Single rotor - GyroStick

[leadfeather]

I want to see if the flexible feather hinge will work on a single rotor design. I volunteered my Slow Stick for this job.

I have had it out once. It barely broke ground and wouldn't stay up but... it didn't appear to roll. I think the blades may be too flexible for the weight. They are restricted from hinging up at the tip only. I am going to stiffen up the blades along the entire length.

I also noticed a definite 2 pulse per rev. 3 or four blades may smooth this out.

Dan

[iter]

Looks interesting! Let us know how it works.

Did you cut up a Slow Stick wing to make the blades?

Ari.

[leadfeather]

The blades are just 3mm flat depron cut and painted to match the original look.

Dan
:)

[leadfeather]

I got a break from the weather and took a break from work. Winds about 5 to 10.

Previously the blades were only taped (with hinge tape) on the top surface to the leading edge spars they were free to swing up, and they were fixed from feathering at the tips. Now I wrapped the leading edge top and bottom with packaging tape. The blades can still flex a little in the feather direction but not a lot. The blades were pre set with bout 5 degrees negative incidence.


I taxied the Gyro Stick around a bit just to make sure all was ok.

Finally I ran up the power up...lift off nice and smooth followed by a nice smooth roll into the ground. :eek: No significant damage, wooo.

Now here is where it gets interesting. The rotor was installed for clockwise rotation. This means the ASL should hav rolled the craft to the right. The craft actually rolled to the left!!!!

I definitely had to think about this for a bit. Here is what I think may be going on. The rotor did not create much if any ASL. The roll to the left was induced by the large diameter propeller, GSW 1147. If this is true, and the roll was dominated by the propeller, then this rotor design may be very effective at reducing ASL.


I am going to replace the propeleler with a smaller one and try again.

Dan
:)

[mnowell129]

Now here is where it gets interesting. The rotor was installed for clockwise rotation. This means the ASL should hav rolled the craft to the right. The craft actually rolled to the left!!!!

:)
Nope. Not true.
Asymmetric lift causes a pitch up, not a roll.
You likely experienced a coning induced roll which
rolls to the advancing blade.
mick

[alfoot]

Mick,

You beat me to it :) .

I was going to point Dan to the earlier thread which discussed the effects of CW versus CCW rotor rotation, and the effects of coning given here:-

http://www.rcgroups.com/forums/showthread.php?t=577093

Keep up with the innovation Dan.

Al

[leadfeather]

Is the rolling to the retreating blade side another one of the common myths?

The rotor is pretty rigid, coning is very small.

Dan

ps Thought about the ASL a bit. Is it the gyroscopic precession that makes it effect pitch?

[mnowell129]

Is the rolling to the retreating blade side another one of the common myths?

ps Thought about the ASL a bit. Is it the gyroscopic precession that makes it effect pitch?
Yes and yes.Look for some more info soon.

[leadfeather]

I put on a smaller propeller and went back out to the field.

First try, short hop, still had a left roll, but much reduced from earlier today with the larger prop.

The spindle for the rotor is a 1/16" music wire and was designed to be tweaked for ground adjustment of rotor disk angle. So tweak I did, I bent it over about 10 degrees to the right looking from behind.

2nd hop, rolled a bit to the right this time, but landed on it's wheels!!!...hmmm getting somewhere. :D

Tweaked it back to about 5 degrees. Ta daaaa. level flight. Now the main problem was I didn't have enough power to sustain flight.

Hops 3 through 8 were about the same, not enough thrust to keep it in the air for more than about 50 or 60 feet. I actually quit flying beacause I couldn't make any more progress, not because of a wreck!! :)

One obsevation I made was that the yaw roll coupling didn't seem very strong (not like the Slow Stick). I would do minor corrections with the rudder but the roll seemed sluggish. I didn't have much time to really check this out though.

Next, a little bit bigger prop since now I think I know how to compensate for the propeller torque now.

Dan
:)

[leadfeather]

Thinking back to the first hops, I'm wondering if the inability to sustain flight and the sluggish roll were due to a forward cg?

I picked up a little bigger prop and installed it. I now have a video camera and tripod so I should be able to video tape the flights.

Dan
:)

[leadfeather]

I added a servo to the rotor module for the GyroStick. The servo was something I had laying around, It's heavy, 49 grams, but hopefully it will handle the loads.

I added a Y connector to the rudder servo output. When the fixed wing is replaced with the rotor module, the roll servo is plugged into the Y connector.

Below is a link to a video clip of the actuator being moved.

Dan

http://www.rcgroups.com/forums/showthread.php?t=618417#post6640095

[alfoot]

Dan, good luck with the roll servo flights.

GyroStick is such a pretty autogyro that she needs to fly!! :)

Al

[leadfeather]

Thanks Al,

I'm hoping to try it New Years day if the weather breaks as forecasted.

Dan

[leadfeather]

It was a bit cold, but other wise a great day for flying. I got a chance to test the roll module added to the rotor of the GyroStick.

Ground handling was fine as expected, since the rudder is still functioning.

Take off required a lot of speed and a lot of nose up. I climbed to about 15 feet and and flew straight ahead for about 300 feet. The roll was very responsive, in fact I was over controlling a bit since the roll response without the roll servo installed previously was so poor.

Since it seemed to be struggling to stay up, I din't want to risk bringing it around for a circuit, instead I just landed straight ahead in the grass.

It looks like the roll control should be fine with the roll servo added. Now I'm trying to decide the next best step to get the rotor lifting a bit better.

Dan

[edi]

How about more or broader blades?

[JochenK]

Dan,

if you use a good 20 g servo with metal gears you can reduce your overall weght by 29 g. That should help.

Jochen

[leadfeather]

Dan,

if you use a good 20 g servo with metal gears you can reduce your overall weght by 29 g. That should help.

Jochen

Thanks Jochen, every little bit helps.

The all up weight is 466 grams on a 30" diameter disk. I calculate 3.375 oz/sqft. Any idea if this is ok or too high? I have room to increase the rotor diameter to 34" Which is about 28% more disk area.

Another possibility is that the blades are twisting (feathering) too much underload. If this is the case, I was thinking of two fixes. A) would be to add some blade area ahead of the spar to get the center of pressure much closer to the hinge, and B) Add a 3rd blade to reduce the blade loading.

If I have time I might try both.

I'm sure one of the conventional rotor designs would probably work on this machine, but the reason I started this particular project was to specifically test out the flex feather hinge rotor idea as a single rotor.

Dan

[rchopper56]

Originally Posted by Leadfeather
Another possibility is that the blades are twisting (feathering) too much underload. If this is the case, I was thinking of two fixes. A) would be to add some blade area ahead of the spar to get the center of pressure much closer to the hinge, and B) Add a 3rd blade to reduce the blade loading.

Dan
Just a thought: You mention that the blades are feathering too much - do you mean that they are going into much negative pitch? If so, adding weight to the blades behind the so called pitch axis will redude their negative pitch. This is the tennis racket effect.
Gene

[leadfeather]

Dan
Just a thought: You mention that the blades are feathering too much - do you mean that they are going into much negative pitch? If so, adding weight to the blades behind the so called pitch axis will redude their negative pitch. This is the tennis racket effect.
Gene

Gene,

Good idea. :) With this strategy, I could also choose to place this weight towards the tip (as well as aft on the blade as you suggest) to reduce the excess feathering to a greater degree near the tip and possibly help form a bit of negative twist (wash in) in the blade...maybe??

It would be great to have a bunch of foam, tubes, tape and a wind tunnel for a day or so. :)

Dan

[mnowell129]

Keep centrifugal force in mind. The aft weight will tend to pull the blade flat.

[leadfeather]

Keep centrifugal force in mind. The aft weight will tend to pull the blade flat.

That is a good point. The centrifugal force should also help reduce the excess feathering.

My concern with the aft weight is that it may cause the blade to flutter.


Dan

[leadfeather]

Another possibility is that the blades are twisting (feathering) too much underload. If this is the case, I was thinking of two fixes. A) would be to add some blade area ahead of the spar to get the center of pressure much closer to the hinge, and B) Add a 3rd blade to reduce the blade loading.


Dan

I think one of the uses of the term feathering is to reduce incidence. This isn't what I meant. What I am describing in the quote above is that the trailing edge may be raising too much relative to the leading edge creating too much negative incidence. I hope this is a bit clearer.

Dan

[mnowell129]

Feathering is generally the motion of a blade when it is rotating around the long axis of the blade. This is in contrast to flapping, the blade tip moving up and down with the root being the hinge. The feathering axis is at right angles to the flapping axis. Feathering in rotor craft (unlike feathering a prop in an airplane) doesn't imply positive or negative. It just describes the rotating motion of the whole blade along its length.
When a blade twists within itself, one end fixed and the other end changing angle, this generally isn't feathering (exception Kaman helicopters use an "aileron" on the flexible blades to force them to feather by twisting, but this is way outside the scope of our discussion) it's just a blade without enough torsional ridigity.
Dan,
You're really fighting an uphill battle. When the blade is free to twist it's just going to unload itself. The key to autorotation with a loaded down aircraft is to hold those blade angles at the right spot so that the blade makes lift and propels itself at the same time. A basic wing just doesn't want to do that. As an experiment take a rectangular wing and try to find a a CG position where a plain rectangle wing will glide forward by itself, an autogyro rotor just being the wing held at the tip being forced to turn in a circle. You really can't find the CG positon that will work without reflexing the airfoil, essentially making a flying wing out of it. About the only solution for the self feathering is to reflex the trailing edge to make the rotor fly at the right angle. (This is essentially the Kaman helicopter solution, a servo flap that makes the blade fly to different locations around the circle, even then the Kaman blades are torsionally rigid enough to not unload themselves under loading.)
I don't mean to be a raincloud but you are running into some basic aerodynamic "givens" that being that a wing cannot be made to make lift and be stable at the same time without some kind of horizontal tail (reflex being a horizontal tail that is really really close to the wing, therefore not very efficient).
I like the idea of the single rotor gyrostick, I think you might have more success by making the blades more rigid in twist and controlling their angle a little tighter.
Apologies for an unsolocited opinion. I'm hoping that I can save you some frustration.
mickey

[leadfeather]

Mickey,

The blades on this rotor are semi rigidly attached to the leading edge spar. They are quit a bit more rigid on the feather hinge than the Cyclone rotor blades were (which worked). I did this on purpose because the disk loading and the blade loading was going to be higher than was the case on the Cyclone.

The rotor design is "close" in that the GyroStick got off the ground although barely.

The initial blade incidence (unloaded) is about 5 degrees down. The rotor does seem to spin up no problem.

1st guess: Too much flex along the feather hinge is just my guess at this time since I can't actually see what is happening.

2nd guess:Another possibility may be not enough flex along the hinge line and the incidence is not negative enough for a flat plate airfoil blade. I hadn't even thought about this until reading about Al's experience with the MonoTwirl.

3rd guess: Another possibility is that the rotor is ok but maybe not enough mast angle. Currently it is set at 8 degrees.

Other ideas?

I think I have plenty of thrust, since as a fixed wing, my Slow Stick climbs straight up no problem.

The feedback is greatly appreciated... it got me across the roll control hurdle!

Dan
:)

[mnowell129]

My experience says that the L/D of a gyro is about 1:1 which means that the thrust needs to be equal to the weight for any kind of decent performance.
At 466 grams (~1lb) you need about the same thrust.
If you guess 5 ounces of thrust per watt of input on a brushless, this is around 80 watts needed.
I found that I needed something north of 65 watts on my 36", 1 pound gyros, so I don't think the math is too far off.
You may be able to go straight up with a wing with the prop/motor pitch speed matching the climb but you may not be making the static thrust required.
I figure you need 60-80 watts delivered to a 3.8 or 4.7 inch pitch prop to provide enough thrust on this size model.
Keep in mind that a rotor in autorotation has the same drag as a parachute of the same diameter.

8° seems marginal on the mast angle. The study I have says 12°-15° is minimum to maintain autorotation.
mick

[leadfeather]

Mickey,

I'll adjust the mast back a bit.

Compared to your machine specs, it looks like my disk might be small and disk loading kind of high. I think I'll also try going to a larger (34") diameter disk. Going bigger than this might be more difficult with the current layout.

Thanks!:)
Dan

[leadfeather]

I did a quick experiment just before dark. I taped a 1/2" wide strip of 3mm foam to the leading edge of the current two blade rotor. I taped it in a way that would not change the flex characteristic of the joint between the foam and the spar. This arrangement should move the center of pressure very close to the spar. I took it outside and held it in to the wind.

Without the modification, ( hinge point 1/8" behind leading edge) the blades would flex and become more negative under load.

With the modification (hinge point 5/8" behind leading edge) I could not see any change of blade incidence at all under load. The rotor spun up noticeably slower too.

As I was expecting, moving the feather hinge aft appears to reduce the negative feathering moment on the blade.

I'm still not sure if I need this. :)


Dan