Tandem gyro plans

[brett s]

For what it's worth CH-47's aren't very stable in yaw at all - turn off artificial stability & they're all over the place unless the pilot is on his toes (literally!)

Have had cabin windows popped out from high slip angles when someone was having a bad day during training with it off.

The only practical choices I can think of to connect your rotors are using a shaft or belt, that speedometer cable will have way too much drag IMO. Look at a typical r/c helicopter tail rotor drive setup for a place to start, should be able to find something you can borrow & make work.

[joe nelson]

Brett,

We've talked about the chinook's slippery rear end before. I did borrow some of the things that we had discussed. I have made the pylons larger in hopes it would add to the longitudinal stability. However, I couldn't get the longitudinal control required with just lateral movement on the aft head alone. If you recall the discussion we had about the location of the dynamic center on tandem helos having such a short arm...I found this to be true and that's why a rudder is needed.

Today, I'm changing to a shaft and double gearbox setup. The reason being is I'll have an advancing blade on each side of the airframe and the ease of rotation of the blades...it turns easier.

Brett, the major question is, how much airplane and how much gyro technology will be needed to make this tandem fly? I'm finding that many things common to the chinook works differently on a gyro. DCP for example. On the chinook it was a function of the cyclic and so far in the tests on the gyro it requires a separate collective input to only the aft rotor.

[brett s]

Have you seen this one:
Piasecki HUP tandem autogyro in flight - YouTube

[joe nelson]

Brett,

I've seen this video many times. Unfortunately, the rotors are a fixed angle and all control is through the rudder and elevators. The coaxial rotors are very short to provide higher rrpm for flight and don't intermesh like mine.

The builder is taking a different approach than I am. He has a flying model and I'm looking to get a workable model to be scaled up for a larger gyro. My goal is not just to get it to fly but to fly with standard flight controls.

The goal is 1000 pounds useful load, 100 mph and 2 hour range.

[joe nelson]

The change to gearboxes has made for a much smoother operation at lower rpm. The rrpm is notably greater than the speedometer cable setup that was used earlier.

While in front of the fan, the tandem responded to the control input but there is still much vibration that wasn't there before at higher rrpm. I wonder if this is resonance or some type of coupling? I'll add some tip weights to see if changes anything.

[joe nelson]

On the test range this morning, the model was tested with "NO" breakage! This is the first time that I have come home with the model in the same condition as I started. With $60 worth of wooden rotor blades installed testing was gentle.

High speed ground tests were performed. The aft part of the airframe rose to a level attitude on the main gear and was fully controlable. I didn't attempt a take off due to severe cold feet...lol. I continued start/stops until the battery was dead.

I couldn't tell what the rrpm was but it was enough to get the aft rotor lifting.

[davh12]

Getting closer Joe. Keep trying.

[joe nelson]

Thanks Dave!

I hope that this isn't a fluke and I'm really making progress! I'll know in a couple of weeks if these mods will be the fix.

There's still a few issues that I'm having trouble with like the C/G range. At this point' the C/G is at the 45% station and I've done a hang test to insure that I have a nose low attitude. All of this is assuming that the model is like a single rotor gyro not a heli.

BTW, how's your project coming? I haven't seen any posts in a while.

[davh12]

May not be a fluke. Just keep trying. I was starting to think that I was crazy until last week-end. How many times did it take Edison to complete the light bulb? Took the Wright Bros. 12 years to get the airplane right. I have may share of crashes and still much to do......rebuild and try it again. Ignore when people tell you that it will not work, unless it's a common sense thing or until mathematics/physics utterly stop you dead in your tracks.

Dave

[davh12]

I never realized just how much the CG affects the airframe

[joe nelson]

My problem is not knowing how much of either helicopter or airplane is needed to fix the my model. Does the CG fall between the two rotors like a helo or at the 25% station like an airplane? I've had it all over the place trying to find out. The only thing that I can assume is, the reason why it wasn't flying was not directly related to CG placement. Now, the CG is at the 45% station and the model has behaved OK...the tail has lifted in a normal fashion but I couldn't pull back on the controls. I was hoping that it would just float off the ground but it wasn't to happen.

It will be another week before I can get back to testing...just keep your fingers crossed, lol. I'm just praying that the model doesn't start looping again...I think that I've fixed that problem.

[davh12]

Gyrocopters have an offset axis for pitching the rotorhead foraward/aft, adjusting relative wind flow up through the rotor disc and in lateral fashion as well. If you look at it from above, do the rotor discs over lap? If they do that poses the question of how much relative wind is going through each of the discs and is one degrading form the the other. Another aspect is hight of each rotor disc, regarding the pylons through which your masts run. I have the gears for the tandem version of my patent that I want to build next year and my spacing between rotor masts, regarding airframe length, will allow both rotor heads to operate indepently with rotorblade tip path plane clearance without synching the gears. But, may rotors will be synced any way just because I want to use one power plant to run both rotors. Further, what angle are the mast pylons oriented? You probably already have gone through this, but I'm just trying to trouble shoot.

If you extended your airframe to allow blade tip clearance and both rotor masts tilted at equal moments, in theory, there should be equal air flow up through both rotor discs.

http://debower.net/gyrocopter.html

[joe nelson]

Dave, the model does have overlapping blades and are tilted back at the same angle. Both rotors are also coupled with small gearboxes to provide the same rrpm. The height between the rotors are what the NACA study recommended at 1/2 a blade length separating the two.

Earlier in my tests, I had the rotors configured in several ways. The coplanar set-up was as you said... with no overlap and no coupling. When the rear rrpm came up to speed it would roll over in the same direction of the rear rotor's rotation. The aft rotor pylon was raised and the speed of the roll over increased. Several test flights resulted in loops of various sizes as I changed the CG.

Both rotors are coupled, now. Each side will have an advancing blade on each side and have the same rrpm. The last test was longitudinally controllable and the tail rose to level flight...no take-off was achieved. What I have is a steerable model that shows a tendency to fly...that's progress I think.

[davh12]

Yep, I see what you mean, does the material that you used for torsion bars/flapping hinges flexible enough? I remember you mentioned music wire. Can there be a possibility that there is not enough flapping action?

Dave

[joe nelson]

Dave,
I've changed the head material to a .018" inch thick polypropylene plastic. It's made up of 4 sheets of the stuff and it appears to flap OK in front of the fan. I will add or substract the number of sheets depending on the amount of droop. 4 sheets seems to be good for the weight of my blades.