcon 46 upgraded helo. Pretty much solved all the problem. But then I put a Hatori tuned pipe on and the monster came out of the engine. What a vast imptovement! I still run 15% and no problems. Same engine I bought with the kit. I had it over-hauled 5 years ago after bringing the ship out of storage. Third flight after over-haul 30 feet in the air and the front bearing seizes up! They do auto-rotate! I do my own over-hauls from now on. My first public flight with my Columbia scheme fuselage and the clutch lining decided to let go. Losing power, drifting aft and sideward, the trike gear touched down and everything tipped over. Paper thin fuselage was nearly cut in half. I took eight months before I pulled the remains out from under my bench for the autopsy! Then I had an epiphany! The flying HOTDOG was born. It is built from a single layer of carbon-fiber cloth with epoxy resin.. It does fly! Watch for it on the OUTDOOR channel program Inside R/C either in October or November.I am so Stoked!!! Another cause for blades to become un-timed is belt tension too low. I have experienced that condition on two occassions. Once in the air and one on the ground. I am SOOOO glad that Hirobo now offers the blades in built-up form rather than do-it-yourself fit version. I am glad to pay the extra bucks.!

Thanks for the forum and I will definitely spread the word!!!
As soon as I build a gallery, I will post pics.

09-12-2006, 08:32 PM
Thanks fellas

Exactly what I wanted.
Mine is also a v1 trike gear factory 0s32 wee black muffler repowered had about 8 flights before it was mine with factroy 0s46 rear cone start. still mint.

If your having success with the Hatori what one is it ?

So where is your cog ?have you played with it and what did it do ???
as my with the kit info is vague.

mine goes tru a little vinegar stroke about half way to 1300 do yours? nothing has flown off at 1300 yet. each 3 blade matched cog in span weight within 0.1g and tracks up to 1300 ok.

What do you think about fitting steerable front wheel for ground taxi ?

Awesome to hear from Hot dog about a v1.00 that's flying !! and on TV - wicked,
I also improved the shroud using thick laminating plastic, temp ok and on the pipe ok in our spring but would like a little cooler run for sumer.
I have a few scraps so will make up something too. It's great to hear how others have done it.
I've fitted a nasty looking little koyosho diverge converge muffler while I find something reliable, powerful, light, and fits inside ! I'm usually running 10-15 % nitro os8 sea level 18-27 degrees c
only after a smooth run !

regards power I know it spools up to 1300 ok kinda similar to my R50 hyper I do have a 50 I could put in it but I don't think it will need it and may break ther rubber band, I want it as a smooth cruise machine I can easily stay in front of and hopefully never require more power !
Ive hovered it for about 10 seconds looking like one handles very hot toast, therefore I tried heading hold, several of us spoke of more speed I was a little scared and have waited until I heard from you folks !!

My tandem flying buddy , who did the 8 tanks is away for a few weeks so will crank it up again when he gets back. He replaced the rear hex head retaining bolt with the next size up in dia as it broke one time while he was spooling up. Lucky !!

Regards Don

09-13-2006, 02:00 AM
I don't think you will ever break the drive belt. Fiber reinforced makes it pretty tough. Mine is still th original one . I am using a Hatori tuned pipe "50 - 3D " P/N 540. Of course the pipe is either on or not on so it is a little peaky. When I was flying for " Inside R/C " folks, the engine was not up on the pipe. I am not sure of my head RPM . i haven't tached it. Definitely OS # 8 plugs! What are you describing as a COG ? If it is the rotorhead, then I have them timed so that both head blades are straight forward inline with the ships keel.Nothing fancy. But make sure your belt tension is good!!!! The only time I ever broke a head bolt was when I used RED LOctite. I only use the BLUE now. When I was first learning, the trike gear gave me a terrible headache. So I built training gear from 1/2" PVC pipe in the shape like standard heli landing gear. I am using a vintage 1985 narowband Futaba PCM radio. It weighs a ton, but I am not sure if I want to go through the learning curve again for the new radios. A pair of Futaba G-153 gyros keep the attide in check. No heading hold function either.

09-13-2006, 03:35 AM
cog = centre of gravity

if you were to balance the chopper length ways with a knife across perpindicular underneath where would this balance point be ? 50% 40 % ? I would guess a little toward the front otherwise why lead pitch on the front head ?

Regards Don

09-13-2006, 09:43 AM
Flying Hotdog...............That is TOO FUNNY! :lol:

09-13-2006, 10:02 AM
Flying Hotdog...............That is TOO FUNNY! :lol:That weiner is hilarious! :lol:

09-13-2006, 01:12 PM
cog = centre of gravity

if you were to balance the chopper length ways with a knife across perpindicular underneath where would this balance point be ? 50% 40 % ? I would guess a little toward the front otherwise why lead pitch on the front head ?

Regards Don

I don't know what my C/G is. It flies with both heads level in a hover like it should. I made pitch adjustments dynamically after a few flights and that is where I leave it. If it ain't broke, why fix it? Besides, as you burn up fuel, your C/G is going to change anyway. Version one ( v 1.00 ) machines with the tank forward of the belt center pulley will gradually pitch nose-up and the opposite would occur on the current production version two ( 2.00 ) ships. From what I have heard about the lead pitch in the front head is to keep down unwanted piroets. It supposedly acts like a train where you have an engine pulling the cars with a caboose bringing up the rear. Am I sounding pretty un-scientific now?

09-13-2006, 05:26 PM
Hi Don,

The chord-wise CG should be roughly the same distance from the leading edge as the bolt hole is from the leading edge. I THINK it is around 30% back, but I'm not sure. I'll try to give it a measure this evening.

As for the span-wise location, it's out at about 55% from the bolt hole. Again, I'll have to measure it.

As for using heading hold on any of the axes, I don't think I would; not until I was comfortable with it in a solid hover. I'm not sure, but I'd think that the feedback process would be a bit too slow with the tandem. I'd recommend going strictly with 'normal' or 'rate' mode until you get comfortable enough to experiment.

OK... so now the real question... what is a "vinegar stroke"?


09-13-2006, 10:42 PM
Ok... I measured the chord-wise and span-wise CG location on my blades.

The chord-wise CG is at 11 mm on a 39mm chord. Thus, it sits at 28%.

The span-wise CG sits at 242 mm on a 476mm blade span (bolt-to-tip). Thus, it sits at 51%.

So, there ya go!