Greg McNair

My Feedback: (3)

Hi guys, my name is Greg McNair, and I am a scale helicopter builder/pilot in Birmingham AL. I am always on the lookout for better technology for my aircraft, and it occurred to me that you guys might be able to help me with an issue. The tailrotor drive mechanisms on these helicopters is typically either wire, torque tube, or belt. When using the torque tube, flexible couplings are required. Wire usually doesn't require it because there is enough flex in the wire to allow some radius bending. Belts obviously don't require couplings. I am building a project that has a 45 degree tailrotor, as shown in the photo here:



It will be going inside a fuselage that looks like this:



Instead of using bulky intermediate gearboxes, etc, I would like to use a flexible drive cable that I can put couplings on at either end, and make a long gradual sweep to make that final 45 degree turn up to the tailrotor gearbox. Do you guys use any type of flexible cable for your prop drives? We have used speedo cables in the past, but one of the problems we have with those is that they tend to "unwind" when loads change quickly. A penny for your thoughts?

Thank you!

tsunami

how thik can the cable be?
or a better question is the size of space wher the cable is

this should help
speedio cables arnt made for this level of stress if you can find a cable for light lifting at a hardwhare store might work but some can be very stiff but theyll tak the fluxing speed better

Greg McNair

My Feedback: (3)

Oops, I left out some important info didn't I? My bad!

The space to be occupied varies among helis. I'm looking for as lightweight a drive system as possible, and if the entire system can be smaller than 10mm diameter, that would be ideal. Length will vary, from 2.5ft up to 4ft, depending on the model size. I can secure the drive system to bulkheads built along the tail of the fuselage. If I wind up with a system that is as heavy as using an intermediate gearbox, torque tubes, and their requisite mounting systems, then I didn't really accomplish anything. And that by the way is my main goal....to be lighter and less complicated than a torque tube setup. Here's what a typical torque tube 45 degree tail drive would require:

*2 torque tubes (One from the main mechs to the intermediate gearbox, the other from that intermediate to the tailrotor gearbox)
*Mounting system, which may include use of a straight piece of pipe/tubing to support the torque tube
*Intermediate (45 degree) gearbox
*Tail gearbox
*4 sets of couplers to attach the torque tubes to the gearboxes and main mechanics

The helicopter in the photo above showing just a "pod and boom" arrangement has a belt driven tail. While this is smooth and quiet, it doesn't lend itself well to scale construction. There are gears and a second belt at the 45 degree upturn that drive the tailrotor gearbox.

One last important tidbit...the tailrotors spin at speeds of ~4000 rpm. This varies among helicopters depending on several factors. I would estimate range to be 3000-5000rpm though

TERBObob

My Feedback: (21)

Greg ,
First , I gotta say that all that heli lingo you were throwing out there made absolutely no sense to me being as to I only run boats ... ( )
Now ... about the flex cable ... you said it unwound ? Might sound lke a dumb question , but was it twisted in the correct direction for your application ? Even a auto speedo will withstand that kind of R's , but not sure what kind of HP power being applied , that we are talking about here ...
Also , this too might sound dumb , but did you have the speedo cable in a tube to re-inforce it from wrapping/unwinding ?
The flex shaft , if ran in the correct direction , would not unwrap , but rather twist tighter and then eventually snap , if too much back presure and too much forward HP was applied .
Again , not knowing the first thing about heli's , you gotta excuse my ignorance in not knowing what kind of blades or what kind of pressure is being applied from them . ( the only thing I DO know about heli's is that when a fellow can fly them right ... MAN ... are they neat to watch ! )
Also , if your heli cable needs to run opposite direction of the flex cables you have , you could have one made via a local shop . ( if unsure as to where , ask your local auto parts store where custom speedo cables are made )
FTR - most boats use 1/4 flex cable - same as what you find in most weed whackers that you use for your lawns .

tsunami

the unwinding as i under stand is from the rotor blades carring too much momentem and when the cable slows it unwinds.encasing it in a copper tube or other stronge tubing that it fit rather close is a very good idea.good luck with your project

Safe_Cracker

Greg McNair, Aren't you a fellow RunRyder member? Anyways a flex cable would work but you will still need a tranny at the 90 deg angle (Tail box). I understand what you are talking about, as long as it is nothing radical and you are not looking for anything spectacular in performance you should be ok. The cable would have to leave the pinion gear (off main gear) then travel down the tail(needs to be affixed every few inches) but once you get top the last section it will still need a gearboax for the 90 deg to the tail rotor. You may get away with a Dremel cable? The only thing I may be concerned about is the tension on the cable during authority input demands. THe cable would need to be secured very well to keep the flex down because there is alot of force acting during tail inputs due to the varing pitch levels. I believe it is possible...Anything is possible! GoodLuck...

Greg McNair

My Feedback: (3)

Safecracker: Yeppers, I am one in the same Greg. I had considered a dremel cable as well. That seems like something strong enough, but i've never applied "stopping" forces to my flex attachment, so I don't know if it could handle it. The gearbox will be standard just like on my other helis, with a 90 degree done via pinion gears. That's not a problem, as I am using existing technology. I'm just trying to find a less complicated and less bulky way to drive the tailrotor than to have precision torque tube arrangements, etc. And don't worry, I won't be flying these "3D" style. Strictly scale application here.

Bob: I apologize for the confusing lingo. The torque tube is a shaft, usually made of carbon arrow shaft or hollow aluminum tubing, which works much like an automobile driveshaft, supported along it's length by bearings, and centered in a tube. Here's a photo to illustrate the intermediate (45 degree) gearbox on the right and the tailrotor gearbox on the left:



The torque tube is driven by a series of gears coming off the engine. Typical power for my application will be in the 1 to 3hp range. It would vary, depending on the size of the aircraft. Imagine if you would having an adjustable pitch 12" diameter two blade prop on the back of one of your boats. Now imagine running the boat up to speed, then quickly reversing the pitch on the prop to slow it down. That would be a very extreme equivalency to what i'm dealing with. Only when I change pitch, I change which direction the nose of the helicopter points. I don't do this fast or too suddenly because I don't fly my particular helicopters in the aggressive aerobatic style that most people have seen. I fly mine more like the real thing, because that's what I am modeling, is a "real" helicopter subject. And real ones don't do sudden tail shifts like that, or at least not often and not intentionally. Now understand that I am not changing the direction of the tailrotor spin, just like you wouldn't be changing the direction your prop spins. You're simply changing the pitch of the blade. This is still hard on the drive unit, so it must be tough enough to withstand those type changes.

I've not yet tried to use any flexible shaft drive, so I haven't personally experienced unwinding, but what I am told is that when a sudden change in tail rotor pitch is applied, (the cable, if winding in the proper direction) will "expand" or unwind to a degree, allowing the tail thrust to drift. I know that the cables used in the past were sheathed. The weedeater flex cable sounds like a great idea. Where can I find this?

Tsunami, you pretty much nailed it on the head. The only thing that worries me about the sheath is if the cable unwinds enough to expand to touch the sidewalls of the sheath, and the resulting friction slowing the blade rpm down enough to lose tail control. Kind of like hitting the brakes and inducing an out of control skid in a car.